Catastrophic flooding of an aeolian dune field: Jurassic Entrada and Todilto Formations, Ghost Ranch, New Mexico, USA

Surveyed outcrops of the Middle Jurassic Entrada Sandstone at Ghost Ranch, New Mexico, show the unusual occurrence of preserved aeolian dune palaeotop ography buried beneath subaqueous strata. The preserved dune remnants have relief up to 35 m, trend NNW, and show internal scalloped cross-strata dipp ing to the WSW, with small sets occurring as both topsets and bottomsets. O utcrop data are best satisfied in computer models by 50 m high, sinuous bed forms that migrated to the WSW, while the sinuosity migrated alongcrest to the NNW. Superimposed small dunes occurred upon the stoss slope, and at the basal lee of the main bedform where they migrated alongslope to the NNW. R emnant dune palaeotopography is buried by onlapping, subaqueous, largely st ructureless sandstones believed to be derived by mass wasting of the upper portions of the dunes and deposited as sediment-gravity flows that infilled between the dunes. Preservation of dune palaeotopography beneath mass-flow deposits, with no evidence for gradually rising water, argues that floodin g of the Entrada dune field was geologically instantaneous. The thickness a nd lithology of the overlying Todilto Formation conform to slight remnant p alaeotopography on the Entrada surface. The Todilto is a laminated limeston e and thinnest over remnant dune crestal areas, but thickens and increases in gypsum content downslope until it abruptly yields to a gypsum mound posi tioned over a remnant interdune hollow. The Todilto laminations are interpr eted as seasonal varves deposited below wave base in a density-stratified w ater body. The flooding event that gave rise to the controversial Todilto w ater body occurred during Entrada time, with Todilto deposition occurring w ithin an already substantial water body.