Please cite this article in press as: Põldsaar, K., Ainsaar, L., Soft-sediment deformation structures in the Cambrian (Series 2) tidal deposits (NW Estonia): Implications for identifying endogenic triggering mechanisms in ancient sedimentary record. Palaeoworld (2015), http://dx.doi.org/10.1016/j.palwor.2014.12.003 ARTICLE IN PRESS +Model PALWOR-274; No. of Pages 20 Available online at www.sciencedirect.com ScienceDirect Palaeoworld xxx (2015) xxx–xxx Soft-sediment deformation structures in the Cambrian (Series 2) tidal deposits (NW Estonia): Implications for identifying endogenic triggering mechanisms in ancient sedimentary record Kairi Põldsaar ∗ , Leho Ainsaar Department of Geology, University of Tartu, Ravila 14a, 50411 Tartu, Estonia Received 15 September 2014; received in revised form 9 December 2014; accepted 19 December 2014 Abstract Soft-sediment deformation structures (SSDS) are documented in several horizons within silt- and sandstones of the Cambrian Series 2 (Domi- nopolian Stage) Tiskre Formation, and some in the underlying argillaceous deposits of the Lükati Formation in NW Estonia, northern part of the Baltoscandian Palaeobasin. The aim of this study was to map, describe, and analyze these deformation features, and discuss their deformation mechanism and possible triggers. Load structures (simple load casts, pillows, flame structures, and convoluted lamination) with varying shapes and sizes occur in the Tiskre Formation in sedimentary interfaces within medium-bedded peritidal rhythmites (siltstone-argillaceous material) as well as within up to 3 m thick slightly seaward inclined stacked sandstone sequences. Homogenized beds, dish-and-pillar structures, and severely deformed bedding are also found within these stacked silt- and sandstone units and within a large tidal runoff channel infill. Autoclastic breccias and water-escape channels are rare and occur only in small-scale — always related to thin, horizontal tidal laminae. Profound sedimentary dykes, sand volcanoes, and thrust faults, which are often related to earthquake-triggered soft sediment deformation, were not observed within the studied intervals. Deformation horizon or horizons with large flat-topped pillows often with elongated morphologies occur at or near the boundary between the Tiskre and Lükati formations. Deformation mechanisms identified in this study for the various deformation types are gravitationally unstable reversed density gradient (especially in case of load features that are related to profound sedimentary interfaces) and lateral shear stress due to sediment current drag (in case of deformation structures that are not related to loading at any apparent sedimentary interface). Synsedimentary liquefaction was identified as the primary driving force in most of the observed deformation horizons. Clay thixotropy may have contributed to the formation of large sandstone pillows within the Tiskre–Lükati boundary interval at some localities. It is discussed here that the formation of the observed SSDS is genetically related to the restless dynamics of the storm-influenced open marine tidal depositional environment. The most obvious causes of deformation were storm-wave loading, rapid-deposition and shear and slumping caused by tidal surges. © 2014 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved. Keywords: Soft-sediment deformation structures; Liquefaction; Tides; Storm-wave loading; Cambrian; Estonia 1. Introduction Soft-sediment deformation structures (SSDS) preserved in the geological record can provide insights into sedimentary his- tory and geodynamic evolution of a sedimentary basin. These structures form due to the application of shock, as for example ground motion would cause during earthquakes. Soft-sediment deformations can also form due to stress applied by cyclic ∗ Corresponding author. Tel.: +372 58 330 832. E-mail address: kairi.poldsaar@ut.ee (K. Põldsaar). storm-waves or loading due to rapid deposition of sediments among many other triggers. Whatever the trigger, the gradual built-up of hydrostatic pressure within the water saturated sed- iments can ultimately lead to a rapid loss of shear resistance and collapse of grain framework (Chaney and Fang, 1991). As a result, sediments become liquefied and prone to deforma- tion until granular strength is regained (Ambraseys and Sarma, 1969). The mechanism and physics of sediment liquefaction have been demonstrated in laboratory tests and discussed in many excellent works by the pioneering researchers in this field (Terzaghi and Peck, 1948; Kuenen, 1958; Seed and Lee, 1966, 1971; Anketell and Dzulynski, 1968; Anketell et al., 1969, 1970; http://dx.doi.org/10.1016/j.palwor.2014.12.003 1871-174X/© 2014 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved.