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The U-Pb age spectrum of detrital zircons in sandstone of the fl uvial Middle Jurassic Ionia Formation in the Michigan basin, United States, resembles the age spectra in Jurassic eolianites of the Colorado Plateau, except that a Neoproterozoic (725-510 Ma) subpopulation present in the eolianites is absent from the Michigan basin strata. The detrital zircon data are compatible with the hypothesis that the Ionia Formation was deposited by northern tributaries of a transcontinental Jurassic paleoriver system that transported detritus from headwaters in Atlantic rift highlands of the Appalachian region to fl oodplains and deltas in the present northern Rocky Mountains region. Previous detrital zircon studies have indicated that sedimented lowlands in that paleogeographic position were defl ated by paleowinds to feed eolian sand southward (in present coordinates) into the Colorado Plateau ergs (sand seas).

On the Colorado Plateau of southwestern Utah, the Lower Jurassic Glen CanyonGroup comprises, in ascending order, the Moenave and Kayenta Formations and the Navajo Sandstone. In southern Nevada and southeastern California, the lithostratigraphic equivalent of the Navajo Sandstone is the Aztec Sandstone. In southern Nevada, the Aztec Sandstone is conformably underlain by four informally recognized stratigraphic units (A-D) of the undifferentiated Moenave and Kayenta Formations. The Glen Canyon Group unconformably overlies the Upper Triassic Chinle Formation above a regional unconformity. In addition to the Petrified Forest and Shinarump Members, the Chinle Formation contains a distinctive limestone-pebble conglomerate at its base.Using the Aztec Sandstone as a distinctive reference unit, the Glen Canyon Group and its relation to underlying and overlying lower Mesozoic depositional sequences are traced southwestward from the Las Vegas extensional domain, along the eastern edge of the relatively unextended Las Vegas Range-Spring Mountains block, into the Jurassic arc terrane of the eastern Mojave Desert. Southwestward, the regional unconformity at the base of the Glen Canyon Group truncates progressively older strata into the arc terrane. Although Middle Jurassic strata have been erosionally or tectonically removed from the Las Vegas extensional basin, volcanic-clast-bearing marginal marine facies of the Middle Jurassic Carmel Formation are tentatively correlated with silicic volcanic, volcaniclastic, and epiclastic rocks of the southern end of the Spring Mountains extensional domain. Stratigraphic and facies boundaries in lower Mesozoic strata potentially serve as important strain markers to test models of Cenozoic extension. Restoration of structural blocks containing lower Mesozoic outcrops to their pre-Tertiary positions is based on restoration of the Las Vegas Range-Spring Mountains block to its preextension position. The reconstruction reveals that (1) the limestone-pebble conglomerate at the base of the Chinle Formation is truncated on the east by the north-south-trending VermilionCliffs paleovalley; (2) the undifferentiated Moenave and Kayenta Formations were deposited in a north-south-trending, incipient foreland basin that deepened to the north; (3) alluvial fans were shed northeastward, at right angles to the Triassic paleoslope, into this basin from the arc terrane; and (4) volcanic centers lying east of the present Colorado River served as the source of volcanic clasts in the Carmel Formation. Marzolf, J. E., 1990, Reconstruction of extensionally dismembered early Mesozoic sedimentary basins; Southwestern Colorado Plateau to the eastern Mojave Desert, in Wernicke, B. P., ed., Basin and Range extensional tectonics near the latitude of Las Vegas, Nevada: Boulder, Colorado, Geological Society of America Memoir 176. 477 on June 20, 2015 memoirs.gsapubs.org Downloaded from 478 J. E. Marzolf w.Bounding Hurricane CliffsUnconformities J-2 J-1

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