RESUME OF STRUCTURAL GEOLOGY OF NORTH AMERICA 



10 



Gulf of Mexico had come into existence. Much salt was precipitated in an 

 evaporite sequence in the Mexican geosyncline as well as along the 

 northern part of the Gulf in Louisiana and Texas. 

 The interior of the continent was extensively emergent. 



Early Cretaceous (Plate 11) 



The Cordilleran geanticline widened and stretched from Rritish Colum- 

 1 bia to Mexico, and from eastern California to central Utah. A broad 

 branch extended across Arizona and central New Mexico into Texas. 

 'Further deformation is noted in northwestern Nevada. 



Basins sank greatly on the west in California, Oregon, and Washington. 

 Volcanism continued there from previous times. The geanticline was 

 flanked on the east by a trough of sedimentation from Alberta to northern 

 Utah into which clastic sediments were shed. The Ancestral Rockies 

 were buried save for a small island in central Colorado. The Mexican 

 ' geosyncline enlarged and sank over 15,000 feet. It received considerable 

 volcanic material from the west. The seas spread over the Coahuila penin- 

 sula to make it a platform, and were more extensive now over the 

 southern and western part of the country than at any previous Mesozoic 

 time. 



The Rocky Mountain sea merged with the Gulf of Mexico, and the 

 Gulf Coastal Plain sediments accumulated to an appreciable extent. Only 

 the northern part of Florida was emergent. It was otherwise a platform 

 and with the Bahama platform made up a large region of carbonate 

 deposition and slow subsidence. It bordered on the south with a volcanic 

 belt in Cuba where a carbonate facies on the north grades into a 

 volcanic facies on the south. 



Late Cretaceous (Plate 12) 



The Late Cretaceous was a time of widespread and intensive crustal 

 unrest along the western margin of the continent. The climatic phase 

 of the Nevadan orogeny occurred at the very beginning of Late Cre- 

 taceous time when most of the batholiths that characterize the belt were 

 intruded. Narrow basins subsided to considerable depths on the west 



margin of the belt where again volcanoes contributed some material. 



The Nevadan belt of orogeny became part of the broad Cordilleran 

 geanticline, along whose eastern margin strong uplift with thrusting 

 occurred. Floods of coarse conglomerate were poured into an adja 

 trough in Utah and western Wyoming, and in places thrust sheets over- 

 rode the elastics. 



The Late Cretaceous seas and deposits were even more widespread 

 over the Rocky Mountains and Great Plains states than those of the 

 Early Cretaceous, and the deposits were much thicker. East of the deep 

 trough in central Utah, only thin deposits had previously accumulated 

 under shelf sea conditions. Now sediments in excess of 5000 feet thick 

 collected over a wide area of the shelf. 



The Mexican geosyncline had shrunk and changed decidedly from its 

 Early Cretaceous form. A trough extending from southeastern Arizona 

 into northern Mexico contains much coarse conglomerate and volcanic 

 material. South of the Coahuila platform a deep east-west trough, the 

 Parras, sank and received over 15,000 Eeet of sediments, mostly lime- 

 stones and shales. 



Florida sank progressively through Late Cretaceous time, tilting south- 

 ward to a trough that centered in Cuba where some 10,000 feet of 

 carbonaceous sediments accumulated. As the carbonates thin northward 

 through Florida, they change into argillaceous and arenaceous facies. Tne 

 Atlantic margin of the continent was widely invaded, and a wedge of 

 sediments that thickens seaward was deposited. The sediments overlap 

 the Lower Cretaceous strata in most places. 



Cretaceous-Tertiary Transition (Plate 13) 



During the latest Cretaceous (Montanan) and earliest Tertiary (Paleo- 

 cene) the main structures of the Rocky Mountains of Canada and the 

 United States came into existence, and Plate 13 has been prepared prin- 

 cipally to show these features. The crustal unrest is known as the 

 Laramide orogeny. The Cordilleran geanticline was broadly deformed 

 with its eastern margin and the adjacent basin deposits of the Triassic, 

 Jurassic, and Cretaceous folded and thrust-faulted. Major overtlrrust 



