Abstract: High-Frequency Cyclicity during Eustatic Sea-Level Rise: Edwards Group of the Balcones Fault Zone

Susan D. Hovorka

Depositional and diagenetic patterns in the Edwards Group (Albian) of South Texas were strongly influenced by interactions among (1) Lower Cretaceous eustatic sea-level rise, (2), high-frequency cyclicity, and (3) gentle warping at the craton edge. The Balcones Fault Zone study area extends from the San Marcos Platform to the Maverick Basin.

High-frequency, upward-shoaling cycles on the San Marcos Platform are composed of basal transgressive condensed packstones; highstand sequences of burrowed wackestone, grain-dominated bar-flank packstone, grainstone bar deposits, and subtidal gypsum; and relative lowstand tidal-flat caps. Ten high-frequency cycles are stacked in two progradational-retrogradational intermediate-frequency cycles to form the Kainer and Person Formations. Strong world-wide Lower Cretaceous sea-level rise created accommodation even during lowstand and resulted in deposition of thick tidal-flat deposits. Prolonged exposure is recognized only at the top of the upper intermediate-frequency cycle (Person Formation).

In the Maverick Basin, changing cycle patterns and lithology during Edwards Group deposition document greater relative subsidence than occurred on the San Marcos Platform. All cycles are subtidal. High-frequency cycles in the West Nueces Formation are wackestone-packstone alternations, shoaling toward grainstone at the top. The overlying McKnight Formation is composed of high-frequency alternations of organic argillaceous limestone and subtidal gypsum. Stacking patterns show two intermediate-frequency upward-shoaling cycles. Cycle patterns are poorly expressed in the Salmon Peak Formation, indicating basin deepening and increased accommodation.

Poor match in intermediate-frequency cycle patterns between the San Marcos Platform and Maverick Basin is attributed to low-amplitude tectonic effects, although correlation uncertainties and facies contrast may also contribute.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas