LITHOSTRATIGRAPHY AND SEDIMENTOLOGY

Site 1194 was drilled through a sequence of hemipelagic drift deposits into underlying slope-sediments of the Northern Marion Platform (NMP) as indicated on seismic profiles (see "Seismic Stratigraphy"). The ~420-m-thick sediment sequence was divided into six units according to variations in sedimentological structures and texture, grain size, inferred organic matter content, terrigenous material content, and biotic assemblages (Fig. F3; Table T3).

Unit I is characterized by 3.8 m of yellow skeletal grainstone, with a sharp boundary to a 114-m-thick hemipelagic mudstone/wackestone interval (Unit II). Unit III is capped by a ferromanganese hardground and consists of a bryozoan/foraminifer dominated packstone/floatstone interval. Unit III was initiated on the upper slope, developing into a middle to inner neritic ramp at the top of the unit. Unit IV is characterized by very fine grained packstone to grainstone with skeletal components, suggesting abrasion during transport. Unit V consists of weakly layered grainstone, with bioturbation often crosscutting the layering. Unit VI, representing acoustic basement, consists of a reddish olivine basalt.

Most of the sediments recovered at Site 1194 are poorly to moderately lithified. Core recovery at this site is good above 117 mbsf and poor below this depth. The drop in recovery corresponds to seismic Megasequence Boundary B/D (see "Seismic Stratigraphy"), which was inferred to be an exposure surface/hardground that correlates with the boundary between lithologic Units II and III.

Lithologic Units

Unit I (0-3.8 mbsf; Pleistocene)

Unit I consists of 3.8 m of pale yellow, well-sorted skeletal grainstone (Fig. F3). Skeletal components are dominated by planktonic foraminifers but include benthic foraminifers, pteropods, echinoids, and gastropods (Fig. F4). The base of this unit is erosional as indicated by the presence of a scoured surface.

Unit II (3.8-117.4 mbsf; Pliocene to Late Miocene)

Unit II consists of 114 m of light olive to greenish gray mudstone/wackestone interbedded with minor packstone intervals (Figs. F3, F5). Skeletal components are dominated by planktonic foraminifers. Benthic foraminifers are common, whereas shell and echinoids fragments are rare. Overall, skeletal components are coarse sand sized, and the matrix is clay to silt sized. Dolomite rhombs are rare. Well-preserved wood fragments are observed at the bottom and the top of Unit II. Black stains ~1 cm in diameter occur throughout this unit. Smear slide analysis revealed that these are nonoxidized framboidal pyrite infilling structures similar to burrows. Bioturbation is extensive in this unit, with common Chondrites and rare Thalassinoides, Taenidium, and Zoophycus.

Scoured surfaces overlain by coarse sand-sized calcareous sediment, typically 1-5 cm thick, are observed at five different levels within this unit (see Fig. F3, column C, 35-105 mbsf, and Fig. F6).

Unit III (117.4-177.3 mbsf, Middle Miocene)

Unit III is 60 m thick and includes two subunits identified by their biotic assemblage (Fig. F3). Both subunits consist of skeletal packstone, comprising coarse sand-sized skeletal components in a dark gray matrix indicating the presence of terrigenous clay. A 1-m-thick hardground (see below) marks the boundary between Unit II and Unit III. This interval is considered to be a depositional hiatus developed through the alteration of Unit III sediments and as such was included in Subunit IIIA.

Subunit IIIA (117.4-158.0 mbsf)

