The lithostratigraphic succession recovered from Hole 1108B consists of 485.2 m of sediments and sedimentary rocks of middle Pliocene to late Pleistocene age. Four lithostratigraphic units were recognized on the basis of sediment or rock type, grain-size variations, sedimentary structures, color, smear-slide and thin-section compositions, bulk mineralogy (X-ray diffraction [XRD]), and calcium carbonate determinations. Only a small, upper part of the succession (60 m) was geophysically logged because of problems coring the hole.
The uppermost 10 m of the succession comprises nannofossil-bearing hemipelagic and pelagic sediment, defined as lithostratigraphic Unit I (see Fig. F1). This is underlain by an inferred unit of talus, represented by a number of sedimentary, metamorphic, and igneous clasts (Unit II) followed by inversely graded sand (Unit III). Beneath this, Unit IV comprises the greater part of the succession and is dominated by coarse- to medium-grained sand/sandstones, siltstones, and claystones. Unit IV is divided into three subunits, based on the predominate fine, medium, and coarse grain size. The succession is disrupted by a fault zone concentrated ~160-180 mbsf.
The recovery in Hole 1108B began with yellowish brown (10YR 5/6 and 6/6), to greenish gray (5G 6/1 and 7/1), slightly mottled nannofossil-bearing calcareous clay. The upper 18 cm of the core is highly disturbed and oxidized. In addition, dark yellowish brown volcaniclastic sand is present at interval 180-1108B-1R-1, 18-20 cm (Fig. F2). This layer has a sharp base and fines upward into very fine nannofossil ooze that is rich in planktonic foraminifers at the top. In addition, a thin layer of disturbed volcaniclastic sand occurs at interval 180-1108B-1R-2, 33-38 cm. Smear slides of the volcaniclastic layers contain common plagioclase, rare quartz, biotite, amphibole, pyroxene, volcanic glass (colorless and brown), rock fragments, carbonate, calcite, and opaque grains (see "Site 1108 Smear Slides"). The XRD analysis of Sample 180-1108B-1R-2, 18-19 cm, indicates the presence of calcite, quartz, plagioclase, and chlorite as major minerals and illite as a minor component (see Table T3). The biogenic component of Unit I is made up of abundant nannofossils, rare planktonic foraminifers, and diatoms. Two determinations of calcium carbonate indicate abundances of 28 and 31 wt% (see Table T4), and thus the sediment is classified as nannofossil clay rather than nannofossil ooze.
The calcareous clay is interpreted as deep-water hemipelagic and pelagic sediment with a high content of volcanogenic clay. The volcaniclastic beds are viewed as possible turbidites composed of redeposited volcaniclastic material (i.e., of epiclastic rather than primary pyroclastic origin). The sedimentation rate is estimated at 15 m/m.y., based on the microfossils present (see "Sediment Accumulation Rate").
Lithostratigraphic Unit II was recognized as the interval from Core 2R through Core 8R in which recovery was restricted to isolated clasts.
The following clast types were recognized in hand specimen: black vesicular basalt, volcanic breccia, granite, gneiss dolerite (metamorphosed to greenschist facies); mylonite (metamorphosed to greenschist facies); and dark volcaniclastic sandstone and siltstone.
Volcanic rocks include three types of basalt and a feldspar porphyry. The first basalt type has a glassy (tachylitic) groundmass. The second type of basalt has a fresh, pilotaxitic groundmass (containing plagioclase laths and rare olivine). The quench texture and freshness of the groundmass in both types imply a young age and submarine eruption. A third lithology is amygdaloidal aphyric basalt, containing chlorite and epidote. The feldspar porphyry contains phenocrysts of alkali feldspar, plagioclase, biotite, and hornblende in a quartzo-feldspathic groundmass, and may have originated as a dike rock.
Plutonic rocks include microgranite, granodiorite (or granite), and epidosite. The microgranite contains plagioclase, hornblende, and quartz, whereas the granodiorite contains plagioclase, quartz, and biotite replacing hornblende. The granodiorite contains chlorite and sericite alteration and has a mortar texture caused by brittle (low P,T) deformation. The epidosite consists primarily of epidote, sericite alteration, and actinolite needles within quartz grains. The granite clasts show a dominantly cataclastic texture.
Metamorphic rocks include mylonite, gneiss, and greenschist facies metadolerite. The mylonite contains stretched quartz, biotite, and chlorite in a very fine grained matrix. The gneiss contains plagioclase, quartz, biotite, apatite, sphene, and zircon. The gneiss also exhibits a foliation defined by biotite laths and shows evidence of shearing marked by sigmoidal porphyroblasts. The metadolerite contains altered feldspar and sporadic epidote.
