| IODP Proceedings Volume contents Search | |||
|
|||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.313.105.2010 LithostratigraphyFigure F1 provides a key to colors and symbols used for figures in this section. Hole M0029A was drilled in the shallow shelf primarily to sample the thick middle Miocene. Seven lithostratigraphic units are defined (Table T2). Figure F2 illustrates the positions of the lithostratigraphic units on the seismic sections. The basal lithostratigraphic unit of Hole M0029A, Unit VII (756.33–747.27 mbsf, late early Oligocene to early Miocene), is siltstone with glauconite sand and thin-walled articulated shells deposited in a low-energy deep offshore environment. Unit VI (747.27–728.55 mbsf; middle early Miocene [late Aquitanian to early Burdigalian]) is a pale brown clayey silt with intercalated very fine and fine sand beds. These sediments represent a river-dominated offshore (prodelta) environment. Units V–III (728.55–640.51 mbsf; middle early Miocene to early middle Miocene) contain a series of granuliferous coarse quartz and glauconite sand packages separated by bioturbated silt. The sand packages are generally sharp based and fine uphole into silt. The sand is interpreted to represent toe-of-clinoform-slope apron systems deposited during lowered sea level and the silt to represent deep offshore deposition during high sea level. Unit II (640.51–325.12 mbsf; middle Miocene [Langhian]) is divided into several subunits. Subunit IID (640.51–602.25 mbsf) contains two packages with poorly sorted medium to coarse glauconitic sand at the base and bioturbated silt on top. The environment of deposition is interpreted as a toe-of-clinoform-slope apron at the base and deep offshore on top. Subunit IIC (602.25–502.01 mbsf) comprises a monotonous succession of very fine sandy silt and silt deposited in a deep offshore environment below storm wave base. Subunit IIB (502.01–448.49 mbsf) is mostly sand, and deposition was likely by sediment gravity flow on a clinoform slope, either within a submarine channel or on an intraslope apron. Subunit IIA (448.49–325.12 mbsf) is another monotonous very fine sandy silt and silt with uncommon sand beds representing offshore and river-influenced offshore environments. Unit I (325.12–3.85 mbsf; late Pleistocene–?late Miocene) was not continuously cored. Spot cores attempted to identify major seismic reflectors. Sediments recovered were likely deposited in a range of shelf settings, from shallow marine shoreface to foreshore, coastal plain, and estuarine. 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. Unit I
Because of difficult coring conditions in sandy sediment, cores in this unit targeted reflectors identified on seismic profiles as post-Langhian unconformities (m4, m3, and m1). No attempt was made to core materials between the reflectors. Subunit IA
Recovery of this subunit was generally very poor, and many cores had no recovery. Cores 313-M0029A-4R through 8R (3.85–17.27 mbsf) contain interbedded and interlaminated micaceous fine sand and silty clay with shell fragments. Cores 313-M0029A-13R through 15R (40.45–50.16 mbsf) contain medium to coarse micaceous, slightly glauconitic sand. There is a change across a sharp surface (Fig. F4) in Section 313-M0029A-15R, 8 cm, from silty medium to coarse sand above to silty clay below. The contact is lined by pebbles. Brown clay and pale blue clay are found in Cores 313-M0029A-16R and 19R. The clay in Core 313-M0029A-19R has a single pebble at its base. Cores 313-M0029A-20R through 24R contain well-sorted fine sand. InterpretationSediments recovered in Cores 313-M0029A-1R through 24R were likely deposited in a range of shelf settings, including shallow-marine shoreface to foreshore, coastal plain, and estuarine. Poor recovery, however, prevents detailed environmental analysis. Two potential unconformities are identified in Cores 313-M0029A-19R and 15R (Fig. F4) based on the clays possibly representing soil environments, although this interpretation is tentative because of coring gaps and sporadic disturbed cores. The upper succession is Pleistocene in age, based on Zone NN21 in Core 313-M0029A-7R. The lower succession, between Cores 313-M0029A-16R and 19R (~50–65 mbsf) is also suggested to be Pleistocene in age. Subunit IB
