The succession cored at Site 1116 recovered mainly clastic sediments and sedimentary rocks, comprising an inferred Pliocene rift succession. The recovery was very poor (21%) and downhole logs were not available. Three lithostratigraphic units are 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 (Fig. F1).
Description: sandstone and
siltstone to claystone
Interval: Cores 180-1116A-1R through 4R
Depth: 0.0-33.8 mbsf
Age: Pliocene
Lithostratigraphic Unit I comprises two parts: (1) a number of isolated clasts of sedimentary rocks of different composition recovered in Core 1R and Section 2R-1; and (2) more coherent intervals of mainly sandstones recovered in Section 2R-CC through Core 4R. The nature of the clasts is summarized first to allow comparison with the more intact intervals beneath.
The following lithologies were recovered as clasts: gray and grayish brown claystone, silty claystone, sandy siltstone, coarse-grained sandstone (with common red grains), and coarse- to very coarse-grained sandstone. The sandstone is well sorted with a calcite cement. Many of the sandstones and siltstones are strongly bioturbated (with Chondrites burrows) obscuring sedimentary structures. Calcite veining is rarely observed. Disseminated organic matter is locally visible in the sandstone and siltstone. Some of the siltstones contain planktonic foraminifers and bioclasts occur in some sandstones.
In addition, the following sedimentary structures were observed within the clasts: inclined lamination, parallel lamination, cross lamination, or convolute lamination of siltstone, and sharp lower and upper bed contacts in siltstone.
Petrographic examination of the sandstones (see Figs. F2, F3, F4, F5; "Site 1116 Thin Sections") show that typically, the grains are angular to subangular, but a small proportion is subrounded to well rounded. Sandstone contains abundant fresh basalt (locally glassy), chloritized basalt, acidic volcanics (with quartz and feldspar phenocrysts), rare dolerite (with pyroxene and plagioclase), and acidic volcanics with brown hornblende phenocrysts. Mineral grains are plagioclase (commonly zoned), biotite, pyroxene, and rare hornblende. Lithic fragments include commonly polycrystalline quartz, calc-schist (with some well-rounded grains), biotite schist, rounded serpentinite grains, and micritic clasts. In addition, bioclasts include bivalve shell fragments and benthic foraminifers. The sandstone typically has a sparry calcite cement. A thin section showed the siltstone to be well sorted with abundant quartz, feldspar, biotite, opaque mineral grains, pyrite, chloritized basalt, planktonic foraminifers, rare muscovite, and chlorite (green and blue types), set in a silty matrix.
In addition, the following lithologies were recovered in the more coherent sedimentary intervals.
Well-sorted, well-cemented sandstone/siltstone is present as normal-graded couplets with sharp bases and graded tops (interval 180-1116A-4R-2, 4-9 cm; Fig. F6). The sandstones are mainly greenish gray to gray, but are locally dusky red in color (Section 4R-CC). Thin sections revealed a composition very similar to that from the clasts described above, including a range of basic and acidic volcanic rocks, together with shallow-water derived bioclasts and fine-grained metamorphic rock (see "Site 1116 Thin Sections").
Siltstone and very fine sandstone is ripple laminated (interval 180-1116A-2R-CC, 4-7 cm), parallel or wavy laminated (e.g., intervals 180-1116A-2R-CC, 7-9 cm; 2R-CC, 13-16 cm; and 4R-1, 10-13 cm) in layers each ~0.5 m thick. Exceptionally, fine-grained silt has an irregular base and appears to be loaded into underlying coarse-grained siltstone. In addition, a synsedimentary injection dike of medium-grained sandstone is present in Section 4R-2.
This facies is burrowed, with sharp upper and lower bed contacts (e.g., interval 180-1116A-2R-CC, 0-4 cm), and commonly contains disseminated organic matter (see Fig. F7). The XRD of fine-grained sediment indicates the presence of quartz and plagioclase as major constituents, and calcite, chlorite, smectite, amphibole, and pyroxene as minor constituents (see Table T3). Calcium carbonate values range from 1.9-4.8 wt% (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen").
In addition, a dewatering structure was observed in which a subvertical conduit is infilled with medium-grained sandstone (Fig. F8).
