Petrography, geochemistry and stable isotopes of carbonate rocks, Lower Cretaceous Alisitos Formation, Los Torotes section, Baja California, Mexico

Abstract

Petrography, stable carbon and oxygen isotopes, and major, trace, and rare earth elements geochemistry of limestones of the Alisitos Formation in the Los Torotes, Baja California were studied in order to document the elemental variations among Member C (MC) and Member E (ME) of the formation and to understand their diagenetic and depositional conditions. The major petrographic types identified are mudstone, wackestone and packstone. The limestones of the Alisitos Formation show significant variations in δ13C and δ18O values (+4.13 to +5.26‰; -14.17 to -6.84‰; respectively). The limestones from Member C (MC) show higher ΣREE content (23 ± 17, n=10) than Member E (ME) (11 ± 5.5, n=9) of the Alisitos Formation. The observed variations in ΣREE content in these limestones are due to the amount of detrital material present in them. The limestones from ME record seawater-like REE+Y patterns, whereas the MC records non-seawater-like signatures (i.e. significant enrichment of middle rare earth elements: MREE). Most of the analyzed samples from the Alisitos Formation show negative Ce anomalies (Ce/Ce*: 0.67 – 0.99, n=17). The limestones show both negative and positive Eu anomalies (Eu/Eu*: 0.58 ‒ 2.91, n=19). The positive Eu anomalies identified in many samples may be due to the influence of hydrothermal fluids or co-precipitation of hydrothermal Fe-sulfide. The limestones of ME exhibit seawater-like-REE patterns suggesting that most of the REEs present in them were derived from seawater. However, the limestones of MC were contaminated by detrital materials which effectively mask the seawater-like signatures. The detrital materials present in the limestones of MC were probably derived from the mafic to felsic source rocks.