Geological Survey Bulletin 845 Guidebook of the Western United States: Part F. Southern Pacific Lines

SHEET No. 8 (click in image for an enlargement in a new window)

In the vicinity of Weimar, about 2 miles east of the Fayette County line, the railroad attains the summit of one of the higher ridges in Colorado County constituting the divide between the Colorado and Navidad Rivers. Southeastern Fayette County, however, has been eroded by the two forks of the Navidad to an area of low relief.

Through Weimar and Schulenburg and halfway to Engle siding the rolling surface of the Lagarto clay is traversed, but about Weimar there is an extensive flat or terrace. Good outcrops are rare, especially near the railroad, but there are shallow cuts in the western part of Weimar and there are exposures at intervals along the streams, especially in some of the bends of the Colorado River. A large part of the area is covered by soil and part is woodland. Remains of the 3-toed horse, Protohippus perditus, and other bones were found in the Lagarto formation at Dripping Springs, 1-1/2 miles north east of Borden siding. Shells derived from underlying Cretaceous formations and minute Foraminifera have been noted.

At Shatto siding the railroad crosses the east branch of the Navidad River, which heads in the low ridges north of Schulenburg but develops into a drainageway of considerable size in the region farther south.

Schulenburg is a rural center for a prosperous agricultural district. Dairying is a thriving industry, and some of its products are utilized at a large plant making evaporated milk, on the western edge of the town. Near by is a mill which produces a nonstarchy flour from cottonseed. Just west of Schulenburg are cuts in light-colored sandstone, and near by are ledges of this rock. These beds dip east and are in the lower part of the Lagarto clay. The general dip of the strata in this region is considerably less than 1°, which is nearly 90 feet to the mile.

Two miles west of Schulenburg the west branch of the Navidad River is crossed, and thence there is a long gentle upgrade that extends nearly to Flatonia. In this interval the Oakville and Catahoula sandstones appear, rising on a dip which is low in angle but steeper than the rise of the land. Both of these sandstones present low ridges and knobs, so that the country has a diversified topography, and as the soil is not very fertile much of the land remains wooded.

The contact between the Lagarto clay and the Oakville sandstone³⁵ is passed just beyond the west branch of the Navidad, whence the railroad follows the low divide between Rock Creek and Mulberry Creek.

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³⁵The Oakville sandstone is characteristically a massive light-gray or yellow rock, in part cross-bedded, in all about 300 feet thick and dipping to the southeast at the rate of about 40 feet to the mile. It is held together loosely by a calcareous cement, commonly crystalline, or, more rarely, it is firmly indurated. The sand grains are, as a rule, fairly coarse and subangular. Light-colored greenish or yellowish limy clays make up an appreciable part of the upper Oakville section, closely resembling in appearance some of the Lagarto clays. For this reason, the Oakville-Lagarto contact is less obvious than might be expected from the characteristic materials of the two formations. However, volcanic ash, both in its original form and altered to bentonite, is apparently restricted to the Oakville sandstone, and the dendrites (plant like forms due to a deposit of manganese oxide) that so commonly occur on the cleavage planes of the Lagarto clays have not been observed in the Oakville clays. Reworked Cretaceous fossils occur as pebbles in both formations. The Oakville contains few marine fossils in place but many remains of vertebrates have been found in it, including primitive horses of several kinds, rhinoceroses, crocodiles, and tortoises, and it is classed as late, middle, and early upper Miocene in age. It is underlain unconformably by the Catahoula sandstone, although the rate of dip of both formations is about the same.

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The landscape of the Oakville outcrop zone along the Southern Pacific Railroad is more subdued than it is from Lagrange northward or to the south. There are several excellent outcrops of this sandstone along the road leading north from Engle to Lagrange. Wild flowers, such as the bluebonnets (Texas State flower) and the mallows, which are the glory of the spring in Texas, are particularly luxuriant upon the sands derived from the Oakville ledges. The Catahoula-Oakville contact is about 3 miles east of Flatonia but is not visible in the lowland of the badly drained area at the headwaters of Mulberry Creek.

