CLIMATIC AND TECTONIC CONTROLS ON LACUSTRINE ISOTOPIC VARIABILITY WITHIN THE MIOCENE HORSE SPRING FORMATION, SOUTHERN NEVADA

The Horse Spring Formation (HSF), found throughout the Lake Mead area, contains several thick lacustrine carbonate units ranging in age from >24 to 12 Ma. Detailed mapping, geochronologic, tephrochronologic and stratigraphic work have allowed us to define a detailed chronostratigraphic framework for these carbonate units. These units have great potential to address two issues: the evaluation of tectonic models of continental rifting and Miocene climate change in the southwest.

We collected 715 stable isotope samples throughout the HSF from lacustrine authigenic carbonates. Here we present data from the Bitter Ridge Limestone (BRL, 13.9-14.4 Ma) and Lovell Wash (LW, 12-13.9 Ma) Members. We focus on data from two measured sections: the Lovell Wash Syncline (615 m thick) and White Basin (670 m thick) areas. The BRL contains 230-250 m of continuous microbialite limestone deposition that we sampled at 1-2 m intervals. δ¹⁸O curves from both locations have very similar trends, confirming our interpretation that these locations represent deposition in a single, large basin. The absolute values vary between the two curves, such that the White Basin values are consistently more positive than the syncline area values. We interpret this to reflect different depositional locations within the same basin, i.e. marginal (White Basin) vs. basinal (LW Syncline) settings.

We also compare the variations of these curves to the Laskar et al. (1993) insolation model and Zachos et al. (2001) marine isotope curves. Consistent variations in the BRL member suggest that Milankovitch cycles control some of the isotopic variability. At the start of LW deposition there is a change to fluvial deposition that may be climatically forced: the Zachos et al. (2001) marine δ¹⁸O curve shows an abrupt positive shift at this same time. This change in facies could also be explained tectonically, but the close association to this isotopic shift is enticing. This fluvial setting is replaced stratigraphically upward by a variety of depositional environments recorded by rapid lateral and vertical facies changes. δ¹⁸O values become highly variable both vertically and laterally throughout this portion of the upper LW. We interpret that this change is due in part to growth faulting, basin partitioning (Lamb et al., this volume), and increased hydrothermal activity.