The succession drilled and sampled at Site 1071 is divisible into four unconformity-bounded sequences of late-middle Miocene to Pleistocene age (Fig. 3). The sequence boundaries are characterized in seismic reflection data by offlap and onlap; the sequences are arranged in a forestepping (overall progradational) pattern. Before drilling, the boundaries were informally designated, from youngest to oldest, as pp3(s), pp4(s), m0.5(s), and m1(s), where "pp" refers to Pliocene-Pleistocene and "m" to Miocene. The "s" has been added to indicate interpretation of the boundaries on the shelf and permit fine-tuning of correlations with surfaces identified as possible sequence boundaries on the continental slope (Leg 150). Each of these surfaces has been verified as a sequence boundary. A fifth reflection, pp5(s), tentatively interpreted as a sequence boundary before drilling, is associated with upward fining of sediments and is reinterpreted to be caused by an increase in paleowater depth; it is clearly of late Miocene age, which is older than initially assumed.
The succession consists largely of sands, silts, and clays, with recovery predominantly from muddy intervals. The presence of sands in poorly recovered or unrecovered intervals is inferred by comparison with the gamma-ray log for the Continental Offshore Stratigraphic Test (COST) B2 well, which was drilled ~950 m to the southwest of Holes 1071A-1071E (a direction approximately parallel to depositional strike). These sediments have been divided into three lithostratigraphic units, primarily on the basis of abrupt changes in the vertical arrangement of lithofacies within the succession (Fig. 4). Unit I (0-134.4 mbsf) is significantly less glauconitic overall than Unit II (143.5-261.90 mbsf), but sediments at the bases of both units are particularly glauconitic. Unit II is also characterized by significantly less calcite than Unit I. The recovered succession from Unit III (261.9-424.2 mbsf) is predominately silty and muddy sands. The base of Unit I corresponds with sequence boundary pp4(s), the base of Unit II is thought to lie no more than a few meters above surface m0.5(s), and the base of Subunit III lies at or slightly below surface m1(s). The physical sedimentology, extensive bioturbation in fine grained sediments, scattered shell fragments, benthic foraminifers, and local abundance of glauconite all suggest that the succession is primarily shallow marine, with cyclicity at several scales related to presumed transgression and regression of the shoreline. An unexpected pattern was observed for a location close to the modern shelf edge; the preserved parts of sequences below both pp3(s) and pp4(s) are predominantly transgressive overall, consistent with pronounced bypassing and erosion at both of these surfaces. Regional seismic data crossing the shelf near the site indicate that transgressive deposits thin seaward across their respective clinoform rollovers/breakpoints, beneath seaward-thickening and offlapping highstand units (regressive). Increasing amounts of offlap of successive sequence boundaries are consistent with increasing amplitudes of glacial-eustatic change from the late Miocene onward (11-0 Ma).
Biostratigraphic resolution at Site 1071 is generally low for calcareous microfossils as a result of strong carbonate dissolution and shallow-water conditions that were unfavorable to these planktonic organisms. Nannofossils provide relatively useful zonations for the late Pleistocene, late Pliocene, and the early Pliocene-late Miocene. Utility of planktonic foraminifers for biostratigraphic zonation is restricted to the earliest Pliocene-middle Miocene. Pleistocene samples examined from Site 1071 yield benthic foraminiferal faunas that vary from inner neritic (0-50 m) assemblages, dominated almost exclusively by Elphidium excavatum, to more diverse upper middle neritic (~50-65 m) assemblages. These variations may reflect paleodepth fluctuations, substrate (finer grained vs. sandier sediments), and/or depositional systems. Miocene benthic foraminiferal assemblages indicate middle neritic paleodepths (50-100 m), possibly with fluctuations within this depth zone. Miocene biofacies changes may reflect paleoenvironmental and paleobathymetric changes that occur within a sequence stratigraphic framework. Organic microfossils also constrain the biostratigraphic framework at this site. Pollen is useful for dating middle to late Pleistocene sediments; dinocysts provide biostratigraphic zonation of the early Pleistocene to late-middle Miocene. Dinocysts suggest rapid sedimentation during the middle through late Pleistocene, the late-late Miocene, and the early-late Miocene. In contrast, hiatuses were identified in the Pliocene through early Pleistocene, middle late Miocene, and late middle Miocene, suggesting thathiatuses during these intervals correspond to stratigraphic surfaces pp4(s), m0.5(s), and m1(s). Site 1071 is not optimally located for dating these surfaces, but approximate ages were obtained as follows at Site 1071 (Fig. 5): pp3(s), late Pleistocene (younger than 0.78 Ma); pp4(s), early Pleistocene possibly latest Miocene (1.1-7.4 Ma); pp5(s)(?), latest Miocene (5.9-7.4 Ma); m0.5(s), late Miocene (7.4-11.2 Ma); and m1(s), late middle Miocene (older than 11.4 Ma).
