| IODP Proceedings Volume contents Search | |||
|
|||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.313.104.2010 LithostratigraphyFigure F1 provides a key to colors and symbols used for figures in this section. Hole M0028A was drilled in the shallow shelf to sample the thick early Miocene succession, as well as the late and middle Miocene. Seven lithostratigraphic units are defined (Table T2). Figure F2 illustrates the position of the lithostratigraphic units on the seismic section. The oldest stratigraphic unit in Hole M0028A (Unit VII; 668.66–662.98 meters below seafloor [mbsf]; ?Aquitanian) comprises dark brown siltstone with thin-walled articulated shells deposited in a deep offshore environment. Unit VI (622.98–611.19 mbsf; Aquitanian to early Burdigalian) is a pale brown clayey silt with intercalated very fine and fine sand beds deposited in a river-dominated offshore (prodelta) environment. The contact with the overlying Unit V (611.19–525.52 mbsf; middle early Miocene [early–middle Burdigalian]) is abrupt and bioturbated. Unit V is divided into three coarse-grained subunits that are all interpreted as deposits of sediment gravity flows supplied from remobilization of updip sediment. Subunits VB and VC have characteristics of debrites, whereas Subunit VA shows more evidence for turbidites. The poorly sorted coarse sediments of Subunit VA are interpreted as deposits from high-concentration flows of coarse sediment deposited at the toe of a degraded clinotherm. Unit IV (525.52–512.29 mbsf; middle early Miocene [middle Burdigalian]) consists of sediment gravity flows, possibly from river flood events, deposited in an offshore environment. Unit III (512.29–335.37 mbsf; middle–late Burdigalian) is divided into four subunits that, overall, mark the shallowing uphole and outbuilding of a storm-dominated and river-influenced delta. Unit II (335.37–223.33 mbsf; middle Miocene) is also divided into four subunits. Subunit IID is poorly sorted and dominantly coarse grained, with a mix of mud, coarse sand, and gravel that is low in mica and organics and deficient in silt to medium sand grain sizes and which may have been deposited at the clinoform rollover. Subunits IIC–IIA represent a series of transgressive and shoreface to offshore and regressive offshore to shoreface sedimentary cycles. Unit I was not cored at Site M0028A. Figure F3 summarizes Units I–VII, and their constituent subunits are described in more detail below. Lithologic descriptions are given according to the order of core numbering, from the top to the base of each lithostratigraphic unit or subunit, whereas interpretations are given in chronologic order, from the base of each unit or subunit uphole. Table T3 describes thin sections that aided in core descriptions. Unit INo core was recovered in this hole, but this unit is correlated with Unit I in Holes M0027A and M0029A based on similar well log character and seismic facies. Unit II
Subunit IIA
Sections 313-M0028A-2R-1 through 8R-2 (223.33–242.13 mbsf) comprise gray-yellow-brown color-banded clay, with Chondrites and pyritic and silty laminae in the upper 15 m (e.g., Section 3R-3, 18 cm; 229.38 mbsf). Small-scale (centimeters) folds and faults indicate deformation of cohesive sediment in the lower 4 m of the clay. An abrupt boundary is observed between clay and underlying bioturbated fine sand containing moderately sorted medium sand with glauconite and thick-walled shells (interval 313-M0028A-8R-2, 100–103 cm; 242.12–242.15 mbsf). Bored cemented sand nodules with glauconitized rims (intervals 313-M0028A-9R-1, 48–53 cm [243.16-243.21 mbsf] and 9R-1, 97–110 cm [243.65–243.78 mbsf]) (Fig. F4) are found in poorly sorted fine and medium sand with glauconite, mica (both 2%), and benthic foraminifers. A further abrupt burrowed boundary with poorly sorted very coarse sand above silty clay is observed in Section 313-M0028A-10R-1 at 7 cm (245.80 mbsf). The coarse sand contains granules that also infill Thalassinoides down to 25 cm below the contact. The underlying brown silty clay