GEOCHEMISTRY OF LOWER CRETACEOUS CARBONATE ROCKS OF NORTHWESTERN MEXICO FOR ASSESSING PALEOCEANOGRAPHIC CONDITIONS: AN OVERVIEW

Geochemical proxies are commonly used to interpret the causes and consequences of natural environmental changes over geological period. An attempt has been made here to infer the geochemical variations on the limestones of the Mural Limestone of Sonora and Alisitos Formation of Baja California that deposited under various depositional environments. The facies characteristic of the Mural Limestone and Alistios Formation suggest that the depositional environments varied from restricted shelf to open shelf. Major oxides of the limestones deposited in marginal marine environments show positive correlation with Al₂O₃, whereas limestones deposited in open marine environment reveal absence of such correlation. Similarly, the limestones from marginal marine setting show non-seawater-like REE+Y patterns, whereas open marine limestones reveal seawater-like REE+Y patterns. The important factor in preservation of seawater REE+Y pattern on ancient limestone is the isolation of the carbonates from the detrital materials, marine or diagenetic authigenic phosphates. The limestones show both negative and positive Eu anomalies. Positive Eu anomalies in marginal marine limestones are influenced geochemically upon terrigenous materials whereas such anomalies in open marine limestones are dependent upon the hydrothermal solutions. The magnitude of Ce anomalies is vary significantly between limestones deposited in marginal marine and open marine environments and such variations are induced by terrigenous materials and/or paleoredox conditions. The limestones deposited in marginal and open marine settings show significant variations in SREE, Al₂O₃, Zr, Sc, REE+Y patterns, and Y/Ho ratios that are influenced by the amount of terrigenous materials. Thus, the ancient limestones having little terrigenous materials deposited under open marine environment are served as reliable tool to understand the REE patterns of ancient shallow seawater and also they serve as a valuable seawater proxy.