In addition, the following information was obtained by study of thin sections of the sedimentary rocks recovered (see "Site 1108 Thin Sections"). Two sandstone pebbles (intervals 180-1108B-3R-CC, 0-4 cm [see Figs. F3, F4, F5, F6] and 5R-CC, 6-8 cm [see Fig. F7]) are lithic sandstones with clasts of basalt, silicic extrusive rock, rare mica schist, quartzite, strongly altered basic extrusive rock, intrusive rock (granite?), and serpentinite. Mineral grains seen in thin section are fresh plagioclase, muscovite, quartz, and rare epidote. Bioclasts are mainly shell fragments and planktonic foraminifers. Another clast of sandstone is fine-grained, relatively well sorted micaceous lithic sandstone containing muscovite, quartz, plagioclase, chlorite, and opaque grains (interval 180-1108B-3R-CC, 16-20 cm). Numerous lithic fragments of basalt are seen. Bioclasts are present as numerous planktonic foraminifers. Scattered glauconite pellets are also seen.
All the clasts recovered are interpreted as talus (i.e., debris-flow or rock-fall deposits). No matrix was recovered, but it may have been washed out or lost during drilling. The probable source was a basement of plutonic igneous and metamorphic rocks combined with a lithified terrigenous sedimentary unit. The adjacent Moresby Seamount is an obvious suitable source location. The relatively fresh basic volcanic pebbles represent additional talus, or milled fragments derived from thin in situ, or local, flows that are probably related to the nearby Woodlark spreading center. The sedimentary rock fragments were possibly derived from the sedimentary cover of the Moresby Seamount, whereas the igneous and metamorphic rocks possibly originated in the underlying basement. The talus is much coarser than the underlying material recovered and may record a significant discrete gravity sliding event that was possibly triggered by movement along the Moresby extensional fault system, as imaged on multichannel seismic profiles. The faulting is assumed to have exhumed the greenschist facies rocks, either on land or on the seafloor, providing that the source material was talus emplaced by gravity flows.
This unit ranges from fine- to coarse-grained unlithified sand, which was recovered beneath the inferred debris-flow deposits and talus of Unit II, but above well-cemented clastic sediments forming Unit IV (Fig. F1). This thin interval is recognized as a separate unit because it differs drastically from the overlying inferred talus in Unit II and the underlying well-lithified sandstones of Unit IV.
Unit III was recovered only in Core 8R. It consists of three inversely graded, medium-bedded sands. The unit comprises three beds. The first is dark gray (N4) silty sand and fine sand; this is followed by medium-grained sand that is, in turn, overlain by medium-grained to coarse-grained sand (Fig. F8). Components identified by visual inspection and XRD analysis are quartz, plagioclase, rock fragments, and shell fragments (<0.5 cm). Minor components are amphibole, chlorite, and probably smectite (see Table T3).
The inversely graded nature of several of the beds in Unit III and the absence of tractional sedimentary structures is consistent with deposits from high-density turbidity currents. The uncemented nature indicates a much less diagenetically advanced state than that of the sedimentary rocks of Unit IV (see "Lithostratigraphic Unit IV"). It is possible that the sands relate to the inferred talus and debris of Unit II, possibly as precursor to its emplacement; however, data are inadequate to test this hypothesis.
Substantial recovery of well-cemented, fine- to medium- and coarse-grained sandstone interbedded with claystones began in Core 9R (72.30 mbsf). Unlike Unit III, Unit IV is well lithified. Because the lithologic composition remains similar throughout, this entire interval is placed within a single lithostratigraphic unit (Fig. F1). However, there is sufficient variation in the relative abundances of different sedimentary rocks to recognize three different intercalated subunits within Unit IV as follows:
The main characteristics of each of the three subunits are described below.
Lithostratigraphic Subunit IVA is characterized by sandstones that vary from coarse to fine grained, together with clay-rich siltstones and silty claystones. There was no recovery in Cores 11R and 12R. However, the base of Core 11R and all of Core 12R were geophysically logged (including gamma-ray, resistivity, and radioactivity logs). The log response of the missing interval (Cores 11R and 12R) indicates the presence of a high porosity facies and is similar to the log response of the underlying interval of Subunit IVA recovered in Cores 14R and 15R, and the upper part of Core 16R. (Note: Only the interval from ~100 to 160 mbsf was logged because of problems in the hole; see "Downhole Measurements".)