is intercalated with rare beds of very fine sand with scour-based, ripple cross-laminae. Plant debris and scattered shells are common, with burrows filled with coarse sand and gravel near the base of Subunit IIA (Section 313-M0028A-12R-1, 40 cm; 252.23 mbsf). InterpretationOffshore silts with rare storm-event beds are cut by an erosion surface at 245.80 mbsf near the base of Subunit IIA. This is overlain by coarse to fine sand with bored nodules and glauconite grains, indicative of condensed deposition and subsequent erosion and exhumation. The environment of sand deposition is possibly shoreface, based on siliciclastic grain size. Reworking of the top of the sand may have taken place in shoreface–offshore transition zone and offshore settings. An abrupt deepening and change to dysoxic offshore environment is marked by clay deposition. Color banding in clays is from discrete depositional events that are interpreted as low-density turbidity currents triggered by storm events. Deformation of cohesive sediment indicates a component of downslope transport, and a clastic sill near the base of the clays may relate to overpressure in underlying sands. Subunit IIB
The top of Subunit IIB is marked by an abrupt coarsening downhole across a surface separating clayey silt above from poorly sorted coarse sand with granules below. There is a core gap (Core 313-M0028A-13R) above well to moderately sorted quartz-rich coarse sand (Section 14R-1; 254.88 mbsf) that fines downhole to medium sand (Section 16R-1; 260.98 mbsf) and then, abruptly, to micaceous silty fine sand (Section 19R-1; 268.82 mbsf). Shell fragments and plant debris are common throughout, and glauconite grains are rare. In Section 313-M0028A-20R-1, 49 cm (272.36 mbsf), there is an abrupt contact surface between silty sand and silty sandstone. From this level, there is a coarsening downhole to weakly graded and bioturbated gravel-rich sandstone containing complete benthic foraminifers (Section 313-M0028A-21R-1; 274.52 mbsf). In Section 313-M0028A-21R-2, 56 cm (276.58 mbsf), there is a change to glauconitic (30%) sandstone, but the contact is not preserved in the core because of drilling disturbance. The glauconitic muddy sand and muddy sandstone is thoroughly mixed, containing mud-filled burrows, scattered pyrite grains, and rare subangular quartz granules (Fig. F5). There are no shells, but molds are present. Cores 313-M0028A-23R and 24R (281.02–285.44 mbsf) are glauconite coarse sand (up to 40%) (Fig. F6) with clay laminae and beds (e.g., interval 313-M0028A-24R-1, 67–78 cm; 284.74–284.85 mbsf) along with indurated and cemented units. InterpretationThe muddy glauconitic sand and glauconite sandstone at the base of Subunit IIB are interpreted as offshore deposits with glauconite grains forming in situ and a background sedimentation of clay. The presence of isolated quartz granules and pebbles remains enigmatic but implies a component of downslope transport. The clay in the middle of the glauconite sandstone (interval 313-M0028A-24R-1, 67–78 cm; 284.74–284.85 mbsf) is firm but not indurated and is bioturbated on top. It is interpreted as the deepest water facies of this glauconite-rich package. The contact with the overlying graded quartzose gravelly sandstone at ~273 mbsf is not preserved in core. However, these poorly sorted sediments containing well-preserved benthic foraminifers are interpreted as high concentration sediment gravity flow deposits (debrites) close to or within a channelized environment in a clinoform rollover position. The overlying succession fines abruptly (Section 313-M0028A-19R-1; 268.82 mbsf) to a silty sand, which is interpreted as a shoreface–offshore transition environment. Above this, grain size coarsens uphole, which is interpreted to indicate a shallowing in the environment of deposition through a shoreface–offshore transition zone to a storm-dominated shoreface. Subunit IIC