Two miles west of Engle siding a summit is attained, beyond which is a long, rolling slope to Flatonia, a thriving town in a community part German and part Bohemian, situated on the outcrop of the lower beds of the Catahoula sandstone.³⁶ Flatonia was named from J. Flato, who kept a store at the original site of the town, 2 miles south of the present one. It has an interesting position topographically, for it is on the notably flat divide between the Colorado River, which heads far west in the outcrop area of the Permian "Red Beds," and the Guadalupe River, which heads on the Edwards Plateau. The town site is just beyond the heads of the fingering tributaries of the intermediate coastal streams, such as the Navidad River. Obar Hill, less than a mile south of Flatonia and rising more than 100 feet above the village, is capped by heavy beds of white sandstone, probably an Oakville inlier. The well at Flatonia, 3,000 feet deep, is supposed to obtain its excellent water supply from the Carrizo sand.

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³⁶The Catahoula consists of soft bluish massive sandstone and interbedded clays, but the sand grains, unlike those of the Oakville, are dominantly rounded and poorly sorted and probably in part of dune origin. The formation is about 200 feet thick and lies conformably on the Fayette sandstone. Near Flatonia the sands are loosely cemented by a calcareous matrix, but elsewhere a blue opalescent quartzite matrix is not uncommon. The clays are greenish gray or yellow, are mostly sandy, and carry limy clay balls an inch or more in diameter similar to those now forming at the salt-water mouths of some large streams. The sands are relatively less abundant in Fayette County than they are to the north, toward the Sabine River, where on the outcrop area there are long stretches of pine woods. Opalized wood and clear impressions of tropical palms are locally abundant. According to E. W. Berry (U. S. Geological Survey Prof. Paper 98, p. 229, 1917), this flora contains no upland or inland types and may be regarded as a strictly coastal assemblage made up of groups comparable with those found along the strand in the present-day Tropics. To the south the formation contains deposits of volcanic materials ejected from vents located possibly in southern McMullen and Duval Counties. Volcanic activity during the Catahoula deposition was not, however, restricted to southwest Texas, for most of the larger fuller's-earth deposits of this region, notably that near Corrigan, in Polk County, are in the Catahoula. The Catahoula outcrop zone in Fayette County is largely overgrown with post oak, and the relief is relatively low compared with that of the Oakville sandstone. The clays weather to a black soil and are very heavy when wet.

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The Fayette-Catahoula contact is about 1 mile west of the crossing of the old San Antonio & Aransas Pass Railway (now Texas & New Orleans) in Flatonia, where the fine-bedded sands of the upper Fayette lie below the greenish-gray compact clays of the basal Catahoula.³⁷ The varied lithology of the Fayette sandstone is expressed in the diversity of the landscape and the vegetation. The soil is more highly colored, as a rule, than that of the Catahoula, and the contrast between the black clay roads and post oak of the lower Catahoula and the red sandy roads and junipers of the upper Fayette is very striking. The configuration of the Fayette outcrop is noticeably different from that of the outcrop of the underlying Yegua formation, though it is not so rugged as that of the Catahoula and Oakville sandstones.

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³⁷There are fine exposures of the formations in road and stream cuts both north of the railroad, toward Muldoon, and to the south, especially in the vicinity of Nickel, where 75 feet or more of kaolinitic shales and concretionary sands and sandstones at the top of the Fayette are overlain by the sandy clays filled with clay balls that characterize the basal Catahoula. The Fayette formation consists of light-gray thin-bedded sands, dark-gray and chocolate-brown clays and lignites, and, in its upper part, thin beds of volcanic ash. The thickness is about 65 feet. The basal Fayette in this general section commonly carries cylindrical concretions of iron oxide an inch or so in diameter oriented at a high angle to the bedding planes.

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A little more than a mile beyond Janice siding the railroad passes from the dominantly sandy Fayette strata to the Yegua beds, which are gray, green, and brown lignitic clays and sandy clays with thin sand deposits. They are highly gypsiferous and, for the most part, nonmarine. They are about 500 feet thick, and all the beds dip to the east at a low rate. The outcrop zone is a region of low hills with gentle slopes. Generally the soil is dark and loamy with scattered mesquite (Prosopis juliflora) and prickly pear (mostly Opuntia engelmanni); more rarely it is a fine sandy loam with a few post oaks. The mesquites growing on Yegua soil in this region are larger than those in the region to the south, probably because of a greater supply of moisture. However, this plant withstands dry weather by sending its taproot as deep as 50 feet to obtain moisture. The mesquite, which begins to be conspicuous in this general region, is a dominant plant in many parts of the region to the west across Texas, New Mexico, Arizona, and southern California. Its beans are an important source of food for grazing animals and they are used for flour by the Indians and Mexicans. The wood is a most useful fuel, and the plant yields a valuable gum; a decoction of the bark is esteemed by the Mexicans as a laxative.

