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- Volume 9, Issue 1, 1997
Basin Research - Volume 9, Issue 1, 1997
Volume 9, Issue 1, 1997
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Sedimentary sequences, seismofacies and evolution of depositional systems of the Oligo/Miocene Lower Freshwater Molasse Group, Switzerland
Authors F. Schlunegger, A. Matter, D. W. Burbank, W. Leu, M. Mange and J. MàtyàsABSTRACT
Magnetostratigraphic chronologies, together with sedimentological, petrological, seismic and borehole data derived from the Oligo/Miocene Lower Freshwater Molasse Group of the North Alpine foreland basin enable a detailed reconstruction of alluvial architecture in relation to Alpine orogenic events. Six depositional systems are recorded in the Lower Freshwater Molasse Group. The bajada depositional system comprises 200–400‐m‐thick successions of ribbon channel conglomerates and overbank fines including mud‐ and debris‐flows which were derived from the Alpine border chain. The alluvial megafan depositional system is made up of massive pebble‐to‐cobble conglomerates up to 3 km thick which reveal a fan‐shaped geometry. This depositional environment grades downcurrent into the conglomerate channel belt depositional system, which comprises an ≈2‐km‐thick alternation of channel conglomerates and overbank fines. The sandstone channel belt depositional system is bordered by the 100–400‐m‐thick overbank fines assigned to the floodplain depositional system. At the feather edge of the basin, 50–400‐m‐thick lacustrine sediments in both clastic and carbonate facies represent the lacustrine depositional system.
The spatial and temporal arrangement of these depositional systems was controlled by the geometrical evolution of the Molasse Basin. During periods of enhanced sediment supply and during phases of stable sliding of the entire wedge, >2000‐m‐thick coarsening‐and thickening‐upward megasequences comprising the conglomerate channel belt, alluvial megafan and bajada depositional systems were deposited in a narrow wedge‐shaped basin. In the distal reaches of the basin, however, no sedimentary trend developed, and the basin fill comprises a <500‐m‐thick series of sandstone meander belt, floodplain and lacustrine depositional systems. During phases of accretion at the toe of the wedge, the basin widened, and prograding systems of multistorey channel sandstones extended from the thrust front to the distal reaches of the basin.
The rearrangement of the depositional systems as a function of changing orogenic conditions created discordances, which are expressed seismically by onlap and erosion of beds delimiting sedimentary sequences. Whereas stable sliding of the wedge succeeded by accretion at the toe of the wedge is recorded in the proximal Lower Freshwater Molasse by a coarsening‐and thickening‐upward megasequence followed by erosion, the opposite trend developed in the distal reaches of the Molasse. Here, fine‐grained sandstones and mudstones were deposited during periods of stable sliding, whereas phases of accretion caused a coarsening‐ and thickening‐up megasequence to form.
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Modelling landscape evolution on geological time scales: a new method based on irregular spatial discretization
Authors Jean Braun and Malcolm SambridgeWe present simulations of large‐scale landscape evolution on tectonic time scales obtained from a new numerical model which allows for arbitrary spatial discretization. The new method makes use of efficient algorithms from the field of computational geometry to compute the set of natural neighbours of any irregular distribution of points in a plane. The natural neighbours are used to solve geomorphic equations that include erosion/deposition by channelled flow and diffusion. The algorithm has great geometrical flexibility, which makes it possible to solve problems involving complex boundaries, radially symmetrical uplift functions and horizontal tectonic transport across strike‐slip faults. The algorithm is also ideally suited for problems which require large variations in spatial discretization and/or self‐adaptive meshing. We present a number of examples to illustrate the power of the new approach and its advantages over more ‘classical’ models based on regular (rectangular) discretization. We also demonstrate that the synthetic river networks and landscapes generated by the model obey the laws of network composition and have scaling properties similar to those of natural landscapes. Finally we explain how orographically controlled precipitation and flexural isostasy may be easily incorporated in the model without sacrificing efficiency.
