588 TRANSACTIONS—GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XXXVIII, 1988

SHORELINE DEPOSITIONAL ENVIRONMENTS OF THE GLEN ROSE FORMATION (LOWER CRETACEOUS) IN THE TYPE AREA, SOMERVELL AND HOOD COUNTIES, TEXAS

Gail R. Bergan1

ABSTRACT

The Comanchean Series in north-central Texas was This tidal flat-salt marsh complex received sediments from both deposited along the margin of a shallow sea as it transgressed terrestrial and marine sources. A broad intertidal zone of northwestward across the Central Texas platform during early vegetated sand and mud flats was dissected landward by small Cretaceous time. A lithologic and paleontologic study of the Glen fluvial creeks and seaward by storm beaches and marine inlets. Rose Formation (uppermost Trinity Division) in its type area was The fine size of the terrigenous elastics indicates a low energy undertaken to define the shoreline sediments deposited dur­ flat with a very gently seaward-dipping substrate. A warm, semi- ing Aptian-Albian time. The studied interval was subdivided into arid to arid climate favored the development of salt marshes and informal units A, B, and C based on lithology and fossil con­ the precipitation of evaporites. The faunal restriction also sug­ tent. The primary focus was on.the lowermost Unit A for pur­ gests the abnormal salinites of tidal marshes, this sequence is poses of fades analysis. considered ocmparable to the Holocene salt marshes on Sapelo Unit A comprises a clastic-to-carbonate transition deposited Island, Georgia, based on similar seaward-fining substrates, in a seaward-fining tidal-flat salt-marsh complex. Eight distinct sedimentary structures, and biological components. facies were identified, including the calcareous sandstone, Unit B (the Thorp Spring Member) consists of a peloidal, calcareous shale, bivalve shale, oyster shale, transitional molluscan packstone-wackestone facies. The fossil assemblage terrigenous-carbonate, dolomite, bioclastic packstone-grainstone, (including Orbitolina texana) suggests slightly brackish to near and shell fragment wackestone facies. These facies were normal marine salinites in a shallow, restricted platform lagoon. deposited in sand flats; small fluvial creeks; an ecologically com­ Thorough bioturbation indicates deposition in the well aerated plex, mud-dominated intertidal flat; and an subtidal, nearshore subtidal zone. Unit C is characterized by coarsening-upward lagoon. Thin bioclastic packstone-grainstones and dolomites of limestones capped with thin intertidal-supratidal deposits, in­ the high energy intertidal and supratidal environments regularly dicated a transitional nearshor lagoon-tidal flat environment. interrupt the terrigenous clastic facies. The microfossils are Repeated inundation with marine waters produced stacked, severely restricted to absent, and the macrofossils are dominated shallowing-upward sequences. The entire studied interval reflects by euryhaline bivalves (including Corbula martinae), gastropods, fluctuating sea levels along a line of section nearly normal to serpulid worm colonies, and dasychladacean algae. Other abun­ the paleoshoreline, and represents the final waning of Glen Rose dant nearshore features include salt-tolerant land plants, seas. dinosaur tracks, desiccation cracks, and localized concentrations of gypsum and barite. 'University of Texas at Arlington