28. 



ROCKIES OF NORTHERN MEXICO 



MEXICAN GEOSYNCLINE 



The Tectonic Map of Northern Mexico by Philip B. King (1947) is re- 

 produed in Fig. 28.1, and on it the belt of Laramide folds can be seen 

 extending southward from New Mexico and Texas into central and east- 

 ern Mexico. Preceding the Laramide orogeny and in about the same 

 region a major basin subsided and received a thick complement of sedi- 

 ments. It is known as the Mexican geosyncline. See Plates 10, 11, and 12 

 for its limits. 



Late Jurassic History 



In the northern part of the geosyncline in Late Jurassic time, sediments 

 2600 to 4800 feet thick accumulated, while in the southern part at least 



5000 feet of beds were deposited. At the beginning of Late Jurassic time, 

 the subsidence and marine invasion was limited to the southern part, 

 where 2000 feet of dark marine clay, lime mud, and sand were deposited. 

 The southward-lying land was evidently not high, but it was stable and 

 a humid climate prevailed (Imlay, 1943). After this stage, the first 

 widespread marine transgression occurred (Devesian stage), and thick 

 salt and anhydrite beds, associated with red clays, sands, and gravels, 

 were deposited. The salt facies was deposited in northern Central 

 America, southern Mexico, and the southern United States. A thick red- 

 bed facies, at least partly of continental origin, was formed throughout 

 much of northern and eastern Mexico at apparently the same time as the 

 salt facies to the south and north. Both facies transgressed a peneplained 

 surface. 



The thickness and extent of the salt layers suggest that the entire Gulf 

 of Mexico was a salt-depositing basin completely enclosed except for a 

 relatively narrow, shallow strait that probably connected with the Atlantic 

 Ocean. 



The material composing the red-beds was probably derived from the 

 Central Stable Begion on the north, where older red-beds cropped out, 

 and from the geanticlinal areas in western and southern Mexico (Imlay, 

 1943). Further sinking of the geosyncline brought on normal marine con- 

 ditions; and lime, clay, and silt were deposited on top of the salt and red- 

 beds. Still later in the late Jurassic time ( Kimmeridgian ) , more red-beds 

 with anhydrite were deposited in the central parts of the geosyncline. 



At this stage in late Jurassic time an uplift formed the Coahuila penin- 

 sula extending southward across western Coahuila and eastern Chihua- 

 hua as far as the Parras basin of southern Coahuila (Kellum, 1944). See 

 map of Late Cretaceous, Plate 12. Coarse elastics that were deposited 

 marginal to it on three sides during latest Jurassic as well as Early Cre- 

 taceous time attest a fairly high topography to the peninsula. 



In the description of the Coahuila system, an orogenic belt was de- 

 scribed, part of which, at least, occupied the position of the later Coa- 

 huila peninsula. It appears that the folded and faulted Permian beds 

 were first intruded, then eroded, and then epeirogenically uplifted before 

 the Upper Jurassic beds were deposited in the area. See Plate 13. The 

 belt of volcanoes on the west that supplied much of the Permian sedi- 



440 