The sandstones typically contain 1.7-7.0 wt% calcium carbonate, whereas intercalated silty claystones are more calcareous (~15 wt%) (see Table T4 and "Organic Geochemistry"). In addition, a sample of clay-rich siltstone from near the base of the succession contains 39 wt% calcium carbonate. The XRD analyses indicate plagioclase and quartz as the major minerals (see Table T3).
Bedding in the sandstones is generally with sharp bases and gradational upper contacts. Colors are subdued, typically greenish gray and dark greenish gray (5Y 4/1 to 5GY 4/1), to a very rare bluish gray (5B 4/1). Bed thicknesses range from thin to thick. An example of a thick-bedded, coarse-grained sandstone is shown in Figure F9. Contrasting thin-bedded sandstones are shown in Figure F10. Commonly, the sandstones are discrete beds separated by silty claystone and clay-rich siltstone. However, repeated thin beds of sandstone of various thickness (<25 cm) were observed. Finer grained sandstones are commonly clay rich (usually termed clayey sandstone in the visual core descriptions).
The majority of the thicker bedded (>30 cm) and coarser grained sandstones are internally structureless. Some of the sandstones exhibit normal grading from coarse- or medium-grained sand to medium- or fine-grained sand. A relatively small number of sandstones show sequences of sedimentary structures that can be identified as the Ta, Tb, and Tc divisions of classical turbidites (Bouma, l962; e.g., Sections 20R-1 and 20R-2). Occasionally, complete Ta-Te divisions were noted within a single-graded sandstone bed (interval 180-1108B-24R-2, 4-36 cm). However, it must be emphasized that the majority of the sandstones defy simple classification using Bouma's (l962) scheme (see below).
Some graded units locally exhibit alternating low-angle planar lamination or cross lamination (e.g., interval 180-1108B-26R-1, 115-120 cm; Fig. F11). There are some exceptions where cross lamination is relatively steeply inclined (i.e., up to 35º, average <20º; e.g., interval 180-1108B-36R-2, 42-45 cm). Convolute lamination was also occasionally observed (e.g., intervals 180-1108B-26R-1, 112-114 cm, and 36R-3, 90-100 cm). A prominent additional feature of Subunit IVA is reverse-to-normal grading (reverse grading passing into normal grading within a single bed) within the medium- to thick-bedded sandstones. Reverse-to-normal grading is present throughout the unit but becomes more common below 245 mbsf. Commonly, individual beds exhibit reverse grading, from medium- to coarse-grained sand near the base, passing upward into normally graded coarse- to medium-grained sand. The reverse-graded intervals are typically structureless.
Rip-up clasts (i.e., intraformational) of mudstone are common within the graded sandstones and are concentrated near the top of individual depositional units (interval 180-1108B-26R-3, 70-80 cm). Mud clasts are typically less than several centimeters in size, but local larger mud clasts (up to 4 cm) were also noted (Fig. F12). In some places, it was unclear if a mud layer was part of a large mud clast or a discrete mudstone bed. Mud clasts are rarely concentrated in repeated thin beds to laminae (Section 32R-3, 86 cm, 105 cm, and 122 cm). Rip-up clasts in places are composed of siltstone rather than claystone (e.g., interval 180-1108B-32R-5, 33.5-83 cm).
In hand specimen the sandstones exhibit numerous angular, to subangular, extraformational lithic grains in the form of reddish lithic grains, quartz grains, and white detrital carbonate grains. The carbonate grains are fragments of bivalves, calcareous algae, bryozoans, echinoderms, and large (i.e., benthic) foraminifers. Coral fragments were rarely noted in Sections 32R-1, 32R-2, and 32R-5. In addition, occasional small intact gastropod shells are present (e.g., Section 15R-2, 109 cm). White, small tubular (noncalcareous) bioclasts (up to 0.8 cm in diameter) were observed locally but remain unidentified. These fossils range from spherical to elliptical in three dimensions with an internal cavity infilled by sediment.
Siltstones are commonly clay rich and occur as intercalated depositional units up to tens of centimeters thick. Colors typically range from dark greenish gray (5GY 4/1) to greenish gray (5BG 5/1). These siltstones exhibit parallel lamination, small-scale low-angle cross lamination, or are structureless. Some siltstones are sandy (Section 16R-1), whereas others are clay rich. Fragments of carbonized wood were locally observed (e.g., interval 180-1108B-15R-CC, 10-15 cm). The siltstones are similar to those described in more detail from Subunit IVB (see "Lithostratigraphic Subunit IVB").
