Geochemistry of Lower Cretaceous limestones of the Alisitos Formation, Baja California, Mexico: Implications for REE source and paleo-redox conditions

Measurement of the major and trace elements were carried out on the Lower Cretaceous limestones interbedded in the volcano-sedimentary Alisitos Formation, northwestern Mexico to understand the source of rare earth elements (REEs) and paleo-redox conditions. The five limestone beds (from the base up, Unit 5 to Unit 9) of the Alisitos Formation show large variations in SiO2 content (0.9-27.9%). A low concentration of CaO is observed in Unit 6 and Unit 8, and high content of CaO is observed in Unit 5, Unit 7 and Unit 9. The limestones are depleted in many trace elements with respect to Post-Archaean Australian Shale (PAAS), whereas Sr shows slight enrichment when compared to PAAS. The concentrations of FREE are higher in Unit 6 and Unit 8 (37.4 +/- 7.5; 46.6 +/- 19.4; respectively) than Unit5, Unit7, and Unit 9 (9.1 +/- 3.2; 11.3 +/- 9.4; 4.2 +/- 2.5; respectively). The limestones of the Alisitos Formation show a non-seawater-like REE + Y pattern with positive Eu anomalies relative to PAAS (0.95-2.47). Variations in FREE, Al2O3, Zr, Sc, REE + Y patterns, and Y/Ho ratios are influenced mainly by the amount of terrigenous materials. The variations in the Eu/Eu*, La/Sc and La/Co suggest that the terrigenous materials included in the lower four limestone beds (from Unit 5, Unit 6, Unit 7 and Unit 8) were likely contributed by intermediate to felsic rocks whereas terrigenous materials from Unit 9 were derived from mafic to intermediate source rocks. The slightly negative to slightly positive Ce anomalies in the studied limestones resulted from variations in the bottom water oxygenation. This was also corroborated by V/Cr and Ni/Co ratios suggesting that the depositional environments experienced large fluctuations in oxygenation conditions ranging from oxic to anoxic conditions during the deposition of limestones of the Alisitos Formation. (C) 2015 Elsevier Ltd. All rights reserved.