The Geology of Dallas County

 The Austin was named by B. F. Shumard in 1860 for limestone beds exposed in and around the capitol city of the state. The formation has gone by several names, and in the early writings of Hill it was variously called the "Rotten limestone", the "Dallas limestone", or the "Austin-Dallas chalk". Once it was established that the beds at Austin and Dallas belonged to the same formation, which could be traced along the surface between the two cities, Shumard's designation was adopted and the other names fell into disuse. The Austin is approximately 600 feet thick in Dallas County. This is near its maximum thickness along the outcrop (Stephenson, 1937, p. 135, Fig. 7).  In Dallas County the Austin formation may be divided into four parts, as indicated below. Figures for thickness are approximate.

The basal member is four feet thick at the first locality, two feet thick at the second. At both places it consists of gray to buff-weathering gritty and pebbly marl, the visible constituents consisting of fish teeth, fish vertebrae, and dark internal molds of gastropods and pelecypods. These fossils are most abundant along the base where they form thin conglomeratic lentils in places. 
 

The Geology of Texas - Vol. 1 - pages 444-451

BLACK PRAIRIE REGION - TYPICAL AUSTIN CHALK

  From eastern Grayson County southwards, the Austin is in general typical. It forms a wide, prominent and important outcrop, the substratum of a part of the fertile black land belt (Black Prairie) devoted largely to cotton raising. On it are located many important towns: Sherman, McKinney, Dallas, Waxahachie, Midlothian, Waco, Temple, Austin and San Antonio. The total chalk in eastern Grayson County is probably as much as 1000 feet thick; near Dallas 700 feet; at Corsicana 425 feet; 480 feet in the Powell field and 440 feet in the Mexia-Richland area (969, p. 331); near Groesbeck, southern Limestone County, 350 feet; in Bell County 550-604 feet; in Milam County about 500 feet; in Williamson County, 325-342 feet; in Travis County, about 420 feet (275-400 in wells) ; in Medina County, about 350 feet; in eastern Uvalde County, 350 feet; western Uvalde County, about 350 feet [F. M. Getzendaner, personal communication].

 Although the literature contains records of 350 to 400 feet of Austin in Bexar County, many geologists now consider that these thicknesses include some rocks of Taylor age. The Austin chalk is stated to be 110 feet thick in eastern Bexar County [L. W. MacNaughton, personal communication], and the 443 feet of Austin recorded (888) in southwestern Bexar County is stated to include the Anacacho. Likewise it is unknown how much, if any, of the upper part of the type Austin chalk in Travis County should be separated from the true Austin. These questions require extensive zonal work and a paleontological redefinition of the type sections.

 At the type locality the lower two-thirds of Austin consists of irregular strata of variable thickness, from thin-bedded to massive, and with often indefinite limits, generally alternately harder and softer. They are composed of a gray-white chalky limestone in the harder layers, and a dark blue or blackish marly limestone or limy marl weathering dead white or light gray, and in texture unevenly flaky or laminated. A few of the limestones are indurated, some are shelly. Some contain much debris of oysters, inocerami and other shells. At certain levels considerable glauconite, dispersed as small specks, occurs; the formation contains imbedded balls, cylinders and irregular botryoidal masses of pyrite with radiating internal structure; and locally veins, seams and joint cracks filled with calcite. In youthful stream cuts vertical cliffs of alternately projecting and receding strata occur; on hillsides and upland prairies a rounded topography prevails. On many patches of upland, headwater erosion is sufficiently rapid to strip the outcrop of soil. 

Generally harder and softer ledges are not topographically well expressed. The upper part of the formation has considerable calcareous marl, in beds up to 30 feet thick, and some very shelly marl (with Exogyra), generally in beds 5 feet or less thick. Such a marl in northern Travis County seems to be characterized by the presence of Exogyra tigrina Stephenson and "Ostrea" centerensis Stephenson, and contains other species listed below. A notable feature of this level is the presence of a wide range and variety of Exogyra, probably referable to several species.

