Cambro-Ordovician Rocks of Central Texas: Application to the Subsurface

Date:: Wednesday, April 23, 3:00 p.m. — Saturday, April 26, 5:00 p.m. (departs from and returns to Convention Center)
Leaders:: Dr. Emilio Mutis, Dr. Bob Trentham and Dr. Emily Stoudt (University of Texas of the Permian Basin, Odessa, TX)
Fee: Professional: Single Occupancy $875; Double Occupancy $700 Student: (limited number available) Single Occupancy $438; Double Occupancy $350
Includes:: Transportation, three nights lodging, all breakfasts, all lunches, Wednesday dinner, refreshments and guidebook
Limit:: 30 persons

Stromatolitic bioherm (algal mound) in the limestone of the Point Peak Member of the Wilberns Formation (upper Cambrian).

Photo courtesy of Emilio Mutis

The “Hill Country” is one of the most popular tourist areas in Texas, providing beautiful scenery, unique food, varied shopping and numerous types of outdoor entertainment. In addition, it displays some of the most spectacular outcrops of Precambrian granites, schists and gneisses in the state. The Texas State Capitol building is built of granite from the Llano uplift, which forms the core of the Hill Country outcrops.

Preserved in a series of down-dropped fault blocks overlying the Precambrian igneous and metamorphic rocks are middle to upper Cambrian sandstones and carbonates of the Riley and Wilberns Formations. These are the oldest Paleozoic rocks in Texas. They formed in terrestrial and shallow marine depositional environments during the first major sea-level rise of the Paleozoic. During Cambrian time, North America was located roughly east-west along the paleoequator, so the depositional setting was tropical and mostly arid. Warm seas were teaming with hard-shelled and soft-bodied organisms that left both shelly faunas and magnificent trace fossils (burrows, feeding trails, etc.) in the sediments. We will visit some outstanding examples of both high-energy sedimentary structures and trace-fossil assemblages in the Hickory Sandstone Member of the Riley Formation.

We will also view outcrops of the overlying Cap Mountain Limestone Member and Lion Mountain Sandstone Member of the Riley Formation. In the 1950s and 1960s, borehole cores were cut in these units as part of a minerals exploration program. We will examine several of these cores to compare them to what can be seen in outcrop. We will discuss the stratigraphic and cyclic framework of the Riley Formation. Because of abundant porosity in the lower Hickory sandstone, it is a major regional aquifer. Recent work by groundwater geologists for a 17-county area in central Texas will be presented in conjunction with the trip. We will also include a stop at a Hickory sandstone quarry that is the source for hydraulic fracturing sand used in west Texas oil and gas operations.

The overlying Wilberns Formation consists of a basal transgressive sandstone (the Welge, another groundwater aquifer), overlain by three limestone units (Morgan Creek, Point Peak and San Saba). Locally, these limestones contain the first Paleozoic reefs, composed of a variety of blue-green algae and scattered sponges. Adjacent nonreefal carbonates display beautiful examples of cyclic sedimentation and storm deposits. We will visit outcrop examples of these features. Finally, we will observe the lower Ordovician Ellenburger Formation, a fossiliferous, shallow-marine carbonate that is equivalent to the highly productive Ellenburger in the subsurface of the Permian Basin. In central Texas, the Ellenburger is a third groundwater aquifer.

Note: This trip can be a good choice for those whose guests may wish to accompany them, but not participate in the geologic discussions. A wealth of shopping, eating and outdoor activities are available in Fredericksburg for those who choose to “remain behind” during the day. It is requested that participants provide their own hard hats for entrance to the quarry. Thursday and Friday evening meals are not included in trip cost. Participants who need lodging in San Antonio on Saturday night are reminded to make arrangements early due to Fiesta activities.

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West Texas Geological Society (WTGS)

Cambro-Ordovician Rocks of Central Texas: Application to the Subsurface

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