PRINCIPAL RESULTS
Site 1132
Site 1132 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, 3, 4, and 6A). The principal objectives were to (1) recover a detailed record of siliciclastic progradation and aggradation to evaluate the complex interaction between 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 the diagenetic history and processes within the Neogene facies in a shelf-edge setting.
The recovered succession was subdivided into six lithostratigraphic units. Unit I (0—113.5 mbsf) consists of bryozoan floatstone and rudstone, alternating with bryozoan packstone and, in the lower part, also bioclastic wackestone with bryozoans. The abundant bryozoan fauna are highly diverse and include a great variety of growth forms. The sediments are unlithified and burrow mottled, and the color is dominantly light gray with thinner pale olive and white intervals. This unit represents a major bryozoan mound complex. Unit II (113.5—158.3 mbsf) consists of uniform bioclastic packstone with a great diversity of bryozoan growth forms. The color is light gray, with minor light olive gray and white intervals. The sediment is burrow mottled and mainly unlithified, but thin, partially lithified beds are present at 130—140 mbsf. Unit III (158.3—250.7 mbsf) consists of unlithified bioclastic packstone and minor wackestone and grainstone, with an interbedded thin package of foraminiferal ooze and chalk. The unit is strongly burrowed and mainly unlithified down to 168 mbsf and partially lithified below that level. The color is dominantly light olive gray with thinner olive, pale olive, and white intervals. Several prominent firmgrounds are the basis for subdivision into five subunits. Unit IV (255.8—437.33 mbsf) was poorly recovered; however, available data suggest that it consists of bioclastic packstone and grainstone with an interval of nannofossil foraminiferal chalk partially replaced by nodular, light to dark gray chert. Unit V (441.5—517.7 mbsf) is also characterized by poor recovery, but recovered material indicates that it consists of bioclastic packstone and grainstone and lacks chert. Glauconite is abundant, and the colors vary between very pale brown and pale yellow. Unit VI (517.7—555.95 mbsf) is topped by a prominent mineralized hardground and consists of lithified bioclastic packstone and wackestone. It differs from overlying units in its colors, prominent firmgrounds and hardgrounds, solution seams, and centimeter-sized bioclasts. The color is pale yellow at the top and passes downward into light gray, pale yellow, red yellow, to pale and dark red at the bottom. The lowest recovered bed contains pebbles of siliciclastic sandstone, rich in lithic fragments.
Calcareous nannofossils and planktonic foraminifers indicate that drilling at Site 1132 recovered a thick Pleistocene—Eocene sequence (~550 m thick), overlying a relatively thin, barren section (~45 m). Calcareous nannofossils from the basal Pleistocene registered a nannofossil event, the "Braarudosphaera Event," which previously was recorded at Sites 1127, 1130, and 1131, indicating a dramatic short-lived change in surface-water conditions over a large geographic area. The thick Pleistocene section contains (~230 m) Zones NN21/NN20 and NN19, and the equally thick middle Miocene section (~300 m) contains Zones NN6 and NN5/NN4. These sections are separated by a thin interval (~20 m) with poor core recovery. Hiatuses are likely within this thin interval, where the recovered nannofossils and planktonic foraminifers suggest late Miocene age. Another similarly thin interval (~20 m) with poor core recovery, which is also likely to contain hiatuses, is recorded between the middle Miocene section and the underlying mainly lower Oligocene. An Eocene age is indicated for the section underlying the lower Oligocene, based on thin-section analysis of two samples at ~530 and ~547 mbsf, respectively. A sharp change in sedimentation rates corresponds to the condensed interval between Pleistocene and middle Miocene sections. The Pleistocene section registered an average sedimentation rate of 175 m/m.y., whereas the middle Miocene showed an average rate of 20 m/m.y. Another condensed section between the middle Miocene and Oligocene successions is indicated by the sparse biostratigraphic datums through this interval. Four main benthic foraminifer assemblages are identified. These indicate upper to middle bathyal paleodepth for the Pleistocene—Oligocene section. In the Pleistocene, a striking, well-preserved assemblage characterized by many large (>1 mm) agglutinated forms is found within bryozoan-rich accumulations. This assemblage probably reflects a diverse, highly dynamic ecosystem that became established at the seafloor at various times during the Pleistocene, corresponding to episodes of bryozoan mound growth. Changes in the composition of this assemblage may relate to sea-level or circulation fluctuations.
