Syntectonic Unconformity Development, Facies Stacking Patterns, and Unroofing History Recorded in Upper Cretaceous Synorogenic Growth Strata, Cordilleran Foreland Basin, Southern Nevada

J. L. Aschoff¹ and J. G. Schmitt^2
1 Jackson School of Geosciences, University of Texas at Austin, Austin, TX
² Department of Earth Sciences, Montana State University, Bozeman, MT

Understanding contractional growth strata is essential for deciphering kinematics of basinward-propagating thrust faults at the interface between evolving wedge-top and foredeep depozones of foreland basin systems. Documenting syntectonic growth strata geometries in concert with detailed analysis of the sedimentology and petrology of syn- and post-growth strata provide further insight into the nature and relative timing of thrust-related deformation. Such analyses of Upper Cretaceous growth strata (Baseline Formation) within the Cordilleran foreland basin of southern Nevada provide an example of the influence of frontal thrust-cored anticlinal ridge growth on evolving alluvial systems. These growth strata contain a syn-growth succession of three relatively thin unconformity-bound packages overlain by a thick, post-growth onlapping succession. Facies within the syn-growth packages include sandy, braided fluvial/sheetflood fan deposits, documenting three closely-spaced thrust propagation events. Each successively younger unconformity-bound package contains more proximal conglomeratic deposits. Paleosols and zones of eogenetic sandstone cement locally demarcate angular unconformities and formed when older syn-growth strata were locally tilted and exposed during fold propagation. Stacking patterns are defined by changes in grain-size, clast type and stratal architecture. Growth strata development accompanied erosion of only the stratigraphically highest levels of the anticline source area, as uplift rates initially outpaced erosional rates. With increased fold relief, new consequent drainage basins developed and debris-flow alluvial fans were established. Thick, post-growth successions of alluvial fan conglomerates progressively onlap syn-growth strata, contain a well-developed unroofing sequence, and document final stages in the erosional denudation and downcutting of the hanging-wall anticline as thrusting and fold growth waned.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005