Depositional Trends in Siliciclastic Deposits of the Stone City Transgressive Systems Tract, Middle Eocene, Texas: ABSTRACT

Strata exposed at Stone City Bluff, Texas, contain shelf sediments deposited during progressive marine deepening (from shallow water depths) and then capped by a maximum flooding horizon. They are part of the early Bartonian (NP16) Cook Mountain transgressive systems tract, exposed in the type section of the Stone City Member of the Crockett Formation. This locality lies on the inner portion of a shelf transect about 175 km (110 mi) shoreward of the Late Eocene shelf-slope margin and probably never farther than 80 km (50 mi) from the shoreline. Sedimentary structures in the sediments change upward, reflecting a shift from tidal current-dominated to storm-dominated deposition. In lower intervals, tidal control is dominant. The transition interval contains isolated storm sands, as sheets with lenses 40 cm (15 in) thick, in mudstones with tidal bedding. These storm beds contain teeth/bone concentrates and some gravel-size clasts at the bases of larger lenses. In higher intervals, storm deposition is dominant in all units, mostly as thin, distal layers of very fine, sand or silt. Amalgamated storm deposits form larger units (e.g., the Moseley bed). Local hardground surfaces are developed where storm washing exposed concretionary layers or on condensed, winnowed zones. Glauconite occurrence and siderite concretion formation are most common in the upper intervals. Storm deposits have dual sediment sources and contain 1) skeletal remains (shells/fish bone) and glauconite pellets concentrated by in-place winnowing, or 2) fine quartzose sand washed in from shallower water settings. Alternation of amalgamated storm deposits and mudstone units with thin storm deposits indicate a cyclic record of storm activity.