Carbonate Ramp Shoreface Coquina and Lagoonal Systems: Holocene of Northwestern Yucatan Peninsula, Mexico

Rankey, Eugene C.; Garza-Perez, Rodrigo; Naranjo-García, Maria Julia

Carbonate ramps are common throughout in the geologic record, and many prolific carbonate reservoirs occur in ancient ramp systems. To better understand the processes, sedimentology and geomorphology of carbonate ramps, this study examines Holocene shallow marine and coastal systems of the semi-arid northwestern Yucatan Peninsula in Mexico.

In the nearshore region considered here, a <5 m deep subtidal zone of thin sediment accumulation dips gently away from the shoreline. Although much of the southern part of the study area in Campeche includes mangrove-rimmed shore, the areas near Isla Arena and the Yucatan shore to the north near Celestún include a sandy shoreface. These shorefaces include an offlapping series of mollusk-rich beach ridges up to 3 km across and 30 km along strike with spit-like geometries that indicate southward transport. The beach ridge complexes near Celestún and Isla Arena form barriers that protect mesotrophic to oligotrophic lagoons that reach up to 2.4 km wide. These subtidal estuary systems include bioturbated muddy, molluscan carbonate sediment rich in organic matter (in excess of 5%). These areas include more mud and have greater freshwater influence away from their open-ocean openings to the south. The lagoons are in turn flanked by brackish water (near the shore, organic matter-rich) to fresh water (inland, marly) mangrove marsh system. Overall, the Holocene sedimentary succession thins from at least 3 m near the coast to its lapout onto Pleistocene bedrock inland.

Collectively, the sedimentologic character and geomorphology of this nearshore system illustrate prograding mollusc-rich shoreface coquinas that occur immediately adjacent to organic matter-rich muddy lagoonal carbonates. This Holocene nearshore sediment wedge is sedimentologically distinct from the skeletal, ooid, or peloid rich shorefaces of the Bahamas, and even shorelines further east in Yucatan. These differences are interpreted to reflect the influences of bedrock elevation, offshore upwelling, longshore transport, and groundwater input, and provide a shoreface analog distinct from existing conceptual models.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013