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Geologische Rundschau

, Volume 78, Issue 3, pp 821–840 | Cite as

In-situ water contents, liquid limits, and submarine mass flows due to a high liquefaction potential of slope sediment (results from DSDP and subaerial counterparts)

  • Gerhard Einsele
Article

Abstract

Water content-depth curves and Atterberg liquid limits support the conception that many finegrained, but lowcohesive thick slope sediments are susceptible to liquefaction and therefore can, as a result of dynamic loading (earthquakes, storm waves), be transformed into mud flows without the uptake of additional water. Usually in the uppermost meters, but frequently also tens or even hundreds of meters below the sea floor, the in-situ water content can be higher or approximately equal to the liquid limit (boundary between the plastic and liquid state of a soil). Under these conditions the sediments have a high liquefaction potential. This situation is particularly common in regions of high sedimentation rates, causing underconsolidation, as well as in sediments rich in silt-size biogenic particles. Whether depthrelated changes in the salt concentration of interstitial waters exert some influence, is not clear. If part of the material is carried unchanged as mud lumps by a liquified matrix, the average »field liquid limit« of the total mass flow can be even lower than the Atterberg liquid limit of small, only finegrained homogenized samples.

The examples presented here are based on the results of the Deep Sea Drilling Project including laboratory studies on land. In some cases, mass physical properties of subfossil mass flows can be compared with those of neighboring slope sediments. In sediments rich in microfossils, the in-situ water contents and liquid limits often decrease versus depth in a similar way due to the onset of slight chemical diagenesis. Thus, in spite of increasing compaction, a high liquefaction potenital may be maintained, at least in certain layers, until finally substantial lithification completely changes all the mass physical properties.

Keywords

Debris Flow Liquefaction Liquid Limit DSDP Slope Sediment 
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

Aus dem Vergleich von Wassergehalts-Sedimenttiefen-Beziehungen und parallel bestimmten Atterberg'schen Fließgrenzen feinkörniger, aber wenig kohäsiver mariner Slope-Sedimente läßt sich folgende Hypothese ableiten: Durch dynamische Scherbelastung (Erdbeben, Sturmwellen) können viele dieser Sedimente ohne Wasseraufnahme von außen in einen breiig-flüssigen Zustand überführt werden und somit Schlammströme (debris flows und mud flows) bilden. Der insitu-Wassergehalt dieser Sedimente ist meistens in den obersten Metern, nicht selten aber auch weit über 10 bis 100 m unter dem Meeresboden höher oder etwa ebenso hoch wie der Wassergehalt der Fließgrenze (Übergang vom plastischen in den flüssigen Boden- oder Sedimentzustand). Es wird postuliert, daß die Sedimente unter diesen Bedingungen ein hohes »Liquefaktionspotential« besitzen. Diese Situation tritt besonders da ein, wo hohe Sedimentationsraten einen unterkonsolidierten Sedimentzustand aufrechterhalten, sowie in Sedimenten, die einen hohen Anteil an Biogenpartikeln in Siltgröße aufweisen. Ob dabei tiefenabhängige Änderungen im Salzgehalt der Porenwässer eine Rolle spielen, ist nicht geklärt. Der durchschnittliche Wassergehalt eines Schlammstroms (field liquid limit) kann nach den Erfahrungen aus terrestrischen Massenverlagerungen sogar unter der Atterberg'schen »Labor-Fließgrenze« der feinkörnigen, breiigen Matrix liegen, wenn festere und wasserärmere Sedimentschollen mitgeführt werden.

Die mitgeteilten Beispiele beruhen auf den Ergebnissen des Deep Sea Drilling Project, die durch eigene Laborbestimmungen ergänzt wurden. In einigen Fällen können subfossile submarine Massenverlagerungen mit benachbarten Slope-Sedimenten verglichen werden. Wenn mit zunehmender Tiefe unter dem Meeresboden die chemisch-mineralogische Diagenese einsetzt, kann bei mikroorganismen-reichen Sedimenten die Situation eintreten, daß in einem gewissen Übergangsbereich die in-situ-Wassergehalte und Fließgrenzen in ähnlicher Weise abnehmen. Dann kann trotz zunehmender Kompaktion ein hohes Liquefaktionspotential, zumindest in Teilschichten, aufrecht erhalten werden, bis schließlich starke Diagenese sämtliche sediment-physikalischen Eigenschaften drastisch ändert.

