A PHOTOGRAMMETRIC REEVALUATION OF SHARED ACCOMMODATION CAUSED BY BIOHERM OCCURRENCE IN THE DESMOINESIAN HERMOSA GROUP, PARADOX BASIN, UTAH

The Paradox basin is an elongate flexural basin that formed during the Pennsylvanian adjacent to the Uncompahgre Uplift, a part of the Ancestral Rocky Mountains. Parasequences within the Pennsylvanian Hermosa Group of the Paradox Basin have been interpreted to record glacio-eustatic fluctuations between open shelf highstands and restricted, inter uplift marginal basins. Previous interpretations of sea level from stratal stacking during the Desmoinesian may be inaccurate due to lateral thickness changes and subsequent unfilled accommodation related to the occurrence of bioherms. Thickened, shallow buildup facies thin laterally (≤6:1) into onlapping deeper water facies, resulting in possible local variation in parasequence thickness, which forms the basis for stacking pattern analysis. The goal of this project is to quantify bioherm occurrence, reevaluate inferred eustatic amplitudes and to use this information to improve basin-scale correlations.

We used photogrammetry to quantify Hermosa Group stratal geometries, bioherm thicknesses, and the interaction of bioherm-stacking in different sequences within a 10 kilometer stretch of the Gooseneck portion of the San Juan River in southeast Utah. To further facilitate correlations, we deployed handheld XRF and Gamma Ray spectrometers on two type sections within the photogrammetry dataset. These data were georeferenced using GNSS GPS and a reflectorless total station and provide centimeter-scale resolution of stratal geometries within the Barker Creek and Akah intervals. These data quantify the lateral continuity of parasequence and sequence thicknesses. This model provides an accurate representation of parasequence and sequence geometries allowing for a clearer interpretation of shared accommodation and Desmoinesian eustatic fluctuation in the southern Paradox basin. We will use the 3D dataset and several subsurface cores to correlate in detail across the southern Paradox basin to Hermosa Group exposures in the San Juan Mountains, southwest Colorado.