Magnetic polarity changes downhole from normal to reverse at 61.4 mbsf in Hole 1071B and at 61.5 mbsf in Hole 1071C were confirmed by analyses of discrete-cube samples. Combined with the analysis of nannofossils, this reversal is considered to be the Brunhes/Matuyama (B/M) boundary (0.78 Ma). Magnetic polarity returned to normal at Cores 1071B-6X and 1071C-3X; however, the B/M boundary was not determined because of low recovery in these intervals. A reversed polarity chron was not found downhole. However, this does not necessarily mean that the polarity is normal, but that remagnetization occurred during the Brunhes Chron. Magnetization intensity after 20-mT AF demagnetization drops from 10 to 100 mA/m above the B/M boundary to about 1 mA/m below the boundary. Magnetization intensity increases to more than 10 mA/m below 68 mbsf, fluctuating downhole, and then decreases to less than 1 mA/m below 200 mbsf. This low magnetization intensity may be related to the dissolution of magnetite and the simultaneous formation of pyrite.
Routine squeezing of whole-round sediment samples showed complex and nonsteady-state interstitial water profiles. Pore waters are significantly fresher than seawater in two intervals, with minima at 30 mbsf and between 261 and 321 mbsf (Cl- to 430 mM, a 23% decrease, and 500 mM, a 11% decrease, respectively; Fig. 6). These salinity minima likely reflect input of fresh (or brackish) water during late Pliocene-Pleistocene drops in sea level and resultant subaerial exposure of the shelf. Pore waters show an abrupt rise in alkalinity, ammonia, and phosphate below 150 mbsf, suggesting that substantial organic-matter diagenesis is occurring in this interval. Operations were highlighted by a spectacular fountain of water that flowed at pressure from the drill pipe upon reaching a depth of ~250 mbsf. The chemistry of fountain water is identical to that of surface seawater and significantly different from that of formation water. This strongly indicates that the fountain was caused by drilling procedures rather than direct drilling of a pressurized aquifer. A plausible explanation is that porous sand intervals were charged with pressurized water when unstable hole conditions caused cave-ins around shallower sections of the drill string.
Hydrocarbon gases were monitored by headspace sampling for every core. Concentrations of C1 were below 9 ppmv, whereas those for C2 through C6 were at or below detection limits. The lack of hydrocarbon gas and the presence of interstitial-water sulfate in cores at depth suggest that significant bacterial gas is not being generated in shallow sediment. Total organic carbon (TOC) increases downhole, averaging 0.3 wt% from 0 to 135 mbsf and 0.65 wt% from 135 mbsf to the base of the sampled section. Organic-matter degradation and siderite precipitation in Units II and III may be related to higher sediment TOC.
Physical properties of primary interest are density and P-wave velocity, which will aid in
traveltime-depth conversion as well as production of synthetic seismograms for linking coring
results to seismic reflection data in both regional and site-specific profiles and logs. Density
measurements were acquired at a variety of resolutions. Wet bulk density data in Holes 1071A
1071C show a gradual overall increase consistent with the effects of compaction; the greatest
downhole increase in density appears in the upper 10 mbsf of Hole 1071A. Several smaller
scale trends in density are approximately coincident with inferred lithologic boundaries.
Although the P-wave logger (PWL) on the multisensor track (MST) was turned off after the
first few cores because of incompletely filled liners, discrete P-wave velocity measurements,
primarily transverse to the core axis, were made with the same frequency as index properties.
Velocities are generally between 1600 and 1800 m/s. Intervals of uniform, or gradually varying,
velocity are separated by abrupt changes at lithologic boundaries. The lithified sandstone at the
base of Hole 1071C has a P-wave velocity in excess of 5000 m/s. It appears to be a thin layer,
however, and its potential to generate a strong seismic reflection remains uncertain. In Hole
1071F, between ~359 and ~370 mbsf, velocities increase downhole from 1800 to 1900 m/s,
and below 370 mbsf, velocities decrease downhole to 1700-1800 m/s. This velocity change at
~370 mbsf corresponds to the boundary between lithostratigraphic Subunits IIIA and IIIB and
lies above the inferred position of sequence boundary m1(s). Natural gamma measurements
were made on the MST at intervals of 20 cm. Coupled with discrete resistivity measurements
(one to two per section), these will assist with future core-log correlations.
Logging-while-drilling data were acquired at Hole 1071G to a depth of only 88 mbsf, when unstable hole conditions prevented further penetration. However, log data acquired at the COST B2, 950 m SSW of Holes 1071A-1071E, provided valuable indications of unrecovered lithologies and depths to key horizons imaged in site-survey profiles.
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