PENNSYLVANIAN BIOHERMS OF THE EASTERN MARGIN OF THE PARADOX BASIN, COMPOSITION, STRATIGRAPHIC POSITION, AND REINTERPRETATIONS

The high cliffs and alpine outcrops of the southern San Juan Mountains expose phylloid algal and Chaetetes dominated bioherms in two sequences within the Pennsylvanian Hermosa Group. In addition, biostromal deposits dominated by either Chaetetes are common, while those dominated by Komia are rare. We reinterpret a feature described as a mud mound by Mack and Miller (1980) as a structural feature resulting from the complex erosion of a fold axis along the Snowdon Fault.

The oldest bioherms occur in the lower Paradox Formation equivalent of the Barker Creek oil and gas interval, on an updip, shallow water fault block, early in the basin’s history. The relative abundance and internal zonation of fossil constituents in two of these bioherms near Molas Pass suggest that the structures formed as topographic highs on the seafloor. Two dimensional abundance data were analyzed using scaled digital images and image analysis software. The bioherm cores contain an average of 64% phylloid algal bafflestone and wackestone in the two outcrops. Phylloid algal abundance decreases vertically and toward the flanks of the mound. The top 0.5-meter of the mounds contains Chaetetes (55%), rugose coral (4%), and mats of the tabulate coral Syringopora (17%), which drape the mound flanks. Other fossil constituents include foraminifera, brachiopods, echinoderm fragments, and less common mollusks. Lowstand siliciclastics fill voids in the mound tops with reworked marine fossils.

Komia- bearing biostromes were described by Girdley (1969), but no distinct bioherms were observed in this study. This Komia-bearing bed occurs in sequence 11 at Coal Bank Pass, which correlated to the upper Akah interval (Perry et al., 2011). Much higher in stratigraphic section, phylloid algal bioherms occur in the prograding Honaker Trail equivalent strata, in sequence 16 of the Hermosa Cliffs (Gianniny and Miskell-Gerhardt 2009). These low amplitude, flat bottomed bioherms also appear to be constructional although access to the mound core is limited by the 200m cliffs. The basinward stepping occurence of these bioherms throughout the deposition of the Hermosa Group mirrors the initial flooding and subsequent progradtion of this margin of the basin.