A principal feature of both the Mountain City and San Marcos North quadrangles, particularly for geologic mapping of the area, is a structural “ramp” between two smajor en echelon faults of the Balcones Fault zone – the Mount Bonnell fault (MBF) to the north and a pair of faults, the San Marcos Springs fault (SMSF) and Comal Springs fault (CSF), to the south. An idealized conceptual diagram of the ramp between en echelon faults is shown below (from Grimshaw dissertation).
A Google Earth image showing the location of the MBF and the SMSF and CSF on the Mountain City and San Marcos North quadrangles is also shown below. The MBF and the SMSF and CMS are described in detail on this webpage.
Source: Grimshaw Dissertation, 1976
Mount Bonnell Fault: A Hinge Fault with Increasing Displacement Northeastward
The MBF has been delineated in a number of mapping projects at The University of Texas at Austin over the past decades. These delineations have been compiled for this webpage. The trace of the MBF in the 7.5-minute quadrangles southwest of Austin is shown diagrammatically below.
The sources for the geologic maps referenced for fault delineations are tabulated below. Other potential sources have not yet been referenced for this summary.
| Oak Hill | Garner, L., and K. Young, 1976. Environmental Geology of the Austin Area: An Aid to Urban Planning. The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 86, 39 p. |
| Signal Hill | Kolb, R., 1981. Geology of the Signal Hill Quadrangle, Hays And Travis Counties, Texas. Department of Geological Sciences, The University of Texas at Austin. Thesis. 100 p. |
| Mountain City | Grimshaw, T., In Progress, Geologic Mapping of Mountain City and San Marcos North Quadrangles. Joint Project with Brian Hunt, UT Bureau of Economic Geology (Formerly Barton Springs Edwards Aquifer Conservation District). Smith, R., 1978, Geology of the Buda-Kyle Area, Hays County, Texas. Unpublished MA Thesis, Department of Geological Sciences, The University of Texas at Austin. |
| Driftwood | Collins, E., 2002, Geologic Map of the Driftwood Quadrangle, Texas. University of Texas at Austin, Bureau of Economic Geology, Open-File Map, OFM0231D, Scale 1:24,000. (Portions were mapped previously by Grimshaw, 1970) |
| Rough Hollow | Collins, E., 2002, Geologic Map of the Rough Hollow Quadrangle, Texas. University of Texas at Austin, Bureau of Economic Geology, Open-File Map, OFM0148D, Scale 1:24,000. (Portions were mapped previously by Grimshaw, 1970) |
| Devil’s Backbone | Collins, E., 2002, Geologic Map of the Devil’s Backbone Quadrangle, Texas. University of Texas at Austin, Bureau of Economic Geology, Open-File Map, OFM0031D, Scale 1:24,000. (Portions were mapped previously by Grimshaw, 1970) |
| (See Three Quads Above) | Grimshaw, T., 1970. Geology of the Wimberley Area, Hays and Comal Counties, Texas. Unpublished MA Thesis, Department of Geological Sciences, The University of Texas at Austin. |
The hinge fault character of the MBF is emphasized in the diagram below. The outcrop patterns along the fault clearly manifest the hinge fault character of the MBF . The hinge point is where the fault ends in the Devils Backbone quad. From that point northeastward through the Driftwood quad, the Glen Rose is down-faulted against Glen Rose, as indicated by the lower arrows in the image above. Continuing northeastward, and starting in the southwestern part of the Signal Hill quad, younger Edwards limestones are down-faulted against the Glen Rose (shown in the upper arrows in the above image, indicating significantly more displacement on the MBF than to the southwest. Quantitative estimates of displacement on the fault have not yet been made, although the amounts clearly increase northeastward as indicated in the conceptual diagram.
San Marcos Springs and Comal Springs: Step Faults without a Hinge Fault Northeastward
The SMSF and CSF have also been delineated in mapping projects at The University of Texas at Austin. These delineations have been compiled for the SMSF and CSF as well as the MBF. The trace of the SMSF and CSF in the 7.5-minute quadrangles southwest of Austin is shown diagrammatically below.
7.5-Minute Quadrangles
The sources for the geologic maps referenced for fault delineations are tabulated below. Other potential sources have not yet been referenced for this summary.
| San Marcos North | Grimshaw, T., 1976, Environmental Geology of Urban and Urbanizing Areas: A Case Study from the San Marcos Area, Texas. Unpublished Ph.D. Dissertation, Department of Geological Sciences, The University of Texas at Austin. Grimshaw, T., with M. Helper, 2013, Geologic Map of the San Marcos North Quadrangle and Adjacent Portions of the Mountain City and San Marcos South Quadrangles, Hays, Caldwell and Guadalupe Counties, Texas. University of Texas Libraries, Texas ScholarWorks. https://repositories.lib.utexas.edu/handle/2152/43985. |
| San Marcos South | (As above.) |
| Hunter | Baumgardner, R., and E. Collins, 1991, Geologic Map of the Hunter Quadrangle, Texas. University of Texas at Austin, Bureau of Economic Geology, Open-File Map, OFM099D, Scale 1:24,000. |
In general, the SMSF and CSF are step faults, with older strata (particularly the Edwards limestones) on the upthrown side and with younger strata, notably the Pecan Gap, on the downthrown side. In this area the exposures between the faults are mostly Eagle Ford and Austin strata.
Cross faults have been mapped between the SMSF and CSF in the city of San Marcos and in the vicinity of Alligator Creek in the Hunter quad. In the first cross fault, the Pecan Gap is downfaulted against Austin strata. In the fault near Alligator Creek, the prominent feature is downfaulting of the Eagle Ford and Austin against older strata such as the Del Rio and Buda. A conceptual block diagram showing the step-fault and cross-fault relationships is shown below.
This interpretation suggests revision of the applicability of the idealized en echelon and ramp structural relationships shown in the idealized conceptual diagram of the ramp structure and en echelon faults The SMSF and CSF are not hinge faults with decreasing displacement to the northeast as indicated in the diagram. Instead, their displacements are probably constant or may actually increase northeastward.
It should be noted that the intervening ramp between the en echelon faults is not impacted by this revised interpretation. Geologic re-mapping of the Mountain City and San Marcos North quads, which is presented on this website, is therefore also not substantially affected by this change of interpretation.