The stratigraphy and age of a sauropod nesting ground containing the first definitive embryonic remains of sauropods preserved inside their eggs is analyzed. The fossil locality, called Auca Mahuevo, occurs in the Anacleto Member of the Río Colorado Formation in Neuquén Province, Argentina. The 5 m thick interval of overbank mudstones containing the fossilized eggs and embryos occurs near the middle of a 35 m sequence of thin, fluvial, concretionary sandstones and thicker units of silty sandstone. Flooding of shallow stream channels deposited overbank silt and mud on the eggs, killing the embryos and initiating the process of fossilization. Egg fragments containing patches of fossilized integument were found as float weathering out of the mudstone on local flats. Complete eggs containing embryonic bones and teeth were quarried from a steep ridge where the mudstone was exposed.
Twelve paleomagnetic samples collected throughout the lower 30 m of the section establish the presence of a Reverse geomagnetic polarity interval. This constitutes the first magneto-stratigraphic characterization for this part of the Río Colorado Formation and for the late Cretaceous sequence of formations that comprise the Neuquén Group. Biochronologic age estimates for the Río Colorado fauna combined with the Reverse polarity determinations for the fossiliferous sediments in the Anacleto Member argue for an age younger than the long Cretaceous C34 Normal, which ends at the upper boundary of the Sartonian and older than the late Campanian. The Reverse interval containing the fossils at Auca Mahuevo is therefore considered to be early or middle Campanian in age, most likely correlative with C33R between 83.5 and 79.5 Ma.