Soft-sediment deformation structures (SSDS) in Gilbert-type glaciolacustine delta

What is it about?

The development of soft-sediment deformation structures in clastic sediments is now reasonably well-understood but their development in various deltaic subenvironments is not. A sedimentological analysis of a Gilbert-type glaciolacustine delta with gravity-induced slides and slumps in north-western Poland provides more insight, because the various soft-sediment deformation structures in these deposits were considered in the context of their specific deltaic subenvironment. The sediments show three main groups of soft-sediment deformation structures in layers between undeformed sediments.

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Why is it important?

In river-dominated deltas with a high sediment supply and with frequent changes of the river water level, gravitational instabilities (which occur in the transitional zone between the high-energy braided river and the low-energy lake) cause in situ deformations within the topsets and foresets due to accretion of mouth bars. Mass-transport processes (slumps and slides) produce folds verging downwards on the foresets and also result in numerous water-escape structures. The deformations developed on the Gilbert-type delta syndepositionally, metadepositionally and/or post-depositionally as a consequence of triggers such as a high sedimentation rate, erosion by waves or currents, ice-sheet loading and seasonal changes in the ablation rate. Plastic deformations developed due to gravity-induced disturbance of rapidly deposited sediments of the delta topsets, by loading, by ground water movement and by dragging. Water-escape structures (WES) developed due to dewatering and elutriation. The deformation structures point to brittle behaviour of the sediment, such as faults, and developed mainly due to gravity-induced mass movement. The transitional conditions from ductile to brittle deformation must be ascribed to a quasi-solid state of the sediment (small extensional fissures within deformed cross-bedded sands). In the bottomsets and foresets of the delta, large-scale recumbent and sheath folds developed, together with pillar structures and faults; in the topsets, deformed cross-bedding and extensional fissures formed. In the zone between the foresets and the overbank deposits, large-scale recumbent and sheath folds developed, as well as clastic dykes associated with sand volcanoes and faults. In the border zone between the overbank deposits and the active distributary channels, pillow structures developed; clastic megadykes intruded all deltaic deposits. The deformed cross-bedding developed due to seasonal changes in the ablation rate, which were responsible for a rising ground water table and for dragging by currents. Differences in the rheological state of the sediments caused the development of single folds, which pass into complex recumbent folds and sheath folds, and finally detached depressions with small extensional fissures. Subaqueous slumps on the delta slope caused deformations inside the slumping material, particularly in the head as well as in the sediments on which the slumping material exerted a pressure. The different kinds of folds resulted from different fluidization rates, which were a consequence of different lithologies. An increase of the pore-water pressure and the development of WES and associated structures in the rapidly deposited sandy glaciolacus-trine sediments were caused by compaction due to folding, flow till deposition, ice-sheet loading and deformation of the banks of the glacial lake. Post-depositional mass movement of consolidated or semi-consolidated deposits (glacial tills) caused: (i) a compression zone and an increase of the pore-water pressure; and (ii) extension zones resulting in the formation of neptunian dykes.