Origin of San Miguel Lignite Deposit and Associated Lithofacies, Jackson Group, South Texas

Abstract

The stratigraphy and depositional environment of the San Miguel lignite deposit, southeastern Atascosa and northern McMullen Counties, Texas, were determined from the analysis of 3 continuous cores and 600 borehole logs. Five units can be recognized by their characteristic composition, texture, sedimentary structures, paleontology, and morphology. Lithofacies 1 is a light-green to gray, cross-laminated sandstone and conglomerate unit. The grain size decreases upward in the bedsets and the unit contains abundant plant and carbonaceous material. These characteristics indicate a fluvial origin. Lithofacies 2, a gray-green bioturbated siltstone and claystone unit, has abundant root structures and was deposited in a coastal grassflats environment. Lithofacies 3 was deposited i a lagoonal environment. It is a green, massive claystone with numerous fossiliferous and marly beds. The fossil mollusks have modern counterparts which flourish in open bays or lagoons. Lithofacies 4 is a coarse, carbonaceous sandstone. Deposition in a barrier-flat environment is indicated by the sequence of roots and bioturbation structures overlying a massively bedded zone. The lignite interval is an alternating sequence of lignite and clay partings. The lignite represents accumulation of plant debris and peat development in a coastal marsh adjacent to a lagoon. The clay partings formed through repeated floodings of the marsh by coastal streams. This marsh was part of the south Texas Jackson Group lagoonal-coastal plain system, an updip equivalent of the south Texas strandplain-barrie bar system. The mining of lignite can be made more efficient by determining the stratigraphy and depositional environment of the lignite and associated units. Analysis of cores and borehole logs makes it possible to predict that the San Miguel lignite will have (1) high sulfur and ash content; (2) low Btu and fixed carbon values; (3) lateral continuity; and (4) relatively constant thickness.