Two possible origins of the clasts in the upper part of Unit I can be considered: (1) it is possible that the clasts were entirely created by drilling disturbance, compounded by the difficulty of spudding into a relatively lithified seafloor; and (2) the clasts could represent gravitational talus. The site is located on the southern flank of Moresby Seamount in an area where seafloor is locally steeply sloping. It is, therefore, possible that the isolated clasts represent talus that was shed down steep slopes on the side of the seamount. However, in either case, because no exotic lithologies (e.g., igneous or metamorphic) are present, both the clasts and intact intervals are correlated as originally parts of essentially the same succession. Around 30 mbsf (Core 4R) the succession is deformed; fault planes cut cleanly across the sedimentary succession, indicating that the faulting postdates consolidation (see "Domain II").
Taken together, the clasts and more intact intervals are interpreted to represent a succession of sandstone, siltstone, and claystone. The presence of normal grading and other structures, including more small-scale cross lamination, suggests that the clastic sedimentary rocks accumulated from low-concentration turbidity currents, the tail of high-concentration turbidity currents, or bottom currents. Similar structures, including small-scale cross lamination, are better developed in Unit III and are discussed more fully under that heading. Dewatering structures are also discussed in that section (see "Lithostratigraphic Unit III"). The paucity of bioturbation and subdued color suggests that subseafloor conditions were relatively depleted in oxygen (i.e., suboxic). On the other hand, some more strongly burrowed intervals were presumably well oxygenated. Water depths were estimated to be from 500 to 2000 m (see "Benthic Foraminifers").
The petrographic evidence indicates that the main source was relatively unaltered basic volcanic rocks, plus acidic volcanics, coupled with a contribution from metamorphic lithologies and serpentinite. The textural maturity of some clasts, and the presence of shallow-water bioclasts, suggest that some of the material was derived from a shallow-water setting before being redeposited by turbidity currents or debris flows into a deep-water setting undergoing hemipelagic accumulation.
Lithostratigraphic Unit II is distinguished by the occurrence of paraconglomerate associated with sandstone and siltstone. The following lithologies are present:
Conglomerate was recovered in two forms: (1) occasional intact pieces ranging in size from 5 to 15 cm (e.g., Fig. F9). This conglomerate is matrix supported and is composed of subangular to subrounded clasts (mostly >1.5 cm in size) of mainly volcanic rocks. The matrix is poorly sorted clayey silty sandstone; and (2) mainly isolated pebbles and cobbles (e.g., Core 5R). The clasts recovered without matrix tend to be more rounded than those with matrix. From core observations, these clasts include red altered basalt, vesicular aphyric basalt, aphyric basalt with green alteration (chlorite?), amygdaloidal basalt (calcite and quartz), feldspar phyric basalt, andesite?, dolerite, and porphyritic acidic volcanic rock. Vesicles and vugs in one basalt clast are infilled with pyrite.
A small number of the igneous clasts were studied in thin section (see "Site 1116 Thin Sections"). These are generally little altered and mainly comprise several different types of basic extrusive rock as follows: (1) basic volcanic with phenocrysts of plagioclase, microphenocrysts of yellowish green amphibole in a flow-textured groundmass of feldspar laths and glass (Fig. F10); (2) basic volcanic with large phenocrysts of plagioclase and microphenocrysts of augite set in a flow-textured groundmass, including plagioclase laths (Fig. F11); (3) basic volcanic with large brown hornblende phenocrysts, plagioclase, and augite in a flow-textured groundmass; (4) mafic volcanic with altered plagioclase and chlorite; and (5) mafic cumulate, with fresh and altered pyroxene.
Some intact sandstone beds are relatively well sorted, have sharp bases and tops and include sand-filled burrows (e.g., interval 180-1116A-5R-1, 108-130 cm). Rare foraminifers are present. Carbonaceous detritus and red altered grains of volcanic rock are very common. In addition, poorly sorted silty sandstone is found adhering to some of the individual clasts. Siltstone was recovered partly as isolated fragments in which it is interlaminated with fine-grained sandstone. XRD analysis of fine-grained sediment indicated the presence of quartz, plagioclase, chlorite, smectite?, calcite, and pyroxene (Table T3).
In addition, data from two thin sections of sandstones document a very poorly sorted texture (see Figs. F10, F12; also "Site 1116 Thin Sections"). Despite this, many grains are subrounded to rounded, especially the basic volcanic grains (Fig. F13). The sandstone contains abundant fresh basalt (locally glassy), chloritized basalt, variolitic basalt, palagonite, acidic volcanics, and plutonic rocks (dolerite and gabbro). Mineral grains are plagioclase (commonly zoned), biotite, pyroxene, and rare hornblende. Lithic fragments are polycrystalline quartz and schist (including chloritic schist). Pyrite is common. In addition, bioclasts include echinoderm plates and both benthic and planktonic foraminifers. The sandstone has a poorly sorted silty matrix.