The top of the section is medium and fine sand with plant debris, concentrically filled burrows (?Cylindrichnus), and bivalve and gastropod (Turritella) shells and shell fragments, which are locally concentrated into layers. The succession gradually fines downhole to silty clay containing thin-walled shells and benthic foraminifers, as well as dispersed plant debris in Section 313-M0028A-34R-1 (308.47 mbsf). InterpretationThe clay that overlies an abrupt surface (Section 313-M0028A-34R-2, 55 cm; 310.53 mbsf) is interpreted to indicate deepening, with coarse sediment starvation during transgression. This is itself overlain by a fining-upward package interpreted as a shoreface–offshore transition to offshore environment, with shell beds and graded sand beds indicating storm activity. The overlying succession gradually coarsens uphole from clay and silt to medium sand and is interpreted as a shallowing-upward trend from an offshore setting to a river-influenced shoreface–offshore transition zone. Subunit IID
During coring, the drill string needed to be pulled out just before the base of Subunit IID. When reinserting the drill string, the bit deviated from the original hole, and core recovery began just above Subunit IID. As a result, most of Subunit IID was cored twice. Through sedimentary logging of both sections, it has been established that Sections 313-M0028A-38R-2, 104 cm, to 42R-1, 10 cm, are repeated by Sections 45R-2, 62 cm, to 48R-3, 40 cm, with the rest of Core 42R and all of Cores 43R and 44R containing drilling slurry and cavings. Here, a synthesis description and interpretation is provided. Below Section 313-M0028A-34R-2, 55 cm (310.53 mbsf), sediment coarsens downhole over 40 cm to a rugose surface in Section 34R-2, 95 cm (310.92 mbsf). An underlying ~2.5 m thick package of very fine sand with common benthic foraminifers and minor glauconite grains (3%) coarsens down to a poorly sorted medium sand with granules resting abruptly on silty clay in Section 313-M0028A-35R-2, 48 cm (313.50 mbsf). The succession grades down to clay with scattered benthic foraminifer and gastropod shells, which abruptly overlies a bioturbated contact (Section 313-M0028A-37R-2, 142 cm; 320.52 mbsf). The underlying very fine sand with silt contains concentrically filled burrows (?Cylindrichnus). Sediments encountered in Sections 313-M0028A-38R-2, 104 cm (overlap with Section 45R-2, 62 cm), to 49R-2, 58 cm (335.37 mbsf), are generally poorly sorted, muddy, glauconitic (up to 40%) coarse sand with common quartz and lithic granules and pebbles. The shapes of pebbles and granules range from well rounded to angular but are commonly subangular. Common mud- and glauconite-filled burrows indicate high levels of bioturbation. Beds with weak normal grading (e.g., Section 313-M0028A-40R-2, overlap with Section 47R-2) and 1–4 cm thick gravel layers (the same one is encountered in Section 40R-1, 64 cm, and in the overlap in Section 47R-1, 78 cm; 327.97 mbsf) preserve the original stratification. Section 313-M0028A-41R-1 (overlap with Section 48R-1) has more bioturbated muddy units, along with rounded mud clasts that contain a bioturbated fabric (e.g., 41R-1, 69 cm; 330.51 mbsf) (Fig. F7). Shell molds are observed (Section 313-M0028A-47R-1; 327.19–328.71 mbsf). The number of matrix-supported granules and pebbles increases from Section 313-M0028A-48R-1, 110 cm, at 331.34 mbsf (overlap with Section 41R-1, 100 cm), to 2%–3% to 5%–10% and locally >20% of the total sediment volume in Section 48R-2, 68 cm, at 332.42 mbsf (overlap with Section 41R-2, 70 cm). Beds of subangular to subrounded gravelly sediment show weak normal grading. Granules and pebbles are dominantly quartz but include quartzite and metasedimentary clasts. The base of the succession is poorly sorted, with a mix of mud, coarse sand, and gravel. It is quartz rich (glauconite = ~5%–10%), with pyritized and mud-filled burrows (Section 313-M0028A-49R-1, 58 cm; 333.87 mbsf). InterpretationSubunit IID is poorly sorted and dominantly coarse grained, with a mix of mud, coarse sand, and gravel. Generally, sediment is low in mica and organic matter and deficient in silt to medium sand grain sizes compared