ABSTRACT: Forward Modeling of High-Frequency, Glacio-Eustatic, Depositional Sequences: An Example from Middle Pennsylvanian Shelf Carbonates of the SW Paradox Basin, Honaker Trail, Utah

Robert K. Goldhammer, E. J. Oswald, P. A. Dunn

Middle Pennsylvanian (Desmoinesian) shelf carbonates in the southwest Paradox basin display three superimposed orders of stratigraphic cyclicity with a systematic, vertical succession of facies, parasequence, and sequence stacking patterns. Fifth-order parasequences (34 parasequences in 645 ft section; averaging 20 ft thick, 40 k.y. duration) are grouped into fourth-order sequences (averaging 100 ft thick, approximately 400 k.y. duration), which in turn vertically stack to define a third-order accommodation cycle (>= 650 ft thick, 2.4 m.y. duration). Fifth-order parasequences are comprised of shallowing-upward packages of dominantly subtidal shelf carbonates with sharp parasequence boundaries (exposure or flooding surfaces). Fifth-order parasequences are packaged into ourth-order sequences (type 1) bounded by regionally correlative subaerial exposure surfaces. These type 1 sequences contain a downdip, basinally restricted lowstand wedge of evaporites and quartz clastics, a regionally correlative transgressive shaly mudstone (condensed section), and a highstand systems tract composed of thinning-upward, aggradational, fifth-order, parasequences. Systematic variation in the thickness of fourth-order sequences (thinning followed by upward thickening), as well as the number of fifth-order parasequences/sequences (decreasing followed by increasing number), define a third-order accommodation trend, which is also regionally correlative. High-frequency parasequences and sequences are interpreted as aggradational allocycles generated in response to glacio-eust tic sea level fluctuations driven by Milankovitch climatic forcing. The fifth-order parasequences record the earth's orbital eccentricity cycle (mean period of 41 k.y.), and the fourth-order sequences result from the long eccentricity cycle (mean period of 413 k.y.). The parasequence, sequence, and facies stacking patterns have been replicated via computer by superimposing composite, high-frequency glacio-eustasy atop regional subsidence using depth-dependent, facies-keyed sedimentation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990