Fracturing and cementation of shallow buried Miocene proximal alluvial fan deposits |
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| Institution: | 1. Universidade do Estado do Rio de Janeiro, Faculdade de Geologia, Departamento de Estratigrafia e Paleontologia, Av. São Francisco Xavier, 524, sala 2020A, Maracanã, 20550–013 Rio de Janeiro, RJ, Brazil;2. Universidade de Aveiro, Dpto. Geociências, GeoBioTec, CESAM, Campus de Santiago, 3810–193 Aveiro, Portugal;3. Department of Palaeontology, Geozentrum, Althanstrasse 14, A–1090 Vienna, Austria;4. Università degli Studi di Urbino “Carlo Bo”, Dipartimento di Scienze Pure e Applicate (DiSPeA) Urbino, Italy;5. CIMA (Centro de Investigação Marinha e Ambiental), Universidade do Algarve, Campus de Gambelas, Faro, Portugal |
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| Abstract: | Fluid flow in fractures and host rocks has been investigated in shallow buried Miocene alluvial fan deposits. A structural, petrographical (optical, CL, SE microscopes and XRD) and geochemical (microprobe and δ18O-δ13C stable isotopes) study has been performed in normal faults affecting Serravalian-Tortonian siliciclastic rocks of the Vallès-Penedès basin. These faults formed during the development of the Vallès-Penedès fault-related syncline, which caused the rotation of the earliest fractures. Faulting occurred continuously before, during and after host rock cementation. Rocks affected by faulting are represented by clay-rich gouges, which formed thanks to the high phyllite clast content within the otherwise clean and mature sandstones and conglomerates. Despite the low permeability of these rocks, cross-fault and fault-parallel fluid flows occurred in most of the faults.Host rocks and veins were cemented by two generations of calcite, i.e. Cc1 and Cc2. Cc1 precipitated from meteoric waters at shallow burial conditions whereas Cc2 precipitated from meteoric waters in a confined aquifer.Palygorskite has been identified in shear zones within the gouges indicating their later formation by interaction of Mg-rich fluids with previous smectites. These fluids probably derived from Miocene seawater expelled from the underlying Transitional–Marine Complex “TMC” by compaction.Sedimentation, fracturing and cementation occurred in a very short lapse time of about 6–7 Ma, between the Serravalian-Tortonian age of the sediments and the end of the extensional tectonics in the Vallès-Penedès fault (Pliocene). |
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| Keywords: | Alluvial fan Diagenesis Normal faults Fluid flow Clay-rich gouges Palygorskite |
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