Browsing by Subject "Geology -- Texas -- Austin -- Maps"

Now showing 1 - 1 of 1

Geologic map of the Austin west quadrangle, Travis County, Texas
(University of Texas at Austin. Bureau of Economic Geology, 1970) Rodda, Peter U.
The rocks exposed in the Austin West quadrangle are Cretaceous marine limestones and clays and Quaternary alluvial deposits. The Cretaceous rocks dip gently eastward and are broken by one large (Mount Bonnell) fault and numerous small, northeast-trending faults comprising the Balcones fault zone. Most faults are downthrown to the east; total displacement across the fault zone is about 1,000 feet. M.ost of the quadrangle is west of the fault zone in the highly dissected hill country; the rocks consist mostly of interbedded hard and soft limestone, dolomite, and marl (Glen Rose, Walnut, and Edwards Formations). In the Balcones fault zone limestone units (Edwards, Georgetown, and Buda Formations) and clay units (Del Rio Clay and Eagle Ford Formation) are complexly faulted and moderately to strongly dissected. In the southeast corner of the quadrangle limestone (Austin Group) and clay (Eagle Ford Formation) crop out at the west margin of the blackland prairie. The Colorado River flows across the area from northwest to southeast, and six terrace deposits (Sand Beach, Riverview, First Street, Sixth Street, Capitol, and Asylum) consisting mostly of sand and gravel parallel the river and occupy successively higher topographic positions; the deposits are more extensixe east of the Mount Bonnell fad. Other alluvial deposits are developed along tributary streams. The limestone units generally are stable and resistant, are difficult to excavate, have mostly moderate to high permeability, and pose few engineering problems; they are sources of crushed stone and other building materials; one unit (Edwards Formation) is an important aquifer. The clay units are weak, unstable, impermeable, and corrosive and generally require special engineering designs. Alluvial deposits are easily excavated, have moderate to low stability and strength, have high permeability, and may need special designs for large structilres; the deposits are important aquifers and sources of sand and gravel.