BENAN, CHEIKH A. AHMED and GARY KOCUREK, Department of Geological Sciences, University of Texas at Austin, Texas 78712
The Middle Jurassic Entrada Sandstone at Ghost Ranch, N. M., records a catastrophic flooding event that resulted in the preservation of dune paleorelief. The Entrada consists of two sandstone units: (1) a lower unit of cross-stratified eolian and sabkha flat deposits, and (2) an upper massive sandstone unit. The uppermost set of cross-strata of the lower unit shows dune paleorelief up to 35 m, trending N25W, with stoss and lee slopes. Small sets occur on the stoss and basal lee slopes, representing superimposed dunes. In computer reconstructions, outcrop data for the main compound set of cross-strata is best matched by a southwestward migrating linear dunes with along-crest migrating sinuosities to the northwest. The massive sandstone unit is interpreted as subaqueous massflow deposits shed from the dune crests during the flooding. A series of massflows progressively onlap the toss and lee slopes of the preserved dunes. The overlying Todilto Formation drapes the remaining dune paleorelief, with limestones thickening and passing abruptly into elongated gypsum mounds situated within remnant Entrada interdune depressions. The basal limestone is varved, suggesting deposition below wave base. The preserved relief, the draping nature of the Todilto, and the varved basal limestone all necessitate a catastrophic and quiescent flooding event that occurred with breaching of coastal dunes during the Curtis Transgression.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah