Keith Young
The Austin area has a great variety of interesting rocks. However, one must go to the Llano Uplift for the oldest rocks in Texas and to the Gulf Coast for the youngest. The following discussion is restricted mostly to the rocks that occur in Travis County or an area of greater Austin.
The discussion below will describe the various formations in and around Austin with the aid of maps and illustrations. Figure 6 serves as a guide towards indicating the lithologic symbols that will be used in the stratigraphic sections found below in this chapter.
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Fig. 6
Symbols for Stratigraphic Sections
Lithology - The Glen Rose Limestone (Fig. 7), particularly the upper part exposed in the vicinity of Austin, consists of alternating beds of harder and softer limestones (mostly biomicrite). The harder beds are more cemented than the softer beds, and are usually thinner than the softer beds, which are marly, consisting of slightly clayey limestone. There are a few harder beds of organic debris or organic growth, the latter usually biolithites.
The Glen Rose Limestone thickens from northwest to southeast in the Austin area, but since the lower part is not exposed, the thickening and thinning is not readily apparent.
Boundaries - In the subsurface of the Austin area the Glen Rose rests disconformably on the Cow Creek Limestone, and there is no Hensel Formation. To the north of the Austin area, the base of the Glen Rose Limestone is gradational with the Hensel Formation, the Hensel being the nearshore lateral terrigenous equivalent of a part of the Glen Rose Limestone.
The Walnut Formation overlies the Glen Rose Limestone. The upper surface of the Glen Rose Limestone in the Austin area is bored by rock-boring pelecypods (Lithophaga sp. and Gastrochaena sp.), indicating that this surface was a hard ground before the deposition of the ensuing formation. Prior to boring and before hardening, dinosaurs sometimes tramped across this surface. This surface has been said to be an erosional surface, but the evidence is debatable. The most marked difference between the upper part of the Glen Rose and the lower part of the Walnut is the hypersaline depositional aspects of the former and the brackish water aspect of deposition of the latter.
Environments of deposition - The Glen Rose was deposited behind (northwest of) the Stuart City reef trend (Fig. 4) and represents a variety of shallow subtidal to supratidal environments; in the Austin area the great preponderance of rock was formed under supratidal conditions. The source of the carbonate sediment was from subtidal organic accumulations of carbonates, which were in turn reworked into supratidal environments by storms, winds, and waves.
Localities - The Glen Rose Limestone can be studied at Mount Bonnell (Fig. 7) and along the new road cuts on the West Loop between the Colorado River and Westover Hills. Other good localities can be visited along the Marble Falls highway (Highway 71) between Oak Creek and Barton Creek.
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Figure 7
Mount Bonnell
The Walnut Formation is comprised of two members in the Austin area, the Bull Creek below and the Bee Caves above.
Boundaries - The lower boundary has already been described. The upper boundary of the Bull Creek limestone is a bored bed (Lithophaga and Gastrochaena spp.) of hard ground on the uppermost biosparite mentioned above.
Environments of deposition - Moore (1961) interpreted the Bull Creek area of deposition as a lagoon. The cycles represent cycles of subtidal deposition, each prograding, and the bed at the top of the last cycle probably represents intertidal shoaling.
Localities - The Bull Creek Limestone Member of the Walnut Formation can be observed in the hills on the West Loop between St. Stephen's School and Bee Cave Road, and along West Loop just west of Westover Hills (Fig. 8).
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Figure 8
Austin City (Emma Long) Park
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Figure 9
Bee Caves Road, Westlake Hills, TX
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Figure 10
Walnut Clay Drive, in Northwest Hills
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Figure 11
Section of Walnut
Whitestone School Area, Travis and Williamson Counties
Environments of deposition - Moore (1961) also interprets the Bee Cave Marl as deposited in a lagoon behind barriers, but with a small source of fine terrigenous sediment that was not available during Bull Creek deposition. The deposit represents mostly subtidal to intertidal grassflats, becoming more calcareous upward.
Localities - The best locality to observe the Bee Cave Member is along the road to City Park at the top of the hill. It also crops out on Bee Cave Road just west of the south end of Red Bud Trail (but watch the traffic), and along Red Bud Trail east of St. Stephen's School (Figs. 8, 9, 10, and 11).
Lithology - The Edwards Formation consists of about 90 to 105 meters (300 to 340 feet) of various kinds of limestone. Most of the Edwards in the Austin area consists of the lowest member, which is about 60 meters (200 feet) thick and consists of dolomite, dolomitic limestone, and hard, gray limestone containing rudists (long, conical bivalves; Fig. 12). Gray to black chert is common.
Member 2 of the Edwards Limestone is about 12 meters (40 feet) of thin bedded, fine-grained, dolomitic limestone, and fine-grained flaggy limestone. Nodular chert is common.
Member 3 of the Edwards Limestone consists of mostly soft, burrowed limestone (micrite) that forms a marly slope. It is from 3 to 5 meters (10 to 15 feet) thick.
Member 4 of the Edwards Limestone is the uppermost member and is about 12 meters (40 feet) thick. It consists of flaggy limestone beds, and a 1-meter thick rudist bed overlain by dolomitic limestone. Its top is a calcarenitic limestone with sparse gluaconite grains.
