Site 999 (12°45'N, 78°44'W; 2829 m water depth) is located on a previously unnamed rise in the Colombian Basin of the western Caribbean (Fig. 1). The name "Kogi Rise" was suggested during Leg 165, and refers to a tribe of the Sierra Nevada de Santa Maria in nearby Colombia. The rise is 150 km northeast of Mono Rise and southeast of the Hess Escarpment, and its crest lies 1000 m above the floor of the Colombian Basin, which is filled with turbidites from the Magdalen Fan and the Hess Escarpment (Burke et al., 1984; Bowland, 1993).
Site 999 is located beneath the Caribbean Current, an extension of the North Equatorial and North Brazil Currents. The Central American Isthmus diverts the warm saline water in this windblown current northward through the Yucatan Channel into the Gulf of Mexico where it is entrained in the Gulf Stream and brought to the subpolar North Atlantic. Several large eddies are present on the flanks of this current, including one in the large embayment between Nicaragua and Colombia (Sverdrup et al., 1942). Through the Cenozoic, changes in the elevation of tectonically controlled gateways have altered the path of surface water through the Caribbean (Berggren and Hollister, 1977; Coates and Obando, 1996). The Aves Ridge and Lesser Antilles arc to the east and the Central American Isthmus to the west have risen and the Nicaraguan Rise to the north has foundered (see summary of tectonic history in Sigurdsson, Leckie, Acton, et al., 1997).
Sediment accumulation at Site 999 was apparently continuous from the late Maastrichtian through the Pleistocene, although there are some condensed intervals. Hole 999A was cored to a depth of 566.1 meters below seafloor (mbsf) and terminated in lower Miocene clayey calcareous chalk. The upper 197.6 m of the hole (to the upper Miocene) was cored with the advanced hydraulic piston corer (APC) and achieved 104.6% recovery. (The figure exceeds 100% because of sediment expansion after decompression.) The remainder was cored with the extended core barrel (XCB) and there was 89% recovery (Sigurdsson, Leckie, Acton, et al., 1997).
The sediments of Hole 999A are divided into three lithostratigraphic units. Unit I extends from Core 165-999A-1H to Section 165-999A-29X-5, 74 cm (0.0-265.1 mbsf; Pleistocene to lower upper Miocene). It consists of nannofossil clayey mixed sediment with foraminifers, foraminiferal clayey mixed sediment with nannofossils, nannofossil clayey mixed sediment with foraminifers and pteropods, clayey nannofossil mixed sediment, and clayey nannofossil ooze. Volcanic ash is found both dispersed and in discrete layers (Sigurdsson, Leckie, Acton, et al., 1997). Three subunits (IA, IB, and IC) are based on variations in the abundance of foraminifers and the occurrence of siliceous microfossils. Unit II extends from Section 165-999A-29X-5, 74 cm, to 38X-2, 63 cm (265.1-346.9 mbsf; lower upper Miocene to lower middle Miocene). It consists of indurated mixed sediments and indurated clays with nannofossils. Two subunits (IIA and IIB) are distinguished by varying abundances of siliceous microfossils plus changes in carbonate content, magnetic susceptibility, and color reflectance. Unit III extends from Section 165-999A-38X-2, 63 cm, to 61X-CC (346.9-566.1 mbsf; lower middle Miocene to lower? lower Miocene). It consists largely of clayey calcareous chalk with foraminifers and clayey nannofossil chalk with foraminifers. Volcanic ash is extremely common in Unit III sediments (Sigurdsson, Leckie, Acton, et al., 1997).