Studies in sedimentary provenance of the intramontane Granada Basin, Southern Spain.

The Granada Basin is an intramontane basin situated within the Betic Orogen of southern Spain, at the westernmost extension of the Alpine Orogenic Belt. The Basin was initiated in the early-mid Miocene and is still active today. The Granada Basin rests upon rocks of the metamorphic Internal Zones of the Betic Orogen, which have evolved in core-complex style, extending and uplifting since the late Miocene, a process which has profoundly affected the evolution of the Eastern margin of the Granada Basin. The sediments of the basin record some of this history, both in conglomerate clast composition and in syn and post-sedimentary deformation of the basin. Sediment was derived from the rocks of the Internal Zone rocks in the Sierra Nevada region. The basin flank has been uplifted progressively during the rise of the Internal Zones core-complex. This has resulted in the westward migration of the depocentre of the basin and the recycling of the eastern basin margin. The Granada Basin is therefore an ideal place to study the evolution of sediment composition, in relation to recycling during a single orogenic phase, and the evolution of maturity and the progressive loss of provenance signature during recycling.
Three conglomeratic fan formations are found on the eastern flank of the basin, which record a transition from marine to terrestrial deposition. In order of decreasing age these are the Dudar, Pinos Genil and Alhambra Formations. Despite a constant provenance, the maturity of the sediments increases with decreasing age. Conglomerate clasts become increasingly rounded and clast composition becomes more mature into the younger Pinos Genil and Alhambra Formations. Quartzose clasts become more common and labile schistose clasts increasingly confined to the smaller grain sizes. Sandstone composition, however, does not become more mature, but marginally less mature. Quartz content does not increase, and lithic component grains do not diminish, despite the increased effects of terrestrial weathering in the younger formations. This disparity between the different grain sizes may be explained by the continuing break-down of conglomerate clasts within the sediment, providing a primary source of sand-size detritus. Provenance indications from sandstone detrital modes are generally confirmed using established discrimination schemes, but in detail, the nature of the Granada Basin sands derived from a high grade metamorphic core-complex source may require the recognition of a distinct provenance type.
Major and trace element geochemistry indicates a general increase in compositional maturity with sandstones, as the quantity of SiO2 and other more immobile elements increase at the expense of mobile elements. These changes are generally consistent with weathering differences between the marine Dudar and terrestrial Alhambra Formations, but also with the possibility of sedimentary recycling. This increase in compositional maturity occurs in the sandstones despite the detrital mode maintaining its immaturity. SiO2 increase is not obviously related to increased quartz content. The geochemical provenance indication given by the sediments is a passive margin regime, incorrect in the present tectonic setting but, perhaps related to the depositional setting of the protoliths to the metasedimentary rocks in the Sierra Nevada source region.
Heavy minerals record an increase in stable species into the younger formations, consistent with increased maturity. Zircon, apatite, tourmaline and, surprisingly, epidote increase in abundance at the expense of garnet and amphiboles. However, for garnet and tourmaline intra-species chemistry reveals no compositional bias that can be related to sedimentary recycling or erosional and depositional weathering differences between formations. Epidote alone shows an increase in Fe poor examples in the Pinos Genil and Alhambra Formations compared with the Dudar Formation.
Isotopic dating of conglomerate clasts and basement rocks from the Sierra Nevada reveals a cooling history for the Internal Zones that reaches back to the late Cretaceous. Most ages fall between 30 and 10 Ma, with the peak of cooling between 18-13Ma. Cooling rates range up to 100˚C/Ma. The Alpujarride Complex, structurally above the Nevado Filabride Complex records an earlier (25Ma , 40Ar-39Ar) and slower cooling rate (35.7˚C/Ma.). There is a complex relationship between cooling in the Internal Zones and sedimentation in the Granada Basin. During the most rapid cooling, sedimentation in the Granada basin had either not begun or was shallow marine carbonate deposition, indicating that there was no relief in the Internal zones at the time. The coarse sedimentation in the Dudar Formation records the uplift and evolution of the Sierra Nevada core-complex, with rapid generation of relief and the exposure of the Nevado Filabride Complex to form the dominant sediment source for the eastern Granada Basin. However, the uplift recorded by the coarse sediments of the Dudar Formation is not directly related to the isotopic cooling ages recorded by this detritus. The cooling ages preserved in the detritus pre-date uplift and sedimentation by up to 10Ma. There is, in this case, a significant lag between source cooling, which was not directly associated with the production of sediment, and the uplift and erosion of the rocks in the Internal Zones of the Sierra Nevada.
On a wider scale, the prolonged history of Betic Evolution and the dominantly left-lateral movement at the boundary between the Iberian and African plates since the Jurassic, suggests that the Orogen may be best explained by invoking an important role for strike-slip tectonics. This may even account for the emplacement of the Internal Zones into the region, and the subsequent deformation of the External Zones.