Boundaries - Both boundaries of the Edwards Limestone in the Austin area are gradational, the Lower Edwards gradually grades into Walnut beds to the north, and the uppermost Edwards Limestone grading into Georgetown beds to the north.
Environments of Deposition - The rocks of the Edwards Limestone represent deposition in a great variety of carbonate environments; reef, lagoonal, shoal, basinal, and supratidal environments of deposition are represented. In addition, many of the limestones thus deposited were altered to dolomite by the invasion meteoric waters shortly after deposition. In the Austin area, along the Balcones fault zone, some dolomitization occurred later, following the invasions of waters high in magnesium sulfate.
Localities - The Edwards Limestone can be observed along the Colorado River below Tom Miller Dam (Fig. 12) and at many localities above Barton Springs on Barton Creek.
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Figure 12
Low Water Bridge Near Red Bud Island
Localities - Most of the Georgetown Formation consists of alternating beds of thin, fine-grained limestone or marly limestone (biomicrite or marly biomicrite = wackestone or marly wackestone). The nearly complete section of Georgetown Formation that was once exposed in the railroad cut at Sixth Street (Fig. 13) has since been removed by the development of Mopac Boulevard, but parts of the formation can still be observed in Johnson Branch and other nearby areas. The Georgetown Formation ranges from 13 meters (40 feet) in thickness in the Rollingwood area to about 20 meters (60 feet) near McNeil, north of Austin, the lower part being replaced by Edwards Limestone to the south.
Environments of deposition - The Georgetown Limestone represents a number of open-shelf, subtidal environments that are differentiated primarily by the faunas that occupied the different environments.
Localities - The top of the Georgetown Limestone can be observed in Shoal Creek at about Martin Luther King Jr. Street (19th Street) (Fig. 14); the formation can also be seen in Johnson Branch and nearby localities near the Sixth Street access to Mopac Boulevard, north of Mount Bonnell Road in Huck's Slough, where it is in fault contact with the Glen Rose Limestone, and in an area known as "Fossil Valley" in Rollingwood.
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Figure 13
Sixth Street Underpass
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Figure 14
Pease Park
Localities - The Del Rio is about 25 meters (75 feet) of dark olive to bluish-gray to yellow-grown pyritic, gypsiferous clay. The clay contains illite, montmorillonite, and kaolinite. Ilymatoqyra arietina (Ršmer), sometimes mentioned as "E arietina by authors, commonly occurs within this for
Boundaries - The lower boundary of the Del Rio is gradational with the Georgetown Limestone, and the transition occurs through one to two meters (several feet). The upper boundary in the Austin area is scoured by, and therefore disconformable with, the basal beach limestone of the Buda Formation.
Environments of deposition - The Del Rio Claystone contains many very small species, and lacks a normal bottom assemblage. It is therefore interpreted to have been deposited in a lagoon with abnormal bottom conditions, as indicated by the large amounts of pyrite. Upon weathering, the pyrite reacts with water to produce sulfuric acid, which in turn reacts with calcite to produce the selenite (crystalline gypsum) that occurs in the weathered zone.
Localities - The Del Rio can be observed along Shoal Creek. It can best be observed along Barton Springs Road.
Localities - The Buda Limestone consists of 11.5 to 16 meters (35 to 50 feet) of nodular, soft and hard limestone (biomicrite and biosparite). The basal bed is a hard limestone composed of oyster shell fragments (oyster shell biosparite or grainstone). Most of the other beds are mollusk limestones softer in the lower part and harder in the upper part (nodular mollusk biomicrites and biosparites or wackestones and packstones).
Boundaries - The lower boundary is locally disconformable, and the upper boundary is disconformable over a broad area. This upper disconformity represents the time during which many formations of the Woodbine Group were being deposited to the north of Austin. The only representative of this group in the Austin area is the Pepper Shale Member of the Eagle Ford Formation, which immediately overlies the broad unconformity.
Environments of deposition - The Buda Limestone in the Austin area represents shallow subtidal and intertidal deposits. The basal limestone represents a shell or shoal of beach that transgressed across the Del Rio Formation and scoured it at the top. The rest of the Buda represents shallow subtidal storm deposits; the different beds seem to have been reworked by storms many times, and most of the fossils have been broken. However, burrowing animals can still be collected.
Localities - The Buda Limestone can best be visited at a variety of outcrops along Shoal Creek (Fig. 15), especially along the Hike and Bike Trail, along Barton Springs
Road at South Lamar (Fig. 16), along Bear Creek just below the old Manchaca-Buda road crossing, and at the Manchaca Road crossing of Williamson Creek (Fig. 17).
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Figure 15
Shoal Creek at Town Lake
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Figure 16
Bouldin Creek at S. Lamar Blvd.
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Figure 17
Williamson Creek
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Figure 18
Watters Park
Boundaries - Both boundaries of the Pepper Shale, the only part of the Woodbine Group in the Austin area. Strata that occur farther north are missing at these boundaries. The lower part of the Woodbine is missing at the Buda-Pepper boundary, and lower part of the Eagle Ford is missing at the Pepper-Cloice boundary.
