In the succession cored at Site 1109, we recovered ~770 m of sediments and sedimentary rocks above basic igneous rocks, interpreted as overlying a Miocene forearc sequence. Eleven lithostratigraphic units were recognized on the basis of sediment or rock type, grain size, sedimentary structures, color, smear slides, thin sections, bulk mineralogy (X-ray diffraction [XRD]), and carbonate content. In addition, geophysical logs, including FMS data, were used to help reconstruct the lower part of the succession, which had poor recovery by rotary drilling.
The lithostratigraphic units recognized are shown in Figure F1. Unit I is composed of calcareous sand, silt, and clay, with volcaniclastic sand and volcanic ash. This unit is divided into two subunits, based partly on the relative abundance of volcanic ash. Unit II is dominated by greenish gray clay interbedded with abundant volcaniclastic sand. A marked feature is the presence of graded couplets of dark gray, fine-grained volcaniclastic sand overlain by greenish gray silty clay and clayey silt. Unit III comprises clayey silt, and silty clay interbedded with clayey silt, to coarse-grained sand containing volcanic, siliciclastic, and biogenic particles. Unit IV is more lithified and is marked by generally finer grained, greenish clayey siltstone and silty claystone with rare foraminifers. Unit V marks a return to slightly coarser grained sediment, with dark gray siltstones and sandstones with common volcaniclastic material. Unit VI is characterized by a clayey siltstone and silty claystone with subordinate thin beds of sandstone/siltstone. Unit VII exhibits a sharp change to coarser grained bioclastic sandstones and limestone (i.e., packstone and grainstone). Beneath this, Unit VIII exhibits an additional major change to dark colored silty clay and clayey siltstone with common plant material and shells. Unit IX is characterized by clayey siltstone and volcaniclastic sandstone that is highly altered and contains goethite concretions. In Unit X, conglomerate containing pebbles of variolitic basalt and dolerite was recovered. The conglomerate lies unconformably above basement dominated locally by dolerite (Unit XI; see "Igneous and Metamorphic Petrology").
All of Unit I was recovered in Hole 1109C. However, the uppermost few meters of the unit were also cored in Holes 1109A and 1109B. These three cores include a number of distinctive marker horizons, notably two individual thin beds of volcanic ash (each ~2 cm thick), calcareous sand, greenish silt layers, and a bed of volcaniclastic sand that together allow a precise correlation between the three holes (Fig. F2; see also "Composite Depths"), confirming that a complete succession was recovered.
The Unit I succession is composed of the following main types of unconsolidated sediment: nannofossil ooze, calcareous clay, calcareous silt, volcaniclastic silt and sand (with rare granules), and volcanic ash. The volcanic ash layers are best displayed in Core 180-1109C-3H from 16.90 to 26.40 mbsf but occur sporadically throughout the whole of Unit I.
Nannofossil ooze, calcareous sand, silt, and clay, interbedded with a small number of volcanogenic layers, are abundant in the upper part of the succession, and defined as Subunit IA.
By contrast, the lower part of Unit I is mainly clayey silt and silty clay, with only rare volcanic ash layers; this interval is defined as Subunit IB.
Similar sediments occur in both Subunits IA and IB and for this reason are described together (as individual lithologies) below:
Nannofossil oozes rich in clay are present only in the uppermost part of Subunit IA. These sediments range in color from brown (10YR 6/4-6/2), in the uppermost 14.5-cm-thick oxidized interval, to greenish (dominantly 5GY 5/2) beneath this interval. The nannofossil ooze is rich in planktonic foraminifer tests and clay. Determinations of calcium carbonate indicate values ranging from 47-68 wt% in Subunit IA and from 15-67 wt% in Subunit IB (see "Organic Geochemistry").
This clay forms a minor constituent of Subunit IA and a major constituent of Subunit IB. The calcareous clay occurs as repeated intervals up to several tens of centimeters thick (e.g., Section 180-1109A-1H-5). Sulfide mottling is ubiquitous. Individual mottles are typically up to several centimeters in size; some follow lithologic boundaries, whereas many occur independently of lithologic variation.
This silt is a minor constituent of Subunit IA and a major constituent of Subunit IB. The silt commonly forms alternating very thin beds (<3 cm) more or less rich in planktonic foraminifers and siliciclastic sand grains (e.g., Section 180-1109C-3H-2). A number of thin foraminifer-rich beds exhibit sharp bases, normal grading, and diffuse tops, passing into nannofossil-rich clays (Fig. F3). Locally, small blebs of volcanic glass occur.
Smear-slide analysis shows that the silt is clay rich and contains quartz, plagioclase, rare accessory minerals (e.g., biotite or pyroxene), and variable amounts of fine-grained volcanic glass particles ("Site 1109 Smear Slides"). Bioclasts are present as ubiquitous calcareous nannofossils, together with sponge spicules, scattered radiolarians, and diatoms. Radiolarians and diatoms are most abundant in several of the most volcanic-glass-rich layers. XRD analysis shows that the silts consist mainly of calcite, quartz, and plagioclase (Table T3).
Carbonate sand occurs sporadically as a minor constituent in Subunit IA. The sand forms relatively coarse-grained, white, granular layers, passing up into fine-grained sand and silt in isolated beds as thick as 25 cm. For example, two beds in intervals 180-1109C-4H-1, 0-28 and 57-61 cm, are composed of coarse granules with abundant fragments of calcareous algae (e.g., Halimeda sp.), planktonic and benthic foraminifers, and shell fragments (Fig. F4).
Discrete ash-fall layers, present in both Subunit IA and Subunit IB, are recognized by their mostly sharp bases (Fig. F5), often bioturbated tops, normal grading, and high content of pyroclastic components. Glass-rich layers are light gray to dark gray, and 0.5-12.5 cm thick. The ash layers are dominated by fresh (i.e., not devitrified) colorless, platy to bubble wall shards that make up 80%-95% of individual layers (Fig. F6). Shards with tubular vesicles and pumice are generally rare. The shards are commonly only a few micrometers thick but range from ~50 to ~300 µm in size. Carbonate and detrital components are minor constituents of ash layers. In addition, Pleistocene ash layers contain less than 10% of co-genetic phenocrysts (i.e., biotite, plagioclase, quartz, amphibole, and clinopyroxene). The ash is most abundant in Subunit IA above 40 mbsf, decreasing downward. Ash persists as sporadic thin layers in Subunit IB (e.g., Sections 180-1109C-5H-5, 5H-7, 6H-2, and 6H-7).
Individual volcaniclastic sediment beds are generally <15 cm thick, but the locally reach 35 cm (interval 180-1109C-3H-7, 34-68 cm). The grain size of the volcanic-ash-rich sediment ranges up to coarse-grained sand with rare granules and pebbles of pumice. Smear-slide analysis confirmed that the volcaniclastic layers are rich in volcanic glass (e.g., Sample 180-1109C-2H-6, 49-53 cm; "Site 1109 Smear Slides"). This is mainly composed of colorless volcanic glass in the form of platy shards and bubble wall shards and is inferred to be of rhyolitic composition.
The volcaniclastic sand beds exhibit sharp bases, normal grading, planar-laminated upper parts, and diffuse tops, passing up into clay-rich silt (Fig. F7). Some beds are structureless (e.g., Core 2H). In addition, wavy lamination was rarely observed (e.g., intervals 180-1109C-2H-7, 36-37 and 47-49 cm). Also, small (<2 mm) shell fragments are commonly observed, mainly in the upper parts of beds.
Silt occurs, both forming the upper parts of graded sand beds and as separate, thin (several centimeters) beds of silt grading up into silty clay and clay. This silt is locally sandy (e.g., Core 2H), with dark volcanic glass fragments scattered throughout. Occasional discrete thin layers of volcanic ash, composed of silt-sized particles, were also observed, commonly including a relatively high clay content.
In addition, several thin, very coarse grained volcaniclastic layers were noted throughout Subunit IB. Sandy gravel occurs mainly in the upper levels of this subunit as a few beds up to several tens of centimeters thick (e.g., Section 180-1109B-1H-3). Also, a concentration of pumice fragments up to several centimeters in size in the interval 180-1109C-5H-4, 73-83 cm, is probably related to a specific volcanic event.
From 36 to 55 mbsf, the sediments are markedly deformed. Bedding ranges from subhorizontal to angles of up to 60º, or rarely 90º (e.g., interval 180-1109C-5H-2, 120-150 cm). Apparent folds with a limb spacing of up to 50 cm were observed (e.g., Section 180-1109C-6H-4; Fig. F8). Some beds are strongly disrupted by layer-parallel extension generating a boudinlike fabric. No penetrative fabric is developed. Further details are given in "Structural Geology."
Unit I is interpreted as a dominantly deep-water, middle-bathyal (500-2000 m; see "Biostratigraphy") hemipelagic succession of calcareous silts and clays interspersed with clastic intervals of several different origins. Only the surface sediment has a lower bathyal (>2000 m) benthic fauna representative of the present water depth (2211 m) at the site.
The thin-bedded calcareous clays and silts were mainly redeposited by low-density turbidity currents. The rare, white carbonate sands are interpreted as derived from neritic shallow-water carbonates (i.e., mainly algal deposits) that were also deposited by turbidity currents. Paleontological data indicate Unit 1B sediments accumulated at ~200 m/m.y. prior to 1 Ma, whereas Unit 1A sediments accumulated at 21 m/m.y. to 0.46 Ma and 67 m/m.y. thereafter (see "Biostratigraphy").
The volcaniclastic intervals are interpreted to be the result of contemporaneous volcanism for which volcanoes of the Trobriand Arc or those within Dawson Strait are the probable source (see "Depositional History"). The well-sorted volcanic ash layers are interpreted as primary pyroclastic airfall deposits, with minimal mixing with other constituents. By contrast, the relatively rare volcaniclastic sand layers are of epiclastic origin and were redeposited by turbidity currents.
