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doi:10.2204/iodp.proc.313.103.2010 LithostratigraphyFigure F1 provides a key to colors and symbols used for figures in this section. Hole M0027A was drilled in the shallow shelf to sample a thick early Miocene succession. The hole also sampled relatively thin and incomplete Pleistocene, late to middle Miocene, early Oligocene, and late Eocene sediments. No Pliocene sediments were found. The succession is divided into eight lithostratigraphic units (Table T2; Fig. F2). Basal Unit VIII (631.15–625.60 meters below seafloor [mbsf]; Eocene) comprises clay deposited in a deep offshore environment. The overlying Unit VII (625.60–488.75 mbsf; late early Oligocene to early Miocene [early Aquitanian]) shows large-scale uphole coarsening from silt through very fine sand to poorly sorted glauconite-rich coarse sand deposited as debrites and turbidites. Cyclic changes in average grain size and glauconite content occur upcore on the scale of 5–10 m, but sedimentary structures are rare, possibly because of bioturbation. The succession is interpreted as the progradation of a clinoform slope succession over deep (>200 m?) apron deposits located seaward of the toe-of-clinoform slope. Unit VI (488.75–355.72 mbsf; late Aquitanian–early Burdigalian) marks the progradation of a thick storm-dominated river-influenced delta (offshore through shoreface–offshore transition to shoreface) over a toe-of-slope apron. A thick shoreface succession (Subunit VIA; ~57 m thick) comprises clean quartz sand. The overlying Unit V (355.72–335.93 mbsf; early–middle Burdigalian) marks an abrupt change in sedimentary facies and mineralogy to poorly sorted glauconite-rich sand with quartz and lithic granules and a poorly constrained environment of deposition in a clinoform rollover position. Unit IV (335.93–295.01 mbsf; middle Burdigalian) comprises an overall deepening-upward succession of shoreface–offshore transition to offshore facies. Units III and IV are separated by an erosion surface that is overlain by 1.5 m of very coarse glauconitic sand interpreted as a transgressive lag deposit. Unit III (295.01–236.16 mbsf; late Burgidalian–early Langhian) comprises deepening- and shallowing-upward packages of silty offshore and shoreface–offshore transition environments with a major storm influence. Unit II (236.16–167.74; Langhian) is a series of deepening-upward sedimentary cycles deposited in environments evolving from shoreface through shoreface–offshore transition to offshore. These cycles are interpreted as incomplete depositional sequences lacking regressive facies successions because of subsequent erosion. The upper part of Unit II is a clay-rich offshore succession. Unit I (167.74–0 mbsf; late Miocene[?]–late Pleistocene, with no evident Pliocene) comprises sand and gravel deposited in a range of fluvial, coastal plain, estuarine, and shoreface environments, with incised valley fills identified. See "Chronology" for refined ages, "Paleontology" for paleodepth estimates, and "Stratigraphic correlation" for comparison of key stratigraphic surfaces identified in cores, well logs, and seismic reflection profiles. 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. Figure F3 summarizes Units I–III and their constituent subunits, described in detail below. Figure F4 summarizes Units IV–VII. Table T3 describes thin sections that aided in core descriptions. Unit I
Unit I is predominantly sand. Core recovery was good only for the uppermost 32 m, and units below this level are poorly characterized. Subunit IAThis subunit comprises mainly gray-brown, quartz-rich coarse and very coarse sand (Fig. F5) with thick interbeds of gray clay. Shell fragments are abundant in the sand. The subunit is coarse to very coarse at the top, grading down to clay at ~18 m and gravel at the base. InterpretationThe succession is interpreted as a transgressive–regressive facies cycle composed mostly of shallow marine (shoreface to shoreface–offshore transition zone) and estuarine sediments. Subunit IBThis subunit is poorly recovered medium to coarse sand with granules. Three