to the surrounding stratigraphy. Overall, the succession becomes more glauconitic (up to 40%) and increasingly stratified uphole, with highly bioturbated mud-rich units with gravel layers or weakly graded beds (0.1–0.2 m thick). Rounded clay clasts that have a bioturbated fabric are interpreted as intraclasts entrained from the substrate. The poorly sorted sand with mud and gravel, with shell molds and rip-up clasts, indicates erosion and reworking in a marine setting. The weak grading of some deposits supports waning sediment gravity flows. Possible interpretations of these deposits at the clinoform rollover include gully-fills and/or outbuilding of small deltas with steep fronts that formed during periods of lowered relative sea level. The lack of plant debris and mica and the low proportion of very fine to medium sand compared to the surrounding stratigraphy suggest that these components may have been transported downdip, with the coarser material and muds from eroded surrounding substrate preferentially retained. The extent of the drainage basin that fed the deltas/gullies is not clear. However, the source for the coarse extrabasinal material need not be tied directly to the hinterland and may have been from older deposits. The environment of deposition above Section 313-M0028A-38R-2, 104 cm (323.21 mbsf), is interpreted to deepen abruptly to an offshore setting, which was organic poor (distal to terrigenous source) before being overlain by very fine sand interpreted as shoreface–offshore transition deposits. Unit III
Subunit IIIA
This subunit is poorly recovered, with most cores retaining small amounts of heavily disturbed material that is water saturated. This succession is distinguished because it is a coarse quartz sand to sandstone with glauconite. Cores 313-M0028A-49R through 54R (335.37–352.97 mbsf) contain poorly sorted, bioturbated silty sandstone with 15%–20% glauconite. Shell fragments are common. In Section 313-M0028A-52R-2, 60 cm (350.59 mbsf), there is an apparent scoured surface. Cores 313-M0028A-54R through 57R (335.37–362.55 m) comprise moderately sorted coarse quartz sand with ~2% glauconite. At the base, Cores 313-M0028A-58R through 67R (365.3–391.38 mbsf) are mostly medium sand with 1%–2% glauconite. The cores are all highly disturbed, and primary sedimentary fabrics cannot be recognized. Core 313-M0028A-67R (389.46–391.38 mbsf) is more cemented, and a strongly bioturbated fabric is evident. InterpretationPoor recovery prevents detailed environmental analysis. Grain size and sorting in the recovered sediment indicate either downslope reworking of a clinoform topset or a shoreface and shoreface–offshore transition setting. Seismic reflection geometries support the latter interpretation. Subunit IIIB
Subunit IIIB is characterized as being quartz sand containing little or no glauconite. The unit was poorly recovered. The top of the section (Cores 313-M0028A-68R through 73R; 391.38–400.30 mbsf) consists of loose poorly sorted medium-coarse and fine sand with scattered granules. Shell hash in well-sorted medium sand is found from Core 313-M0028A-73R (400.07 mbsf) downhole. The shells show evidence of strong in situ dissolution, and the resulting material mimics carbonate crust. Section 313-M0028A-74R-CC (402.48–402.61 mbsf) contains one oyster shell, which is pristine. Cores 313-M0028A-76R through 78R (398.11–408.24 mbsf) are likely disturbed, as the hole was reentered and some overlapping sections exist. The cores preserve a crude fining-downward succession (supported by acoustic image logs). InterpretationThe base of the succession (Section 313-M0028A-79R-2, 70 cm, through Core 73R; 414.67–400.07 mbsf) is a shoreface–offshore transition that coarsens uphole. The upper part of the succession (Core 313-M0028A-73R through Section 67R-2; 400.07–391.38 mbsf) is interpreted as reflecting a change in the environment from the lower to upper part of a wave-dominated shoreface. Subunit IIIC
