General Structural Features and Structure of Oil Fields
Two classes of folding are found in the Tampico Embayment and its western margin, and in the territory north of it. One type of folding (the Sierra Madre) consists of severe plications. The other type (mountain ranges of the coastal plain) consists of gentle folds more or less symmetrical in character.
Sierra Madre Oriental
The Cretaceous rocks of the Sierra Madre Oriental lie in a zone of considerable plication. Asymmetric anticlines with a steep east flank are common. In the second range (westward) the eastern limb of the folds is vertical in many places, for example, the Sierra Nicolás Pérez. In the third range, Sierra Colmena at Mícos (section, Fig. 11; and Frontispiece). the axis of the fold is tilted towards the east. The pressure that caused the folding came from the west; the observations given indicate a lessening of intensity eastward. Baker (5, p. 424) mentions the commonest type of structure in the mountains west of Pánuco as double recumbent fan-shaped folds. He also states that thrust faulting is associated with both fan-folds and isoclines. The strike of the Basement Complex (according to Baker) in El Novillo Cañon is N. 40º W. and the dip is steep toward the northeast. This is the strike of the crystalline metamorphic rocks, only.