Environments of deposition - The environment of deposition of the Pepper Shale is very peculiar. There is no silt, and all of the mollusks are extremely thin-shelled (paper thin) and appear to be mud burrowers. There are no foraminiferans except a few agglutinates. This has led to the interpretation that the Pepper Shale represents deposition in a lagoon near a carbonate terrain with brackish water and no terrigenous source of sediment.
Localities - The only locality at which the Pepper Shale can be readily observed in Austin is in the drainage ditch at the east side of the Missouri Pacific Railroad track on Barton Springs Road (Fig. 19).
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Figure 19
Bouldin Creek - Missouri Pacific drainage canal
Boundaries - The lower boundary is disconformable with Pepper Shale, whereas the upper boundary is gradational with the overlying Bouldin Flags.
Environments of deposition - The Cloice Shale contains a foraminiferal fauna that is restricted, but seems to represent normal salinity; the member may represent lagoonal deposition with more of a terrigenous source than the underlying Pepper Shale.
Localities - Because it weathers easily the Cloice Shale does not occur in good outcrops. The best localities to see it are in the drainage ditch just east of the Missouri Pacific railroad track along Barton Springs Road (Fig. 19) and in the north bank of Bear Creek below the crossing of the old Manchaca-Buda Road.
Boundaries - The lower boundary of the Bouldin Flags Member is gradational to the Cloice Shale and the upper boundary is gradational to the South Bosque Member.
Environments of deposition - The flaggy limestone beds of the Bouldin Member contains bivalves (Inoceramus sp.) in conjunction with large sections of tree trunks (10 to 20 cms in diameter and up to 2 meters long), associated with shales containing a restricted foraminiferal fauna that seems to be of normal salinity. Silver (1963) has interpreted a similar deposit, represented by the Bluebonnet Flaggy Member farther north, as lagoonal.
Localities - The best places to visit the Bouldin Flags in the Austin area are in the drainage canal on the east side of the Missouri Pacific railroad track on Barton
Springs Road (Fig. 19), along Bear Creek below the crossing of the old Manchaca-Buda road, and on West Bouldin Creek just below Milton Street (Fig. 20).
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Figure 20
Bouldin Creek at Milton Street
Boundaries - The lower boundary of the South Bosque is gradational with the underlying Bouldin Flags, but the upper boundary is disconformable with the overlying Austin Formation. At some places in Austin the top of the South Bosque is a "condensed zone". This is a zone of internal molds of many fossils that seems to represent 150 to 200 feet of section in Tarrant County that is characterized by discrete zones of fossils (Adkins and Lozo, 1951).
Environments of deposition - The South Bosque Member, with is wide variety of mollusks and foraminiferans, seems to represent marine, open shelf deposition.
Localities - The South Bosque is a soft rock unit, and because it weathers easily, outcrops are few. It can be observed best along Bouldin Creek just downstream from and around Milton Street in South Austin. The top of the member can be seen in the bank of Shoal Creek at Northwest Park in North Austin, and along Walnut Creek in the vicinity of Watters Park (Fig. 18).
("Pyroclastic Member") and beach rock (McKown Member). Some of these have been intruded by igneous rock.
Type locality - The type locality of the Atco Formation is at the west quarry of the Universal Atlas Cement Company, Atco, McLennan County, Texas, about 8 miles southwest of the City Square of Waco (Fig. 21). Only the lower part of the formation is exposed at the type locality, but there is no better type locality, for no other locality exposes as well such a great thickness of the formation as does this one.
Type description - At the type locality the Atco Formation consists of the following sequence of beds (from K. 0. Seewald, unpublished) (Fig. 21).
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Figure 21
Atlas Cement Company Quarry, South Bosque, McLennan County
| Bed No. | Description | Thickness in meters | in feet |
| 12 | marl, chalky, weathered to a yellowish color |
0.6 | 1.8 |
| 11 | chalk, very hard, white, massive, a projecting ledge | 0.5 | 1.5 |
| 10 | chalk, marly, bluish-white, moderately resistant | 1.8 | 5.4 |
| 9 | marl, chalky, darker gray than above with thin shale stringer at top | 0.6 | 1.7 |
| 8 | alternating beds of chalk and chalky marl, light gray, moderately resistant | 2.7 | 8.3 |
| 7 | chalk, light gray, with thin shale stringer at top | 1.3 | 3.8 |
| 6 | bentonite | 0.5 | 1.5 |
| 5 | chalk, marly, moderately resistant | 0.8 | 2.3 |
| 4 | chalk, moderately resistant with thin shale stringer at top | 1.0 | 3.8 |
| 3 | chalk, marly, with thin shale stringer at top | 0.9 | 2.8 |
| 2 | chalk, marly, with thin shale stringer at top | 1.0 | 3.1 |
| 1 | chalk, moderately resistant with three thin shale stringers | 3.2 | 9.5 |
| TOTAL | 14.9 | 45.0 |
Boundaries - The base of the Atco Member is unconformable as described under Eagle Ford Formation; the top is gradational with the Vinson.
Environments of deposition - The Atco Member represents deposition on an open, shallow shelf, far from the shoreline. The shallowness of the water is testified by numerous oysters, benthonic foraminferans, and inocerami.
