Microbial Carbonates in Central Texas Field Trip
Thursday, March 22 - Sunday, March 25, 2018
Microbes are defined as microorganisms visible only under a microscope. Some examples are bacteria, fungi, molds, algae, and protozoa. Microbial sediments have always attracted the attention of sedimentologists and paleontologists, but in recent years the discovery of large oilfields in microbial carbonate reservoirs has generated renewed interest in these rocks, especially in the environments in which microbial carbonates form and the characteristics that make them good reservoirs. What used to be classified as “algal” is now classified as microbial or calcimicrobial. The older classification of living things that included kingdoms of animals and plants has been supplanted by a scheme that puts all living things into three main branches called “domains” of life – Bacteria, Archaea, and Eukarya. Bacteria, including cyanobacteria, and Eukarya, red and green algae and fungi, are the principal “actors” involved in the formation and diagenesis of microbial carbonates.
Most microbial sediments and rocks are carbonates. Some of the most spectacular examples are found in Upper Cambrian carbonates of Central Texas – microbial buildups and associated facies in the Point Peak and San Saba members of the Wilberns Formation. Point Peak and San Saba outcrops in Central Texas expose some of the best-preserved Cambrian microbial carbonates anywhere. These microbial limestones have not been tectonized and their depositional fabrics and textures have been remarkably well preserved with only minor dolomitization in some stromatolitic and oolitic facies. Thin section study of the microbialites reveals four different calcimicrobes; Girvanella, Epiphyton, Renalcis, and Nuia.
Point Peak and San Saba rocks outcrop around much of the Llano Uplift (a structural dome with its cover removed to form a topographic basin). The best exposures are in the western part of the area in Gillespie and McCulloch Counties, extending from the Doss settlement in the southwest to the San Saba River in the north. Because it is not always easy to get access to outcrops on private land in the Texas Hill Country, this field trip takes advantage of excellent exposures along segments of the Llano and San Saba Rivers, where we have access. The Llano River portion of our trip will be done from kayaks making it even more interesting and exciting.
One day of the trip will be by kayak, starting at White’s Crossing near Mason, Texas. We will see famous exposures of microbial buildups in the Point Peak Member, Wilberns Formation. As we paddle downstream, we will have close-up views of microbial bioherms that have fallen into the river from the surrounding cliffs. As we pass gradually up-section through the Point Peak, we will have a chance to get out on the riverbank to see a variety of sedimentary structures, including mud cracks and flat-pebble conglomerates, some of which are “edgewise” conglomerates formed by strong eddy currents that spun the flat pebbles into accumulations that look like “pinwheels.” Our lunch stop will provide an opportunity to walk over a continuous exposure of stromatolitic and thrombolitic microbialites. After lunch, we will see microbial bioherms with superb stromatolite accumulations at water level. This location offers a chance for close-up photos of stromatolites in cross-section. As we near the end of our float, we will be passing out of the Point Peak and through the San Saba Member of the Wilberns Formation.
The second day of the trip includes an excursion to the US Highway 87 crossing on the San Saba River. After arriving by car, we will walk along the river on both sides of the bridge to examine large ripple marks in grainstones of trilobite-brachiopod hash and microbial bioherms in vertical succession, some with well-developed stromatolitic “capping sequences” as well as thrombolitic and laminar macrostructures that are present at this location.
Base Field Trip cost- $388
Includes transport, 2 Breakfast, 2 lunches, and Dinner Sat)
Lodging at headquarter camp
Group room with multiple beds:
bunk bed (7 spots)- $126
Double (4 spots)- $144
Private Room with 2 single beds:
Three rooms, price per bed (6 beds) $162
There is also a B&B in Mason, Tx that you can arrange lodging at on your own. Contact Pat at First Choice Reservations FirstChoiceReservations@yahoo.com
You must call the HGS office to register 713-463-9476