Ciclos carbonatados submareales-perimareales de alta frecuencia del Aptiense inferior (Fm. San Esteban, cuenca Vasco-Cantábrica): registro de cambio climático

Abstract

El análisis detallado de facies de las calizas de la Formación San Esteban del Aptiense inferior de Cantabria ha permitido caracterizar 15 tipos de litofacies depositadas en ambientes submareales a supramareales. Las facies se organizan en ciclos somerizantes que culminan con superficies de exposición subaérea, evidenciado variaciones relativas del nivel del mar de alta frecuencia/baja amplitud. Se distinguen dos tipos de ciclos en función de las facies y naturaleza de las superficies que los culminan. Los ciclos Tipo-1 están dominados por facies submareales. La secuencia ideal comienza con rudstone bioclástico-intraclástico, seguido de wackestone de Chondrodonta, floatstone de rudistas, boundstone de rudistas y mudstone de milólidos. El techo es una superficie neta con bioturbación y marcas de raíces estilo pseudomicrokarst. Los ciclos Tipo-2 comienzan con floatstone oncolítico-litoclástico, seguido de floatstone de rudistas, packstone-grainstone de foraminíferos, boundstone de rudistas y finalmente rudstone oncolítico, mudstone fenestral y/o estromatolitos laminares a hemiesféricos. Culmina con rasgos subaéreos de desecación y conglomerados calcáreos de cantos negros con microestructura alveolar y rizolitos calcificados. Los ciclos Tipo-1 caracterizaron condiciones ambientales húmedas, mientras que los de Tipo-2 caracterizaron condiciones semi-áridas. La evolución vertical de los ciclos refleja una transición climática de tipo greenhouse a otra coolhouse tras el OAE-1a.

High-resolution facies analysis of Lower Aptian platform carbonates from Cantabria has allowed to characterize 15 lithofacies types, which reflect environmental conditions ranging from subtidal to supratidal. The facies are organized in shallowing-upward cycles capped by subaerial exposures, attesting for high-frequency and lowamplitude relative sea-level fluctuations. According to the facies types and nature of the subaerial capping surfaces, two types of cycles have been differentiated, characterizing respectively the lower and upper part of the succession. Type-1 cycles are dominated by subtidal facies. They ideally start with transgressive bioclastic-intraclastic rudstone, followed by Chondrodonta wackestone, rudist floatstone, rudist boundstone and finally miliolid mudstone. They culminate with burrowed and rooted surfaces that resembles to pseudomicrokarst. Type-2 cycles start with transgressive oncoidal-pebbly floatstone, followed by rudist floatstone, foraminiferal packstone-grainstone, rudist boundstone and ending eventually with either oncoidal rudstone, fenestral mudstone and/or laminoid to hemispherical stromatolites. These cycles are capped by subaerial desiccation and limestone conglomerates rich in black-pebbles with alveolar septal microstructure and calcified rhizoliths. Type-1 cycles developed under humid environmental conditions, whereas Type-2 cycles could have formed under semiarid conditions. The cycle stacking pattern may reflect the sedimentary evolution of the platform during the transition from greenhouse to coolhouse conditions after the OAE-1a.