Cores 313-M0029A-25R through 30R targeted reflector m1 in an attempt to better date and determine its sedimentological character. Core 313-M0029A-25R recovered medium sand with granules that graded down to clay and fine sand in Core 313-M0029A-26R. Core 313-M0029A-27R was empty. Below, Cores 313-M0029A-28R through 30R contained beds ranging from fine sand to clayey silt with organic matter and wood, including a gravel layer at Section 313-M0029A-29R-1, 17 cm. InterpretationThese unsorted, coarse-grained sands with granules interbedded with clay and silty clays are interpreted as coastal plain deposits with possible intervening channels. The gravel bed is a candidate to represent the unconformity and reflector m1. Subunit IC
Cores 313-M0029A-31R through 35R targeted reflector m3 in an attempt to better date and determine the sedimentological character of the seismic reflector. Cores 313-M0029A-31R and 32R recovered no sediment. Cores 313-M0029A-33R and 34R contain deeply weathered clay and silty clay with large (>10 cm) pieces of wood. Below the clay is medium sand (Core 313-M0029A-35R). InterpretationThe deeply weathered clay (Cores 313-M0029A-33R and 34R) may represent separate paleosols (Fig. F5). They are interpreted to represent an unconformity and reflector m3. Subunit ID
Cores 313-M0029A-36R through 41R targeted reflector m4 in an attempt to better date and determine its sedimentological character. Medium to coarse sand in Core 313-M0029A-36R with some shell debris changed to very fine sand and sandy clay with sand granules, woody organic matter, and no fauna in Core 313-M0029A-37R. At the bottom of the succession in Core 313-M0029A-40R was clay and silty sand, and at the bottom of the succession in Core 313-M0029A-41R was fine-grained sand rich in plant debris. InterpretationThe coarsening-upward succession could represent an interdistributary bay to crevasse splay facies succession passing probably upward to marine deposits as indicated by the presence of reworked shell debris. No clear sharp contact attributable to an unconformity and reflector m4 was identified, except by the presence of coarse granules. Subunit IE
The top of the succession (Cores 313-M0029A-44R through 51R) is moderately sorted medium and fine sand barren of shells. A change in Core 313-M0029A-51R to interbedded medium/fine sand and silty clay below lies above a sharp contact with a clean gravel below in Core 313-M0029A-54R (293 mbsf). The underlying succession is sand prone with granules, shell fragments, mica, and significant amounts of plant debris to an abrupt bioturbated contact at the base of the succession (Section 313-M0029A-65R-2, 80 cm). InterpretationThe basal bioturbated contact of silty fine sand above clay (Fig. F6) also marks an abrupt downhole increase in hardness and is interpreted as an unconformity. The overlying succession (Core 313-M0029A-61R to Section 56R-1; 312–295 mbsf) coarsens uphole from silty very fine sand to medium sand, indicating a change from a shoreface–offshore transition to a shoreface setting. The significant amount of mica, plant debris, and organic-rich burrow fills suggests a strong fluvial influence. Gravel in Section 313-M0029A-54R-CC (293 mbsf) and the distinctive overlying interbedded succession of sharp-topped and sharp-based blue-gray clay layers with well-sorted medium sands is interpreted as representing beach/backbeach coastal plain environments (Fig. F7). The cores above Core 313-M0029A-51R to the top of the subunit (Core 313-M0029A-41R) are generally fine- and medium-grained sand without shell fragments, which might be an incised valley fill given the blocky character of the gamma log. The base of this interpreted valley fill (~286 mbsf) may be a candidate for an unconformity and sequence boundary. Unit II
Subunit IIA
Subunit IIA1
Sediments in Subunit IIA1 are dominated by silty clay with occasional thin, sharp-based fine sand beds (convex-up laminae; e.g., interval 313-M0029A-68R-1, 24–30 cm); finely dispersed organic material; and pyrite. A carbonate-cemented siltstone with its base at 49 cm in Section 313-M0029A-72R-1 separates silty clays above from muddy bioturbated fine sand below. InterpretationThe base of the carbonate-cemented siltstone (Section 313-M0029A-72R-1, 49 cm) marks a deepening in environment of deposition. The overlying section was deposited in shoreface–offshore transition and offshore environments. Subunit IIA2
