Environmental significance of gypsum-bearing layers at the Lo Hueco paleontological site (Upper Cretaceous, Cuenca, Spain): petrography, fluid inclusions, and isotopic relations

Lo Hueco (Cuenca, Spain) is an upper Campanian-lower Maastrichtian Fossil-Lagerstatte that has provided more than 8,500 well-preserved macrofossils, including titanosaur sauropod dinosaurs. Although the facies and fossil record point to both fresh and brackish or marine water influences, a detailed study of the sulphate-bearing layers of the site through petrography, fluid inclusions, and isotopes has been undertaken to evaluate the possible marine influence. The two main sulphate units of the Lo Hueco site consist chiefly of bimodal micro-to meso-lenticular gypsum crystals that grew displacively in a clayey-carbonate sediment. The well-preserved lenticular gypsum crystals are primary, as demonstrated by the presence of the original twinning and the absence of hydration textures or anhydrite relicts. Primary fluid inclusions of the lenticular gypsum crystals indicate a vadose environment of formation, with salinities between 1,800 and 14,000 ppm, pointing to a brackish but non-marine environment. Furthermore, gypsum exhibits Sr-87/Sr-86 values between 0.708034 and 0.708120, which are higher than those from marine evaporites of Campanian-Maastrichtian age, indicating a clear influence of fresh water. Gypsum delta S-34 VCDT values (18.1 to 19.0 +/- 0.5 parts per thousand) and delta(OVSMOW)-O-18 values (11.0 to 15.2 +/- 0.5 parts per thousand), on the other hand, are typical isotopic values recorded in marine evaporites of this age. This apparent contradiction between fluid inclusion and Sr isotopic data is probably the result of some recycling from Upper Cretaceous evaporites. Based on all these observations, the sulphate-bearing layers are interpreted as probably formed in a near-coastal saline mudflat of a playa lake. As a whole, this study highlights the importance of combining different proxies when dealing with evaporites formed in brackish-water environments.