Abstract

The Lower Ordovician, passive margin succession of the Diablo Platform of the Tobosa Basin is represented by a second-order supersequence set, which consists of a basal transgressive clastic unit, the Bliss Sandstone, located above the breakup unconformity and overlain by several thousand feet of drift-related, shallow marine platform carbonates. The Bliss Sandstone (225 ft of shallow marine clastics) marks the second-order basal lowstand-transgressive phase and the overlying El Paso Group. (2500 ft of shallow marine platform carbonates) records the second-order highstand.

The El Paso Group. contains several third-order depositional sequences, which have been correlated utilizing biostratigraphy and cycle stacking pattern analyses from the Franklin Mtn locality east to the Hueco Mtns, southeast to the Diablo Arch (Beach Mtn. section at Van Horn, Tx.). The west Texas sequence stratigraphic framework is also correlated with age-equivalent rocks in the Ardmore Basin (Arbuckle Mtns, Ok.) and in the Appalachian Basin (Nittany Arch, central PA and the Great Valley, western MD). Due to late Paleozoic structuring of the Gondwanan passive margin, present-day exposures in Texas occur in an updip shelfal position and lack internal stratal geometries across depositional strike. Therefore, sequences and systems tracts are identified primarily on the basis of the vertical stacking patterns of depositional subfacies and higher frequency fifth-order cycles.

Accommodation plots, or “Fischer plots,” of high frequency cycles gauge systematic shifts in third-order accommodation of two complete third-order sequences, each of which is approximately 2 myr in duration and 200-450 ft thick, within the El Paso Group. This is expressed in the vertical succession of cycle types, systematic changes in cycle thicknesses, plus variations in subfacies as revealed by histograms of subfacies types tied to “Fischer plots.” A complete El Paso shelfal sequence contains a thin lowstand systems tract (LST) of quartz arenite, a thick transgressive systems tract (TST) dominated by upward-thickening, thrombolitic subtidal cycles, and a highstand systems tract (HST) marked by dolomitic, thinning-upward peritidal cycles containing admixed quartz sandstone.

The origin of the Lower Ordovician Diablo fifth-order cycles is best viewed as being driven by a combination of high-frequency, eustatic sea-level oscillations, operating in concert with autocyclic progradation. Both of these mechanisms were forced by third-order accommodation changes. The origin of the third-order sequences appears to be eustatic based on biostratigraphically-constrained correlation of accommodation plots around the periphery of Gondwana, from the Franklin Mtns. and Beach Mtn., to the Arbuckle Mtns, and to the central Appalachians.

In this study a model of El Paso Group facies architecture and a sequence stratigraphic model for Lower Paleozoic passive margin shallow-water platform carbonates are presented. These models focus attention on the vertical and lateral, reservoir-scale cyclic and subfacies architecture of carbonate shelfal deposits within a third-order depositional sequence framework.