What is the llano basin Texas?

The Llano Basin or Llano Uplift is a geologic structural region centered in the Llano Uplift in Central Texas. The basin is an elongated, relatively shallow structural depression that covers parts of about 15 Texas counties. It comprises exposures of the southern portion of the southern and central Llano Uplift, which features some of the oldest exposed rocks in Texas. The Llano Basin is separated from the rest of the state by a series of faults at its perimeter, including the Balcones Fault Zone along its southeastern edge.

These rocks consist mainly of Precambrian (up to 1.1 billion years old) igneous and high-grade metamorphic rocks. There are minor outcrops of Paleozoic sedimentary rocks within the basin. They are primarily metamorphosed limestones, sandstones, conglomerates, and shales folded and faulted during the Ouachita orogeny.

The term "Llano Basin" also may be used to describe a groundwater province with a similar geographic distribution. The Llano Basin extends northwest from San Antonio to Brady and east to Lampasas. The groundwater province overlies the Llano Uplift and is underlain by fractured igneous and metamorphic rocks. The groundwater in this basin generally is highly mineralized.

The Llano Uplift is the eroded core of a much larger late Precambrian mountain chain. The Llano Uplift is made primarily of granites and other igneous rocks, and it is the most stable crustal block in Texas.