| Summary: | The Penedes half-graben represents a natural field laboratory for the study of the link between tectonics and palaeofluids because it exposes numerous outcrops that allow a global and complete diagenetic study of the basin from Mesozoic to present times.
The Penedes half-graben is located in the central part of the Catalan Coastal Ranges (CCR) and results from the superposition of three main tectonic events: (1) the Mesozoic extensional phase which is divided into two Mesozoic rift episodes: the first, Late Permian to Triassic in age and the second, latest Oxfordian to Aptian in age; (2) the Paleocene to middle Oligocene compressional phase which includes the emplacement of ENE-to-NE-trending thick-skinned thrust sheets bounded by SE dipping thrusts with a limited left-lateral strike-slip motion; (3) the late Oligocene?- Neogene extensional phase which split the CCR into a set of ENE-WSW blocks mainly tilted toward the NW, constituting the actual horst-and-graben systems now present at the northwestern Mediterranean.
Samples were taken in 19 different outcrop areas located within three main structural domains: the Gaia-Montmell domain, which represents the footwall block of the SE-dipping major normal faults that bound the north-western margin of the basin, the Central Penedes domain, which comprises the central Miocene syn- and post-rift deposits and the Garraf domain, which comprises a group of small syn-depositional tectonic horsts and half-grabens developed in the Garraf horst during its Neogene evolution.
Based on the macro and microstructural analysis combined with geochemical results from host rocks, fault rocks and fracture cements, the following diagenetic events have been identified: (1) Very early stages characterized by micritization, early irregular micro-fractures resulting from opening in poorly-lithified sediments and early calcite cement precipitations; (2) Progressive burial stages characterized by brecciation, stylolization and dolomitization; (3) Fracturing and cementations characterized by ninth major deformation stages with their related cements, breccias and stylolites, and (4) four karstification events with associated collapse breccias, sediments and cements filling the fracture, vug and cavern porosities.
A depositional control of the δ(18)O values of the syn-rift Mesozoic sediments (Valanginian, Barremian and Aptian) related to the position of the different outcrops with respect to the Mesozoic normal faults is inferred from the values reported in this study. The isotopic values of the Miocene marine facies, depleted in δ(18)O and δ(13)C respect to the expected values for the Miocene seawater, together with the chalky appearance of these limestones, indicate that the Miocene marine limestones were re-balanced under the meteoric diagenetic environment. The meteoric fluid precipitating the calcite cement in the conglomerates of the lower continental complexes was responsible for diagenetically altering the marine host limestone. A different meteoric fluid, more influenced by soil-derived CO2, precipitated the calcite cement present within the upper continental complexes.
The fluids circulating through the fractures attributed to the second stage of the Mesozoic rifting were precipitated from formation waters during the progressive burial of the sediment, in a closed palaeohydrological system. From the Paleocene to the mid-Oligocene the fluids circulating through the compressional fractures had a meteoric origin. Due to the Paleogene compression, Mesozoic rocks were uplifted, subaerially exposed and extensively karstified. Different types of sediments and cements were deposited filling the karstic cavities under the meteoric diagenetic environment.
Related to the syn- and early post-rift stage, the fractures were sealed by meteoric fluids under both, phreatic and vadose zones. The normal faults attributed to the late post-rift stage favoured the upflowing of marine fluids expelled from the compaction of the late Burdigalian to the early Serravallian marine sediments producing the dolomitization of the host rocks and the precipitation of dolomite cements within the fractures. During the late post-rift and related to latest tensional fractures occurrence different types of meteoric fluids circulated through the fractures. These fluids were precipitated from phreatic to vadose cements, agreeing with the uplift of the entire basin and/or with the falling-down of the meteoric water table related to a generalized sea level fall in the Mediterranean area during the Messinian. === La formació de la conca del Penedès està associada a un període extensiu d’edat neògena que provocà l’obertura del Solc de València. El marge nord-oest del Solc de València està constituït per una sèrie de grabens (Penedès, Vallès, Barcelona ...) i horsts (Garraf, Gaià-Montmell, Montnegre ...), el conjunt dels quals formen la Serralada Costanera Catalana. Aquesta serralada resulta de la superposició de tres esdeveniments tectònics principals: (1) l’extensió Mesozoica, compresa entre el Pèrmic i el Cretàcic inferior, (2) la compressió Paleògena, la qual produí la inversió de les principals conques extensives Mesozoiques i (3) l’extensió neògena, compresa entre l’Oligocè tardà i el Miocè mig, la qual generà l’actual sistema de rift de la Mediterrània occidental.
S’han estudiat 19 afloraments localitzats tant en els alts estructurals, Garraf i Gaià-Montmell, com en el sector central de la conca del Penedès. A partir de les dades macro I microestructurals, juntament amb els resultats geoquímics de les roques encaixants, roques de falla i els ciments que reomplen les fractures, s’han identificat els següents estadis diagenètics:
i. un primer estadi diagenètic temprà caracteritzat per la formació de microfractures de morfologies irregulars, formades en un sediment poc litificat i per la precipitació d’un ciment de calcita poc interaccionat amb la roca de caixa.
ii. un segon estadi d’enterrament caracteritzat per la bretxificació i dolomitització de la roca encaixant i la generació d’estilòlits sub-paral•lels a l’estratificació.
iii. nou etapes de deformació amb diferents tipus de rebliments associats a les fractures.
iv. quatre estadis de carstificació caracteritzats per diferents tipus de bretxes de col•lapse, sediments i ciments que reomplen les porositats tipus fractura i vug, generades a partir de la dissolució.
Els fluids relacionats amb l’extensió Mesozoica són característics d’aigües de formació, en canvi, els fluids que circularen al llarg de les fractures compressives paleògenes, són coherents amb fluids d’origen meteòric altament interaccionats amb la roca de caixa. L’extensió Neògena es caracteritza per una circulació predominant de fluids meteòrics no interaccionats amb la roca de caixa. És en l’estadi de post-rift on s’ha definit una dolomitització parcial de l’encaixant produïda per la barreja d’aigües marines i meteòriques. Durant l’estadi de post-rift tardà tingué lloc un esdeveniment de dissolució càrstica molt extens, el qual es relaciona amb la baixada del nivell del mar durant el Messinià.
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