Review of the times of coal accumulation in Mexico with special emphasis on the Sabinas basin, Mexico, where geochronological data are provided

Abstract

Different periods of coal accumulation have been identified in time and space in Mexico. Some examples are the Triassic of Sonora in the San Marcial coal region, the Middle and Late Jurassic in the Tlaxiaco basin, the Lower Cretaceous in the Cabullona Basin in Sonora and the Upper Cretaceous in the Chihuahua and Sabinas Basin (object of the present work).  In the Tertiary, there are several manifestations of coal, the most important are those of the coal region of Colombia-San Ignacio, States of Tamaulipas, Coahuila, and Nuevo León.

The Sabinas Basin coal region contributes more than 90 % of the national coal production. The Sabinas Basin has an area of 37000 km², and its origin is related to the opening of the Proto-Gulf of Mexico and its "tectonic closure”, with the Laramide Orogeny.  The Basin is being delimited by structural highs such as the Coahuila Peninsula, the Burro-Peyotes and the Tamaulipas arch, and faults such as La Babia and San Marcos. Within this basin is the Olmos Formation, also known as the “Coal Formation”, whose origin goes through two formation periods: 1) a marshy environment where the flora was abundant and allowed the deposit of peat with variable thicknesses and quality; 2) a continental and transitional environment: upper intertidal, marginal lagoon, swamp and alluvial plains. Paleobotanical studies of pollen and spores have defined it as a deltaic system of the Upper Cretaceous in a subtropical climatic environment. Due to its stratigraphic characteristics, structural position, spatial distribution and its production of coal, the Olmos formation is a favorable play for gas.

The coal mantles in the Sabinas basin present a “guide-layer” of volcanic tuff that allowed us to date the time of coal accumulation by U–Pb method in zircons. These minerals are refractory, so the age of deposit is not modified at temperature of diagenesis during the transformation of organic matter to coal. The age determined by the U–Pb method in zircons is 76.1 ± 1.2 Ma. This data is consistent with the regional magmatic activity of the area. The subduction to the west of the study area was active from the Cretaceous until the Tertiary with the generation of intrusive and effusive magmas known as the Mexican Laramide Magmatic Arc, which made up the Sierra Madre Occidental. The Laramie magmatic Arc is one of the largest volcanoclastic provinces in the world, and numerous age data have been reported similar the reported in the present work. So, the Sabinas basin, and its coal seams, are a good reference for the combination of geochronological research methods such as paleobotanical and radioactive decay geochronological methods.