Abstract: Significance of Freshwater Limestones in Marine Carbonate Successions of Pleistocene and Cretaceous Age

Robert B. Halley, Peter R. Rose

Freshwater carbonate sediments may be deposited over tens of thousands of square kilometers during subaerial exposure of marine-carbonate platforms. Such deposits, only slightly above sea level, presently cover parts of the Florida-Bahamas carbonate platform. Analogous ancient freshwater deposits can be identified in Pleistocene limestones of the South Florida platform and Cretaceous limestones of the Central Texas platform.

The co-occurrence of a variety of features provides a guide for the identification of freshwater limestones in marine-carbonate sequences. These include: (1) exceptional color (gray or dark gray); (2) calcareous mudstone lithology; (3) single, isolated 1 to 2-m thick homogeneous beds; (4) mottled or burrowed internal structures; (5) irregularly cracked and void-riddled fabric; (6) sparse fossils, usually gastropods and ostracods, and extremely sparse marine fossils; (7) evidence of early lithification; and (8) position at disconformities in carbonate sequences as evidenced by subaerial exposure criteria (leached fossils, caliche, erosional surfaces, etc.).

Recognition of freshwater limestones in carbonate sequences provides the stratigrapher with evidence of disconformities that otherwise might be overlooked. Occurrences of freshwater limestones also imply paleo-freshwater diagenesis, knowledge of which may help the stratigrapher understand or predict the occurrence of porosity related to subaerial exposure and stratigraphic-type hydrocarbon accumulations.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas