The studied area is located in northern Spain (provinces of Burgos and Alava/Araba), north of the Ebro Basin. It includes the two Tertiary basins of Medina de Pomar and Miranda-Treviño (Fig. 1). The continental depositional environment consists of a complex, highly discriminated facies. In an area of 900 km² more than 20.000 m of sections were measured.
Aerial photographs, microfacies analysis, paleosols, paleolakes and tectosedimentary megacycles were used to correlate the outcrop sections, build a three-dimensional depositional model and reconstruct the facies changes and the paleogeographic evolution of the area under study. These results, in turn, allowed the analysis of the tectonic development of the southern Basco-Cantabrian Basin (uplift, subsidence, halokinesis, reactivated basement faults).
Twenty-two different types of lithofacies were identified and combined in eleven facies associations (FA):
The paleogeographic reconstruction suggested the presence of four tectonic stages (Fig. 2): In the Middle Eocene subduction in the Biscaya area ended in a continent-continent collision between Europe and the Iberian Peninsula. The tropical Ilerdian-sea retreated to the northwest, resulting in the formation of an early local braided plain (Fig. 2-A and B). On top of this plain, in the marginal trough of the diapir of Salinas de Rosío a large, deep, meromictic, hydrological closed lake developed. A marginal fan delta was formed by the rising of the diapir of Salinas de Rosío. Finally the progradation of the lake margins led to the deposition of a regressive shallowing-upward sequence (Fig. 2-C). The vegetation of the banks was dense and the climate was warm and subhumid. The oscillation of the coastlines indicates pulsating halokinesis and climatic variations (stage I: tectonically induced halokinesis).
At the Eocene-Oligocene boundary the final uplift of the Pyrenees took place. At the same time the Ebro-High inverted into a foreland basin (Riba et al. 1983). The two basins were detached from the basement and overthrusted on Keuper sediments as piggy-back basins over the subsiding Ebro Basin. During uplift and folding of the marginal mountain ranges nine alluvial fans developed at the basin margins, draining transversally towards the center of the basins (Fig. 2-D). The thickness of these sediments reached up to 1500 m. The diapirs in the basin center developed their own fluvial systems, draining in the opposite direction towards the basin margins (stage II: first uplift and folding stage, continuous movement).
During the Eocene and Oligocene the climate changed in the Ebro Basin from subhumi to semiarid and arid. Paleosols are better developed in the southern alluvial fans than in the northern ones. This observation indicates the presence of a more humid climate in the south than in the north. Consequently the formation of the alluvial fans was diachronous, with the earliest uplift beginning in the southern mountain ranges (collision zone Ebro Basin/marginal piggy-back basins).
During the Oligocene the area was characterized by a braided river system. The great thickness of overbank deposits indicates high subsidence. Drainage was longitudinal, parallel to the basin axes, eastwards. This "Paleo-Ebro" incised into the rising Sobrón-anticline. Only the northern alluvial fan of Quintanilla la Ojada remained active near the basement fault of Losas. In the basin centers ephemeral, holomictic, shallow lakes of different sizes developed. In the Miranda-Treviño Basin a saline ridge separated the lakes, which were either low-energy swamps or ponds, or high-energy, hydrological open lakes with an onkoid-algae-flora. Climate was semiarid (Fig. 2-E). A new uplift and folding phase, which occurred in the Lower Miocene, caused renewed deposition of coarse gravel units with a thickness of up to 250 m (stage III: second uplift and folding stage) (Fig. 2-F). These sediments were finaly tectonically tilted and folded (stage IV: Upper Miocene ?).
The presence of different megacycles in the alluvial fans reflects an overlapping of compressional tectonics (resulting in variable uplift in the hinterland and subsidence in the basin), a reactivation of basement faults (basement fault of Losas) and local halokinesis (diapirism, saltpillows). Only the basinwide correlated basal prograding cycle, exposed in nearly all alluvial fans, could be interpreted as controlled by climatic changes. En-echelon folds in the marginal mountain ranges, basement faults, and a salinar decollement level are possible indicators of strike slip movements. At the southern margin of the Miranda-Treviño-Basin halokinesis (and strike slip faults ?) caused massive normal faulting in spite of a general compressive tectonic framework. Basin subsidence was high.
The clastic deposits include a great variety of paleosol features: prismatic calcrete, powder-calcrete, concretions, septaria, pisoids, root-concretions, root-casts, Microcodium, textural inversion, mottling, different vugs, peloids, alveolar-texture, corroded grains, and peds. Pedogenic superimposition of lacustrine-palustrine units is indicated by pseudomicrokarst, Microcodium, pisoids, different vugs, mottling, and prismatic calcrete.
In the area under study four different paleosol units were distinguished. The first pedogenic superimposition is exposed at the lake margins of the diapiric rim syncline. The lake finally emerged and the second pedogenic unit was formed in the overlying distal floodplain sediments with more than 70 different horizons of calcrete glaebuls and septaria. The third paleosol unit is exposed in the allmost basinwide prograding basal unit of the alluvial fans. Stacked calcrete horizons, with a lateral extension of more than 10 km, were locally found above this unit (fourth paleosol unit). The main part of the overbank deposits, developed in inactive zones of the distal floodplain, contain inmature, unstratified paleosols. It was not possible to establish correlations between these paleosols.
Different paleosol stages were differenciated in siltstones, sandstones, conglomerates and lacustrine limestones. In mudstones and fine grained sandstones all transitions between unstratified soils with poorly developed grey and coloured mottling and massive calcretes, more than 4 m thick, are exposed. On the opposite, in coarse grained sandstones and conglomerates the connecting link between mature and immature pedogenic units is missing.
Paleorelief and paleosol stages show a clear relationship (Fig. 3). In the inner fan zones no soils can be formed because of the strong erosion and high sedimentation rates. In the middle fan area weakly developed unstratified soils are exposed. In inactive fan zones massive calcrete was rarely developed. In the basin centers widespread paleosols (concretions, septaria and mature calcrete) were formed, frequently associated with paleolakes. The investigated area is characterized by paleosols, developed under subhumid and semiarid regimes. The existence of a paleoclimatic change from subhumid to semiarid and arid during the Eocene-Miocene period can be proved. Foraminifers in sandstones and conglomerates (up to 15 % in thin sections) were interpreted as reworked marine faunas of the Lower Tertiary and Upper Cretaceous. In spite of an aquatic transport in high-energy, fluvial channels, they suffered no destruction. Marine foraminifers of the Upper Cretaceous and Lower Tertiary, found in continental sediments of the Oligomiocene, demonstrate the possibility of facial and temporal misinterpretation of continental sediments.
Key words: Ebro-Basin, continental Tertiary, paleosols,
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