Sections 313-M0029A-72R-1, 49 cm, through 73R-3 (343.81–349.70 mbsf) are muddy and silty very fine and fine sand becoming silty clay below. Section 313-M0029A-75R-1 (353.31 mbsf) is silty clay. The succession coarsens downhole to medium sand, with many large shells in Sections 313-M0029A-76R-2 and 76R-3 (355.84–356.44 mbsf). The section then fines downhole to clayey silt rich in plant debris with common Turritella and rare thin-shelled bivalves (Section 313-M0029A-81R-2; 371.17 mbsf). The succession coarsens downhole to fine sand in Section 313-M0029A-83R-2 (376.16 mbsf). These successions are interrupted by three normally graded beds:
InterpretationThis subunit represents river-influenced predominantly offshore environments at the base (Sections 313-M0029A-83R-2, 99 cm, through 78R-2; 377.15–360.94 mbsf), with river-influenced shoreface–offshore transition environments on top (Sections 313-M0029A-78R-1 through 75R-1; 360.94–353.31 mbsf). Deposition was punctuated by coarser graded sand beds (intervals 313-M0029A-83R-2, 0–99 cm [376.16–377.15 mbsf]; 82R-1, 53–90 cm [372.36–372.73 mbsf]; and 79R-2, 50–90 cm [364.46–364.86 mbsf]) that represent either individual turbidity current deposits of coarser material and/or episodes of lowered sea level. Subunit IIA3
From Section 313-M0029A-84R-1 (377.71 mbsf) to Section 313-M0029A-107R-1, 35 cm (448.49 mbsf), the succession is overall silt and silty clay with plant debris and mica along with beds that alternate in degree of lamination and bioturbation (Chondrites). This interval includes a few thin (centimeter thick) sand beds. In Section 313-M0029A-94R-1, a downhole coarsening from silty clay to sandy silt is separated by a sharp bioturbated erosion surface from silty clay below. The base of Section 313-M0029A-107R-2, 127 cm, has a faint surface across which the silt contains rare glauconite grains. Core 313-M0029A-108R is slightly glauconitic silt on top, and glauconite grains increase in size and number downsection. InterpretationThe section represents deposition in the outer limits of the shoreface–offshore transition zone or the shallowest depths of the offshore. Subunit IIB
Subunit IIB1
Subunit IIB1 increases in glauconite concentration and grain size from silt to silty sand with granules down to an inclined surface at Section 313-M0029A-108R-1, 124 cm (449.00 mbsf), which truncates the underlying clayey silt. Sections 313-M0029A-108R-2 through 111R-CC (448.49–460.52 mbsf) are overall poorly sorted slightly shelly silt. A 60 cm thick fine sand succession with cross-bedsets occurs in Section 313-M0029A-110R-2 (Fig. F9). In Cores 313-M0029A-112R through 118R, the section coarsens downhole from silty fine sand to poorly sorted slightly glauconitic medium sand. A bioturbated surface in Section 313-M0029A-118R-1, 25 cm (478.61 mbsf), separates medium sand above from siltstone below. InterpretationThe bioturbated basal contact is interpreted as a sequence boundary. The overlying poorly sorted and immature coarse sediments likely represent debris flow and turbidity current deposits on a clinoform slope, possibly in a channelized context (e.g., erosion surface at Section 313-M0029A-108R-1, 124 cm). Subunit IIB2
The top part of Subunit IIB2 is siltstone. Sediments between Sections 313-M0029A-120R-1 and 126R-2 (478.61–502.01 mbsf) contain two packages that irregularly fine downhole. Medium sand with coarse to very coarse grains (Core 313-M0029A-120R; 481.41 mbsf) fines downhole to clayey silt to Section 313-M0029A-122R-2, 52 cm (489.54 mbsf). Cores 313-M0029A-121R through 122R (484.46–487.6 mbsf) contain numerous sharp-based erosive sand beds that grade normally to sandy silt, commonly with parallel and cross-lamination and plant debris near the bed top. The second fining-downward package is found between Sections 313-M0029A-122R-2, 52 cm, and 126R-2, 80 cm (489.54–502.01 mbsf). Sediments in this interval contain a variety of sedimentary structures, including parallel and cross-lamination, possible ripple lamination, and contorted and convolute lamination. InterpretationThe poorly sorted but well-stratified nature of the sediments and the lack of storm-related sedimentary structures suggest sedimentation took place below the mean storm wave base. Normal grading cross-stratification style suggests rapid deposition by turbidity currents. The environment of deposition is likely a clinoform slope with sediment gravity flow, either within a submarine channel or intraslope apron environment. Subunit IIC