Several arguments in favor of either a primary origin, or one by drilling disturbance, can be considered.
Drilling disturbance (APC): (1) the deformation is highly incoherent such that individual lithologic units cannot always be recognized on both limbs of individual "folds;" (2) some, but not all, of the deformation is focused within several millimeters of the core liner, suggesting that at least some coring-related disturbance is present; and (3) seismic data for Site 1109 imply that the sedimentary succession is nearly flat lying, although minor faults are imaged on seismic profiles in the vicinity (see "Drill Sites" in the "Leg 180 Summary" chapter), and normal faults were observed deeper in the succession at Site 1109 (see "Structural Geology").
Primary origin: (1) Many of the structures are sharply truncated at the edges of cores without disruption, suggesting they are considerably larger than the core diameter; (2) the structures occur within high-recovery APC cores, and are not restricted to the upper or lower parts where drilling disturbance is anticipated; and (3) similar deformation features were not observed above or below the discrete deformed interval. Paleontological evidence shows that reworking of biota is significant in the deformed interval, but no zonal markers are deleted or duplicated (see "Biostratigraphy").
Unfortunately, the disturbed interval is too shallow to be imaged by FMS data, which could have conclusively differentiated between the alternative origins. On balance, a primary origin is preferred, suggesting that the seafloor experienced a phase of sediment instability, probably leading to sediment redeposition as a slump that was perhaps triggered by faulting at depth.
Unit II is characterized by repeated normal-graded beds of volcaniclastic sand, silt, silty clay, clayey silt, and clay, together with rare carbonate packstone and volcanic ash. Unit II corresponds to an interval of relatively high magnetic susceptibility (see "Physical Properties"). This unit is the first to be covered by geophysical and FMS logs (see "Downhole Measurements").
The top two cores of this unit (10H and 11H) were recovered by APC, whereas the remaining seven cores (12X through 18X) were obtained using the XCB. Visual inspection indicates that the APC cores are undisturbed, whereas the XCB cores become progressively more disturbed downward. Cores 10H, 11H, 12X, and 13X were recorded as moderately disturbed; Core 15X as moderately disturbed; Core 16X and 17X as highly disturbed, and finally Core 18X was logged as drilling biscuits. Notably, no marked change was observed in the appearance of similar lithologies in the lower, APC, and the following upper, XCB cores, confirming that the lithologies described below from XCB cores are primary and not the result of drilling disturbance.
Most of the clay and silty clay occurs within the upper parts of thin-graded beds (couplets) as described below. However, structureless clay is also locally present in intervals as thick as tens of centimeters. The sand is homogenous, or occurs as very thin beds. Compared to Unit I, levels of calcium carbonate are much lower (2.8-31 wt%). In general, the more clay-rich zones are more calcareous (10-31 wt%), whereas siltstones are less calcareous (~10 wt%; see "Organic Geochemistry").
An ubiquitous feature of Unit II is the occurrence of repeated greenish colored couplets of fine-grained sand, grading progressively upward into silt and silty clay, and finally into clay. The sands, and silts are slightly darker in color (5Y 4/1-5/1) than the clay and silty clay (5GY 5/1-7/1).
The bases of individual couplets are sharp, locally with evidence of slight scouring sediments beneath. Rhythmic couplets dominate all but one of the cores recovered from Unit II. The clays and silty clays forming the upper parts of individual couplets are markedly bioturbated, as indicated by Chondrites burrows, whereas the sands and silts are commonly less bioturbated (Fig. F9). For example, 25 couplets occur in Core 10H. Because these couplets are from APC cores (with complete recovery), a primary (i.e., not drill disturbed) origin is certain. Core 12X, the first XCB core, contains 52 couplets, individually ranging from 1-3 cm to 10-20 cm, with an average thickness of ~10 cm. The fact that these couplets recovered by XCB are effectively identical with those observed in the overlying two APC cores confirms that they are of primary origin.
Smear-slide analysis of the fine-grained sands and silts reveals the presence of quartz, muscovite, clay (unidentified), volcanic rock fragments, volcanic glass, rare pyroxene, planktonic foraminifers, nannofossils, rare radiolarians, carbonate, and opaque grains (see "Site 1109 Smear Slides"). In addition, XRD analysis indicates the presence of calcite, chlorite, quartz, plagioclase, augite, illite, and amphibole. Indeed, the XRD data indicate a marked change at the top of Unit II with the appearance of ferromagnesian minerals (augite and amphibole) and clays of mixed-layer type (not identified in detail).
The regular couplets are interrupted from 150.5 to 151.9 mbsf by a single interval of greenish gray silty sand interbedded with three very thin to thin volcaniclastic sand beds (each 1-4 cm thick; Section 17X-1). Each of these thin sand beds exhibits a sharp base, and normal grading from sandy silt at the base to very fine silty clay at the top. An interval of sandy silt, 0.9 cm thick, directly beneath (Section 17X-2), comprises sandy silt with volcanic granules. Bioturbation is ubiquitous, with sulfide-rich burrow infills. Smear-slide analysis shows the presence of quartz, biotite, pyroxene, clay (unidentified), volcanic rock fragments, volcanic glass, planktonic foraminifers, rare siliceous spicules, and pyrite.
Very thin (<3 cm) volcanic ash beds are very rare. Beds exhibit sharp bases composed of fine- to medium-grained sand, grading up to silt (e.g., interval 180-1109C-13X-1, 90-95 cm). These ash layers are seen in smear slides to include colorless, acidic volcanic glass, together with some co-genetic phenocrysts ("Site 1109 Smear Slides"; see Fig. F10). The glass is similar to that found disseminated within the graded couplets described above.
Two graded carbonate beds (12 and 10 cm thick) were observed in Section 13X-3 (e.g., Fig. F11). Also, a single bed of medium-grained packstone, 30 cm thick, was observed in the interval 180-1109C-14X-1, 0-30 cm. This carbonate sand contains abundant calcareous algae, planktonic foraminifers, and shell fragments. The sand is bioturbated throughout with burrows infilled by sandy silt imparting an overall dark gray color. A calcium carbonate analysis yielded a value of 85 wt% (see "Organic Geochemistry"). In addition, a single clast of carbonate packstone was recovered from interval 180-1109C-11H-1, 0-7 cm.
Unit II, of early Pleistocene age, is a rhythmic succession of volcaniclastic sand-silt and silt-clay couplets: these are interpreted as repeated deposits from low-density turbidity currents in deep water (500-2000 m; middle bathyal). Sedimentation rates were very high, estimated as 225 m/m.y. (see "Biostratigraphy"). The remarkably uniform thickness and grain-size variation of the couplets, without thicker bedded and coarser grained turbidites, could have several possible explanations. These include (1) the fine pyroclastic material was derived from a relatively remote source that accumulated upslope and was then redeposited by turbidity currents and (2) frequent triggering of turbidity currents by volcano-tectonic events or glacioeustatic-related processes. The single interval of more thickly bedded and coarser grained silt, sand, and granules (Core 18X) is viewed as rare deposits from high-density turbidity currents.
In addition, the provenance of a few beds was from a shallow-water origin, as shown by the presence of the rare packstones with calcareous algal and shell fragments. The co-occurrence of common planktonic foraminifers indicates mixing of neritic and pelagic sediment before the final deposition of the packstones as calcareous turbidity current deposits (i.e., as calciturbidites). The volcanic glass in the rare ash layers and admixed with the volcaniclastic turbidity current deposits is of similar acidic (rhyolitic) origin, suggestive of derivation from the Trobriand volcanic arc or volcanoes in the Dawson Strait area as it is for Unit I higher in the succession.
The geophysical and FMS logs confirm that Unit II is composed of alternations of more or less clay- and sand-rich intervals (see "Log Unit L2"). Thin, graded sand beds are well imaged by the FMS and marked by sharp bases and diffuse tops. The geophysical logs (e.g., gamma ray) indicate the presence of a few sand layers from 156 to 161 mbsf that were not recovered by coring. A carbonate-rich horizon also exists around 161 mbsf that may correspond to one of the thin calciturbidites observed in the cores. Dips of 5º-8º mainly to the south-southwest are common lower in this unit (see "Downhole Measurements"). This dip might reflect the paleoslope at the time of deposition but is more likely to record postdepositional tilting toward the depocenter of the Woodlark rift basin.
In addition, specific variations in log response (e.g., in the gamma log and photoelectric logs) can be interpreted to indicate the presence of several repeated, discrete cycles (156-161 and 192-200 mbsf) each beginning with calcareous sediment (0.4-2.0 m thick), followed by sand (1.0-4.0 m thick), then clay (up to 5.0 m thick). These cycles could indicate cyclical sedimentation controlled by glacioeustatic sea-level change in the source area, with warmer, wetter interglacial periods being marked by enhanced continental weathering and runoff; alternatively, they could reflect tectonic-controlled events.
Unit III is composed of clayey silt and silty clay, with subordinate sand and rare calcareous intervals. Relative to the surrounding units, the recovery was greatly reduced, probably reflecting the difficulty of XCB coring indurated, but unlithified, sediments (see "Operations"). The recovered sediment was markedly affected by drilling (see "Core Descriptions" contents list). Recovery was low in two cores (24X and 25X), from which mainly unconsolidated medium- to fine-grained sand was recovered, but this interval can be documented using geophysical and FMS logs.
Greenish gray (5GY 4/1-5/1) silty clay is the dominant sediment type in Unit III. The silty clays are extensively burrowed throughout (mainly Chondrites). Smear slides reveal the presence of quartz, feldspar, volcanic glass, carbonate grains, foraminifers, radiolarians, nannofossils, and sponge spicules (see "Site 1109 Smear Slides").
