Rare Carbonate Eolianites In the Middle Jurassic Lower Sundance Formation, Bighorn Basin, North-Central Wyoming

Distinctive, 11m–thick, large–scale, cross–stratified sets of ooids deposited in the Middle Jurassic Sundance Formation in the Bighorn Basin of northern Wyoming were previously interpreted as high–energy subtidal deposits. In fact, they are eolian in origin caused by the deflation of emergent ooid shoals after structural uplift and eustatic sea level fall during the late Callovian. Although many examples of Paleozoic and Quaternary carbonate eolianites exist, the oolitic dunes are the first Mesozoic eolian carbonates documented in the Western Interior. Primary indications for an eolian origin of the cross–strata are:

1. Sedimentary structures consisting of climbing translatent stratification produced by migrating wind ripples of alternating laminations of ooids and silt–sized quartz (“pinstripe” lamination) in the cross–strata, corrugated bedding planes, basal foresets tangential to the underlying surface, and adhesion structures at the base of the unit.

2. Petrographic evidence including coarsening–upward sequences of broken and abraded ooid grains, dissolution–compaction of grains in the vadose zone, fungal hyphae in the pore space, and intergranular micrite of vadose origin.

Relationships of the cross–stratified ooid bodies with encasing lithofacies demonstrate that the intact cross–stratified oolitic limestones were deposited as isolated bedforms on an emergent deflation surface during a regression of the Sundance Sea. Although the preservation potential of carbonate eolianites is low, partial meteoric lithification of the cross–strata before being subjected to low–energy conditions during a subsequent transgression allowed for burial and preservation.

Misinterpretation of other cross–stratified carbonates as subtidal marine grainstones may account for the scarcity of documented Mesozoic eolian carbonate examples. The evidence from the sedimentary structures, petrography, and stratigraphic relationships confirms the eolian origin of these strata, in specific; and provides a process for further interpretation of carbonate eolianites in the Mesozoic of the Western Interior, in general.