PRINCIPAL RESULTS
Site 1130
Site 1130 was located to intersect and characterize Neogene cool-water carbonate shelf edge sequences and the nearshore portion of a Paleocene?–middle Eocene progradational siliciclastic wedge (seismic Sequences 2, 4, and 6A). The principal objectives were to (1) recover a detailed record of siliciclastic progradation and aggradation to evaluate the complex interaction among Paleogene sea-level fluctuations, accommodation space, and subsidence, (2) determine the facies characteristics, sea-level response, and paleoceanographic history of a Neogene cool-water carbonate succession in a shelf-edge setting, and (3) evaluate diagenetic history and processes within the Neogene facies in a shelf-edge setting.
The sediments recovered at Site 1130 were divided into four major lithostratigraphic units. Unit I (0–261.43 mbsf) is a light gray to pale olive, strongly bioturbated, unlithified to partially lithified bioclastic packstone. This succession is punctuated by unlithified bioclastic wackestone layers and occasional nannofossil foraminiferal ooze to chalk intervals. The upper part of this unit is characterized by a repetitive uphole textural change from matrix-supported to grain-supported sediment. Lighter intervals in the otherwise monotonous packstones are fine grained and foraminifer rich, whereas the generally coarser and darker intervals contain more bioclasts. The lower part of Unit I is typified by gradual color changes ranging from light gray to olive gray, with a few abrupt transitions. Soft sediment deformation structures interpreted as slumps are present at the base of Unit I. Unit II (261.43–328.86 mbsf) is dominated by strongly bioturbated, white to light gray nannofossil foraminiferal chalk with a wackestone to packstone texture. Disseminated black grains (pyrite, glauconite, and other unidentifiable grains) are scattered throughout the unit. Unit III (328.86–369.5 mbsf) consists of chert layers (silicified nannofossil planktonic foraminiferal ooze) interbedded with intervals of white nannofossil planktonic foraminiferal ooze. The light to dark gray, partially translucent chert fragments contain burrows filled with white, fine grained, lithified planktonic foraminiferal packstone to grainstone. Unit IV (369.5–386.51 mbsf) is dominantly a calcareous sandstone, but with compositional and textural variability encompassing red bioclastic glauconitic wackestone, sandy bryozoan grainstone, pink, coarse-grained bivalve grainstone, and red, very coarse-grained calcareous sandstone.
Calcareous nannofossils and planktonic foraminifers indicate that three biostratigraphic successions were recovered at Site 1130: (1) Pleistocene–upper Miocene (0–327 mbsf), (2) upper–middle Oligocene (327–~355 mbsf), and (3) a sandy limestone of unknown age from the bottom of Holes 1130A and 1130C. Preliminary results indicate that part of the Pliocene section is either missing or highly condensed and that the major disconformity between upper Miocene and Oligocene sediments represents a hiatus of at least 15 m.y. The sedimentation rate for the middle–upper Oligocene was 10–30 m/m.y., although this should be viewed with caution because of poor core recovery. The rate fluctuated between 15 and 30 m/m.y. during the Pliocene–Miocene, and reached as high as 240–260 m/m.y. during most of the Pleistocene. The three benthic foraminiferal assemblages recognized correspond respectively to the Oligocene, Pliocene–Miocene, and Pleistocene and indicate a shallowing-upward trend during these time periods. The Oligocene and Pliocene–Miocene assemblages are typically cosmopolitan, middle bathyal assemblages, whereas the Pleistocene contains a mixed assemblage representing upper bathyal paleodepths and containing many well-sorted specimens from shelf environments.
Long-core measurements revealed a deep episode of normal magnetization and then a sharp reversal to normal polarity at 199.9 mbsf, interpreted as the Brunhes/Matuyama boundary. Intensities of magnetization varied from 0.01 to 1 mA/M, with an overall decreasing trend downhole. Within this general trend are fluctuations with dominant wavelengths of a few tens of meters. The ratio of ARM/IRM, a measure of the relative importance of single-domain to multidomain behavior, decreases downcore, suggesting that the role of biogenic magnetite diminishes. Coupled with the decrease in magnetization, this strongly suggests that fine-grained magnetite of biogenic origin is preferentially dissolved.