The compositions of a number of thin sections of the sandstones and siltstones studied are summarized in "Site 1108 Thin Sections". All the sandstones are lithic; however, there is a marked difference in composition of the clasts and related mineral grains between sandstones in the upper and lower parts of the succession, with the change found at ~330 mbsf. However, it must be emphasized that this depth is only approximate as the change was observed in only a few thin sections of sandstone (see "Site 1108 Thin Sections"). There is no marked change in sedimentary structures, grain size, or physical properties. Also, there is no marked change in physical properties, corresponding to a depth of 330 mbsf (see "Physical Properties").
Sandstones in the upper part of Unit IVA (e.g., interval 180-1108B-15R-1, 11-14 cm) are lithoclastic, with small subangular to angular grains of fresh and altered basalt, schist, and altered siliceous extrusive igneous rock (Fig. F13). Common mineral grains are mainly quartz, feldspar (mostly plagioclase), muscovite, biotite, and epidote, set in a cement of fine calcite spar. Serpentinite lithoclasts and chromite are rarely observed.
Some of the sandstones (e.g., interval 180-1108B-26R-3, 7-16 cm) contain planktonic foraminifers as well as bioclasts of shallow-water origin, including calcareous algae and bryozoans. These biogenic components are in addition to numerous lithic grains of siliceous and calcareous schist, altered basalt, altered siliceous extrusive igneous rock, micritic carbonate (partly recrystallized), and mineral grains including fresh plagioclase, hornblende, biotite, quartz, and rare serpentinized olivine.
One other sandstone bed (interval 180-1108B-22R-6, 20.5-22.5 cm) includes rare grains of with a sheared mylonitic texture. These grains occur within a typical assemblage of relatively fresh basaltic grains, more altered silicic volcanic grains, mica schist, quartzite, and associated mineral grains (quartz, feldspar, and muscovite).
In several thin sections, grains of clinopyroxene were noted. They are probably associated with the basaltic clasts that commonly contain pyroxene phenocrysts (e.g., interval 180-1108B-23R-3, 35-36.5 cm). Indeed, an isolated clast of fresh olivine basalt was recovered in a nearby part of the succession (interval 180-1108B-26R-1, 0-2 cm). Elsewhere, the sandstones include well-rounded clasts of olivine basalt (e.g., interval 180-1108B-27R-5, 61-63 cm) in which the basalt grains show variable degrees of alteration. Basaltic grains, typically well rounded, were also observed to be abundant in several thin sections of sandstones that were recovered near the base of Subunit IVA (intervals 180-1108B-32R-3, 61-64 cm, and 34R-2, 126-128 cm).
By contrast, sandstones below ~330 mbsf (e.g., interval 180-1108B-36R-3, 52-56 cm) contain a mixture of lithoclasts of basalt, altered dolerite (epidote rich), acidic volcanic grains, chloritic grains (altered basic glass), rare coarse granitic rock fragments (characterized by intergrown quartz and feldspar, fresh and altered), plagioclase (commonly zoned), biotite, minor muscovite, hornblende, pyroxene, and rare olivine, together with bioclasts (e.g., bryozoans and calcareous algae), rare lithoclasts of quartzose siltstone, and diagenetic pyrite. In addition, microcrystalline quartz that may be chert was present but rare (e.g., interval 180-1108B-45R-CC, 8-10 cm). Planktonic foraminifers are also invariably present. Textures range from matrix to clast supported within individual sandstones, which are commonly poorly sorted with mainly angular grains.
In addition, a few thin sections of fine-grained sandstones were studied, mainly from the lower part of the Subunit IVA succession (e.g., interval 180-1108B-28R-3, 28-31 cm). They contain a mixed assemblage of planktonic foraminifers and detrital grains, including mineral and rock fragments set in a fine micritic matrix. These sediments are well sorted, with abundant small muscovite grains oriented parallel to lamination, and occasionally include benthic foraminifers (e.g., interval 180-1108B-39R-1, 94-96 cm).
In summary, sandstones from the upper part of Subunit IVA (i.e., above 350 mbsf) contain a high proportion of metamorphic-derived lithoclasts and related mineral grains (i.e., schist, quartzite, phyllite, polycrystalline quartz, muscovite, epidote, and rare chromite). Highly altered igneous-derived grains are also present. The lower ~150 m of the succession recovered contain large amounts of material derived from both basic and acidic volcanic rocks, with a minor contribution from both basic (dolerite) and acidic (granite) plutonic rocks. By contrast, metamorphic-derived material (e.g., schist, polycrystalline quartz) is greatly diminished relative to the above 330 mbsf (see "Site 1108 Thin Sections").
