La Formación Almadich en la Sierra de Mariola: caracterización litológica, bioestratigráfica, geoquímica y mineralógica (Aptiense Inferior, Cordillera Bética, Alicante)

Abstract

The Almadich Fm. (lower Aptian) is a characteristic feature of the distal parts of the Prebetic Zone (External Zones, Betic Cordillera) and crops-out extensively in the so-called Prebetic of Alicante. It is a hemipelagic unit intercalated between the shallow carbonate platform materials of the Llopis Fm. (lower Aptian) and the Seguilí Fm. (upper Aptian). We have carried out a multidisciplinary study of the Agres section in the Sierra de Mariola (Alicante), including stratigraphy, biostratigraphy, geochemistry (C and O stable isotopes, trace and major elements, biomarkers) and mineralogy, embracing the uppermost part of the Llopis Fm. and the Almadich Fm. The biostratigraphic study included planktonic foraminifers and calcareous nannofossils, and integrated previous data on ammonites leading to a precise dating of the late early Aptian to early late Aptian interval. The early Aptian Oceanic Anoxic Event (OAE 1a) has been identified from the integration of the C-isotope stratigraphy and biostratigraphy. Our data indicate enrichments and oscillations in the redox-sensitive trace elements concentrations within the OAE 1a interval. Calcite is the dominant mineral, although quartz is abundant in some levels, and clays are minor components, composed of muscovite and illite. Biomarkers are dominated by n-alkanes, derived from marine and continental plants, isoprenoids (pristane and phytane) of marine origin, hopanes, derived from bacteria, and steranes, originated from eukaryotic organisms; lycopane is also locally present, and could indicate anoxic conditions during sedimentation. The data integration has allowed the recognition of the timing of events related to the establishment of hemipelagic conditions in the distal Prebetic during the Early Aptian: a first episode of environmental change that occurred during the deposition of the carbonate platform (Llopis Fm.) gave place to the deposition of the Capa de Agres unit, under more open marine conditions with a higher nutrient content in the waters. Subsequently the platform was affected by a drowning event, which was coeval with a tectonic pulse and the local emersion of the Capa de Agres unit. After the following transgression the Almadich Fm. was deposited. During the deposition of the Almadich Fm. the OAE 1a occurred, linked to a global environmental perturbation, which affected mainly oceanic environments. Nevertheless, this event clearly affected the distal parts of the Prebetic, as has been established. This study documents the interaction between regional and global factors controlling the sedimentation during the early Aptian in the Prebetic.