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Guidebook to the Geology of Travis County

Chapter 1 : The Geologic History of the Austin Area

Diana Grunig

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The geologic history of any area is interpreted by studying its rocks and fossils. Where those rocks and fossils do not outcrop, the history of the area may be interpreted, though only broadly, by extrapolating data from other areas and by studying samples brought from deep in the ground by tests drilled for oil or water. In the Austin area the older rocks are not exposed, and the historical interpretation of those more ancient times is less exact.

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(up to 600 million years ago)

Not much is known of the Precambrian history of the immediate Austin area. Precambrian rocks do not crop out in Travis County, and the deepest rocks that have been reached by drilling can be identified as Paleozoic or as metamorphic rock of undetermined age. To the northwest, in the nearby Llano region, outcrops expose rocks rich in history -- a sequence of events including (1) deposition of sedimentary and volcanic debris in a broad basin, (2) deformation of the lithified sediments by faulting and folding, (3) metamorphism of the deformed rocks and their intrusion by igneous rock bodies, and finally, (4) the intrusion of large granitic masses about a billion years ago. The processes of faulting, folding, and intrusion recorded in the Llano rocks must have built a mountain range there, but the effects of nearby Precambrian mountains are not recorded in the much younger rocks of the Austin area. The next 400 million years saw the destruction of the mountain range by erosion. (For a more detailed picture of the geologic history of the Llano region, a good start is Guidebook 13, by Barnes and others, 1972, published by the Bureau of Economic Geology, University of Texas at Austin).

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(from 600 million to 225 million years ago)

During the Paleozoic Era the Austin area was part of the Ouachita Geosyncline, the rocks of which were folded and faulted into an ancient mountain range in the late Paleozoic (Figs. 1, 2). The Ouachita system of rocks stretched in a broad and gentle arc from the Ouachita Mountains, for which they are named, in Arkansas and Oklahoma, across Travis County, to Texas' Big Bend region and in the Marathon Uplift, but they are known only from well data in intervening areas. Paleozoic rocks on the edge of the geosynclinal basin, updip from the strata that underlie the Austin area, are exposed in the Llano region to the northwest.

Location of Ouachita Structural Belt(36K)
Figure 1
Location of Ouachita Structural Belt

Rocks of early Paleozoic age are known from the Llano region, in outcrops, and in wells just to the southeast of the outcrop area. They record a time of principally shallow marine shelf and shoreline environments of deposition, with both clastic and carbonate sediments. Early Paleozoic time in the Austin area was probably similar to this, though by the start of the Ordovician time, the Ouachita geosyncline trough may have been established; with the existence of the Ouachita trough, sedimentary deposits in the Austin area became relatively thicker than in the Llano region, and deposition may have been in relatively deeper water.

Central Texas in the Upper Pennsylvanian(54K)
Figure 2
Central Texas in the Upper Pennsylvanian

Through middle and late Paleozoic time great volumes of sediments were deposited in the Ouachita trough. During Pennsylvanian time these geosynclinal sediments were compressed, deformed, and uplifted into a great mountain range. These mountain ranges supplied sediment to the flanking seas. By the end of the Paleozoic the period of Ouachita mountain building was over. The seas no longer covered the Austin area. The region was underlain by folded Paleozoic rocks to the northwest and metamorphic rocks to the southeast. Figure 3 shows the extent of known Paleozoic rocks in the Austin area.

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(225 million to 136 million years ago)

No known rocks from the first part of the Mesozoic Era remain in the Austin area or in nearby Llano. The whole region was probably a land area, and erosion the dominant geologic activity of the time. As the old Ouachita mountain belt was being reduced to lowlands, the area to the southeast began to subside to form the Gulf of Mexico geosyncline. To the southeast of Travis County, Jurassic rocks are encountered in deep wells.

