Origin of Faults and Extension Fractures in Cretaceous Carbonates, Northeast Mexico

By

HOOKER, John, MARRETT, Randall, and LAUBACH, Stephen

The University of Texas at Austin

The timing of extension fractures and faults can be problematic, particularly in a fold-thrust belt. The temptation exists to associate all structures in the rocks with the period of the most intense deformation (tectonic folding). However, fractures also commonly develop during post-tectonic exhumation. The additional possibility that fractures and faults developed prior to tectonism commonly is ignored.

The Cretaceous carbonate systems of the Sierra Madre Oriental in northeastern Mexico provide faults and fractures related and unrelated to tectonism. The fractures there are associated with isoclinal folds in basinal facies strata, and are kinematically linked with layer-perpendicular stylolites, suggesting the fractures formed during folding. Yet within isoclinal folds in shelf facies strata, several lines of evidence suggest that most fractures formed during early burial diagenesis. These fractures are concentrated in necks between soft-sediment boudins, have anomalously high kinematic apertures for their lengths, are crosscut by shallowly developed solution-collapse features, and locally contain sediment. The faults in the region extend bedding, but because bedding is now vertical, their hanging walls have moved up with respect to their footwalls. Therefore they make kinematic sense in association with either extension due to early gravitational spreading or thrusting related to tectonic folding. In faults studied, changes in bed thickness have been found, suggesting that they are syn-depositional growth faults. Also, their extension directions, in restored bedding, extend roughly perpendicular to the margin of the Coahuila carbonate platform, parallel to its slope. These observations suggest much of the faulting and fracturing occurred prior to tectonic folding.