The high percentage of gypsum in the Yegua strata makes the soil unfvorable for many crops and the water unsuitable for stock, so that the outcrop area of the formation is rather thinly settled. About 2-1/2 miles east of Waelder the railroad deflects to the north and follows the divide between Sandy and Copperas Creeks, crossing the rather ill-defined contact between Yegua and Cook Mountain strata about 1 mile east of Waelder.

Sandstone of the Cook Mountain formation makes a ridge of moderate prominence that extends far to the north and south of the railroad. The formation includes sands and glauconitic marls and clays,³⁸ some of them lignitic but for the most part of marine origin. The higher beds are locally fossiliferous, and many of the species were early correlated with those of the sands of the Claiborne of Alabama. The glauconitic beds of the Cook Mountain formation are highly colored by the oxidation of the iron, and in many places the formation carries an appreciable content of phosphate, which serves as a fertilizer. The Cook Mountain greensand soil is highly productive, and cotton, corn, and garden truck are successfully grown along its outcrop. The basal member of the Cook Mountain is the Sparta sand, probably nonmarine, carrying less iron than the higher beds. Its outcrop zone, from 1 to 2 miles wide, is crossed by the railroad on the down grade to the valley of Bee Branch, 4 miles west of Waelder. The vegetation varies with the formation; the mesquite here is not so large nor so numerous as in the Yegua area, but the oaks are very much more in evidence, especially on the lower, more sandy beds.

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³⁸The mineral glauconite, also called greensand, is a silicate of iron and organic and alumina deposited in the sea through chemical agencies.

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On leaving the outcrop zone of the Sparta sand member of the Cook Mountain formation the railroad bends considerably to the south, and for the next 5 miles, or nearly to Harwood, it crosses the Mount Selman outcrop diagonally. The strata dip to the west-northwest at a low angle. In a general way the Mount Selman beds resemble the Cook Mountain formation, consisting of glauconitic sands, marls, and clays, but they contain a greater number of indurated, irony beds, so that the hills and ridges are higher and the soil a more intense red. The soil is productive and well adapted to truck farming, and a large part of the 25,000 acres planted in tomatoes in Texas in 1930 was upon an outcrop of the upper member of the Mount Selman. The medial sand member, known as the Queen City sand in northeastern Texas, contains clay deposits, some of which carry fossil leaves, and the series is, for the most part, nonmarine. This sand member is uniform in character, and in the Winter Garden region in southern Texas it is sufficiently thick and pure to carry water of a quantity and quality second only to that in the Carrizo sand. Post oak and blackjack oak are the characteristic trees growing on the heavier sands, but where a little clay is mixed with the sand the forest growth is varied, and in well-watered areas the underbrush is heavy.

The lowest member of the Mount Selman formation along the Southern Pacific Railroad has been separated as the Reklaw. It consists of ferruginous sands and brownish and grayish glauconitic micaceous clays of marine origin, possibly less than 100 feet thick. The railroad crosses the outcrop zone, about 1 mile wide, at the small village of Harwood. The soil and landscape sharply reflect the lithology of this member, the clay outcrop forming a low flat belt and a heavy soil which in wet weather makes very difficult roads, whereas the ferruginous, concretionary beds at the base give much more relief and better-drained though much rougher roads. The hilly character of this region is due to the capping of these resistant basal beds of the Reklaw upon the soft, readily eroded Carrizo sand. Both the basal ferruginous layers and the clays above carry fossils. The ferruginous beds contain impressions of Venericardia and Corbula, which in places are fairly common; the glauconitic clays carry locally, notably on the Colorado River near Bastrop, a well-preserved and varied coral and molluscan fauna.

