Abstract
The Horse Spring Formation (HSF) was deposited in the Lake Mead region near the southwest edge of the Colorado Plateau (CP) from approximately 26 - 12 Ma, well before and during major extension from 17 - 8 Ma. Current research focuses on how the basins formed during large-offset extension and how climate and tectonics affected sedimentary facies at 100 ky and 1 my time scales. Here we highlight five emerging hypotheses. (i) Initiation of HSF deposition coincides with a major shift to a warmer global climate, the end of the Chuska erg on the CP, and start of incision related to CP uplift. Was incision on the CP and deposition on the CP margins driven by climate change and uplift of the CP? (ii) The lower HSF is dominated by siliciclastics, but there was a shift to carbonate and back to siliciclastics and gypsum from 18-15.5 Ma, and a shift from swamps and fluvial facies to lakes at approximately 17 Ma. This timing largely overlaps the global mid-Miocene climatic optimum (MMCO) that peaked at 17 - 14 Ma, but also coincides with the start of extension at 17 Ma. Were these carbonate lakes formed by a warmer, wetter climate or did new relief from faulting increase spring activity? (iii) The end of the lake-dominated period at 15.5 Ma coincides with progradation of alluvial fans across the basin from the hanging wall of major faults, and the start of landslides from their footwalls. 15.5 Ma is near the peak of the MMCO and the peak of footwall exhumation on the main detachment fault. Does peak of detachment faulting produce asymmetric alluvial fan/landslide facies across the basin that are enhanced by a warmer, wetter climate? (iv) The final HSF carbonate lake is the Bitter Ridge limestone (14.5 - 13.5 Ma) lying only in western Lake Mead that changed from open to closed, then abruptly ended at 13.5 Ma. Did this lake form by local faulting changes and increased groundwater flowing toward the extended Lake Mead region at the front of the Sevier highlands? (v) The HSF overall forms two cycles of sedimentation that culminate in carbonate lakes that abruptly end (17 and 13.5 Ma) when sedimentation changes to siliciclastics and gypsum. The two cycles are superimposed on the MMCO. We suggest that the end of the two cycles were driven by faulting initiation and reorganization, respectively, while the MMCO produced warmer - wetter conditions that enhanced carbonate lake formation and gravel progradation.