STRUCTURAL AND SEDIMENTOLOGICAL DEVELOPMENT OF FOOTWALL GROWTH SYNCLINES ALONG AN INTRAFORELAND UPLIFT, EAST-CENTRAL BIGHORN MOUNTAINS, WYOMING

Structural, sedimentological, and provenance data from Paleogene synor ogenic deposits of the east-central flank of the Bighorn Mountains pro vide new information about the development of footwall growth syncline s, the evolution of fault-related folds, and the erosional unroofing h istory of intraforeland uplifts, Three conglomerate units, the upper c onglomerate member of the Fort Union Formation and the Kingsbury and M oncrief Members of the Wasatch Formation, are incorporated within an a symmetric, east-verging growth syncline in the footwall of the main ra nge-bounding thrust system, Three stages of footwall deformation are r ecorded within these conglomerates. Analysis of mapped progressive unc onformities, retrodeformed balanced cross sections, and conglomerate c last composition data define these stages as part of a continuum of de formation associated with the development of footwall growth synclines . Development of an anticline-syncline pair marked the earliest stage of growth syncline formation (stage I), Rotation of the shared fold li mb resulted in amplification of the growth syncline, Fine-grained, syn orogenic sediment derived from easily eroded Mesozoic mudstone bypasse d the growth syncline during this stage, By the end of Lebo Shale depo sition, an average of 12.1% of shortening and 6.46 km of uplift had oc curred along the range margin, Continued growth syncline development w as marked by the deposition of the Kingsbury Conglomerate, The Kingsbu ry Conglomerate was derived from resistant, middle and lower Paleozoic carbonate strata in the uplifted source terrane, Intraformational unc onformities, recording as much as 55 degrees of bed rotation, were dev eloped within the Kingsbury Conglomerate as fold limb rotation occurre d coeval with deposition, Cross sections indicate that during this ear ly stage of fault-related folding, an average of 16.9% shortening and 8.12 km of uplift occurred along the eastern flank of the Bighorn Moun tains (end of stage I). The intermediate stage (stage II) of footwall growth syncline development involved partial truncation of the growth syncline by the advancing thrust faults and deposition of the Moncrief Conglomerate, The lower portion of the Moncrief Conglomerate was rota ted basinward in the developing growth syncline, The final stage of de formation (stage III) was dominated by the thrust faulting of middle a nd lower Paleozoic strata eastward over steeply dipping Mesozoic strat a and rotated Eocene synorogenic conglomerate. During this stage of de formation, the Moncrief Conglomerate was deformed, as the initially bl ind thrusts propagated into the near-surface conglomerate deposits, tr uncated the entire footwall syncline, and overrode the synorogenic con glomerate package, Cross sections in areas where this final stage of d eformation is well developed indicate that an average of 24.1% shorten ing and 9.7 km of uplift had occurred along the eastern margin of the Bighorn Mountains, The caliber of synorogenic deposition in the Powder River basin was linked directly to the lithologic composition of the Bighorn Mountains, Approximately half of the 3.6-km-thick source-strat igraphic section of the eastern Bighorn Mountains was eroded prior to accumulation of conglomerate, The majority of this eroded material was derived from Mesozoic mudstone and poorly indurated sandstone that we re incapable of generating coarse detritus, The first Paleogene conglo merates deposited along the east-central Bighorn Mountains, therefore, do not represent the initiation of Laramide uplift, but instead repre sent the exposure of coarse-clast-forming rock types from the lower ha lf of the hanging-wail stratigraphic section (i.e., the Mississippian Madison Limestone and Ordovician Bighorn Dolomite).