IS THE TRANSITION IN “HOT WATER?”; PETROGRAPHIC ANALYSIS OF THE MARINE TO NONMARINE TRANSITION IN THE PENNSYLVANIAN HONAKER TRAIL FORMATION AND CUTLER FORMATION, PARADOX BASIN, SOUTHWESTERN COLORADO

Trimble Canyon north of Durango, Colorado, exposes the transition from the marine Pennsylvanian Honaker Trail Formation to the terrestrial Cutler Formation. The age of the Cutler Formation at this location is unconstrained and may straddle the Pennsylvanian-Permian boundary. Fault systems and potentially porous strata in this area host commercially important geothermal resources. Here we document the transition between these two units and identify strata with higher porosities and hence geothermal resource potential.

This study includes a stratigraphic section of the top 80 meters of the Honaker Trail Formation and 50 m of Lower Cutler non-marine strata. The upper section of the Honaker Trail Formation is composed of 3-4 sequences composed of alternating fluvial, deltaic, and shallow water limestones. The overlying 50 meters of basal Cutler Formation contains 3 alternations. Each alternation contains coarse-grained hematite-stained fluvial channel deposits with micaceous mudstones, and fine-grained sandstone, with rhizohalos and insect burrows. This stratigraphic succession is indicative of the prograding clastic wedge which overfilled the eastern Paradox Basin.

Petrographic analyses are based on 26 samples and thin sections which represent the range of lithologies in this section. Porosity was estimated for carbonates, mud-rich sandstones, and medium-grained fluvial sandstones. Initial results indicate that highest porosities were found in the upper-fine to medium grained fluvial deposits of the Honaker Trail Formation. These fluvial sandstones may host additional geothermal resources outside of the fault zone.