Deformation of sediments and sedimentary rocks
Davidson-Arnott Ed. Meddelelser om Groenland "Communications on Greenland" , v. Tectonics, v. India, v. In: M. Jones and R. Preston Eds. London Spec. Frontiers of Earth Sci.
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Soft-Sediment Deformation: Recent Flood Evidence
Mazumder, D. Evans and A. Collins, Eds. Professional Paper D, DD Springer- Verlag, Berlin, p. Kluwer Academic Publishers, Dordrecht, p. Binghamton, New York. Structural Geol. Geologie en Mijnbouw, v. Prasad, K. Chennai, India.
Soft Sediment Deformation
Book Review. Marzo and C. Puigdefabregas Eds. South Amer. Earth Sci. In: P. Scholle and D. Spearing Eds. ORD, D. OWEN, G. Sedimentary Res. POPE, M. Sedimentary recorders of earthquakes in Appalachian basin. Geolines, v. ROeE, S. RUST, B. Journal of Sedimentary Research, v. These sediments are the result of a braided fluvial system characterized by plane parallel laminae, desiccation cracks, ripples, and cross bed sets, antidunes, groove and flute casts. Several types of soft-sediment deformation structures were recorded in Losero deposits and these occur interbedded with undeformed strata throughout the entire stratigraphic interval.
The lateral extent of the deformation is of several hundred meters long and the deformed bed involves the mid-upper part of the 30 meter thick succession. Deformation affects about two meters of coarse-grained sandstone, fine-grained sandstones, and mudstones.
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This deformation is represented by layers with asymmetric and symmetric folds, dish structures, pseudonodules, sandstone dikes, flame structures, syn-sedimentary normal microfaults and convolute laminae. It can be described as a gradient system of unstable density layers.
Soft-sedimentary deformation structures observed in the Losero Formation are interpreted as seismites, developed as a result of in-creased pore pressure and vertical or horizontal stresses induced by seismic activity. Seismites are soft-sediment features produced by paleoearthquakes. These are formed after sediment deposition, before sediment compaction, and during the sudden breakdown of a loosely packed, water-saturated grain framework with low to zero shear resistance as a result of tectonic and sedimentary processes Rossetti and Santos, ; Alfaro et al.
The term "seismites" proposed by Seilacher , is commonly used in the literature to describe deformation of unconsolidated sediments caused by earthquakes Mills, ; Maltman, ; Collinson, Soft-sediment deformation structures are very important in paleogeographic reconstruction, as they indicate seismic activity, and are also essential components in the analysis of the conditions affecting the deposition of sediments Sims, ; Allen, ; Mills, ; Seth et al.
However, their origin is still questioned because the same or similar structures can be produced by non-seismic processes autokinetic triggering mechanisms : storm waves, gravitational or rapid sediment accumulation, cyclic loading from vortices, flow conditions approaching the plane-bed phase, flood surges, flash floods, or from frictional drag of saturated sand Selley, ; Hendry and Stauffer, ; Lowe, ; Owen, ; Wells et al.
To help understand whether soft-sediment deformation structures are sedimentation controlled or seismically induced allokinetic triggering mechanisms , a combination of various field criteria, including water escape features, hydroplastic deformation, liquefaction and fluidization features, and seismic activity related to episodic fault motions, normally of magnitude 5 or greater, is described by different authors Sims, ; Obermeier, ; Jones and Omoto, ; Moretti and Ronchi, ; Kundu et al. Soft-sediment deformation structures have been attributed to porepressure changes, mainly caused by fluid escape during fluidization and liquefaction Lowe, ; Owen, Liquefaction features are mentioned as the most common type of soft-sediment deformation related to earthquakes Allen, ; Obermeier, This liquefaction can be induced by different triggering agents earthquakes, overloading and storm waves, among others.
Earthquake-induced structures represent different sedimentary responses to seismic shaking and depend on several interrelated factors e. This work presents the first report of a suite of soft-sediment deformation structures in fluvial deposits of Eocene-Oligocene age, which accumulated in a sedimentary basin located in the Sierra de Guanajuato Figure 1.
The aim of this paper is to document the nature and distribution of the deformation structures preserved in the Losero Formation, to discuss the processes responsible for their genesis and analyze the possible influence of paleoseismic events as the triggering mechanism for sediment deformation.
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Lapierre et al. The sedimentary Cenozoic succession was deposited in an extensional tectonic setting, represented by synvolcanic conjugated sets of normal faults. The Guanajuato conglomerate presents a thickness between and m and was divided by Edwards into two members. The upper member has a thickness that varies from to m, increasing toward the southeast, consisting of a matrix-supported conglomerate.
The Losero Formation has been considered as the base of the Oligocene volcanic cover, overlying the Guanajuato conglomerate, and has been previously described by a number of geologists e. The Losero Formation consists of brown and green fine to coarse-grained sandstones intercalated with mudstones, accumulated in a fluvial environment by predominantly turbulent flow and it represents the distal facies of the Guanajuato conglomerate Puy-Alquiza et al.
The possibility that these deposits are related to volcanic activity is not discarded considering that some lithofacies contain volcaniclastic material derived from the redeposition of the products of volcanic activity. The Losero Formation consists of clastic rocks dominated by sandstones and mudstone, with internal erosional surfaces. The succession is dominated by sets with plane-parallel bedding alternating with laterally-extensive sets with cross-stratification. Recognized lithofacies and depositional architectural elements are presented in Tables 1 , 2 and 3.
The formation has been divided into two informal members Puy-Alquiza et al. The lower member, about 5 m thick, rests on a surface of Eocene conglomerate Guanajuato Conglomerate Formation. The most distinctive lithological feature of this member is a coarse- to fine-grained sandstone intercalated with brown mudstone deposits. The upper member rests conformably on the lower member, and is characterized by a 25 m thick brownish red and green sandstone intercalated with mudstone.
Table 2 Sedimentary facies associated with soft-sediment deformation structures in the lower member of the Losero Formation. Sedimentary facies Description Interpretation Coarse-grained sandstone with normal gradation facies Sg. Brownish red sandstone, with normal gradation. Massive medium- to coarse-grained sandstone.
Scour fill. Medium- to fine-grained sandstone with plane-parallel laminae facies Sh. Brownish red, medium- to fine-grained sandstone with planeparallel laminae. Thickness of individual laminae sets range from 0.
Some sets of laminae show undulations and small cusps indicative of syn-sedimentary deformation, probably induced by water escaping during deposition. Evidence of tractive flow in upper flow regime Harms, Medium-grained sandstone with cross lamination facies St.
Atlas of syn-sedimentary deformation
Green medium-grained sandstone with cross lamination. The majority of the sets have a very low angle and seem to be a laterally extensive combination of trough and tabular types. Thickness of individual cross bed sets ranges from 0.