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

Abstract

Fresh-water lime 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, are presently found covering portions of the Florida-Bahamas carbonate platform. Analogous ancient fresh-water limestones 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 fresh-water limestones in marine carbonate sequences. These include: (1) exceptional color (grey or dark grey); (2) lime mudstone lighology; (3) single, isolated 1 to 2 meter thick homogeneous beds; (4) mottled or burrowed internal structures; (5) irregularly cracked and voidriddled fabric; (6) rare fossils, usually gastropods and ostracods, exceptionally rare 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 fresh-water limestones in carbonate sequences provides the stratigrapher with evidence of disconformities that might otherwise be overlooked. Occurrences of fresh-water limestones also imply paleo-fresh-water diagenesis, knowledge of which may help the stratigrapher understand or predict the occurrence of porosity related to subaerial exposure and stratigraphic-type hydrocarbon accumulations. 'U.S. Geological Survey, Fisher Island Station, Miami Beach, Florida Energy Reserves Group, Houston, Texas DEPOSITION AND DIAGENESIS OF THE FORT TERRETT FORMATION (EDWARDS GROUP) IN THE VICINITY OF JUNCTION, TEXAS