STRATIGRAPHIC ANALYSES ON LARAMIDE SYNOROGENIC DEPOSITS: UNROOFING OF THE BEARTOOTH PLATEAU, MONTANA & WYOMING

Synorogenic sedimentary rocks exposed on the northeast perimeter of Beartooth Mountains, Montana and Wyoming, record tectonism and deposition in the central Rocky Mountains during the Laramide orogeny. Synorogenic strata in the western Bighorn Basin are variously mapped as Paleocene Fort Union Formation and Eocene Willwood Formation. This study uses a multi-disciplinary approach with petrographic and stable isotope datasets that were collected from the strato-type section of Beartooth synorogenic units.

Stable oxygen and carbon isotope data were collected from calcite cement and pulverized-clast subsamples at measured intervals with the composite section. Recycled marine carbonate clasts have the highest δ¹⁸O values, -5.79‰ to 10.43‰, whereas Paleogene fluvial cements have the lowest values from -11.76‰ to -1.18‰ (V-PDB). Assuming early fluvial cements formed in equilibrium with shallow groundwater, and an isotopic lapse rate of 2.9‰/km, the precipitation source was likely from the plateau elevated 4.0 ± 0.5 km asl, or ~3.5 km above the elevation of the early Eocene Bighorn Basin. This elevation difference is higher than the modern difference of 1.5 to 2.5 km. δ¹³C values from fluvial cements range from -3.63‰ to 2.61‰ (V-PDB) with little systematic change, which is consistent with the presence of a warming climate by the early Eocene.

Petrographic data from sandstones in the western Bighorn Basin show calcite cements constituting up to 40% of proximal sandstones. A high initial porosity with uniform intergranular sparite growth has been interpreted as a primary diagenetic calcite, supporting the use of stable isotopes as a proxy for paleoclimate. However, some samples displayed grains with carbonate replacement. Provenance of sandstone modal compositions indicate the proximal sediments were derived from a recycled orogen and exhibit an unroofing sequence; this is consistent with other Laramide basins where Mesozoic through Archean source rocks progressively eroded during the Paleogene. Further aspects of this study include the synthesis of magnetic-reversal stratigraphy for the type section to provide better age control. These interdisciplinary datasets aim to correlate proximal sedimentation off the Beartooth Plateau with the well-defined formations within the Bighorn Basin.