On the western edge of Harwood the shale at the base of the Mount Selman formation gives place abruptly to the Carrizo sand, one of the most characteristic of the Texas Tertiary formations. The airplane maps show this formation as a solid pale-gray ribbon picoted along the margins by the darker pattern of the Reklaw above and the Indio beneath. The sand is coarse and almost pure white, and consists of nearly pure quartz grains, loosely packed and readily weathered, blowing about the fields and resisting cultivation. The bright-colored indurated layers occurring at intervals in the sand and the local capping of the Reklaw ferruginous strata along the eastern edge of the outcrop break down into rugged, castellated shapes, contrasting sharply in color and relief with the soft, dazzling white sand. Rather scrubby oaks are the most conspicuous trees, and the few and difficult roads wind through long uninhabited stretches broken only here and there by a lumber or goat camp. In certain Carrizo areas where the sand is coarser and less pure it is adapted to the growth of some kinds of garden truck. The Carrizo sand is the underground reservoir that furnishes the water to irrigate many fields and gardens, notably in the Winter Garden area of western Dimmit and Zavala Counties and in the extensive trucking district south of San Antonio, particularly the strawberry farms near Poteet, in northern Atascosa County. In the Winter Garden area, where the annual rainfall is less than 25 inches and the native vegetation was mostly mesquite and nopal, irrigation by water from the Carrizo sand has made a garden spot that abundantly justifies the name.

Along the Southern Pacific Railroad the Carrizo outcrop is a rather monotonous belt of sand and scrubby oak extending from a point just west of Harwood to and beyond Ivy siding. Iron Mountain, one of the higher hills, consisting largely of hard brown sandstone, causes a deflection of the railroad to the north for some distance east of Ivy siding, beyond which it passes onto the outcrop zone of the Indio formation. There are fine exposures of the basal sandstone of the Carrizo in the cut through the divide a mile west of Ivy, which reveals about 30 feet of coarse, mostly soft massive sandstone containing considerable ironstone and notably cross-bedded.

These beds are underlain by a thick body of softer sandstones and clays of the Indio formation. This constitutes the surface of a wide area about Luling, where, however, the strata are mostly covered by alluvium, especially in the flat that extends west to the San Marcos River.

The outcrop of the Indio formation is wider along the Southern Pacific Railroad than that of any other of the Tertiary formations, largely because the railroad crosses it diagonally in the northerly bend near Luling and also in the southward deflection of the tracks west of that place. Most of this outcrop zone is gently rolling and rather featureless, though it presents the threefold division of a lower and an upper clay and shale series, locally of marine origin and fossiliferous, separated by nonmarine sands and sandy shales. The soils of the Indio formation vary with the lithology. Some of them are almost as sandy, though not so coarse, as the Carrizo sand; others are almost as heavy and black as the Midway soils; but most of them respond to cultivation. Blackjack oak and post oak are the common trees on the more sandy soils, and mesquite predominates on the clay soils.

The large "tank farm" of the Magnolia Petroleum Co., a mile east of Luling, indicates the proximity of the Luling oil fields. Before the discovery of petroleum here in August, 1922, Luling was a small, easy going German community, concerned chiefly with the price of cotton, on which the material welfare of the inhabitants depended. The discovery of oil at a horizon lower than that of any other source then producing in the Texas fields not only changed over night the resources and status of the community and of its individual members, but opened new possibilities of finding deep productive sands throughout the State. The Luling field was discovered by Edgar B. Davis, a shoe manufacturer of Brockton, Mass., who drilled to the Edwards limestone, against the counsel of other oil operators, at a cost of $200,000. The large fortune which he gained from his success was shared with the rapidly growing community; streets were paved, clubs and orphan homes were developed, and the Foundation farm of 1,200 acres was established near by with a trust fund of $1,000,000 for experimental farming for the benefit of the people of the region.

There are three productive oil fields near Luling—the Salt Flat field, to the northeast of the town; the Darst Creek field to the southwest; and the Luling field, to the northwest. There are also some smaller pools. All have the same geologic relations.

The many derricks of the Salt Flat oil field are conspicuous north of the tracks a few miles east of Luling. In 1930 this field produced from 15,000 to 27,200 barrels a day, with a total of 7,305,000 barrels for the year, from about 330 borings. Their average depth is 2,700 feet, and most of the oil comes from the Edwards limestone, which has been lifted by a fault with upthrow of about 375 feet. The field was discovered in 1928 and up to the end of 1930 had produced 21,116,554 barrels of oil.³⁹ The field is about 2,000 feet wide at most and 6 miles long, extending along the main fault, which trends northeast. The surface beds are sands and shale of the Indio formation, which is 700 feet thick in this vicinity.⁴⁰ The relations are shown in Figure 5.