The paraconglomerates are interpreted as debris-flow deposits, based on the presence of the poorly sorted silty matrix, matrix support, and the relatively angular shape of some of the clasts. The minimal recovery of pebbles and granules in some intervals (e.g., Section 6R-CC) probably represents remnants of thick beds of paraconglomerate of which only a few pebbles and cobbles were recovered. The paraconglomerate was originally interbedded with sandstone and siltstone. Clasts of laminated siltstone are interpreted as fragments of originally coherent beds created by drilling rather than as primary clasts. The associated silty sandstone and minor siltstone are thus interpreted as original interbeds deposited by turbidity currents, based on the sharp bases of beds and normal grading. They might represent either discrete interbeds of low-density turbidity currents or the upper parts of high-density turbidity currents.
In addition, much of Unit II is represented by isolated well-rounded pebbles and granules of igneous rocks and rare micritic carbonate. For example, in Section 5R-1, small pebbles within sandstone are overlain by isolated well-rounded pebbles; this interval is interpreted as an original >1-m-thick conglomerate bed. The well-rounded pebbles and granules clearly underwent previous reworking, probably related to wave or current action in a shallow-water setting, before being deposited within debris flows into a deep-water setting. Notably, shallow-water-derived benthic foraminifers (inner neritic) are recorded within some clasts in Unit II (Core 5R; see "Benthic Foraminifers").
This sequence of sandstone and conglomerate suggests differing sediment gravity-flow events in which the matrix-supported conglomerates were deposited by debris flows, whereas the sandstones and siltstones were deposited from high- and low-density turbidity currents. Such composite depositional units are characteristic of relatively steep, proximal settings (e.g., see Miall, 1984).
Petrographic study of a small number of clasts suggests that the main source was an arc-type terrane including basalt, dolerite, and andesite. One interesting aspect is the variety of basic volcanic rock types present. Several of these could have alkaline affinities (although titanoaugites are not present). These extrusives could be shoshonitic (e.g., similar to the high-K lavas from Lusancay and Woodlark Islands; see Smith, 1976; Johnson et al., 1978). Study of thin sections indicates the provenance was varied and included acidic volcanic, metamorphic, and ophiolitic rocks in addition to the mainly basic volcanic rocks that formed clasts. Also, the textural maturity, especially of the basaltic grains, and admixing with bioclasts, indicates derivation from a high-energy shallow-water setting.
This unit is marked by a return to a finer grained succession of sandstone and siltstone without conglomerate. The upper part of this unit was recovered only as drilling breccia, whereas the lower part (below Core 13R; 110.6 mbsf) is better preserved and includes evidence of dewatering and sediment instability. Lithologies are calcareous. A small number of calcium carbonate analyses of claystones and siltstones indicate low values of 2.4-3.8 wt% (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen").
The sandstone is fine to medium grained, to locally coarse grained, and calcite cemented. The succession is tectonically tilted at up to 45º (see "Structural Geology"). In the upper part of the unit the sediment is mainly greenish gray, but below 110 mbsf this changes to reddish brown (e.g., Core 17R).
Most beds exhibit sharp, locally scoured bases (e.g., interval 180-1116A-15R-3, 16-84 cm) and are normal graded. Some sandstones are nearly structureless (see Fig. F14). In places, sandstone/mudstone couplets are present, which exhibit normal grading from fine sand to clay. Slight inverse grading was rarely noted at the base of sandstone beds (interval 180-1116A-16R-2, 10-22 cm). Planktonic foraminifers are locally concentrated at the tops of individual beds (e.g., interval 180-1116A-15R-2, 11-11.5 cm). Near the base of the recovered succession a single bed (>2.5 m thick) grades from coarse- to fine-grained sandstone. The sandstones are moderately to locally strongly bioturbated with both Zoophycos (interval 180-1116A-17R-1, 41-53 cm) and Chondrites.
