Sedimentation and Structure, Jackson Group, South-Central Texas

ABSTRACT The lower and middle Eocene formations of the Gulf Coastal Plain reflect major cyclic advances and retreats of the sea, but the upper Eocene formations (Jackson group) represent shorter cycles of alternating marine to nonmarine conditions of deposition. Detailed surface and subsurface studies of the Jackson rocks in the Karnes County area of south-central Texas show that the lithologic character and thickness of the stratigraphic units reflect conditions of deposition that bear a distinct relation both to regional structure and to local faults and associated features. The relation of lithology and thickness to regional structure is illustrated by studies of two units of the Jackson. A map showing thickness and structure of the Stones Switch sandstone member of the Whitsett formation shows an area of thick sand on the outcrop along the present course of the San Antonio River. The sedimentation there was deltaic or littoral in character. Down the dip is a thinner zone of sand, but still farther down the dip is an elongate belt of thicker sand that parallels the strike of the rocks and was presumably an off-shore bar. The thickness of sand diminishes seaward from this off-shore bar. A sand-percentage map of the McElroy formation, in the middle of the Jackson group, shows that at and near the outcrop the formation consists chiefly of clay with a few interbedded sands and is of a near-shore, possibly estaurine origin. Farther down the dip is an elongate zone where the formation is predominantly sand that parallels the off-shore bar in the Stones Switch and lies a few miles southeast of it. Local anomalous thickening and thinning of individual units indicates that faulting produced topographic irregularities that locally, and temporarily, affected deposition. On top of the anticline in the downthrown block of the "down-to-the-coast" Coy City fault, sand units of the Stones Switch are thinner than the regional norm, but they are somewhat thicker on the downthrown blocks of the Fashing and Hobson "up-to-the-coast" faults. Outcrops a short distance north of the Fashing fault show that a coarse-grained sandstone was laid down locally as a channel-type deposit in the upper part of the Whitsett formation. The outcropping beds of the lower part of the Jackson (Wellborn and Caddell formations) cut by the Falls City fault apparently were not affected by their proximity to the fault, indicating that movement along the fault did not occur during the deposition of those beds.