Abstract

The Maastrichtian Muerto Formation was deposited in the distal part of the once-contiguous Sierra Madre foreland, which developed over the rifted eastern margin of Mexico. In a segment of the now deformed foreland known as the La Popa Basin, the Muerto Formation is presently exposed adjacent to an evaporite diapir known as El Gordo Diapir. Muerto strata represent the first significant influx (1970 ft, 600 m thick) of sediment derived from the growing thrust belt to the west and transported eastward by a large deltaic system. The Muerto Formation in the vicinity of El Gordo Diapir contains three facies assemblages: shelfal/shoreface, tidal, and back-barrier/estuarine. The shelfal/shoreface facies assemblage ranges from offshore shale to middle shoreface sandstone. The tidal facies assemblage includes ebb-tidal delta and flood- and ebb-tidal channel lithofacies. The estuarine facies assemblage includes oyster bank, and bioturbated washover/flood-tidal delta facies. The Muerto Formation exhibits lateral thinning towards the diapir from 2280 ft (695 m) to 220 ft (67 m) over a distance of 1.25 mi (~2 km). The thickness trends and geometry of the Muerto near El Gordo Diapir are similar to that of the Oligocene Frio Formation near Spindletop Salt Dome; in contrast, the geometry of the Frio Formation adjacent to the Nash Salt Dome is dissimilar to the Muerto Formation. Comparison with other structures in the Gulf of Mexico indicates that topographic expression of El Gordo Diapir and the relationship between sedimentation rates and halokinetic processes rates were primary controls on stratal geometry; halokinesis associated with El Gordo had lesser syndepositional influence on the Muerto Formation. This has significant implications for hydrocarbon migration and accumulation in salt flank traps.