19. 



LATE CRETACEOUS 

 AND EARLY TERTIARY 

 ROCKY MOUNTAIN SYSTEMS- 

 THE LARAMIDE OROGENY 



DEFINITION OF LARAMIDE OROGENY 



Geologists to date in the Rocky Mountains have discovered a succession 

 of dynamic events through late Mesozoic and Tertiary time. At first, a 

 single, rather violent orogeny was visualized, but now numerous uncon- 

 formities, coarse conglomerates, and structural relations attest a condition 

 of unrest in the general Rocky Mountain region from middle Mesozoic 

 to the present. The single and intense orogeny in the Rocky Mountains 

 ; which they visualized was called the Laramide Revolution, and this was 

 supposed to have occurred precisely at the close of Cretaceous time, or 

 at the beginning of Tertiary time. 



Some geologists advocate dropping the term Laramide because of 

 the many recognized deformational pulses and the current concept that 

 crustal deformation is continuous. Limits cannot logically be set, they 

 contend. The writer believes, however, that since the usage is so deeph 

 ingrained in the literature that it is better to try to define the term ar- 

 bitrarily, and furthermore, finds the attempt helpful and not confusing. 

 For the purposes of this book the following nomenclature will be used: 



Orogenic events during Eocene time — Late Laramide 

 Orogenic events during Paleocene time — Mid-Laramide 

 Orogenic events during Montana time — Early Laramide 



Any orogenic phases older than Montana or younger than Eocene 

 will not be called Laramide, and, where desirable, new orogenies will 

 be defined. The Cedar Hills orogeny of central Utah of Colorado age 

 falls in this category. The disturbances in late Mesozoic time were gener- 

 ally precursory to the climatic ones of the very Late Cretaceous or the 

 Early Tertiary. 



BELTS OF DEFORMATION 



Major Divisions 



The map, Fig. 19.1, has been prepared to show the mountain systems 

 of the Laramide orogenic belts. Two major divisions of the systems have 

 been pointed out in the literature, namely, a western composed of ranges 

 formed of the thick sediments of the Paleozoic and Mesozoic troughs, and 

 an eastern composed of ranges and intermontane valleys formed of the 

 shelf sediments and the crystalline basement complex (Fig. 19.2). The 

 generalization needs scrutiny from both a spatial and time aspect, else 

 a number of misconceptions will arise. This will be done in the following 

 several chapters. 



General Characteristics 



Thrust faults and folds are the most characteristic structures of the 

 Laramide Rocky Mountains. In the eastern division great asymmetrical 

 anticlinical ranges dominate. Those that have been uplifted so much 



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