Site 1131

Site 1131 was the second and intermediate site of a planned three-site transect through a spectacular set of late Neogene clinoforms immediately seaward of the present-day shelf edge. Site 1131, located on the upper slope in 333.6 m of water, intersected the best record of the middle part of the clinoform sequence. The principal objective of this transect was to collect detailed, high resolution profiles through a late Neogene shelf-edge (high energy) to upper slope (low energy) succession deposited within a cool-water carbonate environment and to determine the response of this type of depositional system to Pliocene–Pleistocene sea-level fluctuations.

Three major lithostratigraphic units were identified at Site 1131. Unit I (0–25.0 mbsf) consists dominantly of bioclastic packstone, floatstone, and rudstone. The upper part of Unit I is composed of a massive, homogeneous, light gray to olive gray, unlithified, bioclastic packstone, which includes a significant bryozoan component. The lower part of Unit I is a heterogeneous interval of unlithified bryozoan floatstone and rudstone punctuated by thin (decimeter scale) layers of wackestone, packstone, and grainstone. Unit II (25.0–531.7 mbsf) consists of a very thick (>500 m), homogeneous succession of light gray to olive gray, bioclastic packstone, grainstone, and wackestone. In general, lithification and diversity of the bioclastic component increase downhole, and large miliolid foraminifers appear in the lower part of Unit II. Unit III (531.7–607.4 mbsf) is separated from Unit II by an unconformity representing a major hiatus. Below the unconformity, recovery was incomplete in a succession consisting of dark gray, silicified nannofossil ooze beds or lenses within olive gray, partially to strongly lithified bioclastic grainstone containing blackened grains and glauconite.

Two biostratigraphic successions were identified at Site 1131: (1) an expanded Quaternary interval more than 510 m thick, underlain by a thin and conformable Pliocene (?) interval, and (2) a middle and lower Miocene section lacking hiatuses within the resolution of available biostratigraphic data. These successions are divided by a disconformity at 532 mbsf that spans ~10 m.y. An environmental crisis produced an unusual nannofossil assemblage dominated by Braarudosphaera bigelowii near the base of NN19 at ~522 mbsf. A similar event was observed in the same stratigraphic interval at Site 1127. Carbonate microfossils, especially foraminifers, were strongly affected by overgrowths, cementation, and recrystallization below 60–100 mbsf. These effects are consistent with the pore-water geochemistry and the onset of lithification. One main Pleistocene upper bathyal assemblage of benthic foraminifers is recognized above 100 mbsf. This assemblage also contains a large proportion of small neritic tests (63–150 Ám), probably redeposited from the shelf. Poor preservation prevented detailed faunal analysis below 100 mbsf.

Magnetic measurements of archive-half cores established a magnetostratigraphy to a depth of 320 mbsf, which includes the Brunhes and uppermost Matuyama Chrons. Below this, drilling disturbance disrupted the record and no further magnetostratigraphic data were obtained. Rock magnetic properties show minimum downhole variability and are consistent with single-domain biogenic magnetite, except for cores at intermediate depths (70–150 mbsf) that display the typical behavior of ultrafine-grained (superparamagnetic) greigite. Magnetic susceptibility is dominated by contributions from the diamagnetic carbonate matrix.

Composite section construction from Holes 1131A and 1131B indicates that a continuous record was not recovered. Overlap between cores in adjacent holes was maintained to ~55 mcd, until poor recovery in Core 182-1131C-6H resulted in a short (few centimeters) data gap. Below 70 mbsf, low recovery from extended core barrel (XCB) cores resulted in reduced core overlap and prohibited splicing below 78 mcd. The primary parameters used for correlation were natural gamma-ray emissions, GRA bulk density, and color reflectance. Comparison of data between holes revealed a low degree of similarity, making correlations difficult.