further divisions are possible. Subdivision A (Cores 313-M0027A-15H through 19H; 26.29–32.77 mbsf) comprises clean, well-sorted sand with rare gravel beds and no shells (Fig. F6). This upper succession may represent a beach or foreshore environment. Subdivision B (Cores 313-M0027A-20H through 26X; 32.77–46.52 mbsf) was very poorly recovered and is composed of poorly sorted sand with abundant subangular granules. Poor recovery makes environmental interpretation impossible. Subdivision C (Cores 313-M0027A-27X through 34X; 46.52–77.92 mbsf) is stratified and poorly sorted with subangular sand and granules (Fig. F7), with some burrows filled with dark organic material (organic matter is otherwise rare). Locally, well-developed cross-bedding occurs, indicating unidirectional currents. InterpretationThe lowest succession (Subdivision C) is interpreted as fluvial. Seismic lines and well logs support the interpretation that Core 313-M0027A-27X may have been drilled into an incised valley and the sediments captured here reflect the transgressive fill of the valley. Subunit ICThis lithologically variable unit comprises thin interbeds and interlaminae of sand and clay. Sediments in this poorly recovered interval show abrupt downcore facies changes. Plant debris and large fragments of wood are common (e.g., interval 313-M0027A-38X-1, 59–97 cm; 77.61–77.99 mbsf). InterpretationThis interval may represent estuarine environments. Subunit IDThis subunit is very poorly recovered. Two cores near the top (Cores 313-M0027A-50H and 51H; 95.32–110.83 mbsf) are nonmarine and exhibit paleosol fabrics (Fig. F8). Below the paleosol fabrics, sand (Cores 313-M0027A-53H through 56H; 134.97–162.49 mbsf) is generally poorly sorted coarse to very coarse quartz with small-pebble conglomerate. Large gaps in recovery separate cores (e.g., between Cores 313-M0027A-50H and 51H, 51H and 53H, and 54H and 56H; 96.12–110.57, 110.83–134.97, and 139.24–162.37 mbsf, respectively). InterpretationThese sediments likely represent fluvial environments, with possibly some shoreface deposits at the base of Core 313-M0027A-57H. Unit II
Recovery of this unit was good overall (70%), except for the top 23 m, where it was poor (16%). Broadly, the unit comprises clay and silt, with beds of sand and granules occurring prominently in the lowest 20 m. Well-sorted fine and very fine sand also occur at the top of the unit (e.g., interval 313-M0027A-58H-2, 0–118 cm; 175.19–176.35 mbsf). The underlying clay is interbedded with 1–2 cm thick silt beds and laminae of alternating dark and pale gray (e.g., Core 313-M0027A-65X; 192.11–193.61 mbsf). Sporadic very fine sand laminae occur (Fig. F9). Below this level (Core 313-M0027A-65X; 193.61 mbsf), the sediment is relatively homogeneous clay with only weak and rare color banding. Pale and medium gray-brown color-banded clay recurs in the underlying thickly interbedded clay and bioturbated silty clay (Cores 313-M0027A-67X through 69X; 197.91–207.06 mbsf). Burrows are present throughout the clay-dominated section; these are millimeter scale and dominantly horizontal where color banding is preserved and centimeter scale where color banding is disrupted. Bedding is contorted at a range of scales through the interval (Cores 313-M0027A-65X through 69X; 192.11–207.06 mbsf) (e.g., small-scale soft-sediment folding at 201.49 mbsf; Section 313-M0027A-68X-1, 53 cm) (Fig. F10). Homogeneous silty clay with common shell layers (bivalve and gastropods), foraminifers, and diatoms occurs below (Cores 313-M0027A-70X through 75X; 207.06–219.26 mbsf). The lowest part of this lithologic unit comprises a series of at least four ~5–10 m thick sedimentary cycles bounded sharply at their bases by beds of very coarse sand and/or granules. Examples are Sections 313-M0027A-75X-2, 50 cm (218.21 mbsf); 80R-1, 10 cm (225.46 mbsf) (Fig. F11); 82R-1, 1 cm (231.47 mbsf); and, at the base of the unit, 83R-2, 26 cm (236.16 mbsf). Burrows commonly pipe coarse sediments down into underlying fine sediments (Fig. F12). Each cycle coarsens downhole, and the lower two cycles are glauconitic at the base. Successive