Abstract: Preservation of Aeolian Linear-Dune Relief During
Catastrophic Flooding: Middle Jurassic Entrada and Todilto
Formations, Ghost
Ranch, New Mexico, USA
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