Following the success and popularity of a 2023 spring field trip in the Texas Hill Country, organizers plan another – expanded -- outing for this year.
“We hope to extend the field trip over a few days and are currently working on the details and logistics,” said Molly Turko, president of the AAPG Petroleum Structural Geology and Geomechanics Division.
The May trip to the Llano fault system, “Bluebonnets, Red Granite and Faulting too,” was led by Thomas E. Ewing, 2023’s Robert R. Berg Outstanding Research Award winner. The next foray will be led by Peter Hennings, senior research geologist with the Bureau of Economic Geology at the University of Texas at Austin, and David Ferrill, institute scientist at Southwest Research Institute.
The May field trip was based out of Marble Falls and was attended by a number of scientists, many well-known in the structural geology world, Turko said.
About the Region
“The outcrops surrounding Marble Falls provide a prime opportunity to discuss various structural geology concepts vital to petroleum systems, carbon capture, critical minerals, and even geothermal. These concepts include the kinematics of faulting, fault seals/leaks and fluid migration, fault-related reservoir traps, and the impact of mechanical stratigraphy on faulting and fracturing. Many of the attendees have worked in a variety of industries and listening to their insight at each of the different outcrops was priceless,” she said.
“The region we visited is known as Llano Country, and it is the most extensive exposure of Precambrian (Proterozoic) rocks in Texas. It is overlain by a substantial Cambrian and Lower Ordovician succession of carbonates and clastics, which made up the Great American Carbonate Bank, followed by a very thin Middle Paleozoic and thick Pennsylvanian section. The fault timing in this region is tightly constrained to a short interval in the Pennsylvanian. Plummer (1950) presented evidence for fault motion beginning in the Morrowan during deposition of the Marble Falls limestone, through the Atokan during deposition of the Smithwick and into the Desmoinesian during deposition of the Strawn. The faulting was a result of the Pennsylvania Orogeny, an event that shaped many of the prominent oil and gas basins we drill in today including the Ft. Worth, Anadarko, and Permian Basins. Faults surrounding Llano Country help us to understand the tectonic history that shaped these basins and act as analogs for faulting within them,” Turko explained.
She said the mix of attendees and varying levels of expertise made the outing as enjoyable as much as educational.
“It was great to see discussions amongst the various degrees of expertise, including graduate students, petroleum geologists and expert structural geologists. It was a prime opportunity to discuss ‘structural geology 101’ on one hand, and then switch to complex structural concepts that we are still debating today,” she said.
Stops Along the Way
Turko said the three stops on the trip offered a variety of concepts and applications for discussion.
“Stop 1 offered the opportunity to see a major fault with several hundred feet of throw where the Proterozoic Town Mountain Granite was juxtaposed against the Cambrian age Hickory Sandstone. This fault, along with many others of the Llano fault system, are primarily classified as normal faults, however the situation is more complex in several ways,” Turko explained. “Faults of the Llano system characteristically dip around 70-80 degrees, which is steeper than the classical Andersonian fault theory of 60 degrees for normal faults. Two possibilities could be entertained: 1) that the system is nearly strike-slip in character, or 2) the faults are steepened due to pervasive basement fracturing or jointing. Debating these options in the field and looking for evidence that supports one option or the other makes for a fun day!”
The major fault at Stop 1 consists of a gouge zone as much as four-feet wide and multiple slip planes littered with slickenlines. The Town Mountain Granite makes up the footwall and has been “grusified” (weathered granite). Extensive fracturing on each side of the major fault makes up the damage zone which showed evidence for fluid flow along the fault.
“Stop 2 was at the infamous Hoover Point which is located at the southwestern end of Backbone Ridge, high ground that is underlain by Cambrian and Ordovician carbonates and sandstone. The graben is bounded by the northeast-trending Roaring Spring fault on the northwest, and east-northeast trending Bald Knob fault on the east, creating a wedge-shaped geometry. The roadcuts here provide an excellent exposure of the interior of this graben near its southwestern tip. The stratigraphic units exposed include the Cap Mountain limestone, the conspicuous glauconitic sandstone of the Lion Mountain, the thin yellow-brown sandstone of the Welge, and the mixed carbonate of the Morgan Creek limestone. The outcrop includes about 261 faults that strike about 43 degrees and dip about 72 degrees in either direction. The outcrop can be divided into seven structural domains and includes exciting structures such as normal faults, possible bed-parallel thrust faults, oblique slip, reverse faults, inversion features, and small-strike slip faults. The outcrop has quite a neat story to tell and deciphering the structural history amongst peers makes for a great time!” Turko explained.
“This is also the spot we picked to take our group photo!” she added.
“Stop 3 included a larger normal fault – about 200 feet of throw – where the Hickory Sandstone is juxtaposed against the Cap Mountain limestone. This was a great spot to discuss mechanical stratigraphy and how two mechanically different layers responded to the same fault. The Cap Mountain consists of interbedded limestone and limy siltstone with most beds less than 20-centimeters thick. Fracture intensity and additional small faulting occurs in the Cap Mountain which also makes up the hanging wall of the fault. The Hickory here consists of even-bedded course sandstone – about 80 centimeters thick – separated by recessive silty (possibly limy) sandstone around 60-centimeters thick. We tend to see less fracturing within the Hickory sandstone. The fault zone consists of gouge several centimeters thick and dips about 74 degrees to the southeast. Unlike stop 2 (which was a bit too dangerous to cross the highway), stop 3 provided a nice opportunity to put your finger on the fault!”
Lessons Learned and to Build Upon
”A day in the field beats a day in the office, and this field trip was no exception to the rule. Each of the stops provided an opportunity to discuss several key structural geology concepts while also telling an interesting story on the tectonic history of Llano Country. We looked at the impact of faulting and fracturing on a reservoir, and how faults can act as seals or conduits, an important concept for both petroleum and carbon capture. Faults can also help to create structural traps which help to capture fluids in a reservoir. We also discussed the role of mechanical stratigraphy and how something like a granite may behave differently than a sedimentary rock under the same stresses. If the structures alone weren’t enough to keep the participants engaged, the debates on the structural styles and tectonic history were enough to keep our curiosity piqued,” Turko recounted.
“Our leader, Tom Ewing, did a fabulous job writing up the guidebook and explaining the debates on strike-slip verse normal faulting, and flexural loading due to thrust sheets from the southeast verse tectonic stress from the southwest during the Pennsylvanian Orogeny. Overall, it was a great spring day to be in the field and we look forward to planning another trip for 2024!” she added.
To stay up to date on the latest AAPG PSGD activities please follow their LinkedIn page or email them at [email protected] to get put on their mailing list.