234 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



more feet in the Marathons. The trough in which these Carboniferous 

 sediments accumulated appears to have extended uninterrupted from the 

 Ouachita to the Marathon and Solitario regions. This Pennsylvanian 

 trough is referred to as the Llanorian geosyncline. The Fort Worth 

 ( Strawn ) and Kerr basins seem to be expansions of the geosyncline over 

 the margin of the foreland. 



The land area of Llanoria, southeast of the Llanorian geosyncline, ap- 

 pears to have been composed largely of crystalline rocks and probably 

 stood as a highland or mountain area during a large part of Paleozoic 

 time. For the most part, the former highland is now buried beneath 

 Cretaceous and younger strata, and the hypothesis of its former existence 

 is based largely on evidence supplied by the composition of the Paleozoic 

 sediments in the geosyncline (Miser, 1929; King, 1937). 



Both Pennsylvanian clastic and Devonian cherty formations thicken 

 southeastward across the Llanorian geosyncline in the Marathons; lime- 

 stones are replaced by shales or cherts; and the clastic deposits contain 

 grains of schistose or granitic rocks, pebbles of vein quartz, and cobbles 

 of igneous rocks. The distance south at which the land lay during Paleo- 

 zoic time is unknown, but it may have been 100 or more miles away. 

 Examine Fig. 14.10. 



Phases of Marathon System 



Early Pennsylvanian Phase. The lowest of the Pennsylvanian forma- 

 tions, the Tesnus, was deposited in the Llanorian geosyncline, probably in 

 early Pennsylvanian time (King, 1937). It is a great mass of inter- 

 bedded sandstone and shale in thin and thick beds, nearly barren of fos- 

 sils. In the southeastern part of the basin it exceeds 6500 feet in thickness, 

 and it is predominantly sandstone with many arkose layers and several 

 prominent massive layers of white quartzite. In the northwestern part of 

 the basin, it is about 300 feet thick and is nearly all black shale with a few 

 sandstone beds. The Tesnus, the Dimple limestone, and the lower part 

 of the Raymond formation make up the flysch facies — a European term 

 to signify sediments deposited during the time of a rising hinterland and a 

 sinking geosyncline. The Dimple limestone is over 1000 feet thick in the 

 Marathon basin, and thins southward. The Ilaymond formation is a mass 

 of arkosic sandstones and shales 3000 feet thick. 



Overthrusting in the southern part of the Marathon area began at this 

 stage, as is suggested by a remarkable layer of mudstone in the upper 

 part of the Haymond, in which are embedded large blocks of older rocks. 

 The blocks are believed to have been derived from the erosion of ad- 

 vancing thrust sheets and to mark the first strong compression in the 

 region (King, 1937). 



Late Pennsylvanian Phase. The uppermost Pennsylvanian formation, 

 the Gaptank (Upper Pennsylvanian in age), consists of conglomerate and 

 sandstone derived from the erosion of rising folds. The strong deformation 

 to which the Paleozoic rocks of the Marathon basin have been subjected 

 apparently culminated after the deposition of this Upper Pennsylvanian 

 formation. The Permian rocks of the Glass Mountains to the northwest 

 rest, at least in places, with great angular unconformity on the disturbed 

 older beds. See section M-M'-M", Fig. 14.9. The structural features con- 

 sist of close folds that trend northeast and are overturned to the north- 

 west, and several thrust faults. The faulting culminated on the northwest 

 in the nearly flat-lying Dugout overthrust, with a known displacement of 

 more than six miles. Farther southeast the other thrusts have miles of dis- 

 placement and some are folded and therefore older than the frontal fault 

 (P. B. King, 1937). 



COAHUILA SYSTEM 



Known Geologic History 



Exposures of Late Pennsylvanian (?) and Permian rock in the south- 

 western part of the Mexican state of Coahuila, some 250 miles south of the 

 Marathon region of Texas, are believed to reveal a continuation of the 

 Llanorian geosyncline and the approximate position of the west margin of 

 Llanoria. In the Acatita-Las Delicias area, according to Kelly (1936) and 

 R. E. King et al. (1944), a series of sediments and interstratified igneous 

 rocks over 10,000 feet thick was deposited in a subsiding trough. The 

 sediments came from the landmass of Llanoria, and the lava flows, sills, 

 fragmental igneous material, and graywacke came from the west. The 

 volcanics are rhyolite, andesite, and basalt flows and tuffs. 



Late Pennsylvanian ( ? ) limestones, possibly in part of reef origin, were 

 deposited simultaneously with products of volcanic activity. Coarse 