Localities - In the Austin area the Atco Member can best be observed along Shoal Creek above Northwest Park, along west Bouldin Creek above Milton Street, along Walnut Creek above the North Lamar Bridge, on west Bouldin Creek at Oltorf Street, and along the Missouri Pacific railroad cut just north of Oltorf Street.
Type locality - The type locality of the Vinson is on Vinson Creek, about 0.8 kilometer(one-half mile) from Onion Creek on the Bluff Springs Road toward Interstate 35 from Bluff Springs, southeast of Austin, Travis County, Texas. The formation was named by Durham (manuscript) for Vinson Creek.
Type Section - Only the upper 15.5 meters (46.5 feet) of the approximately 27 meters (80 feet) of the Vinson Member are exposed at the type locality (Fig. 22). The lower part of the Vinson is exposed just west of Interstate 35 along Harper Creek, just south of the Colorado River, Austin, Texas. A complete section of the Vinson Member is not known. The type section (Fig. 22) follows:
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Figure 22
Vinson Creek
| Bed No. | Description | Thickness in meters | in feet |
| 15 | limestone, hard (biosparite) | ||
| 14 | limestone (micrite), chalky with shaly layers | 3.1 | 9.5 |
| 13 | limestone (micrite), chalky with shaly layers | 2.9 | 8.0 |
| 12 | limestone (micrite), shaly and chalky | 0.2 | 0.5 |
| 11 | limestone (micrite), chalky | 0.2 | 0.5 |
| 10 | limestone (micrite), shaly and chalky | 0.4 | 1.0 |
| 9 | limestone (micrite), chalky | 1.3 | 4.0 |
| 8 | limestone (micrite), shaly and chalky | 0.3 | 0.8 |
| 7 | limestone (micrite), shaly and chalky | 1.4 | 4.2 |
| 6 | limestone (micrite), shaly and chalky | 0.5 | 1.5 |
| 5 | limestone (micrite), hard and chalky | 1.6 | 4.5 |
| 4 | limestone (micrite), shaly | 0.8 | 2.5 |
| 3 | limestone (biomicrite), hard and chalky with Ehynchostreon ? sp. aff. subotbiculata (Damarck) and Pychodonte- sp.). | 1.3 | 3.9 |
| 2 | limestone (biomicrite), hard and chalky | 0.2 | 0.5 |
| 1 | limestone (micrite), hard and chalky base at water level in Vinson Creek | 1.3 | 3.7 |
| TOTAL | 15.5 | 46.5 | |
Localities - The lithology of the Vinson Member varies from limestone to chalky limestone to chalk, and a few thin beds may contain a small amount of clay and thus be shaly. Sometimes the section is almost entirely soft chalk; in other areas the section may be harder chalk with a conchoidal fracture. Storm beds with ripped up limestone fragments and shingled Inoceramus fragments are abundant. The Vinson differs from he Atco in having thinner shale beds and greater amounts of chalk.
Boundaries - The Vinson Member is conformable with the Atco M ember below and the Jonah Member above.
Environments of deposition - The Vinson Member represents environments on a broad, shallow, open, marine shelf well removed from the shoreline. The shallow water deposition, probably less than 30 meters, is attested to by many oysters and inocerami.
Localities - Waller Creek, in Austin, Texas, flows over Vinson from about 47th Street all the way to the Colorado River. The hard chalk with a conchoidal fracture can be observed on the grounds of the Hancock Golf Course. Storm beds of isolated or shingled fragments of Inoceramus shells can be observed at many places along Waller Creek. The Rhynchostreon ? sp. aff. suborbiculata (Damarck) beds and Pychondonte sp. occur below 38th Street in Waller Creek and again about 7th Street. The chalky facies with a minute undescribed species of Lopha are best observed at the south bank of the valley of Onion Creek about 200 yards east of Interstate Highway 35. Here also can be seen remarkable gullying controlled by jointing.
Type locality - The type locality of the Jonah Member is on the left (Jonah) bank of the San Gabriel River at the crossing of the old road from Jonah to Hutto, Williamson County (Fig. 23). The member was studied, but not named by Marks (1950) and named by C. 0. Durham (unpublished) (Murray, 1961), and further studied by Kenneth 0. Seewald (unpublished).
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Figure 23
San Gabriel River at Jonah
Type section - The type section described below is modified from Marks (1950, pp. 121-122 pl. 26):
| Bed No. | Description | Thickness in meters | in feet |
| 10 | limestone (biosparite), gray, massive, undulating bedding, with Actinostreon travisana (Stephenson) | 1.0 | 3.2 |
| 9 | limestone (micrite), fissile, clayey, with Actinostreon travisana (Stephenson), and Pychodonte aucella (Romer) | 0.1 | 0.4 |
| 8 | limestone (biosparite), gray, undulating bedding, massive, with Actinostreon travisana (Stephenson) | 0.7 | 2.0 |
| 7 | limestone, shaly, with Hemiaster texanus (Romer) and Pychodonte aucella (Romer) | 0.2 | 0.6 |
| 6 | limestone, gray, with undulating bedding | 0.3 | 0.8 |
| 5 | limestone, shaly, with Hemiaster texanus (Romer) and Pychodonte aucella (Romer) | 0.3 | 1.0 |
| 4 | limestone, chalky, alternating with thin shaly beds | 2.2 | 6.8 |
| 3 | limestone (micrite), shaly, gray, fissile, undulating beds | 0.8 | 2.2 |
| 2 | limestone (micrite), hard, gray, (sparite cement scattered) | 0.7 | 2.0 |
| 1 | limestone (micrite) alternating with shaly limestone | 0.5 | 1.4 |
| TOTAL | 6.8 | 20.4 |
Base of section, but not base of formation.
