Geochemical Traces and Sequence Stratigraphy Analysis during the Upper Jurassic in Southern Iberia

Abstract

Near the region of relative movements between Iberia and Africa, the South Iberian margin was structured in epicontinental and epioceanic environments during the upper Jurassic. This fact determined favourable conditions to record traces of interactions between tectonics and eustasy. Ecostratigraphic and sequence stratigraphy analysis show evidences of this (Marques et al., 1991; Olrriz et al., 1991, 1993). During the Lower and lower Upper Kimmeridgian (Platynota to Acanthicum Zones) two depositional sequences have been identified, the lower one been tectonoeustatic and the upper eustatic in origin. The studied interval is represented by rhythmic successions of marly limestones and limestones 50-115m thick in which some intercalations of marls are significant but irregularly distributed in the Lower (lower and upper Platynota Zone) and Upper Kimmeridglan (lower Acanthicum Zone). Lithologic differences are related with paleogeographic location within the shelf system (onshoreoffshore gradient), bottom topography and local conditions favouring buildup growths (RodriguezTovar, 1993). Sequence stratigraphy characterization has been realized on the basis of facies analysis, stratal patterns, microfacies, grain size and mineralogy; skeletal components have been studied according with ecostrat igraphic approaches. The present research is focused on the geochemical analysis of seven sections which were selected in order to give a representative image of the heterogeneity of depositional conditions on the investigated shelf system. The studied sections belong to three sectors in the South Iberian paleomargin (East Algarve Basin, Central Prebetic and Eastern Prebetic) which are differentiated mainly according with subsidence, tectonic activity and their relative proximal-distal location. The studied deposits belong to typical relative proximal depocentres (mid shelf sediments?) where the chemical composition even was strongly influenced by the weathering of source areas, reflecting fluctuations that, in short-time intervals, evidence the relative heterogeneity and instability which characterize proximal water masses. We consider that, as a whole, the geochemical composition of the source areas was similar in the analyzed sectors of the South Iberian margin. Moreover the proven geochemical differences between sections are interpreted to be related with the local paleogeography and/or with variations in those factors controlling the physicochemical characteristics of the basins on which the sediments were accumulated.