NEOPROTEROZOIC DIAMICTITE-CAP CARBONATE SUCCESSION FROM EASTERN SONORA, MEXICO

Glacially derived diamictites that are abruptly overlain by unique carbonate strata (cap carbonates) are a common component of Neoproterozoic stratigraphic successions worldwide. While these features have been noted in Neoproterozoic successions throughout western and eastern North America, glacial diamictites have not been reported south of Death Valley, California, despite well-exposed Neoproterozoic successions in Sonora, Mexico (e.g., the Caborca section). Here, we report the discovery of a thin diamictite-cap carbonate succession from eastern Sonora, near Sahuaripa, Sonora, Mexico. The diamictite is found at the base of the section in fault contact with Mesozoic rocks. Only 20 meters are exposed (with the top 2 to 4 m well exposed), so the original thickness of the diamictite is currently not known. It has a brown fine-grained matrix with outsized carbonate and granitic clasts of unknown affinity; the upper meter consists of finely laminated mudstone with lone-stones (interpreted here as dropstones). The dropstone layer is abruptly overlain by a very finely laminated dolostone unit (~30 m thick) that may represent the basal Mina el Mesquite Formation.

Carbon isotopic chemostratigraphy of the finely laminated dolostone above the diamictite reveals a strong negative anomaly (-5 per mil PDB) that is characteristic of cap carbonates worldwide. Carbon isotopic values rise to +10 per mil PDB over the next ~400 m of section in overlying carbonate strata. The pattern recorded here is thought to be most characteristic of post-Sturtian time (post ca. 700 Ma); post-Sturtian (but pre Marinoan, ca. 590 Ma) carbon isotopic values commonly attain values greater than +8 per mil. Thus, we tentatively conclude that the diamictite is Sturtian in age (ca. 750-700 Ma) and may correlate with the lower carbonate clast diamictites in the Kingston Peak Formation, Death Valley, California. If so, the base of the Sahuaripa succession may pre-date the formation of the Neoproterozoic-Cambrian Cordilleran Miogeocline.