Localities - The Jonah Member in the type area is recognizable because of the thick biosparites at the base and the top. It is less chalky than the underlying Vinson and the overlying Dessau Members. It can be mapped by its more rubbly outcrop and the larger clasts within its rock fragments. In other words, the Vinson and Dessau are largely mudstones, whereas the Jonah is large wackestones, packstones and grainstones; allochems are usually fossil fragments.
Boundaries - The lower boundary of the Jonah is conformable with the Vinson Member. The upper boundary of the Jonah is a bored and glauconitic corrosion zone that is recognizable over 40 or 50 miles of the outcrop. It marks the base of the upper chalk of older literature.
Environments of deposition - Both the Vinson and Dessau members are relatively free of burrowing mollusks, and the large specimens of Inoceramus, a bivalve, may indicate that the substrate was so soupy that burrowing mollusks could not obtain traction for the foot. This is not true of the Jonah Member. Burrowing mollusks, such as Idonearca Sp.,, abound, and the presence of Actinostreon, H m aster, and Spondylus guadalupae (Romer) attest to a shallow, open, marine shelf well removed from shore.
Localities - In the Austin area the Jonah is around 8.3 meters (25 feet) thick, thickening even more towards the type locality. To the south this coarser member is less prominent and more difficult to map. The Jonah can be seen in the Austin area just above the fault on Onion Creek upstream from Bob's Store in the Bluff Springs area, just south of Little Texas (Goodnight Lane) on South Congress, just below the bridge on the Hutto Road at Pflugerville, Travis County, and above the Vinson on Vinson Creek (Fig. 22).
Boundaries - The lower boundary of the Dessau Member is locally disconformable with the Burditt Member. This local disconformity at the top of the Dessau can be associated with uplift in the vicinity of an ancient volcano in the Austin area now called Pilot Knob. The Dessau Member thins toward this ancient volcano, which raised the sea floor, resulting in less deposition and finally erosion in its vicinity.
Environments of deposition - The Dessau Member was deposited on a broad, open marine shelf far removed from the shore, as attested by the many oysters and inocerami.
Localities - The Dessau Formation is best exposed on Brushy Creek just below the old iron bridge on the Hutto- Manda Road. Nearer Austin it can be observed at the old Sprinkle bridge over Little Walnut Creek (Fig. 24), below Dessau Road on Walnut Creek and its tributaries (Fig. 25), and on Rinard Creek at the old Turnersville Road crossing just east of Bluff Springs (Fig. 26), where it is much thinner than at the other mentioned localities.
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Figure 24
Little Walnut Creek and Old Sprinkle Bridge
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Figure 25
Dessau Road off Walnut Creek
Boundaries - The lower boundary of the Burditt Member is locally disconformable; the upper boundary is gradational to the Pflugerville Member.
Environments of deposition - The environment of deposition is an open, shallow (as attested to by the many oysters) marine shelf well removed from the shoreline.
Localities - The Burditt Member weathers to a slope and is not easier observed. The best locality in the Austin area is just upstream from the Elgin Highway (Highway 291) on Little Walnut Creek. The formation can be observed from there all the way up Little Walnut Creek to a locality where it is faulted out just beyond the old Cameron Road crossing, now known only from the old bridge supports. This is its type locality (Adkins, 1933).
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Figure 26
Rinard Creek at Old Turnersville Road Crossing
Boundaries - The boundaries of the pyroclastic formation are conformable with the overlying beds except along Onion Creek in the vicinity of Pilot Knob, where mud flows have been truncated by erosion before the deposition of younger Pflugerville and McKown members. Off Stasney Lane on Williamson Creek the pyroclastic formation truncates Vinson Member where the Vinson has been blown out of the crater prior to the deposition of the ejecta.
Environments of deposition - The pyroclastic rocks were deposited by ash falls from explosions of volcanoes; the ash fell into marine water and settled to the bottom to become clay. Later some of these clay beds were reworked by currents and waves to produce beach rock composed of fragments of claystone. Earthquakes accompanying the volcanism caused muds to flow down the sides of the volcano, producing mud flows that not only involve the pyroclastic rocks, but also involve the upper part of the Dessau Member (Fig. 27).
Localities - Explosion craters are known at Pilot Knob, under the bed of the Colorado River near Huston-Tillotson College, under St. Edwards University, at the intersection of West Bouldin Creek and Ben White Boulevard, along Williamson Creek just off of Stasney Lane, and in the Castlegate subdivision of South Austin - 6 separate explosion craters in all are known, each producing a certain amount of pyroclastic rock, but none as much as Pilot Knob. The best localities to observe pyroclastic rock are at the Lower McKinney Falls in McKinney Falls State Park (Fig. 27), in the valley between Elroy Road and Pilot Knob, downstream from the old Turnersville Crossing of Rinard Creek (Fig. 26), along Onion Creek below Bluff Springs, and along the rim rock north of Pilot Knob on Elroy Road south of Highway 183 (Fig. 28).
