700 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



GUANAJUATO 

 RIVER 



Tpc-^ RIVER QoU 



Fig. 43.3. Cross section at city of Guanajuato, Mexico, after Edwards, 1955. Qal, Alluvium; Tpc, 

 Pliocene conglomerate; Tmv, Young volcanic rocks, Miocene; Tomb, Bufa ss., Oligocene-Miocene; 



weathered granite is intrusive into the Triassic (?) shales and schists. 

 Other plutons are dioritic and monzonitic in composition, and all are 

 believed to be pre-conglomerate by Edwards. 



Uplift, faulting, erosion, and volcanism followed the folding and intru- 

 sions. Although the volcanics do not occur interstratified in the sequence 

 a great pile of them is believed to have existed nearby because derived 

 fragments constitute a large part of the conglomerate. 



The red conglomerate at Guanajuato City is named after the city. There 

 it is about 5000 feet thick but thins northeastward and southwestward. 

 Volcanic fragments form more than half of the deposit, with granite, 

 diorite, limestone, and chert making up the rest. Granite fragments in- 

 crease upward and compose 35 percent of the mass near the top. This 

 indicates an increasing exposure of the granitic pluton in the source area. 



The source of the conglomerate was a highland northeast of Guanajuato 

 City where silicic volcanics capped shales and limestones of Cretaceous 

 age. The highland was also an area where a granitic pluton had intruded 

 the Cretaceous strata (Edwards, 1955). 



The Guanajuato conglomerate is late Eocene or early Oligocene in age. 

 Similar red conglomerates presumably of the same age occur at Zacatecas 

 and Taxco. 



The conglomerate is overlain by the tuffaceous Rufa sandstone into 

 which it is transitional. The sandstone is about 50 feet thick. 



The great Miocene (?) volcanic epoch followed, which was initiated 

 by the deposition of massive, bedded tuffs more than 1500 feet thick 

 in the Guanajuato area. Then followed normal faulting, which produced 

 the tilted block and graben structure so strongly evident today. The slip 



Toe, upper part of Guanajuato conglomerate; Teoc, lower part of Guanajuato conglomerate; upper 

 Eocene-Lower Oligocene; MES, Mesozoic sedimentary rocks and coarse-grained silicic, intrusive rocks. 



on several of the northwest striking faults is as much as 3000 feet. See 

 cross section of Fig. 43.3. 



Fold Belt in Central America 



The fold belt is shown to include gentle folds in the states of Tabasco 

 and Vera Cruz along the north side in a region recognized as coastal plain 

 by some. 



As the fold belt is traced eastward into Guatemala and Rritish Hon- 

 duras, continental Jurassic and Lower Cretaceous beds are involved. 

 These beds probably covered much of the "nuclear region" of Central 

 America (Imlay, 1944; Roberts and Irving, 1957). Toward the close of 

 Early Cretaceous time subsidence and marine embayments resulted in the 

 deposition of limestone and dolomite approximately coextensive with the 

 underlying terrestrial beds. Deposition continued in most places until 

 Late Cretaceous when the folding occurred. 



The earliest Tertiary beds are coarse clastic rocks of the Sepur formation, 

 whose composition shows that they were derived from a wide variety of sources 

 including crystalline rocks, Permian limestone and quartzite, and limestone 

 and volcanic rocks of Mesozoic age. The Sepur strata were probably deposited 

 during orogenic movements in Late Cretaceous and Eocene time. Intrusions of 

 granodiorite and diorite that accompanied the orogeny cut the Cretaceous rocks 

 throughout eastern Guatemala and Honduras. Folds, largely trending eastward, 

 were developed in the Cretaceous rocks and appear also to have involved rocks 

 as young as those of Sepur age. 



The orogenic movements culminated in thrust faulting, first mapped in the 

 Departamento de Huehuetenango, which thrust the Permian rocks over the 

 Todos Santos formation and the Cretaceous limestone. The extent of the thrust- 

 ing is not known, and many such faults may be present in central Guatemala 

 (Roberts and Irving, 1957). 