Beginning in Core 313-M0028A-79R (412.47 mbsf), more competent and finer grained cores continue the fining-downward trend. There is also a marked increase in the amount of mica and plant debris present. Moderate bioturbation is present throughout, including simple vertical burrows. Cores 313-M0028A-80R through 88R (416.32–445.80 mbsf) record fining downhole with predominantly fine to very fine sand at the top and silt common at the base (Fig. F8). Most of the cores are dominated by subparallel and subhorizontal laminations and thin beds. Core 313-M0028-80R (416.32–419.38 mbsf) picks up hints of lamination and sand beds that generally increase in frequency and thickness downhole. There are many distinctive two-part sand beds showing slight grain-size change at a lamina scale (Fig. F9). These beds have sharp bases overlain by clean quartz sand (no silt or mica); above is silty sand with subparallel low-angle lamination (Fig. F9). Some of these beds have current ripple lamination and possible symmetrical/aggradational ripples at the top. Shells and plant debris are concentrated near the tops of beds (Fig. F9). Bed tops are typically bioturbated to a range of intensity. Cores 313-M0028A-89R through 98R (445.82–476.35 mbsf) are clay-rich silt with sand. The succession of sediments is largely monotonous from top to bottom. The lower cores have somewhat more clay-rich microlaminated beds that show a greater contrast with the intercalated fine sand beds. Burrowing is intense in sand beds but absent in the clay. Teichichnus is a recognizable burrow. InterpretationSubunit IIIC is interpreted to record an initial uphole fining and deepening followed by coarsening and shallowing from a river-influenced offshore to a river-influenced shoreface–offshore transition back to a river-influenced offshore environment with the change in trend around 445 mbsf. Cores 313-M0028A-98R through 89R (476.35–445.82 mbsf) were deposited in an offshore marine environment. The scarcity of shells and abundance of lamination suggest bottom waters were poorly oxygenated, except during deposition of sand (Fig. F10). Sand-sized sediment is interpreted to have been delivered to the seafloor by storm-induced density or geostrophic currents in a river-influenced offshore setting. The abundance of mica and plant debris in Cores 313-M0028A-88R through 80R (445.80–416.32 mbsf) suggests sediment supply had a strong fluvial influence. The two-part sand beds are interpreted as combined-flow storm deposits. The lower clean and slightly coarser quartz sand is interpreted as having been deposited from geostrophic (unidirectional) currents that transported sediment entrained from the coeval shoreface to deeper water. The upper sediment was locally agitated, and deposits from (orbital) currents set up by storm waves formed low-amplitude hummocks, current ripples, and late-stage symmetrical (wave) ripples before organisms could start to mix the sediment again. The succession is interpreted as having been deposited in the shoreface–offshore transition zone in a river-influenced setting. Subunit IIID
The top of the section (Cores 313-M0028A-99R through 100R; 476.32–482.47 mbsf) documents a minor fining-downward succession from fine sand to sandy silt. Silty very fine sand and very fine sandy silt continue downhole (Core 313-M0028A-101R to Section 105R, 32 cm; 482.42–494.94 mbsf) (Fig. F11). Interval 313-M0028A-105R-1, 32–54 cm (494.94–495.16 mbsf), contains a sharp-based sand bed with a brecciated contact with fractures filled by sand (not drillers mud). Another sharp-based bed of normally graded sand is found between Section 313-M0028A-105R-2, 100 cm (497.16 mbsf), and the base of Section 105R-2 (497.67 mbsf). Silty very fine sand and sandy silt continues in Cores 313-M0028A-106R through 108R (497.67–505.94 mbsf). There is a small amount of sand at the base of Core 313-M0028A-108R (505.94–506.00 mbsf). Beginning in Core 313-M0028A-109R (506.825 mbsf), bioturbated poorly sorted silty fine and medium sand contains rare bivalves, scattered pebbles, and granules. This lithology grades to fine sand with scattered pebbles and granules and