Geology of the De Queen Formation of Arkansas

ABSTRACT Beds of the De Queen Formation are exposed in gypsum quarries within Pike and Howard Counties, Arkansas. The formation lies within the Lower Cretaceous Trinity Group which crops out in an east-west band across southwestern Arkansas. The De Queen Formation consists of a lower sulphate facies and an upper predominantly siliciclastic facies. The lower facies is approximately 40 ft (12 m) thick and is composed of interbedded gypsum, claystone, and limestone. This lower facies is equivalent to the subsurface Ferry Lake Anhydrite of the Gulf Coastal Plain. The upper facies is also approximately 40 ft (12 m) thick and contains interbedded clastics, limestones, and minor evaporites. Upper beds of the De Queen are equivalent to the lowermost beds of the subsurface Mooringsport Formation. During Ferry Lake-De Queen time a wide lagoon was located behind an extensive reef stretching around the Early Cretaceous shelf edge. This reef formed a barrier which restricted circulation and led to the deposition of the gypsum beds of the De Queen Formation and the Ferry Lake (later recrystallized to anhydrite during burial). Individual evaporite beds may be traced downdip from the outcrop across southern Arkansas into Louisiana and Texas. The regional extent of these evaporite beds reflects the variable geographic breadth of the lagoon during deposition of the Ferry Lake. Gypsum beds of the outcrop are the equivalent of the more widespread anhydrite beds of the Ferry Lake Anhydrite. Faunal assemblages, sedimentary structures, and trace fossils, which include a large number of dinosaur tracks, are important to the interpretation of depositional environments of rocks of the De Queen Formation. Much of the lower half of the formation was deposited in a shallow-subaqueous setting, while depositional environments of beds within the upper half of the De Queen varied between subaqueous and exposed conditions.