Third-Order Sequences and Parasequence Stacking Patterns of Lower Ordovician Platform Carbonates of the El Paso Group, Franklin Mountains, West Texas

GOLDHAMMER, R K, P. J. LEHMANN, and P. A. DUNN, Exxon Production Research Co., Houston, TX

The Lower Ordovician passive margin succession of the Franklin Mountains is represented by the second-order Sauk C supersequence .set consisting of the Bliss Sandstone (225 ft of shallow-marine clastics) and the overlying El Paso Group (2500 ft of shallow-marine platform carbonates). The Bliss marks the second order basal lowstand-transgressive phase and the El Paso Group records the second-order highstand. The El Paso Group contains several third-order depositional sequences, which in this updip, shelfal position lack internal stratal geometries along dip. Thus, sequences and systems tracts are identified solely on the basis of the vertical stacking patterns of depositional subfacies and higher frequency, fifth-order parasequences (ave. 44,000 yrs/parasequence; 8 ft/parasequence)."Fi cher plots" of high-frequency parasequences gauge systematic shifts in third-order accommodation of two complete third-order sequences (each approx. 2 myr duration, 200-450 ft thick) within the Arenigian portion of the El Paso Group (i.e., Chamizal through Ranger Peak formations). This is expressed in the vertical succession of parasequence types, systematic changes in parasequence thicknesses, plus variations in subfacies as revealed by histograms of subfacies types (e.g. % peritidal laminite/parasequence, % thrombolitic bioherm/parasequence, etc.) tied to "Fischer plots." A complete El Paso shelfal sequence contains a thin lowstand (LST) of quartzarenite, a thick transgressive systems tract (TST) dominated by upward-thickening, thrombolitic subtidal parasequences, and a highstand syste s tract (HST) marked by dolomitic, thinning-upward peritidal parasequences containing admixed quartz sand. We have investigated the mechanics of third-order sequence formation and contrasted allocyclic models with autocyclic models for high-frequency parasequence formation through 1-D and 2-D forward modeling.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)