New stratigraphic and geochronologic data from the lower Horse Spring Formation, Longwell Ridges, Lake Mead, Nevada: tectonic implications for the Lake Mead extensional domain
Wednesday, September 28, 2005
4:00 - 5:00 pm
LFG 102, UNLV Campus
Dr. Paul Umhoefer
Associate Professor and Graduate Program Coordinator
Department of Geology, Northern Arizona University
http://jan.ucc.nau.edu/~pju/
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 <16.8 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 ~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 ~15.5 Ma in both
areas, and continue up section to ~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
~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 - ~14.4 Ma, and a second stage of
mixed-mode strike-slip + normal faulting + local(?) N-S contraction
along the sinistral Lake Mead fault system from ~14.4 to ~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.
Updated September 14, 2005