Description:

_Journal of Structural Geology 33 (2011) 271e279_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Recognizing soft-sediment structures in deformed rocks of orogens_ _John W.F. Waldron*, Jean-Fran?ois Gagnon Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 ESB, Edmonton, AB, Canada T6G 2E3_ _article info_ _Article history: Received 31 January 2010; Received in revised form 13 May 2010; Accepted 20 June 2010; Available online 30 June 2010_ _Keywords: Soft-sediment deformation Slope processes Liquefaction Folds Overprinting_ _abstract_ _Soft-sediment deformation structures are common on passive continental margins, in trenches at subduction zones, and in strike-slip environments. Rocks from all these tectonic environments are incorporated into orogens, where soft-sediment deformation structures should be common. However, recognizing soft-sediment structures is dif?cult where superimposed tectonic structures are present. In seeking characteristic features of soft-sediment deformation, it is important to separate questions that relate to physical state (lithi?ed or unlithi?ed) from those that address the overall kinematic style (rooted or gravity driven). One recognizable physical state is liquefaction, which produces sand that has much lower strength than interbedded mud. Hence structures which indicate that mud was stronger than adjacent sand at the time of deformation can be used as indicators of soft-sediment deformation. These include angular fragments of mud surrounded by sand, dykes of sand cutting mud, and most usefully, folded sandstone layers displaying class 3 geometry interbedded with mud layers that show class 1 geometry. All these geometries have the potential to survive overprinting by later superimposed tectonic deformation; when preserved in deformed sedimentary rocks at low metamorphic grade they are indicators of liquefaction of unlithi?ed sediment during deformation._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Soft-sediment deformation is a widespread phenomenon in a variety of tectonic settings including passive continental margins, subduction zones, and strike-slip environments. Because basins from these environments occur in orogens, soft-sediment structures would be expected to be equally common in the deformed rocks of orogens. However, separating soft-sediment deformation structures from those induced by later tectonism is challenging._ _This article stems from a discussion in the early 1980s between the senior author and Paul F. Williams, to whom this issue is dedicated, about the nature of structures in deformed clastic sedimentary rocks at low grade in central Newfoundland (Williams, 1983). Subsequent work by Paul and his students (e.g., Elliott and Williams, 1988) elsewhere in Newfoundland indicated that folds previously interpreted as synsedimentary had in fact formed much later in the deformation history. In the process, they showed that many of the features previously used as indicators of ‘soft-sediment deformation’ are invalid. Maltman (1994a) lamented the lack of clear criteria for recognizing the products of soft-sediment_ _? Corresponding author. Tel.: ?1 780 492 3892; fax: ?1 780 492 2030. E-mail address: john.waldron@ualberta.ca (J.W.F. Waldron)._ _1 Present address: Shell Canada Limited, 400 4th Avenue S.W., P.O. Box 100, Station M, Calgary, Alberta T2P 2H5, Canada._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2010.06.015_ _deformation where later tectonic overprints are present, a problem which has concerned both sedimentary and structural geologists for many decades. Since that time much has been learned about processes that deform sediment on present-day continental margins. The purpose of this article is brie?y to review the occurrence of present-day soft-sediment deformation in environments that have the potential for preservation in future orogenic belts, and to suggest some geometrical criteria for the recognition of soft-sediment structures, particularly folds, in ancient orogens, where they have been overprinted by later tectonic events._ _2. Soft-sediment deformation: de?nition_ _We de?ne soft-sediment deformation, following Maltman (1984, 1994b) as any deformation, other than vertical compaction, of a sediment or sedimentary rock that is achieved by rearrangement of the original sedimentary particles, without internal deformation of those particles or of any interstitial cement. Deformation occurs primarily by the mechanism of grain-boundary sliding. Soft-sediment deformation, thus de?ned, passes imperceptibly into sedimentary processes such as debris ?ow. In general, processes like slumping, that leave some of the original bedding of previously deposited sediment, are included in ‘soft-sediment deformation’ whereas those that largely destroy pre-existing structures, such as debris ?ow, are generally regarded as_ _272_ _J.W.F. Waldron, J.-F. Gagnon Journal of Structural Geology 33 (2011) 271e279_ _sedimentation processes, but the distinction is arbitrary (Maltman, 1994b). In sandstones or conglomerates where grains have low sphericity, soft-sediment deformation typically produces no penetrative fabric at grain-scale. Where sedimentary grains have inequant shapes, however, it is possible for soft-sediment deformation to produce a fabric, though it is usually weak. Inequant grain-shapes are almost universal in ?ne-grained sediments (silt and mud) and indeed, ?ne-grained sedimentary rocks typically display a fabric (?ssility), resulting from the preferred orientation of inequant grains during compaction. Because a fabric acquired during synsedimentary deformation might conceivably be emphasized mimetically during later tectonic deformation, the presence of a related fabric cannot unequivocally be taken as evidence that a structure is of tectonic origin._ _3. Occurrence of soft-sediment deformation_ _Deformation of unlithi?ed sediment occurs in numerous present-day environments, including unstable terrestrial slopes, areas of rapid marine and marginal marine sedimentation such as deltas, and sedimentary basins that are cut by active faults. In many such areas, soft-sediment deformation is a major