The top of Subunit IIIA consists of a 1-m-thick interval, which has a sharp, scoured surface coated with a 1-cm-thick reddish brown crust (Fig. F7). A 10-cm-deep and 2-cm-wide fissure filled with fine-grained pale yellow carbonate sediment occurs in this surface. The dark reddish brown crust is composed of wavy phosphatic and ferromanganese laminae, as can be seen in thin section (Fig. F8). The underlying 1-m-thick interval is extensively dolomitized (Fig. F3, column C, and Fig. F9), with up to 40 wt% dolomite (see "Geochemistry"). Dolomite appears to be mostly confined to skeletal particles, and no dolomitized cement was observed. This dolomitized interval consists of a pale yellow skeletal grainstone with quartz, bryozoans, small benthic foraminifer fragments, and molds of gastropods. Other skeletal components are not identifiable in hand samples, possibly because of recrystallization and/or fragmentation. Thin section analysis reveals that the hardground is composed of well-preserved organisms mixed with angular quartz. Cements consist mainly of syntaxial overgrowths on high-Mg calcite particles that later recrystallize to single-crystal low-Mg calcite (Fig. F10). Calcite cement, typical of the vadose zone, was not found. Glauconitized bioclasts are common in this interval, which has a distinct uranium peak in the downhole natural gamma ray log (see "Downhole Measurements") that is typical of hardgrounds. A second interval, which was not recovered in the cores, has a similar natural gamma ray signature in the downhole logs at ~160 mbsf (Subunit IIIB). This second interval was included in the lithologic column of this site (Fig. F3, column D).

From 118.4 to 158 mbsf, Subunit IIIA consists of 30 m of dark gray packstone overlain by a 10-m-thick interval of skeletal floatstone (Figs. F11, F12). Skeletal components within Subunit IIIA are dominated by well-preserved bryozoans. Bivalve fragments are common, and benthic foraminifers are present.

Subunit IIIB (158.0-177.3 mbsf)

The main difference between Subunits IIIA and IIIB is the near-total absence of bryozoans and the predominance of benthic foraminifers in the latter interval. This subunit is composed of skeletal packstone with clay and coarse sand-sized skeletal components dominated by small benthic foraminifers. Although most skeletal fragments are well preserved, minor amounts of silt-sized, unidentifiable skeletal components first occur downcore in this subunit (Fig. F13).

Unit IV (177.3-331.1 mbsf; Early-Middle Miocene)

This unit consists of 153.8 m of silt-sized packstone to grainstone (Fig. F3). It is capped by a 1-cm-thick layer of grainstone with a sharp contact to Subunit IIIB. Skeletal components are not identifiable in hand samples because of extensive fragmentation and/or recrystallization, which is the characteristic feature of this unit. Unit IV was divided into two subunits based on mineralogy and clay content.

Subunit IVA (177.3-264.5 mbsf; Late Early-Middle Miocene)

This subunit consists of light olive-gray silt-sized packstone (Figs. F14, F15). The bottom of the interval has a packstone/grainstone texture. No skeletal components are identifiable except for an interval at ~230 mbsf, which is rich in bryozoans. Dolomitic rhombs are abundant in smear slides, suggesting partial dolomitization or transport of dolomitized material. This interval is rich in clay (up to 50 wt% noncarbonate components; see "Geochemistry;" Fig. F3, column C) and is well sorted.

Subunit IVB (264.5-331.1 mbsf; Early Miocene)

This subunit is distinctive because of the common occurrence of angular quartz sand (Figs. F16, F17). It consists of well-cemented, silt-sized grainstone that is partially dolomitized (see Fig. F3, column C). The interval is moderately well sorted and shows neither bioturbation nor layering. Glauconite is common and is often observed to be infilling burrows. Clay is absent.

Unit V (331.1-421.1 mbsf; Early Miocene)

This 90-m-thick packstone unit (Fig. F3) features centimeter-scale dark layers. Clay was not observed in Unit V. Bioturbation is common but not extensive as shown by the partial preservation of layering. The burrows and layering distinguish Unit V from previous units. Based on grain size, quartz content, and color variation, this unit was divided into two subunits.

Subunit VA (331.1-372.1 mbsf)

The distinctive feature of Subunit VA is the dominance of silt-sized components (Figs. F18, F19). The sediments are primarily greenish gray packstone with skeletal components that are not identifiable in a hand sample because of extensive fragmentation/recrystallization. One wood fragment was found.

