Geology of Mexico: An Overview

Donald P. McGookey

Physiography
A considerable part of Mexico has elevations above 2000 meters. There are four large mountainous trends: Sierra Madre Occidental, Sierra Madre Oriental, Trans-Mexican Volcanic Belt (TMVB), and Sierra Madre del Sur.

Plate Tectonics
Mexico is a patchwork quilt of microcontinents (terrains) that accreted to the North American continent from north to south starting in the late Paleozoic and continuing into the early Cenozoic. In total the blocks comprise an isthmus between the Pacific Ocean and Gulf of Mexico. They are separated from the continent by an east-southeast left-lateral wrench system that extends from southeast California to the east coast called the Mojave-Sonora Megashear. Exposures of the shear zone are poor because of widespread cover by Tertiary extrusives and younger valley-fill. The existence of this feature is not accepted by everyone, but considerable scattered evidence argues for the interpretation.

Magmatism
As the blocks docked and moved southwest with the rest of the continent, they over road the east side of the Pacific oceanic crust. This subduction resulted in continuous magmatic activity during the Mesozoic and Cenozoic that has migrated from north to south. Along the west side of the Sierra Madre Occidental trend there are exposures of a Jurassic-Cretaceous batholith. This mountain trend is primarily a very thick pile of Late Eocene to early Miocene extrusives dominated by Oligocene ignimbrites. The current subduction underlies the Sierra Madre del Sur and generates magmas responsible for the Trans-Mexican Volcanic Belt and the Central America volcanic arc (El Chichon).

Gulf of Mexico Basin
As South America moved south during the breakup of Pangaea the Yucatan block was pulled away from the southeast part of Texas forming a large basin with an oceanic crust, but no evidence of a sea-floor spreading ridge. The movement of the Yucatan block apparently happened in two stages. An early Mesozoic opening formed the basin. This was the basin at the time of mid- Jurassic salt deposition. An additional late Mesozoic movement is indicated by a large central area that is salt-free.

Mining
The rich copper deposits of Arizona continue south into the state of Sonora. If you are looking for the Treasure of Sierra Madre, I suggest you look there. In the northeast part of the country there are a number of Mississippi-valley type mineral deposits (fluorite, lead, zinc and barite). There is very little coal.

Oil and Gas
In the Gulf of Mexico basin there are five megagiant (over 5 Billion BO) fields. One is in east Texas and four are in Mexico. The Golden Lane fields along the west edge of the Gulf were discovered in the early 1900’s by drilling seeps. These fields produce from Jurassic-Cretaceous reefs that fringe a large carbonate bank similar to the Horseshoe Atoll of the Midland Basin. In a deep trough west of the bank debris from the reefs formed a thick pile that resulted in the Posa Rica fields. The Reforma areas fields of southeast Mexico are along the east side of a salt basin. The large A. J. Bermudez field developed during the 1970’s produces from Cretaceous carbonates in a highly faulted anticline that drapes over a salt pillow. The last megagiant field is offshore west of the Yucatan peninsula along the east side of a large deep water salt basin. It extremely large (over 15 billion BO) and bizarre. It resulted from Mid-Tertiary overthrusting of Mesozoic carbonates. The main pay is a 1000 meter thick section of breccia at the top of the Cretaceous that was originally deposited on the slope west of the peninsula.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009