Silty claystones are common and range in color from greenish gray and dark greenish gray (5Y 4/1 to 5GY 4/1), to dark gray (N4), to olive gray (5Y 5/4), and black (5Y 2.5/2). The silty claystones are mainly structureless, but they are commonly bioturbated to variable extent, giving rise to a distinctive mottled appearance. Discrete silt laminae are rarely present. Planktonic foraminifers are commonly scattered throughout (e.g., interval 180-1108B-19R-1, 43-80 cm). The darkish claystones are commonly massive and devoid of trace fossils (e.g., interval 180-1108B-23R-4, 83-150 cm). In addition, the lower part of the subunit includes rare thin (<20 cm) intervals of reddish brown claystones (e.g., interval 180-1108B-36R-4, 21-23 cm; 2.5YR 4/4). The claystones are similar to those described in more detail from Subunit IVB (see "Lithostratigraphic Subunit IVB").
Two pebbles of medium- to coarse-grained sandstone were recovered in Core 13R. This was the entire recovery in the core and, thus, it is unclear if the pebbles were derived from an interbedded conglomerate, if they fell from above (i.e., from Unit II), or if they represent drilling artifacts (i.e., pieces of sandstone milled to clasts by rotary drilling). For this reason, the pebbles in Core 13R are included within Subunit IVA (i.e., mainly coarse clastic sediments) rather than with Subunit IVC (see conglomerates discussion in "Lithostratigraphic Subunit IVC").
The sandstones locally contain scattered granule-sized clasts. These occur exclusively within the thicker (>30 cm), coarser grained sandstones that are common near the base of individual beds. For example, a rounded pebble (2 cm × 3.5 cm) of coarse- to very coarse-grained, well-cemented sandstone was recovered in Core 13R. This sandstone contains clasts of quartz and rock fragments. In addition, rare small clasts of reddish, altered basalt clasts were also noted (e.g., interval 180-1108B-13R-CC, 0-7 cm).
Relatively homogeneous volcaniclastic layers are very rare, but local thin-bedded siltstones are rich in volcaniclastic material. For example, a thin bed of light gray (5YR7/1) sandy, silty volcaniclastic material is in interval 180-1108B-14R-2, 54-59 cm. Smear-slide analysis of this layer revealed common colorless glass, rare brown glass, and common bioclasts (e.g., echinoderm fragments). A large amount of colorless (rhyolitic) glass shards is present together with bioclasts, including planktonic foraminifers. Elsewhere, volcaniclastic material forms a subordinate constituent of thin-bedded (<5 cm) siltstones (e.g., Sample 180-1108B-23R-1, 99.5-100.5 cm) as documented by smear-slide analysis (see "Site 1108 Smear Slides").
Subunit IVB is distinguished from Subunit IVA by greenish gray to dark gray and black foraminifer-bearing clay-rich siltstone interbedded with thin- to medium-bedded, mainly fine- to medium-grained sandstone. The XRD analysis documents the presence of chlorite, illite, and smectite as the clay minerals present (see Table T3). Determinations of calcium carbonate indicate that the fine-grained sediments of this subunit are highly calcareous, with CaCO3 values >25 wt% recorded (see Table T4).
The part of the succession between 140 and 200.2 mbsf shows abundant evidence of tectonic deformation within mainly silty claystone and clay-rich siltstone, characterized by bedding dips up to 35º, low-angle shearing, brecciation, and ubiquitous slickensides that are commonly concentrated along joints and lithologic boundaries (see "Structural Geology"). The structural deformation has particularly distorted bedding contacts and disrupted depositional units. A combination of structural, paleomagnetic, and physical properties evidence indicates that an important, inclined, predominantly dip-slip fault zone is present and is concentrated ~158-173 mbsf (Cores 18R and 19R). The orientation of this fault zone is unknown. However, it is probable that up to 200 m of the succession were cut out by faulting. Below 200.2 mbsf, within more competent lithologies, the intensity of tectonic deformation falls off markedly, although occasional (normal) faults cut the succession.
The following lithologies are present in Subunit IVB:
These sediments are greenish gray (5GY 4/1), bioturbated (with variable intensity) and contain scattered planktonic foraminifers (e.g., interval 180-1108B-19R-1, 0-43 cm). There are subtle differences between the relative abundance of claystone and siltstone, as observed both within and between individual depositional units.
The clay-rich siltstones are typically present as relatively thin to medium beds <15 cm thick. These sediments are commonly strongly sheared and tectonically fragmented, obscuring primary sedimentary structures. Chondrite burrows are ubiquitous throughout many of the claystones (e.g., Section 16R-1). In addition, large Zoophycos-type horizontal burrows are locally present (e.g., Sections 19R-2 and 19R-3). In places, entire intervals, including successive beds, are bioturbated (e.g., interval 180-1108B-16R-1, 0-84 cm). Darker (organic-rich?) layers are mainly claystones with minimal silt and are mainly devoid of trace fossils (e.g., interval 180-1108B-19R-4, 82-100 cm).
