Resumen de Tesis Doctoral: Sismoestratigrafía y evolución tectonosedimentaria del Margen Continental Norcatalán (Mediterráneo Occidental, NE de España) durante el Cenozoico

ALEJANDRO ALBERTO TASSONE

Abstract


The integrated analysis of the onshore and offshore geological record in the Northern Catalan continental margin allows to establish the major factors which controlled its Cenozoic upbuilding and evolution. The structural processes, which took place along successive tectonic stages, together with the coeval vertical movements and eustatic variations were critical for the generation of major erosion and sedimentation areas. All these factors as a whole, controlled the accommodation and the sedimentary supply throughout the margin upbuilding and resulted in its sedimentary architecture. Two sectors (Northern and a Southern) have been defined for the studied area, upon the basis of differences in their structure and sedimentary record. The main Eastern Pyrenean Paleogene contractive structures (axial zone antiformal stack, Figueres - Montgrí thrustsheet, Vallfogona thrust) and their coeval foreland basin were traced in the offshore areas, as far as the shelf edge. Extensional structures occur related to two major fault systems, which strike NE-SW to NNE-SSW and NW-SE and later shaped the divergent Catalan continental margin. The NE-SW fault system would have originated during a Late Oligocene-Earlier Miocene faulting stage, which would have resulted in most of the recorded crustal extension and thinning. The NW-SE faults have been at least partially active as strike-slip faults during the Paleogene, with displacements which are difficult to precise. However, these faults behaved as normal faults during Middle Miocene to Quaternary and though they display less important displacements than those of the NE-SW system, they are closely related to extensive volcanic activity, which suggests its connection with the lower lithosphere. The Cenozoic stratigr a p hy records the successive influence of sedimentary and erosive processes related to 1) the upbuilding of the Pyrenean orogene and the evolution of its foreland basin; 2) the activity of the NE-SW to NNE-SSW fault system as well as the lithospheric uplifting related to the NW Mediterranean rifting and 3) the later thermal subsidence of the continental margin developed during the Middle Miocene-Quaternary, which was opposite to the isostatic rebound of the Pyrenees and its foreland. Changes of sea level, together with other environmental factors, have extensively controlled the present architecture of the submerged margin since Late Miocene. On the basis of the predominance of one or more of those factors, the Cenozoic record of the North-Catalan margin has been split into two, upper and lower parts. The lower part is composed by four tectonostratigraphic units (seismic units 1 to 4), related to the main tectonic events. The upper part is made up by two depositional supersequences (units 5 and 6), which are controlled by changes in sea level and environmental factors. Tectonostratigraphic units are as follows: Unit 1: syntectonic and coeval to the compression which gave rise to the Pyrenees. Corresponding sediments are those deposited in the foreland basin. Unit 2: syntectonic in relation to the NE-SW structures. Sediments belonging to Subunit 2a are synrift, while those of Subunit 2b belong to the late rifting stage. Deposits of Unit 3 in the Southern zone are postrift while in the Northern zone they are associated to the valley infill during lithospheric uplifting related to Oligocene rift generation and possibly to the beginning of NW-SE fault system activity. Unit 4 in the Southern sector shows the evidences of the ceasing of the extensional process, while in the Northern sector, sediments are syntectonic with the starting or persistence of the NW-SE system activity. The two upper supersequences (Units 5 and 6) consist of sequences 5.1 and 5.2 (3rd order), 6.1 (3rd order) 6.2 (5th order) and sequences 6.3 and 6.4 (7th order). During their development, the margin underwent alternatively regressions (seq. 5.2, 6.2, 6.3 ), transgressions (seq. 5.1, 6.3, 6.4) and aggradation (6.2). The occurrence and development of the different sedimentary bodies and depositional systems forming the sedimentary system tracts in both the Northern and Southern sectors was related to eustatic variation and environmental factors, resulting in different arrangements among the sedimentary system tracts identified within each sequence. Eight arrangements have been defined and a certain pattern can be recognized for their areal distribution when comparing Northern with Southern sectors. The conspicuous difference between the volumes of sediments supplied during different periods at the Northern and Southern sectors, and the tectonic control due to Pre-Neogene and Pre-Pliocene structure on later sedimentary infill are remarkable features of the studied zone. The accumulation of Miocene to Quaternary sediments related to the more widespread subsidence stage at the continental margin was modified by the generation of local depocenters resulting from the excavation of topographical depressions and the more or less simultaneous activity of the NW-SE faults. Some of these low laying zones were superimposed and thus accentuated pre-existing contractive Paleogene depressions. In some cases this fact controlled the development of significant de-pocenters as well as the location of some conspicuous Neogene and Recent submarine canyons at the continental margin. These c a nyons have played a relevant role from north to south as sedimentary traps and barriers. At a former stage, essentially pre-Pliocene, the main sedimentary trap would have been the L’ Escala paleocanyon, while during Pliocene and Quaternary the La Fonera canyo n would act as the main sediment barrier. The Cap de Creus canyon originated during the Quaternary, acting as a trap. The relationship of some of these canyons with sedimentary processes and onshore depositional systems can be observed at the whole system of L’ Escala canyon, with its head inside the emerged continent (Fluvia river basin), its transport segment and its accumulation system at the base-of-slope.

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