167 Preliminary Report

Site 1011 (Proposed Site CAM-2A)

Site 1011 is the landward site of the Baja Transect, which crosses the California Current at about 30°N (Fig. 1). Site 1011 was drilled to study both surface-water properties and water-column structure for the upper Miocene to Quaternary interval. It was also drilled to sample a sedimentary section to acoustic basement to determine the nature of the basement and to gather information on the opening and subsidence of Animal Basin.

The sedimentary sequence at Site 1011 consists of an apparently continuous, 281.5-m-thick interval of Quaternary to upper Miocene sediments. They are divided into four lithologic units. Unit I (0.0-25.0 mbsf) consists of upper Quaternary (0.0-1.0 Ma) siliclastic sediments, predominantly clays and silt, with vitric ash layers and graded quartz sand beds. Unit II (25-204 mbsf) is characterized by an increase in calcium carbonate and consists of upper Miocene to Quaternary (1.0-7.9 Ma) interbedded silty clay and nannofossil ooze on a decimeter to meter scale. Volcanic ash layers occur above 118 mbsf and below 184 mbsf. Graded sand beds occur above 98 mbsf. This unit is divided into three subunits. Subunit IIA consists of interbedded nannofossil ooze and silty clay. Subunit IIB is defined by a marked decrease in calcareous nannofossil content and increase in silty clay. Subunit IIC reflects an increase in lithification and in calcareous nannofossils. In Unit III (204-262
mbsf) the dominant biogenic component changes from calcareous nannofossils to siliceous microfossils. Subunit IIIA consists of upper Miocene (7.9-9.2 Ma) interbedded clayey diatomites and nannofossil diatomites. Subunit IIIB is defined by a lithologic change to almost pure diatomites (9.4-9.5 Ma). Unit IV (262-276 mbsf) was poorly recovered and consists of indurated siltstone and sandstone. Basement was reached at 276 mbsf and recovered 1.89 m of fine-grained vesicular basalt.

Detailed comparisons between the magnetic susceptibility and GRAPE density records generated using the multisensor track (MST), and high-resolution color reflectance measured using the Oregon State University system, demonstrated complete recovery of the sedimentary sequence down to 150 mbsf.

The section includes an upper 132-m-thick sequence containing abundant planktonic foraminifers, calcareous nannofossils, few benthic foraminifers, and rare to absent diatoms and radiolarians from the late early Pliocene to the Quaternary (Fig. 3). This is underlain by a 48-m-thick sequence of late late Miocene to early Pliocene age characterized by rare and sporadic assemblages of planktonic foraminifers, few to abundant calcareous nannofossils, and generally uncommon diatoms and radiolarians. Below this interval, a 110-m-thick sequence of rapidly deposited diatom-rich sediments of late Miocene age was recovered. These sediments contain abundant diatoms and radiolarians. Calcareous nannofossils and planktonic foraminifers are rare or absent. Benthic foraminifers assemblages are less consistently present. The base of the sedimentary sequence is assigned to the middle late Miocene diatom Denticulopsis dimorpha Zone, indicating an age of approximately 9 Ma.

Dominant cold species of planktonic foraminifer, diatom, and radiolarian assemblages exhibit evidence of strong upwelling conditions during the late Miocene. Domination of temperate foraminiferal species in early late Pliocene through early Pleistocene assemblages, and rare occurrences of diatoms and radiolarians, indicate warm temperate to cool subtropical conditions with a weakening of the upwelling system. The late Pliocene to Quaternary planktonic foraminiferal assemblages indicate cooler conditions with major sea-surface temperature changes related to glacial-interglacial episodes. Fewer occurrences of subtropical forms suggest generally cooler conditions than during the early and early late Pliocene.

A magnetostratigraphy could not be obtained. An interesting feature of the paleomagnetic data at this site is the steplike reduction of magnetic intensity between 1.5 and 2.5 mbsf. The intensity drop corresponds to the decrease in magnetic susceptibility in the same interval because of a strong decrease in concentration of magnetic minerals within 1 m. Dissolution of fine magnetic minerals is caused by diagenetic sulfate reduction in these highly organic sediments.

Significant amounts of biogenic methane gas were observed. Headspace methane concentration increased with increasing depth and reached a maximum (6347 ppm) at 89 mbsf and a high concentration (2505 ppm) at around 190 mbsf. No significant higher weight molecular hydrocarbons were observed, indicating that the methane is of biogenic origin and not significant for safety and pollution investigations. The carbonate content varies strongly between 1 and 73 wt%, and is generally high in lithostratigraphic Unit IIA. The organic carbon contents is high (0.3%-6.6%) throughout the section. The correlation between organic carbon and the C to N ratio suggests a predominantly terrigenous origin of to the organic material.

Chemical gradients in the interstitial waters (Fig. 4) reflect organic matter diagenesis via sulfate reduction, an increase in dissolved sulfate at greater depth, the dissolution of biogenic opal, and the influence of authigenic mineral precipitation. The decrease in dissolved calcium in the upper sediment, coincident with the sulfate decrease and the alkalinity increase from sulfate reduction, and the nonlinear relationship of calcium and magnesium suggest that authigenic mineral precipitation is significant in influencing the geochemical profiles.

Porosity decreases with depth in the upper 150 mbsf and shows an inverse correlation with P-wave logger (PWL) velocity. The lowest porosity values (55%) were measured in a very clay-rich interval (140-150 mbsf). Porosity increases slightly from 55% to 70% below 150 mbsf. At about 204 mbsf, the grain density values drop from around 2.7 to 2.5 g/cm3, reflecting the transition from clay-rich sediments with well-preserved calcareous nannofossils to siliceous sediments, marking the boundary between lithostratigraphic Units II and III.

Color reflectance data allowed for real-time prediction of sedimentary opal content. Using a regression equation generated from site-survey reflectance and opal measurements, we were able to simulate major lithologic units. Low opal content coincided with lithostratigraphic Units I and II, which included silty clay, and nannofossil ooze and chalk. Opal maxima and minima corresponded to interbedded diatomite and clay of lithostratigraphic Unit IIIA. The highest predicted opal content matched Unit IIIB, a diatomite. Opal levels were low in the siltstone, silty clay, and sandstone of Unit IV.

Core images were captured using the ODP color digital imaging system. Colors were measured and reported in the CIELAB system and appear to correlate with GRAPE density, possibly reflecting the carbonate component.

Logging at Hole 1011B consisted of two full passes with the Triple Combination tool string (density, neutron porosity, resistivity, and natural gamma ray) and one full pass with the Formation MicroScanner-Sonic (FMS-Sonic) tool string. Hole conditions were excellent, so the recorded log data were of excellent quality. The log physical property data closely matched the measured core density and porosity over the core-log data overlap; the log sonic velocity data were unfortunately not reliable because of the very high porosity sediments and consequent low impedance contrast. The log variations clearly delineate the major and minor lithologic boundaries. A sharp reversal to higher porosities below 200 mbsf is caused by an upper Miocene diatomite interval; this lithologic boundary represents a strong seismic reflector that may be useful for regional correlations. The FMS data revealed clear carbonate-clay interbedding that can be matched with the core MST data, particularly over the high sedimentation rate section below 200 mbsf.

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