At Site 998, located on the Cayman Rise at a water depth of 3180 m (Fig. 1), we recovered a 638-m-thick, largely complete sedimentary section that extends from the lower Eocene to the Pleistocene. More than 500 tephra layers were identified at this site, interbedded with nannofossil and foraminiferal ooze, chalk, and limestone. Tephra fall layers were most abundant in the Miocene to Pleistocene sediments of lithologic Units I, II, and the upper parts of III. The Miocene fall layers contain abundant colorless glass shards of rhyolitic composition, with <10% crystals of plagioclase, quartz, biotite, hornblende, and clinopyroxene. In the older sections of Site 998 the abundance of ash turbidites increases dramatically with a prominent peak in the early Eocene (Shipboard Scientific Party, 1997a). These layers are typically altered and consist of a mixture of biogenic components (foraminifers and radiolarians) and volcaniclastic material (altered glass, plagioclase, quartz, biotite, Fe-Ti oxides, and igneous rock fragments).
Site 999 is located on a small rise at a water depth of 2827 m in the Colombian Basin ~150 km northeast of the Mono Rise (Fig. 1). It lies some 1000 m above the adjacent floor of the Colombian Basin and thus is outside of the influence of turbidite deposition from South America. It is also isolated by a saddle from redeposited sediment originating on the Hess Escarpment to the west. The relative stability of this site compared to nearby tectonic features such as the Hess Escarpment and Lower Nicaraguan Rise make this site ideal for a long-term record of tephra deposition.
An apparent complete and continuous sedimentary sequence of upper Maastrichtian to Pleistocene age was recovered in two holes that penetrated to 1066 meters below seafloor (mbsf). More than 1200 tephra fall layers were identified at this site, defining two prominent peaks in explosive volcanism during the middle to late Eocene and early to middle Miocene. Miocene to Pleistocene layers are interbedded with nannofossil clayey mixed sediment, clayey nannofossil mixed sediment, and clayey calcareous chalk of lithostratigraphic Units I, II, and III. They are similar in composition to the layers from Site 998, consisting of abundant rhyolitic glass shards, plagioclase, biotite, amphibole, rare quartz, and clinopyroxene. However, many of the layers are extensively altered with glass shards that are being converted to Mg-rich smectite. In addition, pyrite coating on glass shards is a common feature in many of the Miocene layers (Shipboard Scientific Party, 1997b). The extent of alteration does not appear to be systematic downcore. For example, many of the upper Miocene tephra fall layers are extensively altered, yet the majority of lower to middle-Miocene layers that constitute the major peak in ash abundance are relatively unaltered.
Site 1000 is located at a water depth of 916 m. This area is part of a series of carbonate shelves and isolated carbonate banks that form the Northern Nicaraguan Rise, extending from the coast of Honduras in the west to Jamaica in the east. An apparently complete sequence of late Miocene- to Holocene-age sediments was recovered by drilling to 696 mbsf. Tephra fall layers are interbedded with periplatform sediments and intervals of redeposited periplatform/pelagic and neritic carbonate sediments transported from the nearby shallow carbonate banks (Shipboard Scientific Party, 1997c). The majority of the tephra layers are extensively altered, although some contain colorless bubble-walled shards. Phenocryst phases include feldspar, biotite, and, more rarely, hornblende, Fe-Ti oxides, and quartz.