Jurassic Paleogeography and Paleoclimate of the Central and Southern Rocky Mountains Region

Abstract Jurassic units of the central and southern Rocky Mountains were deposited in a retroarc to craton-margin basin. During Jurassic (Pliensbachian-Middle Oxfordian) time, the region drifted north, occupying a paleolatitude range of 5 to 25 degrees north, which is equivalent to the modern intertropical convergence zone and trade wind belt. Outcrop evidence suggests the intensely hot, arid climate was marked by rare storm events. Widespread eolian deposits show sand transport by modified trade winds. The primary modification appears to have been deflection of winds along the western magmatic arc to blow parallel to the arc trend and length of the basin; additional distortion of trade wind patterns were by intense heating over the continental desert and local effects of remnant interior uplands and the shoreline. Sands composing the eolian units were largely derived from multi-cycle cratonic deposits. Reconstruction of the Early Jurassic (Pliensbachian-Toarcian) paleogeography of the region indicates that the eastern and southern parts of the basin were occupied by an eolian sand sea that extended to the magmatic arc. The western portion of the basin was occupied by a shallow sea in which carbonate sediments and clastics from the western tectonic lands and from the craton accumulated. Eolian sand sea accumulation appears to have ended in the Bajocian with south-progressing net deflationary conditions that possibly were initiated by a marine transgression over source areas to the north. Bajocian to Early Callovian (Middle Jurassic) paleogeography consisted of a northern portion of the seaway (characterized by a shallow carbonate province in the west, terrigenous accumulations in the east) and a tongue of the seaway in the south. The tongue of the seaway was the site of broad sabkha in the east that graded west to tidal flats and shallow hypersaline marine conditions. Marginal to the southern extent of the seaway were areas of eolian sand seas, which show halting advances over the adjacent sabkha and tidal flats. Regional retreat of the seaway in the later Middle Jurassic (Early Callovian) lead to north and northwest eolian sand sea progradation and establishment of widespread eolian environments over the region through the Middle Callovian. Red beds flanking the eolian sand seas to the north and west represent very shallow, hypersaline marine to sabkha environments, in part developed on distal portions of alluvian systems that prograded from the tectonic lands in the west. Normal marine conditions extend south only as far as the central northern portions of the region. Middle Callovian through Middle Oxfordian (Upper Jurassic) deposits show a major transgression and the establishment of the most widespread interior seaway of the Jurassic. Transgression of the sea resulted in drowning of the eolian sand sea under a variety of marine environments. Complex lithofacies in the northern area for the Late Jurassic seaway consist of a western sandy portion and an eastern heterogeneous assemblage of shale, sandstone, and minor limestone. The southern area was characterized by shallow marine to tidal flat to restricted marine environments. The southern terminus of the basin remained largely continental, showing complex interfingering of eolian, coastal and inland sabkha, and tidal flat deposits.