plant debris down to a bioturbated contact with underlying clayey silt in Section 313-M0028A-110R-2, 110 cm (512.29 mbsf). InterpretationThe bioturbated poorly sorted sand with granules that lies above the well-marked surface defining the base of the subunit accumulated at the toe of a clinoform slope as sediment gravity flow deposits. Average grain size increases toward the top of Core 313-M0028A-109R (506.82 mbsf). From 505.94 mbsf uphole, the succession fines, and the two prominent sand beds in Core 313-M0028A-105R (494.94–497.67 mbsf) may represent storm deposits or wave-modified turbidites in an offshore toe-of-slope setting. The brecciated basal contact of the upper bed has evidence of hydraulic fracturing and remobilization of sandy sediment into fractures due to pore water overpressure. Cores 313-M0028A-104R and 103R (494.53–488.52 mbsf) are low in sand, shell fragments, and plant debris, indicating an offshore environment. Cores 313-M0028A-99R and 100R (477–482 mbsf) have interbedded bioturbated and laminated sand beds, indicating the influence of storm events and/or river flood events in what is interpreted as a river-influenced shoreface–offshore transition environment. Unit IV
Subunit IV is thin (13.23 m). The upper part of the unit is bioturbated silty clay with thin beds of sand that either coarsen or fine uphole. Plant and shell debris are concentrated in laminae. Downhole, scattered glauconite grains (~2%) appear first in Section 313-M0028A-112R-1, 62 cm (516.59 mbsf), in fine and medium sand. The proportion of glauconite increases downhole to ~40% at a sharp and burrowed surface in Section 313-M0028-113R-1, 68 cm (519.70 cm) (Fig. F12). Burrows are filled with glauconitic medium sand. Below this level is a return to silt and clay, with downhole increasing glauconite sand (<50%) and quartz sand content. Clay clasts and clay-lined burrows are present in Section 313-M0028A-114R-2 (523.57–524.65 mbsf). InterpretationThe lower part of the unit is interpreted as a mix of sediment gravity flow deposits that entrained clay clasts and background hemipelagic deposits. The decreases in glauconite and grain size uphole to a prominent burrowed surface in Section 313-M0028A-113R-1, 68 cm (519.70 mbsf), indicate a reduction in downdip sediment supply. The burrowed surface itself could have originated from sediment starvation and/or the onset of predominantly erosive sediment-laden flows and is a candidate unconformity. The overlying sandy sediment fines uphole; mica and plant debris support the presence of a river-influenced sediment source. Bioturbation obscures evidence for the physical depositional processes; however, the graded boundaries of some sand beds supports interpretation of river flood events, probably with storm influence. Unit V
Subunit VA
Core 313-M0028A-115R and those below are predominantly composed of glauconite (40%–60%) sand with scattered granules. Locally, 80% glauconite is reported in Section 313-M0028A-116R-1 (525.65 mbsf). Sediment is poorly sorted, with a muddy matrix in Cores 313-M0028A-115R and 116R. However, in Core 313-M0028A-117R to Section 119R-1, 140 cm (527.5–532.62 mbsf), beds of medium glauconite-quartz sand are inclined up to ~15° (apparent dip) (Fig. F13). The cross-stratification is delineated by common muddy laminae, which are crosscut by clay-filled Chrondrites. Cross bedsets are on the order of 100 cm thick. Normal grading from gravel to medium sand is well preserved in a bed in interval 313-M0028A-118-2, 60–70 cm (530.38 mbsf) (Fig. F14). InterpretationSubunit VA is interpreted as comprising a series of turbidites because of the normally graded beds, the well to moderately sorted sediment, and the relatively steep angle of bedding. Turbidites built dune-scale migrating bedforms on the toe-of-clinoform slope. Burrows and clay laminae show that the dune-scale bedforms migrated episodically, rather than from sustained flows, possibly in a channelized context. Poorly sorted deposits with floating granules and no evidence for bioturbation are interpreted as intercalated sandy debrite sourced from glauconite-rich deposits higher on the clinothem. Subunit VB