The Brunhes/Matuyama boundary was found at ~180 mbsf, yielding a sedimentation rate of ~230 m/m.y. There were indications of the same intensity fluctuations observed at earlier sites; therefore, there is a possibility of high-resolution stratigraphy within the Brunhes Chron after further shore-based work. The top of the Jaramillo Subchron was found at 230 mbsf, which is consistent with this sedimentation rate. Magnetic anomalies were found associated with hardgrounds with mineralized crusts, and with lithified skeletal limestones. In some cases, this suggests that diagenesis occurred extremely early, within 100,000 yr of sedimentation.
As at Sites 1126, 1128, and 1130, only low concentrations of methane were detected at Site 1132. Of the four sites, Site 1132 has the highest methane content, with a maximum value of 54 ppm. Unlike the other low-methane sites, hydrogen sulfide is present at Site 1132 in low concentrations. Calcium carbonate content is between 85 and 95 wt%, with values at the higher end of the range near the surface and declining gradually toward the lower end of the range with depth. Organic carbon values are primarily in the range of 0.3—0.6 wt%. Nitrogen concentrations are all less than 0.1 wt%, and sulfur is present, at low concentrations, in only a few samples in the upper 90 mbsf.
Site 1132, similar to the previous sites except 1128, is influenced by the presence of high salinity fluids. However, unlike the neighboring Site 1130, the rate of salinity increase with depth is uniform and shows no evidence of nonsteady-state conditions. Site 1132 possibly had an initially higher organic carbon content, producing slightly higher sulfate reduction rates and a more extended sulfate reduction zone. In turn, carbonate diagenesis is more active, causing carbonate precipitation in the upper part of the profile, whereas the lower part is characterized by carbonate dissolution. As with Site 1131, Site 1132 displays a sodium/chloride anomaly in the upper part of the profile. Site 1132 is characterized by a constant seawater composition of both conservative and nonconservative interstitial water constituents in the upper 30 mbsf, suggesting active flushing with seawater. With the completion of the slope transect from Site 1130 to 1132, we can draw initial conclusions about the horizontal distribution of brine in this area. It appears that salinity values at Sites 1130 and 1132 stabilize at about the same depth below the sea surface, suggesting a horizontal top of the main brine body ~520 meters below sea level (mbsl).
High-quality NGR and gamma-ray attenuation porosity evaluator (GRAPE) bulk density data were obtained from the MST. Variations of NGR and GRAPE bulk density, supplemented by index properties measurements of porosity and thermal conductivity, form the basis for the five PP units recognized at Site 1132. Unit 1 (0—5 mbsf) is characterized by very low (<5 cps) NGR and a high initial bulk density (>1.9 g/cm3), which declines rapidly with depth. This unit corresponds to a thin package of grainstones that caps lithostratigraphic Unit 1. Unit 2 (5—140 mbsf) has an overall increase in bulk density with depth (1.65—1.90 g/cm3). There is a corresponding decrease in porosity from 55% to 43%, but with considerable variation, probably associated with lithological variability. High-amplitude cyclic variation in both NGR (>20 cps) and bulk density (0.2 g/cm3) is also observed throughout this unit. Unit 3 (140—248 mbsf) is characterized by a decline with depth in GRAPE bulk density and porosity and by a continued but more regular cyclicity in both NGR and GRAPE bulk density. Unit 4 (~248—520 mbsf) is recognized solely on the basis of a change in NGR from 25 to <5 cps at 248 mbsf. Similar characteristics are present in limited core recovery from 516 to 520 mbsf, with cherts having the anticipated high values of P-wave velocity. Unit 5 (>520 mbsf) is poorly characterized because of limited recovery and drilling disturbance. NGR shows a shift back to higher values (10—20 cps), but there are considerable fluctuations associated with the much greater variety of lithologies present.
