Weno and Pawpaw Formations 19 



West Texas, abundant lower; in Central Texas (South Bosque, Round 

 Rock, Austin) it is abundant in the basal half of the formation but rare or 

 wanting above. It also occurs in the top of the Georgetown limestone at 

 Austin. 



(4) Turrilites brazoensis Roemer occurs in the lowest five feet of the 

 Del Rio Clay at South Bosque and at Austin ; its main zone of abundance is 

 in the top of the underlying Georgetown limestone. 



Zones (1) and (2) are Grayson; zones (3) and (4) are Mainstreet; or 

 briefly the upper part containing Gryphea mucronata is Grayson, and the 

 lower part containing Exogyra arietina is Mainstreet. The Mainstreet 

 equivalents locally have sandy levels in Central Texas, as for instance at 

 the top of the Georgetown limestone at South Bosque and in the E. arietina 

 slabs at Austin. Westward from the turning point of its outcrop in Bexar 

 County the Del Rio becomes sandier and more flaggy and contains sandy 

 slabs with Nodosaria and other fossils. At the south end of the Quitman 

 Mountains the Del Rio is transitional, showing a mixture of sand and clay 

 facies, and at Cerro de Muleros the Mainstreet, represented by Bose's sub- 

 division 7 is sandstone. North of the turning point the typical Central 

 Texas section continues to the Brazos, where the Del Rio differentiates into 

 two portions by the intercalation of limestone strata basally; this basal 

 part becomes upper Mainstreet and the upper marlier part becomes Gray- 

 son. The lower Mainstreet however continues as a limestone into Central 

 and West Texas. 



Marl facie*: 



The upper Mainstreet formation in McLennan County is transitional 

 from clay to marl, and at South Bosque is rather calcareous. North of the 

 Brazos this calcareous marl contains limy seams which become more prom- 

 inent and take on the interbedded appearance of the Mainstreet limestone 

 of North Texas. 



Limestone facie*: 



The typically developed Mainstreet limestone between the Brazos and 

 the Red Rivers shows this lithological facies. There is considerable inter- 

 bedded marl as far north as Gainesville. Eastward from Gainesville, down 

 the Red River Valley, at Cedar Mills, Denison, Bennington, Bokchito and 

 near Hugo, the marl is more reduced in amount, and the formation consists 

 of compact, slightly marly, massive shell breccia or indurated limestone 

 with typical Mainstreet fossils. 



