THE MORONI FORMATION: IMPLICATIONS FOR MID-TERTIARY PALEOTOPOGRAPHY

Evolution of the paleotopography from the inferred elevated terrain of the Late Cretaceous Sevier thrust belt to the Late Cenozoic Basin and Range province is not well understood. One interpretation of the mid-Tertiary topography envisions it consisting dominantly of broad plains, with little relief except in a swath along the Nevada-Utah border, onto which mid-Tertiary volcanic rocks were erupted (Best and Christiansen, 1991; Gans et al, 2001). Another interpretation suggests collapse of the Sevier belt with the formation of half grabens, at least along the eastern margin in the NE Utah-Wyoming region (Constenius, 1996).

Rocks assigned to the Eocene-Oligocene Moroni Formation in Sanpete Valley, central Utah, overlie the Charleston-Nebo thrust plate and also Uinta Basin Tertiary rocks, and have been examined in the southern Cedar Hills. Moroni rocks form an alluvial fan sequence, more than 230 m thick, no older than 38.5 Ma, and overlain by an ash-flow tuff dated at 34.3 Ma. The alluvial sediments are dominated by volcanic detritus but with a minor component of quartzite and limestone; contemporaneous silicic ash is intermittently present. The quartzite and carbonate clasts must have been derived from Sevier thrust sheets or their orogenic conglomerates, whereas the detrital volcanic material came from coeval, or near coeval, volcanic deposits possibly associated with the Tintic center to the northwest or an older center to the west. The distribution of the Moroni deposits on the Charleston-Nebo thrust sheet and the northern end of the San Pitch Mountains suggests a paleovalley through which the Moroni sedimentary detritus was funneled to the Cedar Hills. The alluvial fan depositional environment of the Moroni Formation in northern Sanpete Valley indicates paleotopography with relief of at least 1500 m (Mt. Nebo massif), which is most easily explained by early development of basin and range morphology.