Clayey silts are very common as normal-graded, thin to very thin beds (<5 cm) and laminae (<1 cm thick), with small-scale burrowing, especially toward the top of individual thin beds. The clayey silt is slightly darker in color than the interbedded silty clay (5GY 5/1 and 6/1). In places, the clayey silt contains scattered sand grains commonly concentrated in burrows. Smear slides reveal mainly planktonic foraminifers, nannofossils, clay, and silt.
Thin beds of sand occur repetitively in some intervals (e.g., three thin beds in Section 21X-2). Such sands are graded and rarely show planar laminae (e.g., Sections 23X-5, 23X-6, and 23X-7), or cross-laminae (interval 180-1109C-23X-4, 10-13 cm). Also, occasional intervals contain beds of clay-rich sand up to several tens of centimeters thick (Sections 20X-1 through 20X-CC; Fig. F12). However, close inspection shows that such sands are not homogeneous but occur in repeated thin beds several centimeters thick.
In addition, a small number of thin beds are marked by sharp, scoured bases overlain by fine- to medium-grained sand, then passing up into clayey silt and silty clay. These coarser grained, graded units are intercalated with the finer grained, graded clayey silts described above. Examples of thicker and coarser graded beds occur at interval 180-1109C-20X-2, 40-45 cm. By contrast, some sand beds are structureless. Near the base of Unit IV, the sand is unusually dark in color (N4) and contains fragments of wood (interval 180-1109C-24X-1, 0-28 cm). At the base of the unit, fine-grained sand with volcanic rock fragments is also common.
In smear slides, the sands are seen to contain variable abundances of quartz, plagioclase, biotite, muscovite, volcanic fragments, volcanic glass, pyrite, carbonate grains, and plant debris ("Site 1109 Smear Slides"). In addition, XRD analysis revealed variable abundances of calcite, plagioclase, quartz, augite, chlorite, amphibole, and illite (Table T3). This assemblage remains little changed from Unit II (see "Lithostratigraphic Unit II"). Samples of silty clay and clay-rich silt were found to contain 4-26 wt% calcium carbonate (see "Organic Geochemistry").
A thin packstone-wackestone layer was noted in interval 180-1109C-21X-CC, 38-45 cm. This layer consists of alternating grain-supported carbonate (packstone), matrix-dominated (wackestone) carbonate laminae, and thin beds (Fig. F13).
Two thin sections of volcaniclastic sandstone (intervals 180-1109C-21X-1, 40-42 cm, and 26X-CC, 18-20 cm) shed important light on provenance as they contain mixed volcanic, metamorphic, and biogenic components ("Site 1109 Thin Sections"). The volcanic-derived material is variably altered basalt, acidic extrusive rocks, chloritized volcanic glass, zoned plagioclase, quartz, hornblende, and biotite. The metamorphic-derived constituents are polycrystalline quartz, mica schist, and rare serpentinite (Figs. F14, F15). In addition, the bioclastic component includes echinoderm plates, shells, calcareous algae, bryozoans, corals, benthic foraminifers (including rotalines and Amphistegina), micritic intraclasts, planktonic foraminifers, phosphatic grains, and pyrite, all set in a micritic matrix.
In addition, a single clast of acidic volcanic rock was noted in interval 180-1109C-22X-1, 0-5 cm.
Unit III, of late Pliocene to early Pleistocene age, is interpreted as a deep-water (mid-bathyal 500-2000 m; see "Biostratigraphy") succession of pelagic and hemipelagic silty clay interspersed with several types of sediment-gravity-flow deposits. The bottom conditions were continuously aerobic, as shown by the ubiquitous bioturbation (i.e., mainly Chondrites and Zoophycos). However, the rare occurrence of wood fragments indicates redeposition of terrestrial plant debris by gravity flows.
The background silty clays are interspersed with relatively fine grained, clayey silt deposited from low-density turbidity currents. The sand beds are interpreted as deposits from higher density turbidity currents. Carbonate-rich sands and silts rarely reached the site by redeposition of shallow-water deposits, probably also by turbidity currents. The single clast of acidic volcanic rock is seen as having reached deep water by gravity flow, rather than representing primary pyroclastic lapilli.
The smear-slide analysis revealed that both the fine- and coarse-grained sediments contain variable amounts of fresh plagioclase, quartz, glass, volcanic grains, biotite, and hornblende. The XRD data additionally indicate the presence of a significant amount of mixed-layer-type clays (i.e., probably smectite-chlorite mixed layer) suggesting a distinctive provenance for Unit III.
A relatively coarse grained unit was inferred from 219 to 233 mbsf and was designated as log Unit L2 (see "Downhole Measurements"). This corresponds to an interval of very limited recovery in Cores 24X and 25X, in which sand, with silt and mud intraclasts, volcanic rock, and wood fragments were recovered. Thin-section study indicated derivation from a source area including evolved arc-type volcanics, metamorphic rocks, and shallow-water carbonates.
Unit IV is dominated by relatively uniform, greenish, clayey silt and silty clay, with scattered planktonic foraminifers. Additional minor lithologies are sporadic volcaniclastic sand and silt, and rare volcanic ash. Recovery was generally good; however, some cores are quite disturbed with the development of drilling biscuits (e.g., Fig. F16). In addition, Unit IV is slightly affected by normal faulting (see "Structural Geology"). The unit is well documented by both geophysical and FMS logs (see "Downhole Measurements").
Details of lithologies follow.
Silty clay and clayey silt, the dominant lithologies, are quite well indurated, with scattered foraminifer tests and detrital grains. Alternating very thin beds are more or less rich in foraminifers. The paler layers are generally richer in calcium carbonate, based on smear-slide analysis. The silty clays are extensively mottled by burrows throughout. Zoophycos burrows are generally subordinate to smaller scale Chondrites burrows but are rarely conspicuous (e.g., Section 30X-1). Some of the larger Zoophycos burrows are infilled with sand grains. Sulfide mottling was commonly observed.
In smear slides, quartz, feldspar, biotite, volcanic glass, opaque grains, pyrite, foraminifers, and nannofossils were observed (see "Site 1109 Smear Slides"). One thin section indicates the presence of quartz, feldspar chlorite, epidote, pyrite, acidic volcanics, and planktonic foraminifers (interval 180-1109C-28X-1, 4-8 cm) ("Site 1109 Thin Sections"). Analyses of calcium carbonate revealed values of 8.7-39 wt% (see "Organic Geochemistry").
XRD analysis indicates the presence of calcite, plagioclase, and quartz in most samples, with, in addition, chlorite, illite, augite, and amphibole appearing near the base of the unit (see Table T3). Unlike Unit III, mixed-layer-type clays were not detected.
Sedimentary structures, other than bioturbation, are rare. Rarely, traces of grading and parallel lamination have survived extensive bioturbation (e.g., Section 27X-5). Also, mainly in the lower part of the unit, thin couplets (<5 cm thick), composed of clayey silt grading into burrowed clay, are preserved.
There are infrequent occurrences of thin-bedded sand and silt. These beds are characterized by sharp bases, normal grading, and common occurrences of parallel lamination. In addition, there are sporadic occurrences of coarse- to medium- and fine-grained sand. The sand is mainly present in the upper and lower parts of the unit. Relatively coarse grained sand is present in interval 180-1109C-28X-1, 142-150 cm, and in Cores 31X and 32X. Examples of fine-grained sand are seen in intervals 180-1109C-27X-4, 141-142 and 149-151 cm, as well as in 31X-1, 0-148 cm. Elsewhere, the sand is seen only as disseminated diffuse patches throughout clay and silty clay, with no preserved bedding (e.g., Section 30X-3). Very rarely, convolute lamination was observed in the lower part of the unit (e.g., interval 180-1109C-29X-3, 43-50 cm). Locally, small shell fragments are also seen within sandy silt (Sections 28X-2 and 32X-5).
In smear slides, the sand is observed to consist mainly of volcaniclastic constituents with particles of colorless volcanic glass, in addition to quartz, plagioclase, biotite, and hornblende, together with particles of organic and inorganic calcite.
Very rarely, concentrations of volcanic glass as thin partings were sufficiently abundant for the interval to be termed volcanic ash in the "barrel sheets" (see the "Core Descriptions" contents list; e.g., intervals 180-1109C-30X-1, 106-108 cm; 31X-1, 148-150 cm; and 31X-2, 0-4 cm). Study of smear slides indicates the presence of quartz, biotite, clay, volcanic rock fragments, volcanic class (colorless), hornblende, and minor pyroxene, together with foraminifers, nannofossils, and rare siliceous spicules (see "Site 1109 Smear Slides").
Volcanic glass shards are turbid under plane-polarized light and speckly under crossed nicols, suggesting advanced devitrification. Many volcanic glass fragments have diffuse edges. The glass within basaltic lithoclasts is similarly altered. In addition, many individual crystals above 360 mbsf are irregular, internally turbid (due to alteration), with embayed margins, locally gradational to clay. Many of these crystals are subrounded and abraded, in contrast to well-preserved euhedral morphologies below 380 mbsf (see "Lithostratigraphic Unit VI").
Unit IV records deep-water hemipelagic calcareous deposition and fine-grained turbidites deposited in upper bathyal depths (150-500 m; Sections 30X through 39X), then mid-bathyal depths (500-2000 m; Sections 27X through 30X) during middle Pliocene to late Pliocene time. Sedimentation rates were initially 160 m./m.y. but decreased to 65-70 m./m.y. (see "Biostratigraphy"). Bottom-water conditions remained aerobic throughout. Occasional turbidity currents with sand and silt-rich loads reached the depositional site leading to deposition of sand and silt. Sands and silts were subsequently burrowed, such that they are rarely preserved as discrete beds. The sands and silts and the disseminated detrital grains (mainly quartz, plagioclase, biotite, and hornblende) are all of volcaniclastic origin. In contrast to Unit III, volcanic glass is a minor constituent. Rare thin, glass-rich layers are seen as volcanic ash that was reworked by gravity flows and may not record contemporaneous volcanism.
