Oxfordian‐Kimmeridgian (Jurassic) dinoflagellate cysts from the Chiricahua Mountains, Arizona

Abstract

Upper Jurassic strata exposed within the northeastern Chiricahua Mountains, southeastern Arizona, contain a transitional to open marine facies consisting of supratidal sediments, intrabasinal turbidites, pelagic sediments, and submarine volcaniclastics. This sequence is underlain unconformably by the Permian Concha Limestone and is thrust to the southwest over the Cretaceous B isbee Group. These marine strata were deposited during a period of active rifting accompanied by a northwest transgressive incursion into a half‐graben depositional depression known as the Bisbee Basin. Progressive infilling of Cretaceous Bisbee Group sediments resulted in a succession of Upper Mesozoic strata that is unique to this region of the southwestern Cordillera.

Fifteen genera and thirteen species of dinoflagellate cysts date this sequence from middle Oxfordian to early Kimmeridgian. Ammonites Idoceras striatum (Imlay), Perisphinctes ("Discosphintes"), and Dichotomosphinctes cf. D. wartae (Bukowski) substantiate this age, as well as questionably establishing the Oxfordian‐Kimmeridgian boundary within this section.

The presence of dinoflagellate cysts, in conjunction with Tethyan ammonites, indicates a time of normal‐open marine conditions within this region and documents the northernmost incursion of the Tethyan Seaway into this region of southeastern Arizona.