LARAMIDE OROGENY

a series of mountain-building events that affected much of western North America in Late Cretaceous and Early Tertiary time (the Cretaceous period ended 66.4 million years ago and was followed by the Tertiary period). Evidence of the Laramide orogeny is present from Mexico to Alaska, but the main effects appear centred in the eastern portion of the Cordilleran Geosyncline from southern Nevada to the Northern Rockies and Northern Cordillera in western Canada, in the Central Rockies of Montana and Wyoming, in the Southern Rockies of Colorado and New Mexico, and in southern Arizona, southwestern New Mexico, and northern Mexico. The evidence consists of great eastward-directed thrust faults and folds with only slight basement involvement in the eastern portion of the Cordilleran Geosyncline from Nevada northward to British Columbia; initial vertical uplift accompanied by the development of flanking, coarse clastic basin sediments and unconformities in the Central and Southern Rockies; and acidic plutonic intrusions ranging from 50,000,000 to 70,000,000 years in age that are much smaller in bulk than those that accompanied the Nevadan orogeny, with the exception of the portion in southern Arizona. Clastic wedges that were derived from Laramide uplifts in the Cordilleran Geosyncline were shed eastward into parts of Wyoming and Utah. The Laramide orogeny originally was believed to mark the CretaceousTertiary boundary. It is now considered to have been a polyphase orogeny consisting of many disparate pulses of deformation that varied in intensity and age from place to place in western North America. Events ascribed to the Laramide range in date from Late Cretaceous to as late as Oligocene time (the Oligocene epoch occurred from 36.6 to 23.7 million years ago). Laramide igneous intrusions, however, are generally centred around the Cretaceous-Tertiary time boundary.