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Abstract: Nature, Timing, and Paleogeographic Consequences of Laramide Deformation in the Northeastern Hanna Basin, Wyoming

Jason A. Lillegraven

Overturned, dinosaur-bearing uppermost Cretaceous strata along the Hanna Basin's northern margin bear arkosic sandstones, suggesting that the Precambrian core of the Granite Mountains was exposed locally prior to Paleocene time. Previously unrecognized, right lateral strike-slip faults, out-of-the-basin thrusts, and complex splays of a Laramide thrust zone exist along the basin's northern boundary; all involve the late Paleocene-earliest Eocene Hanna Formation. The top of the Hanna Formation is cut by the Shirley thrust. The Shirley thrust's eastern end dies in a zone of complex deformation within the Hanna Formation at the basin's northeastern corner. Development of the massive anticlinorium immediately east of the Hanna Basin can be linked to paleontologically atable elements within the Hanna Formation of the northeastern basin.

Although polyphasic, the main pulse of deformation of the northern Hanna Basin was late in Wyoming's history of the Laramide orogeny, beginning not significantly before the middle Wasatchian. The eastern Hanna Basin served throughout latest Cretaceous and Paleocene time as a narrow conduit for eastwardly flowing stream systems that drained the vastness of the greater Green River Basin. Intense deformation of the eastern Hanna Basin (and adjacent landscape to the north and east) matches temporally with advent of Lake Gosiute. A new drainage divide probably developed during early Eocene time in vicinity of the eastern Hanna Basin, initiating localized reversal of general stream directions (i.e., back to the west), thus contributing to the origin of Lake Gosiute.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada