Constraints of applying strontium isotope stratigraphy in coastal and shallow marine environments: insights from Lower C
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RESEARCH PAPER
Constraints of applying strontium isotope stratigraphy in coastal and shallow marine environments: insights from Lower Cretaceous carbonates deposited in an active tectonic setting (N Iberian Basin, Spain) M. Isabel Benito1,2 · Pablo Suarez‑Gonzalez1 · I. Emma Quijada3 · Sonia Campos‑Soto1 · Marta Rodríguez‑Martínez1 Received: 28 August 2020 / Accepted: 27 October 2020 © Universidad Complutense de Madrid 2020
Abstract The Lower Cretaceous Leza Formation is an essentially carbonate unit deposited at the northernmost active margin of the Cameros Basin (N Spain) under an extensional tectonic regime. This unit is composed of freshwater, marine-influenced, marginal-marine and hypersaline marine carbonate facies, interbedded with variable amounts of alluvial deposits, mainly derived from the erosion of the Jurassic substrate. 87Sr/86Sr, δ18O and δ13C analyses were obtained from carbonate facies of the Eastern and Western sectors of the basin. δ18O values follow the expected trend in both sectors: they are more negative (down to − 7.9‰) in freshwater carbonates and more positive (up to + 2.8‰) in marginal-marine to hypersaline facies. However, independently of the seawater or freshwater influence, in the Western Sector the 87Sr/86Sr values (0.707373–0.707801) are significantly lower and closer to the published Lower Cretaceous seawater 87Sr/86Sr ratios, than those of the Eastern Sector (0.707988–0.709033), where the overall marine influence was relatively high and the alluvial input low. These data strongly suggest that 87Sr/86Sr ratios were mainly controlled by those of the riverine freshwater arriving to the coastal and marine areas after the weathering and erosion of the Jurassic carbonates or siliciclastic rocks, in the Western and Eastern sectors, respectively. Thus, data indicate that, in coastal and shallow marine carbonates, the influence of the riverine water on the 87Sr/86Sr ratios should be systematically evaluated. This is particularly necessary in active tectonic settings, where the uplifted areas are significantly prone to weathering and erosion and where alluvial fan systems commonly developed, eventually discharging into coastal and shallow marine areas. Keywords Cameros basin · Coastal wetlands · Carbon and oxygen isotopes · Water–rock interaction Resumen La Formación Leza es una unidad esencialmente carbonática del Cretácico Inferior depositada en el borde norte de la cuenca de Cameros (N de España) en un contexto tectónico extensional. Está formada por facies carbonáticas de agua dulce,
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s41513-020-00142-z) contains supplementary material, which is available to authorized users. * M. Isabel Benito [email protected] 1
Departamento de Geodinámica, Estratigrafía y Paleontología, Universidad Complutense de Madrid, 28040 Madrid, Spain
2
Instituto de Geociencias IGEO (UCM-CSIC), Madrid, Spain
3
Departamento de Geología, Universidad de Oviedo, 33005 Oviedo, Spain
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