Abstract
Abstract We present new geochronologic (magnetostratigraphy, fission‐track and 40Ar/39Ar radioisotopic dates, biostratigraphy) data constraining the age of three separate sequences and a composite section from Guanajuato, Mexico. Those data make this one of the most complete and precisely age‐calibrated sequences in North America spanning the Hemphillian/Blancan North American Land Mammal “Age” (NALMA) boundary interval, and the data further constrain the timing and pattern of the Great American Biotic Interchange (GABI). In total, 196 samples (77 sites) were used to construct the magnetic polarity stratigraphies, with eight fission‐track analyses and four new 40Ar/39Ar radioisotopic dates. The sections sampled are possibly latest Miocene to Late Pliocene in age, spanning about 2.7 m.yr. (∼5.5–2.7 Ma), from Chrons C3n.4n or C3r to C2An.1n. The radioisotopic dates and magnetostratigraphic correlations indicate the Hemphillian/Blancan NALMA boundary at Guanajuato in central Mexico is ∼4.7–4.8 Ma, and it pro...
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