Lower Cretaceous (Comanche) Rocks of Southeastern Oklahoma and Southwestern Arkansas

ABSTRACT The Lower Cretaceous (Comanche) rocks of southeastern Oklahoma and southwestern Arkansas rest upon rocks of many different ages ranging from pre-Cambrian to Permian, but the floor they lie upon is a remarkably uniform plane with minor irregularities in spite of the folds and faults in the Paleozoic rocks. The floor represents a part of the Ouachita peneplain that was tilted slightly southward and submerged by the Cretaceous sea. A pronounced unconformity, though less striking than the one at the base of the Lower Cretaceous, separates the rocks of this series from the overlying Upper Cretaceous series. Its plane truncates all the several formations of the Lower Cretaceous, the youngest formations in Oklahoma, and the oldest in Arkansas. The Trinity formation, the basal unit of the Lower Cretaceous, contains beds in Arkansas that do not extend westward far into Oklahoma, owing to a westward overlap of the upper part of the Trinity over the lower part of the formation. The Trinity of Oklahoma is thus for the most part younger than the Trinity of Arkansas. Two contrasting relations thus exist in the Cretaceous of southeastern Oklahoma and southwestern Arkansas—(1) a westward overlap in the basal formation (Trinity) of the Lower Cretaceous and (2) an eastward overlap of the Upper Cretaceous across the truncated edges of the Lower Cretaceous. The westward overlap in Lower Cretaceous time was caused by a downwarping of the Texas-Oklahoma embayment that lies between the Llano and Ouachita uplifts. The eastward overlap in Upper Cretaceous time was evidently caused by a regional movement that formed the Mississippi embayment.