Sections 313-M0029A-126R-2, 80 cm, to 161R-2, 37 cm, is a largely monotonous succession of very fine sandy silt and silt. At the top (Cores 313-M0029A-127R and 128R), sporadic centimeter-scale very fine sand beds contain faint laminae, and Teichichnus burrows decrease in thickness downhole (Fig. F10). Beginning in Core 313-M0029A-129R, the deposits become homogeneous, slightly micaceous very fine sandy silt with rare thin-walled shells, Teichichnus burrows, and plant debris. There are several ~20 cm thick heavily bioturbated intervals with burrow infills containing glauconite sand, quartz sand, shell debris, and occasional granules. This lithology continues to the bottom of Core 313-M0029A-159R. In Sections 313-M0029A-160R-1 to 161R-2, 37 cm (597.31–602.33 mbsf), silt deposition is interrupted by sharp-based beds (3–14 cm thick) of medium sand that grades normally to silty very fine sand. At the base of the unit, sandy silt overlies muddy to granuliferous medium sand assigned to Subunit IID. InterpretationSediments in Subunit IIC were deposited in a deep offshore environment below mean storm wave base. The intervals with glauconite-filled burrows might represent periods of lower sedimentation rates. The graded beds are interpreted as dilute turbidity current deposits. Subunit IID
This subunit contains two similar glauconite sand packages that fine uphole to silts at the top. A silt below Section 313-M0029A-167R-2, 42 cm, is used as the basis for dividing the subunit into Subunits IID1 and IID2. Subunit IID1
Section 313-M0029A-161R-2, 37 cm, to 163R-2, 40 cm (602.23–608.37 mbsf), are dominantly muddy glauconite sand, and the amount of glauconite and quartz increases downhole. Beds of glauconitic sandy mud indicate the presence of original stratification. Shell fragments are scattered throughout, and quartz granules have a patchy green coating, but no plant debris or mica are observed. Physical structures include subparallel to cross-lamination, and some laminae have a more chaotic and crosscutting pattern. In Sections 313-M0029A-163R-2, 40 cm, to 167R-2, 42 cm (608.37–620.59 mbsf), glauconite (from 30%–40% to ~5%) and mud decrease downhole. In Core 313-M0029A-165R-2 (614.1–615.66 mbsf), there is a concentration of thin-walled shells in poorly sorted sand and an isolated and articulated thin-walled shell. Quartz grains and granules become increasingly well rounded downhole through this unit. There are more cemented horizons, and light and dark green glauconite grains are observed. In Section 313-M0029A-167R-2, 42 cm (620.59 mbsf), there is an abrupt and bioturbated surface of coarse-grained glauconitic sand on brown clayey silt. The burrows are horizontal and vertical and filled with coarse glauconite and quartz sand. InterpretationThe deeply bioturbated surface in Section 313-M0029A-167R-2, 42 cm (620.59 mbsf), marks an abrupt increase in the supply of coarse sediment. This surface is a candidate unconformity or a correlative conformity and is interpreted as a sequence boundary. The overlying poorly sorted sand, glauconitic sand, and muddy sand are interpreted as sediment gravity flow deposits. The preservation of delicate thin-walled shells suggests deposition by cohesive laminar flow deposits (debrites). The common crosscutting concave-upward laminae may indicate dewatering of saturated turbidites soon after deposition or could be a bioturbation fabric (Teichichnus type). Some remnants of tractional structures and weak normal grading support the interpretation of turbidity current deposits. The environment of deposition is interpreted as a toe-of-slope apron system. The succession increases in mud content and gradually decreases in quartz and glauconite sand content in Section 313-M0029A-161R-2, indicating a progressive abandonment of downslope transport. Subunit IID2