cycles become notably thinner downhole. InterpretationAt the base of the unit, fining-upward sedimentary cycles are interpreted as having been deposited in transgressive shoreface evolving through shoreface–offshore transition to offshore environments. These cycles are interpreted as incomplete depositional sequences in which regressive facies successions are missing and, presumably, subsequently eroded. The highest part of the unit represents shoreface–offshore transition and offshore environments subject proximally to episodic influx of sand due to storm events and distally to episodic input of mud, also possibly a result of storm-related processes. The thick clay-prone succession is interpreted as an offshore environment in a topset position. Soft-sediment folding indicates postdepositional disruption of the succession. This could be caused by downslope sediment transport, although an alternative interpretation is deformation while in a cohesive state, perhaps related to overpressure in the underlying sands (see "Unit II" in "Lithostratigraphy" in the "Site M0028" chapter). Fine sand at the top of the unit (Cores 313-M0027A-58H and 59H) is interpreted as shoreface–offshore transition and shoreface and signifies shallowing. Unit III
Recovery of this interval was excellent (~99%). The top of the unit (Sections 313-M0027A-83R-2, 126 cm, to 87R-2, 156 cm; 236.16–249.76 mbsf) comprises medium gray to dark gray-brown clay with faint laminations defined by weak concentrations of plant debris. A thin sharp-based horizon comprising calcilutite occurs in interval 313-M0027A-84R-3, 76.5–80 cm (244.375–244.41 mbsf) (Fig. F13). The clay passes down into a lithologically more varied succession of sandy mud and silty sand and gravel in Cores 313-M0027A-88R through 96R (249.76–274.21 mbsf). Sandy mud gradually coarsens downhole to a sharp basal contact separating granule-rich sandy silt above from silt (containing complete turritellid gastropods) below (Section 313-M0027A-89R-1, 40 cm; 253.21 mbsf) (Fig. F14). The silt succession sharply overlies glauconitic shelly and silty sand across a surface marked by fragmented shells (Section 313-M0027A-89R-2, 85 cm; 255.16 mbsf) (Fig. F15). A further abrupt and weakly bioturbated surface separates coarse glauconite-rich sand with granules and sandy silt above from silt below (Section 313-M0027A-90R-1, 33 cm; 256.09 mbsf). The underlying silt has discrete beds of clayey silt and sandy silt with parallel to subparallel lamination and little evidence of bioturbation (Sections 313-M0027A-90R-1, 33 cm, to 95R-1, 14 cm; 256.09–271.25 mbsf). This part of the succession is initially dominantly silt, with shell-lined burrows and rare shell-hash layers (Cores 313-M0027A-91R and 92R; 258.91–265.01 mbsf), but coarsens downhole with an increasing number and thickness of silty very fine to fine sand beds with shell fragments. Beneath a marked surface, the succession changes to coarse sand with granules, pebbles, and woody debris (Section 313-M0027A-95R-1, 14 cm; 271.25 mbsf). Below a 1 m coring gap, the succession is sandy silt and silty sand containing normally graded sand beds with concave-downward lamination and low-angle truncation surfaces (e.g., Sections 313-M0027A-96R-1, 22 cm, to 96R-1, 39 cm; 274.38–274.55 mbsf) (Fig. F16). Overall, the succession continues to fine downhole and is principally dark brown clay with thin interbeds of very fine sand, silt, or shells. Sand and shell beds thin downhole from thin beds to thick laminae. Where undisturbed by drilling, they are sharp based and graded normally. Core 313-M0027A-100R (286.36–289.41 mbsf) represents overall the finest grain size in this sediment package. The base of the unit occurs in Section 313-M0027A-102R-2, 105 cm (295.01 mbsf), where ~1.5 m of coarse to very coarse poorly sorted glauconitic (2%) sand with plant debris and angular quartz sits sharply on a prominent burrow-galleried erosion surface cut into the silt of Unit IV (Fig. F17). InterpretationThe major erosion surface at the base of the subunit in Section 313-M0027A-102R-2, 105 cm (295.01 mbsf) is overlain by ~1.5 