Geochemistry of the Mural Formation (Aptian-Albian) of the Bisbee Group, Northern Sonora, Mexico

The elemental content (major, trace and rare earth elements) of 35 Aptian-Albian limestone samples from the Mural Formation has been determined to provide information on depositional conditions and provenance. The limestones of the Mural Formation show large variations in terrigenous and carbonate contents (1.2 to 423% and 57.7 to 98.8% respectively). Small variations are observed in CaO concentrations in the Tuape Shale, Cerro La Puerta and Mesa Quemada members whereas large variations are found in the Cerro La Ceja, Los Coyotes and Cerro La Espina members. The majority of the limestones show high values of Th. Sc and Zr. Large variations in Sigma REE content are observed among different members of the Mural Formation. Most limestones from the Mural Formation record non-seawater-like REE+Y signatures. The limestones show large variations in Ce anomalies which may be due to mixing of sediment components (biogenic and authigenic phases) and detrital materials including Fe-colloids from fluvial input. Most of the limestones show positive Eu anomalies, but some samples show negative Eu anomalies (Eu/Eu*: 0.42 to 2.62). The large variations in terrigenous percentage, high Al2O3 and Sigma REE contents, high La-N/Yb-N ratios, low Y/Ho ratios and non-seawater-like REE patterns suggest that the observed variations in Sigma REE contents are mainly controlled by the amount of detrital sediments in the limestones of the Mural Formation. The limestones of the Mural Formation were deposited under both coastal and open shelf environments, and they exhibit non-seawater-like REE + Y patterns. The presence of terrigenous materials in these carbonates as contaminants effectively masks the seawater signature due to their high concentration of the REE. Thus, trying to decipher the palaeoceanographic conditions represented by ancient carbonate rocks should be done cautiously since limestones deposited under open marine environments may also be contaminated by some amount of terrigenous particles. The presence of small quantities of terrigenous materials in the limestones can also reveal source rock information. The La/Sc, La/Co, Th/Sc, Th/Cr, Th/Co and Cr/Th ratios suggest that the terrigenous materials present in the limestones were mainly derived from a nearby exposed basement of intermediate to felsic igneous rocks. (C) 2010 Elsevier Ltd. All rights reserved.