ABSTRACT

Regional lithofacies mapping in South Texas and stratigraphic relationships, which are best documented in the Edinburg fault block, suggest the presence of a nested series of submarine canyons cutting the southern flank of the Frio (Oligocene) Norias delta system. Canyon fill consists of thick sequences of mudstone, locally containing interbedded siltstone and sandstone, that abruptly replace the normal section of deltaic coastal-barrier and delta-flank barrier-bar sandstones and prodelta/shelf mudstones. Individual canyon fills are several miles wide and as much as 2,000 ft (600 m) thick, making them comparable in scale to the late Quaternary Mississippi Canyon. Canyon excavation and filling appear to have been recurrent processes during deposition of the lower and middle Frio depositional complex of South Texas; however, the best documented canyon fills predate cutting and infill of the Hackberry canyon complex of eastern Texas and Louisiana. The Edinburg canyon system, like its counterparts in the Cenozoic deposits of the Gulf Coast and Niger delta, occupies a paleogeographic setting at the flank of a major deltaic depocenter. This, and similar canyon complexes offer deep exploration targets that have commonly been only sparsely tested and may have gone largely unrecognized.