ABSTRACT

Lower Cretaceous shallow-water limestones were dredged from between 2300 to 3000 m (7500-9800 ft) below sea level along the northeastern slope of Abaco Knoll in the northern Bahamas, and crystalline dolomites were dredged from between 3600 to 4000 m (11,800-13,000 ft) below sea level along the southwestern slope of the knoll. Among the shallow-water limestones is a coral-rudist boundstone, which represents one of the first pieces of an Early Cretaceous reef to be recovered in the northern Bahamas. The depositional setting of this reef could either have been a patch reef on an open platform or part of a platform-margin complex.

The dredged intervals are below the top of a drowned shallow-water platform identified on a nearby multichannel seismic reflection line. The recovered carbonates confirm that the knoll is a drowned Early Cretaceous carbonate platform and suggest that the platform on Abaco Knoll was part of a broader Early Cretacous megaplatform, which extended west beneath Northeast Providence Channel. Shallow-water limestones of Barremian to Aptian age from the northeastern slope of the knoll indicate that the platform was drowned sometime after the Aptian.

Early Cretaceous stratigraphic horizons at Abaco Knoll and nearby DSDP Site 98 are vertically offset. Depending on the depth of recovery for the dredged rocks, relief between the horizons is at least several hundred meters, and might be as large as a kilometer. This relief can be explained by either: 1) a fault or series of faults along the southwestern flank of Abaco Knoll, or 2) depositional relief between an Early Cretaceous platform (i.e. Abaco Knoll) and an adjacent deep-water basin (i.e. Northeast Providence Channel). The recovery of inferred shallow-water dolomites from the southwestern slope of Abaco Knoll, seismic stratigraphic reinterpretations, and previously published magnetic anomaly patterns suggest that faulting is the more likely explanation.