The clay-rich siltstones and silt-rich claystones are mainly structureless (e.g., interval 180-1108B-19R-4, 0-20 cm), but are locally graded (e.g., interval 180-1108B-20R-1, 10-23 cm), cross laminated (e.g., interval 180-1108B-16R-2, 102-115 cm), convolute laminated (e.g., interval 180-1108B-26R-1, 112-121 cm), or parallel laminated (e.g. interval 180-1108B-20R-1, 130-145 cm; Fig. F14).
Some siltstones are packed with planktonic foraminifers (e.g., interval 180-1108B-19R-2, 35-39 cm). Pyrite concretions occur locally (interval 180-1108B-18R-4, 0-67 cm). Also, some small woody fragments were noted. Chemical analysis indicates that the calcareous claystones of Subunit IVB are rich in organic matter, which is probably of terrestrial origin (see "Organic Geochemistry").
In thin section the typical foraminifer-rich silty claystone was seen to comprise abundant planktonic foraminifers set in a muddy micritic matrix. The XRD analysis of Sample 180-1108B-17R-2, 70-73 cm, shows calcite as the major mineral and quartz and plagioclase as minor minerals (see Table T3). In addition, a fine-grained foraminifer-rich calcareous interval from Subunit IVA (interval 180-1108B-16R-CC, 1-3 cm) comprises planktonic foraminifer packstone with a calcite spar cement. Small detrital grains of quartz, feldspar, plagioclase, muscovite, and biotite are also present. Pyrite was noted within individual foraminifer tests.
Sandstones are mainly fine to medium grained and thin to medium bedded and are intercalated with the silty claystones and clay-rich siltstones. The majority of the sandstones are graded, with sharp bases (commonly scoured) and gradational tops. They are very similar to the sandstones described above from Subunit lVA and are, therefore, not described in detail here. Partial Bouma sequences were rarely observed and in places show amalgamation (interval 180-1108B-20R-2, 16.5-25 cm). Typical sedimentary structures within the thin- to medium-bedded sandstones are shown in Figures F14, F15, F16, and F17. In addition to siliciclastic sediment, local disseminated volcanic clasts (Fig. F18) and glass shards (brown and colorless) were identified by use of a hand lens (e.g., interval 180-1108B-16R-2, 0-14 cm), and in a number of the smear slides (see "Site 1108 Smear Slides").
Rare, very coarse grained sandstone (up to 2 mm) was recovered in interval 180-1108B-16R-2, 14-19.5 cm. This sandstone forms the base of relatively thick, graded sandstone beds, and is included within Subunit IVB rather than within the conglomerate forming Subunit IVC.
First, a distinctive horizon of granule-sized conglomerate is present in interval 180-1108B-34R-2, 0-69 cm. This is a single sharp-based unit of granule conglomerate, grading into coarse sandstone at the top (Fig. F19). This interval is structureless and contains shell fragments. The color is subdued dark gray (5Y 4/1).
In the lower part of Unit IV, a second conglomerate interval is interbedded with the succession (interval 180-1108B-41R-1, 36-142 cm). This interval is composed of matrix-supported sandstone strewn with subrounded to rounded lithic fragments that are concentrated near the base of the unit. Clast sizes range from coarse-grained sand to pebble with most being in the 3-5 mm grain-size range. Sedimentary clasts are more rounded relative to igneous ones. In addition to lithogenous clasts, shell fragments and other calcareous fossil clasts are scattered through the conglomerate. Colors of this conglomerate are red, black, white, and green. The matrix is more grayish in color.
A third, thin conglomeratic interval is present lower in the lower part of Unit IV (interval 180-1108B-47R-1, 29-66 cm, through 47R-CC, 0-24 cm). This comprises 0.2 m of polymict paraconglomerate containing both intraformational and extraformational clasts. The base of this unit was not recovered by drilling, whereas the top of the paraconglomerate is directly overlain by fine-grained sandstone. In marked contrast to Subunit IVA, colors are dark gray to dark red and very dark gray. The extraformational clasts include subangular, subrounded to subangular, to locally rounded basalt, and other altered extrusive igneous rocks. The intraformational clasts are coaly fragments of small pebble size. In addition, large shell fragments were observed only in the working half of the core. The matrix to the paraconglomerate is fine-grained sandstone.