Subunit VB (372.1-421.1 mbsf)

This subunit is composed of well-sorted, light greenish gray to pale yellow skeletal packstone (Figs. F20, F21). The major difference between Subunits VA and VB is that, in the latter, skeletal components are coarse sand-sized. Planktonic foraminifers are the dominant skeletal particles for the upper part of the subunit with larger benthic foraminifers increasing in importance downsection and becoming dominant in the 2-m interval immediately overlying the basement. In this 2-m interval, a robust, thick-walled shallow-water form of the larger foraminifer Lepidocyclina dominates (Fig. F22). This foraminifer occurs in discrete, parallel-laminated fining-upward sequences. About half of the Lepidocyclina are filled with glauconite, indicating potential reworking of the species within Subunit VB. Quartz is rare to absent.

Unit VI (421.1-? mbsf)

This unit consists of 3 m of olivine basalt basement (Figs. F3, F23, F24). The lithology is a reddish black, amygdaloidal extrusive rock with olivine, feldspar, and epidote. Several generations of cracks filled with calcite are visible.

Discussion

Unit I

The predominance of coarse-grained skeletal particles, the absence of mud, the high degree of sorting, and the erosive boundary to Unit II suggest that Unit I is winnowed. This unit was likely to have been deposited on a current-swept seafloor. It is laterally equivalent to Unit I at Sites 1192 and 1193 (see "Lithostratigraphy and Sedimentology" in the "Site 1192" chapter and "Lithostratigraphy and Sedimentology" in the "Site 1193" chapter).

Unit II

The scoured surfaces overlain by a centimeter-scale normal graded bed of coarse carbonate sand present within Unit II are interpreted to be gravity flow deposits. This unit was likely deposited in a distal, hemipelagic setting (see "Biostratigraphy and Paleoenvironments") with frequent input of coarser sediment from gravity flows. It is laterally equivalent to Unit II at Site 1193 (see "Lithostratigraphy and Sedimentology" in the "Site 1193" chapter).

Unit III

The hardground surface separating Units II and III was likely formed in a submarine setting. Evidence to support this interpretation includes boring by organisms, phosphatization, and a positive peak in the uranium curve of gamma ray analysis (see "Downhole Measurements"). However, the fact that the hardground is submarine does not, in itself, rule out the possibility of subaerial exposure prior to drowning. The question of subaerial exposure of this hardground requires further investigation (e.g., using stable isotopes). Biostratigraphic data indicate that this surface represents a maximum time gap of 4 m.y. Subunit IIIA represents shallow-water deposition possibly on a neritic ramp (~30-50 m water depth) (see "Biostratigraphy and Paleoenvironments") with bryozoans being the dominant organisms. Subunit IIIB was deposited in a deeper setting (~120-200 m) (see "Biostratigraphy and Paleoenvironments"), such as on a neritic upper slope. Skeletal components are dominated by small benthic foraminifers. The sharp surface at the base of Subunit IIIB is interpreted to be the top of a firmground as supported by the fact that its gamma ray signature resembles that of the previously discussed hardground (see "Downhole Measurements"). In both subunits the clay fraction is relatively important and the skeletal fragments are still preserved, indicating a low- to medium-energy environment.

Unit IV

Physically abraded silt-sized particles are the dominant bioclasts in Unit IV. This is indicative of long-distance transport in a high-energy environment or possibly of bioerosion. The source for this fine carbonate material could be the NMP or a more distant, time-equivalent carbonate platform. Unit IV was deposited in a hemipelagic setting at water depths ranging from 120 to 200 m (see "Biostratigraphy and Paleoenvironments"). This range in water depth, together with the seismic geometry of seismic sequences that show gently inclined reflections, imply a slope depositional environment for Unit IV. Overall, a shallowing-upward trend is seen between Units IV and III; water depths of Subunit IVB are weakly constrained, whereas those water depths of Subunits IVA and IIIB ranged between 120 and 200 m. Subunit IIIA was deposited in 30-50 m water depth.

