Stratigraphy and Sedimentology of Kincaid Formation, Midway Group (Paleocene), Upper Rio Grande Embayment, Texas: ABSTRACT

Abstract

ABSTRACT Sedimentary rocks of the Kincaid Formation crop out along the northern and western edges of the Rio Grande Embayment. Siltstones are exposed at the type locality of the Kincaid Formation along the Frio River in Uvalde County, Texas. To the east and south the Kincaid Formation changes facies to richly fossiliferous carbonate rocks; however, basinward, the Kincaid Formation grades into a shale facies which contains interbedded units of fine-grained sandstone. At the type locality of the Kincaid Formation approximately 30 ft (9 m) of massive siltstone grades upward into a very silty limestone unit. The outcrop is characterized by four resistant units of tightly cemented siltstone and resistant limestone which separate beds of less resistant, massive siltstone. Bedding is poorly defined throughout the section largely as a result of intensive bioturbation. The grain size of the siltstone increases upward ranging from medium to coarse. Clay content in the siltstone decreases upward and the amount of calcareous material increases upward. The upper four to six feet may actually be considered a silty limestone. There is a dramatic facies change along the outcrop both to the east and to the south of the type section. To the east, the Kincaid Formation is composed of limestone which is glauconitic and highly fossiliferous. The siltstone present at the type locality thins to the east and is absent less than 20 mi (32 km) away. Eighty mi (129 km) to the south along the Rio Grande River approximately 45 ft (14 m) of limestone and shale comprise the Kincaid Formation. The limestone is glauconitic and highly fossiliferous and is very similar to the limestone exposed to the east of the type locality. These early Paleocene sediments are interpreted as shallow marine in origin. The siltstone represents a shallow sublittoral shoreface environment whereas the limestone to the east and south were deposited in shallow near shore environments beyond the reach of clastic deposition.