m of very coarse glauconitic sand interpreted as a transgressive lag deposited in a shoreface environment. This sand grades abruptly uphole into the weakly laminated brown clays with thin beds of silt and shells that represent a shoreface–offshore transition environment characterized by deposition of thin storm beds under low-oxygen conditions, reaching a maximum water depth and a maximum flooding at ~288 mbsf (Core 313-M0027A-100R). Above this level, the succession contains more frequent interbeds of very fine sand and is interpreted as a regressive shoreface–offshore transition zone. Further coarsening-upward into sediments with discrete very fine sand beds containing hummocky cross stratification indicates a storm-dominated shoreface–offshore transition setting. The storm beds become increasingly amalgamated, and the coarse sand in interval 313-M0027A-95R-1, 14–68 cm (271.25–271.79 m), suggests a shoreface environment. The coarse sand is overlain by a prominent surface. The surface is interpreted as a flooding surface, without a transgressive lag (Section 313-M0027A-95R-1, 14 cm; 271.25 mbsf). The overlying well-stratified silt-rich succession (~15 m thick) with discrete shell-rich layers, commonly in life position, indicates a generally low-energy offshore environment, with individual beds interpreted as deposits of distal storm events. An interval with less sand or silt may be the deepest environment in this setting (Section 313-M0027A-92R-2; ~263 mbsf) and may represent the maximum flooding surface (MFS). Above this level, the uphole trend is shallowing with greater influence of storm activity to an abrupt contact with glauconitic coarse sand with thick-shelled bivalves (Section 313-M0027A-90R-1, 33 cm; 256.19 mbsf), indicating a high-energy shoreface environment. This contact is interpreted as a sequence boundary between shoreface–offshore transition deposits below and shoreface deposits above (see "Stratigraphic correlation"). This thin unit fines slightly uphole and then coarsens uphole to an abrupt flooding surface 0.93 m higher, which may indicate preservation of sea level lowstand deposits. A 2 m thick silt succession, rich in gastropod shells, is abruptly overlain by a dark brown sandy mud that has a fining, deepening-upward trend to another abrupt contact (Section 313-M0027A-88R-1, 68 cm; 250.44 mbsf), above which a clayey silt to silty clay also shows a weak deepening-upward trend in an offshore setting. The overall significance of these contacts at the top of meter-scale fining-upward silt-prone units (shoreface–offshore transition zone to offshore) is not clear (i.e., parasequence or sequence versus autogenic). Clays and silts at the top of the unit (236.16–249.76 mbsf) are interpreted as offshore with very minor reworking of silt and very fine sand from updip. The following Units IV–VIII are summarized in Figure F4 and described hereafter. Unit IV
Core recovery for this unit was excellent (98%). The unit comprises mostly medium gray to gray-brown silt, more or less rich in very fine sand, with abundant macroscopic dispersed plant debris and mica grains and scattered gastropod and bivalve shells. The succession is strongly bioturbated. Rare thin beds of parallel-laminated fine sand represent discrete depositional events (e.g., in intervals 313-M0027A-107R-2, 24–29 cm [Fig. F18], and 110R-2, 80–83 cm; 309.45–309.50 and 319.16–319.19 mbsf). A normally graded interval of sandy silt to shelly very fine to fine sand has an abrupt basal contact modified by burrows (Thalassinoides) that penetrate the underlying silt to 39 cm (Section 313-M0027A-114R-2, 133 cm) (Fig. F19). Very fine sandy silt continues down to an abrupt contact with the underlying unit (Section 313-M0027A-116R-1, 91 cm; 336.07 mbsf). InterpretationThe succession is interpreted as having been deposited mostly in deepening-upward packages of river-influenced shoreface–offshore transition to offshore setting (prodelta), with graded shell beds (storms) and high silt and plant debris content (rivers). The thick graded sandy interval overlying an abrupt surface at 331.85 mbsf (Section 313-M0027A-114R-2, 133 cm) (Fig. F19) is interpreted to represent abrupt shallowing from an offshore silt below to shoreface sand above and may be a sequence boundary. Unit V
