Slope currents and contourites in an eastern boundary current regime: California Continental Borderland
Rebecca S. Robinson, Janette M. Murillo De Nava, Donn S. Gorsline, 2007. "Slope currents and contourites in an eastern boundary current regime: California Continental Borderland", Economic and Palaeoceanographic Significance of Contourite Deposits, A. R. Viana, M. Rebesco
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Analysis of piston cores from lower slopes of central and outer basins of the California Continental Borderland shows the presence of structures ranging from starved silt ripples and lenses to erosion surfaces and truncated burrows to cross-bedded units that testify to reworking of the sediments by bottom currents. Adjacent basin-floor piston cores do not reveal these structures, but exhibit the usual bioturbated hemipelagic mud and turbidites. Slope sediments generally contain more silt than the adjacent basin-floor clay silts. The slope grain-size distributions are multimodal as a result of mixing of hemipelagic mud and thin (millimetre scale) silt layers as a consequence of the sampling intervals at centimetre scale. Carbonate and organic carbon contents tend to be low during the glacial periods but variations from this pattern occur that are probably related to the shift in the upwelling associated with the California Counter Current between interglacial and glacial conditions. The evidence for reworking is most abundant from late in Marine Isotope Stage 5 (MIS 5) to early Stage 4 (MIS 4), and it decreases into the Holocene. No structures are observed in the Holocene sections of the slope cores. The temporal distribution of the reworking structures can be explained either by changes in the degree of bioturbation (bottom-water oxygenation), which would work to erase the structures, or variation in the intensity of lower slope circulation through time. The observed pattern is in rough agreement with documented changes in bottom-water oxygenation conditions for these basins. However, the occurrence of reworking is also dominantly within glacial intervals, where lower sea levels produce stronger circulation within the basins as a result of exposure of banks and resulting restriction of cross-sections of the deeper flow pathways. These observations add to the increasing evidence that sediment transport by bottom currents is not restricted to the intensively studied western boundary current drift deposits. The action of bottom currents on eastern boundary slopes can introduce subtle effects on what are commonly assumed to be continuous records.
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There has lately been a growth in the number and level of studies of contourite deposits. Most recent studies of contourites have two major lines of interest. One, propelled by the oil industry's continuous move into increasingly deep waters, concerns their economic significance. The other involves the stratigraphic/palaeoceanographic record of ocean circulation changes imprinted on contourite deposits that can be a key to understanding better the climate-ocean connection. The application of many different theoretical, experimental and empirical resources provided by geophysics, sedimentology, geochemistry, petrology, scale modeling and field geology are used in the 16 papers of this volume, proposing answers to those two main aspects. The papers are subdivided into two major categories (economic interest and stratigraphic/palaeoceanographic significance), with case studies ranging from well-documented drifts to new examples of modern and fossil series, involving a large diversity of geographic and physiographic scenarios worldwide.