DO LACUSTRINE INUNDATION PATTERNS RECORD EARLY EXTENSION IN THE LARAMIDE FORELAND PROVINCE? INSIGHTS FROM COMPARISONS WITH MODERN BASEMENT ELEVATIONS

Lake basins can be sensitive recorders of tectonic processes because they nearly universally occur in areas of localized relative subsidence. Lake basins formed within and adjacent to the North American Cordillera in a variety of tectonic settings during the Paleogene. In the contraction-thickened hinterland of the orogen, lake basins that formed atop folded and faulted Paleozoic bedrock are spatially and temporally related to early extension and magmatism that accompanied or followed removal of the Farallon flat slab and/or delamination of the North American mantle lithosphere. In contrast, Paleogene lakes in the foreland formed initially in a contractional setting, evolving largely within the confines of broad downwarps abutted by reverse fault-bounded “Laramide” basement arches. Evidence for Eocene extension in the foreland analogous to that characterizing the hinterland is lacking, and it is widely believed that the foreland did not experience the magnitude of crustal thickening recorded in the hinterland. Several Neogene lake basins resulted from modifications of basement structures, but are largely viewed to have resulted from post-Paleogene processes. It remains uncertain whether any of these structures formed during the Eocene, coeval with the end-stages of the Laramide orogeny.

We used recent geochronology and mapping in conjunction with a recent compilation of basement elevations to more closely examine the spatial relationship between lacustrine inundation and crustal deformation across nine time slices from the late Paleocene through the Miocene. Early Paleocene lakes correspond to the deepest structural positions across the region, suggesting that they formed due to direct structural downwarping. Early Eocene lakes are similarly largely confined to structural lows during the main phase of Green River Formation lakes. After 49 Ma, however, lacustrine inundation increasingly occurred in areas where basement elevations exceed the regional mean. In some cases, lake strata lie directly atop basement arches, and, in the case of the Rattlesnake Hills, are interbedded with local volcanic rocks. We hypothesize that the observed shift in the location of tectonic lakes from basin floors to structural highs resulted from previously unrecognized Eocene extension and collapse of gravitationally unsupported basement arches. Research is underway to refine the chronology of events and to assess whether the observed “landscape inversion” occurred due to Farallon slab removal or other tectonic processes.