This unit is poorly recovered (64%). The lithology is typically very poorly sorted glauconite-rich (typically 3%–10%) coarse to very coarse sand. The maximum observed glauconite concentration in this unit occurs in Core 313-M0027A-120R (~342 m), where it is estimated at ~20% (Fig. F20). K/Th ratios in the spectral gamma ray log indicate a significant change to lower glauconite concentration at about the middle of the unit; this likely corresponds to Section 313-M0027A-121R-1, 110 cm (345.40 mbsf), where glauconite sand content decreases downhole from ~20% above to ~5% below Section 313-M0027A-122R-1, 28 cm (347.64 mbsf). Downhole, carbonate-cemented laminae and rare burrows are present, and soft mud is less prevalent. An abrupt contact with a ~15 cm thick carbonate-cemented bed lies at the base of the unit (interval 313-M0027A-125R-1, 132–147 cm; ~355.45–355.49 mbsf) (Fig. F21). InterpretationThe environment of deposition of the coarse to very coarse glauconitic sand succession with carbonate cement concentrated at the base is poorly constrained. One interpretation is that sand is reworked by physical and biological processes in a transgressive shoreface to offshore setting. Seismic reflection data indicate a clinoform rollover position, and alternative environment of deposition interpretations include gulley fills or small deltas. Maximum condensation may correspond to the most glauconite rich interval (~342 mbsf). Unit VI
Subunit VIAOverall, 66% of this subunit was recovered. The unit was recovered least effectively where the lithology, as determined from downhole log signatures, is most variable: in the uppermost 10 m and the lowermost 20 m. The subunit comprises mostly gray quartz-rich coarse–medium sand with ~3% black nonorganic detrital grains (Fig. F22), wood debris, and shell debris. There is a general downhole decrease in grain size from coarse to medium sand and an increase in bioturbation. Cylindricnus (distinctive concentrically backfilled burrows) occurs near the top of the subunit (e.g., interval 313-M0027A-127R-1, 60–95 cm; 360.16–360.51 mbsf). Sorting is moderately good, although granules occur throughout some intervals (e.g., interval 313-M0027A-129R-1, 60–100 cm; 366.26–366.66 mbsf). Thick beds exhibiting sets of low-angle inclined stratification alternate with more homogeneous bioturbated intervals (Fig. F22). Near the base of the subunit, silt laminae are present (e.g., interval 313-M0027A-144R-1, 119–121 cm; 409.55–409.57 mbsf) (Fig. F23). InterpretationThe lower, slightly muddy sands are interpreted as (lower) shoreface deposits at the base of cleaner (upper) shoreface deposits. Low-angle cross-lamination, burrows, and shell fragments support an upper shoreface to foreshore environment sustained over >30 m, which may indicate that sediment supply kept pace with relative sea level rise. Subunit VIBAbout 95% of this unit was recovered. The uppermost 40 cm comprises brown very fine sand and sandy silt (Sections 313-M0027A-145R-2, 120 cm, to 145R-CC, 10 cm; 414.11–414.51 mbsf). The bulk of the subunit is composed mostly of dark brown silt and clayey silt with common mica flakes and abundant dispersed plant debris (Fig. F24). Thin-shelled bivalves, commonly intact, occur throughout. Well-defined thin beds of very fine sand occur sporadically in the top 25 m and show strong parallel lamination or climbing-ripple cross-lamination (e.g., Sections 313-M0027A-148R-1, 421 mbsf [Fig. F25], and 149R-2, 425 mbsf). The succession is well bioturbated at the top but is more weakly bioturbated in the middle, with weak parallel fine lamination and streaky fabric predominant in Sections 313-M0027A-152R-1 (434 mbsf) through 164R-2 (472 mbsf). A trace of glauconite grains occurs in Core 313-M0027A-161R. Diatoms are present in smear slides. Below ~468 mbsf (Core 313-M0027A-163R), corresponding to a prominent seismic reflector (see "Stratigraphic correlation") and a downhole decrease in