Siliciclastic sedimentation and sequence stratigraphic evolution on a storm-dominated shelf: the Lower Ordovician of the Central Iberian Zone (NE Jaén, Spain)

Abstract

The Lower Ordovician of the southernmost part of the Central Iberian Zone (southern Spain) is a 1000-m thick, transgressive, largely siliciclastic succession deposited on a shallow epicontinental shelf. Facies analysis reveals lithofacies representing a variety of depositional environments, ranging from foreshore-shoreface (Facies S 0), proximal to distal parts of the shoreface (Facies S 1, S 2), proximal to distal shelf (Facies H 1, H 2, H 3), outer shelfzone or continental slope (Facies M). The vertical arrangement of the facies is characterized by the presence of high-frequency regressive cycles (partial shelf progradational sequences), themselves interrupted by strata resulting from catastrophic phenomena (storms), producing higher order autocycles of transgressive–regressive character. Major regional unconformities and the vertical distribution of the facies delineate successive genetically related units in relation to relative sea-level changes. Two second-order megacycles have been distinguished. The first of them (MC1), Tremadoc?-Lower Arenigian in age, comprises one third-order sequence (Ar-1). The lower part of the second megacycle (MC2), deposited during the Arenig-Llanvirnian, displays a trangressive stratigraphic architecture. Within this megacycle, three third-order sequences are proposed (Ar-2, Ar-3, Lln-1), in which a transgressive systems tract and a highstand systems tract can be distinguished. Facies attributable to lowstand systems tracts are restricted to the base of the Ar-2 sequence, at the boundary between megacycles.