This section (Cores 313-M0028A-122R through 126R) starts in indurated, poorly to moderately sorted, muddy glauconitic sandstone with common quartz granules and rare pebbles. Granule and pebble grains tend to be less rounded than sand-grade grains. There are remnants of the original physical structures (subhorizontal lamination and normal grading) and stratification (muddy beds) that have been largely obscured by bioturbation. Shells, mica, and plant debris are rare to absent throughout the succession. Below a coring gap, there is a change downhole at the top of Section 313-M0028A-127R-1 (546.47 mbsf) to unlithified coarse glauconitic sand. The amount of glauconite decreases from 30% in Section 313-M0028A-127R-1 (546.47 mbsf) to 5%–10% in Section 132R-1 (563.26 mbsf). Overall, the lithology remains medium to coarse sand, with dispersed quartz granules, concentrations of pebbles (up to 15%), and common weak normal grading in beds. In Sections 313-M0028A-133R-1 through 135R-2 (564.77–573.48 mbsf), there is poorly sorted medium to coarse sand with variations in the proportion of glauconite (7%–20%) and granules (1%–5%). Poorly sorted medium to coarse glauconitic sand with granules is interbedded with silt in Section 313-M0028A-136R-1 (573.92 mbsf) and a siltstone in Section 137R-1 (576.02 mbsf). From Section 313-M0028A-141R-1 (583.07 mbsf), moderately to poorly sorted medium to coarse sand is less glauconitic (2%–3%), with steeply dipping silty beds (apparent dip up to 20°). In Section 313-M0028A-144R-1 (590.25 mbsf), well-sorted medium sand overlies a rare thin (5 cm) silty clay layer. Below the silty clay is a return to coarse sand with granules. Section 313-M0028A-145R-1 (592.22 mbsf) fines downhole to medium sand with a silty matrix, and in Section 145R-3, 82 cm (594.67 mbsf), there is a gradational change to coarse sand with granules. InterpretationSubunit VB is dominated by moderately to poorly sorted coarse sand-prone sediment with dispersed granules and pebbles and local concentrations of gravel. Poorly sorted coarse sediments are interpreted as deposits from high-concentration flows of coarse material eroded from updip positions that was deposited at the toe of a clinoform in an apron setting. Weak normal grading supports the partial transformation of debris flows into turbidity currents. The general poor sorting can be attributed to mixing through bioturbation and/or cohesive debris flow deposition. The overall succession of cores is coarsening uphole, with more silty layers and medium sand at the base that provide stratification, to largely unstratified, moderately sorted, coarser sand at the top, with greater amounts of granules and glauconite grains. Subunit VC
Cores 313-M00028A-149R through 151R consist of medium and fine sand with mica, plant debris, and shell fragments and foraminifers, which is a significant compositional difference from Subunit VB. Other changes include some granules in Core 313-M0028A-150R and glauconitic medium quartz sand in Section 152R-1, 0 cm (610.52 mbsf), becoming increasingly glauconitic down to a deeply (60 cm) and intensely burrowed (Thalassinoides and Ophiomorpha) surface in Section 313-M0028A-152R-1, 76 cm (611.60 mbsf), below which is a slightly silty nannofossil-bearing clay (Fig. F15). InterpretationThe deeply bioturbated basal surface of Subunit VC in Section 313-M0028A-152R, 76 cm (611.28 mbsf), marks a major increase in grain size and is interpreted as a sequence boundary separating Units V and VI. The sand in Subunit VC is evidence of sediment remobilization downslope into a toe-of-clinoform-slope apron derived from different sediment source areas compared to Subunit VB, perhaps eroding from different clinothem top successions. Unit VI
