POTENTIAL CONSTRAINTS ON THE TIMING OF HALOKINETIC MEGAFLAP DEFORMATION; BIOSTRATIGRAPHY AND PALEOECOLOGY OF PERMIAN/PENNSYLVANIAN CARBONATES, BIG GYPSUM SALT ANTICLINE, SW COLORADO

The recognition of the onset of halokinetic deformation and megaflap formation is typically identified with seismic data in the subsurface. Here we provide biostratigraphic and paleoecological data from surface exposures which constrain the initiation of halokinetic deformation on the southeastern wall of the Big Gypsum salt anticline in southwestern Colorado. These data support the seismic and outcrop interpretations of Rowan et al. (2016), which suggest earliest salt movement after the deposition of the arkosic Lower Cutler Formation facies (Late Pennsylvanian to Permian). This study focuses on carbonate/clastic sequences which occur stratigraphically above the Desmoinesian/Missourian Honaker Trail Formation on the southeast megaflap described by Deatrick et al. (2014).

Here we describe a new measured section in the Lower Cutler Formation arkosic arenites and marine carbonates, which contain the highest stratigraphic occurrence of marine fossils with biostratigraphic potential. This section records sixty-two meters of strata which contain alternations of coarse felspathic litharenites and non-marine red siltstones with fossiliferous calcareous shales, nodular wackestones, packstones, and quartz and micaceous grainstones, which form five marine/non-marine parasequences. Marine fossils include abundant brachiopods, pelecypods, gastropods, crinoids, and three types of bryozoa, but the section lacks fusulinids. High abundances, but low faunal diversity, may suggest restricted lagoon conditions. We are currently analyzing carbonate samples for conodonts, to tie to the conodont biostratigraphic framework of Ritter and Barrick (2010). Coarse biostratigraphic constraints are provided by the brachiopod Composita subtilita and the pelecypods Aviculopinna paracuta, Nucula girtyi, and Wilkingia terminalis, which range from Pennsylvanian to Permian.

 These initial results suggest that the youngest marine conditions predate the onset of deformation and uplift of the Lower Cutler facies, and that these sediments are most likely Permian, but could be as old as Pennsylvanian at this locality. The arkosic Cutler Formation facies also range from middle Pennsylvanian to Permian, which supports this interpretation.