Abstract: Late Carboniferous Mixed Carbonate-Siliciclastic Shelf Sequences, Paradox Basin, Utah: Controls and Sequence Attributes
Gary L. Gianniny, J. A. Toni Simo
The Lower Desmoinesian (Late Carboniferous) Barker Creek and Akah productive intervals exposed in the incised meanders of the San Juan river illustrate that cyclicity, facies and geometries of a mixed system vary by the interaction of sea level history, relative position on the shelf, syndepositional topography, sedimentary processes, siliciclastic influx, and differential compaction and erosion. All of these parameters combine to change, from sequence to sequence, facies successions in both dip and strike sections.
The 165 m thick section comprises 31 exposure-bounded sequences grouped into 3 sequence sets. Sequence sets are bounded by fluvial incision, or very coarse non marine sandstones. The partially exposed basal sequence set is characterized by 9, thin, subtidal and subtidal-intertidal sequences with peloidal and skeletal limestone facies. Sequence set two consists of 16 sequences, and sequence set three contains six subtidal sequences. Sequence thickness in sequence sets two and three vary from 27 to 1 meters. These sequences form thinning and thickening bundles. Beginning and end of the bundles change from section to section (e.g. in an updip column a bundle commonly starts one sequence earlier than a downdip column). These changes in bundling reflect the effects of progradation and shin ling, position on the shelf, and topography and compaction around carbonate buildups. Sequence thickness and facies diversity are positively correlated. Thick sequences show a more complete succession with a lower onlaping wedge of siliciclastic sandstone, an up-dip transgressive open-marine carbonate, an early highstand prograding mixed carbonate-siliciclastic sandstone and siltstone, and a late highstand succession of carbonate buildups, oolitic shoals and stromatolites. Quartz-rich sandstones in both the low-and highstand are volumetrically important in thick sequences. Thinner sequences lack of highstand carbonate buildups and mixed carbonate-siliciclastic sandstones, and peloidal and skeletal limestone, and algal laminates dominate.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France