Below the major lithologic change in Section 313-M0028A-152R-1, 76 cm (611.28 mbsf), across a deeply burrowed surface (Fig. F15), the cores contain a thick succession of pale brown nannofossil-bearing silty clay and clayey silt with common sand and silt lamination (Fig. F16). Also present are postdepositional, and commonly precompactional, concretions. There is parallel and ripple lamination, and sandier beds are commonly normally graded and locally inversely graded at their base. Burrows are predominantly simple meniscate backfilled forms identified as Taenidium (Fig. F17). Pyrite replacement of burrow fills is common, and pyrite shapes along the burrow margin suggest fecal pellets (?Ophiomorpha). Overall, the amount of pyrite and the number of nodules decreases downhole. In places, laminated intervals alternate with intensely bioturbated intervals. Mica and plant debris are typically found in very fine sand beds, but shell fragments or molds are absent from the succession (Fig. F18). At the base of the succession is a ~2.5 m thick glauconite sand that coarsens down from very fine to medium-coarse sand grade. This bed or bedset abruptly overlies a sandy siltstone, with glauconite-filled burrows reaching 87 cm beneath the contact. InterpretationThe fining-upward glauconite sand at the base of this unit is interpreted as turbidity current deposits or river flood–related graded beds, with the abrupt increase in grain size across a deeply burrowed surface indicating a sequence boundary. The abundance of plant debris and mica indicates a fluvial source, and the environment of deposition is interpreted as a river-dominated deep offshore (prodelta) setting. There is no evidence of hummocks or other indicators of storm influence in the laminated beds, and this is taken as evidence of river-dominated deposition rather than reworking by storms. This inference is supported by the very fine sand beds with gradational upper and lower boundaries. Unlike other similar facies in this hole and in Holes M0027A and M0029A, this unit seems to lack abundant colloidal organic matter. This relatively quiet water environment was periodically subject to dysoxic bottom waters that excluded burrowing fauna. At times of better oxygenation, the infauna was dominated by horizontally mining deposit feeding organisms. Unit VII
This unit comprises dark brown siltstone with abundant plant debris, mica, and beds concentrated with shell fragments (Fig. F19). InterpretationThe brown siltstone is interpreted as a river-influenced deep offshore environment. Computed tomographyA total of 86 whole-core samples from Hole M0028A were analyzed with the computed tomography (CT) scanner in order to acquire two-dimensional (2-D) and three-dimensional (3-D) images. CT scan images were acquired along two perpendicular axes of the whole core (see "Lithostratigraphy" in the "Methods" chapter) to evaluate the dip angle of laminae and the intensity and character of bioturbation (Fig. F20). ConclusionThe 674.34 mbsf deep Hole M0028A washed through the upper 223 m of stratigraphy (no core in Unit I). Silt- and clay-prone Units VI and VII are interpreted as river-influenced deep offshore (prodelta) successions (Fig. F21). The overlying stratigraphy (Units V and IV) is interpreted as deposits remobilized from a heavily degraded clinothem that led to the development of a composite and progradational apron system dominated by debrites, with turbidites forming a toe-of-clinoform-slope toeset. Unit III is a thick clinothem that is interpreted to mark the progradation and aggradation of a mixed storm-river delta body. Basal toe-of-clinoform-slope apron coarse sand, which is interpreted to be related to the degradation of a clinothem, is overlain by offshore silts through shoreface–offshore transition interbedded sand and silt to poorly recovered sand interpreted as shoreface deposits. Deposits of the overlying Unit II form an aggradational stack of topsets close to the clinoform rollover position that preserve multiple truncated offshore to shoreface (regressive) and shoreface to offshore (transgressive) packages and poorly sorted glauconite-rich coarse sand. These shallow-water deposits underlie a clay, which is the youngest core recovered. |
|||