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Tectonic geomorphology of the Ash Hill fault, Panamint Valley, California
Authors Alexander L. Densmore and Robert S. AndersonABSTRACT
Panamint Valley, in eastern California, is an extensional basin currently bounded by active, dextral‐normal oblique‐slip faults. There is considerable debate over the tectonic and topographic evolution of the valley. The least‐studied structure, the Ash Hill fault, runs for some 50 km along the valley’s western edge, and active strands of the fault continue south into the neighbouring Slate Range. Vertical displacement on the fault is valley‐side up, creating topography that conflicts with the gross morphology of the valley itself. We use this topography, along with kinematic and geological markers, to constrain the Quaternary slip rate and orientation of the Ash Hill fault. The fault offsets all but the active channel deposits in the valley, and slickenlines indicate a strike‐slip to dip‐slip ratio of 3.5:1. An offset volcanic unit dated at 4 Ma provides a minimum slip rate of 0.3±0.1 mm yr−1, and a long‐term strike‐slip to dip‐slip ratio of 5.2:1. Slip on the fault has warped a palaeolake shoreline within the valley. Simple elastic dislocation modelling of the vertical deformation of the shoreline suggests total fault slip of ≈60 m, valley‐side up. The shoreline probably dates to 120–150 ka, implying a late Quaternary slip rate of 0.4–0.5 mm yr−1. We suggest two possible mechanisms for the apparently anomalous slip behaviour of the Ash Hill fault. The fault may be a listric structure related to the proposed low‐angle fault underlying Panamint Valley. Alternatively, the Ash Hill fault is a high‐angle fault, implying that the valley is currently bounded by high‐angle dextral‐slip faults. Lack of detailed subsurface information precludes any knowledge of the true relationships between the presently active faults.
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The Rioja Trough (N Spain): tectosedimentary evolution of a symmetric foreland basin
Authors A. Muñoz‐Jiménez and A. M. Casas‐SainzABSTRACT
The Rioja Trough is the foreland basin of the western Pyrenees (to the north) and the Cameros‐Demanda Massif (to the south). This E–W elongated trough is about 120×35 km. It was filled with Tertiary continental deposits (upper Eocene to upper Miocene), reaching thicknesses between 2500 and 5000 m. Both margins of the Rioja Trough are large thrusts with horizontal displacements of more than 20 km basinward. Rocks that fill the basin originated in alluvial fan and playa‐lake environments, with conglomerates in the proximal sectors grading into sandstones, mudstones, lacustrine limestones and evaporites in distal sectors. The Tertiary series are horizontal in the central parts of the basin, with several E–W monoclines caused by north‐verging thrusts in the basement of the basin. Near the basin margins, the Tertiary units are folded and thrusted, with several syntectonic unconformities. Calculated velocities for the Cameros‐Demanda thrust range from 0.02 to1.1 mm yr−1 (average 0.7 mm yr−1). The sedimentation rate near the southern basin margin varies between 2 and 20 cm 1000 yr−1 (average 10 cm 1000 yr−1). Deposition in the Rioja Trough was strongly controlled by tectonic activity throughout the Tertiary. Eight tectosedimentary units (R1 to R8) have been characterized. These are bounded by angular unconformities at the margins and breaks in the vertical trend of the sedimentary record toward the basin centre. Every tectosedimentary unit (except R6 and R8) shows a fining‐upward/coarsening‐upward trend, corresponding to tectonic retrogradations and progradations, respectively. The main source area during the Palaeogene was the Cameros‐Demanda Massif, whose unroofing sequence was strongly dependent on tectonic activity. During the Neogene a longitudinal WNW–ESE drainage system, with short alluvial fans in the northern and southern margins, developed. The final shape and the evolution of the Rioja Trough are the result of crustal flexure in the northern border of the Iberian plate, linked to the emplacement of the southern Pyrenean thrust system, and intraplate thrusting with basement uplift at its southern margin.
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Volumes & issues
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Volume 36 (2024)
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Volume 35 (2023)
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Volume 34 (2022)
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Volume 33 (2021)
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Volume 32 (2020)
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Volume 31 (2019)
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Volume 30 (2018)
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Volume 29 (2017)
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Volume 28 (2016)
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Volume 27 (2015)
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Volume 26 (2014)
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Volume 25 (2013)
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Volume 24 (2012)
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Volume 23 (2011)
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Volume 22 (2010)
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Volume 21 (2009)
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Volume 20 (2008)
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Volume 19 (2007)
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Volume 18 (2006)
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Volume 17 (2005)
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Volume 16 (2004)
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Volume 15 (2003)
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Volume 14 (2002)
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Volume 13 (2001)
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Volume 12 (2000)
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Volume 11 (1999)
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Volume 10 (1998)
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Volume 9 (1997)
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Volume 8 (1996)
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Volume 7 (1994)
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Volume 6 (1994)
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Volume 5 (1993)
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Volume 4 (1992)
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Volume 3 (1991)
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Volume 2 (1989)
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Volume 1 (1988)