Brown clayey silt is found below an abrupt and bioturbated surface in Section 313-M0029A-167R-2, 42 cm (620.59 mbsf). The clayey silt contains many thin-walled shells. In Core 313-M0029A-168R (621.71 mbsf), the succession changes to bioturbated glauconite-bearing silt, which coarsens downhole into poorly sorted glauconitic medium to coarse sand. Cemented medium-grade glauconite sandstone is present in Core 313-M0029A-172R. Variably cemented glauconite sand continues from Core 313-M0029A-173R to the top of Core 175R. Thick-walled shells occur throughout this interval but show signs of severe dissolution. Glauconite grains are coarse and fragmented. Silty coarse glauconitic sand abruptly changes to bioturbated glauconitic silt and silt in Section 313-M0029A-175R-1, 50 cm. InterpretationThe abrupt grain size increase across the basal surface indicates an increase in energy and can be interpreted as a sequence boundary. The floating very coarse sand grains and granules in silt suggest deposition of debris flows, though bioturbation at several levels indicates that the individual deposits must have been thin. Glauconite sand appears in pulses uphole, becoming dominant in coarse cemented units in Core 313-M0029A-175R and above. There are no physical sedimentary structures preserved in the coarse sandstones (or burrows in the most coarse beds), and the depositional process is unclear. The most likely explanation is multiple sediment gravity flow deposits in a toe-of-slope apron setting. Unit III
This unit was poorly recovered at Site M0029. Unit III begins with glauconitic siltstone. Several large vertical burrows and Teichichinus are identified in the upper 70 cm. Siltstone decreases in glauconite content downhole and is largely confined to burrows. Glauconite grains are coarse and fragmented. Thick-walled shells occur but show signs of severe dissolution. Glauconite and quartz sand, including quartz granules, increase toward the base of Unit III to a sharp surface in Section 313-M0029A-179R-2, 28 cm (650.13 mbsf). InterpretationThe abrupt influx of sand at the base of Unit III on a bioturbated contact may be interpreted as an unconformity at the fringe of a toe-of-slope fan or at a sediment bypass surface. The overlying siltstone is interpreted as a deep offshore setting. Unit IV
Core 313-M0029A-179R is bioturbated silt with glauconite sand–filled Thalassinoides burrows at the top, and the sediment passes down into intensely bioturbated silt with floating coarse and very coarse quartz sand grains. The succession is dominantly siltstone with dispersed plant debris, sand-filled burrows, and rare thin-walled shells. The succession gradually coarsens downhole and increases in glauconite and quartz sand through Core 313-M0029A-183R (661.36–664.29 mbsf). InterpretationThe fining- and thinning-upward trend at the base of Unit IV is interpreted to represent reduction in sand supply to a toe-of-slope setting in response to updip transgression. The sedimentary expression of this at the lower clinoform break in slope is a return to silt and clay deposition with coarse-grained material stored on the topsets of clinothems. Floating coarse grains in silt suggest a cohesive mud flow depositional process, and bioturbation indicates that the mud flow deposits must have been thin. There is a possible unconformity at the top of the unit. Unit V
Subunit VA
This succession begins in brown bioturbated silt with traces of plant debris, and grain size gradually increases downhole in glauconite and quartz sand through Core 313-M0029A-183R (661.36–664.29 mbsf) through silty medium glauconitic sand to coarse sandstone with many subangular granules, very coarse quartz sand grains, and articulated shells and well-preserved benthic foraminifers. The proportion of glauconite varies. From Core 313-M0029A-184R to Section 313-M0029A-189R-2, 15 cm (664.41–675.06 mbsf), the succession is coarse grained with poorly sorted medium glauconite sand with quartz and silty glauconitic sand, with shell fragments, benthic foraminifers, and an articulated shell. Bed stratification is poorly preserved. Physical structures are rare, although convolute and subparallel laminations were observed. In Section 313-M0029A-189R-2, 15 cm (675.06 mbsf), an abrupt and bioturbated contact between medium and fine silty glauconitic sand above and silty clay below exhibits glauconite and quartz sand in burrows. Concretions are distinctive, as they