Abstract

Latest Cretaceous and Paleocene strata (Ferris and Hanna formations) preserved in southern Wyoming’s Hanna and Carbon basins record sedimentary response to episodic uplift of surrounding Laramide structures and high frequency changes in local sea level over a 12 million year period. During the latest Cretaceous (ca. 66 Ma, lower Ferris Formation) an eastward-flowing axial drainage system was established (as a conduit from the Greater Green River Basin) and persisted until the middle Paleocene (ca. 58 Ma, lower Hanna Formation). The axial system acted independently of proximal systems and responded to tectonic activity (associated with episodic uplift of the Rawlins Uplift and Simpson Ridge anticline) and high frequency (10s–100s k.y.) base level changes not detectable in proximal, coarser-grained facies. Longer-term (1–3 m.y.) cycles are manifested in the proximal basin deposits as changes in paleosol composition and fluvial channel fill composition. A previously undocumented southern embayment of the Western Interior Sea (WIS) periodically flooded this axial drainage in response to local, high frequency (fourth- and fifth-order) forced regression-transgression cycles. Estuarine deposits preserved within 15–90 m-thick paleovalley fills in both formations contain tidal and brackish-water indicators that reveal the influence of the WIS throughout the Paleocene.

The predominantly eastward flow of the axial drainage through the Hanna Basin periodically reversed in response to episodic uplift of Simpson Ridge anticline during deposition of the middle and upper Ferris Formation (early Paleocene: 65-63 Ma). At these times, sedimentation in the Carbon Basin ceased, leading to a dramatically thinner (about one tenth the thickness of the Hanna Basin) section and lacustrine drowning of the Hanna Basin. A basin-wide unconformity (ca. 62 Ma) separates the lower Hanna Formation from underlying strata in both the Hanna and Carbon basins. This unconformity probably formed in response to tectonic quiescence/rebound of the basins and is manifested as the base of incised valley fills in the axial system and coarse-grained fluvial channel fills and paleosols in the northern Hanna Basin.

Complete tectonic separation of the eastward-flowing axial drainage from the Laramie Basin to the east occurred during the middle Paleocene (ca. 58 Ma) and is represented by a widespread lacustrine flooding surface in the Hanna Basin. This damming was a result of tectonic uplift of the Freezeout Hills and associated Laramide structures to the northeast of the Hanna Basin and contributed to formation of coal swamps adjacent to lacustrine margins.