density, the succession gradually becomes more coarse grained, with sediment grading from dark brown silt into brown fine sandy silt and eventually to greenish gray fine glauconitic sand with rare burrows (Core 313-M0027A-167R; 478 mbsf). Below this level, glauconite sand becomes progressively more abundant, and bioturbation and shell content increases to an abrupt change across a bioturbated surface to coarse, poorly sorted glauconitic sand in Section 313-M0027A-168R-2, 55 cm (483.64 mbsf) (Fig. F26), that continues to the base of the subunit. InterpretationThe lower part of the subunit is interpreted as having been deposited in relatively deep water by the reworking and transport of materials derived from the top and slope of the previously deposited clinothem via sediment gravity flows into a toe-of-clinoform-slope apron setting. The upper part of the succession, characterized by high amounts of plant debris and mica, low intensity of bioturbation, and common thin-shelled bivalves, is interpreted as having been deposited in a poorly oxygenated, silt-dominated, river-influenced offshore (prodelta) setting. Thin beds of very fine sand with planar and climbing-ripple cross-lamination are interpreted as deposits of river flood events. Unit VII
Subunit VIIAThis thin subunit is characterized by coarse grain sizes and strong glauconite enrichment. Lithology ranges from glauconitic quartz sand with coarse quartz granules and clay beds to quartz-rich glauconite sand. Sorting is moderate to poor, and sediments are locally carbonate cemented (intervals 313-M0027A-171R-1, 31–63 cm, and 171R-2, 40–130 cm; 489.70–490.01 and 491.39–492.19 mbsf). Bioturbation is variable, and, where weakly developed, sediment is well bedded and occasionally normally graded (e.g., interval 313-M0027A-174R-1, 110–121 cm; 494.86–494.97 mbsf) (Fig. F27). The maximum glauconite concentration is estimated as ~80%–90% in interval 313-M0027A-172R-1, 31–63 cm (489.70–490.01 mbsf). At several levels, pale brown clay with nannofossils occurs in thin beds and as burrow fills, including Chondrites. Bedding is predominantly horizontal, but locally the clay and sand interbeds are inclined to ~15° (apparent dip) (interval 313-M0027A-171R-2, 66–105 cm; 491.55–494.91 mbsf) (Fig. F28), possibly representing migration of a dune-scale bedform. The transition to the underlying subunit at Section 313-M0027A-175R-3, 10 cm, is subtle. InterpretationThe coarse, poorly sorted glauconite sand at the base of the unit is interpreted as a relatively deep shelf (~100 m water depth) sediment-starved deposit with abundant in situ deepwater glauconite. This becomes mixed with increasingly coarse quartz sand and granules uphole to the abrupt facies change in Section 313-M0027A-170R-1, 109 cm (488.75 m), which marks the top of the unit. Graded beds with angular quartz and rounded glauconite in inclined beds and the well-stratified succession with nannofossils in clay laminae suggest a sediment supply via sediment gravity flows (turbidity currents and debris flows) in a deepwater apron environment (i.e., seaward of the toe-of-clinoform break in slope). Sediment was likely sourced from a clinoform rollover location that may either have been exposed or flooded. These deposits may have a channelized context. Subunit VIIBThis thick subunit (125.69 m thick) is characterized by strong glauconite sand enrichment in fine sand or a matrix of silt. Gradual cyclic changes in average grain size and glauconite content occur downcore on the scale of 5–10 m, but there is also a large-scale downhole trend from very fine sand at the top to silt near the base. Glauconite sand content locally reaches as high as 60% (e.g., Core 313-M0027A-188R) but is generally in the range of 5%–20%. Quartz granules occur rarely (e.g., at a sharp contact between very fine sand and clayey silt in Core 313-M0027A-201R). Trace mica occurs throughout. The sediment fabric is structureless overall and presumed to be intensely bioturbated; locally, muddy laminae are observed (e.g., Core 313-M0027A-200R). An interval of glauconite-rich well-sorted fine sand with ripple-scale