STRATIGRAPHIC INTERPRETATION OF LARGE SPHEROIDAL CHERTS OF THE UVALDE SALIENT OF THE BALCONES ESCARPMENT

Cretaceous exposures within the Edwards Plateau are separated gulf-ward by the Miocene age Balcones Escarpment. Extensive erosion has revealed areas of Upper and Lower Cretaceous formations while localized areas of high hydraulic gradient reveal Triassic and Paleozoic outcrops. Well indurated chert is characteristic of the Escarpment and the gravels below. Chert spheroids are common within Hensel Sands of the Lower Cretaceous and reports of sizes up to eight inches are reported. Through a series of tightly spaced faults, the Uvalde Salient is a transition between erosion-resistant West Texas formations and the Gulf Plain. Immediately south of the Tectonic Hinge Line and located shoreward of the euxinic Maverick Basin, the Salient preserves both terrestrial and marine sediments. The Basin is thought to have been continuously submerged and well sheltered.

Large chert spheroids have been located within the Salient with surfaces of exceptionally hard Ilymatogyra arienta characteristic of the Del Rio Clay-Salmon Peak formation interface. The spheroids were found within a surface exposure with faults separating Del Rio, Buda and Salmon Peak strata. These oysters are well bonded into the Salmon Peak. This surface and the continuous chert structure within have protected the spheroid from erosion. The chert is in an onion-skin configuration arranged in the characteristic Cretaceous couplets of Salmon Peak, West Nueces, McKnight, and Glen Rose. At the center, a 3 by 30 mm object is preserved within an organic fossiled case is surrounded by a very fine-grain sandstone attached to a limestone cobble. The object has the distinctive shape of a trampled Metoposaurus egg that was subsequently preserved in Cow Creek formations. The single eight–inch diameter spheroid available for study appears to uniquely preserve a continuous record from late Triassic to mid-Cretaceous.