232 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



I 1 oermun 1---; -I : . 7 ";j cretaceous \lll^ T voL«mcs [ 



*•* TERTIARY INTRUSIVES »""■ 



PRE-PERMIAN PALEOZOIC STRATA 

 FOLDED AND FAULTED 



PENNSYLVANIAN THRUSTS 



CONTOURS BASE COMANCHE SER. 

 ? MILES 



Fig. 14.8. Structure map of the Marathon uplift. After King, 1937. Black Peak thrust is post- 

 Cretaceous. A number of Cretaceous outliers in the Paleozoic area not shown. 



region lies on the edge of the Mexican highlands physiographic province 

 which merges with the Great Plains on the east. Structurally, the region 

 is a broad dome of Cretaceous rocks, from whose central part the Creta- 

 ceous cover has been stripped away, leaving an area of low country in 

 the center, the Marathon basin. See Figs. 14.1 and 14.8 and cross-section 

 M-M'-M", Fig. 14.9. Here, strongly folded Paleozoic rocks are exposed. 

 The Paleozoic rocks in the basin, and in the Glass Mountains which flank 

 it on the northwest, have a thickness of 21,000 feet. The greater part of 

 them was laid down in a subsiding trough commonly referred to as the 

 Llanorian geosyncline. The oldest rocks are Upper Cambrian sandstones 

 and shales, whose base is not exposed. Overlying them are 2000 feet of 

 Ordovician rocks composed of shaly limestone and shale, with some beds 

 of chert. The Ordovician is overlain by the Caballos novaculite, possibly 

 of Devonian age, which reaches 600 feet in thickness. The Caballos no- 

 vaculite is over-lain by a great series of clastic rocks of Pennsylvanian age, 

 as much as 12,000 feet thick in the southeastern part of the area but 

 much thinner in the northwest. 



Llanoria and the Llanorian Geosyncline 



The belt of folded sedimentary rocks of the Ouachita Mountains ex- 

 tends around the Llano uplift to the Marathon region and thence south- 

 westward across the Solitario near the Rio Grande and on into Mexico. 

 See Plate 8. The early Paleozoic trough lay about 100 miles north of the 

 present mountains (Rarton, 1945). See Fig. 14.10. In Permian time, a 

 trough of geosynclinal proportions existed in Coahuila, 200 miles south of 

 the Solitario. 



Pre-Carboniferous sediments of the Marathon and Solitario uplifts, 

 like those of the Ouachitas, are rather thin and include much clastic ma- 

 terial. They are composed of sandstones, conglomerates, boulder beds of 

 debated origin, and impure limestone with much shale, chert, and, con- 

 spicuously, novaculite. Some of the sediments evidently accumulated at 

 no great distance from shore; others such as the shales may have been 

 carried much farther away from their source. In the foreland areas of 

 both the Ouachitas and Marathons, the sediments are mostly limestones. 

 It is generally concluded that the early Paleozoic sediments came from a 



