Palaeoenvironmental inferences of a Mexican Konservat-Lagerstätte (Tlayúa Quarry; Lower Cretaceous) based on the geochemistry of rare earth elements

Abstract

The Tlayúa Quarry is considered one of the most important Cretaceous outcrops in Mexico. Its well-preserved palaeobiota has allowed this outcrop to be considered a Konservat-Lagerstätte. Geochemical proxies such as major, minor and trace elements were evaluated from selected Zones of the Tlayúa Quarry, in order to infer some features of the depositional environment in which Tlayúa limestones originated. Aluminum, Thorium and Zirconium concentrations in addition to a low Phosphorous content evidence little terrigenous contamination, as well as a phosphate-free environment. Additionally, proxies such as Fe vs ΣREE, Mn vs ΣREE, Al vs ΣREE, Fe vs Nd_SN/Yb_SN, Al vs Nd_SN/Yb_SN, and Ce/Ce∗ vs Eu/Eu∗ show low correlation values (r² less than 0.25 in all cases with n = 128 and p < 0.05), thus suggesting a negligible influence of diagenetic effects or terrigenous contamination. These conditions are necessary for carrying out reliable palaeoenvironmental interpretations. Rare Earth Elements + Yttrium (REE+Y) concentrations were normalised to the Post-Archean Australian Shale standard to evaluate REE+Y patterns and anomalies. Following traits were identified: a slightly enrichment of heavy REE relative to light REE and moderate REE; positive La/La_SN, Gd/Gd_SN and Y/Y_SN anomalies; negative/positive Ce/Ce_SN anomalies; superchondritic and chondritic Y/Ho ratios. Here, we described that most of our limestone samples show the influence of marine waters and well oxidising conditions. However, freshwater signals that have been reported for some regions of the Tlayúa Quarry were also verified. Geochemical data are in agreement with the depositional environment evidenced by microfacies.