Field Trips
AEG-TX Section Summer Meeting, 2007 (Here)
July 27-28, 2007
AEG Jahns Lecturer: John Moylan
Field Trip: Trinity River - ASACE Central City Project, Fort Worth, TX
AEG Texas Section Spring Meeting (Here)
Longhorn Caverns
Hi everyone! This is to remind you about our upcoming summer meeting. If you check out the latest edition of Texas Highways, there's a feature article about Longhorn Caverns that makes it seem pretty interesting. Hope to see you all there, come out and meet our National President.
Please RSVP to me by July 8, and let me know if you can join us by the pool Friday night in Marble Falls.
Marie Marshall Garsjo
AEG-Texas Section Chair
817-509-3757
See the Recent AEG-TX Newsletter for the details. A summary of that information is presented below:
The Next Meeting....
Hot! Summer Meeting
July 16, 2005
Longhorn Cavern, Texas
Join us for the Texas Section's second annual: it-is-too-hot-out, let's-go-underground summer meeting!
As we learned in the 2003 July AEG Death March., it is way too hot to spend much time above ground. Last year we went to Inner Space, this year we will meet up Friday night in Marble Falls and visit Longhorn Caverns Saturday. The trip will be led by the team of Brenda McCaleb, our Vice Chair, and Dr. Chris Mathewson, Texas A&M.
We will arrange for continuing education credits.
Ms. McCaleb's thesis: The Surface Expression of Longhorn Cavern, Burnet County, Texas, completed December 2000, covered this area in detail. Dr. Mathewson was her advisor. They will present a two-part discussion prior to the field trip. No doubt, you’ll come away with a better understanding of this complex and interesting area. See details below:
The meeting will start with an icebreaker poolside at the hotel Friday night, July 15, at 7:00 PM. This is your opportunity to meet the national AEG President, and for him to meet us.
Saturday morning, 9:00AM - Leave hotel, and drive by the Marble Falls Fault, located on FM 1431. It has a total displacement of 3,600 feet, bringing Pennsylvanian limestone units in contact with Precambrian granites.
1st stop - Town Mountain Granite Quarry. Stop at rest area across street, for a roadside discussion of the Llano Uplift and local geology. The granite dome mined here was used in the State Capital Building in Austin and many other construction projects.
2nd stop, 9:30 AM - Hoover Point Scenic Overlook. It is located approximately 10 miles west of Marble Falls, along FM-1431. To the southwest you overlook the Colorado River and Precambrian metamorphic rocks. The road cut across the street has a great exposure of the Cambrian Moore Hollow Group and two drag folds.
3rd stop, 10 AM - Longhorn Cavern. The surface above the cavern is normally closed to visitors but we have been given permission to access this area. There are 17 sinkholes above the cavern, and they display a multitude of "text book" karst features and structural fracturing. A few of these karren features include grooves along structural fractures (grikes) up to 4 feet deep, rills indicating sheet flows, and etched pits within massive rock.
10:45 AM - After touring the surface, we will go underground in Longhorn Cavern.
1:00 PM - Lunch at Dock's in Marble Falls. They have a presentation room reserved for us.
After Lunch: Board meeting, 1 hour. Anyone who is interested is invited to attend.
Other points of interest in the area, while we are waiting for dusk to fall:
Inks Lake Vista - exposes the Vista Springs Gneiss
Inks Lake - Valley Spring Gneiss and Packsaddle Schist
Saturday Night:
Drive 65 miles west to Mason, Texas and observe bats in flight. The Eckert James River Bat Cave is 17 miles west of Mason. It is one of the largest bat nurseries in the country, with about 4 million female bats inhabiting the site from May through September. About an hour or two before sunset, bats begin to flutter and chirp around the mouth of the cave. Slowly, a stream of bats emerges flying in a large circle, low to the ground, just outside the cave entrance. These bats gradually spiral upwards and form a dark funnel, reaching several hundred feet into the evening sky. The bats at the top of the spiral break off, forming columns of bats that stream out over the countryside. This seemingly impossible torrent of bats forms a densely packed "bat tornado" for about an hour. It quite a sight.
Where: Best Western Marble Falls
1403 US Highway 281 Marble Falls, TX (830) 693-5122
Fee: $25 ($15 for students)
Make your reservations soon! We have a few rooms reserved for Friday night at a reduced rate of $89 plus tax, and they need to be booked by July 1. Ask for the
AEG rate. Please join us for more AEG fun, geology, and camaraderie!
Please RSVP to marie.garsjo@ftw.nrcs.usda.gov, or call her at 817-509-3757.
If you have any questions, contact Brenda or Chris at: bmccaleb@hotmail.com,or mathewson@geo.tamu.edu
Fall Meeting--October 22!!
