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Geologische Rundschau

, Volume 67, Issue 1, pp 15–37 | Cite as

Tectonic control of Late Pliocene molasse sedimentation in a portion of the Jhelum Re-Entrant, Pakistan

  • C. F. Visser
  • G. D. Johnson
Aufsätze

Abstract

A Late Pliocene interval of the Neogene Siwalik (Himalayan) molasse exposed in the Jhelum Re-entrant, Pakistan, is examined for lateral variations in the environment and rate of fluvial sedimentation. A correlative pair of volcanic ash horizons within the Upper Siwalik section defines an isochronous interval of fluvial deposition. Stratigraphic sections measured through the interval at various localities in the Jhelum Re-entrant are analyzed in terms of fluvial stratigraphy and interval thickness, or net sedimentation rate. The stratigraphy indicates a fluvial system of laterally migrating streams leaving behind fining upward sedimentary cycles of lateral and vertical accretion deposits. Fully developed sedimentary cycles display point bar sands (lateral accretion deposits) of the stream channel environment topped by mudstones (vertical accretion deposits) of the overbank (floodplain) environment. The detailed sequence of vertical accretion deposits in these cycles indicates that the floodplain environment consisted of a stream-proximal, sedimentation-dominant zone and a stream-distal, pedogenesis-dominant zone.

Broad lateral variations in the rate of molasse sedimentation appear to be related to the large-scale syntaxial tectonics of the northwestern Himalaya. Sedimentation rate increases toward the apex of the re-entrant, indicating that the syntaxial structure controlled the focus of molasse sedimentation during the examined interval of Late Pliocene time. In addition, sedimentation may have been influenced by mild flexures within the molasse basin.

Keywords

Pliocene Molasse Molasse Basin Sedimentary Cycle Sedimentation Rate Increase 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Zusammenfassung

Im Gebiet von Jhelum (nördliches Pakistan) reicht das Vortief der neogenen Siwalik-Molasse als Sporn weit in den Faltenbogen des nordwestlichen Himalaya. Die Verfasser untersuchten hier einen Abschnitt des Spätpliozän mit besonderem Hinblick auf laterale Wechsel der fluviatilen Ablagerungen und Sedimentationsraten. Die Obere Siwalik-Molasse enthält hier zwei Leithorizonte mit Tuff, wodurch die Altersgleichheit der fluviatilen Zwischenlagerungen sichergestellt wird. Die Schichtfolge dieser Zwischenlagerungen ist an mehreren Lokalitäten in bezug auf Sedimentationstypen und-raten analysiert worden. Die vorgefundenen Ablagerungszyklen deuten auf ein System von seitlich wandernden Flußläufen. Vollständige Zyklen zeigen Flußbettsedimente (meist Ufersandbänke) gefolgt von feinkörnigen Überflutungssedimenten. Die Feingliederung der Überflutungsfazies zeigt stromnahe Ablagerungszonen einerseits und pedogenetische Randzonen andererseits.

Über das gesamte Untersuchungsgebiet ergibt sich eine klare Abhängigkeit zwischen Raten der Molasseaufschüttung und der Faltenbogentektonik des nordwestlichen Himalaya. Die Ablagerungsraten nehmen in Richtung des Bogenscheitels deutlich zu. Daraus wird geschlossen, daß während des untersuchten Zeitabschnittes die regionale Bogentektonik den Schwerpunkt der Molassesedimentation maßgeblich beeinflußt hat. Flache Flexuren innerhalb des Molassebeckens sind ebenfalls in diesen Zusammenhang zu stellen.

Résumé

Une avant-fosse de la molasse du néogène Siwalik (Himalayen), en éperon dans l'arc plissé du nord-ouest de l'Himalaya, dans la région du Jhelum, Pakistan, a été étudiée quant à ses variations latérales environnantes et son taux de sédimentation fluviale. Dans la section du Siwalik supérieur, deux horizons-repères de tuffs volcaniques délimitent une période isochronique du dépôt fluvial. Les séries stratigraphiques de cet intercalaire ont été analysées en divers points en fonction de la stratigraphie fluviale et de son épaisseur ou en fonction du taux de sédimentation. Les cycles de sédimentation indiquent un réseau fluvial à migration latérale avec dépôts latéraux et verticaux, de plus en plus fins vers le haut. Les cours semblent être en transition avec le type traditionnel de haute et basse sinuosité. On démontre que l'ensemble des dépôts verticaux sont différentiés selon une zone proximale où la sédimentation domine et une zone distale où la pédogenèse domine.

De larges variations latérales dans le taux de sédimentation semblent dues à la tectonique globale de l'éperon associée à la syntexis du nord-ouest himalayen. Une tendance vers une augmentation du taux de sédimentation / subsidence en direction de l'axe de l'éperon délimite un lieu de sédimentation molassique pendant l'intervalle étudié. De plus, l'auteur suggère que la sédimentation du bassin de l'éperon ait été influencée par de légères flexures à la périphérie de la structure régionale.

Краткое содержание

В области Игелюм, севе рный Пакистан, неоген овые моляссы Сивалик дале ко вклиниваются в склад чатые дуги северо-зап адных Гималаев. Авторы изучали отрезок позд него плиоцена, обраща я особое внимание на чередова ние флювиальных отложен ий и скорость осадкон акопления на крыльях. Верхние слои молясса Сивалик содержат два ведущих горизонта, состоящих из туффов, что разреша ет проводить возраст ную корреляцию флювиаль ных отложений. Свиты этих прослоев в различных районах исследовали по типам осадков и скорости их накопления. Удалось у становить, что эти отложения происходят от боковы х притоков рек, меняющ их свои русла. Осадки русел ре к — чаще всего прибрежн ые песчаные банки — по крытые осадками различного гранулометрическог о состава, — сохранили циклы полностью. При провед ении более мелкого страти графического подраз деления фаций этих райодов установили с одной ст ороны зоны речных отл ожений, а с другой — почвенные краевые зоны. Вся исследованная об ласть показывает явн ую зависимость между ск оростью осаждения моляссов и тектоникой складчат ых дуг северо-западных

Гималаев. Из этого сле дует, что в течение дан ного исследованного отре зка времени региональна я тектоника дуг сильн о повлияла на осадкообразовани е моляссов. Плоские фле ксуры внутри бассейн ов молясса также связаны с этим периодом.

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Copyright information

© Ferdinand Enke Verlag Stuttgart 1978

Authors and Affiliations

  • C. F. Visser
    • 1
  • G. D. Johnson
    • 1
  1. 1.Department of Earth SciencesDartmouth CollegeHanoverUSA

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