LEG 101

The Bahamas

The Bahamas carbonate platform represents one of the closest modern analogs of ancient carbonate deposits exposed in mountain belts around the world. Study of the stratigraphic and structural evolution of this platform contributes to the record of eustatic sea level changes, paleoceanography, sclerochronology and climatic changes, and aides in constraining vertical tectonics and establishing three dimensional facies models of platform flanks.

During Leg 101, eleven sites (Sites 626 to 636) were drilled, investigating the shallow-water carbonate banks and the intervening deep-water troughs. In particular, two different hypotheses for the long-term crustal evolution of the Bahamas were tested. The "megabank" hypothesis holds that the modern archipelago is underlain by a single flat-topped shallow-water platform on which the present system of banks was built, whereas the "graben" hypothesis suggests that the present topography reflects the underlying horst and graben related to Mesozoic rifting of North America and Africa.

Operations during Leg 101 proved that a prominent acoustic unconformity identified beneath the southern Blake Plateau correlates with the top of a drowned, shallow-water carbonate platform of Middle Cretaceous age. Correlation of these results to both the industrial Great Isaac 1 well and Site 626 suggests that such a platform underlies all of the northwestern Bahamas. Platform drowning occurred in steps during the Middle Cretaceous. During the Late Cretaceous and Tertiary, north- and west-facing platform flanks prograded tens of kilometers. However, faulting and/or regional tilting remain possible mechanisms controlling the long-term platform-basin pattern.

Drilling to address carbonate-slope evolution provided support for the concept of high-stand shedding, a depositional response to sea-level fluctuations opposite to that of siliciclastic systems. Bahamian slopes were generally covered by muddy cement, whereas graded sand and rubble characterized turbidite aprons at toes-of-slope and on basin floors. Slope bypassing by turbidity currents and slumps were more efficient on steeper slopes, but all slopes drilled during Leg 101 are accreting rapidly. Therefore, the transition to truly erosional depositional environments on carbonate platform flanks must occur at declivities over 12 degrees.