Provenance and Sediment-Dispersal System in Tectonically Active Rapidly Evolving Foreland Basin, Western Interior

Nazrul I. Khandaker, Carl F. Vondra

The Upper Cretaceous Frontier Formation, along the mobile edge of the Western Interior foreland basin, is composed mainly of clastic sediments and was deposited during the initial Late Cretaceous transgressive-regressive phases of the Western Interior seaway across Wyoming. The formation contains many persistent bentonite beds and several sandstone packages in its lower part and a thin, lenticular lithic wacke-polymictic conglomerate association at its upper contact (Torchlight Sandstone Member). Abundant granule to cobble-sized clasts of andesite, granite, chert, and quartzite are set in a poorly sorted sand-to-granule grade volcaniclastic matrix. There is a lithologic continuity of this volcaniclastic unit across the Bighorn Mountains into the Powder River basin. A high-energy distr butary complex of sizable areal extent is invoked for the deposition of this linear conglomerate facies. Geochemical investigations of the whole-rock andesite clasts and bentonite allowed more precise definition of character, tectonic setting, and evolutionary stages of sedimentary distributive provinces. Bentonites and andesites are strongly enriched in strontium and barium, but only mildly enriched in heavy rare earth elements and high field-strength elements. These analyzed rocks have trace element characteristics similar in a general way to those of typical orogenic volcanics; they show some significant differences in detail. Composition of volcaniclasts and paleocurrent data indicate a proximal sediment source for the extrabasinal detritus within the Frontier Formation. The possibil ty of a contribution from a Mesozoic volcanic center in the neighborhood of southwestern Montana is strongly favored. The products of this volcanism constitute an assemblage of deep crustal to mantle(?) derived rocks, and their composition record time-integrated enrichment in light over heavy rare earth elements. The localization of the volcanism in the Late Cretaceous, however, is related to the development of fractures, some of which resulted from reactivation of old fracture zones in response to plate motion. Subsequent propagation of this fracture zone in a northwest-southeast trend apparently controlled the distinctive andesite dispersal within the foreland basin proper. The volcaniclastic rocks in terms of their field and compositional aspects document sedimentary responses to the ectonic partitioning of the Western Interior foreland basin, and the chemical nature of the volcanic activity places a significant constraint on the type of lithospheric involvement in foreland basin development.

The observed field and geochemical data have significant tectono-sedimentologic implications pertaining to intraforeland deformation and pulses of tectonic and volcanic activity. This critical information can be used to reconcile the relative role of allocyclic and autocyclic control on synorogenic sedimentation within the Western Interior foreland basin.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.