ABSTRACT

Paralic deposits of Late Cretaceous age are represented in Trans-Pecos Texas by the Aguja Formation in the Big Bend region and the correlative San Carlos Formation in the Sierra Vieja region. Although both units are broadly progradational, each records a single widespread transgressive pulse. This transgressive event began in Early Campanian time, and reached a peak during the "middle" Campanian. Regressive (progradational) deposits consist of prodeltaic shale and delta front siltstone and claystone, overlain by coalesced lenticular fluvial-dominated deltaic sand bodies. Trangressive deposits consist of local storm-dominated shoal sandstone, thin laterally extensive oyster-rich sublittoral sand sheets, and shelf mud. Transgression occurred relatively rapidly and was expressed differently depending on the distance traversed landward of the previous shoreline. Coal and lignite deposition occurred in restricted interdistributary areas during progradation, and in more widespread areas during peak transgression when the strandline was neither advancing nor retreating. There is no evidence for barrier island development during transgressive events; hence, the coal and lignite accumulated along a muddy non-barred coast in marshes directly adjacent to the sea. The coal-bearing strata consist of an alternating series of dark gray root-mottled carbonaceous claystone with sideritic "ironstone" concretions, coal or lignite, and well-laminated light gray pyritic shale with calcareous concretions bearing marine invertebrates. The cyclic alternation of these facies suggests the repeated inundation of coastal marshes by marine waters, followed by mud accumulation, subsequent regrowth of the marsh, and the deposition of lignite or coal.