Leg 101--An Overview

During Leg 101, the inaugural leg of the Ocean Drilling Program, the JOIDES Resolution drilled 19 holes at 11 sites in the Bahamas. Grids of high-resolution seismic profiles provided information crucial for both site selection and regional stratigraphic interpretations. The first major scientific objective was to understand the long-term evolution of the bank-and-basin pattern that currently characterizes the Bahamas archipelago. Drilling and seismic surveys have indicated considerable platform expansion and retreat rather than stationary upward growth. Upbuilding in the Late Jurassic-Early Cretaceous was followed by drowning and retreat in the mid-Cretaceous and by renewed expansion in the Cenozoic. At Site 627 on the southern Blake Plateau, the stratigraphic succession consists of shallow-water platform carbonates/evaporites (late Albian), marly limestones of a mixed terrigenous/carbonate shelf (latest Albian-middle Cenomanian), carbonate ooze of an oceanic plateau (Campanian-Miocene), and finally the turbidite apron of an advancing platform (Neogene). The top of the upper Albian shallow-water platform is characterized seismically by an acoustic unconformity, an amplitude contrast, and a velocity transition (from 2.8 to 4.2 km/s). Jump correlation of a similar acoustic horizon underlying the Straits of Florida (Site 626) with the Great Isaac 1 well on Great Bahama Bank approximately 60 km away suggests that deep-water carbonates in the Straits are also underlain by a mid-Cretaceous shallow-water platform. Delineating the evolution of platform flanks was the second major scientific objective of Leg 101. Modern facies belts were traced back through the Neogene record in two slope transects. A gentle (2°-3°) slope (north of Little Bahama Bank) is characterized by fine-grained sediments and erosional gullies. Coarse material bypasses the slope in turbidity currents and is deposited on debris aprons. With an increase of slope angle to 10°-12° (southeastern Exuma Sound), the zone of maximum accumulation shifts from the slope to the debris apron, probably because of increased turbidity-current activity. Slumps, debris flows, and turbidites are unusually abundant in the lower and middle Miocene sections and are probably caused by tectonic activity (the Abaco event ) superimposed on long-term slope evolution. Detailed stratigraphy of the two slope transects supports highstand shedding, the concept that maximum input of platform sediment to the deep sea occurs during highstands rather than lowstands of sea level. Pleistocene glacial periods as well as postulated eustatic lowstands in the Messinian and Pliocene correspond to hiatuses or intervals of slow sedimentation on platform flanks, and perhaps to karst horizons on the platform tops. Diagenesis of periplatform ooze is rapid. Both high-magnesium calcite and aragonite decrease with depth, and dolomite precipitates. However, lithification is discontinuous and incomplete to burial depths in excess of 200 mbsf and ages of 10-15 Ma