WEDGE KINEMATICS AND PALEOGEOGRAPHY OF THE SEVIER RETROARC: BIOSTRATIGRAPHIC CONSTRAINTS FROM THE LOWER CRETACEOUS NEWARK CANYON FORMATION

A synorogenic sedimentary system preserved in the Sevier hinterland of the North American Cordillera provides a surface proxy for the geotectonic and paleogeographic evolution of the retroarc. This study focuses on the poorly-constrained Lower Cretaceous(?) Newark Canyon Formation (NCF), the only syncontractional unit in the hinterland of the Sevier orogenic belt. Herein, we present new floral biostratigraphic ages, coupled with a detailed lithostratigraphic framework (~200 meters), for exposures of the NCF in the southern Fish Creek Range of east-central Nevada. Within the lowest 60 meters of the section, occurrence of the biostratigraphically sensitive Charophyte species Clavator grovesi grovesi suggests an Upper Berriasian (~140 Ma, and possibly older?) age, which is known from the uppermost Tithonian (latest Jurassic)-lowermost Valanginian of the Swiss Jura, Spain, and southern England, and the Upper Berriasian of the Lakota Formation in the Western Interior Basin of North America. The tectonic, paleogeographic, and stratigraphic implications of these data are three-fold: 1) Deformation in the Central Nevada thrust belt and Eastern Nevada fold belt was ongoing by the earliest Cretaceous, coeval with the Cordilleran magmatic lull and before initiation of thrusting in the Canyon Range and Willard sheets of the western Sevier fold-thrust belt. This interpretation suggests that changes from super-critical to sub-critical taper predicted by lengthening of the orogenic-wedge are not solely responsible for “internal” deformation of the hinterland, and an overall west-to-east propagation along a master decollement is preferred; 2) Topographic elevations and geographic connectivity remained relatively consistent between the hinterland and foreland throughout much of the Early Cretaceous, implying that uplift of the Late Cretaceous “Nevadaplano” post-dated at least one significant phase of upper-crustal shortening/thickening in the hinterland; 3) Regionally, the sub-Cretaceous unconformity may represent far less time (0-15 m.y. vs. 30-40 m.y.) than previously proposed, suggesting that these data have major implications for our understanding of subsidence migration across belts of protracted orogenesis.