Magnetostratigraphic Dating: Insights to Late Cenozoic Deposition and Dolomitization of Little Bahama Bank

MCNEILL, D. F., and P. K. SWART, University of Miami, Miami, FL, and V. C. VAHRENKAMP, Koninklijke/Shell Exploratie en Productie Laboratorium, Rijswijk, The Netherlands

Magnetostratigraphic dating of a transect of cores across Little Bahama Bank (LBB) provides refined insight into timing and anatomy of platform deposition since late Miocene. Age-dating of four cores, with biostratigraphic tie-points, shows a gently northward dipping platform from late Miocene to about mid-Pleistocene (0.73 Ma) as a result of higher growth/accumulation of the southwestern platform margin. During the Brunhes normal chron (0.73 Ma to present) the platform evolved into a nearly horizontal surface, coincident with the onset of nonskeletal sedimentation. Sediment accumulation since late Miocene is consistent with third-order scale sea level events recognized elsewhere in the archipelago. Refined dating also permits additional insight into facies evolution during platform s oaling. Time-slices of platform facies at magnetic reversal boundaries show a well-circulated gently dipping shelf with skeletal packstones and wackestones and a reefal rim on the leeward side during late Miocene/early Pliocene. Lowering of sea level about 2.4 Ma deposited lagoonal mudstones and wackestones over much of the platform prior to prolonged subaerial exposure (2.4-0.8 Ma). Deposition during Pleistocene sea level changes resulted in peloidal wackestone and mudstone units over this portion of the platform.

Magnetostratigraphic dating coupled with Sr-isotope ages from LBB dolomites significantly improves the late Cenozoic depositional/diagenetic framework. Dolomitization episodes at about 2.2-3.4 and 1.6 Ma correspond with a prolonged period of subaerial exposure and formation of large aquifer systems and perhaps providing a mechanism to pump dolomitizing fluids (seawater) through the platform. A distinct northward slope of the dolomite may be explained by either: (1) a northward-dipping platform top, which supported different aquifer levels during fluctuating sea level lowstand events, or (2) the thickness of the freshwater lens which in turn is controlled by differences in permeability and platform slope between the northern and southern margins.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)