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The AAPG/Datapages Combined Publications Database
CSPG Special Publications
Abstract
Regional Paleogeography and Tectonics
Middle Jurassic Stratigraphy, Sedimentation and Paleogeography in the Southwestern United States
Abstract
The San Rafael Group (SRG) encompasses all Middle Jurassic strata on the Colorado Plateau. Depositional environments were strongly influenced by, or directly related to the southward advance, highstand and subsequent northward regression of the shallow Sundance seaway. SRG strata unconformably overlie the Navajo Sandstone (Lower Jurassic) in the western Colorado Plateau, whereas to the southeast, in New Mexico, a more profound unconformity separates the SRG from the Chinle Group (Upper Triassic). At the type area in Utah the SRG consists of (ascending) the Carmel Formation (shallow marine deposits), Entrada Sandstone (eolian), Curtis Formation (shallow marine, arid coastal plain, sabkha) and Summerville Formation. Southeastward in New Mexico the group consists of the Entrada Sandstone, the carbonate-evaporite sequence of the Todilto Formation, the Summerville Formation and the Bluff Sandstone (mostly eolian). The Todilto was deposited in a paralic salina fed indirectly by the Curtis Sea to the north, a relationship supported by sulfur-isotope data. Following the highstand Summerville strata prograded across the vanishing water bodies and finally eolianites accumulated. Cessation of eolian deposition was not synchronous across the basin as SRG deposition came to a close, thus the upper part of this unit is, in places, of fluvial origin. The base of the overlying, fluvially dominated Morrison Formation is picked on lithologic rather than lithogenetic factors in the San Juan Basin, plus the presence of an unconformity (sequence boundary). Lithology and biota of the Morrison suggest the onset of more humid climates, which is consistent with the northward drift of Laurasia following the initial breakup of Pangaea.
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