 The Austin consists of beds of impure chalky limestone, containing 85 per Cent or more of calcium carbonate, interstratified with beds of softer marl. It is usually of an earthy texture, free from grit, and on fresh exposure softer, so that it can be cut with a hand saw, but on exposure more indurated. In thin slices the material shows calcite crystals, particles of amorphous calcite, finely crystalline calcareous material, foraminiferan shells and fragments, fragments of the prismatic layer of Inoceramus often in great abundance, debris of pelecypods, gastropods, echinoids, and other organic fragments. The material has the typical crystalline structure of limestone. Some slices show abundant glauconite specks; some show a sparse to medium amount of "spherical bodies" (see page 365) ; and some show a finely crystalline texture almost devoid of organic material. Typical analyses show calcium carbonate 82 per cent; silica and insoluble silicates 11 per cent; ferric oxide and alumina 3 per cent; magnesia 1 per cent.

 The water-filled subterranean chalky limestone is usually of a blackish-blue to bluish-gray color, as in most cores. The air-dried material is generally glaring white and of a matte texture. The dried marls may be more blackish or bluish. They weather mostly into abrupt slopes capped by harder ledges. Some ledges become indurated and crystalline; others, less crystalline, weather into irregular small conchoidal flakes with an earthy fracture. The harder strata have an irregular, ragged conchoidal fracture. Locally in the more massive layers, there occurs a large conchoidal flaking, superficially resembling exfoliation. On prolonged disintegration, the Austin weathers into a black residual soil, characteristic of the Black Lands belt of east-central Texas. Locally as near Pilot Knob, the Austin is metamorphosed, and occurs as a porous, redeposited and recrystallized limestone in medium beds, soft enough on fresh exposure to be sawed, nearly pure, and producing an excellent building stone. The German Lutheran Church just north of the Capitol at Austin is built of this stone. Formerly the ordinary Austin was somewhat used as a building stone, but its softness, marly partings and iron stain make it less desirable than other stones available in central Texas.

 The outcrop covers the southeastern one-fourth or more of Grayson County. Marly lime is more prominent in the lower half of the formation, and medium bedded to thick massive chalky limestone in the upper half. In Dallas County the base of the chalk capping soft upper Eagle Ford shales, clays and flags, forms the prominent west-facing White Rock escarpment which proceeds southwards to the Brazos. It is well exposed in road cuts and cement plants on the Fort Worth road about 5 miles west of Dallas.

 In the quarry of the Texas Portland Cement Company 3 miles west of Dallas, the base of the Austin chalk, just above the Eagle Ford contact, is marked by a layer of phosphatic pebbles (1530, p. 148, and PL XXVII-A). The basal part, 150 feet of the formation, consists of heavy-bedded, massive chalk layers separated by thin shaly layers, the most resistant beds being contained in the basalmost 50 feet (1454, p. 19). The basal part contains an abundance of nodular, spherical or cylindrical masses of pyrite.

 The middle part, about 250 feet thick, has fewer massive layers, and is characterized by thick, and often indurated shaly layers which show a fine lamination. This part does not show in stream cuts as marked expression of projecting and receding layers as does the basal part. The uppermost part contains more shaly limestone and less chalk. The colors are predominantly blue and yellow. Some sandy strata occur. At the Austin-Taylor boundary, there is a sharp transition from massive flaggy chalk (containing "large ammonites,' most of them Parapuzosia) to gray shale, some of it calcareous.

 In the Waco region the basal chalk is well exposed in Cameron Park. Here it consists of medium to thick massively bedded strata with some alternating receding ledges. By undercutting of Bosque River large blocks fall down the slopes and disintegrate. Flaking and exfoliation are extensive. In the cuts of Brazos River across the Austin chalk outcrop, considerable small scale faulting, with the development of V- and A-shaped grabens and horsts, is present. Such local faulting occurs in White Rock Creek near the Harrington well.