Unit IV is interpreted as a single depositional unit, which on land would correspond to a single lithostratigraphic formation. This conclusion was reached for the following reasons: (1) sediments of similar type (i.e., lithology and sedimentary structures) are present throughout the entire unit; (2) the study of thin sections shows that all levels of Unit IV exhibit similar composition, although the relative abundances of constituents vary locally; and (3) all the sediments are well cemented by calcite and exhibit a similar diagenetic state.
Notwithstanding the above similarities, the relative abundances of coarse-, medium-, and fine-grained sediments vary systematically throughout Unit IV and allow us to subdivide them into three subunits as described previously.
We interpret all of the sandstones as turbidites. These predominate in Subunit IVA but are also a subordinate component of Subunit IVB. A deep-water origin is indicated by a well-preserved, diverse fauna of benthic foraminifers that indicate bathyal depths (>1000 m; see "Benthic Foraminifers"). A relatively small number of Bouma sequences (Bouma, l962) were identified: either complete Ta-Te or partial Tb-Te, or rarely Tc-Te divisions. However, such classical turbidites are rare. More commonly sandstones are massive, show irregular cross lamination, or reverse-to-normal grading on different scales. In addition, rare amalgamated beds may record deposition from successive turbidity currents. Provisionally, these sandstones are interpreted as deposits from high-density turbidity currents, and they require further postcruise study. In addition, a small number of structures (e.g., sharp-topped planar laminae) are suggestive of deposition under the influence of bottom currents (cf. Pickering et al., l989).
Within Subunit IVB, the succession is considerably finer grained and more calcareous, with abundant silty claystone and clay-rich siltstone, rich in nannofossils, dispersed planktonic foraminifers, and (inferred) organic matter. We interpret this interval as mainly hemipelagic silts and clays. The sandstones with partial Bouma divisions (Tb, Tc, and Td) are interpreted mainly as classical turbidites, whereas the thinner bedded more silty beds are viewed as deposits from low-density turbidity currents (cf. Piper, l978). The subdued hues of gray green and gray to black, with minor disseminated pyrite, are suggestive of relatively low-oxygen bottom conditions at various times. The presence of relatively fine grained sediments in Subunit IVB could relate to factors that include sediment input from the source area, relative sea-level change, or autocyclic effects (i.e., evolution of the depositional system).
The three coarser grained intervals of conglomerate that form Subunit IVC are provisionally interpreted as high-density turbidity current and/or debris-flow deposits. Possible reasons for their occurrence could include relative sea-level change, increased clastic sediment input to the basin, or concentration in local channelized units.
The provenance of all the clastic sediments of Unit IV was from a continental area exposing mainly extrusive igneous and metamorphic rocks. Plutonic igneous and ophiolitic rocks (i.e., indicated by rare chromite grains and serpentinite fragments) were also present in the source region. Possible source areas are Moresby Seamount, Normanby Island, adjacent islands, and areas generally to the west that are now submerged but were land during the Pliocene, including the mainland of Papua New Guinea. The ubiquitous reddened lithic grains may well represent material weathered by laterization on land. The calcareous bioclast fragments were derived from shallow-marine (e.g., coral and algae) and shelf depths (e.g., benthic foraminifers), and then redeposited into a deep-marine setting.
Finally, based on evidence of microfossils (see "Biostratigraphy"), the sedimentation rate of the uppermost part of Subunit IVA (90-155 mbsf) is estimated as 324 m/m.y. (see Fig. F30). An apparent sedimentation rate for Subunit IVB (155-200 mbsf) is estimated as 18 m/m.y. but is probably due to the removal of as much as 200 m of the succession by the fault (see "Structural Geology"). The lower sandstones and conglomerates of Subunits IVB and IVC accumulated at ~409 m/m.y. It is notable that the apparently slow depositional rate of Subunit IVB is an artifact of faulting that cut out up to 200 m of the succession. On the other hand, an original difference in sedimentation rates between the finer grained Subunit IVB and the coarser grained Subunit IVA above and below may have existed but cannot be quantified
A generalized downhole variation in grain size is shown in Figure F20. Sedimentation recorded at Site 1108 began in the middle to late Pliocene with very rapid accumulation (409 m/m.y.) of fine-, medium-, and coarse-grained sandstones, and minor conglomerate, interpreted as mainly deposits from turbidity currents and debris flows. The composition of the sandstones is dominated by clasts and mineral grains derived from basic and acidic volcanic rocks, with a minor contribution from both basic and acidic plutonic rocks (dolerite and granite). Shallow-water-derived carbonate and quartzose siltstone are also present. A possible source of the shallow-water-derived sediments is the upper crustal units of the Moresby Seamount, assuming it was then exposed above sea level. However, this was possibly too small to provide all the clastic material and may not have existed as a discrete feature. It is probable that sediment was also contributed from the present areas of the D'Entrecasteaux Islands (e.g., Normanby Island), where a similar range of rock types is exposed (Hill et al., l992), and from any adjacent emergent areas. An alternative possible source for shallow-water carbonate might have been the Trobriand Platform or Ergun Reef to the north. However, these settings are unlikely because the carbonates recovered at Site 1108 are invariably mixed with abundant lithic clasts, including metamorphic constituents, for which a westerly or southwesterly derivation is likely, as discussed above.