contain quartz grains with a shattered fabric, although quartz grains are pristine outside the cemented areas. InterpretationThe sharp-based sandstone in Section 313-M0029A-189R-1, 15 cm, is interpreted to be an unconformity, with the overlying coarse-grained unit an apron system that extended seaward of the lower clinoform break in slope. The main depositional processes in operation are inferred to be debris flows, although evidence of more sedimentary structures and bed stratification toward the top suggests the presence of turbidity current deposits. The top of this coarse-grained package is not sharp; rather, there is a gradational fining and thinning of beds uphole, indicating a sediment supply response to transgression. Subunit VB
Below a bioturbated contact in Section 313-M0029A-189R-1, 15 cm (673.61 mbsf), silty clay contains glauconitic and quartz sand–filled burrows. The silty clay has rare benthic foraminifers and thin-walled shells, along with a trace of mica and plant debris. Silty clay continues through Section 313-M0029A-192R-2 (675.06 mbsf), where there are patches and beds of glauconite sandy mud and muddy sand with glauconite concentrated in burrows but with no observed quartz grains associated with the glauconite. Core 313-M0029A-193R-1 and 193R-2 both contain burrowed erosion surfaces. Below the surfaces, brown silt continues with dispersed very fine mica and traces of fine shell debris. Glauconite-filled burrows are concentrated in the top 16 cm of Section 313-M0029A-194R-2, where the glauconite is fine sand sized and occasionally forms larger aggregates. The base of the unit is placed at the base of Section 313-M0029A-196R-1, 93 cm (695.84 mbsf), below which there is gradational change to glauconitic sandy silt. InterpretationSilty clay is interpreted as a deep offshore environment. The absence of quartz grains with the glauconite may indicate that the glauconite is forming in situ. Alternatively, glauconite concentrations may represent submarine fan/apron deposits that were remobilized down the clinoform slope from source areas with no quartz. The two erosion surfaces in Core 313-M0029A-193R likely represent significant sequence stratigraphic surfaces. Subunit VC
Sections 313-M0029A-196R-1, 93 cm, to 199R-CC, 11 cm (695.84–707.17 mbsf), contain glauconitic sandy silt and silty glauconite sand and sandstone with 5% quartz, including floating quartz granules. Glauconite granules are pale green (immature), whereas the smaller glauconite grains are almost black (mature). The amount of quartz decreases down through the cores to 10% in Core 313-M0029A-199R. Interbeds of fine sand and silt are observed near the base of the subunit together with ripple-scale cross-lamination that could be spreiten and, as such, the result of burrowing rather than physical processes. Grain size abruptly decreases between Sections 313-M0029A-199R-2 and 200R-1 from quartzose fine sand above to siltstone below. InterpretationThe abrupt grain size increase at the base of Subunit VC from siltstone to sand is interpreted as a sequence boundary. The silty glauconite sand with quartz-rich intervals and granules in Cores 313-M0029A-199R through 197R is interpreted to be a series of debrites in an apron system seaward of the toe-of-clinoform slope. Subunit VD
Subunit VD is predominantly silt with varying amounts of glauconitic sand and rare ripple-scale cross-lamination. Glauconite is commonly concentrated in burrows together with rare shell fragments and benthic foraminifers. Granules of pale green and subangular glauconite are observed with fine sand grains of dark green glauconite. In interval 313-M0029A-205R-1, 79–82 cm (720.10–720.13 mbsf), a fine to medium glauconite and quartz sand forms a weakly normally graded bed. InterpretationThe silt is interpreted as a deep offshore deposit, whereas the glauconite patches and beds are interpreted to be an in situ accumulation. The base of the unit includes more sand grains and a normally graded sand bed, which indicates a component of downslope transport of clastic sediment via turbidity currents. Subunit VE