hazard for human populations (e.g., Brunsden and Prior, 1984). To reduce the associated risk, signi?cant research and engineering effort have been devoted to the prediction and even prevention of soft-sediment deformation (e.g., Hearn and Grif?ths, 2001)._ _Actualism suggests that analogous structures must exist in ancient sedimentary rocks, and studies in undeformed sedimentary basins have typically been successful in identifying the products of soft-sediment deformation where contemporary tectonic structures are absent (e.g., Collinson, 1994; Strachan and Alsop, 2006; Strachan, 2008). In many such areas, soft-sediment deformation is associated with gravitationally driven movement of sediment down slopes that were formed during sediment deposition. The acquisition of deep seismic re?ection pro?les from continental slopes has enormously increased our knowledge of such structures on passive continental margins (e.g., Bilotti and Shaw, 2005; Morgan, 2003; Morley and Guerin, 1996; Morley, 2003)._ _Two challenging and distinct groups of questions arise in the discussion of soft-sediment deformation processes recorded in ancient rocks. The ?rst group relates to the mechanical state of the sediments at the time of deformation: where they compacted, cemented, or otherwise lithi?ed, and how strong were they? The second group of questions relates to the larger scale driving forces that led to deformation. This group includes questions like ‘was deformation entirely due to gravitational instability of slopes or was part of the differential stress for deformation supplied by tectonic movements at depth?’. The second group of questions is often summarized as a dichotomous choice between ‘gravity-driven’ and ‘tectonic’ deformation. However, this statement of the dichotomy makes a false distinction, because gravitational forces acting on slopes are responsible for a large part of the differential stress even in clearly ‘tectonic’ deformation processes. For example, in foreland fold and thrust belts, surface slope is a large component of the ‘critical taper’ required for the self-similar growth of a tectonically driven deformed zone (Dahlen et al., 1984; Davis et al., 1983). A clearer distinction can be made on kinematic grounds between deformation that is ‘super?cial’ because it occurs above a basal detachment that is linked to the surface in both upslope and down-slope directions, and deformation that is ‘rooted’ in a shear zone or fault zone at depth (Fig. 1)._ _Even with this clari?cation, confusion between the two groups of questions is still common. Most practising geologists have a tendency, once it is shown that deformation occurred in unlithi?ed sediment, to assume that it occurs by super?cial, down-slope gravity-driven processes. However, soft-sediment deformation is clearly occurring at many present-day plate boundaries, where the driving force and overall kinematics are driven by rooted, tectonic processes. Furthermore, under some circumstances it is possible for pockets of unlithi?ed sediment to become mobilized during deformation in otherwise lithi?ed sedimentary packages undergoing tectonic deformation (Phillips and Alsop, 2000). Conversely, it is possible for quite large slabs of lithi?ed rock to move, and even deform internally, in a scenario where movement occurs entirely down-slope, displaying ‘super?cial’ kinematics. Fig. 1 presents an idealized, two dimensional representation of the spectrum of deformational processes, with four end-member environments._ _End-member A (super?cial, unlithi?ed) represents super?cial down-slope movement of unlithi?ed sediment in slumps and gravity slides. End-member B (rooted, unlithi?ed) includes deformation of unlithi?ed sediments in_ Ключевые слова: formed, special, case study, orogens, criterion, question, stronger, contact, demonstrated, sedimentary rock, matter, fold-and-thrust belt, layer thickness, interpretation, deformed, folds, surface, sediments, morley, robertson, sedimentology, soft, morgan, bulletin, ha, relative strength, slump, soft-sediment deformation, antalya complex, fold formed, mudstone layer, strength, williams, scale, softsediment structure, journal structural, maltman, scotia, geometrical criterion, typically, tectonic process, structural geologist, bell, wilson, post-lithication deformation, evidence, lithied material, sandstone-lled dyke, hinge, angular, deposited sediment, rooted, canada, sediment deformation, journal, strachan, introduction, recognition, sandstone, rocks, schultz, differential stress, nova scotia, pickering, strength reversal, intraclast conglomerate, soft-sediment, welded contact, fabric, process, rooted deformation, folded, eds deformation, up-slope extension, lithied, geological, slope, class geometry, study, publication, london, surrounding sand, liquefaction doe, surrounded, concave re-entrants, mudstone body, gagnon, soft-sediment fold, pajari, brunswick, soft-sediment origin, environment, feature, clear, distinction, waldron, class fold, case, formation, taylor, interbedded mud, newfoundland, collinson, ?ow, structural, unit, deformation structure, product, van rensbergen, style, weak layer, deformation, dyke, wiley, soft sediment deformation, mobile shale, geology, conglomerate, alexander, sediment surface, class layer, subduction zone, cleavage, sediment, clastic, metamorphic grade, ramsay, deformed rock, structural geology, unlithi?ed sediment, zone, gagnon journal, common, deformation occurred, wa, america bulletin, class, vast majority, amundsen, lithied rock, society, belt, superimposed, deformation process, structure, tectonic, mudstone, mud, debris, strain, time, international union, fold, sedimentary, layer, horne, geometry, passive, reversal, group, outcrop, relationship, tighter curvature, unlithi?ed, basin, super?cial, alsop, -f, geological society, eds, folding, basal detachment, hesse, review, down-slope movement, interpreted, unlithied sediment, mudstone bed, shape, liquefaction, characteristic, bed, geological deformation, soft sediment, continental, mud layer, smith, lithi?ed, sandstone body, nova, sand, margin, overprinting, area, rock, kenney, fold hinge, deposited