Above 330 mbsf there is a marked incoming of metamorphic rock fragments and related mineral grains, in addition to the igneous and other grains summarized above. A possible explanation for the change in composition is that sediments were derived from source areas that experienced pervasive extensional faulting (i.e., detachment faulting) such that deeper, metamorphic levels of the crust were exposed through time (i.e., high-level extrusive and intrusive rocks were unroofed and eroded first, whereas deeper metamorphic rocks were not unroofed until later) resulting in an overall "reverse stratigraphy" of composition as observed in Hole 1108B.
In addition, the serpentinite and rare chromite grains that occur throughout the middle to upper Pliocene succession are assumed to have been derived from ultramafic ophiolitic rocks of the Papuan ophiolite belt (Davies and Jaques, l984). Also, the metamorphic rocks can possibly be correlated with the Kagi metamorphics that regionally underlie the Papuan ophiolite belt and are interpreted as a lower Tertiary subduction-accretion complex (Rogerson et al., l987). The shallow-water bioclastic debris (including algae and bryozoans) presumably accumulated at shelf depth and were then mixed with land-derived clastic sediment. Some of the lithic grains were clearly rounded in a fluvial or shallow-marine setting.
From ~1.95 to 2.58 Ma (late Pliocene) there was an apparent drop in sedimentation rate to ~24 m/m.y., corresponding to an interval of finer grained deposits from turbidity currents (Subunit IVB). However, this is probably an effect of faulting.
Later, during the time interval 1.71-1.95 Ma (late Pliocene), represented by the upper part of Subunit IVA, the sedimentation rate increased markedly to ~324 m/m.y. and then reduced slightly to an estimated 138 m/m.y. from 1.25 to 1.71 Ma (see "Sediment Accumulation Rate"). During this entire time interval, there was a return to predominant deposition from high-density turbidity currents.
Following, at ~60 mbsf, there is an abrupt change from well-consolidated lithologies to completely unconsolidated sand (Unit III). To explain this sudden change in degree of consolidation, sediment may have been deposited then eroded, possibly by mass flow processes, gravity flows, or bottom currents.
This was then followed by emplacement of talus as thick as ~50 m. The coarse, angular nature of the clasts recovered, including basalt, metadolerite, mylonite, and gneiss, suggests that the obvious provenance relates to the proximity of the Moresby Seamount. In addition, the commonly cataclastically deformed and sheared nature of some of the clasts, including mylonite and gneiss, suggests the Moresby detachment fault system as the specific source. As noted above, metamorphic rocks probably became available for erosion following exhumation related to extensional faulting. The time of emplacement of the talus is only poorly constrained by the late Pleistocene age of Unit I above (to 1.25 Ma) and the late Pliocene-early Pleistocene age (1.25-1.71 Ma) of the upper part of Unit IV beneath (see "Biostratigraphy"). The talus differs strongly from the relatively well sorted predominantly silt-sand grain size of the mainly Pliocene succession beneath, suggesting that it might relate to a discrete tectonic event (e.g., a pulse of movement along the Moresby extensional detachment system).
Finally, during the late Pleistocene (to 1.25 Ma) deposition was restricted to calcareous, nannofossil-rich clay with minor silt and sand containing a high content of volcanic ash. This is assumed to be pyroclastic ash derived from the Trobriand volcanic arc (including the Amphetts Islands and Egum Atoll) or rift-related volcanoes in Dawson Strait. The ash is mainly reworked, reflecting a gravitationally unstable setting adjacent to Moresby Seamount. Sedimentation rates are very much lower (15 m/m.y.) than for most of the Pliocene succession (Unit lV), despite the additional input of pyroclastic ash and proximity to Moresby Seamount. This area was possibly submerged during this time, and thus, not a significant sediment contributor, or the basin had become isolated from surrounding land areas by formation of tectonic barriers (i.e., sub-basins and highs related to rifting).