DETERMINING STYLES OF HANGING-WALL DEFORMATION IN CONTINENTAL EXTENSION, THE LOVELL WASH MEMBER OF THE HORSE SPRING FORMATION, LAKE MEAD AREA, NEVADA

The Lake Mead area is one of the best places in the world to study continental rift processes. In the last four decades, several key papers from this area have put forth important theories of extensional processes, including the idea of simple detachment faulting followed by major extensional breakup of the hanging wall (Spencer et al., 2001) versus the sequential development of a rolling hinge or domino-style faulting or a hybrid of these two (e.g., Brady et al., 2000). Consensus, however, has not been reached and competing theories remain. Umhoefer et al. (2010) proposed a two-stage model of detachment faulting followed by breakup of the hanging wall by transtensional and normal faults. The Horse Spring Formation (HSF; >24 to 12 Ma), with excellent exposures and numerous tuffs, records Basin and Range extension in the Lake Mead area, making it ideal to test models of rift processes. Our work suggests that the chronostratigraphy and stratigraphic relationships of the HSF do not support a model of continuous progressive deformation of the hanging wall through time and space but instead suggest cycles of deformation where a single large basin is broken up by faults into smaller subbasins over a short time interval, less than 100 ky.

The HSF contains 4 members that record two such cycles. The oldest Rainbow Gardens Member lake is broken up into smaller basins as recorded by the Thumb Member. A second, younger cycle begins with the Bitter Ridge Limestone (BRL) Member that records ~200 m of continuous, homogenous deposition of stratiform lacustrine microbialites over half a million years. At about 13.9 Ma, the lake is replaced by widespread fluvial deposition recorded in a series of siltstone and sandstone packages of the Lovell Wash (LW) Member. Both the lacustrine and fluvial sequences are uniform across a few tens of kilometers. Shortly after this, however, deposition changes to mixed siliciclastic and carbonate deposition with numerous rapid lateral and vertical facies changes. In the LW Syncline area, fault-bounded basin margins, growth faults, and large intra-formational clasts of BRL require the development of local structural and topographic relief during LW time. Recent detailed mapping and stratigraphic work in the White Basin area suggests that syndepositional faulting occurred mainly in this area from 13.43 to 12.9 Ma.