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Figure 27
Lower McKinney Falls, McKinney Falls State Park
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Figure 28
Rim Rock Section (2500 feet west of Pilot Knob School)
Localities - Although commonly called "basalt", the igneous rock in the Austin area is not true basalt, but consists of two rock types, nepheline basanite and olivine nephelinite.
Boundaries - There are two types of boundaries, usually not observable. Some of,the igneous rock is intrusive, cutting the rocks, and some of it has flowed out over the ground surface to be covered by later marine deposits (McKown or Sprinkle).
Localities - Igneous rock in place is not easy to see. The south end of Pilot Knob is the best exposure of igneous rock in place, and at South Pilot Knob on Bluff Springs Road igneous rock can be seen that has not moved more than two or three feet since it solidified.
Type locality - The Pflugerville Member is named for the village of Pflugerville north of Austin, Travis County, where the formation underlies the Immanuel Lutheran Church. The type locality is selected at the curve in Cameron Road on levels (presumably bedding planes) at which Exogyra erraticostata and Actinostreon travisana (Stephenson) are concentrated. These may have represented submarine hard grounds. On Little Walnut Creek, just upstream from the Manor Highway (Highway 291), there is a shell hash about 5 centimeters thick near the middle of the member (Fig. 29). In some parts of the Pilot Knob area the member thins to as little as 3 meters (10 feet), and in other areas it is entirely replaced by beach rock (McKown Member). On Jane's Farm at Onion Creek the Pflugerville Member discordantly overlies pyroclastic and Dessau mud flows (Fig. 30).
Boundaries - The lower boundary of the Pflugerville Member is gradational with the Burditt Member below, and the upper boundary is gradational with the overlying Sprinkle Claystone. The upper boundary may have a concentration of internal molds of Idonearca sp. in living position (Fig.31).
Environments of deposition - Beds of the Pflugerville Member represent a broad, shallow, open marine shelf. The numerous oysters testify to the shallow deposition, which probably took place in less than 30 meters of water. This member contains one of the finest and most beautiful foraminiferal faunas in the Cretaceous of Texas.
Localities - The Pflugerville Member weathers rapidly. It can be easily observed at the type locality along the curve on Cameron Road on the south side of Walnut Creek (Fig. 32). Until developers destroy the outcrop the Pflugerville can also be easily seen on the slopes of Little Walnut creek above the stained waterfall about 200 meters upstream from Manor Highway bridge (Highway 291) (Fig. 29), just north of Reagan High School. The base is exposed at the old Turnersville Road crossing of Rinard Creek (Fig. 26).
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Figure 29
Little Walnut Creek just north of Manor Highway
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Figure 30
Section at Jane's Farm, Mouth of Marble Creek
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Figure 31
Springdale Road at Little Walnut Creek
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Figure 32
South Bank of Walnut Creek at Old Sprinkle Road
Type locality - The McKown Member type locality is the McKown Quarry on the north side of Onion Creek approximately 1.6kilometers (I mile) above the Lockhart Highway (Highway 183)(Fig. 33).
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Figure 33
KcKown Quarry on Onion Creek
Type section - The type locality exposes a complete section of the McKown Member, a total of about 18 meters (55.5 feet).
| Bed No. | Description | Thickness in meters | in feet |
| 25 | gravel, golf ball size and larger chert and limestone pebbles | 1.6 | 4.7 |
| 24 | caliche (weathered profile on montmorillonitic clay) | 1.5 | 4.5 |
| 23 | clay, dark gray, montmorillonitic | 2.5 | 7.6 |
| 22 | limestone, shell beach, (biosparite or shell frag- ment grainstone), nodular bedding, corrosion zone at top | 2.5 | 7.6 |
| 21 | limestone, same as bed 22, but evenly bedded | 1.2 | 3.8 |
| 20 | limestone, same. as bed 22 | 1.2 | 3.9 |
| 19 | limestone, same as bed 22, but with Exogyra errati- costata (Stephenson) through- out and corrosion zone at top | 2.1 | 6.3 |
| 18 | limestone, same as bed 22, but rubbly and burrowed at top with corrosion zone | 0.5 | 1.6 |
| 17 | conglomerate of reworked and water-worn fragments of the pyroclastic member | 0.1 | 0.4 |
| 16 | limestone, same as bed 22, but evenly bedded | 1.4 | 4.4 |
| 15 | limestone, same as bed 22, but evenly bedded | 1.4 | 4.2 |
| 14 | limestone, same as bed 22 | 0.5 | 1.6 |
| 13 | limestone, (biosparite) with large (4 to 10 centi- meters) horizontal, (1 to 2 centimeters thick) fine grained limestone (micrite) clasts and with pronounced stylolite at top | 0.4 | 1.2 |
| 12 | limestone, same as bed 22 | 0.7 | 2.1 |
| 11 | limestone, same as bed 13 | 0.4 | 1.4 |
| 10 | limestone, same as bed 22 | 0.4 | 1.2 |
| 9 | limestone, same as bed 13 | 0.4 | 1.2 |
| 8 | limestone, same as bed 22 | 0.4 | 1.2 |
| 7 | limestone, same as bed 22, and with pronounced styolite at top | 1.1 | 3.2 |
| 6 | limestone, same as bed 13 | 0.4 | 1.2 |
| 5 | limestone, same as bed 22 | 0.5 | 1.6 |
| 4 | limestone, same as bed 13 | 0.3 | 1.0 |
| 3 | limestone, same as bed 22 | 0.5 | 1.6 |
| 2 | limestone, same as bed 13 | 0.4 | 1.2 |
| 1 | limestone, same as bed 22 | 1.2 | 3.6 |
| 0 | green, reworked ,pebbles of clay | ||
| TOTAL THICKNESS OF McKOWN: | 18.0 | 55.7 | |
Localities - The lithology of the McKown Member is primarily a calcarenitic limestone (oyster shell biosparite or oyster shell grainstone). It is clean with little micrite and no clay. The micrite clasts in bed 13 and similar beds are about the only exception. Fragments of reworked Pyroclastic Member are not uncommon at some levels.
