Abstract

Although the multiple causes of stratigraphic cyclicity have received much attention for over a century, the interactions among these causes remain problematic especially when different paleoclimatic settings are considered. Here we demonstrate that limited supply of both carbonates and siliciclastics could not fill available accommodation space in some settings during the Pennsylvanian icehouse climate. Consequently, thickness-based stacking patterns provide a poor record of changes in accommodation space, and should neither be used to correlate stratigraphic sections across the shelf, nor to predict exposure surfaces and associated diagenetic processes.

This study describes the Middle Pennsylvanian (Lower Desmoinesian Akah and Barker Creek intervals) mixed carbonate-siliciclastic shelf strata of the Paradox Basin within a sequence stratigraphic framework. Thirty-five kilometers of spectacular strike-oblique outcrops along the San Juan River, near Mexican Hat, Utah, reveal a complex mosaic of facies in a range of both continuous and discontinuous facies belts that step shelfward and basinward. The distribution of facies geometries and compositions define multiple, at times overlapping and interacting scales of stratigraphic cyclicity. The 165 m section contains 55 to 63 flooding surface-bounded parasequences (average thickness is 2.7 m, range 0.5-10 m) bounded by flooding surfaces, grouped into 25 subaerially-bounded sequences, and into three sequence sets bounded by surfaces of exposure and fluvial incision. Thirteen lithofacies are recognized and integrated in a low-angle bank-model in which lithofacies distribution was controlled by circulation, turbulence, salinity, and sediment growth potential rather than a simple linear facies relationship between water depth and distance from the shore of typical basin models. Lithofacies successions show 50 different vertical patterns. The low number of repeated lithofacies successions (8/50 patterns repeated, maximum number of repetition of one pattern is 4) is interpreted as the result of lateral substitutions among coeval lithofacies. Similarly, neither parasequence thickness and facies diversity, nor parasequence thickness and the maximum paleowater depth inferred from lithofacies successions show a correlation, indicating that thicknesses of shallowing-upward successions are not controlled by changes in water depth, and that the lithofacies record of changing water depths is incomplete. Geometries and lithofacies successions within parasequences and sequences suggest that deposition occurred during sea-level rise, virtually ceased as a function of sediment starvation during maximum inundation, and resumed during relative sea-level fall. The recognition of different bounding surfaces and the comparison of trends in interpreted paleowater depth, internal geometries, and degree of facies shifts across the different surfaces has provided a stratigraphic framework that improves our understanding of the depositional systems.

These examples illustrate that the distribution of lithofacies within parasequences and sequences are highly variable as a result of lateral lithofacies substitutions and “non-Waltherian” vertical lithofacies transitions during Pennsylvanian high-amplitude composite relative sea-level changes. In addition, the recognition of unfilled accommodation space demonstrates that thickness-based stacking pattern analyses can not provide reliable proxies of accommodation history unless they are integrated with patterns of two- and three- dimensional facies composition, geometry, and stratal surface type.