Abstract Diploma-Thesis

TILMAN BUCHER (1997): The geology of the Sierra de Santa Casilada northwest of Monasterio de Rodilla (Burgos province, Northern Spain) - sedimentology, tectonics, paleosoils- (unpublished Diploma Thesis, Geology Department University of Freiburg, 99p.)

The studied area is situated along the N1-highway northwest of Monasterio de Rodilla (Burgos province, Northern Spain). An area with a size of 6 km² was mapped in a scale 1:10.000. This region between the Ebro - and the Duero basins is called the Bureba. The area is located at a contact between folded mesocoic sediments of the southern Basco-Cantabrian basin and undeformed sediments of upper tertiary age. The marine sediments of the upper Cretaceous were examined to reconstruct the facies changes in the southern part of the Basco-Cantabrian basin. Secondly, the tectonic structures in the mapped area were interpreted within the regional context. Furthermore paleosoils and well sorted ostracods in the tertiary sediments were examined.

The oldest rocks of upper Triassic age (Keuper) found are clays and gypsum in a sebkha-facies. In an extensional regime a horst and graben structure was formed during the upper Jurassic/lower Cretaceous. This lead to early diapirism and erosion of the jurassic sediments in the mapped area. Coloured sands and clays of the Utrillas formation (Alb) rest unconformably on the Keuper sediments. They formed a shallow, northward dipping ramp on which the northern Castillian carbonate plattform built up during the marine ingression in the Cenoman (upper Cretaceous). Several transgressive- and regressive cycles can be observed in the marine sediments of the upper cretaceous. Ammonite bearing marls mark the maximal transgression during the lower Turon, but most of the sediments where formed under shallow marine conditions. An initially low energy environment in a restricted plattform setting changed to a higher energy environment of an open plattform with high bioclastic input. The youngest marine sediments found are of upper Santonian age.

During the alpidic orogeny the mesocoic rocks of the Basco-Cantabrian basin in the foreland of the Pyrenees were folded and transported over the subsiding Ebro- and Duero basins. Old WNW-ESE-striking normal faults were reactivated and inverted as thrust faults. At the same time the evaporitic Keuper sediments were mobilised and rose in antiforms and along thrust planes. This lead to erosion and deposition of coarse clastic sediments in alluvial fans during the upper Eocene/Oligocene. These sediments were folded and show strong pedogenic overprinting, which can be seen in massive caliche crusts with well developed structures like microcodium.

Late E-W trending compressive structures in some cases seem to be the result of the diaprism. Others were formed during a late orogenic stage in the Miocene; the San Pedro high, a flower structure in the paleozoic basement below the Duero basin, blocked the southward transport of the mesocoic rocks of the Basco-Cantabrian basin and induced the escape and the thrusting of themapped area as part of the "Plataforma burgalesa" to the southeast along strike-slip faults.

During the Miocene the tectonic activity ceases. The existing relief is unconformably covered with clastic sediments of a distal alluvial fan together with lacustrine and evaporitic sediments in a playa setting. In the lacustrine succession a phenomenon of stacked ostracod carapaces was observed. Up to 8 shells are perfectly sorted and stacked into each other. Up to 5 closed carapaces stacked into another (both valves together!) were found. A high percentage of the population is affected by this phenomenon. A pure physical sorting process can therefore be excluded. What biologic and/or taphonomic processes participate cannot be said. This problem will be discussed in detail later.