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

Abstract

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 ky. 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 fourth-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-eustatic sea level fluctuations drivenmore » by Milankovitch climatic forcing. The fifth-order parasequences record the earth's orbital eccentricity cycle (mean period of 41 ky.), and the fourth-order sequences result from the long eccentricity cycle (mean period of 413 k.y.).« less