ROUCH, LINDA S., and L. J. WEBER, Mobil Exploration and Producing Technical Center, Dallas, TX

ABSTRACT: Alternative Sea Level Curves and Resultant Stratigraphic Geometries: High Resolution Computer Simulations Applied to the Middle Pennsylvanian (Desmoinesian), SE Utah

Computer simulation experiments are used to evaluate stratigraphic relationships of Middle Pennsylvanian (Desmoinesian) strata in southeast Utah. Relative changes in sea level are investigated along the southwest margin of the Paradox depocenter. Facies descriptions and stratigraphic cross-sections in the vicinity of the Aneth field indicate that the carbonate platform and surrounding basin underwent linear and uniform subsidence during deposition of the Desert Creek and Ismay intervals. Eustasy remains the controlling mechanism.

Three Desmoinesian sea level curves are tested: the Goldhammer Curve (I), the Van Eysinga Curve (II), and the Ross and Ross Curve (III) reveal different duration, periodicity, and amplitude. Each curve is input and simulations are run. Simulations that approximate observed stratal geometries are considered realistic. A cross-section which transects the Aneth carbonate platform, slope and basin will serve as the stratigraphic reference section.

Distinct differences exist among the three computer simulations. All simulations are based on a constant rate of subsidence (0.11 meter/1000 years) across the platform and throughout the duration of the simulation. Where periodicity differs but amplitudes are similar (Curves I and II) the major difference is in sequence thickness. Where amplitude varies but periodidty is similar (Curves II and III), distinctly different geometries occur. Each curve assumes a different duration for the Desmoinesian (Curve I, 3.2 my: Curve II, 10 my; and Curve III, 8.5 my). Longer duration produces a greater number of sequences.

A second and important variable that effects these models is the carbonate accumulation rate across the platform. The rate was varied by changing the lagoonal damping across the platform through time. This serves to exaggerate or subdue depositional topography. In the case of Curve I certain key features (bioherms and margin geometeries) could not be reproduced using carbonate accumulation rate to produce such subtle features as bioherm structures and margin geometeries. In the case of Curve III carbonate accumulation rates greater that accepted limits were needed to keep up with sea level rise. These simulations indicate that the

sea level curve proposed by Van Eysinga (1975) produces platform to margin sequences that resemble the stratigraphic reference section.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.