246 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



STRATIGRA PHY 



z 



UJ 

 H 

 (/} 



>- 

 W 



=-- 

 < 



< 

 > 



-1 

 >- 



n 



H 

 PL. 





SERIES 



STAGE 



GROUP 



FORMATION 



MEMBER 



UJ 



CISCO 





THRIFTY 

 GRAHAM 







CANYON 





CADDO CREEK 

 BRAD 



GRAFORD 

 WHITT 



Home Creek Ls 

 Colony Creek Sh 

 Ranger Ls 

 Placid Sh 

 Winchell 

 Cedarton Sh 

 Adams Branch 

 Upper Brownwood Sh 

 Palo Pinto 

 Keechi Creek 

 Salesville 



Lake Pinto Ss 



a 

 OS 



pa 



i= 

 O 

 — 



STRAWN 







East Mountain Sh 



Capps Ls 



LONE CAMP 



Garner 



Brazos River cong. 

 Mingus Sh 

 Thurber Coal 



MILLSAP LAKE 



Grindstone Creek 



Goen Ls 

 Santo Ss 

 Buck Creek Ss 



Lazy Bend 

 (Restricted ) 



Brannon Bridge Ls 

 Hill Creek 



LAMPASAS 



KICKAPOO 

 CREEK 





u » 



£ o 



H fl 



J tfc. 



b 



O 3 



§•§ 



Rayville 

 Parks 

 Caddo Pool 



Kickapoo Falls Ls 

 Dickerson Sh 



ATOKA 





Smithwick 



Lower "Caddo Ls" 



Lake Ss pay 







a 

 o 



p <- 

 5 o 



co 3 

 o 



Big Saline 

 Upper Marble Falls 



Brister 

 Lemons Bluff 



Gibbons cong. 



MORROW 





Comyn of Subsurface 

 Lower Marble Falls 



Aylor 

 Sloan 



SPRINGER 











Fig. 15.9. Pennsylvanian stratigraphy of the Llano uplift. After Cheney and Goss, 1952. 



Concho Arch. In late Mississippian time the orogeny of the hinter- 

 land of the Ouachitas resulted in the depression and fill of the Fort 

 Worth and Kerr basins marginal to the later belt of compression. This 

 resulted in the development of a broad, pronounced asymmetrical arch 

 involving the previous formations and the top of the Precambrian. The 

 situation is illustrated in the lower cross section of Fig. 15.10. Subsidence 

 continued through the Atoka and Kickapoo elastics (subdivisions of the 

 Lampasas according to Cheney and Goss (1952). The asymmetrical arch 

 is called the Concho. With the deposition of the thick Permian sedi- 

 ments of the Midland basin (second cross section, Fig. 15.10) the arch 

 becomes a very strong and large feature. The present contour of the 



Precambrian surface reflects the arch essentially as it was at the close 

 of Permian time. See Fig. 15.1. It pitches gradually to the north-north- 

 west and reaches to the Matador arch. 



Bend Axis. The Permian and Upper Pennsylvanian beds overlap the 

 Concho arch from the west in the manner illustrated in Fig. 15.11. As far 

 as these beds are concerned an axis of down tilting to the west is in- 

 volved, and this has been called the Bend axis or arch. There may be 

 an arch in the Upper Pennsylvanian beds but probably not in the 

 Permian. 



Llano Uplift. The southeast end of the Concho arch was so high 

 that all beds were stripped off down to the Precambrian before the in- 

 vasion of the Cretaceous seas, which spread a cover of coastal plain 

 sediments widely over the south and east flanks of the arch. These sedi- 

 ments have since been mostly removed from the Precambrian and a 

 domal area known as the Llano uplift results. This is the most prominent 

 feature evident on the geologic map of Central Texas. 



The Pennsylvanian history of the site of the Llano uplift is somewhat 

 more involved than the cross-sectional representation of the Concho 

 arch in Fig. 15.10. According to Cheney and Goss (1952): 



Mississippian outcrops in the Llano region transgress the truncated Ordo- 

 vician Ellenburger group. Drilling has shown an increasing loss of section west 

 of the Llano uplift so that, as a result of both erosion and non-deposition, 

 Upper Pennsylvanian (Canyon) marine sediments locally overlap Cambrian 

 rocks in and near northeast Menard County. Farther west and northwest, 

 Middle Pennsylvanian beds rest on truncated Mississippian and Ordovician 

 or older rocks in a large region, heretofore called the Concho arch, where 

 local as well as regional tectonic features had developed mainly along trends 

 varying from north-northeast to northwest. Thin Middle Pennsylvanian marine 

 sediments of the Lampasas and Strawn series deposited across this base-levelled 

 region are chiefly limestones and shales of the platform type in contrast to 

 thick basinal type deposits on the east and south. 



A system of large faults extending northward from the present Llano 

 uplift into die Fort Worth basin developed during very late Lampasas 

 time. The faults, well known from surface mapping in the Llano uplift, 

 have now been followed by geophysical work and drilling for more than 

 100 miles northward into the Forth Worth basin. Some of the faults have 

 displaced upper Lampasas and older beds as much as 1100 feet in the 



