Introduction

The accompanying map, Figure 1, is a geologic map which depicts the outcropping of Paleozoic and Precambrian age rocks, and where these rocks subcrop beneath the younger Cretaceous age rocks in Gillespie County. The definition of a subcrop is an area within which a body of rocks occurs directly beneath an unconformity. The map does not distinguish where the Paleozoic and PreCambrian age rocks subcrop or outcrop on the map. To determine where the rocks outcrop, one would need to refer to the surface geologic map as shown on Figure 2A.

Since the majority of Gillespie County is covered with Cretaceous age rocks, Figure 2A, this map helps to understand the geology beneath the Cretaceous. Should the Cretaceous not contain groundwater, then this map is helpful in determining where groundwater may be present at greater depths and from which units.

Precambrian

The Precambrian age rocks are the granites and metamorphic rocks associated with the Llano Uplift. They outcrop in the northern and northeastern portion of Gillespie County (Figure 2A). Some extend to the south and are directly beneath the Cretaceous (Figure 1). In these areas they are overlain by the Cretaceous age Edwards and Hensel aquifers. The Precambrian rocks are the oldest rocks in the county (1 to 1.2 billion years) and are not generally identified as aquifers. Hence, the odds of encountering additional groundwater beneath the Cretaceous are low, where the Precambrian underlies it.

Cambrian Age Riley Formation

The Riley Formation of the Moore Hollow Group is comprised of three members, with youngest to the oldest being Lion Mountain, Cap Mountain and Hickory. The total thickness of the Riley if all members are present in their entirely can be in excess of 800 feet.

The oldest member, the Hickory, is predominately a sandstone with a maximum thickness of 300 feet, and serves as an aquifer where it is adjacent to the area where the Precambrian subcrops beneath the Cretaceous. It is present beneath the Wilberns formation and Ellenburger group, and other Paleozoic rocks, but at great depths and probably does not contain potable water.

The Cap Mountain and Lion Mountain members are both generally limestones and occasionally produce groundwater where the units are fractured.

Cambrian Age Wilberns Formation

The Wilberns Formation of the Moore Hollow Group is made up of four members with the youngest to the oldest being the San Saba, Point Peak, Morgan Creek and Welge.

The San Saba is a carbonate unit comprised of limestone and dolomite. It is an aquifer and often linked with the younger Ordovician age Ellenburger, however in Gillespie County the Ellenburger provides significantly greater groundwater yields than does the San Saba.

The Point Peak member is a mix of limestone and shale and usually does not produce groundwater. The Morgan Creek is a limestone, and like the younger Point Peak does not produce groundwater, unless the units are fractured. If so, then these non aquifer members can produce small amounts of groundwater.

The oldest member of the Wilberns formation is the Welge, which is a thin sandstone (≈ 20 feet thick). It is capable of producing moderate to good yields (> 25 gpm) of groundwater. This is usually in the Cherry Springs and Doss areas in Gillespie County.

Ordovician Age Ellenburger Group

The Ordovician Age Ellenburger Group is comprised of the three formations being the Honeycut, Gorman and Tanyard. All three formations are carbonates with dolomite being the dominant lithology. Limestone is secondary. The Ellenburger is highly fractured in some areas of Gillespie County. In these areas, groundwater yields can be extremely high (> 1000 gpm). The City of Fredericksburg gets the majority of its water from the Ellenburger.

Pennsylvanian Age Marble Falls and Pennsylvanian Undiff.

The Pennsylvanian Age Marble Falls is present in a very small area in eastern Gillespie County. It is predominantly a limestone and provides low to moderate amounts of groundwater.

The Pennsylvanian Undifferentiated is present in the southwestern portion of Gillespie County where it is preserved in the Kerr Basin. The Kerr Basin extends from southwestern Gillespie County, into Kerr and Bandera counties. This unit is comprised of an assortment of lithologies with shales representing the dominate one. As a result, little groundwater has been produced from the unit.

Geologic Structure

The subcrop and outcrop units shown on Figure 1 are adjacent to each other.  This is due primarily to the faulting that occurred at the end of the Pennsylvanian Period, approximately 300 million years ago.  This is termed the Ouachita Tectonics, which resulted in the formation of Pangea, the super-continent.  This collision of land masses produced the faulting, which we see today in the subcrops and the outcrops. Some of the faults have displacements of 2000 feet. That is the reason the Ellenburger, which is approximately 400 million years, is adjacent to the Precambrian with an age of greater than 1 billion years. The cross sections presented on the District’s website, under the Geology Tab, provides examples of the faulting and their displacements.

After the Pennsylvanian Period, the area under went a long period of erosion, approximately 160 million years, until the Cretaceous seas transgressed the area approximately 140 million years ago. The Hensel sand was the first Cretaceous unit deposited in Gillespie County. With regression of the Cretaceous seas, the area has been exposed and under gone additional erosion. This was accelerated with the Edwards Plateau being uplifted approximately 20 million years ago. The current Hill Country terrain is a result of this fairly recent event. 

Acknowledgements

The geologic units shown on Figure 1 are based primarily on driller’s logs from wells drilled in Gillespie County. The District has in its database an excess of 10,000 wells from which the map was developed. In addition to these 10,000 plus wells, 200 plus wells have geophysical logs, which aided in determining the geologic units beneath the Cretaceous age rocks. This effort is an ongoing process with the addition of new wells and additional data. Consequently, the map may and most probably will change as additional data is obtained.

Figure 2A shows what geologic units are exposed at the surface in Gillespie County. As sited on Figure 2A, the source for that data is from Bureau of Economic Geology, a part of the University of Texas. This map is the portion of Llano Sheet that contains Gillespie County.  The Llano Sheet covers a large area of the Texas Hill Country.

As previously mentioned, information depicted on the map shows what geologic units are present at the surface or those that outcrop. This information was an enormous aid in developing the subcrop map shown on Figure 1. The various geologic units shown on Figure 2A are identified on the legend, Figure 2B. The Llano Sheet has a multitude of geologic units as shown by the legend (Figure 2B). However, Gillespie County has only a portion of these units.

The Bureau of Economic Geology is and always has been an excellent source of geologic information which covers reports, studies, and maps of the State’s geology.