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

Abstract

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 low-amplitude 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.