The most striking organic geochemical results from Site 1131 were the high concentrations of methane (C1) and hydrogen sulfide in the upper part of the section. In comparison to Site 1127, gas pockets were less abundant, and the concentration of C1 in both gas pockets and headspace samples at Site 1131 was lower. Hydrogen sulfide concentrations, however, were comparable at both sites. Atypically, C1/C2 values first increase and then decrease with increasing depth in Hole 1131A, essentially mirroring the methane profile as C2 varies little throughout the section. Calcium carbonate content is uniformly high (86–94 wt%). Organic carbon is less than 0.4 wt% down to 125 mbsf, with higher values (up to 1.0 wt%) at greater depths.

Site 1131 exhibited similar inorganic geochemical features as Site 1127, with the exception of more extreme changes in pore-water geochemistry profiles because of increased oxidation of organic material. For example, alkalinity reached a maximum value of 137 mM, compared to 106 mM at Site 1127. In contrast to Site 1127, although there was significant depletion in the concentration of sulfate, measurable concentrations remained even in the zones of highest alkalinity, suggesting that organic material was the limiting factor in the production of hydrogen sulfide at Site 1131. Excess sulfate in the upper portion of the core suggests that the oxidation of hydrogen sulfide originated lower in the section. In the upper 300 m, there was depletion in the concentration of calcium and magnesium and enrichment in Sr, indicating the recrystallization of aragonite and HMC. This was confirmed by mineralogical analyses, which showed that HMC had disappeared by 50 mbsf. Aragonite gradually decreased with increasing depth, and vanished at the Pliocene/Pleistocene boundary. Ratios of sodium to chloride were in excess of the ratio in seawater throughout the Pleistocene section, indicating that the waters responsible for the high-salinity fluids had participated in the precipitation and dissolution of halite.

Physical properties measurements at Site 1131 correlate well with lithologic changes observed in the sedimentary section and provide the basic data for core-log correlation to reconstruct poorly recovered intervals. Correlation of downhole gamma-ray logs with NGR data from the multisensor track (MST) indicates that moderate to poor recovery at Site 1131 may have aliased and concealed in cores the cyclic fluctuations that are clearly visible on downhole logs. Physical properties data indicate that the cored interval is divisible into two units. Unit 1 (0–31 mbsf) is mainly defined by a large increase in NGR toward the base of the unit, corresponding to the lower limit of the bryozoan floatstone/rudstone comprising lithostratigraphic Subunit IA. This NGR peak is also observed on downhole logs, with spectral gamma data indicating that it reflects uranium content and, accordingly, is either the result of increased organic matter content or carbonate diagenesis. P-wave velocity and bulk density also increase toward the base of this unit. Unit 2 (31–532 mbsf) is characterized by gradual increases in porosity, P-wave velocity, and bulk density, primarily reflecting increased compaction, but also showing data excursions corresponding to coarser and more lithified horizons. More indurated layers within the generally homogenous sediments of Unit 2 have increased velocities and bulk densities, and generally lower NGR. Below 532 mbsf, core recovery was too low for physical properties characterization of the deeper part of the sedimentary section.

Three successful logging-tool strings were deployed in Hole 1131A. Downhole logging data indicate that the uppermost part of the cored section (0–28 mbsf) correlates with bryozoan rich horizons described in core. As was noted with the physical properties data, a uranium peak at the base of this unit reflects increased organic content and/or carbonate diagenesis and may indicate a firmground. The remainder of the cored section to 575 mbsf contains porosity-density crossovers that coincide with low PEF values and high resistivity and sonic values. These crossovers are interpreted as probable chert layers. In contrast, low-porosity peaks associated with density, PEF, sonic, and resistivity peaks may represent possible firmgrounds.

Integration of coring results with high-resolution site survey seismic data at Site 1131 confirms that the mounded features visible in the uppermost part of the seismic section are in fact bryozoan-dominated mounds. The spectacular clinoform geometry corresponding to the remainder of the cored interval consists of bioclastic packstone, grainstone, and wackestone, with variable lithification as the dominant factor controlling seismic data amplitude and impedance variability. The lithostratigraphic and biostratigraphic boundary toward the base of the section coincides with a major regional unconformity and sequence boundary.


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