Abstract
The pioneering and energetic work of Ernie Anderson in the Lake Mead region provides a rich database that has spawned several, competing tectonic models of Tertiary deformation in this geologically complex area. Testing these models relies in part on a complete and accurate picture of the regional stratigraphy, yet this picture is far from complete. In particular, the Tertiary Horse Spring Formation (HSF)--that appears to record the onset of extension and partitioning of the region into small, confined basins--is relatively poorly dated and facies relationships are only documented thoroughly in the South Virgin Mountains (Beard, 1996). Ongoing 1:5000 scale mapping, including geochronologic, structural, and stratigraphic studies, in the Longwell Ridges (LR) area provide a more solid chrono- and lithostratigraphic framework for the Rainbow Gardens (RBG), Thumb, and Bitter Ridge Limestone (BRL) members of the HSF. Detailed measured sections throughout the region show rapid lateral facies and thickness changes in the Thumb Member, much like those described by Beard (1996) in the South Virgin Mountains, supporting the idea that the Thumb records the onset of extension. Ten new Ar-Ar dates further support the facies heterogeneity of the Thumb and BRL Members and show that (1) either the RBG extends later in time than previously recognized or it was mapped incorrectly and it is actually of Thumb age; (2) the BRL, which was previously constrained between 13.2-13.5 Ma, is actually somewhat older, with a new sanidine age of 14.3 Ma deriving from the middle of the member in its type locality; (3) units exposed between LR are contemporaneous with rocks exposed in Echo Wash to the east and below the main body of the BRL to the west; and (4) a striking, basin-bounding buttress unconformity is strongly time-transgressive. Detailed mapping reveals sets of faults that postdate deposition, suggest strain partitioning between normal and strike-slip faults, and may shed light on the nature of Miocene deformation in the area. In order to test the tectonic models for this region, we argue that continued detailed chronostratigraphic studies tied to detailed mapping must accompany the structural work that has characterized the area up until now.