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(136 million to 65 million years ago)

Most of the rocks that crop out in the Austin area are Cretaceous. In brief, the history of the Cretaceous Period in the Austin area is the story of gradual though intermittent northwestward encroachment of the sea that filled the subsiding Gulf of Mexico geosyncline to the southeast.

Paleozoic Rocks, Austin Area(23K)
Figure 3
Paleozoic Rocks, Austin Area
(Simplified from Flawn, 1961)

1-Metamorphic rocks, subsurface, Paleozoic and Precambrian
2-Rocks of the Ouachita Geosyncline, subsurface, Mississippian through Pennsylvanian
3-Rocks of the Ouachita Geosyncline, subsurface, Cambrian through Devonian
4-Rocks of the shelf and the margins of the Ouachita Geosyncline, subsurface, Mississippian through Pennsylvanian
5-Rocks of the shelf and the margins of the Ouachita Geosyncline, outcrop, Mississippian through Pennsylvanian
6-Rocks of the shelf and the margins of the Ouachita Geosyncline, outcrop, Cambrian through Devonian

Earliest Cretaceous time is recorded in rocks of the Trinity Group of the Commanchean Series. The generalized paleogeographic setting for the deposition of these rocks is shown in Figure 4. The eroded lowland over which the Cretaceous sea encroached, was not flat, and local differences of relief add to the complication of interpreting Cretaceous events. The Sycamore Conglomerate, the oldest formation of the Trinity Group, probably represents the fluvial systems that fed the first marine Cretaceous sediments into the Gulf of Mexico geosyncline (the Hosston and Sligo Formations, found in the subsurface to the southeast of the Sycamore Conglomerate). After a period of slight uplift and erosion, the Hammett Shale and the Cow Creek Limestone were deposited on the older Sycamore Conglomerate and on Paleozoic surface to the northwest. The Hammett Shale and Cow Creek Limestone marine shale and some marine sandstones and conglomerates; the second comprises, in many places, a cross-bedded beach rock. After a period of erosion of the upper Cow Creek Limestone, the Cretaceous sea once again deposited sediments on underlying older Cretaceous rocks and encroached on the remaining exposed Paleozoic surface to the northwest. The Hensel Sandstone and the Glen Rose Limestone were deposited during this time. The Hensel Sandstone represents fluvial, tidal, lagoonal, beach, and nearshore marine environments, the marginal deposits of the northwestward-advancing sea. The Glen Rose rocks represent sediments deposited on a very large, very shallow marine shelf. Outcrops around Austin show a wealth of interfingering carbonate environments, changing in time and space, from subtidal lagoonal flats, to shoals, to beaches, to marshes and ponds at or just above sea level. The youngest rocks of the Glen Rose contain many indications of very shallow-water and supratidal environments of deposition, including evaporates, dinosaur tracks, and mud cracks.

Central Texas in the lower Cretaceous(47K)
Figure 4
Central Texas in the lower Cretaceous

The Walnut Formation of the Fredricksburg Group was deposited on the Glen Rose Limestone. The contact between these two formations in the Austin area is not erosional, as it is in the Llano region, but the very uppermost Glen Rose has been bored by marine mollusks, indicating lithification of the Glen Rose and the return of a marine environment before the deposition of the Walnut. The oldest Walnut rocks contain some terrigenous clastic material, clay and sand-sized limestone grains, probably brought in from the northwest. This is the Bull Creek Member of the Walnut Formation. Overlying this is the Bee Caves Member, followed by the Cedar Park Member which represents the rapid advance of a marine marsh environment. Next came deposition from a sequence of alternating marine marsh and open marine environments (the upper clays of the Walnut Formation), extending as far south as an oolite bar (the Whitestone Lentil) which developed across the northern part of Travis County. These formations are overlain by the youngest formation of the Fredricksburg Group, the Edwards Limestone, which was deposited from a northwesterly transgressing sea. The geography of the Austin area during the time of deposition of the Edwards may have looked like Figure 5.