cross-lamination occurs in Cores 313-M0027A-191R and 192R. Ripple cross-lamination also occurs in other sand-rich intervals (e.g., Cores 313-M0027A-202R and 205R). Further downsection, primary parallel lamination is developed (Cores 313-M0027A-215R through 217R). Thin-shelled bivalves (e.g., Core 313-M0027A-207R) and benthic foraminifers are common (e.g., 3% in Core 313-M0027A-192R) throughout Cores 313-M0027A-188R through 198R and 210R through 217R. Woody debris (e.g., Core 313-M0027A-191R) is common at several levels, and distinct burrow forms are also recorded (e.g., Diplocraterion in Cores 313-M0027A-186R and 198R, ?Teichichnus in Cores 190R and 198R, Chondrites in Core 221R, and possible Zoophycos in Core 223R). Pyrite is common as a diagenetic mineral, observed to have replaced shell material, in Cores 313-M0027A-218R through 222R. The base of the unit is marked by a highly bioturbated contact between silty sand above and silty clay below. InterpretationThe basal strata of the unit comprise sandy silt coarsening down to silty sand on an abrupt and highly bioturbated (Thalassinoides) contact with pale green silty clay from the underlying unit. Bottom water oxygen content is interpreted as reduced in the lower ~20 m of the succession based on burrow type and sulfide mineral precipitation. Glauconite sand is interpreted as a mixture of in situ grains and grains reworked from shallower water. The environment of deposition of the overlying strata is interpreted as coalesced fans beyond the toe-of-clinoform slope with sediment deposited from density flows. Unit VIIIThis unit represents only the top part of a likely much thicker unit that was not drilled. Gray-brown bioturbated silty clay with trace mica and scattered glauconite sand is moderately well bioturbated with identifiable Chrondrites and Planolites (Fig. F29). Rare thin-shelled bivalves and foraminifers are present. Disseminated pyrite is present, locally replacing burrows. The top of the section has burrows piping down glauconitic sand ~30 cm from the overlying unit (Fig. F30). InterpretationThe environment of deposition is interpreted as deep offshore with dysoxic bottom waters. Computed tomography scansComputed tomography (CT) was used on 38 whole-core samples from Hole M0027A to acquire two-dimensional (2-D) and three-dimensional (3-D) images. An example scan from Section 313-M0027A-69X-1 (204.01–205.51 mbsf) is illustrated in Figure F31, together with the line-scan image of the split core, magnetic susceptibility and density data from the multisensor core logger (MSCL), and an acoustic image from the borehole. This comparison enables better definition of sediment characteristics, structures, and lithologic boundaries. CT images shown were acquired in two perpendicular planes (see "Lithostratigraphy" in the "Methods" chapter). Hole M0027A lithostratigraphic summaryLate Eocene deep offshore silty clay (Unit VIII) is abruptly overlain by late early Oligocene to early Miocene glauconitic clastics (Unit VII) that overall coarsen from silts to sands but comprise smaller scale cyclicity in grain size and glauconite content (Fig. F32). These are interpreted to be deepwater apron deposits seaward of the lower clinoform break in slope that are overstepped by less glauconitic toe-of-slope and clinoform slope deposits (Subunit VIIA). Unit VI marks the progradation of a storm-dominated river-influenced delta, which is abruptly capped by poorly sorted coarse-grained deposits in a clinoform rollover position. The overlying succession (Units IV and III) comprises a series of deepening-upward packages, with less common shallowing-upward packages in storm-dominated shoreface, shoreface–offshore transition, and offshore settings in clinothem topset position. Unit II, which comprises a series of top-truncated deepening-upward sequences from shoreface through shoreface–offshore transition to offshore settings, overall fines uphole to a thick clay-prone succession. Unit I comprises a range of fluvial, coastal plain, estuarine, and shoreface environments. |
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