Interpretation of the conventional logs (gamma and porosity) is in keeping with the lithostratigraphy. The sediments are clay rich above 599 mbsf, with peaks in the thorium and potassium logs generally corresponding to rare volcaniclastic sandy layers (~340 mbsf; within log Unit L5; see "Log Unit L5"). The FMS data reveal more and less clay-rich intervals (e.g., less clay rich at ~250 mbsf) and more sandy layers (e.g., ~341 mbsf). Thin resistive layers on the FMS can in general be correlated with calcareous, sandy parts of turbidites in the cores, although one-to-one correlation is difficult, even where recovery is high, as there is little distinctive bed-thickness variation. Around 305-320 mbsf, dips of 5º-10º to the south-southeast and south-southwest are observed (see "Downhole Measurements").
A boundary between Unit IV and Unit V was adopted between Cores 180-1109C-39X and 40X, based on the significant appearance of volcaniclastic sand layers in Cores 40X and 41X. In Hole 1109D, Unit V occupies Cores 180-1109D-1R through 4R (see Fig. F1). The conventional logs (i.e., gamma) indicate a mainly clay-rich deposit with occasional peaks in the radioactivity logs (i.e., Th) correlating with volcaniclastic intervals observed in the cores. The FMS data for Cores 180-1109D-1R through 4R are not usable because of poor hole conditions.
Cores down to 375.7 mbsf were recovered by XCB, whereas cores from 352.8 mbsf onward were obtained by RCB. Thus, there was an overlap of ~23 m between the intervals recovered by XCB and those by RCB (from 352.8 to 375.7 mbsf). The recovery by RCB was good to excellent. Note: Correlation of lithologies using physical properties data indicate an offset of ~0.5 m between the values of mbsf quoted for Holes 1109C (APC/XCB) and 1109D (RCB) (see "Hole-to-Hole Correlation"). The excellent recovery by rotary drilling reflects the relatively uniform, well-cemented calcareous nature of the succession.
Overlap between the parts of the succession recovered by XCB and RCB drilling proved useful in resolving uncertainties as to whether repetitive silt-clay units, ~5 cm thick, were either primary thin beds of more or less clay- and silt-rich composition, or drilling biscuits (a drilling artifact). The controversial silt-clay units are well developed throughout the XCB-cored succession but disappear in the overlapping interval recovered by rotary drilling, effectively proving an origin of the repetitive silt-clay intervals as a drilling artifact. Apparently, the silty clays were more lithified than the clayey silts, and the latter were fluidized by XCB drilling and injected as homogenous silty material every few centimeters to form characteristic drilling biscuits (see Fig. F16). Notwithstanding this, occasional graded sand-silt and silt-clay graded beds of primary origin are indeed present within the rotary cores, and these can also be recognized between drilling biscuits in that part of the succession recovered by XCB.
Unit V is characterized by relatively uniform, greenish gray, clayey siltstones and silty claystones interbedded with volcaniclastic silt and sand layers, rare volcanic ash, and quartz-rich sand layers. The core recovery was excellent (>90%); however, cores are disturbed with the development of drilling biscuits (see the "Core Descriptions" contents list).
The silty claystone is olive green, calcareous, burrowed, and silty, with scattered detrital sand grains. Zoophycos and Chondrites burrowing is commonly present. Calcium carbonate content in the silty claystones of Unit V range from 21 to 32 wt%. By contrast, the volcaniclastic sediments are relatively low in calcium carbonate (<1 wt%; see "Organic Geochemistry").
Study of smear slides of the silty claystones indicates the presence of quartz, clay, plagioclase, volcanic glass, pyrite, planktonic foraminifers and nannofossils, inorganic calcite crystals, and scattered siliceous sponge spicules (see "Site 1109 Smear Slides"). XRD analysis indicates the presence of calcite, plagioclase quartz, illite, chlorite, amphibole, and augite, together with smectite. In addition, alkali feldspar was rarely identified (see Table T3).
Unit V contains seven volcaniclastic beds within Cores 180-1109C-40X through 41X and 58 volcaniclastic layers in Cores 180-1109D-1R through 4R. The thicknesses of these beds range from 1 to 10 cm. The bases of individual volcaniclastic sand beds are mainly sharp and scoured, whereas diffuse tops are common. In places, the bases of individual beds are disrupted by bioturbation (e.g, Fig. F17). Smear-slide analysis reveals a mixture of colorless volcanic glass and sand-sized phenocrysts of amphibole, pyroxene, plagioclase, quartz, and biotite. The volcanic glass grains range from 50 to 300 µm in size. The finer grained glass is mainly composed of platy shards, whereas the coarser material is commonly highly vesicular glass.
A small number of thin sections of volcaniclastic sandstones (see "Site 1109 Thin Sections") reveal the presence of zoned plagioclase, quartz, biotite, pyroxene, hornblende, rare microcline, and basaltic, andesitic, and acidic volcanic grains, also colorless volcanic glass (Fig. F18), all set in a micritic matrix with planktonic foraminifers. Some of the volcaniclastic beds also contain biogenic fragments (e.g., echinoderms). Volcaniclastic grains are also scattered throughout adjacent silty claystone and clayey siltstone. Occasional burrows are filled with volcaniclastic sand.
Unit V contains four volcanic ash layers 1-7 cm thick. An ash layer in Section 180-1109C-41X-1 is composed of almost entirely (>90%) less altered, colorless volcanic glass. The main glass morphologies are platy and bubble wall shards. Highly vesicular glass is rarely observed. The grain size ranges from 100 to 200 µm. In addition, an ash layer in interval 180-1109D-4R-2, 71-72 cm, is characterized by abundant highly vesicular glass (Fig. F19). Phenocrysts of pyroxene, amphibole, biotite, and plagioclase are only rarely observed within the volcanic ash, in contrast with the volcaniclastic sandstone described above.
Unit V, of middle Pliocene age, accumulated in an upper bathyal (150-500 m) setting at a sedimentation rate of ~200 m/m.y. (see "Biostratigraphy"). Although relatively condensed, this unit is seen to considerably thicken downslope toward the depocenter of the rift basin (see "Drill Sites" in the "Leg 180 Summary" chapter). The bioturbated silty clay and clayey siltstones are interpreted as background calcareous hemipelagic and maybe fine-grained turbidite sediments on a well-oxygenated seafloor. By contrast, the numerous graded glass-rich sands (with a few ash layers) are seen as volcanic-derived material (i.e., of epiclastic origin) that was redeposited by turbidity currents. They probably relate to a discrete eruptive event in middle Pliocene time, as discussed in the summary section of this chapter. In addition, the volcaniclastic sands are interpreted as epiclastic in origin, and derived from a calc-alkaline arc-type terrain.
The conventional log unit data suggest that Unit VI is relatively fine grained (recognized as log Unit L5; see "Log Unit L5"); however, this masks the occurrence of discrete thin volcaniclastic beds that characterize Unit V.
Unit VI is distinguished from Unit V (see "Lithostratigraphic Unit V"), by clayey siltstone and silty claystone, with subordinate thin beds of fine- to medium-grained sandstone/siltstone. The unit is the thickest encountered at Site 1109 (~175 m) and is remarkably homogeneous except for a progressive increase in carbonate below 520 mbsf. It is largely equivalent to log Unit L6 (see "Log Unit L6"). The hole conditions were good throughout this interval, yielding high-quality logs.
Recognition of the boundary between Units V and VI was not straightforward; no marked lithologic change is apparent in the cores for 10 m above and below the boundary picked at the base of Core 4R (387.6 mbsf). However, several lines of evidence suggest that an important boundary does in fact exist around 390 mbsf: (1) In smear slides and thin sections, the bulk composition of the sediment above and below 380 mbsf is similar, derived from a volcanic arc-like provenance. However, volcanic glass, lithic fragments, and individual crystals are less altered below 380 mbsf, and no metamorphic-derived material (e.g. muscovite schist; polycrystalline quartz) or mixed-layer clays are present. (2) Magnetic susceptibility and porosity drop sharply over <5 m (see "Magnetic Susceptibility" and "Density and Porosity"). (3) Significant changes in sediment character are identified on geophysical logs (see "Interpretation," also see "Downhole Measurements").
Details of the individual sedimentary lithologies present in Unit V follow.
The silty claystone is olive green (10Y 6/2), calcareous, and silty, with scattered detrital sand grains. This silty claystone is highly burrowed, commonly with concentrations of medium- to coarse-grained sand within individual relatively large (centimeter-sized) burrows (mainly Zoophycos). Elsewhere, Chondrites burrowing predominates, giving rise to a weak color mottling. Small shell fragments are rare (interval 180-1109D-7R-3, 10-50 cm).
Determinations of calcium carbonate indicate relatively constant levels above 520 mbsf of 20-30 wt%, increasing to 30-40 wt% below that, with an average of 25-30 wt%. The calcium carbonate abundance generally increases downward (see "Organic Geochemistry").
Study of smear slides shows that the silty claystones consist of quartz, clay (unidentified), feldspar, volcanic glass, and pyrite, as well as foraminifers and nannofossils, inorganic calcite crystals, and siliceous sponge spicules (see "Site 1109 Smear Slides").
XRD analysis indicates the presence of calcite, plagioclase, and quartz as the dominant constituents of most samples (see Table T3). Amphibole is present in many samples, whereas pyroxene was rarely detected. The clay minerals are dominantly illite, with variable occurrences of chlorite; also, smectite is restricted to high in the unit (above Core 7R). In addition, K-feldspar and aragonite are locally present (see Table T3).