Boundaries - At some localities the lower boundary is gradational with the Dessau Member. At other localities the lower boundary is gradational with the pyroclastic member. But along Onion Creek at several localities, especially on Jane's Farm and McKinney Falls State Park the McKown Formation rests discordantly on the mudflows of the Dessau and "Pyroclastic" Member. Just above the Highway 183 bridge across Onion Creek the McKown rests with sharp disconformity on the Dessau.
Environments of deposition - The McKown Member represents the beach facies of the old volcano of Pilot Knob. At some localities it grades laterally into "Pyroclastic" Member beach facies. The beach environment was most greatly developed on the north and east side of the old volcano; beach facies of other old igneous plugs in the area, such as those at Thrall and Kimbro, are best developed on sides ranging from northwest to southeast. Just upstream from the Highway 183 bridge on Onion Creek the McKown has a number of beds that contain algal fragments intercalated among strata of the beach rock (White, 1960).
Localities - The best localities to visit the McKown Formation are along Onion Creek on the grounds of McKinney Falls State Park. The two falls (Upper and Lower McKinney Falls) are held up by the McKown Formation, and just above the Lower Falls an old beach berm has been preserved and re-exhumed. The many quarries on Onion Creek below McKinney State Park, which have been utilized by the Texas State Highway Department for many years, are in the McKown Formation (Fig. 33). This limestone is easily crushable, but then case hardens to produce a good aggregate. The J. K. Ross, old McKinney, and other early ranch buildings along this part of Onion Creek are and were constructed from stone of the McKown Member. Some of the stone was hand chiseled, but stone in other buildings has been sawed. McKown is also exposed on the Jane's Farm (Fig. 30) and in the Rim Rock section on Elroy Road (Fig. 28).
Localities - The Sprinkle Formation is a calcareous claystone. The primary clay mineral is montmorillonite with sodium dominant over calcium. The calcium carbonate ranges from around 10 percent to over 40 percent. The Sprinkle is about 100 meters (300 plus feet) thick, but varies, being thinner on the tectonic highs coincident with and adjacent to igneous plugs.
Boundaries - The lower boundary of the Sprinkle is usually conformable with the Pflugerville Member of the Austin. At Little Walnut Creek below Springdale Road (Fig. 31) the boundary is marked by a sudden decrease in calcium content and a number of internal molds of Idonearca sp. (a burrowing clam) in living position. In the Pilot Knob vicinity the Sprinkle may be separated from members of the Austin Formation by a local disconformity associated with tectonic activity of the old volcano.
Environments of deposition - The Sprinkle Claystone represents deposition far offshore in a shallow marine environment. The fossil oysters and other pelecypods certainly indicate deposition in depths of less than 60 meters, and probably less than 30 meters.. The source of the clay is not local and is most likely due to the slow outfall of volcanic dust produced by numerous volcanoes in the ancient mountains of the western United States at that time.
Localities - The best localities to see Sprinkle Claystone are fresh road cuts. The formation is so soft and weathers so rapidly that there are few old classic localities. Therefore, it is necessary to watch for fresh exposures. The base of the Sprinkle is usually observable on Little Walnut Creek about 35 meters (100 feet) downstream from the Springdale Road (Fig. 31).
Localities - The Pecan Gap ranges from a marl to a chalk. In the Austin area it would mostly be called a marl, with calcium carbonate content ranging from around 25 to over 75 percent. At Cele in northeastern Travis County, and at Normann's Crossing of Brushy Creek, it is a true chalk. The chalky areas are usually associated with topographic highs on the Cretaceous sea floor, as that associated with the Kimbro structure in eastern Travis and adjacent Williamson Counties.
Boundaries - The lower boundary of the Pecan Gap Chalk is sharp on the Sprinkle, but is probably not disconformable. The upper boundary is slowly gradational into the Bergstrom Claystone and cannot usually be distinguished within plus or minus 10 feet.
Environments of deposition - The environment of deposition for the Pecan Gap Formation is offshore open shallow sea. The many species and specimens of the clam Inoceramus and the numerous oysters indicate depths of not more than @O meters and probably less than 30 meters. Carbonate deposition usually exceeded clay deposition.
