Tectonic setting and syndepositional deformation of molasse and other nonmarine-paralic sedimentary basins

Abstract

Molasse is a distinctive sedimentary facies consisting of alluvial and shallow marine deposits derived from source areas undergoing rapid uplift and erosion. The characteristic setting of molasse is within or adjacent to fold belts, and most molasse basins can be classified as foreland basin (external) or intermontane (internal) in type. Thick alluvial and shallow marine sequences in cratonic settings (e.g., taphrogenic troughs) have been classified as molasse by some workers, but this is not generally accepted.Most molasse is synorogenic, and the interdependence of sedimentation and tectonism results in structural and stratigraphic complexity. Intermontane basins generally are tensional in origin and are bounded by high angle (commonly normal) faults. Facies distributions adjacent to each fault depend on relative rates of differential movement, erosion, and sedimentation; accelerated source area uplift causes segmentation and progradation of alluvial fans and generates coarsening-upward sedimentary cycles. Complications are introduced if tectonism occurs in discrete pulses and if basins enlarge themselves along systems of stepped faults.Basins of compressional origin, such as those in a foreland setting, show a greater variety of internal complexity. Facies belts and trends of maximum thickness move progressively out from the core of the fold belt in response to a migration of the zone of active deformation. Conversely, broad structural warps may develop in response to alocally increased sedimentary load, such as a major delta. Folds or thrust faults growing within the basin cause local thinning or unconformable relationships, and may breach the surface so as to isolate parts of the alluvial plain as a synorogenic intermontane basin. Uplift of the fold belt may be discontinuous, causing the generation of stacked sedimentary megacycles. Basin margins commonly are characterized by intraformational angular unconformities and syndepositional folds.Rates of alluvial sedimentation and tectonic movement measured in modern environments are one to two orders of magnitude higher than rates deduced from ancient nonmarine sequences. This lends support to the idea that much sedimentation may be very episodic in nature, but it could also imply that alluvial sequences are deposited much more rapidly than current interpretations of the ancient record would imply, based as they are on relatively crude dating techniques. Only in some wrench-fault basins do calculated sedimentation rates compare with those measured in modern environments. -A .Les taux de sedimentation alluviale et de mouvement tectonique dans des milieux recents sont d'un a deux ordres de grandeur plus eleves que les taux deduits pour les sequences non marines anciennes. Ceci supporte I'idee qu'une grande partie de la sedimentation pourrait Ctre de nature tres episodique mais pourrait aussi impliquer que les sequences alluviales se sont deposees beaucoup plus rapidement que les interpretations courantes des dep6ts anciens le laissent supposer, puisqu'elles sont ba...

Sedimentary response to tectonism in the Campanian–Maastrichtian succession, Anambra Basin, Southeastern Nigeria

“…The thickening of the sand bodies in the landward part of the basin and the proximal fluviatile-distal marine character are consistent with facies architecture in fault-controlled asymmetrically subsiding basins (Miall, 1978;Burns et al, 1997).…”

“…It is therefore difficult to make a case for normal eustatic control on the cyclicity observed within the succession. The simultaneous development of m-scale shallowingupward subtidal cycles within the coal-bearing formations in the more distal parts of the Anambra Basin is rather consistent with sedimentation in an asymmetrically subsiding basin (Miall, 1978;Artwater, 1987).…”

Anatomy of a strike-slip fault controlled sedimentary basin, Permian of the Southern Pyrenees, Spain

“…Miall, 1978), but, unfortunately, conclusive data on the horizontal movement along the Ges Zone are lacking, although a substantial dextral offset cannot be excluded. Therefore, it is tentatively proposed that due to negligible net vertical movements and resemblances with well-described strike-slip systems, the Ges Fault Zone acted as an oblique-slip zone with a large horizontal component.…”

Stratigraphy of the late Proterozoic Murdama Group, Saudi Arabia

“…(as discussed below), this rate is seen to suffice. Miall (1978) also pointed out that rates of sedimentation in the modem Mississippi delta are an order of magnitude higher at 5 to 10 m/1,000 yr, demonstrating the episodic nature of sedimentation.…”

“…Rates of sedimentation in subsiding basins were considered by Miall ( 1978). The focus of his discussion is on "molasse," a term little used today that is defined as "a distinctive sedimentary facies consisting of alluvial and shallow marine deposits derived from source areas undergoing rapid uplift and erosion."…”