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Hydrofracture System in Glaciolacustrine Unit at Novigrad Sea SW Coast, Croatia

The Middle Pleistocene glaciolacustrine allo- unit of Novigrad Sea coastal section is represented with poorly cemented varved-like calcisiltites and rippled calcarenites (MARJANAC, 2012), though few well-cemented horizons are distinct. This allo-unit extends for about 2 km along the coast and documents glacial recession as well as younger glacial advance. The hydrofracture system comprises a variety of small-scale faults, folds, disrupted layers and clastic dykes (Fig. 1). All structures occur concentrated in zones up to 2 m wide and up to 3 m visible vertical intervals. Host sediment is locally strongly disturbed and clay beds were broken and discontinued due to the high pressure of escaping water through the sediment. Narrow clastic dykes representing water escape pipes, up to 1 m in length, are locally visible and are filled with sand-size sediment (Fig. 1a). They occur in association with numerous small-scale reverse faults and small-scale folds (Fig. 1b). Laterally, within the same interval occur soft-sediment deformations with characteristic morphology of liquefaction/fluidization-induced seismites (Fig. 1c) found elsewhere in Pleistocene lacustrine sediments (MARJANAC et al, 2001 ; MORETTI & VAN LOON, 2014 ; ÜNER, 2014). Hydrofracture dykes of inclined step-like morphology and thin sediment infill occur locally and are best visible in fine-grained intervals while they die out below a coarser grained interval (Fig. 1d). Hydrofracture systems, in general, resemble water-escape features and „represent a visible expression of the passage of pressurized meltwater through subglacial to ice-marginal environments” (PHILIPS et al, 2013). Nevertheless, this hydrofracture system in the glaciolacustrine unit of Novigrad section is likely not just a result of a significant pressure produced during glacial advance, which is documented by the younger Novigrad moraine (subglacial till) deposited over the glaciolacustrine sediments (MARJANAC, 2012), but also a result of glacioseismic events related to deglaciation.

glaciolacustrine sediments, hydrofracture system, Novigrad Sea

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