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Sedimentary Structures in Modern Carbonate Sands of the Bahamas: Abstract
A layer of unconsolidated Recent sediment. 0-20 feet in thickness, lies discon-formably on a karst surface of Pleistocene limestone in the Bahamas. Geologists may think of this layer as an embryonic strati-graphic formation deposited during the past 5000 years as part of a transgressive hemicycle initiated by post-glacial sea-level rise. Although much has been learned about the sediments exposed on the sea floor, we are only beginning to study cores and understand in three dimensional terms the stratigraphy, paleontology, and sedimentology of the formation.
Emphasis is placed on sedimentary structures in Bahamian carbonate sands, particularly (1) surface forms such as ripples, dunes, bars, and linear furrows that can be studied on air photos and by underwater inspection; and (2) internal structures (burrows and stratification) that can be studied on the 1 square foot surfaces of box-cores. Three types of strata formed by bottom traction occur: avalanche deposits formed at the angle of repose on the lee sides of advancing ripples or embankments; accretion deposits formed at lower angles; and nearly horizontal sheet deposits. These three deposit types are interpreted as representing in the order named an increase in the velocity of current flow tangential to the bottom.
The spectrum of sedimentary structures preserved at any one site is diagnostic of the geologic environment, although individual types commonly are not.
Structures similar to those found in the Bahamas may be seen in many ancient limestones. Examples from the Ordovician, Devonian, Mississippian, Permian, and Pleistocene are cited.
Acknowledgments and Associated Footnotes
1 Columbia University, New York
Copyright © 2006 by the Tulsa Geological Society