Upper Jurassic Paleogeography of the Four Corners Region

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ABSTRACT The Entrada sandstone (Jurassic) of New Mexico and Arizona is largely eolian, having been deposited in two extensive dune fields (one in each state). The wind which carried Entrada sand blew from N.N.E. to S.S.W., almost parallel with the N.E.-S.W. shoreline across Utah, Colorado, and Wyoming. A low range of hills in southwestern and central Colorado controlled deposition in the pocket dune field in northwestern New Mexico, in Entrada time. The Todilto Formation, immediately above the Entrada, accumulated in two stages: (1) an interior, arid or semi-arid climate lake, covering roughly northwestern New Mexico, was the site of deposition of a dark, fetid limestone, up to about 30 feet thick; (2) later, a white sands dune-and-lake deposit (gypsum sand) covered a somewhat reduced area. Reconstruction of the Jurassic dune field leads to the suggestion that a high pressure cell, such as the Pacific cell which controls modern California weather, also existed off of the west coast in Jurassic time. The existence of such a cell would explain the arid, or semi-arid, climate which prevailed in the study area through much of Jurassic time. Most of the information contained in the present report has been derived from studies of eolian cross-bedding, eolian ripple marks, rock thicknesses, facies changes, other types of cross-bedding, ancient dune surfaces, rock colors, and grain size. The wind-type ripple marks are perhaps the first ever reported from the rock section.