The Geology of Tarrant County

" The Mainstreet limestone totals about 50 feet in thickness at Fort Worth.

The formation with little variations from top to bottom, is composed of regularly alternating strata of straw-colored marl and chalky, or hard fairly pure limestone. The bands do not usually exceed one foot in thickness and the formation has a deceptive similarity to the Fort Worth limestone.

The Mainstreet limestone can be identified and its levels distinguished by means of the fossil sequence, which in part is as follows:  Ostrea sp., aff. marcoui Boese, Ostrea carinata (?) Lamarck., Ostrea quadriplicata Shumard, Lopha sp., Ostrea subovata Shumard., Exogyra arietina Roemer., Exogyra sp., Gryphea sp., Pecten texanus Roemer, Pecten subalpina Boese, Pecten wrightii Shumard, Pecten roemeri Hill, Pecten spp., Spondylus cragini Whitney., Lima wacoensis Roemer., Protocardia vaughani Shattuck., Pholadomya shattucki Boese., Ptychomya ragsdalei Cragin "

The Geology of  Denton County

"MAINSTREET paleontology:

  Turrilites brazoensis (plate 5). This large, spirally, coiled ammonite marks the Main Street not only in North Texas but appears in a corresponding zone in rocks of the same age wherever they are exposed in central and western Texas. The species is a fairly abundant one and it is unusually favorable as a horizon marker because of its abundance. The relative abundance decreases somewhat toward the north although specimens occur in southern Oklahoma. The preservation is usually in the form of a mud cast and it is seldom that the suture pattern or annual growth marks will be preserved.

  Pachymya austinensis (plate 11). This is the largest of the clams preserved in the lower Cretaceous rocks of Texas. It is readily recognized by its large size and characteristic contours. In Central Texas the mud cast often shows the shell markings but in Denton County the specimens are usually smooth.

  Holectypus limitis (plate 13). This flattened sea urchin is the second species in this genus of importance in these rocks. It is distinguished from the species occurring in the Goodland by differences in size, slight differences in outline, and structural differences of a technical nature which need not be gone into here as the two species, because of their wide separation -in the column, should never be confused with each other. "

Integrated Albian-Lower Cenomanian Chronostratigraphy Standard, Trinity River Section

"The upper Albian / Lower Cenomanian boundary is identified in the uppermost meter of the Main Street Limestone at a cycle boundary... "  

The Geology of Texas - Vol. 1

MAINSTREET FORMATION

Nomenclature. -- The formation was named by Hill (788, pp. 302–303, 330–331) in 1894, the type locality being on East Main Street, Denison, Texas. The formation is exposed in a cut at the St. Louis and San Francisco Railway station, and in cuts of Pawpaw Creek and of railways within the city limits. Synonyms: Choctaw limestone, (Cragin, 325) "; Bennington limestone (Taff, Atoka folio No. 79, p. 6, 1902) ; "Exogyra arietina" limestone (Taff, 1575).

Stratigraphic position and contacts.--- Stephenson places an unconformity (?) at the base of the Main Street, and describes the basal stratum as follows: "Greenish-gray, sandy, calcareous, clay marl with scattered white clay pebbles and ferruginous concretionary clay nodules along the base, some of which suggest having been mechanically included .... 1.25 to 1.5 feet." The Main Street-Grayson contact has not been well investigated, but is possibly conformable.

Facies.--- Three main facies are known: (1) the usual marl-lime facies, in southern Oklahoma, and in Grayson and Cooke counties; (2) rudistid caprock, as near Fort Stockton; and (3) red sandstone-quartzite with fossils, at El Paso.

⁴¹Literature.—North-central Texas: Hill, 803; Stephenson, 1530; Winton, 1789. South-central Texas: Cuyler, 383; Hill, 803; Taff, 1575. Trans-Pecos Texas: Adkins, 12; Böse, 129. Paleontology: Roemer, 1331; Cragin, 325; Adkins and Winton, 9; Adkins, 13; Scott, 1389.

* Hand-written notation by Professor Cragin: date of publication, April 5, 1895. See also: Cragin, Am. Geol., 16: 165, footnote, September, 1895.

Areal outcrop; local sections.--- Taff says that the Main Street is about 10 feet thick in Bryan and Marshall counties, Oklahoma. In Marshall County (174, p. 44) it is 10 to 20 feet thick, and consists of heavy-bedded, brown, hard, semi-crystalline limestone with subordinate interbedded layers of marl. There is almost as much marl as limestone in most sections. The limestone is more massive, and the marl more reduced, near the base of the formation. As farther south, Kingena wacoensis is common in the basal part, and Exogyra arietina is common in the upper part. In Grayson County it is 8 to 23 feet thick, and around Denison about 9 to 12 feet thick. In the railway cut, 3 miles west of the Union Station at Denison, Stephenson measured 8¾ to 9 feet of irregular layers of limestone underlain by a massive limestone ledge, which rests on a clay marl with apparently rolled pebbles at its base. In a head-waters branch of Pawpaw Creek, just east of the underpass under the Missouri, Kansas and Texas tracks south of the Union Station at Denison, Stephenson recorded 9.5 to 10 feet of Main Street, consisting of an upper 8 feet of irregular nodular limestone strata interbedded with soft yellow marl layers, and a lower 1.5 to 2 feet of gray, faintly laminated sandy marl with evidences of unconformity at its base. At the Choctaw Creek bridge on the Denison-Bonham road, the Main Street is 17 feet thick, all limestone. In the Pottsboro cut-off, about 4 miles west of Denison, it is 11 feet thick; 1 mile south of Fink it is 13¾ feet thick, and at another nearby locality, about 8 feet thick; in Rock Creek, northwestern Grayson County, it is 23 feet thick. The basal part contains Alectryonia quadriplicata, its highest occurrence. The Main Street is generally coarsely crystalline, bedded, brecciated white limestone, which because of its ferruginous content turns deep yellow or yellow-brown on oxidation and hydration. Both marls and lime-stones are locally somewhat sandy. In a railway cut in the eastern part of Denison, an excellent exposure of the topmost Main Street layer in contact with the Grayson, contains many fossils. On the trestles of the Missouri, Oklahoma and Gulf Railway across the small headwaters of Pawpaw Creek, about a mile east of Denison, there are fossiliferous Grayson exposures with the Main Street contact exposed. In the extreme northeastern corner of Cooke County, 2 miles west of Dexter, the Main Street is a bed of light-blue limestone 10 to 15 feet thick (803, p. 282).

