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The growth of magnetite into a stable single-domain structure allows a remanent magnetization to be acquired, resulting in a chemically remagnetized unit (Hirt et al., 1993; Mc Cabe et al., 1989). (1998) presented evidence of a strong or detectable chemical remagnetization in the limestone-marl sequences of their study area, the Vocontian Trough (southeastern France). The occurrence of illite is associated with chemically remagnetized rocks in these studies, and the presence of smectite is associated with primary magnetizations or comparatively weaker chemical remagnetizations. Illitization from smectite or illite precursors is common in naturally deformed rocks (e.g., Vrolijk and van der Pluijm, 1999), offering the potential for radiometric dating of deformation (van der Pluijm et al., 2001).Cored samples were cut to 2.2 cm length with a dual bladed saw at the University of Michigan.Broken samples were glued back together with alumina cement and all specimens were labeled with Velvet underglaze nonmagnetic temperature-resistant paint.The studied clay samples in central Mexico were collected along the same Aptian–Albian shale horizon and in all cases these samples provided well-defined Ar-Ar illite ages that were younger than deposition.This is in good agreement with textural observation showing only one generation of authigenic illite in these rocks (Fitz-Díaz et al., 2014).