650 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



H-I 14. NORTHEASTERN TEXAS. 



According to Hill/^^* the Upper Cretaceous of northeastern. Texas is character- 

 ized as follows : 



The upper Cretaceous or Gulf series rests unconformably upon the Comanche series and 

 may be divided into five conspicuous formations, the beds of which grade into one another by 

 imperceptible transition. The lowest of these north of the Brazos, the Woodbine, consists of 

 ferruginous sands and clays. Above this is a formation characterized by bituminous clays, 

 the Eagle Ford. Then follows a conspicuous formation of chalk, the Austin chalk. Succeeding 

 this is a formation of marly clay, the Taylor marl. The uppermost formation, the Navarro, is 

 composed of unindurated bed's of glauconitic sands, clays, and a little chalk, as will be described 

 later. There is no sedimental break between' these formations, the whole series being prac- 

 tically continuous. All are characterized, with certain exceptions, by their general lack of 

 consolidation, little or no hard rock being found. The beds grade into one another so gradually 

 that partings are mappable only with the greatest difficulty. * * * 



The Woodbine formation apparently rests unconformably upon the Grayson marls and 

 Main Street limestones of the Denison beds of the Washita division, at the top of the Comanche 

 series. The upper beds pass, by inseparable transition, from sands into sandy clays and finally 

 into the bituminous clays of the Eagle Ford formation. This transition is so gradual that no 

 exact line of separation can be drawn between the Woodbine and Eagle Ford formations. I'he 

 parting is arbitrarily established at the zone of Exogyra columieUa, which is considered as the 

 top of the Woodbine formation. 



The formation has a thickness of at least 600 feet in the northern section and diminishes 

 southward until it disappears by thinning out and overlap at the Brazos. On Aquilla Creek 

 north of Waco the formation is 45 feet thick. According to Taff, it is 200 feet thick in the 

 Cleburne section. On Cottonwood Creek northwest of Hillsboro it is 95 feet thick. In the 

 Fort Worth section its thickness is about 300 feet and in the Denison section about 500 feet. 



The Eagle Ford formation is essentially a bitumiabus clay, accompanied in places by thin 

 laminated clay limestones and nodular septaria of blue limestone. These clays are notable in the 

 Texas section, where the other clay formations are mostly of a marly nature by reason of their 

 bituminous character, resembling the shales of the Cretaceous formations of the Rocky Mountain 

 region and the northern part of the Great Plains. Even these dark-blue clays become more 

 calcareous in the upper layers and in the extension of the beds south of the Brazos, but north 

 of that stream they form most of the miedial and lower portion of the beds. From Dallas north 

 the nodular septaria are of large size and abundant. In age these beds are identical with the 

 Benton formation of the Meek and Hayden section. On Red River, in the Paris section, their 

 thickness is estimated at 600 feet; in the Sherman section, 600 feet; in the Dallas section, about 

 480 to 500 feet; near Midlothian, 350 feet; in the Waco section, south of South Bosque, 200 

 feet; in the Austin section, 30 feet; on the Nueces, 200 feet. 



The Austin chalk consists of beds of impure chalk containing 85 per cent or more of car- 

 bonate of lime, interstratified with softer beds of marl. It is usually of an earthy texture, free 

 from grit, and when freshly exposed easily cut with a handsaw. Under the microscope the 

 material shows calcite crystals, particles of amorphous calcite, and the shells of foraminifers, 

 mollusks, echinoids, and other marine organic debris, such as usually constitute chalk for- 

 mations. 



Throughout the area included in this report the Austin chalk rests upon 5 to 10 feet of 

 finely laminated and usually arenaceous, calcareous, argillaceous yellow to bluish material 

 constituting the top of the Eagle Ford formation and ordiaarily termed the Fish beds. The 

 formation passes upward into the Taylor marls without stratigraphic break. Southeast of 

 Waxahachie, at the upper contact or parting between the Austin chalk and the Taylor marl. 



