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Several major seismic reflectors in the deep western North Atlantic have been calibrated according to age and physical and lithologic nature by JOIDES drilling. These reflectors result from geologically abrupt changes in depositional conditions and lithofacies. Within the limits of biostratigraphic resolution, the reflectors are approximately but not strictly synchronous, and sediment accumulation, although commonly changing in rate, was continuous across the seismic boundaries. Major reflectors include horizon B, which ranges between Hauterivian and Barremian in age and correlates with an upward change from limestone to black clays coincident with a rise in the calcite compensation depth (CCD). In middle to late Maestrichtian time, a brief, sharp depression of the CCD ca sed widespread deposition of chalks that correlate with horizon A. This reflector commonly conforms to preexisting topography, a fact which suggests its pelagic origin. Widespread deposition of sediments enriched in biogenic silica occurred during the Eocene, and diagenesis formed chert beds in the upper lower to lower middle Eocene section. The top of these cherts matches horizon AC, which is one of the most laterally extensive reflectors in the western North Atlantic. Across the western Bermuda Rise, and overlying reflector, horizon AT, correlates with the top of a sequence of turbidites deposited prior to and during uplift of the rise in the latter half of the Eocene. Limited biostratigraphic data at JOIDES boreholes suggest that the reflector is diachronous; thi probably results from gradual westward offlap of the turbidites as the Bermuda Rise was uplifted. One major reflector, horizon AU, is not within a continuously deposited sedimentary section, but corresponds to a major unconformity eroded between late Eocene and early Miocene time by abyssal currents along the lower continental rise. Sedimentation patterns mapped from the distribution and spacing of these reflectors are used to interpret the paleo-oceanographic conditions in the basin.
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