Mid-Jurassic events in the Western High Atlas (Morocco)

Abstract

ZusammenfassungBei Kartierarbeiten und Profilaufnahmen im Westlichen Hohen Atlas in Marokko beobachtete Winkeldiskordanzen, Schichtlticken und progressive M~ichtigkeitsentwicklungen deuten auf mitteljurassische tektonische Aktivit~iten hin. Untersttitzung erh~lt diese Hypothese durch Rotsedimente des Dogger im Ktistenbecken yon Agadir-Essaouira, die hier im Gegensatz zur allgemeinen Meeresspiegelentwicklung zum Absatz kamen.In der tektonischen Entwicklung ergibt sich ein Rift-Stadium yon Perm bis Unterjura, das yon einero Golf-Stadium von Oberjura bis Mittelkreide abgel6st wird, bevor sich durch Inversion in der Oberkreide und seit dem Jungterti~ir das heutige Hochgebirge bildete. Im Mesozoikum wird Zerrungstektonik fiberlagert durch vom Atlantik ausgehende Vertikaltektonik. Sie ffihrte bereits im Mittleren Jura zu einem NNE-SSW verlaufenden Hochgebiet, das auch im heutigen >>massif ancien<< des Westlichen Hohen Atlas gipfelt und die Aufkl~irung der sedimentfiren Vorgeschichte des Gebirges erheblich erschwert. AbstractAngular unconformities, time gaps, and progressive increase of thickness in the Mesozoic sedimentary pile of the Western High Atlas, Morocco, point to mid-Jurassic tectonic pulses in this area. Red beds, Middle Jurassic in age, were deposited in the nearshore Agadir-Essaouira basin in contrast to transgressive tendencies of the eustatic curve. These phenomena are easily explained by uplift and erosion in the central part of the mountain belt even in the Middle Jurassic. Thus, a rift stage (Permian to Early Jurassic) in the tectonic history of the Western High Atlas was followed by a gulf stage (Late Jurassic to Middle Cretaceous) until later the high mountain range generated by inversion in Late Cretaceous and Neogene to recent times. Tensional forces that created the mobile rift belt were superimposed by vertical uplift during the Middle Jurassic that gave rise to a NNE-SSW stretching high. Even in the present-day high mountain range this area, called the >>ancient massif<<, is strongly elevated. Thus, the evaluation of the geological history of the Western High Atlas seems much complicated. R6sum6Dans les couches mdsozoiques du Haut Atlas occidental (Maroc), des discordances angulaires, des lacunes stratigraphiques ainsi que des variations progressives d'dpaisseur, tdmoignent d'activit6s tectoniques au Jurassique moyen. C'est ainsi qu'au Dogger, des couches rouges se sont ddpos6es dans le bassin c6tier dAgadir-Essaouira en opposition avec les tendances transgressives g6n6rales de la courbe eustatique. Ces ph4nom6nes s'expliquent facilement par des processus de soul~vement et d'drosion dans la partie centrale de la zone mobile atlasique au Jurassique moyen.Fhistoire tectonique a comport4 une p4riode de rifting (du Permien au Jurassique inf6rieur) suivie par une p6riode o6 le golfe marin atlasique s'est constitu4 (du Jurassique supdrieur au Les forces d'extension, responsables de la naissance de la zone mobile du rift atlasique, ont dtd suivies au Jurassique moyen par un soul~vement vertical en rela...

Post-rift vertical movements and horizontal deformations in the eastern margin of the Central Atlantic: Middle Jurassic to Early Cretaceous evolution of Morocco

“…Names of wells are from Bouatmani et al (2007) and (2) the Toarcian unconformity, which descends downsection to the NE (e.g., Medina 1995). A wedge geometrically similar to the one just described is formed by the post-rift part of the succession, that is, by Middle Jurassic to Lower Cretaceous sediments which overly the break-up unconformity (e.g., Stets 1992). These sediments show a fan geometry with dips to the NW in the S, becoming northward in the N (Figs.…”

“…5, 6). The corresponding formations are basically continuous over the entire area and show a pattern of decreasing thicknesses from the SW to the NE (Stets 1992;Zühlke et al 2004). …”

“…5, 6) is of particular interest because of the map-view wedge formed by Triassic to Lower Cretaceous sediments. The wedge links the MAM in the E with the Essaouira-Agadir basin in the W thereby providing relevant information on the kinematics of coeval upward and downward vertical movements (e.g., Stets 1992). In contrast to other regions, no evaporites are documented in the area and differential vertical movements must have a tectonic origin rather than being driven by halokinesis.…”

The Atlas System

“…1, but other data on the Late Permian and Triassic extension in western Morocco can be found in Medina (1991), Jalil (1999, Tourani et al (2000), Medina et al (2001), El Arabi E.H. (2007. References on the Jurassic sedimentary formations comprise Bouaouda (1987), Stets (1992), Ourribane et al (1999), Martin-Garin et al (2007); and on the Cretaceous sedimentary formations, see Taj-Eddine (1992), Algouti et al (1999), Labbassi et al (2000), Nouidar & Chellai (2001.…”

Characterization of the Cretaceous aquifer structure of the Meskala region of the Essaouira Basin, Morocco

“…1). The basin has been the subject of several geological studies (Roch, 1930;Duffaud, 1960;Choubert and Faure-Muret, 1962;Ambroggi, 1963;Souid, 1983;Laville and Fedan, 1989;Medina, 1984Medina, , 1985Medina, , 1988Medina, , 1989Medina, , 1994Stets, 1992;Broughton and Trepanier, 1993). In brief, the stratigraphy and structure of the Essaouira Basin are primarily due to two major tectonic periods and penecontemporaneous and intervening sedimentation cycles.…”