engleski

Mass transport deposits (MTDs) in a shallow-marine succession of the Dinaric Foreland Basin

Mass transport deposits (MTDs) are common in relatively deep and tectonically active settings and represent distinct “events” in the development of a sedimentary basin. The present study from the Dinaric Foreland Basin documents a rare example of a 300 m thick shallow-marine succession characterised by about 50 MTD units interbedded with calcilutites and calcarenites of the offshore and offshore transition zone. The succession crops out in the vicinity of Novigrad in northern Dalmatia, Croatia, and represents the infill of a sub-basin that developed in the wedge-top part of the evolving foreland during the Middle Eocene to Early Oligocene (Ćosović et al., 2018). Four types of MTDs, ranging in thickness from 20 cm to 600 cm and extending for more than 600 m have been identified: (1) slumped offshore and offshore transition strata, commonly displaying convoluted bedding and signs of sediment torsion ; (2) slides comprising blocks of clast supported beachface conglomerates and/or bioturbated shoreface calcarenites encased in offshore strata ; (3) blocky-flow deposits comprising matrix-supported conglomerates with large blocks of deformed shoreface and/or offshore transition strata ; and (4) debrites comprising massive, matrix- supported conglomerates. Large calcarenite and/or conglomerate blocks in MTD types 2 and 3 were incorporated as consolidated blocks into the mass transport, suggesting significant erosion of older units. The study succession shows several transgressive-regressive cycles and progressive deepening of the sedimentary basin with an accompanying increase in MTD incidence, especially of types 2–4. Small-scale slumps of type 1 occur throughout the succession and commonly within transgressive offshore deposits. Where parasequences can be recognised, other types of MTDs tend to be associated with regressive offshore transition and offshore deposits. Relative sea- level fluctuations probably stemmed from non- synchronous uplift and subsidence of neighbouring blind thrust anticlines, possibly leading to coeval transgressions and regressions on opposite limbs of the same synclinal sub-basin (Mrinjek et al., 2012 ; Ćosović et al., 2018). The multitude of MTDs in the uppermost part of the succession was likely favoured by basin floor steepening associated with increasingly frequent uplift pulses, possible emergence and erosion of the basin bounding growth anticline. Contrarily, basin deepening and ramp extension during transgressive stages could have favoured mass-flow transformations. A generic relationship between relative basin depth and MTD type is suggested, whereby slides would be indicative of a somewhat shallower environment compared to blocky flow deposits and debrites. Such alternation of various types of MTDs in a shallow-marine succession attests the colossal power of syn- sedimentary tectonics during basin development and may give a clue on small scale transgressive regressive cycles when flooding surfaces and/or correlative conformities cannot be readily identified.

Mass transport deposits, Dinaric Foreland Basin, debrites, blocky flows, slumps

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