Subordinate, thin to very thin sandstone and siltstone beds, typically <6 cm thick, and rarely more than 10 cm thick, are present throughout the claystone. Where still well preserved, these sandstones exhibit sharp bases, locally with scouring into silty clay beneath (e.g., intervals 180-1109D-11R-3, 68-72 and 87-89 cm; see also Fig. F20). Several of these scour features could be flutes (e.g., interval 180-1109D-7R-3, 79-81 cm). Coarse- to fine-grained, very thin beds and thin laminae of sandstone with sharp bases and tops were rarely observed (Fig. F21). In addition, occasional occurrences of several volcaniclastic, graded beds and laminae were noted. Very rarely, granules of lithic fragments, quartz, and ferromagnesian minerals are present at the base of normal-graded beds (interval 180-1109D-13R-2, 46-47 and 123-124 cm). Typical sandstones are graded over several centimeters and pass up into clayey siltstone and then into silty claystone. Rarely, plane laminations are observed (e.g., interval 180-1109D-9R-3, 20-23 cm).
The sandstone and siltstone are commonly highly burrowed, obscuring original bedding and internal structures. Where intense burrowing has taken place, all intermediaries are seen between intact thin-graded beds (as above) and patches of disseminated sand within claystone (e.g., Section 180-1109D-20R-4). In the lower part of the unit scattered shell fragments, occasional intact small gastropods, and carbonate grains are observed (e.g., Cores 22R through 23R).
In smear slides, the sandstones and clayey siltstones are observed to contain quartz (commonly undulose), fresh plagioclase, biotite, and common green hornblende, together with pyrite, foraminifers, and nannofossils. Pyrite both fills foraminifer tests and occurs as scattered framboids. Some samples contain devitrified volcanic glass. The relative abundance of ferromagnesian grains differs from sandstone to sandstone, with some being relatively richer in biotite or hornblende, respectively ("Site 1109 Smear Slides").
Study of thin sections confirmed the presence of the constituents mentioned above ("Site 1109 Thin Sections"). However, the abundance of quartz relative to feldspar was overestimated in the smear-slide analysis, and several of the sandstones are arkoses. Additionally, thin sections revealed the presence of zoned orthoclase, common small devitrified fragments of basic and rare acidic extrusive rocks, rare benthic foraminifers, and shell fragments. Small biotite laths are commonly concentrated in laminae. There is also rare evidence of repetitive partings with sharp bases and tops, and small-scale ripples (interval 180-1109D-22R-1, 57-60 cm).
Five volcanic ash layers occur between 465 and 485 mbsf. They are mainly composed of colorless acidic glass, but a minor amount of brown glass shards is present as well. Shards in these layers exhibit thin platy and bubble wall morphologies. Most of the glass shards are fresh, with minor signs of devitrification in some places.
This unit can be interpreted with confidence as recovery was excellent (for the "barrel sheets," see the "Core Descriptions" contents list). Unit VI, of early to middle Pliocene age (see "Biostratigraphy"), accumulated as hemipelagic carbonate clay in a quiet upper bathyal setting (150-500 m), interspersed with the occasional arrival of a mainly low- but, rarely, high-density turbidity current. The estimated sedimentation rate, at 312 m/m.y., was relatively high (see "Biostratigraphy"). The source was dominantly volcanic related (basalt-andesite), but minor amounts of neritic carbonate material (e.g., gastropods) also reached the site of deposition. There is evidence of minor current reworking to produce repetitive lamination and rare small ripples. The seafloor was well oxygenated and was extensively bioturbated, to such an extent as to largely or completely obliterate primary depositional structures, and a slope setting is inferred.
The geophysical logs (e.g., gamma logs) are interpreted to indicate the presence of more and less sandy and clay-rich layers. Carbonate content increases below 500 mbsf, based on the photoelectric log data. On the FMS (static normalized), Unit VI is distinguished by a well-layered structure, with numerous 5- to 10-cm-thick, more resistant layers. These show sharp bases and gradational tops and correlate with the turbidity current deposits seen in the cores. In contrast to the variation in the units above and below, the relatively monotonous log response attests to relatively constant depositional conditions within Unit VI. Bedding is less apparent near the base of the unit (below 505 mbsf).
Unit VII is recognized on the basis of a generally coarser grain size and markedly increased carbonate content, up to 75 wt%, based on chemical analysis (see "Organic Geochemistry"). The recovery was initially very good (around 80%) but decreased sharply downward to around 10%-25%. This results in considerable uncertainty concerning the identity of the intervals that were not recovered. These deficiencies were largely rectified by the log data, especially the photoelectric log that indicates changes in carbonate content (see "Downhole Measurements").
Unit VII consists almost entirely of well-cemented mixed carbonate-siliciclastic rocks. The interval from 599 to 672 mbsf is divided into three parts: the upper part of the unit was classified as calcareous (bioclastic) sandstone (570-599 mbsf, with 40-50 wt% calcium carbonate); the middle part as slightly calcareous sandstone (599-643 mbsf, with ~20 wt% calcium carbonate); whereas the lower part is bioclastic limestone packstone-grainstone) (643-672 mbsf; 40-80 wt% calcium carbonate).
The intergradational lithologies found in Unit VII follow.
Greenish gray (5GY 4/1) calcareous silty sandstone occurs extensively in the upper part of the unit. This sandstone contains ~20-25 wt% calcium carbonate (see "Organic Geochemistry"). The sandstone varies from fine to medium grained at the top of the unit (Cores 25R-1 through 3), becoming coarser downward (Fig. F22). The sandstone is moderately to strongly burrowed. Large Zoophycos burrows are commonly infilled with coarse siliciclastic grains, as observed in overlying units (see "Lithostratigraphic Unit VI"). The fine- to medium-grained sandstone is mainly well sorted. Planar lamination, cross-lamination, wavy lamination, and herring-bone-type lamination (e.g., Section 34R-6) are developed.
The study of smear slides and a small number of thin sections indicates that many of the sandstone are rich in feldspar, quartz, biotite, hornblende, and pyroxene (in decreasing order of occurrence), together with organic and inorganic calcite and foraminifers ("Site 1109 Thin Sections"). Devitrified volcanic glass and tabular crystals of orthoclase were occasionally observed. In addition, carbonaceous detritus is present (e.g., Core 25R). Thin-section analysis additionally indicated the presence of lithoclasts of basic-acidic igneous rock and rare altered basic glass (e.g., Fig. F23).
XRD analysis reveals the presence of calcite, quartz, plagioclase, amphibole, and illite, with occasional pyroxene and aragonite (Table T3). This composition is little changed from Unit VI, except for the abundant calcite.
Most of the mainly calcareous rocks observed (with subordinate volcaniclastic grains, as above) are termed packstones and grainstones. Analyses of calcium carbonate are in the range of 45-75 wt% (see "Organic Geochemistry"). The carbonate component is seen in smear slides ("Site 1109 Smear Slides") and a small number of thin sections ("Site 1109 Thin Sections") to be composed of shell fragments, micritic grains, minor calcite spar, planktonic (and rare benthic) foraminifers, rare echinoderm plates, bryozoans, and nannofossils. The packstone-grainstones are well sorted and cemented by calcite spar. Pyrite occurs as scattered cubes in the matrix and as infills of foraminifer tests. Both planktonic and agglutinating benthic foraminifers were noted. Exceptionally, small angular lithoclasts of plagioclase-hornblende-phyric andesite with a dark unaltered mesostasis were noted, together with very small fragments of altered basalt and isolated olivine crystals (Sample 180-1109D-32R-2, 101-103 cm). The appearance of the plagioclase and amphibole crystals is very similar to that of the scattered grains that occur much more widely throughout this unit.
A number of coarse-grained sand beds of granule- to pebble-size carbonate particles were observed (Fig. F24). For example, paraconglomerate in the intervals 180-1109D-27R-1, 8-22 and 65-90 cm, comprises shell fragments, calcite, coral, calcareous green algae (oncolites), calcareous red algae (rhodoliths), echinoderm plates, and detrital grains. The shells are thin and are mainly the remains of bivalves, a few of which are still articulated. Several rugose coral fragments were recovered with an intact calyx (e.g., intervals 180-1109D-29R-4, 4-8 and 37-39 cm; Fig. F25). These coarser bioclastic intervals are poorly sorted and cemented by calcite spar, with admixed siliciclastic grains, as revealed by a smear slide ("Site 1109 Smear Slides").
In the upper part of the unit, two pebbles of basalt were recovered (Section 27R-1); also lower in the unit (e.g., intervals 180-1109D-27R-4, 22-23 cm, and 32R-2, 102-103 cm), rare small (<1 cm) well-rounded, altered, basalt pebbles were recovered. In addition, small (<0.3 mm) well-rounded lithic clasts were observed in thin sections (see "Site 1109 Thin Sections").
The siliciclastic-carbonate sediments reflect mixing of sediment derived from volcanic-related and neritic carbonate source materials. The sands show evidence of current reworking in a well-oxygenated high-energy environment. Paleontological evidence indicates shallow marine (i.e., neritic; <150 m deposition), based mainly on evidence of benthic foraminifers (see "Benthic Foraminifers"). The carbonate grains were largely derived from corals and algae, presumably reefal, although we have no evidence of the location, or morphology, of any contemporaneous reefs.
The volcaniclastic components were mainly derived from basic-intermediate composition extrusive igneous rocks, as indicated by the occurrence of small basalt-andesite lithoclasts. The well-rounded shape of many of these clasts suggests that these were reworked and/or eroded before final deposition. In contrast to overlying units, volcanic glass-rich intervals were not observed.
One problematic aspect is the apparently abrupt nature of the change from shallow-water deposition in Unit VII to deep-water (upper bathyal) deposition in Unit VI. This may in part reflect reworking, such that neritic sediments were reworked to water depths of >150 m in the upper part of the unit (without recording the water depth of final deposition).
