Outcrops south of the San Saba River crossing are gray limestone of the uppermost Cambrian, overlain by Ellenburger group rocks of Ordovician. From San Saba to Brady the highway crosses Ordovician strata. Brady itself lies on the souther tip of a band of Pennsylvanian age rocks, which form the bedrock over a wide area to the northeast.
An additional site to investigate is the Wilberns fm. cyanobacterial reef deposits on the San Saba River and highway 87 south of Brady. A virtual field trip of the reefs is available at: http://www.lgsweb.org/field05g.html
Northwest of Brady, in the town of Paint Rock, excursions can be arranged to see pictographs painted on the limestone cliffs bordering the Concho River. The paintings span a time ranging from prehistoric to the end of the nineteenth century.
mile 0.0 (30o 45.590; 98o 40.510) - Junction of 29, 16, 71
mile 1.0 (30o 45.647; 98o 41.522) - crossing Pecan Creek; granite outcrops can be seen in the creek bed.
mile 2.8 (30o 46.160; 98o 43.202) - highway 71 to Brady on right
mile (30o ; 98o ) - mile (30o ; 98o ) - Willow Creek
mile 11.3 (30o ; 98o ) - Valley Spring
mile 16.2 (30o ; 98o ) - outcrops of Hickory Sandstone Member of Riley Fm. (Cambrian) on both sides of road and in quarry.
Rocks of the Hickory Member of the Riley Formation represent the earliest transgression of the Llano area during the Phanerozoic. The thickness of the Hickory, which consists primarily of coarse-grained, crossbedded quartz sandstone and interbedded, burrowed siltstone, varies substantially across the Llano area due in large part to the irregular upper surface of the underlying Precambrian. The age is in the Bolaspidella trilobite zone of late Middle or early Late Cambrian age.
At this stop only the upper part of the sequence, approximately 5.5 m of the Hematitic Sandstone Facies (alternating units of festoon-bedded and bioturbated sandstone) and 12 m of overlying Even Bedded Sandstone Facies (predominantly burrowed fine-grained sandstone and siltstone), is exposed. Hematitic sandstones are poorly sorted and contain linguloid brachiopods and abundant hematite ooids that are nucleated around a variety of skeletal and siliciclastic grains. These deposits have been interpreted to represent migrating tidal channels.
mile 22.2 (30o ; 98o ) - Pontotoc
mile 25.2 (30o ; 98o ) - channel in Hickory Sandstone on right
mile 31.1 (30o ; 98o ) - Fredonia
mile 38.3 (30o ; 98o ) - San Saba River
mile 39.2 (30o ; 98o ) - stromatolitic bioherms in Point Peak Member of Wilberns Formation (Middle Cambrian) on both sides of road.
mile 46.9 (30o ; 98o ) - Hickory exposed in stream cut on right side of road. Here the member comprises thin-bedded quartz sandstone in thin wedge sets. Bioturbation, adhesion ripples, and small crossbed sets are common.
mile 52.0 (30o ; 98o ) - Stromatolitic bioherms in San Saba Member of Wilberns.
mile 54.9 (30o ; 98o ) - Outcrops of Threadgill Member of Tanyard Formation, Ellenburger Group (Lower Ordovician), on both sides of road.
This outcrop has limestones and dolostones of the basal Ellenburger Group (eastern half of outcrop) and associated collapse megabreccias of uncertain age (western side of outcrop) . Intact host carbonates are predominantly burrowed brachiopod-trilobite wackestones that are interrupted by thin (0-15 cm), scour-based lenses of intraclastic breccia . Burrowed wackestones probably represent low energy, restricted shelf deposits, whereas intraclastic beds may represent storm deposits. Clasts in these deposits are small (1-3 cm) with irregular margins and are interpreted as submarine- cemented sediments that were differentially winnowed, forming a lag. The upper 1.4 m of the unbrecciated portion of the outcrop is a tan dolostone, which upon closer examination can be observed to have the same bioturbation and bioclastic component as that contained in underlying limestones.
Ellenburger carbonates are relatively free of siliciclastic detritus. The Ellenburger Group is an example of an epeiric sea deposit, with water depths varying only a few feet between the Llano region and West Texas.
mile 56.2 (30o ; 98o ) - Staendebach Member of the Tanyard Fm outcrops on both sides of the road. This formation contains abundant grainstone of ooids and pellets
mile 61.5 (30o ; 98o ) - intersection with highway 87
mile 63.0 (30o ; 98o ) - Brady fossil location
mile 64.0 (30o ; 98o ) - Brady central plaza
The specimens here are Pennsylvanian, chiefly crinoids, corals, and bryozoans. While crinoid stems account for the largest portion of the mass, careful searching could produce crinoid heads, cephalopods, and trilobites.
A high-energy calcareous algal reef complex within the Wilberns Group is spectacularly exposed in the riverbed and along the southern bank of the San Saba River at its crossing with Highway 87. Key relationships to be observed include the geometry and internal structure of a major calcareous algal reef complex and the relationship between the reef and inter-reef facies, the latter comprising intraclastic-trilobite-crinoid grainstone.
The reef complex is made up of a lower, more continuous biostrome (several hundred meters wide and at least 4 m thick) and an upper zone of elongate bioherms approximately 20-25 m wide and at least 70 m long. Buildups are separated by 1-15-m-wide grain stone- filled channels , some of which are over 6 m thick. The average trend of the large reef "spurs" is N28E, whereas contained stromatolitic columns are oriented oblique to this master trend (N50E), and wave-ripple axes in interreef areas are oriented approximately perpendicular to this trend.
Both the lower biostrome and the elongate bioherms are made up of 20-100 cm diameter discrete stromatolite columns that are a minimum of 50 cm high. Most but not all stromatolite columns are distinctly elongate, with average trends slightly oblique to the trend of the elongate bioherms. Internal structures of the stromatolite columns include the typical convex-up lamination, distinctly more dense outer walls, and selectively dolomitized burrows (borings?) that are ubiquitous throughout this shallow-water subtidal setting. The simple convex-up internal lamination fabric and abundant bioturbation observed in these stromatolites are in marked contrast to internal structures present in the deeper-water bioherms of the Llano River White's Crossing outcrop.
The importance of the elongation developed in these bioherms is as a general indicator of high-energy conditions at the time of growth and as an indicator of shoreline trend. Many studies of similar reef structures to date have indicated that elongation of these large stromatolite-reef spurs is perpendicular or at a high-oblique angle to shoreline trend, being sculpted by the action of wave surge and tidal currents. This relationship, combined with regional depositional trends and isopachs, would suggest that at least the local depocenter was to the southwest.
Stromatolites are typically thought of as organo- sedimentary structures formed by trapping and binding action of filamentous blue-green algae. This perception was largely developed through work on Recent stromatolites from places like Shark Bay, Western Australia, and the Bahamas /South Florida area, and the application of this work to ancient stromatolites, largely of Proterozoic age.
The reef structures and bioherms we will observe at the San Saba and Llano River (White's Crossing) areas are not stromatolites in the true sense, mainly because they owe their accretion in large part to the syndepositional calcification of the organic sheaths of the algae (now more properly referred to as cyanobacteria), and thus represent in situ precipitated growth forms more similar to the reefs we are familiar with in the Phanerozoic.