This succession begins in poorly sorted sand beds interbedded with sandy silt. Overall, the succession coarsens downhole to muddy medium sand with clay laminae and normal grading in discrete 2–5 cm thick beds. The coarse component is increasingly well sorted and well rounded downhole, and glauconite also increases downhole to an abrupt and bioturbated base in Section 313-M0029A-208R-1, 9 cm (728.55 mbsf). InterpretationThe abrupt bioturbated boundary in Section 313-M0029A-208R-1, 9 cm (728.55 mbsf) (Fig. F11), is interpreted as an unconformity, and the overlying coarse-grained glauconitic is interpreted as having been deposited from downslope sediment gravity flow. Discrete 2–5 cm thick graded sand beds separated by clay laminae are interpreted as turbidites. Unit VI
The Unit VI succession is a pale brown (tan colored) clayey silt with intercalated very fine and fine sand beds. There are several nodules in the upper part of this succession that fizz mildly with hydrochloric acid. The nodules commonly have deformed laminae above and below, with truncated laminae above. This interval has alternating units of interlaminated and bioturbated clayey silt with sand. In the interlaminated units, sharp-based sand beds are commonly rich in mica and plant debris and are normal and/or inverse graded. Parallel lamination, along with rare cross-ripple lamination, is observed in sand beds. There are common Taenidium burrows and also Chondrites, Planolites, and Teichichnus. A change in color around Section 313-M0029A-212R-2, 119 cm (744.76 mbsf), marks a transition from pale clayey silt to dark brown siltstone with rare shell fragments, trace plant debris and mica, a loss of lamination/bedding, and a decrease in clay content. Core recovery is poor at the base of the unit, although a very fine sandstone is found in Core 313-M0029A-214R. InterpretationThe base of the unit is taken to be below the glauconitic very fine sandstone recovered in Core 313-M0029A-214R. The overlying succession fines and becomes paler uphole into an interlaminated unit interpreted as river-dominated offshore (prodelta) deposits with sediment gravity flow deposits, possibly flood events. Deformed laminae around the nodule demonstrate precompactional carbonate precipitation, and truncation of deformed laminae above the nodules indicate that nodule growth took place just below the water/sediment interface. A more glauconitic unit is identified on well logs (K/Th ratio), and with the rock pieces recovered in Core 313-M0029A-214R, an unconformity may be inferred at 745.90 mbsf. Unit VII
Section 313-M0029A-214R-1, 52 cm, through Core 217R (747.27–756.33 mbsf) is siltstone with glauconite sand grains, thin-walled articulated shells, trace mica, and plant debris. InterpretationThe succession is interpreted as a low-energy deep offshore environment. ConclusionsMiocene clinoforms in Hole M0029A were cored in a more seaward position than in Holes M0028A and M0027A. Sedimentary packages summarized here correspond well with sequences resolved by seismic stratigraphy (see "Stratigraphic correlation"). Unit VI shows evidence for development of open-shelf, shallow-water dysoxia in the form of weak or absent bioturbation, millimeter-laminated bedsets, and pyrite precipitation. We developed a facies model for deepwater toe-of-slope sequences from observations from Unit V to Subunit IID. Above a basal surface interpreted as a sequence boundary, sediment gravity flow deposits represent lowstand. The section fines uphole during transgression and highstand to offshore bioturbated silt. The majority of the section consists of silt deposited in offshore environments, including in situ glauconite. These fine-grained sediments are fossiliferous. Coarse-grained intraslope units within the silt-prone Unit II are either turbidite lobes or channel fills. Spot coring may have identified Pleistocene sequences. Smear slidesAnalysis of 56 smear slides from Hole M0029A shows variations and general trends in the relative abundance of sand, silt, and clay (see "Site M0029 smear slides" in "Core descriptions"). Sand, silt, and clay composition curves broadly correspond to major lithostratigraphic units (Fig. F12). Silty sediments dominate; therefore, siliceous and calcareous microfossils such as diatoms, silicoflagellates, sponge spicules, and nannoplankton are common in Hole M0029A (absent in Subunit IE and Unit VI). Photomicrographs of typical smear slides are shown in Figures F13, F14, F15, and F16. |
|||
