High-Resolution Paleoclimatic Insights from Oncoid Lamination: Preliminary Results from Eds, Ebsd and Sims (campanian, Allueva Formation, Spain)

The Allueva Formation (middle–upper Campanian, Upper Cretaceous) is an up to 500-700m-thick continental succession formed during the initial stages of Alpine tectonic that gave rise to the Iberian Ranges of NE Spain. While this unit is predominantly composed of terrigenous alluvial deposits, it locally contains an up to 50m-thick lacustrine to palustrine carbonate stratigraphic interval that include several oncoid-rich horizons. Previous petrographic and isotope-ratio mass spectrometry (IRMS) studies on these oncoids allowed to identify distinct alternating laminae patterns of micrite and sparite, but reported minimal isotopic variation (<1‰) without identifiable cyclicity. However, if humid-arid alternations controlled oncoid lamination, the apparent isotopic homogeneity may reflect insufficient analytical resolution rather than a true absence of cyclicity. This study applied a multi-proxy approach to address this question by integrating the following high-resolution analytical techniques: (1) Energy Dispersive Spectroscopy (EDS) to map the spatial distribution of key elements; (2) Electron Backscatter Diffraction (EBSD) analyses to characterize crystallographic properties, crystal size distributions, and orientation patterns; and (3) Secondary Ion Mass Spectrometry (SIMS) to acquire micrometer-scale stable isotope measurements (δ13C and δ18O) across individual sub-laminae. EDS data reveal cyclic variation in silica content in micritic layers, indicating periodic detrital input during wetter intervals in contrast to the crystalline layers where detrital elements are lacking. EBSD analysis further supports this observation by showing randomly oriented and highly misoriented micritic crystals, likely caused by incorporation of organic matter during crystallization, which is consistent with biologically induced precipitation during wetter intervals. In contrast, crystalline layers are highly co-oriented and show minimal misorientation attributable to crystal merging during diagenesis, suggesting abiotic precipitation during drier intervals. SIMS data yield mean values of -5.06‰ for δ18O (σ=0.99‰) and -5.61‰ for δ13C (σ=1.33‰), consistent with previous IRMS studies, but appear to reveal cyclic patterns that were previously unidentified. The integration of these techniques provides a robust multi-proxy framework, potentially revealing high-resolution climatic fluctuations that affected the formation of these oncoids in the Iberian Ranges during the Campanian.