engleski

Contrasting styles of sedimentation on the opposing limbs of blind-thrust synclinal troughs in an evolving orogenic wedge-top basin (Eocene Dinaric Foreland, Croatia)

The Eocene–Oligocene Dinaric peripheral foreland basin in the northern Dalmatia developed by a SW-directed orogenic thrusting and comprised two distinct palaeogeographic zones: a shallowmarine proximal foreland where a calciclastic succession of neritic to terrestrial sediments known as the Promina Beds was deposited, and a deep-water distal foreland accumulating turbiditic deposits known as the Dinaric Flysch. The proximal foreland evolved gradually into a thrust wedge-top (‘piggy-back’) basin, while the tectonically quieter distal foredeep was filled up with the Dinaric Flysch and was eventually onlapped by the Promina Beds. The progressive tectonic shortening of the orogenic thrust wedge caused formation of a series of SW-vergent blind-thrust growth folds within the wedge-top basin, resulting in its compartmentalization into an array of narrow, high-relief marine sub-basins (synclinal troughs), where the main part of the Promina Beds succession was deposited. An illustrative example of these troughs is the Novigrad sub-basin, a SE-trending synclinal swale formed close to the orogen front and filled with sediments in the middle to late Eocene. The two opposing limbs of the asymmetric synclinal trough show markedly different styles of sedimentation. The steeper NE limb hosted coarse-grained shoal-water to Gilbert-type deltas, whereas the gentler SW limb hosted a wave-dominated gravelly shoreline (foreshore zone) with sandy shoreface, heterolithic sand-mud offshore transition and muddy offshore zone. The offshore-transition and offshore deposits here are commonly intercalated with foreshore-derived, gravelly debris-flow and slump deposits, which represent resedimentation pulses triggered most probably by a normal faulting of the syncline limb growing in its relief. A similarly contrasting style of sedimentation is recognizable on the opposing limbs of other synclinal troughs in the Dinaric wedge-top basin in northern Dalmatia, with the steeper NE limb shedding sediment gravity flows and the gentler SW limb hosting a littoral system prone to gravitational collapses and resedimentation. This evidence indicates a wedge-top basin turning into a system of blind-thrust growth folds as a result of tectonic contraction. The structural development in such a deformation system is intricate, as it may involve both in-sequence and out-of-sequence thrusts, as well as back-thrusts and related pop-up ridges. In addition, the entire thrust wedge-top basin may be episodically uplifted piggy-back by its soling master thrust and be episodically subsiding due the crustal load of orogen thrust-sheets. The inherent shortterm variability in the syndepositional tectonic development of a wedge-top basin bears several important stratigraphic ramifications as to the facies record and relative sea-level changes: (1) drastic facies changes may occur over short distances away from the orogenic front ; (2) a forced or normal regression on one limb of a synclinal trough may be coeval with a transgression on the other limb ; (3) some of the marine transgressions and regressions may occur synchronously on both limbs of the synclinal trough, irrespective of the difference in local bathymetry and sedimentary environment ; and (4) progressive unconformities may be a common feature of the basin-fill succession. This Dinaric case study may be relevant to a palaeogeographic development of the wedge-top parts of the ‘inner’ Carpathians and other orogenic belts.

contrasting depositional styles; blind thrusts; Eocene; Promina Basin; Croatia

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