Résumé

Les courbes du contenu en eau en fonction de la profondeur et les limites de liquidité d'Atterberg, établies dans des sédiments fins et peu cohérents déposés sur des pentes sousmarines amènent à formuler l'hypothèse que de nombreux sédiments de ce type sont susceptibles, en réponse à des sollicitations dynamiques (tempêtes, séismes) de se transformer en coulées de boues (mud flows) sans addition d'eau. D'ordinaire, dans les premiers mètres, mais souvent aussi à des dizaines, voire des centaines de mètres sous le fond de la mer, le contenu en eau peut être supérieur ou à peu près égal à la valeur limite de liquidité (transition entre les états plastique et liquide des sols ou des sédiments). Dans de telles conditions, les sédiments possèdent un potentiel élevé de liquéfaction. Cette situation est particulièrement fréquente dans les régions à taux de sédimentation élevé entraînant une sousconsolidation, ainsi que dans des sédiments riches en particules biologiques de la taille des silts. Une influence éventuelle d'une variation de la salinité des eaux intersticielles avec la profondeur n'est pas évidente. Si une partie des matériaux est déplacée en bloc sous la forme de loupes de boue incluses dans une matrice liquéfiée, la limite de liquidité moyenne «de terrain» peut être inférieure à la limite d'Atterberg, établie sur des échantillons de petite taille homogènes et à grain fin.

Les exemples présentés ici sont basés sur les résultats du Deep Sea Drilling Project, complétés par des opérations de laboratoire. Dans certains cas, les properiétés physiques d'ensemble de volumes subfossiles déplacés en masse peuvent être comparées à celles des sédiments de pente voisins. Dans les sédiments riches en microfossiles, le contenu en eau et les limites de liquidité décroissent de la même manière avec la profondeur, en raison d'une légère diagenèse chimique. De la sorte, un potentiel de liquéfaction élevé peut se maintenir en dépit d'une compaction croissante, jusqu'à ce que, finalement, une lapidification complète modifie toutes les proeriétés physiques de masse.

Краткое содержание

На основании сопоста вления соотношения с одержания воды в седиментах и гл убины их залегания, а т акже определения границы текучести по методу Atterberg'a на мелкозернистых, но мало когезивных седи ментах морского склона можн о высказать следующе е соображение: В резуль тате динамической на грузки (землетрясения, волны штормов) при сколе мно гие из этих седиментов могу т перейти в состояние жидкой кашицы даже без поглащения в оды извне и т.о. образов ывать грязевые потоки из ил ов (debris flows & mud flows). Содержание воды в эт их седиментах in-situ почти всегда выше, или такое же как и содержание во ды у предела текучести, т.е. перехода почв, или с едиментов из пластического к те кучему состоянию. Счи тают, что при этих условиях седименты обладают повышенным потенциа лом разжижения. Такая ситуация наблюдается в тех слу чаях, когда при большо й скорости осадконако пления, седимент нахо дится в состоянии ниже состо яния консолидации, а т акже в седиментах, характеризующихся в ысоким содержанием биогенных частичек, в еличина которых лежи т в размерности силта. Играет-ли при эт ом какую-либо роль глубоко идущие измен ения содержания соле й в поровой воде — еще не выяснено. Как известно из перен оса массы материковых ос адков, среднее содерж ание воды в потоках грязи (field liquid limit) мо жет в случае кашеобра зной матрицы лежать ниже А ттербергового предела текучести, ус тановленного в лабор атории, если переносятся и тв ердые, бедные водой гл ыбки седимента.

Приведенные примеры составлены п о литературным данным и дополнены со бственными лаборато рными исследованиями. В нек оторых случаях можно сопоставлять субфос сильные подводные пе реносы массы с соседними сед иментами на склонах. Е сли с возрастяющей глубин ой захоронения седим ентов в них протекают процес сы химико-минералоги ческого диагенеза, то в случае седимента, богатого микроорганизмами, может создаться ситу ация, при которой в нек ой известной переходно й области содержание воды in-situ и предел текучести п онижаются параллель но.

Тогда, даже при возрас тающем уплотнении, со храняется высокий потенциал ра зжижения. особенно в известных частях гор изонтов до тех пор, пок а усиленные процессы диагенеза н е изменят радикально все физические свойства седиментов.

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Copyright information

© Ferdinand Enke Verlag Stuttgart 1989

Authors and Affiliations

  • Gerhard Einsele
    • 1
  1. 1.Geological InstituteUniversity of TübingenTübingenF. R. Germany

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