An experimental nonmagnetic cutting shoe was used for alternate cores on Cores 182 1130A-3H to 7H, and an entire nonmagnetic core-barrel assembly was used on Cores 182-1130B 3H to 7H. Both the nonmagnetic shoe and core-barrel assembly produced reductions in radial component contamination, but the effects were not as dramatic as at Site 1128.
Construction of the composite section from Holes 1130A and 1130B indicates that a continuous sedimentary record was not recovered at Site 1130. Low recovery in Core 182-1130A 3H and no recovery in Core 182-1130B-3H produced a gap at 22 24 mbsf. The record below this is apparently continuous to ~191 mcd (all Pleistocene), at which point core overlap becomes minimal to nonexistent, disrupting continuity of the spliced section. The primary lithologic parameters used for correlation at Site 1130 were natural gamma emissions and a ratio of the 700:400 nm color reflectance data.
Only low concentrations of methane were detected, with most samples having less than 7 ppm (maximum = 11.3 ppm). Calcium carbonate content ranges from 80.9 to 93.2 wt% in lithostratigraphic Units I and II, with most samples varying between 85 and 90 wt%. No carbonate analyses were conducted in Units III and IV (328.9–386.51 mbsf) because of low core recovery.
Site 1130 is influenced by high-salinity pore fluids, as was the case at Sites 1126 and 1127. The maximum rate of salinity increase, 5.7/m, is the highest observed during Leg 182. The steep gradient down to ~32 mbsf, and then the constant salinity concentration (83) below, suggest nonsteady-state conditions. In contrast to Site 1127, the sulfate reduction zone is incomplete and confined to the upper part of the profile, and the degree of sulfate reduction is ~40% less than at Site 1127. These differences are probably caused by a lower initial organic carbon content at Site 1130. As a consequence of the decreased sulfate reduction activity, interstitial waters are saturated with respect to strontium sulfate, limiting pore-water strontium concentrations.
Sediment physical properties data closely reflect Oligocene–Holocene lithologic variations observed in recovered sediments and downhole logging data. Three physical properties units were recognized primarily on the basis of trends in NGR. The homogenous bioclastic packstones of Unit 1 (0–43 mbsf) are characterized by low variability in all measured parameters. The large increase in NGR from 5 to 35 cps at the base of Unit 1 does not correlate to any lithologic change, but is roughly coincident with an increase in P-wave velocity variability, suggesting that it results from diagenesis. Unit 2 (43–254 mbsf) is dominated by cyclic variations in NGR that appear to be related to Milankovitch cyclicity on a 100-k.y. frequency from 43 to 175 mbsf and on a higher 41 k.y. frequency for the remainder of the unit. This change in frequency occurs near the Brunhes/Matuyama boundary and is clearly seen in downhole logging data from this site. From 199 to 254 mbsf in Unit 2, increased variability in P-wave velocity corresponds to a series of firmgrounds observed in the recovered sedimentary section. The upper boundary of Unit 3 (254–335 mbsf) correlates well with a distinct change in lithology (lithostratigraphic Unit II) from packstone/wackestone to nannofossil oozes and chalks. NGR and P-wave velocity show low variability within Unit 3, whereas bulk density shows a general increasing trend.
Three large-scale units were identified in downhole logging data from Hole 1130C, based on shifts in conventional and FMS logs, with boundaries at 258 and 329 mbsf. The interval above 260 mbsf is characterized by a near-constant background level of gamma radiation of 20 API, with individual spikes reaching 40–45 API. The boundary at 260 mbsf is defined at a sharp downhole decrease in the uranium gamma-ray log, and the boundary at 329 mbsf corresponds to the change from carbonate sediments above to siliciclastic deposits below.
Integration of the various datasets with the regional seismic stratigraphic interpretation provides a sequential record consisting of an Eocene? progradational wedge deposited at shelf water depths, overlain, after an indefinite hiatus, by deep-water late Oligocene oozes that have since been irregularly silicified. The record then contains another hiatus of ~20 m.y., before deposition of Miocene–Pliocene oozes that pass up into a packstone unit, as a continuation of a shallowing upward trend. A further short hiatus was then followed by rapid deposition of a thick sequence of Pleistocene wackestones and packstones at upper slope/shelf-edge depths.