Paleoenvironments and Trace Fossils of Large Aggrading Delta Margin Embayment--Upper Woodbine Formation of Northeast Texas: ABSTRACT

Abstract

ABSTRACT The broad Lewisville embayment of northeast Texas covers an area of some 30,000 km2; it developed during the latter half of Cretaceous (Cenomanian) Woodbine deposition as a result of reduced clastic influx and bypassing of sediment toward the south, where deltaic systems persisted. Strike-oriented shoreline sandstone bodies separated by finer grained shelf and backbarrier sediments are the dominant feature of the Lewisville Member. This marine-dominated succession reflects significant aggradation which kept pace with vertical rates of delta sand accumulation in adjacent areas along depositional strike. Classical regressive beach sequences are rare in outcrops of the strike-oriented sandstones. Instead, there is evidence of substantial modification by tidal channel incision, or by partial reworking during episodes of low-energy marine inundation. External and internal geometry of these sandstones is thus highly complex. Trace fossils are abundant and indicate locally discontinuous sedimentation. Thalassinoides and Ophiomorpha are dominant, but escape burrows characterize some beds. Oyster reefs developed in tidal channels and interbar depressions, and along bay or lagoon margins. Clays and silts of coastal lake, bay, and lagoon origin reflect seasonal variation in sediment supply and water chemistry. Small flood-tidal deltas and washovers developed in places along the seaward margins. Fluvial facies are prominent in outcrop but are not reflected in subsurface sandstone isoliths, presumably because of the volumetric dominance of the strike-oriented systems. Bed-load and mixed-load streams supplied fan deltas and small bayhead deltas respectively. Fan delta sandstones, with irregular Gilbertian foresets, but lacking the more typical upward-coarsening pattern, show a varied suite of trace fossils, including Gyrochorte, arthropod tracks, and delicate feeding traces.