HYPERSALINITY AND STROMATOLITIC BIOHERM GROWTH IN THE DEVONIAN ELBERT FORMATION OF THE SOUTHERN SAN JUAN MOUNTAINS, COLORADO

Here we describe newly excavated stromatolitic bioherms found in the upper part of the McCracken Sandstone member of the Devonian Elbert Formation in southwest Colorado near Durango. Although stromatolites have been described in this formation before, new road cuts reveal previously undocumented biohermal mounds. These bioherms may provide insights to microbial carbonate reservoirs similar to those in offshore Brazil.

The eight meter-thick stratigraphic succession in the McCracken Sandstone Member at this locality includes a basal light brown to reddish brown quartz arenite overlain by thin layers of siltstone, shale, and dolomitized peloidal grainstones with bidirectional current ripples. Above these are shaley sandstone and dolomudstone which form a continuous marker bed beneath the bioherms.

The bioherms are 1-3 meters thick and six meters long, with crests 6-14m apart. The bioherms have a relief of 0.5-1.5 meters between the crest and intermound low. Etchings on fracture faces highlight crinkly laminated microbial laminae, rip-up clasts, and calcite-filled fenestral porosity. Rip-up clasts of lithified blocks of boundstone, and sand filled fractures indicate intermittent subaerial exposure between episodes of bioherm growth and halite hoper molds indicate evaporitic conditions. Estimated primary porosity based on thin sections of these boundstones ranges from 0 to 20% which has later been occluded by calcite spar. Dolomite crystal sizes in the boundstones are approximately 40-50 microns suggesting a synsedimentary or penecontemporaneous origin. Biomoldic pores are lined with a coarser crystalline dolomite (~ 230 microns) indicating a second dolomitization event.

The transition from sandstone in the bottom of the outcrop to dolomite and boundstone suggest that the area was undergoing a transgression during the late Devonian. Basal shallow clastic-dominated shore facies are overlain by intertidal evaporitic facies with the stromatolitic bioherms and halite hopper-molds. This deepening trend continues into open marine facies of the Devonian Ouray Limestone. Previous workers have demonstrated erosional relief along the base of the Elbert Formation which could have provided the accommodation space and preservation of these carbonate bioherms.