Abstract

The Hoback Basin is the northern extension of the Green River Basin, being separated from the latter by the Rim, a low topographic divide. It is bounded on the northeast by the Gros Ventre Range and on the west by the Hoback Range. The Hoback Formation, which attains a thickness of 15,000 feet, was thought to be a folded sequence of lower Tertiary rocks.

Palynological samples from the lower 5000 feet (from the Hoback conglomerate downward through the Coaly Sequence) indicate the presence of Cretaceous rocks. The age of these rocks is thought to be Maastrichtian and Campanian, and possibly Santonian. Lack of palynological control makes precise dating difficult.

Samples collected above the Hoback conglomerate that now forms the base of the Hoback Formation are Paleocene and Eocene in age, based on palynological evidence. Precise dating of this sequence had been accomplished with vertebrate remains by Dorr (1958, 1969), who dated it late Torrejonian to Lysitian (late middle Paleocene to middle early Eocene).

Uppermost Cretaceous (Danian) and lower Paleocene (Puercan and Dragonian) rocks are presumed to be missing beneath the basal Hoback conglomerate.

In the west, the Game Hill Fault separates the Coaly Sequence, which is late Cretaceous, from a Hoback Formation segment that was dated as late Torrejonian (or early Tiffanian) by Dorr (1958).