Interpretation of the geophysical logs, particularly the photoelectric log, reveals additional information on lithologic variation within Unit VII that was not apparent from this generally poorly recovered section (see "Comparison of Core Data with Results of Downhole Measurements"). The upper part of the unit (599-643 mbsf) contains intervals that on geophysical and FMS logs are interpreted as well-bedded sandstones, and these correlate with volcaniclastic sandstone in the cores. The highest values of calcium carbonate as inferred by the photoelectric log occur in the lower part of the lithostratigraphic unit around 643-675 mbsf. Small peaks in the uranium and potassium logs can be correlated with thin intervals rich in biotite and ferromagnesian minerals (e.g., pyroxene and olivine). Also, rare minerals (e.g., zircon), if present in trace amounts, could influence the log response. The FMS image of the above carbonate-rich interval is highly resistant and "blotchy," interpreted as concentrations of bioclastic material and isolated, angular clasts yielding bright patches.
Unit VIII is characterized by an abrupt change (i.e., at interval 180-1109D-35R-4, 74 cm) from bioturbated, mixed volcaniclastic-carbonate rocks (sandstone-packstone) to much finer grained, massive calcareous silty claystone. Colors are mainly dark greenish gray (5GY 4/1). Interpretation of the geophysical and FMS log data suggests that the succession contains sand/mud intercalations, although the hole conditions were poor (see "Downhole Measurements"). For the total unit, bioturbation ranges from absent (e.g., Core 35R) to only very weak, in contrast to all the overlying units. Calcium carbonate varies up to 65 wt% (see "Organic Geochemistry").
The lithologies present in Unit VIII follow.
The most abundant lithology of Unit VIII is greenish calcareous clayey siltstone, with variable abundances of thin shells and plant material (Fig. F26). A weak bedding fabric is defined by subparallel orientation of shell fragments. Monosulfide mottling is occasionally present. The shell fragments are the remains of bivalves and occasional gastropods. They range from isolated to concentrations of fragmented, randomly oriented shells (e.g., intervals 180-1109D-36R-1, 20-23, 43-45, and 87-89 cm). Rarely, shells are still articulated (e.g., interval 180-1109D-36R-6, 0-110 cm). Occasional slightly coarser grained intervals (interval 180-1109D-37R-2, 73-149.5 cm), and rare wavy laminae of sand grains are also present (i.e., interval 180-1109D-39R-3, 49-86 cm).
In smear slides, the clay-rich siltstones are seen to comprise quartz, plagioclase, mica, hornblende, glauconite, possible orthoclase, clay (unidentified), fine-grained basic volcanic rock fragments, and pyrite framboids, together with organic and inorganic calcite, and scattered small foraminifers (including globigerines; see "Planktonic Foraminifers").
In addition, XRD analysis of siltstones and one sample of sandstone revealed the presence of plagioclase, pyroxene, quartz, amphibole, smectite, and illite. Smectite appears for the first time since the upper part of Unit V (Table T3).
Slightly finer grained intervals of silty claystone also occur (e.g., Section 36R-CC). The silty claystone includes local thin (<3 mm) anastomosing laminae composed of finely divided carbonaceous fragments (lignite; e.g., interval 180-1109D-38R-1, 121-133 cm; Fig. F27).
Sandstones are present as rare intervals up to 40 cm thick (e.g., interval 180-1109D-36R-1, 0-7 cm). These sandstones generally exhibit sharp bases (e.g., interval 180-1109D-38R-1, 7-12 cm) and contain common laminations rich in wood debris (described below). In addition, fine-grained basic rock fragments are present (interval 180-1109D-38R-1, 7-12 cm; see "Site 1109 Thin Sections").
In smear slides ("Site 1109 Smear Slides") and a small number of thin sections ("Site 1109 Thin Sections"), the sandstone is arkosic, with angular grains of plagioclase (zoned), quartz (undulose), sericite (i.e., unresolvable mica), biotite, hornblende, rare pyroxene and microcline, organic and inorganic calcite, pyrite, and organic (woody) debris (e.g., Sample 180-1109D-38R-4, 10-14 cm). In addition, common lithoclasts of altered glassy basalt, and rare acidic volcanic rock fragments are observed (e.g. Samples 180-1109D-38R-2, 12-18 cm.
A 49-cm-thick dark bluish gray well to moderately sorted, fine-grained volcaniclastic sandstone bed occurs in interval 180-1109D-39R-3, 0-49 cm. A thin section of the sandstone shows it to contain angular grains of quartz, feldspar, clinopyroxene, amphibole, acidic volcanics, glass shards, and rare intraformational siltstone ("Site 1109 Thin Sections"). The glass shards and acidic volcanic fragments are commonly highly altered.
Terrestrial organic matter ranges from laminae composed of fine-grained plant-derived material (i.e., macerals), to discrete fragments (i.e., woody stems). The plant-rich material is sufficiently diagenetically advanced to be termed lignite (low-grade bituminous coal). Terrestrial matter is well developed (e.g., intervals 180-1109D-37R-2, 65-75 cm, and 38R-2, 10-25 cm) as thinly interlaminated coaly material and claystone (see Fig. F28). Unusually, burrowing has disrupted these coal-claystone laminations.
A number of coaly intervals are composite when examined in detail, being composed of alternations of lignite-dominated laminations, claystone-dominated laminations, and fine-grained sandstone-dominated laminations (i.e., intervals 180-1109D-38R-1, 91.5-133 cm, and 38R-2, 0-99 cm. In one case, such a composite unit indicates a generally fining upward unit (i.e, interval 180-1109D-39R-3, 49-86 cm).
Rare thin intervals were described in the cores as being dominated by small carbonate rhombs. Smear-slide analysis confirms that these carbonate rocks (e.g., interval 180-1109D-37R-1, 7-12 cm) are silt-sized dolomite rhombs with rare quartz, plagioclase, biotite, and accessory minerals.
A number of lines of evidence suggest that Unit VIII accumulated in a marginal lagoonal setting: (1) The sediments are mainly fine grained with abundant clay, suggesting deposition in quiet waters. (2) The shell debris shows little, or no, reworking by currents. (3) Stenohaline (i.e., normal marine salinity) organisms (e.g., corals and echinoderms) are absent, again in contrast to Unit VII. However, the presence of a few benthic foraminifers tolerant of reduced salinities (see "Benthic Foraminifers") indicates that some connection to the open sea existed. The possibility that the foraminifers were washed in by storms is unlikely as they occur disseminated through fine-grained sediments, and the shell deposits do not include coquina (shell concentrates) characteristic of storm influence. The thin-shelled bivalves and occasional gastropods also may include forms tolerant of reduced, or fluctuating, salinity. (4) The abundant woody material indicates that a copious supply of plant material was available. (5) The near absence of burrowing indicates that the near-surface sediments were characterized by low-oxygen conditions, consistent with a quiet, protected depositional setting. Such conditions would also favor the diagenetic preservation of fine woody and plant material. And (6), the local occurrence of dolostones is indicative of hypersaline conditions. The presence of volcaniclastic sandstone, altered glass, and rare acidic volcanic pebbles is consistent with a marginal, near-land setting.
Interpretation of the log data is hampered by poor hole conditions. However, the FMS images reveal more resistant and less resistant intervals up to several meters thick that are interpreted as sand/mud alternations. In contrast to the underlying units, conglomerate is not imaged (see "Lithostratigraphic Unit IX"). In addition, on the radioactivity logs, elevated uranium (682-707 mbsf) is interpreted as an indication of anoxic conditions, in keeping with the large amount of reduced organic matter present. Despite the poor hole conditions, the FMS data indicate the presence of sandy and muddy alternations locally, in keeping with the core recovery.
Unit VIII, therefore, is seen as forming in a marginal lagoonal setting subject to salinity fluctuations, ranging from near marine, to locally freshwater, and evaporitic. The nature of any contact between the inferred lagoon and the open sea is unknown, but might have been igneous basement associated with fringing reefs. This setting is suggested by the presence of rare basaltic clasts within Unit VII and by coral in Unit VIII, above. A more detailed scenario is discussed in the concluding section (see "Depositional History").
Unit IX is characterized by a marked change in sediment type, away from the dark-colored organic-rich fine-grained sediment of Unit VIII to a more uniform greenish gray (5G 5/1) clayey siltstone. The recovery was poor, both in amount and quality (see "Core Descriptions" contents list). However, the geophysical and FMS logs indicate that this unit contains a substantial amount of conglomerate that was not cored.
The lithologies observed within the limited core recovery follow.
The dominant lithology is uniformly fine- to medium-grained clayey siltstone, with minor mottling (e.g., interval 180-1109D-42R-2, 75-76 cm). Colors are typically greenish gray (5G 6/2) to orange brown (5Y 5/3 to 5G 5/2). The clayey siltstone generally lacks bedding, although faint layering is seen in places (e.g., Section 41R-3). Minor small-scale burrowing is locally observed (e.g., interval 180-1109D-42R-2, 60-63 cm). In smear slides, the clayey siltstone is seen to contain undulose quartz, feldspar, biotite, clay (unidentified), volcanic rock fragments, accessory minerals, and minor inorganic calcite. In addition, silty claystone was also observed, mainly at the base of the unit. This is vaguely mottled and contains burrows filled with blue-green (5G 6/2) smectite-rich clay, based on XRD analysis (Table T3).
The clayey siltstone is intercalated with several thin intervals of very dark brown, slightly altered coarser grained siltstone and very fine grained sandstone. These sediments are mottled and faintly burrowed, with occasional impressions of rootlets.
Study of smear slides ("Site 1109 Smear Slides") indicates the presence of abundant plagioclase, quartz, sericite, volcanic rock fragments, accessory minerals, and iron oxide. Euhedral zoned plagioclase crystals were also noted. Highly altered volcanic rocks and rare, little-altered basaltic clasts are also present. In places, the siltstone to fine-grained sandstone contains ferruginous concretions (see "Goethite Concretions").
The XRD analysis confirmed the presence of plagioclase (albite), pyroxene, K-feldspar, quartz, and smectite. A high relative abundance of feldspar and plagioclase (albite) and the presence of smectite was noted in this unit (Table T3).
