Abstract
New stratigraphic studies and 10 new tuff ages refine the lower Horse Spring Formation at Longwell Ridges (LR) in the Lake Mead area. A lower unit was deposited slowly from <17.5 to 15.68 Ma at the unfaulted NE-trending basin margin as a wedge of conglomerate interfingering laterally and vertically with sandstone and gypsum. An upper unit was deposited rapidly from 15.68 to approximately 14.4 Ma with a lacustrine limestone overlain by a thick fluvial sandstone. Correlation of LR strata to the south Virgin Mountains (SVM) strata (Beard, 1996) provides new ideas on Lake Mead evolution. The lower unit of LR looks to be pre extensional with facies and slow sedimentation that are compatible with the margin of the uppermost Rainbow Garden lake in the SVM. An abrupt change to a lacustrine limestone at 15.68 Ma in LR is correlative to an unconformity and the widespread lower Thumb Limestone at SVM; increased sedimentation and local megabreccias in the SVM indicate initiation of extension. Widespread fluvial clastics ended lake deposition by approximately 15.5 Ma in both areas, and continue up section to approximately 14.4 Ma. The 15.68-14.4 Ma faulting was part of the south Virgin-White Hills detachment system related to uplift of the Gold Butte Block. Many changes occurred at approximately 14.4 Ma: (i) in the SVM, progradation of conglomerates and local megabreccias were shed from Gold Butte and northeastern areas; (ii) major initiation of sinistral strike-slip faults of the Lake Mead system; (iii) major lake deposition began in the LR and White Basin as thick gypsum and the Bitter Ridge Limestone; (iv) slowing of sedimentation in the hanging wall basin of the south Virgin-White Hills detachment suggests slowing of faulting. These conclusions suggest a first stage of extension on the south Virgin-White Hills and Grand Wash Faults from 15.68 to approximately 14.4 Ma, and a second stage of mixed-mode strike-slip + normal faulting + N-S contraction along the sinistral Lake Mead fault system from approximately 14.4 to approximately 8 Ma. The initial detachment faulting may have triggered major lower crustal flow and a complex upper crustal response in the second stage as combined E-W strike-slip faulting and extension, and N-S shortening. The latter suggests that the models of Ernie Anderson may be largely correct, but that the complex "occlusion" or tectonic escape may have been a response to an earlier phase of detachment faulting.