EVOLUTION OF MIDDLE-UPPER JURASSIC DEPOSITIONAL SYSTEMS, SOUTHERN WESTERN INTERIOR

Middle Jurassic eolian, shallow marine, lacustrine and arid coastal plain deposits are part of a westward thickening sedimentary prism that accumulated in a retroarc foreland basin that extended from Oklahoma-Nebraska on the east to Idaho-Nevada on the west. These strata of the San Rafael Group are an unconformity-bounded succession assigned to the (ascending order) Carmel, Entrada, Curtis/Todilto, Summerville and Bluff formations. Type 1 sequence boundaries are at the Carmel base on the Navajo Sandstone and the Curtis/Todilto base on the Entrada. The Entrada and Bluff sandstones represent vast ergs during times of regionally low sea level. Entrada dunes were driven by southerly blowing winds, but by Bluff time North America had drifted north into the zone of prevailing westerlies, so Bluff dunes were driven by easterly blowing winds. Todilto deposition took place in a salina with a maximum surface area of 88,000 km2 that formed in response to the Curtis transgression to the northwest. The Summerville Formation and Recapture Member of the Bluff represent extensive arid coastal plains. The entire San Rafael Group merges southward into the basin-edge erg deposits of the Zuni Sandstone. The base of the Salt Wash Member of the Morrison Formation is a basinwide unconformity (tectonosequence boundary) that corresponds to a significant tectonic reorganization of the Jurassic basin. In Morrison time, deposition changed to fluvial deposits derived from the uplifted arc terrane to the west and locally from the Mogollon highlands to the south. In southwestern NM and southeastern AZ, Jurassic marine deposition took place in the Bisbee basin, and this confirms location of the Late Jurassic Mogollon highlands in west-central NM and east-central AZ. Morrison climates were substantially wetter than those during San Rafael Group deposition, and the Morrison fluvial system evolved from an extensive fluvial braidplain to a vast muddy floodplain, but not an extensive playa lake.