442 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



ments sank, and the area became the Mexican Upper Jurassic and Creta- 

 ceous geosyncline. 



Northeast of the Coahuila peninsula, there is evidence of a narrow 

 promontory, called the Oriental geanticline by Imlay. The structure for 

 the most part was barely emergent. 



Early Cretaceous History 



During Early Cretaceous time, the Mexican geosyncline sank over 

 12,000 feet and received its greatest load of sediments. The contours of 

 the tectonic map, Plate 11, were drawn on the basis of thicknesses given 

 by Imlay (1944), and in conformity with the Coahuila peninsula and the 

 Orental geanticline as outlined by Kellum and Imlay. The Coahuila pen- 

 insula was overlapped considerably, and by Aptian time (middle Early 

 Cretaceous) it was completely submerged and for the rest of Creta- 

 ceous time was a platform on which 1500 feet of lagunal deposits ac- 

 cumlated. 



The land along the western margin of the Mexican geosyncline, at 

 least in the northern part, had probably suffered intense deformation dur- 

 ing the late Early Cretaceous. Over 10,000 feet of beds of Aptian age lie 

 along its eastern margin in central Sonora. The outcrops farthest west 

 consist of andesite flows, tuffs, and agglomerates, but this volcanic facies 

 is replaced to the east by a marine limestone, shale, and sandstone facies. 

 According to R. E. King (1939), there is evidence of great oscillations in 

 level of the sea, with repetition of cycles of marine and continental de- 

 posits. 



In northern Sonora, thick coarse conglomerates of the same age as the 

 volcanics of central Sonora occur. They increase in thickness southeast- 

 ward from Risbee, Arizona, and are at least 5000 feet thick only 30 

 miles to the south. The boulders are in part large and angular and, to- 

 gether with the large volume, show that the sea was bordered by steep 

 shores, and that the southwestern landmass was suffering active defor- 

 mation. The deposition and causative orogeny was rapid, because the 

 time represented by the conglomerates and related sediments is but a 

 fraction of Cretaceous time (Imlay, 1939). The presence of finer clastic 

 sediments, as well as coal, higher in the section indicates times of 



lowered lands, broader littoral zones, and marginal swamps. The litho- | 

 graphic character of the Risbee group shows that the landmass to the | 

 south was composed of Precambrian gneisses and schists, and Paleozoic 

 quartzites and limestones similar to those outcropping at present in the 

 Risbee district and locally in northern Sonora (Imlay, 1939). 



Still farther west than Sonora in Raja California, Lower Cretaceous 

 rocks have been identified, and their nature is significant regarding the 

 belt of orogeny west of the Mexican geosyncline. They occur along the 

 west shore of the northern part of the peninsula and consist of conglom- 

 erates, quartzites, tuffs, and agglomerates, with thick lava flows inter- 

 bedded. They are cut by dikes and large stocks. In some localities, the 

 intrusive rocks predominate over the sediments and pyroclastics, and in 

 places there is much metamorphism. Unaltered, or but little altered, sand- 

 stones and shales appear in places, and limestone also occurs. The meta- 

 morphosed Cretaceous rocks may be equivalent to schists and coarse, 

 massive, white granite that are widely distributed southward down the 

 peninsula. The granitoid rocks that intruded the Lower Cretaceous beds 

 are probably early Late Cretaceous in age. Chapter 30 is devoted to the 

 geology of Raja California and Sonora, and should be consulted for 

 further details. 



The composition of the Lower Cretaceous rocks is that of the volcanic 

 archipelago type, and indicates an associated orogenic belt. They appear 

 to be separated from the deposits of the Mexican geosyncline by the Oc- 

 cidental geanticline, but not enough is known of the distribution and litho- 

 logic variations of the Lower Cretaceous strata in this region to dem- 

 onstrate the interpretations. Refer further to Chapter 30. 



Late Cretaceous History 



Upper Cretaceous deposits are widely distributed from the Santa Ana 

 Mountains southeast of Los Angeles (Woodford, 1939) throughout the 

 length of Raja California to Todos Santos. They are separated from the 

 strata of Early Cretaceous age by a period of intrusion and metamor- 

 phism, and were themselves faulted and in places folded before the 

 Cenozoic sediments were laid down. For further details, see Chapter 

 30. 