On passing south, the Main Street changes in thickness and in lithology. In northern Denton County it is 14 to 15 feet thick, in the southern part of the county about 20 feet. In the Clear Creek section, 5 miles northeast of Denton, it is 17 feet thick. The heavily iron-stained, massive, thick ledges of the Red River section give way in Denton County to a limestone almost indistinguishable from the Fort Worth limestone: the limestone strata are thinner, more evenly bedded, whiter, and alternate with light-colored marls.

In Tarrant County the Main Street increases from 25 to about 50 feet from north to south; in Johnson County it is nearly 50 feet. From Denton County southwards, the Main Street forms conspicuous and extensive upland prairies, next in size to those of the Fort Worth limestone. Good and nearly complete sections of the formation occur in Buffalo Creek at Cleburne, along Deer Creek between Burleson and Crowley, and near Crowley.

From Hill County southwards, the Main Street becomes thinner, and past Bell County is consolidated with other members into the top of the Georgetown limestone. In southern Hill County, west of Aquilla, it is about 35 feet thick, in McLennan County about 25 feet, and in Bell County about 20 feet. A few feet is recorded at Austin. Its thickness in the lower Pecos Valley, where the George-town is thick, is unrecorded. Over its thinned southern extension, it maintains its typical facies and fauna: Turrilites brazoensis, Exogyra arietina (in the top), and Kingena wacoensis.

An important part of the Main Street is the upper few feet, which form the "transition zone" to the overlying Grayson. Some misunderstanding of this portion is evident in the literature. In Grayson County the Grayson is at many places badly overwashed, and the Main Street is full of Exogyra arietina from bottom to top; in Travis County the Main Street limestone is reduced in thickness and is inconspicuous, and the Grayson (Del Rio) is full of Exogyra arietina. Those writers who considered Exogyra arietina to be diagnostic of a single formation correlated the Main Street with the Del Rio (and as a further deduction, the Grayson with the Buda). Speculatively the Woodbine has also been correlated with the Buda. To establish a correlation between these formations, the best starting point is the "transition zone" at the Main Street-Grayson contact. This is well exposed at Shoal Creek and 18th Street, Austin; at the fault a mile east-northeast of Georgetown; on Lampasas River 4¾ miles east of south of Belton; at numerous places near South Bosque; at Grayson Bluff northeast of Roanoke; and in a railway cut a half-mile southeast of the Union Station at Denison. Persistent and abundant fossils in the zone are large. Typical specimens are Turrilites brazoensis, Exogyra arietina, Kingena wacoensis (basally), Stoliczkaia n. sp. (numerous fine ribs), and Mantelliceras two n. spp. ("Acanthoceras aff. canningtoni " Böse). Study of this persistent level indicates that it is identical throughout its range. It everywhere overlies hard Main Street limestone and underlies Grayson marl or clay. This indicates that the Grayson and the Del Rio are at the same level at their base, and the pyritic micromorphs are the same, from Denton County to Del Rio and Terlingua.

The Main Street in the Fort Stockton area is a rudistid caprock, containing many of the reef-dwelling genera found in the middle caprock. At Kent it is neritic limestone, with banks of Exogyra arietina. At El Paso it is a thick, red-brown quartzite, evidently a marginal facies, containing in the upper part in thin, shaly layers, poorly preserved Exogyra arietina, Exogyra whitneyi? (juvenile), and Hemiaster calvini.

Paleontology.--- The Grayson is one of the most distinctive Comanchean formations. As Hill and Cragin pointed out, it and the Main Street stand out from the lower and middle Washita faunally. The last quadrituberculate and compressed Pervinquieria occur in the Main Street; none is known from the Grayson. Stoliczkaia, already appearing in the Denton, has several species in the Main Street and Grayson. Prohysteroceras and Macraster have not been reported from above the Pawpaw. Mantelliceras, a Cenomanian genus, appears in the "transition zone." The dying out of Albian genera lends to this boundary the appearance of considerable change; the fact that the section is probably fuller here than at many localities published elsewhere makes the placing of the boundary more difficult; on the other hand all sections now require closer zonal study. Turrilites brazoensis is a practical marker for the Main Street.

Scott (1389) correlates the upper Washita, including the Main Street, with the Upper Albian in Europe, which he claims is greatly extended in Texas. It is possible that the Main Street contains a fuller development of zones than the Upper Albian in England and France, a situation which would make the Albian-Cenomanian boundary hazier in Texas than in western Europe.

It is significant that the highest "Pervinquieria" in Texas occur in the Main Street. However, the Buda and apparently the Grayson are distinctly Cenomanian in fauna. This correlation awaits more extensive and accurate zonal studies.