April Luncheon - Bo Henk, TCU Dept. of Geosciences

The purpose of this study is to determine the depositional environments of the Woodbine Group in North Texas, Prior interpretations have included fluvial, deltaic and shallow marine environments for the Dexter, Lewisville and Arlington Fms, respectively. Our investigation utilized two USGS cores and numerous outcrops. The USGS has taken two cores, the USGS GC-2 core from Dallas County and the USGS GC-4 core from Fannin County, Both cores penetrated the upper 150 feet of the Woodbine. Sedimentology and Ichnology were utilized to determine depositional environments. The same holds true for the numerous outcrops and measured sections at Lake Grapevine and elsewhere in Tarrant and Denton counties. Prior work from previous investigators, combined with new measured sections were used to construct a clearer understanding of the Stratigraphy. Outcrop work, compared with earlier investigations, was greatly enhanced by developing new techniques to clear the outcrop faces allowing for much higher resolution of sedimentary and biogenic structures. Outcrops included sections from the Dexter, Lewisville and Arlington intervals. Much of the sand

bodies formerly described as fluvial braid plane to delta front are now interpreted to be of shallow marine and tidally influenced origin. The sand bodies earlier described as shallow marine continue to hold true but with much more detail in the breakout of trace fossils, sedimentary structures and resultant environments. This study's findings is that the Woodbine is now considered to be deposited along a shallow water marine shoreline with back barrier lagoon or bay deposits of shoreline marshes with root grounds and carbonaceous shales, muddy and sandy tidal flats and compound dunes with predominant flood tidal features, flood tidal deltas, muddy tidal flats with organics and root grounds incised by sand and organic filled tidal channels, sand flats incised by tidal inlets with high energy fill, lower and middle shoreface sand bodies that are heavily bioturbated and a calcite cemented oyster lag deposit that represents a significant transgressive surface of erosion. Findings from this study should be helpful in interpreting the Woodbine in the subsurface along the western margins of the East Texas Basin.