Speksnijder, A., 1985. Anatomy of a strike-slip fault controlled sedimentary basin, Permian of the southern Pyrenees, Spain. Sediment. Geol., This study is mainly concerned with a 55 km long, east-west oriented strike section through a thick succession of dominantly clastic post-Variscan deposits in the Spanish Pyrenees south of Andorra. A subdivision into three formations can be made: two formations of Stephanian age, composed of volcanic rocks and fluviatile sediments respectively, and one of probable Permian age which has been studied in detail and is entirely composed of terrestrial redbeds. These three formations are characterised by large variations in thickness along strike. Thicknesses vary from 0 to 800 m in the volcanics, from 0 to 300 m in the Stephanian fluviatile sediments and from 0 to 1500 m in the Permian redbeds. The geometry of the sedimentary bodies in all three formations suggests deposition in an east-west oriented graben-like depression, a conclusion which is confirmed by detailed sedimentological analysis of the Permian rocks.The vertical succession of Permian sediments can be subdivided in six major basinfill sequences which unconformably overlie each other. Internally the basinfill sequences are built up of one or more coarsening-upward/fining-upward cycles. The genesis of these cycles is directly related to dip-slip movement along the basin boundary faults, whereas the unconformities between the basinfill sequences indicate that periods of basin subsidence and sedimentation were followed by periods of deformation and erosion. These changes in tectonic regime as reflected in the sedimentary sequences are interpreted to indicate alternating phases of transtension and transpression in a strike-slip zone. Other sedimentological observations substantiate this interpretation. The basin depocentre shifted in space and time, which resulted in the observed large variations in sediment thickness along strike, together with eastward paleocurrent directions along the basin axis.Transtension was accompanied by outpouring of andesitic lavas and drape folding on a large scale. During transpressional phases normal faults changed character to reverse faults while folding, uplift and subsequent erosion resulted in the formation of intrabasinal unconformities. Alternation of transpressive and transtensive conditions should be the result of movement along non-planar fault surfaces in a major strike-slip zone. A separate structural study of the Variscan basement of the studied basin revealed the * Present address: Koninklijke/Shell Exploratie
Permo-Triassic rift basins offer important hydrocarbon targets along the Atlantic margins. Their fill is dominated by continental red beds, comprising braided fluvial, alluvial fan, aeolian, floodplain and lacustrine facies. These relatively lightly explored basins span both the Atlantic and Tethyan domains and developed above a complex basement with inherited structural fabrics. Sparse data in offshore regions constrain understanding of depositional geometries and sedimentary architecture, further impeded by their deep burial beneath younger strata, combined with the effects of later deformation during continental breakup. This paper provides results from a multidisciplinary analysis of basins along the Atlantic margin. Regional seismic and well data, combined with geochemical provenance analysis from the European North Atlantic margins, are integrated with detailed outcrop studies in Morocco and Nova Scotia. The research provides new insights into regional basin tectonostratigraphic evolution, sediment fill, and reservoir distribution, architecture and quality at a range of scales. Regional seismic profiles, supported by key well data, indicate the presence of post-orogenic collapse basins, focused narrow rifts and low-magnitude multiple extensional depocentres. Significantly, Permo-Triassic basin geometries are different and more varied than the overlying Jurassic and younger basins. Provenance analysis using Pb isotopic composition of detrital K-feldspar yields new and robust controls on the sediment dispersal patterns of Triassic sandstones in the NE Atlantic margin. The evolving sedimentary architecture is characterized by detailed sedi-mentological studies of key outcrops of age equivalent Permian-Triassic rifts in Morocco and Nova Scotia. The interplay of tectonics and climate is observed to influence sedimentation, which has significant implications for reservoir distribution in analogue basins. New digital outcrop techniques are providing improved reservoir models, and identification of key marker horizons and sequence boundaries offers a potential subsurface correlation tool. Future work will address source and seal distribution within the potentially petroliferous basins.
ZusammenfassungEine strukturelle Analyse im Variszikum der zentralen Siidpyren~ien zeigt mehrere Deformationsphasen, yon denen die Haupffaltungsphase mit WNW-ESE gerichtetem Streichen die iilteste zu sein scheint. Die Interpretation und Entzerrung der zur Hauptphase geh/Srigen Strukturelemente l~if~t aber erkennen, dat~ eine Verstellung der sediment~ren Schichtung vor der Hauptfaltung stattgefunden hat. Eine FriSahphasenfaltung ist auch aus der r~iumlichen Verteitung yon Lineationen im Arbeitsgebiet zu erkennen, die die Existenz zweier friihvariszischer Faltensysteme andeutet. Diese sind durch weite NNW-SSE und WSW-ENE Faltenz~ige mit subvertikalen Achsenebenen und subhorizontalen Faltenachsen gekennzeichnet. Im Gegensatz zu den Hauptphasenfalten haben sie w~ihrend der Deformation keine Schieferung parallel zur Achsenebene ausgebildet.Friihphasenfalten in unterschiedlichen Orientierungen sind auch aus anderen Gebieten der variszischen Zentralpyren~ien beschrieben. Eine Neninterpretation bestehender Karten und weiterer Daten legt jedoch nahe, dal~ auch in diesen F~illen eher zwei Deformationsphasen der Hanptfattung vorausgegangen sind als nur eine, wie meist vorgeschlagen. Wiederum ist aus den Interferenzfiguren der zwei Faltensysteme sowie aus Gel~ndedaten zu schliel~en, dal~ die Achsenebenen sehr steil sind und N-S oder E-W streichen, aber 6rtlich kann eine starke Uberpr~igung durch alpine Deformation (insbesondere Uberschiebungen) vorliegen.Eine Friihphasenfaltung in den variszischen Pyren~ien nimmt eine wichtige Position ein im Rahmen eines dynamischen/kinematischen Modelles for die pal~iozoische Entwicklung der gesamten Mobilzone der Pyren~ien. Dieses Modell basiert auf einer abwechselnd kon-und divergenten, rechtsh~ndigen Horizontalverschiebung am NE Rand der iberischen (Mikro-)Platte.Das Vorkommen von Friihphasenfalten steht in Beziehung *) Authors' address: A. SPEKSNIJDER, Department of Structural Geology, State University Utrecht, Budapestlaan 4, Postbus 80021, 3508 TA, Utrecht, The Netherlands; Present address: Koninklijke/Shell Exploratie en Produktie Laboratorium, Volmerlaan 6, 2288 GD, Rijswijk (ZH), The Netherlands.(1) zum Ubergang von divergenten zu konvergenten Horizontalverschiebungen (fiir die NNW-SSE streichenden Falten), und (2) zu einer anf'~nglichen schr~igen Kollision der iberischen und europ~iischen Platten (fiir die WSW-ENE streichenden Falten) vor der eigentlichen Hauptfaltungsphase. AbstractDetailed structural analysis of part of the Variscan southcentral Pyrenees revealed the occurrence of several deformation generations, of which the most important one, called the mainphase folding and striking WNW-ESE, seems to be the oldest. Directional analysis of structural elements related to mainphase folding (sedimentary bedding, mainphase cleavage, small-scale foldaxes and intersection lineations) shows, however, that sedimentary bedding must have been non-planar before mainphase deformation took place. This observation suggests that premainphase folding occurred as well, and indeed the areal d...
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