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³⁹Southwest Texas Oil Scouts Assoc., Bull. 1, 1930.

⁴⁰Below the Indio are 280 feet or more of tough clays and glauconitic sands of the Midway group (basal Tertiary), which lie unconformably on Navarro clay, here about 500 feet thick and containing some thin beds of sandstone. Next below are about 500 feet of Taylor clays and chalky clays, which extend to the Austin chalk, 228 feet thick, underlain by the Eagle Ford shale, 30 feet; Buda limestone, 60 feet; Del Rio clay, 40 feet; and Georgetown limestone, 60 to 110 feet, lying on the Edwards limestone. (McCullum and Cunningham.) See also table, p. 65.

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FIGURE 5.—Section across Salt Flat oil field, Caldwell County, Tex. Tertiary: Ti, Indio formation; Tm, Midway formation. Cretaceous: Knt, Navarro and Taylor clays; Ka, Austin chalk; Kw, Georgetown limestone overlain by Del Rio clay and Buda limestone; Ke, Edwards limestone

A few miles southwest of Luling is the Darst Creek oil field, discovered in 1929 and also drawing from the Edwards limestone, the depth of which in the first well was 2,605 feet. The oil here, as in the other fields near by, is believed to have migrated up the dip and become trapped in the upper porous member of the Edwards limestone, here sealed off by a fault that has brought it up against impervious strata. This fault is estimated to have a displacement of about 525 feet. It trends northeast, parallel to the Luling and other faults. (Brucks.) The production in 1930 ranged from about 15,000 to 36,452 barrels a day and amounted to 11,424,000 barrels for the year from about 253 wells. The productive area is about 4-1/2 miles long and in places 3,500 feet wide, covering about 1,500 acres.

In the Luling field, which is nearly 8 miles long and half a mile wide, the oil occurs in the upper part of the Edwards limestone, at an average depth of about 2,100 feet. This field reached a peak production of about 11,134,000 barrels in 1924, with 567 wells, but in 1928 it had diminished to half that amount, and in 1930 it yielded 3,692,000 barrels, or about 10,000 barrels a day. The total production to the end of 1930 was about 50,000,000 barrels. The oil has a gravity of 26° to 29° Baume.⁴¹

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⁴¹According to Brucks the structure, which is similar to that in the Salt Flat and Darst Creek fields, shows a faulted monoclinal block with northeast trend. The upthrow is about 450 feet on the southeast side, and at the surface the basal clays of the Indio formation are brought up into contact with a medial sandy member. The fault is well exposed where it crosses the San Marcos River about 6 miles northwest of Luling.

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Some interesting data have been obtained from borings in the Luling field as to underground temperatures. In general the temperature in mines and borings is found to increase with depth at an average rate of about 1° for every 60 feet, below the first few feet. In two 2,250-foot holes near Luling the temperature was found to be 120°, which indicates a rate of increase of 1° in slightly less than 45 feet. At Pierce Junction, near Houston, the temperature in a 3,300-foot hole was 130° and in a 4,303-foot hole 146°.⁴²

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⁴²Hawtof, E. M., Am. Petroleum Inst. Bull. 205, pp. 62-108, 1930.

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In this field the Eagle Ford shale is 35 feet thick; the Buda limestone, 40 feet; and the Georgetown, a compact limestone, 50 feet. The normal dip of the strata in this region is only 1°-2° SE. Deep test borings seeking oil-bearing beds at lower horizons show that the Edwards limestone, of which the upper part is porous, is about 730 feet thick. It is underlain by 1,450 feet of Glen Rose limestones and clays (Brucks) and Trinity sands, the Trinity lying on pre-Cambrian schist at a depth of about 4,790 feet. (See table, p. 65.) Into this schist drilling was continued nearly to 8,000 feet without the slightest prospect of reaching its base or of finding petroleum.

The San Marcos River, which is crossed 3 miles west of Luling, has its principal source in the great springs at San Marcos, 30 miles northwest, at the foot of the Edwards Plateau. (Turn to sheet 9.)