Fossil traces of the whitestone limestone and associated strata of the walnut formation, lower Cretaceous, south-central Texas
McCrevey, John Alfred
Warme, John E.
Master of Arts
The abundant burrows and borings in the pelletoidal oolitic Whitestone Limestone and the Cretaceous marls and limestones which physically surround it were studied in an attempt to integrate the ichnology with the known stratigraphy and paleontology into a paleoenvironmental synthesis. Distorted and compacted burrows are common in the marls because these substrates remained soft during reworking by infauna. The lowest unit, the Bee Cave Marl, was burrowed and reburrowed by thalassinidean crustaceans in a subtidal environment. The highest unit, the Keys Valley Marl, has a distinctive fossil trace assemblage that includes Rhizocoral1ium, thalassinidean crustacean burrows, and unbranched burrows that gently spiral about a vertical axis. The latter burrows were probably produced by crabs in an intertidal environment. Small oyster reefs in both marls have bored shells and intraclasts. Thus borers lived in the environment and were able to function on the hardened substrates present in the subenvironments in and around oyster reefs. A limestone sequence deposited between the two marls underwent varying degrees of submarine lithification. Burrows of thalassinidean crustaceans in a limestone wackestone, the Cedar Park Limestone, have distinct walls. The coarse internal fill of the burrows contrasts with the wackestone matrix. The burrows are not distorted by soft sediment compaction, but are broken by early compaction faults and styolitization. Because no borings are present it is probable that the Cedar Park Limestone hardened internally while remaining soft at the sediment-water interface. Two limestone grainstones, the Whitestone Limestone and the Edwards Formation (the rudist bearing limestones beneath the Whitestone Limestone) have a distinctive trace assemblage that includes burrows and borings. Internally the laminations and cross bedding are intersected by soft sedit ment escape burrows. The borings, produced by annelids, sipunculids, bivalves, and sponges, occur only at the top of each grainstone. Because no evidence of subaerial exposure is preserved at either one of these bored horizons, they probably represent surfaces of submarine lithification. In this study, the morphology of the fossil traces in the marls provides a criterion for distinguishing between subtidal and intertidal strata. Also, the morphology and style of preservation of the traces provides evidence for the early lithification of the limestones.