We will tour scoured Cretaceous limestone beds exposed in the Guadalupe River below Canyon Lake, brought to you in 2002 courtesy of 30 inches of rain in two days and flows equal to or above the 100-year flood. The Guadalupe trip, under the direction of Boyd Dreyer, will meet in Gruene. The trip includes an extraordinary view of limestone, micro and macro. Spectacular faults, fractures, fossils, casts, and vugs are close up and scrubbed clean. And for the "big-picture" fans, a panoramic view of some amazing ramp faulting. Mark your calendar and bring your hiking boots. Again, we.ll arrange CEU credits. What more could you want?!
The photo shown in the Newsletter was taken from: Ferrill, D.A., Sims, D.W., Franklin, N., Morris, A.P., and Waiting, D.J. 2005, Structural Controls on the Edwards Aquifer/Trinity Aquifer Interface in the Helotes Quadrangle, Texas. Southwest Research Institute final report prepared for Edwards Aquifer Authority and U.S. Army Corps of Engineers. Many thanks!
Gulf Coast Subsidence
The following web links and summary, penned by Bob Traylor, gives you a quick overview of the Gulf Coast subsidence issue. Gaining a better understanding of this geologic process is key for Gulf Coast geoscientists and decision makers. It's alarming how quickly things are changing and what the long-term consequences will be.
Recently, the National Geodetic Survey (NGS) officially released:
NOAA Technical Report 50: Rates of Vertical Displacement at Benchmarks in the Lower Mississippi Valley and the Northern Gulf Coast (www.ngs.noaa.gov)
This is one of the most important documents to be released by the NGS. It gives vast insight into the coastal subsidence problem. Here's a quick review of the report.
Do not be intimidated by the length of the NOAA Report. The text is quite readable, brief, and to-the-point. Most of the report consists of tables and graphs of tedious, but vastly important, leveling data sets. These data, the collection and analytical methodologies, and subsequent conclusions have been exhaustively peer-reviewed by NOAA and the US Geological Survey.
Very clearly, the report establishes coastal subsidence as proceeding at an alarming rate regardless of mitigating intervention. That is to say while anthropocentric causes, such as fluid withdrawal, may have local dominance and is a force to be reckoned with, the principal driving forces of coastal subsidence are geological (tectonic) components. To understand, quantify, and predict subsidence velocities we must first pursue height modernization as quickly as possible with orthometric heights tied to the North American Vertical Datum (NAVD) 88. In the near future, serious planning decisions will need to be made regarding coastal management programs, ground water usage, facility permitting, infrastructure redesign, flood control, and large-scale intervention through levee emplacement. If we are unable to differentiate the various components of coastal subsidence, we cannot make wise planning decisions.
From a geological perspective, the report's graphing of sequential first-order leveling campaigns shows the surging of active faulting and the subsequent elastic crustal response to the surges. This is fascinating data and provides enormous insight into how coastal faults move and tectonic subsidence bowls develop. Decades of leveling data show slow-motion, aseismic earthquakes.
The report does not address subsidence velocities in Texas. Nonetheless, one cannot assume that everything stops at the Sabine. In my opinion, coastal tectonic instability is most prevalent in Texas from the Brazos to the Sabine. Instability diminishes toward the Central Texas Gulf Coast. The Lower Texas Gulf Coast is relatively stable tectonically. A major destabilizing component is salt.
Much of the early Gulf Basin was underlain by Jurassic-aged salt. For the Texas Gulf Coast the core of that salt basin was the Houston Salt Basin, which is the focus of the greatest sediment loading and coastal instability that we see today. The present-day Lower Texas Gulf Coast was probably underlain by comparatively little to no salt; moreover, it has a thinner sediment load. As a matter of fact, the Maverick Basin, which is the northern most extension of the failed Rio Grande spreading arm, is underlain by Jurassic clastics. The greatest sediment loading occurred during the Eocene and was dominated by large rafting blocks and subsequent mini-basin infilling. These rafting blocks, which often spanned several counties, probably glide planed on geopressured shales and had little to do with salt-evacuation tectonics. It appears that the tectonic effect of differential loading has relatively stabilized for the South Texas Gulf Coast.
Conversely, salt and geopressured shale tectonics are ongoing today along the Upper Texas Gulf Coast, where there is a very thick sediment load, perhaps upwards of 50,000 - 60,000 feet of sediment and salt. Massive sediment loading occurred though the Eocene, Oligocene, and Miocene for the Upper Texas Gulf Coast. That differential loading is still being accommodated, not only by internal-basin salt/shale tectonism, but by external-basin lithospheric movement as well. In other words, the basement crustal lithosphere is depressed by the huge sediment load. Needless to say, Gulf Coast Basin tectonics are quite complex and difficult to understand, but like it or not, those tectonic components have an overwhelming influence on the present-day coastal plain of Texas and Louisiana.
In understanding Gulf Coast Basin geology, eustasy is often cited as the key mapping concept, such as in sequence stratigraphy, but as we learn more about the Gulf Coast Basin, relative (local) sea level depends less on eustasy and more on the dynamics of sediment loading and its related tectonics. NOAA Technical Report 50 is yet another eye-opener to the importance of tectonics in mapping Gulf Coast geology. I reccomend this report as interesting reading for those involved in the subject and for those who are not. |
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