Abstract
Cores from 190 mechanic boreholes that cut through the Miocene deposits of Montjuic mountain (Barcelona) were studied. Part of these deposits are well lithified (sandstone, conglomerate) while other parts are not (marl, sand and gravel). There is a significant lateral and vertical gradation between the lithified and non-lithified parts. In the eastern sector of Montjuic, these deposits form a 100 m thick sequence made up of 7 coarsening-upward cycles of progradant deltaic origin. Below these cycles there is a massive alluvial unit that extends downward until it reaches a Paleozoic basement made up of metamorphic rocks identified in the subsoil of the Barcelona harbour. In the western sector of Montjuic, the boreholes cut through a 80 m thick sequence made up of 4 fining-upward cycles of retrogradant alluvial origin with minor marine deposits at the top. This sequence is developed over the continental deposits of the Early Miocene (Burdigalian) found in the subsoil of the western edge of Montjuic. The deltaic sequence of the eastern sector corresponds to a higher stratigraphic position than the alluvial sequence of the western sector. Both sectors are bounded by a listric fault of NE-SO strike, which may have caused the sinking of the eastern sector and its tilting toward the NW. This fault would be related to the extensional fault system that formed the coastal Catalan margin during the Neogene period. The entire Miocene unit of Montjuic was also folded and form a gentle anticline of NO-SE strike. This fold would have originated before the faulting and can be interpreted as a synsedimentary structure originated by a differential compaction between the early cemented alluvial and deltaic sediments that form the fold core and the unconsolidated sediments that form its limbs. Both the anticline and the fault were subsequently unconformably covered by the Quaternary deposits that cover the mountain of Montjuic.
Highlights
Cores from 190 mechanic boreholes that cut through the Miocene deposits of Montjuïc mountain (Barcelona) were studied
The deltaic sequence of the eastern sector corresponds to a higher stratigraphic position than the alluvial sequence of the western sector
Both sectors are bounded by a listric fault of NE-SO strike, which may have caused the sinking of the eastern sector and its tilting toward the NW
Summary
Los materiales que forman Montjuïc se originaron durante el Mioceno medio (Serravaliense según Parcerisa, 2002) como sedimentos aluviales y deltaicos depositados sobre un substrato de rocas mesozoicas y del Mioceno Inferior (Parcerisa et al, 2008). El Mioceno Superior representa una etapa de erosión de la zona litoral catalana ligada a la caída del nivel del mar Maditerráneo ocurrida durante este periodo (crisis messiniense), en la que se reciclan parte de los sedimentos de Montjuïc. Las principales capas de arenisca que forman ambos sectores han sido objeto de numerosas explotaciones a cielo abierto, que han sido activas desde los inicios de la construcción de la ciudad de Barcelona (canteras romanas) hasta finales de los años 60 del pasado siglo en que fueron definitivamente clausuradas (Roca i Blanch, 2000). En estos frentes se pueden observar las capas de arenisca de la parte superior de la montaña, así como la cobertera cuaternaria que en algunos casos llegan a superar los 10 m de espesor. Es una cobertera coluvial que se compone principalmente de arcillas rojas que engloban gravas y bloques de arenisca, y contienen abundantes nódulos edáficos de carbonato
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