Hemipelagic deposits in a Miocene basin, California: toward a model of lithologic variation and sequence

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Summary Deep marine basins began to form along the continental margin of central California in late Oligocene time as a result of the initiation of a translational plate boundary. Deposition of transgressive shallow-marine sand (Vaqueros Formation) was followed by deposition of a thick (> 500 m) massive clay unit (Rincon Shale). Rapid subsidence (> 2000 m) coincident with rising sea-level created sediment-starved basins by late early Miocene time ( c. 18 Ma). Typical of many of these borderland basins was the Santa Barbara basin, located at least 50 km from the Miocene strandline. In this basin, between late early and latest Miocene time (18–5.5 Ma), was deposited a heterogeneous sequence of highly biogenous sediment (Monterey Formation) consisting of mixed diatomaceous, foraminiferal-coccolithic, and terrigenous debris generally rich in organic matter (mean 8%). Overlying latest Miocene and Pliocene sediments (Sisquoc Formation) contain increasingly abundant clay debris. The Miocene biogenous sequence reflects hemipelagic sedimentation from highly productive coastal waters during an extended period of generally low terrigenous influx. Accumulation of terrigenous debris decreased markedly in the early Miocene (from 11 to 1 g/cm 2 ·10 3 yr) and was low throughout the middle Miocene, increasing somewhat at about 8 Ma and markedly after 5.5 Ma; this pattern is partly related to global sea-level changes but also resulted from high rates of subsidence and tectonic events. Biogenous silica accumulation peaked (mean 2.5 g/cm 2 ·10 3 yr) in the late early Miocene (18–15 Ma) and again in the late Miocene and early Pliocene (8–3.5 Ma) and was 5 to 25 times slower in the interval 15–11 Ma; these variations are thought to reflect primarily variations in the intensity of upwelling and associated production of diatoms. Accumulation rates of biogenous calcite gradually declined from a late early Miocene peak through the early late Miocene and virtually ceased in latest Miocene (8–5.5 Ma), probably due to dissolution. Massive stratification indicates that bottom waters were oxygenated until latest early Miocene time ( c. 16 Ma), when expansion of the oxygen-minimum zone or tectonic formation of a sill within the oxygen-minimum zone produced marked lowering of oxygen levels resulting in the dominance of laminated stratification between 16 and 5.5 Ma. Abundance of organic matter is closely associated with the abundance of clay and of calcite but is inversely correlated with silica content. Organic matter is also about twice as abundant in massive or discontinuously laminated beds (indicating good to moderate oxygenation) than in associated beds with varve-like lamination (indicating minimal oxygenation). These relations indicate that grain-size outweighed the influence of low-oxygen bottom waters in preserving organic matter.