Cordilleran foreland basin evolution in response to interactive Cretaceous thrusting and foreland partitioning, southwestern Montana

Cretaceous strata of southwestern Montana indicate that the large, first-order Cordilleran Foreland Basin was tectonically partitioned by a complex of second-order intraforeland uplifts that occupied sites of eventual classic Laramide intraforeland uplifts. The intraforeland uplifts and intervening basins formed a block-like surficial pattern throughout the evolving Cretaceous foreland. Preliminary stratigraphic and sedimentary petrologic evidence suggests that some of the same structures were active during Jurassic time. Thus, the Laramide orogeny in southwestern Montana can be viewed as the structural culmination of a long (at least 100 m.y.) history of basementinvolved deformation. The developmental history of the Cordilleran Foreland in southwestern Montana is divisible into three stages: (1) an early stage of embryonic foreland development in Late Permian-Late Jurassic time, characterized by carbonate shallow-marine sedimentation with local coarse siliciclastic input from intraforeland structural elements; (2) a middle stage of fully developed, first-order foreland-basin subsidence in Early-early Late Cretaceous time, characterized by siliciclastic-dominated nonmarine and shallow-marine sedimentation; major reactivated intraforeland uplift (and subsidence); and eastward encroachment of the Cordilleran (Sevier) thrust belt; and (3) a late stage, during latest Cretaceous-early Eocene time, characterized by impingement of the frontal Cordilleran thrust belt upon major intraforeland uplifts (Laramide foreland uplifts) and nonmarinedominated synorogenic sedimentation. The early stage (Late Permian-Late Jurassic) of foreland development was temporally associated with terrane accretion and other tectonic events in the Cordillera. The onset of first-order basin subsidence and a coeval increase in intraforeland tectonism were temporally associated with a second phase of Cordilleran tectonism and eastward migration of the Cordilleran thrust belt. Deposition of the Cretaceous Kootenai, Blackleaf, and lower Frontier Formations in southwestern Montana occurred during this middle stage of foreland-basin evolution. Source areas consisted of the Cordilleran fold-thrust upland, intraforeland uplifts, and the unstable craton. Thus, the Cretaceous stratigraphic sequence in southwestern Montana was produced by the interaction of Cordilleran and intraforeland tectonic events. The entire Kootenai-Blackleaf-lower Frontier sequence developed during a single cycle bounded by tectonic maxima (Cordilleran and intraforeland) with an intervening period of tectonic quiescence and continued foreland-basin subsidence. However, this longer term (∼20 m.y.) sequence was punctuated by higher frequency (∼3 to 4 m.y.) events, most clearly signalled by intraforeland reactivation. The first-order behavior of the Cordilleran Foreland Basin in southwestern Montana is partly explained as the flexural response to thrust-sheet loading of a viscoelastic lithosphere. However, explanation of foreland-basin evolution in Montana requires a model that incorporates an additional, interactive mechanism for broad-scale reactivation of second-order (i.e., intraforeland) elements.