Sandstone Architecture of Upper Miocene and Pliocene Shoreface, Deltaic, and Valley-fill Complexes, Macuspana Basin, Southeaste

Upper Miocene and Pliocene shoreface and wave-dominated deltaic depocenters in the Macuspana Basin are related primarily to deep-sealed growth faults and large-scale rollover structures associated with shale evacuation. Early Neogene thrusting south of the basin triggered evacuation of Oligocene shale along northeast-down listric faults in the east and southeast basin margin. Episodic motion along these growth faults intermittently halted progradation of sandy shorezone systems and provided accommodation for thick (locally >500 m [>1,640 ft]) aggradational shoreface complexes with stacked successions of sandy upper-shoreface deposits. Sediment accumulation rates, facies distribution, and sandbody stacking patterns in the Macuspana Basin were a function of base-level changes induced by tectonic activity, local subsidence in fault blocks, variations in sediment supply, and eustasy. Base-level change in the south end of the basin, caused by tectonic activity along the Chiapas Foldbelt, is inferred from upper and middle Miocene angular unconformities associated with more than 150 m (>492 ft) of missing section and deposition of (incised) valley-fill systems in narrow (commonly <5 km [<3 mi] wide) belts locally containing more than 100 m (>328 ft) of gross sandstone. Abrupt thickening of upper Miocene shoreface successions and stabilization of paleoshorelines along growth faults associated with significant withdrawal and northwestward expulsion of mobile Oligocene shale in the south-central part of the basin resulted in hundreds of meters of aggraded upper-shoreface deposits. Retrogradational successions above these thick, aggradational sandstones contain thin intervals of upward-thinning sandstones reflecting basinwide transgression. Sandstones in these retrogradational sections originated from transgressive winnowing of southeast-retreating shorelines. Modern analogs include the Santee Delta and Cape Romain in South Carolina, U.S.A., and the Grijalva Delta in Tabasco, Mexico. A second-order, 5.0-m.y.a, flooding surface in the lower Pliocene defines the top of geopressure and serves as a regional reservoir seal for the main upper Miocene productive interval. Shales above this flooding surface, as much as 350 m (1,148 ft) thick, locally interfinger with lower Pliocene, highstand progradational successions that reach the westward margins of the basin. Early Pliocene valley-fill deposits in the basin are dominated by narrow, northwest-trending sandbodies. Gross-sandstone trends in these systems cut across fault trends, suggesting that accommodation for these deposits was provided by either sea-level fall or intermittent uplift from the Chiapas Foldbelt.