COMPOUND REGRESSIVE AND TRANSGRESSIVE OVERLAP 625 



beginning of Woodbine deposition (Dexter sands) would allow a certain 

 amount of erosion, probably by the streams which later spread out the 

 Dexter sands. The presence of dicotyledonous plants and the absence 

 of marine organisms indicate that these sands were spread by streams. 

 Sometimes a clay marks the transition from the Grayson marls. The 

 presence of glauconite in the lower beds suggests that at first they were 

 deposited in a shallow sea, and that only during the progress of deposition 

 of the sands did emergence occur. If this is the case, there can be no 

 serious break between the Grayson marls and the basal Dexter sands, 

 and from the description of the sections there appears to be none. False 

 bedded structure is a characteristic feature of these sands, whose thickness 

 is approximately 160 feet. 



It was apparently during this period of emergence that the erosion at 

 Austin took place. This is believed to be the case, because the succeeding 

 beds of the Woodbine (Lewisville beds) carry a marine fauna, and hence 

 mark the readvance of the sea. The Lewisville beds consist of laminated 

 lignitic sands and clays, interstratified with brown sands, ferruginous 

 sandstones, and argillaceous shelly sandstones, aggregating 100 feet. The 

 fauna of this bed, listed by Hill,* is peculiar, in that it i6 unknown above 

 or below this horizon. This indicates a considerable period to have 

 elapsed before the readvance of the sea took place. The higher beds of 

 the Woodbine are sands and clays, often fossiliferous, and pass upward 

 into the overlying Eagle Ford formation. 



The Eagle Ford formation of Texas is essentially a bituminous clay. 

 It rests directly on the Buda limestone in central Texas, having there 

 become a flaggy argillaceous limestone. The thickness of the formation 

 varies considerably, from 250 feet on the Eio Grande to 600 feet in north- 

 ern Texas, with varying thicknesses at other points. In the Austin region 

 it is only 30 feet thick, but here only the upper beds of the formation rest 

 upon the post-Buda erosion plane. 



In southern Kansas the typical Dakota sandstone is followed by lignitic 

 sands, bituminous shales, and saliferous and gypsiferous shales with 

 marine fossils, followed by 350 to 400 feet of shales and limestones with 

 the typical fauna of the lower Colorado or Benton group, Inoceramus 

 labiatus predominating. 



In the Front Eange region of Colorado these shales (Benton) vary in 

 thickness from 500 to 700 feet, while farther north, in the Bighorn 

 Mountains they increase to 1,300 feet, and in the Black Hills to 1,600 

 feet.f Throughout most of the region the characteristic fauna with 



* Hill : Loc. cit, p. 314. 



t N. H. Darton : Bull. Geol. Soc. Am., vol. 15, pp. 379-448. Professional paper no. 32, 

 U. S. Geological Survey. 



