Algarve Basin Research Articles

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

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.

Heat flow and deep temperature in South Portugal

Heat-flow density determinations were made in South Portugal. The region of study covers the Algarve Basin, the South Portuguese Zone, part of the Ossa Morena Zone and part of the Lusitanian Basin.

Stratigraphic implications of palynological studies on berriasian to Albian deposits from western and southern Portugal

One section in the Algarve Basin (southern Portugal) and several sections in the Western Portuguese Basin (Lisbon region) have been investigated palynologically with emphasis on the use of dinoflagellate cysts in stratigraphy. As a result, the stratigraphy of both basins can be defined more accurately. In the Western Portuguese Basin the existence of Jurassic and Late Valanginian-Early Hauterivian strata could not be established from dinoflagellate cyst associations. The existence of a hiatus comprising at least the Upper Barremian was confirmed but a continuous Late Aptian-Early Albian sequence in the basin could not be unambiguously identified. In the Algarve Basin the Upper Aptian is well-developed.

A review of Alpine tectonics in Portugal: Foreland detachment in basement and cover rocks

The Alpine foreland in Portugal was deformed by compressional tectonism during the Miocene. In the NNE-SSW oriented Lusitanian Basin, most folds and thrusts in the Meso-Cenozoic cover are oriented ENE-WSW, parallel to the Alpine front in the Betic Cordillera, and verge towards the north-northwest and south-southeast. The thrusts are connected by lateral ramps: most of these are oriented NNE-SSW to N-S and show sinistral movement, and some are transpressional. The lateral ramps result from reactivation of older extensional faults related to crustal thinning of the continental margin. In the E-W oriented Algarve Basin a simpler basin inversion occurred, with older E-W normal faults reactivated as essentially pure thrusts. In both basins Alpine structures formed above décollements in the Hettangian evaporite-clastic complex. Variscan basement was also deformed by ENE-WSW reverse faults during Miocene time. The similarity in orientation and style of the basement structures to those in the cover suggests that they also occurred by detachment, but their larger scale indicates that the detachment is deep and involves much of the crust. Thus, we interpret the Central Cordillera, in which basement rocks are thrust over Miocene sediments on both sides, as a “pop-up” of crustal scale, elevated above downward-flattening faults that dip towards each other and merge into a single deep detachment. Alpine structures in the Iberian foreland are therefore similar in structural style to those of the Appalachian and Laramide forelands of North America and the Alpine foreland of northwest Europe.