Abstract

The last widespread episode of marine sedimentation in the Cordilleran miogeocline occurred during the Early Triassic when about 2,000 meters of Lower Triassic rocks accumulated at the depocenter within the Meade plate of southeastern Idaho. The Dinwoody Formation is the oldest Triassic unit in the miogeocline and encompasses an initial Griesbachian transgression that spilled rapidly eastward out of the basin onto the Wyoming shelf. Subsequent basinward progradation of silty terrigenous sediments continued through the Dienerian and included a westward-thinning tongue of red beds assigned to the Woodside Formation. The end of Dinwoody deposition is marked by a Smithian transgression. The Meekoceras-bearing lower limestone of the Lower Triassic Thaynes Limestone that defines the top of the Dinwoody in the Meade plate was deposited during this widespread flood.

Four complete stratigraphic sections and two composite sections of the Dinwoody provide control for a coordinated conodont biostratigraphic and lithostratigraphic study. Correlation of the measured sections by conodonts from biozones 1 through 4 provides a satisfactory test within the Dinwoody depocenter for part of the Lower Triassic zonation.

Conodont color indicates the thermal maturity of the Dinwoody ranges from 1.5 to 2.5 and generally increases toward the depocenter. These values are within the appropriate range for hydrocarbon generation and preservation.

The interbedded calcareous siltstones, mudstones and limestones of the Dinwoody Formation record “basinal”, outer-shelf and inner-shelf depositional environments within a persistently subsiding basin centered in southeastern Idaho. The maximum thickness of the Dinwoody is 733 meters near Blackfoot River Reservoir and the formation thins away from that locality in all directions.

Most Dinwoody limestones and associated lithologies within the Meade plate reflect a shallow shelf environment with bioclastic packstones as the prevalent carbonate lithology. This, and other supportive evidence, indicates that differences in circulation and terrigenous sediment supply were more important controls on sedimentary facies than water depth.