Recognizing soft-sediment structures in deformed rocks of orogens

• Published in: Journal of structural geology 2011-03, Vol.33 (3), p.271-279
• Main Authors: Waldron, John W.F., Gagnon, Jean-François
• Format: Article
• Language: English
• Subjects: Deformation; Dykes; Folds; Fragments; Indicators; Liquefaction; Mud; Overprinting; Rocks; Sand; Slope processes; Soft-sediment deformation; Tectonics
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2010.06.015

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 difficult 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 (lithified or unlithified) 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 unlithified sediment during deformation. ► Soft-sediment structures are difficult to recognize where tectonically overprinted. ► Distinguish lithification questions from kinematic questions. ► Liquefaction causes sand to have lower strength than surrounding mud.► Class 3 folds and angular re-entrants indicate deformation during sand liquefaction. ► These geometries in low-grade rocks are indicators of soft-sediment deformation.