 The base of the chalk, and locally the hard Eagle Ford flags, form the crest of the west-facing Bosque escarpment across McLennan  County. The Bosque is deflected parallel to this scarp and follows it northward to the point where the Brazos cuts through the scarp. Along the scarp, there is considerable faulting in disconnected lines. The medial part of the chalk consists of medium bedded chalky limestone softer marly layers, weathering white. The Austin-Taylor contact occurs in and near the Baylor University Campus. Here the topmost chalk stratum is a massive chalky limestone containing Inoceramus undulato-plicatus and other species, and is followed with a sharp lithological break by blue-black Taylor shale. Stephenson considers that about the upper 250 feet of chalk is here absent by subsequent erosion. The top of the chalk occurs in several creeks just east of the Waco-Austin highway. In these creeks Dr. Pace has demonstrated a persistent line of faulting, possibly the eastern limit of the Balcones fault zone in this area.

 Southward from Waco, the typical hard Austin chalk is overlain by a chalk marl of variable thickness, generally referred to the Taylor because of its foraminifera. In or near the base of this marl at many localities, large ammonites of the genera Parapachydiscus and Parapuzosia occur, and their zone of abundance marks approximately the top of the Austin chalk. Unfortunately their exact zonation has not yet been published. They occur in several localities southwest of Robinson and Rosenthal, and east of Temple, west of Holland in the branches of Darr's Creek, north of Austin in Big and Little Walnut creeks, on the Rio Grande at Tequesquite Creek between Del Rio and Eagle Pass, and near San Carlos, Coahuila. Some of these "cart-wheel" species reach large size; two of them have been described from Tequesquite Creek by Scott and Moore. The eastern border of the chalk is exposed near Garland, east of a line between Waco and Eddy, in Deer Creek, in northern Bell county near Little Flock Church east of Temple, in various creeks west of Holland, in Brushy Creek south of Hutto, and in various creeks in Travis County.

 In San Antonio the excavations in Brackenridge Park represent the Austin chalk at levels between 100 and 150 feet above the base. The rock is evenly bedded in strata of from six inches to several feet in thickness. It is light gray, tinged with yellow. Oxidized pyrite nodules are present. Near the base of the quarry, a layer rich in Gryphaea shells occurs. In this county the upper 200 feet of the Austin is a soft bluish calcareous clay or mud. In these beds Stephenson found Scaphites sp., Placenticeras sp., and a large Baculites.

 The Austin in Medina County (992, pp. 46-48) consists of  alternations of soft chalky limestones and marls, argillaceous shales, or clays. Near the top the formation is mostly chalky limestone. The basal half of the Austin as far west as Hondo River contains ledges which are highly glauconitic. The upper part especially has vertical cylindrical concretions of pyrite with radiating structure. The basal 75 feet is quite highly indurated and lithologically somewhat resembles the Buda, but lacks the calcite veins characteristic of that formation. At a level about 275 above the base there is a layer of Gryphaea aucella shells, about 4 to 5 feet thick, which persists across Medina and Bexar counties, and is possibly the same as the prominent Gryphaea layer above the middle of the formation at Austin.

 In the Uvalde area the Austin consists of soft, white and yellow chalky limestone and limy marl alternating. Two large exposures are on the east side of Nueces River opposite Soldiers Camp Spring, where the upper 150 feet is exposed; and a basal 200 feet in the high bluff on the west side of the Nueces between the Southern Pacific Railroad and the West Nueces. Gryphaea aucella Roemer, Inoceramus aff. digitatus Sowerby, and Mortoniceras texanum (Roemer) are recorded from the Austin in this area.

BURDITT MARL

 Nomenclature.—Hill included this chalk marl with the Austin chalk, and stated that the top is transitional to the Taylor. Taff (1574, p. 353) segregated the upper marly lime zone of the Austin chalk, and considered it lithologically transitional to the Taylor marl. This chalk marl is here called Burditt, from Burditt School, Travis County, and the type locality is along Little Walnut Creek downstream from the Austin-Cameron road.