Unit V

Unit V may represent the initial sedimentation on the underlying basement surface and thus maybe a transgressive system tract. This interpretation is supported by the ratio of planktonic to benthic foraminifers showing a gradation from larger benthic, shallow-dwelling species to small benthic species and eventually to a planktonic foraminifer-dominated environment (see "Biostratigraphy and Paleoenvironments"). This transition takes place over a 20-m interval. The general absence of terrigenous components in this unit is striking, especially when compared with Unit V at Site 1193, where the first lithology over basement is dominated by siliciclastics (see "Lithostratigraphy and Sedimentology" in the "Site 1193" chapter). This can be explained by the fact that Site 1194 was located on a basement high that became an island just before the transgression and, thus, during subsequent flooding, had a small siliciclastic source area (see "Seismic Stratigraphy" and Fig. F3 in the "Leg 194 Summary" chapter). A fining-upward trend is seen between Subunits VB and VA, indicative of a deepening-upward sequence. Conditions at the water/sediment interface might have been slightly dysaerobic, an assumption that could explain the darker layers that have been partially burrowed by organisms. The base of the unit is rich in benthic foraminifers, and planktonic foraminifers become increasingly important. Thus, Unit V is interpreted as a shallow marine transgression over a topographic high, eventually developing into an outer neritic setting.

Unit VI

Thin section analysis of the basement revealed that it consists of an olivine basalt. The well-crystallized porphyries of olivine suggest that this rock was intruded close to the surface, but was not a basaltic flow. It is unclear whether this unit represents Paleozoic extrusive basaltic rocks subsequently metamorphosed during the Tasman orogeny or extrusive rocks linked to early rifting of the Coral Sea Basin (Late Jurassic-Early Cretaceous) (Struckmeyer and Symonds, 1997). However, the general absence of metamorphic minerals and the overall good preservation of the rock seems to favor a synrift or slightly prerift emplacement.

The following geological events were recorded by the succession drilled at Site 1194:

  1. During the Early Cretaceous, rifting led to the opening of the Coral Sea. During this phase and/or later, the volcanic basement was subaerially exposed and eroded (Unit VI) and a high was created at the location of Site 1194 (see "Seismic Stratigraphy") that, before final flooding, became an elevated island separated from the mainland to the west.
  2. During the early Miocene, sea level flooded this elevated area (Subunit VB). However, flooding might have occurred earlier if the resulting sediments were subsequently eroded. As water depth increased, the sediment source became more distant, and thus finer-grained sediments were deposited at Site 1194 (Subunit VA). Seafloor conditions may have been somewhat dysaerobic as shown by the presence of laminations. Additionally, abraded skeletal carbonate particles observed in the recovered sediments may have come from the nearby NMP.
  3. During the early to early middle Miocene, sediments were abraded and fragmented to silt size during transport. Rare quartz grains indicate a minor terrigenous influence within this interval. Paleoenvironment data obtained from benthic foraminifers (see "Biostratigraphy and Paleoenvironments") indicate a deepening upward toward more clay-rich intervals that is followed by a shallowing-upward succession.
  4. In the late middle Miocene, sea level fell and a middle to inner neritic ramp environment developed, where a bryozoan-dominated facies was deposited. Paleowater depth in Subunit IIIA ranges from 30 to 50 m, placing the seafloor in the euphotic zone (Subunit IIIA).
  5. A late Miocene (~8-11.5 Ma) sea level lowstand was probably not low enough to expose Unit III to subaerial conditions, but shutdown of the adjacent carbonate factory led to a depositional hiatus. Seismic sequence geometry indicates that flooding of Unit II did not result in immediate deposition but that a time lag occurred between flooding and onset of sedimentation, presumably as a result of current activity (see "Seismic Stratigraphy"). A phosphatic hardground was formed.
  6. During the late late Miocene to Pliocene, hemipelagic, fine-grained sediments were mixed with coarse sand-sized distal mass flow deposits to form Unit II.
  7. In the late Pliocene to Pleistocene, strong bottom-water currents swept the seafloor, creating winnowed deposits of skeletal grainstones (Unit I). This process is still active today.

NEXT