The Lower Triassic Dinwoody Formation, the oldest Triassic unit in the Cordilleran miogeosyncline, rests unconformably on underlying Permian rocks. The type section of the Dinwoody is on the northeastern flank of the Wind River Mountains in Wyoming. As this locality is near the eastern margin of the Early Triassic depositional basin, the formation is relatively thin (52 m) and largely composed of clastic sediments. To the west, the Dinwoody thickens abruptly and the amount of interbedded limestone increases.

In the Bear Lake-Montpelier region of the southeastern Idaho overthrust belt, the thickness increases to about 380 m. Although significant amounts of interbedded carbonate are present here, intertonguing red beds and evidence of periodic shoaling also characterize this transitional area. In northwest Utah, the Dinwoody thickens to 410 m and includes numerous interbeds of limestone.

Conodont biostratigraphic zonation of the Dinwoody Formation permits detailed correlation and establishes the timing of the Early Triassic transgression eastward over the Wyoming shelf. This short-lived event was followed by westward progradation of terrigenous sediments from the shelf into the depositional basin.

On the basis of color change of conodont elements, which relates to depth and duration of burial, thermal maturity of the Dinwoody Formation increases southwestward. The color

End_Page 566------------------------------

alteration index (CAI) ranges from 1 at the type section to 1.5 to 2 in transitional and basinal areas. Because CAI values of 1 to 2 correspond to the thermal interval of oil generation, a vast area of Lower Triassic rocks has productive potential.

End_of_Article - Last_Page 567------------