Edwards Paleogeography, Central Texas(281K)
Figure 5
Edwards Paleogeography, Central Texas

The encroaching Cretaceous seas had by this time covered almost all of the Llano uplift, and marine circulation was still restricted by the development of large reef trends. The Edwards Limestone is composed of rocks from medium- to high-energy, shallow-water, marine environments reefs and flank deposits of reef detritus, oolite shoals, inter-reef deposits of carbonate mud and sand, and lagoonal carbonate mud and sand, and lagoonal carbonate mud and evaporates in the Kirschberg area, plus many associated supratidal rocks.

The Washita Group, the last of the Comanche Series, was deposited on top of the Edwards Limestone. By this time the remaining islands in the Llano region had been drowned, and the Cretaceous sea in Texas was part of the sea that stretched all the way to the Arctic and was known as the Cretaceous Rocky Mountain Geosyncline. The San Marcos Platform was still a controlling feature of Cretaceous topography, and the formations of the Washita Group are thinner in the Austin area than they are in the nearby Tyler and Maverick basins. The oldest unit, the Georgetown Formation, consists of shaly limestone and fine-grained limestone deposited in shallow marine waters. Above this, the Del Rio Formation is a shale from a restricted marine shelf environment, and the overlying Buda Limestone is also thought to have been laid down on a broad, low energy, shallow, shelf behind the Stuart City Barrier Reef.

Following the floundering of the Stuart City Barrier Reef, the Gulf Series of the Cretaceous in the Austin area was inaugurated by the deposition of the terrigenous sediments of the Woodbine and Eagle Ford Groups. The detrital material introduced into this carbonate system was probably derived from the Ouachita Mountains in Arkansas and to the east, since following the drowning of the last of the Llano islands, there was no nearer source.

Carbonate deposition returned to the area with the deposition of the Austin Group, locally known as the Austin Chalk. Austin Group sediments were deposited in shallow and near-shore marine environments. Some of the topographic highs that controlled the distribution of those environments were provided by a belt of volcanoes which erupted in the Austin area and to the south and west, interlayering lava flows, volcanic ash beds, and wave-reworked volcanic material with the carbonate sediments.

The Taylor Group, which was deposited next, is a marine calcareous clay, and the Navarro Group, the last of the Cretaceous, consists of marine marl and carbonaceous shale.

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(65 million years ago to present)

The Cenozoic history of the Gulf of Mexico geosyncline is dominantly the story of gradual withdrawal of the seashore to its present position, accompanied by the deposition and distribution of marine sediments, then sediments deposited by deltaic and shoreline processes and finally terrigenous sediments deposited by fluvial systems. Each successively younger fluvial system is farther to the southeast than the last. The Austin area, already on the margin of the geosyncline at the start of the era, was not an area of major sedimentary deposition during the Cenozoic. Possibly it was a site of deposition in the Paleocene (the earliest Cenozoic); by Oligocene (about middle Cenozoic), the area may have been on a low land area, a minor contributor of sediment in the Gulf. In the next epoch, the Miocene, the major movement along the Balcones Fault Zone began, in the belt where volcanoes of the late Cretaceous had stood. Any earlier-deposited Cenozoic sediments and the upper Cretaceous rocks began to be eroded from the Austin area. Northwest of the fault zone, on the upthrown side, erosion continued until today the lower Edwards Formation is the youngest rock unit exposed. In the fault zone, younger rock units are preserved in down-dropped fault blocks. Southeast of that, the undisturbed updip ends of the very gently-dipping late Cretaceous and Cenozoic formations make broad bands of outcrops, each band younger than the last as you go southwest towards the coast.

Onto this general picture the geologic processes of the most recent period, the Quaternary (beginning up to 2 or more million years ago), have superimposed only the dissection of the edge of the upthrown fault block by the Colorado River and other streams, and the deposition of broad, thin, fluvial terrace deposits on the plains of the downthrown block.

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