Strandline Sedimentation of Carbonate Grainstones, Upper Pleistocene, Yucatan Peninsula, Mexico: GEOLOGIC NOTES

An ancient beach-ridge plain along the eastern Yucatan Peninsula is underlain by a body of carbonate grainstones 150 km long, 0.5 to 4 km wide, and 3 to 10 m thick. Primary structures and textures suggest that these calcarenites are a regressive sequence deposited in the nearshore and beach zones of a high-energy coast environment. Their accumulation built a strand plain along the prograding Yucatan coast during the late Pleistocene high stand of sea level. In typical vertical sequence (from bottom to top) the upper Pleistocene calcarenite deposits consist of: (a) lower shoreface--low-angle cross-bedded, burrowed bioclastic, fine-coarse calcarenite with calcirudite lenses; (b) upper shoreface--multidirectional trough cross-bedded, bioclastic, pelletoid, and oolitic, fine to coarse calcarenite with calcirudite layers containing shells, corals, caliche lithoclasts, and intraclasts of beachrock; and (c) foreshore-backshore--parallel-laminated oolitic, bioclastic, and pelletoid, fine to medium calcarenite with rhizocretions in the upper part and caliche crusts at the top. In the southern part of the study area the strand-plain grainstones overlie nonbedded bioclastic calcarenite and micrite that were deposited in the offshore between the mainland and a coral barrier reef which fringed the shelf edge. This section overlies thin, discontinuous caliche clast-shell-coral calcirudites (transgressive lag), which lie on a pholad- and sponge-bored subaerial crust (caliche) developed on older limestones. Farther north the strand-plain calcarenites lie directly on the subaerial crust. The shoreface section is largely storm deposits composed of material derived both from offshore and from the shoreline. Storm waves also probably deposited much of the foreshore calcarenite. Thinly coated ooids that are concentrated in the beach and inner-shoreface deposits probably were coated in the high-energy nearshore. These coastal-zone limestones show that significant volumes of porous and permeable carbonate grainstone can be accumulated by seaward advance of the shoreline during high stands of sea level. Ancient strandline grainstones are potential hydrocarbon reservoirs in updip carbonate sequences.