Two logging runs were attempted at Site 1132. The triple combo successfully logged 560 m, whereas hole conditions limited the FMS/Sonic tool to only 70 m. Logging data closely reflect lithologic variations observed in the upper 245 mbsf of the section. Below this depth, downhole logs enable the characterization of sedimentary sequences in low recovery intervals and assist in intersite correlations. Logging data were subdivided into four units on the basis of variations in the collected datasets. Unit 1 (0—242 mbsf) is characterized by an increase in the magnitude and variability of gamma-ray values that are mainly the result of increased uranium concentrations. High and variable uranium contents continue for the remainder of the unit, with an abrupt decrease at the boundary with Unit 2. In the open-hole logged interval below 104 mbsf, separation of the porosity and density curves indicate a somewhat higher noncarbonate fraction than is present in the remainder of the hole. This noncarbonate fraction is only slightly enriched in K and Th. The base of Unit 1 correlates well with the base of lithostratigraphic Unit III. Unit 2 (242—437 mbsf) is characterized by a return to low gamma-ray values and increased variability in the magnitude of density and porosity variations, possibly caused by chert layers. In addition, resistivity logs from Unit 2 indicate that drilling fluid invasion occurred within this partially lithified packstone interval, suggesting higher porosity than in surrounding units. In the lower part of Unit 2, both porosity and density values increase, as does the degree of fluid invasion. The base of Unit 2 correlates well with the base of lithostratigraphic Unit IV, and is characterized by increases in resistivity and decreases in gamma-ray values, porosity, and density. Unit 3 (437—524 mbsf) is characterized by nearly constant density and gamma-ray values and variable porosity. Resistivity logs show low variability and indicate that drilling fluid invasion occurred in this interval, although to a lesser extent than in the units above and below. Unit 3 correlates well with lithostratigraphic Unit V, which, despite low recovery, appears to be dominated by a homogenous interval of lithified bioclastic grainstones. The upper boundary of Unit 4 (524—537 mbsf) is marked by a sharp increase in all parameters. Within the limited section logged, resistivity measurements show increased drilling fluid invasion. The gamma-ray increase in Unit 4 is dominated by increased Th and K, indicating the presence of terrigenous minerals within the carbonate sediments. Unit 4 correlates with lithostratigraphic Unit VI.
Regional seismic stratigraphic correlation shows that the Pleistocene bryozoan mound complex, represented on seismic imagery by a distinctive mounded facies, occurs in a narrow band immediately below the shelf edge along much of the western Great Australian Bight. Lithostratigraphic data indicate that bryozoans are also important constituents of underlying nonmounded packstone and grainstone facies corresponding to the lower parts of seismic Sequence 2. Seismic Sequences 3 (middle-late Miocene) and 4 (early Oligocene—middle Miocene) are characterized by bioclastic grainstones, packstones, and wackestones, with minor foraminiferal ooze. Increased amplitudes toward the base of Sequence 3 probably represent increased impedance contrast associated with interbedded silicified horizons, and disconformity surfaces visible on the high-resolution seismic within both sequences appear to correlate with firmgrounds and hardgrounds. Lithostratigraphic and biostratigraphic data show that the small mounds immediately overlying the distinctive Sequence 7 progradational wedge are composed of lithified bioclastic packstone and wackestone of Eocene age. The only samples recovered from Sequence 7 are carbonate-cemented sandstone fragments forming the uppermost sequence boundary, with the underlying friable sandstones being too poorly cemented for recovery.