A tidal origin for coarsening-upward sequences above two Wilcox lignites in East Texas

Abstract

Conventional palaeogeographic reconstructions show a large river system occupying the East Texas Embayment during the deposition of the Wilcox Group. Coarsening-upward sequences above Wilcox lignites in East Texas have been interpreted as floodplain splays. However, the bedding types, sedimentary structures and shallow scour surfaces in a coarsening-upward sequence above the main seam at the Martin Lake Mine in Panola County indicate deposition in water stirred by tidal and wave activity. The bedding types include sand streaks in mud and lenticular, wavy and flaser bedding. The sedimentary structures include abundant wave-ripple lamination and large-scale crossbeds with thin, closely-spaced mud drapes. These crossbeds suggest traction currents of short duration alternating with periods of slack water. Pollen from broadleaf hardwood trees is the most common element of the palynoflora from the coarsening-upward sequence at the mine. The upper reaches of riverine estuaries along the SE coast of the United States provide a modern analogue for the environment of deposition of the Wilcox sediments at the Martin Lake Mine. Here tidal currents extend inland to places where freshwater aquatic plants and forests border the estuaries. The same bedding types and sedimentary structures that occur in the sediments at the Martin Lake Mine occur in a coarsening-upward sequence above a lignite at the Big Brown Mine in Freestone County. Recognition of the tidal origin of the coarsening-upward sequences suggests an embayed coast or a shoreline within the East Texas Embayment during the deposition of the strata above the lignites.