A PALEOGENE ANTECEDENT STREAM THAT BEVELED RISING, STEEPLY TILTED, CRETACEOUS STRATA ALONG THE EASTERN EDGE OF THE HANNA BASIN DURING LARAMIDE DEFORMATION

Along the thrust-faulted, NE margin of the Hanna basin (south-central Wyoming), thin (<5-m) conglomerates mapped as Paleogene Hanna Formation rest on steeply tilted Cretaceous rocks (Steele Shale, Allen Ridge Fm, and Lewis Shale) along the southern and western edges of the Freezeout and Flat Top anticlines. During the Laramide orogeny, subsidence of the footwall of a west-vergent blind thrust accommodated a 7-km-thick package of growth strata (Hanna and Ferris Formations).

Our working hypothesis is that the angular unconformity is syntectonic-- it developed when an east-flowing, antecedent river was able to maintain its gradient across soft, deforming rocks above the thrust. The river system—presumably the same one that deposited fluvial growth strata of the Hanna Formation on the foot wall-- beveled the folding Cretaceous strata of the hanging wall, preventing development of positive topography. Wildcat Top, a short, prominent ridge 1 km east of and rising 50 m above the nearest conglomerate, is an ancient, inter-fluvial remnant. Fifteen kilometers east of the conglomerates, near-horizontal Triassic and late Paleozoic rocks lie about 100 m above the beveled Cretaceous rocks. These relationships suggest that, after crossing onto the hanging wall, the river flowed southward, away from the resistant strata that emerged at the culmination of the Flat Top anticline. Similar syntectonic stratal and geomorphic relationships have not been previously described from Laramide structures, but several studies have inferred that weathering and erosion greatly diminished the topographic relief of rising Laramide blocks.

Controls on the distribution of sedimentary facies in the Hanna Basin could have resembled those described from the Axhandle piggyback basin in the thrust belt of central Utah: the hanging wall block acted as a valve controlling conditions in the basin to the west. Paleobotanical studies have indicated a cool, humid paleoclimate with 130-150 cm annual precipitation prevailed in the Hanna Basin during coal deposition. Coal beds may record episodes when seismic activity slowed river flow over the hanging wall, causing water tables to rise. Lake deposits dominate the uppermost Hanna Formation and probably record rise of the block above base level and full blockage of eastward flow.