Thickness- and Facies-Incomplete Parasequences; Pennsylvanian Ice-House of the Paradox Basin, Utah

Simo J. A. Toni and Gary L. Gianniny

High resolution outcrop stratigraphy along the spectacular strike-oblique outcrops in the San Juan River, Utah has shown that sediment supply of both carbonates and siliciclastics lagged behind changes in accommodation space and led to unfilled accommodation space and incomplete facies successions.

The Lower Desmoinesian Barker Creek and Akah productive intervals show a complex mosaic of mixed carbonate-siliciclastic facies that define multiple, at times overlapping and interacting, scales of stratigraphic cyclicity. The 165 m thick section contains numerous (55-63) parasequences grouped into 25 subaerially-bounded sequences and 3 sequence sets bounded by surfaces of significant fluvial incision. Twelve facies combine to form 50 different vertical facies successions within parasequences; the low number of repeated successions is interpreted to be the result of lateral facies substitutions and non-Waltherian patterns. Similarly, parasequence thickness and facies diversity do not correlate, suggesting either that the thickness of the shallowing upward facies' successions is not co trolled by changes in water depth, or that the facies record of water depth is incomplete. 10-15% of the parasequences are subtidal facies capped directly by subaerial exposure surfaces, making these facies-incomplete parasequences indicative of non-Waltherian successions. Neither parasequence or sequence thicknesses correlate with inferred bathymetric maxima.

The Paradox Basin illustrates that during Pennsylvanian, high-amplitude composite relative sea level changes the distribution of facies within parasequences is highly variable as a result of lithofacies substitutions and non-Waltherian facies transitions. Stacking of thickness patterns do not provide reliable proxies of accommodation history.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California