Several intervals (e.g., intervals 180-1109D-40R-1, 40-80 cm; 40R-CC, 0-20 cm; and 41R-1, 80-150 cm) of clayey siltstone and siltstone (see "Interpretation") contain goethite concretions, <1.5 cm in diameter, as confirmed by XRD analysis (Table T3). Some of the concretions are perfectly rounded and densely packed within silty claystone (Fig. F29). When sliced, they are seen to comprise rusty reddish brown iron oxide impregnating host clay-rich siltstone. In other cases, the goethite-rich concretions have a polygonal shape (e.g., Section 41R-1).
Several small (<1.5 cm) rounded concretions of fine-grained carbonate were noted within yellowish brown silty clay and clayey silt (i.e., interval 180-1109D-40R-3, 0-100 cm). Several of the concretions exhibit tiny internal radiating calcite veins. Elsewhere, angular fragments of similar carbonate material were noted (i.e., intervals 180-1109D-40R-3, 100-150 cm, and 40R-4, 40-42 cm). In one interval, similar carbonate material was seen to be reworked as angular fragments within clayey siltstone (interval 180-1109D-40R-4, 40-42 cm).
The core catcher of Core 39R contains two pieces of dark gray fine- to medium-grained basic igneous rock (basalt or microdolerite). In addition, a small number of igneous clasts were recovered between sediment. A clast of aphyric basalt was recovered in Sections 39R-CC and 40R-CC. Also, several subrounded to subangular dolerite pebbles were retrieved (i.e., from Sections 39R-CC, 41R-1, 41R-3, and 41R-CC), which are most likely to be related to conglomerates as inferred from the FMS data. The basaltic clasts generally exhibit fractures infilled with a white, fine-grained alumino-silicate. Similar vein-infilling material was observed within dolerite from Unit XI beneath and identified as the zeolite mineral, natrolite (see "Igneous and Metamorphic Petrology").
The greenish gray to orange brown clayey siltstones, silty claystones, and fine-grained sandstones are interpreted as nonmarine deposits in a probable coastal swamp environment (mangrove swamp?). The following lines of evidence support this interpretation: (1) Marine fossils are absent, in contrast to all the overlying units. (2) The sediments include traces of rootlets. (3) The goethite concretions are interpreted as pisoliths, typical of "bog iron ore" (Blatt et al., 1980). The pisoliths presumably developed beneath the sediment surface in a nonmarine setting. (4) Freshwater biota are locally present (i.e., charophytes in Core 39R). (5) The occasional carbonate concretions are similar to caliche. The internal radiating fine cracks are typical of diagenetic growth where volume reduction has taken place. In addition, the local presence of fragments of similar carbonate material probably indicates that some of the caliche was reworked after formation, consistent with a contemporaneous origin. Caliche forms within soils, in areas affected by alternating wet- and dry-season conditions (Easton and Klappa, 1983). However, in this case caliche development was only incipient, as only scattered concretions occur. (6) Smectite is common, based on XRD analysis (see Table T3). Analysis of squeezed pore water indicates a decrease in Mg and increase in Ca, which is attributed to a combination of uptake of Mg to form diagenetic clay minerals and expulsion of Ca related to albitization of feldspar (see "Inorganic Geochemistry"). Such smectite can form within soils (i.e., pedogenic smectite) affected by an alternating, seasonal wet-dry regime (Bjorlykke, 1983). And (7), the sediments also show evidence of color mottling (e.g., within burrows) that can be interpreted as the result of reduction during diagenesis, again consistent with a swampy setting.
In addition, interpretation of the FMS data suggests that the occasional clasts of basalt recovered from Unit VIII were derived from substantial conglomeratic intercalations similar to those in Unit X (see "Lithostratigraphic Unit X"). Interpretation is complicated by poor hole conditions, with diameter varying strongly on a meter-by-meter scale (based on FMS caliper data; see "Downhole Measurements"). This variation itself suggests that highly variable lithologies are present, with the narrow-hole-diameter intervals possibly representing consolidated sand and conglomerate, whereas the wider hole diameters may represent soft silts and muds. Assuming this, the conglomerate wanes, whereas the mud/clay content increases upward.
The highly altered igneous material identified in the sandstones is interpreted as highly altered mesostasis of basaltic rocks. Therefore, it is likely that the conglomerates were dominated by basic igneous rock clasts, as in Unit XI below. The alteration of the grains, especially of feldspars and ferromagnesian crystals, mainly took place during diagenesis, as in typical red-bed formation. The abundance of plagioclase relative to quartz suggests provenance mainly from basic intermediate, rather than acidic, volcanic rocks.
In summary, therefore, Unit IX is seen as having accumulated in a coastal swamplike setting, subject to tropical weathering. The probable depositional setting is discussed in more detail in the depositional history section (see "Depositional History"). The silty clays, silts, and sands were interbedded with conglomerate, based on interpretation of the log data. The swamp was possibly recharged with river water during seasonal high rainfall, as in coastal Papua New Guinea (PNG) today. The coastal area is seen as backed by an elevated hinterland including basic igneous rocks. Weathering and alteration continued after deposition within a reactive diagenetic setting, characterized by redox-controlled reactions, possibly accelerated during dry-season conditions.
Within Core 43R through Section 45R-2, there was minor recovery of conglomerate, individual clasts of basalt, and minor sandstone and claystone. Analysis of FMS data suggests that these lithologies were derived from a major interval of conglomerate ~30 m thick overlying a basement of dolerite rocks (see "Igneous and Metamorphic Petrology" and "Downhole Measurements"). The contact was estimated to lie near 772 mbsf, equivalent to the base of Section 45R-2 (see "Downhole Measurements").
Conglomerate occurs in Sections 43R-1, 43R-3, 43R-CC, 44R-1, and 45R-3). The conglomerate recovery in Core 43R is represented by pebbles (from 3 cm × 3 cm to 3 cm × 6 cm) of dark gray basalt. Pebbles in conglomerate from Section 44R-1 are smaller (from 1 cm × 1 cm to 3 cm × 5 cm). The conglomerate is composed of well-rounded pebbles and granules in a matrix of medium- to coarse-grained sandstone. The larger pebbles within the conglomerate exhibit onionskin alteration. In addition, the smaller pebbles, granules, and sand exhibit various stages of reddish oxidation (Fig. F30).
In thin section, the granules and sand are seen to comprise well-rounded grains of altered variolitic basalt and dolerite, altered plagioclase and pyroxene, and rare grains of fine- to medium-grained acidic igneous rocks (i.e., devitrified rhyolite), microcrystalline quartz (devitrified acidic glass), and chloritic grains (altered basic volcanic glass?) cemented by sparry calcite ("Site 1109 Thin Sections;" Fig. F31). XRD analysis of minor silt- and clay-sized material suggests the presence of smectite (see Table T3).
In addition, isolated clasts of basalt and dolerite were recovered from Sections 42R-1, 43R-1, 43R-3, 43R-CC, 44R-1, 45R-1, and 45R-2. The individual clasts recovered range from well rounded to rounded to angular (although the shape of isolated clasts in part may be an artefact of drilling). Also, a number of individual, rounded clasts of very altered sandstone were recovered in Section 44R-1. These are medium grained and include plagioclase and ferromagnesian minerals.
Finally, ~82 cm of clayey siltstone was recovered from within the conglomerate (Section 43R-2). This sediment is structureless, with angular, poorly sorted, sand-sized material and traces of mottling.
The relation of the recovered conglomerate and clasts and matrix can be correlated with the FMS data. The base of the conglomerate is inferred to be located around 773 mbsf, based on a sharp change from variable to higher and more uniform resistivity. The contact is irregular and probably represents an unconformity. The FMS response of the conglomerate is a "blotchy" resistive image that may correspond to individual rounded clasts (<30 cm thick) within a matrix of low resistivity material that may be sandstone, siltstone, or claystone.
The conglomerates and individual clasts of basalt and sandstone are interpreted to indicate the presence of conglomerate overlying a dolerite basement of inferred intrusive origin (see "Igneous and Metamorphic Petrology"). Two possibilities for the origin of the conglomerate are that the conglomerates represent beach or fluvial deposits. Several factors favor a fluvial origin: (1) marine fossils (e.g., shells) were not observed; (2) the FMS data indicate the presence of conglomerate throughout Unit X, with finer grained, clastic sediments between the clasts, rather than as a single thick, well-sorted interval, as predicted for a beach unit; and (3) the overlying Unit IX is interpreted as terrestrial, swamp setting with conglomerate deposited away from a shoreline.
In addition, the composition of the clasts indicates a predominant derivation from basaltic and dolerite igneous rocks, with a subordinate additional input from acidic extrusive and possible intrusive rocks that were not otherwise recovered. The material is strongly altered (either subaerially or during diagenesis), as indicated by the abundance of smectite.
The lithostratigraphic Unit XI dolerite is described in "Igneous and Metamorphic Petrology".
The main results are summarized in a series of figures. A sedimentary log of the succession and the downhole variation in grain size is displayed in Figures F32, F33, and F34. Trends in the sediment composition according to smear-slide data are indicated in Figure F35. Finally, the presence and relative abundance of volcanic ash and volcaniclastic layers are shown in Figure F36.
The depositional history at Site 1109 involves an overall transition from terrestrial to marginal (coastal/lagoonal) to shallow-marine, and then deep-marine (bathyal) accumulation. The succession recovered is sufficiently complete to interpret the depositional history, except for Unit IX, which has to be inferred using logging data. The age of the upper part of the succession is well documented (Units I to VI inclusive). However, Units VIII through X lack diagnostic fossils. For the well-dated parts of the succession, rates of deposition were inferred to range from 21 to 312 m/m.y. (see "Biostratigraphy").
The overall scenario inferred through time follows.
