The geology of the MCQ encompasses many components and levels. At the highest level is the regional geologic setting, which includes the basement rocks deep in the earth — the Texas Craton and the Ouachita Foldbelt. At the next level are the local components that extend elsewhere in central Texas, such as the Balcones Fault Zone and the Cretaceous limestones, clays and related rocks that comprise the bedrock above the basement. Finally, there are the geologic features that are found specifically in the MCQ. Many of these are described in the “Salient Geologic Features” webpage.
Geologic Description of the San Marcos Area in 1976 Dissertation
A description of the geology of the San Marcos area is included in Dr. Grimshaw’s 1976 dissertation as an appendix. It covers in a brief and succinct way many of the topics below, including geologic context (regional setting), stratigraphy, structural geology, and geomorphology. These and other aspects of the geology have been updated and expanded for this website.
Relationship of Geologic Features of the Dissertation Area
A paper was published as an outgrowth of the geologic mapping and related work for Dr. Grimshaw’s dissertation. The paper focuses on structural geology and its significance for geomorphic and hydrologic aspects of the area. The abstract and citation are shown below. Click here for a copy of the paper.
Abstract
Detailed geologic mapping in the Balcones fault zone in the San Marcos area has revealed a structural style that may have had profound effect on the geomorphologic and hydrologic evolution of the area. Two major in echelon step fault zones are present in the area, and a highly faulted ramp structure has formed in the zone between the in echelon fault zones.
Differential erosion of rock units in the ramp structure may have determined the course of a stream which captured the Blanco River from an easterly flow direction into the Onion Creek basin to its current southeasterly flow direction. Subsequently, the Blanco may have “tapped” the Edwards aquifer by down-cutting or side-cutting action at or near the San Marcos Springs location. Thus, both the capture of the Blanco and the current location of San Marcos Springs may have been indirectly caused by the local structural setting between the two major en echelon fault zones of the Balcones system.
Similar major ramp structures are apparent by map inspection in at least three other locations in the Balcones fault zone, one near Austin and two west of San Antonio. A fourth structure may also be present near New Braunfels.
Grimshaw, T., and C. Woodruff, 1986, Structural Style in an En Echelon Fault System, Balcones Fault Zone, Central Texas: Geomorphologic and Hydrologic Implications. In Woodruff, C., and P. Abbott, The Balcones Escarpment Geology, Hydrology, Ecology and Social Development in Central Texas. Published for the Annual Meeting of the Geological Society of America, San Antonio, Texas, p. 71-75.
Regional Geologic Setting
[In preparation]
Stratigraphy
[In preparation.]
Structural Geology
[In preparation.]
Geomorphology
[In preparation.]
Hydrology
[In preparation.]
Environmental Geology
[In preparation.]