165 Preliminary Report


The Cretaceous/Tertiary (K/T) and ocean history goals of ODP Leg 165 were accomplished through drilling at five sites. Highlights of the leg included the recovery of K/T boundary clays and ejecta deposits (at Site 1001 and perhaps Site 999), and the recovery of igneous basement from the Caribbean Oceanic Plateau (Site 1001). Beyond any of our pre-cruise goals or expectations, we discovered a spectacular record of Eocene and Miocene explosive volcanism at Sites 998, 999, 1000, and 1001, which is unmatched in its magnitude and chronostratigraphic resolution. This includes the first documentation of arc volcanism along the Cayman Ridge. Also recorded above the excellently preserved basement/sediment contact in the two holes at Site 1001 is a mid-Campanian volcanic episode, probably the waning stages in the formation of the basaltic plateau that is the foundation of the Caribbean Plate. The age, physical characteristics, and geochemistry of the underlying basalts will bear importantly on the tectonic history of the Caribbean. The vesicularity of the basalts, the benthic microfossils found in sediments resting on the flows, and the magnetic directions recorded by the flows indicate the plateau was rapidly subsiding and near the paleoequator in the mid-Campanian. Geochemical results indicate that dispersed volcanic ash (excluding discrete ash layers) is a significant constituent (typically 10% to 20%) of the Caribbean sedimentary record. The ash alteration products strongly influence the composition of pore waters and may be an important source of silica for the vast accumulation of cherts that abound in the Eocene marine record.

Important paleoceanographic events recovered by drilling include the K/T boundary, the "late Paleocene thermal maximum" (LPTM), and a middle/late minimum in carbonate accumulation ("carbonate crash"). All three holes from which the K/T boundary interval was recovered contained an unusual limestone layer that directly overlies clays and claystone. The limestone, though only a few centimeters thick, is anomalous in that it appears more massive, more indurated, and lighter in color (white to very light gray) than any other limestone recovered during the leg. The LPTM also marks a change in lithology and physical properties, characterized by a faintly laminated claystone unit, less than one meter thick, with significantly lower carbonate content than surrounding chalks and limestones. The spatial distribution of a middle/late Miocene minimum in carbonate accumulation ("carbonate crash") was extended from a regional anomaly in the Pacific to an interbasinal tropical anomaly. In addition to these anomalies, the transect of sites with depths ranging from 916 mbsf (Pedro Channel, Site 1000) to 3260 mbsf (Hess Escarpment, Site 1001) and with continuous depositional sequences offers the opportunity to study Neogene water mass history and circulation across the basin. Finally, Site 1002 from the anoxic Cariaco Basin contains a unique tropical counterpart to high latitude ice cores for studies of large and abrupt climate changes in the latest Quaternary.

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