SEQUENCE STRATIGRAPHY OF COASTAL MIXED CLASTIC-CARBONATE FACIES: PERMO-CARBONIFEROUS OF NORTH CENTRAL WYOMING, USA

Dott (1996) argued for the coexistence of sequence- and event-scale stratigraphy. The Amsden Fm and Tensleep Sst are well known for interstratification of clastic and carbonate facies. The case is made herein that these two units taken together constitute a stratigraphic sequence in the sense of Sloss (1963); furthermore, the infra-formational stratigraphy is best regarded as parasequence (PS) stratigraphy.

Amsden lower contact and Tensleep upper contact have characteristics expected for sequence boundaries. Lower contact with lower Carboniferous Madison Grp is deeply karsted with eolian and fluvial clastics infilling a highly irregular erosional surface. Upper contact with the middle Permian Goose Egg or Phosphoria Fm is also highly irregular erosional surface with stratal truncations below and stratal onlaps above. In addition, the upper sequence boundary progressively truncates deeper in to Tensleep PSs from south to north in Bighorn Mtns exposures.

PS definition and correlation are established for the upper member (Ranchester Lst) of the Amsden and for the Tensleep Sst. The Amsden lower two members (Darwin Sst and Horseshoe Sh) are poorly exposed and include a complex mosaic of mostly clastic facies; a transgressive surface is placed above a pisolitic paleosol in the Horseshoe. Nine PSs (1-9) are recognized in the Ranchester with statistical analysis of ascending facies transitions indicating an ordered succession of offshore clastic mudstone à restricted marine subtidal carbonate à peritidal carbonate à carbonate paleosol. Twelve PSs (A-L) make up the Tensleep. The transition interval from the Ranchester to overlying Tensleep is a complex mosaic of clastic and carbonate facies: offshore clastic mudstone à restricted marine carbonate or shoreface/foreshore à variegated mudstones. Eolain facies stratigraphically appear in PS B in northern exposures and PS D or E in southern exposures. Ordered ascending facies transitions include: open to restricted marine subtidal carbonates à peritidal carbonates à eolian sandstones or à shoreface/foreshore sandstones à eolian sandstones and eolian sandstones à deformed eolian sandstones. PS 1-9 exhibit a stacking and facies patterns indicating onlap (TST) to the north. By contrast PS A-L exhibit offlap (HST) to the south and southeast across the Bighorn Mtns.