Localities - Good outcrops of Pecan Gap are rare because the formation weathers so easily. Fresh cuts along the west side of Walnut Hill (Fig. 34) are usually good collecting sites. Outcrops of Pecan Gap Chalk can be seen at Cele in northeastern Travis County, and just upstream from Normann's Crossing of Brushy Creek.
(BIGGER)
Figure 34
Walnut Hill
Localities - The Bergstrom Formation is a montmorillonitic claystone. Calcareous content ranges up to 25 percent, but the latter figure is only at the base near the Pecan Gap Chalk. Higher in the formation the calcium carbonate percent is frequently less than six and may be as low as 3 or 4 percent. In the area around New Sweden, Manda, and Kimbro, the upper part of the Bergstrom is characterized by large irregularly shaped, sometimes septarian, sometimes calcite filled concretions. These may contain fossils. These concretion zones are apparently associated with a slightly higher sea floor. Total thickness of the Bergstrom ranges from 100 to 125 meters roughly 300 to 375 feet).
Boundaries - The boundaries of the Bergstrom Formation are seldom seen. The lower boundary is gradational with the Pecan Gap Chalk. The upper boundary is said to be disconformable, but this disconformity is not apparent (Fig. 35) (Young, 1965). The disconformity was once exposed at Noack (Fig. 35), but has since been covered by slump.
Environments of deposition - The Bergstrom Claystone represents deposition in an open, offshore, shallow seaway. The many large specimens of Exogyra ponderosa and occasional large rudistids testify to deposition in the neritic zone. The source of the montmorillonite is thought to be slow outfall of dust from numerous volcanoes then extant in the western United States.
Localities - Like the other clay formations, the Bergstrom weathers rapidly, and is best exposed in fresh cuts on newly constructed roads. There is a good outcrop in the west bank of Onion Creek about 100 meters upstream from the old Moore and Berry's Crossing; this was one of Helen Jeanne Plummer's favorite collecting sites for Foraminifera. Bergstrom Claystone can also be observed in the north bank of the Colorado River below the Hornsby Cemetery and at the ramp down to the Colorado River at Del Valle.
(BIGGER)
Figure 35
Bergstrom-Corsicana boundary once exposed 7/8ths mile west of Noack, Williamson County. Texas
From Young, 1965, figure 3
Localities - The Corsicana Formation is another montmorillonitic claystone with sodium dominant over calcium, and it is just as unstable as the Sprinkle and Bergstrom Formations. The Corsicana is calcareous at many levels, seemingly largely due to the presence of minute calcareous foraminiferans. The source of the clay is thought to be the outfall from the numerous late Cretaceous volcanoes in the western part of North America.
Boundaries - The boundaries of the Corsicana are seldom observed. The lower boundary can be observed at the top of the cliff just below the thin, platy siltstones below Jones' Crossing (Bastrop Highway). It is gradational with the Kemp Formation.
Environments of deposition - The environment of deposition of the Corsicana Formation should be the same as that for the Bergstrom Formation.
Localities The lower part of the Corsicana Formation can be observed in the south bank of Gilliland Creek just upstream from Texas Highway 973. The best exposure is at the large bluff just north (downstream on Onion Creek) of Jones' Crossing (Bastrop Highway bridge).
Like the other later Cretaceous formations the Kemp Formation is montmorillonitic claystone, but it contains at some levels the thin, flaggy, quartz siltstones that differentiate it from the Corsicana Formation. Since there are several feet of these siltstones at the base of the Kemp, there is a small basal scarp in eastern Travis County, and the formation can be mapped more easily than the Bergstrom.
Boundaries The lower boundary of the Kemp is gradational to the Corsican. The upper boundary is disconformable with the Paleocene Midway Formation. The latter boundary is usually burrowed; the burrows are filled with overlying Midway glauconites, and there are fragments of worn Cretaceous fossils for several feet into the overlying Midway Formation.
Environments of deposition - The Kemp Formation represents deposition in an open, offshore, shallow seaway, and the increasing amount of quartz silt indicates an additional source other than volcanic dust.
Localities - About the only place the Kemp Formation can be observed in the Austin area is where the lower part is exposed in the top of the bluff below Jones' Crossing (Bastrop Highway) on Onion Creek.
Localities - The higher terrace deposits are coarse-grained, gravelly, reddish deposits. The lower deposits may also be red, but they are a finer-grained, usually sandstones and sandy siltstones.
Boundaries - Terrace deposits rest disconformably upon older deposits.
Environments of deposition - The terrace deposits are all alluvial, representing various fluvial regimes including channel, overbank, point bar, chute, and natural levee. The deposits represent former levels of the Colorado River and its tributaries. Terrace deposits of tributary streams contain rock particles similar to the country rock in their vicinity.
Localities - Terrace deposits can be observed at many localities. Higher gravels are best observed along Ed Bluestein Boulevard opposite Tracor. Lower deposits can be observed wherever a tributary channel cuts through the floodplain, as off Bolm Road, along Boggy Creek, and near the Colorado River at Waller and Shoal Creeks.
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Chapter 3
