Isotopic chemostratigraphy and biostratigraphy of Lower Cretaceous Alisitos Formation (Punta China section), Baja California, Mexico

Abstract

Nine units of carbonate rocks are recognized, of which the highest five units are examined here under biostratigraphical and chemostratigraphical aspects in order to infer mid‐Cretaceous OAE intervals in Punta China section, Baja California. The biota in Units 5, 6, 7, and 8 show low diversity, while Unit 9 is characterized by a high diversity of microfossils. Benthic foraminiferal assemblage and rudist genera identified in Unit 9 indicate a lower Albian age. Lithofacies and fossil assemblages suggest that the depositional environment was a shallow marine shelf or platform adjacent to caprinid buildups. The limestones of the Alisitos Formation show both negative and positive δ13C values that range between −0.9 and + 7.5‰. Large variations in δ18O values are noticed in the limestone units (−7.9 to −2.2‰). The 87Sr/86Sr ratios of the bulk limestone samples vary between 0.706662 and 0.707305. Most of the limestones have lower 87Sr/86Sr ratios that may be the result of inclusion of less radiogenic strontium from the volcanic rocks. One limestone sample from Unit 7 has a relatively pristine 87Sr/86Sr ratio (0.707305) whose numerical age is 111.61 Ma. The carbon‐isotope curve reveals the presence of two episodes of oceanic anoxic subevents in the studied section. The lower part of Unit 9 has significant negative carbon isotope excursion (CIE) that correlated with the Paquier subevent. Similarly, another negative CIE is also observed in the upper part of Unit 9, which suggests a correlation with Leenhardt subevent.