Before the onset of the recorded deposition at Site 1109, the area formed part of the forearc region of the Miocene Trobriand Arc, related to southward subduction of oceanic crust within the Solomon Sea (Lock et al., 1987; Davies et al., 1984). This scenario is discussed more fully in the "Leg 180 Summary" chapter. The volcanic front is inferred to have been located south of Site 1109, in the vicinity of the Moresby Seamount, although Pliocene andesitic volcanoes also existed to the west-northwest and east-northeast of the site in the Amphlett Islands and Egum Atoll (Smith and Milsom, 1984). The outer arc high is located about 100 km to the northeast of Site 1109, which is therefore located in the arc-proximal part of the forearc basin. Forearc basins are expected to be dominated by arc-related volcaniclastic sediments, as suggested by drilling of the Tonga and Izu-Bonin forearc basins (Boe, 1994; Taylor, Fujioka, et al., 1990) and study of well-exposed analogues emplaced on land (e.g., the Late Cretaceous Dras-Kohistan arc complex, Himalayas; Robertson and Degnan, 1994). Trobriand Arc-derived volcaniclastic sedimentary rocks were not observed. This may suggest that the basalt clasts were locally derived from magmatic rocks associated with dolerite emplacement rather than from the Trobriand magmatic arc.
Before the onset of the recorded deposition at Site 1109, the inferred arc-proximal forearc region was uplifted. Regional seismic analysis and exploratory hydrocarbon drilling indicates that the Cape Vogel area, including the Trobriand depocenter to the northwest of Site 1109, was uplifted and deformed in middle- to late-Miocene time. In the west, the northern and southern margins of this basin were stratigraphically inverted at this stage.
The dolerite at the base of Hole 1109D are low-K tholeiites (see "Igneous and Metamorphic Petrology"). Following intrusion, the entire magmatic complex was uplifted and subaerially eroded in a tropical climate, and fluvial conglomerates were deposited in a marginal, terrestrial (swampy) setting (Unit IX).
Seismic mapping of the morphology of the erosional unconformity (see the "Data Report: Marine Geophysical Surveys of the Woodlark Basin Region" chapter) indicates that Site 1109 is located in a north-south-trending depression that presently deepens to the south (see Fig. F9 in the "Data Report: Marine Geophysical Surveys of the Woodlark Basin Region" chapter). Beneath the unconformity, the strata forming the southern flank of the forearc basin dip northward at ~10º. The local east-west relief is up to 0.2 s, estimated as 200-250 m (assuming a seismic velocity of the lower section of 2.0-2.5 km·s-1), with a width of around 5 km. The depression is interpreted as a paleovalley, thus explaining the 60 m of terrestrial clastics within Units IX and X at the base of the sedimentary succession as a silt-sand-gravel-filled channel system. The possible direction of flow in this paleovalley is discussed in the "Leg 180 Summary" chapter in the light of the regional paleogeographical setting. Notwithstanding this, Unit X can be interpreted as an aggrading channel-fill succession interbedded with clay, silt, or sand. A further implication of the textural maturity of the conglomerate is that the hinterland was perhaps of relatively low relief (less than several hundred meters), consistent with only limited emergence of the former forearc region.
The inferred swamp deposits (Unit IX) can be viewed as overbank deposits, with a broad valley of low relief relative to stream courses that were stabilized by dense vegetation. Relief decreased with time, or the base level of erosion rose; hence, the decrease in conglomerate and transition to deposits of Unit VIII. Later, the inferred remanent paleovalley was inundated and became a marine lagoon (Unit VIII) subject to fluctuating salinity ranging from freshwater to hypersaline.
With further subsidence, marine transgression occurred, giving rise to a shallow carbonate-dominated shelf setting (Unit VII). Small highs in the basement of up to several hundred meters (see Fig. F9 in the "Data Report: Marine Geophysical Surveys of the Woodlark Basin Region" chapter) could have existed as small islands at this stage, possibly fringed with coral reefs and thus providing the extrusive igneous and shallow-water carbonate clasts (e.g., coral) observed in Unit VII. No evidence of a barrier (e.g., bar or reef) between the inferred lagoonal and shallow-marine carbonate deposits is preserved at Site 1109D. A possible explanation (assuming that sediment transport was by then to the south) is that the drowned river valley formed a natural inlet that, together with small islands, formed a protected coastal setting without construction of a sedimentary barrier.
The area continued to subside such that overlying sediments entirely accumulated in deepening water (from upper bathyal to middle and then to lower bathyal). Thick (~180 m), rapid (~300 m/m.y.) deposition of hemipelagic mud, silt, and minor sand turbidites ensued during early to middle Pliocene time (Unit VI). This interval contains relatively unaltered lithoclasts consistent with derivation from the uplifted and rapidly eroding Trobriand forearc region. Sediments at ~380 mbsf correspond to an important boundary between two inferred mega-sequences of differing sediment type and provenance.
There was then an interval marked by an important input of volcaniclastic sand and silt, with abundant finer grained mud and thin-bedded silt turbidites (Unit V). Unit IV records fine-grained calcareous sediments with sporadic turbidity current depositions. Unit III is a succession of pelagic and hemipelagic silty clay interspersed with several types of deep-water (middle bathyal 500-2000 m) sediment-gravity-flow deposits. An interval of sand obtained near the base with poor recovery (Cores 180-1109C-24R and 25R) probably represents an important interval of turbidity current deposition, comprising material from arc-type volcanic, metamorphic, and shallow-water carbonate settings. This was followed by an interval of characteristic volcaniclastic silt and sand turbidites (Unit II), with small (5-10 m) carbonate sand-clay-rich cycles. The probable source of the metamorphic detritus is the D'Entrecasteaux Islands (Davies and Warren, 1988), or southeastern PNG (Davies, l980).
Finally, the Pleistocene (Subunits IA and IB) was marked by additional calcareous nannofossil-rich pelagic deposition, interspersed with volcaniclastic silt, sand turbidites, and volcanic ash (see "Role of Volcanism"). The entire early Pliocene-Quaternary succession includes sporadic (allodapic) calciturbidites derived from shallow-water carbonate sources. Sedimentation rates apparently decreased markedly to ~21 m/m.y. between 1.0 and 0.46 Ma. Possible explanations include diminished sediment input possibly caused by erection of topographic barriers between the site of deposition and the source area, or slowing of net sediment accumulation owing to enhanced bottom-current activity during Quaternary time.
A total of 48 volcanic ash and 377 volcaniclastic layers were recovered at Site 1109, with a combined total thickness of 10.07 m compared to the total sediment thickness of 772.9 m. The stratigraphic distribution and number of volcanogenic ash layers observed per core, as well as the total thickness of volcanogenic ash layers per core, are shown in Figure F36. Three pronounced maxima are apparent: (1) in the middle Pliocene; (2) in the early Pleistocene; and (3) in the late Pleistocene-?Holocene. These three volcanic episodes are characterized by different transport processes and possibly different sources, as follows:
In the lower Pliocene sequence (Unit VII), ash fallout layers are absent. In Unit VI, five ash fallout layers are present between 465 and 485 mbsf. These are mainly composed of colorless silicic glass, with a minor contribution of brown glass shards. These shards exhibit thin platy and bubble wall morphologies; most are fresh with only minor signs of devitrification.
An important change in the nature of volcanogenic deposition took place in the middle Pliocene where the volcanic material of Unit V (i.e., 352-385 mbsf) is dominantly in the form of volcaniclastic turbidites. Total crystal content is high (up to 50%) relative to the abundance in the inferred ash-fall deposits. Also, volcanic lithoclasts, terrigenous (e.g., metamorphic derived) and bioclastic material are present, indicating an epiclastic origin. In marked contrast, in Unit VI and below most of the glass and phenocrysts are moderately to highly altered.
Three ash-fall layers are present in Unit IV (middle Pliocene), which are characterized by common to abundant vesicle-rich glass shards.
Finally, Pleistocene Units I and II record several volcanic episodes, represented by 33 volcanic ash layers together with four ash-rich turbidites. Primary ash-fall layers, reworked to a variable extent, are predominate in this uppermost interval at Site 1109.
The middle Pliocene and Quaternary glass-rich layers record explosive acidic volcanism, inferred to have been sited in the Trobriand Arc to the south (including Amphetts Islands and Egum Atoll), or eastern PNG including the D'Entrecasteaux Islands (Johnson et al., 1978; Davies et al., 1984). In addition, the epiclastic turbiditic sands clearly originated within the confines of the Woodlark Basin area. Two alternatives are that the glass originated in calc-alkaline volcanism of the Trobriand Arc that is related to southward subduction of oceanic crust from the Solomon Sea (Davies et al., 1984), or from calc-alkaline of eastern Papua and/or peralkaline volcanism of the Dawson Strait area that is attributed to rifting of the Woodlark Basin rift (Smith, 1976; Johnson et al., 1978; Stolz et al., 1993). Eastern Papuan Peninsula has been volcanically active from the middle Miocene to the present day, and there have been eruptions on many of the surrounding islands and in the marine basins (Smith and Milson, 1984). However, these alternatives can only be discriminated following postcruise major and trace element analysis of glass shards and related phenocrysts.
Site 1109, therefore, documents the rift-subsidence history of the Pliocene-Pleistocene Woodlark Basin. Taken as a whole, the sedimentary succession can be (provisionally) subdivided into two megasequences, separated by important changes in lithology, physical properties (e.g., magnetic susceptibility), log units, and seismic character around 370-380 mbsf. The provenance of the lower megasequence was probably mainly from the then emergent Trobriand Arc/forearc. This area formed the proximal part of the Miocene arc/forearc region that was uplifted, eroded, suppled detritus, and later submerged.
By contrast, the provenance of the upper, onlapping megasequence that includes altered calc-alkaline volcaniclastic material, local metamorphic detritus, smectite, and mixed-layer clay was possibly mainly the calc-alkaline volcanic and metamorphic terrains represented by the D'Entrecasteaux Islands and eastern PNG. However, the two possible provenances probably also interfinger to some extent.