Intraformational rip-up clasts are locally present in coarse-grained sandstone. Occasionally, these are large (up to 5 cm) and irregular in shape. In one case, large mud clasts have disintegrated, giving rise to a network of small cracks into which sand was injected (see "Interpretation"). Elsewhere, small well-rounded granule-sized rip-up clasts are seen locally near the top of a bed (e.g., interval 180-1116A-16R-1, 0-42.5 cm).
The sandstones include unusually well developed sedimentary structures. A number of sandstone beds exhibit well-developed climbing-ripple lamination (e.g., Section 9R-1, and interval 180-1116A-17R-2, 14-24 cm; Fig. F15). Some individual cross-laminated intervals are truncated upward by overlying cross lamination, or common planar lamination (e.g., interval 180-1116A-14R-1, 21-28 cm; Section 16R-2; Fig. F16). Cross lamination in places dips in contrasting directions within a single bed. In addition, wavy or convolute lamination (e.g., see Fig. F15) is locally present (Fig. F17). Individual laminae are commonly composed of carbonaceous detritus (e.g., Section 14R-1). Some intervals show alternations of ripple lamination and wavy lamination without clear, sharp basal contacts.
Sand injection structures are well developed in the lower part of the unit (Figs. F18, F19). These structures are of both high angle (i.e., sandstone dikes) and low angle (i.e., sandstone sills) with respect to primary bedding. Most of the injection structures are composed of subhorizontal, fine- to medium-grained sandstone sills (up to 1 cm thick) within finer grained sandstone or siltstone. However, local injection of coarse-grained sandstone into underlying siltstone was observed in one case (e.g., interval 180-1116-15R-3, 0-8 cm). In most cases the origin of the injected coarser grained sediments is not apparent (i.e., whether local or far removed).
Study of seven thin sections of sandstone from Unit III revealed that the sandstones are feldspathic and rich in rock fragments, corresponding to lithic sandstones or subarkoses (see "Site 1116 Thin Sections"). Typically, grains are angular to subangular and poorly sorted, but a small proportion is subrounded to well rounded, especially the basalt grains. Sandstone contains fresh basalt (locally glassy), chloritized basalt, palagonite, felsic volcanics, rare dolerite, and gabbro. Mineral grains are plagioclase (commonly zoned), biotite, pyroxene, and rare hornblende. Lithic fragments are common polycrystalline quartz, schist (including commonly biotite schist; Fig. F20), serpentinite, and micritic and rare sandstone clasts. In addition, bioclasts include bivalve shell fragments, echinoderm plates, benthic foraminifers, and bryozoans. The sandstone typically has a fine-grained calcareous matrix.
These sediments occur as two types of beds: (1) thin interbeds of siltstones and silty claystones. These beds are sharp based, mainly structureless, and are commonly graded from silty clay to clay. Parallel lamination is observed locally (interval 180-1116A-16R-1, 54-56 cm). The upper parts of individual thin beds are bioturbated, and small foraminifers are common. Several thin claystone beds show both sharp top and bottom contacts. (2) Thicker intervals of silty claystone are present near the base of the drilled succession. For example, one thick (>90 cm thick) silty claystone is graded with laminated siltstone restricted to the basal 5 cm (interval 180-1116A-16R-4, 90-95 cm). In addition, small-scale injection of sand into claystone was noted (interval 180-1116A-16R-3, 53-60 cm; Fig. F21), and small irregular fluid-escape structures are also present (Section 16R-CC).
The XRD analysis of the fine-grained sediment indicates the presence of plagioclase and quartz as major constituents, and chlorite, smectite?, calcite, illite, pyroxene, and amphibole as minor components (Table T3).
Unit III accumulated during the Pliocene at middle bathyal depths (500-2000 m; see "Benthic Foraminifers"). Deposition took place in a suboxic setting characterized by abundant detrital organic matter input. Based on the occurrence of bioturbation, the sediments range from well-oxygenated to poorly oxygenated. The reddish color of the sandstones lower in the succession was possibly influenced by diagenesis.
The thick-bedded sandstones with inverse grading at the base are interpreted as deposits from high-density turbidity currents. The parallel lamination and ripple lamination in the upper part of these beds is suggestive of reworking by the tail of a turbidity current (see Allen, 1982). In addition, some of the sandstones can be interpreted mainly as classical turbidites. Beds exhibiting grading, and climbing-ripple and planar lamination can be inferred to be Ta, Tb, and Tc, Td divisions of the Bouma sequence (Bouma, 1962). Notably, the climbing-ripple lamination indicates high sediment fallout from suspension. However, complete Bouma-type sequences are not developed. The rare thick-bedded, graded claystones with graded siltstone bases are interpreted as deposits from large-volume, low-density turbidity currents. In addition, numerous sand/silt/clay couplets are interpreted as low-density turbidity current deposits (Piper et al., 1978). Units that show alternations of ripple lamination and wavy lamination are interpreted to suggest deposition from pulsating currents and perhaps bottom currents (Pickering et al., 1989).
