New Mexico Geological Society Annual Spring Meeting — Abstracts

The Jurassic to Paleogene stratigraphic succession of the Magdalena basin, encompassing the upper Magdalena Valley and Cordillera Oriental of Colombia, is similar in lithology and age to correlative strata of southwestern New Mexico and southeastern Arizona. This broad
similarity suggests an equivalent tectonic evolution, probably related to the breakup of Pangaea, for these now widely separated regions of South and North America. Early to Middle Jurassic continental arc magmatism, related to subduction along the western margin
of Pangaea, is recorded by thick intermediate volcanic and coeval intrusive rocks in both Colombia and southern Arizona. Upper Jurassic to Lower Cretaceous marine, continental and mafic volcanic deposits in southern New Mexico and Arizona record an extensional
basin, the Borderland rift system (= McCoy, Bisbee, and Chihuahua basins) associated with the opening of the Gulf of Mexico. A similar succession of marine strata with mafic intrusions is present in the Cordillera Oliental. Abrupt local changes in thickness of correlative units may indicate normal fault control of sedimentation. Neocomian strata are absent in the upper Magdalena basin and the Cordillera Oriental (Santander and Floresta massifs) and on some fault-bounded blocks of southwestern New Mexico.

In Early Cretaceous time, volcanism abruptly decreased and extension concluded in both areas, indicated by onlap of formerly active normal faults. A shift from continental to marine environments during Aptian-Albian transgression is recorded in both areas by siliciclastic and carbonate deposits. After the Albian, marine conditions prevailed until the Maastrichtian in Colombia, whereas in New Mexico and Alizona post-Albian to Coniacian marine deposits are present only north of the former rift basin.

Crustal shortening and attendant inversion structures, uplifts, shifting depositional loci and reversal of drainages took place in both areas beginning late in the Cretaceous. In the southwestern United States, upper Campanian alluvial and volcanic deposits indicate the beginning of the Laramide orogeny, attributed to flattening of the Farallon slab beneath North America. Magmatism asssociated with subduction, basin inversion and sedimentation in intermontane basins prevailed dUling the Late Cretaceous to Eocene. In contrast, the Late Cretaceous-Paleogene shift to continental environments in the Magdalena basin was slow and subtle. Accretion of oceanic terranes from Campanian to early Paleogene, associated with oblique convergence between the Caribbean and South Amelican plates, caused uplifts and coarse deposits restricted to the western part of the Magdalena basin. During the Paleogene, coarse alluvial deposits interfingered with minor coastal-plain deposits on a regional southwest-northeast trend. Abrupt changes in sandstone composition, facies, and thickness among correlative units also indicate the presence of uplifted massifs or blocks in the region of the present Cordillera Oriental east of the Magdalena Valley.

We infer that the Borderland rift and extensional basins of the Magdalena Valley and Cordillera Oriental formed as a once-contiguous lift system coordinate with opening of the Gulf of Mexico and proto-Caribbean basins. The north-trending lifts failed, whereas the Gulf of Mexico and proto-Caribbean opened, ultimately to form oceanic lithosphere in latest Jurassic and earliest Cretaceous time. Subsequent shortening of the failed rift basins, albeit for different reasons, caused their inversion and dramatic changes in depositional patterns at the end of the Cretaceous.