The sand-injection structures are seen as the result of injection of coarse-grained sand into finer-grained sediment at varying angles. Such sand-injection structures may result from a high sedimentation rate that in turn causes entrapment of fluid. Elevated pore-water pressure then results, leading to sudden liquefaction and injection of fluidized sand into adjacent more coherent muddy sediments (Lowe, 1975). Such fluidization may simply relate to high sedimentation rate, but may also be tectonically triggered.
The petrographic evidence indicates essentially the same provenance as indicated by samples from Units I and II, with a mixture of clasts and mineral grains from basic and acidic volcanic rocks, plutonic rocks, and some metamorphic rocks. Input of subordinate shallow-water-derived carbonate persists to the bottom the hole.
Site 1116 drilling was aimed to penetrate the deeper parts of a synrift sedimentary succession, based on seismic stratigraphic interpretation. The whole of the drilled interval accumulated at middle bathyal depths (500-2000 m) depths during Pliocene time. An early? to middle Pliocene age is inferred down through Core 17R, below which the age is undetermined. The average sedimentation rate was >70 m/m.y. for the entire succession. Several factors indicate that Quaternary sediments are absent from Site 1116. (1) Site 1116 sediments were dated as middle Pliocene or older (see "Biostratigraphy"); (2) all the material is well lithified, suggesting that superficial sediments were removed; and (3) physical properties data show relatively high near-surface velocities and low porosities. These data are consistent with the removal of a Pleistocene to upper Pliocene section (see "Physical Properties"). A possible explanation is that younger sediments were deposited, but then removed, perhaps related to tectonic uplift of the Moresby Seamount.
The recovered section is interpreted as a single deep-water succession dominated by gravity flow and interbedded fine-grained sediments. The gravity deposits range from debris-flow deposits and possible high-density turbidites in the case of the paraconglomerates (Unit II), to classical turbidites with partial Bouma divisions, to mud turbidites (Units I and III). In addition, minor redeposition by the tail of high-concentration turbidity currents or by bottom currents may have occurred. The well-rounded clasts in the conglomerate, including sporadic shallow-water benthic foraminifers, were redeposited from an inferred high-energy beach or fluvial setting and were finally redeposited within debris flows and possible high-density gravity flows (Unit II). Bottom-water conditions were at least relatively oxidizing in view of the common bioturbation. However, subsurface anoxia was probably developed as indicated by the presence of disseminated organic matter. Slightly elevated levels of sulfur (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen") correlate with the common occurrence of pyrite. The presence of dewatering structures and sand-injection structures (small dikes and sills) indicate the development of elevated pore pressure. Dewatering and fluidization was possibly seismically triggered, and indeed, there is much evidence of faulting at this site (see "Structural Geology").
The petrographic evidence indicates the clasts were derived from a dominantly calc-alkaline arc setting. The voluminous basaltic and acidic volcanic rocks are assumed to have been derived from the Miocene Trobriand Arc (including Amphelts Island and Egum Atoll). The former axis of this arc is believed to be located close to the present day location of Moresby Seamount and thus, a relatively proximal origin is possible. The basic volcanics may include unusual relatively alkaline basalts related to arc rifting. The serpentinite, rare chromite, and some at least of the dolerite and gabbro grains, may have an ophiolitic origin, related to derivation from deeper levels of the Papuan Ultramafic Belt, which is believed to have been emplaced over a region including the D'Entrecasteaux Islands and Moresby Seamount in Paleocene to Eocene time (prior to inception of the Trobriand Arc; Davies and Jaques, 1984). In addition, the common schist can possibly be correlated with the Owen Stanley or Kaga Metamorphics, interpreted as an early Tertiary accretionary complex that regionally underlies the Papuan Ophiolitic Belt (Rogerson et al., 1987). Thus, material was apparently derived from all parts of the regional tectonostratigraphy.
In conclusion, the overall sedimentary regime is consistent with heterogeneous proximal sources feeding a subsiding rift basin in the Pliocene.