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Slumps as indicators of basin geometry and development - examples from the Promina Beds in northern Dalmatia

The Promina Beds in northern Dalmatia are a more than 2000 m thick calciclastic succession of Middle Eocene–Early Oligocene age, constituting the infill of the piggyback part of the Dinaric Foreland Basin (Fig. A, B ; MRINJEK et al., 2011 ; ĆOSOVIĆ et al., 2018). Progressive thrusting due to the collision between Africa and Europe compartmentalized the basin and led to the formation of an array of narrow marine sub-basins, separated from one another by ridges developed on blind-thrust growth folds. These sub-basins had steep flanks, whereby dynamic synsedimentary tectonic activity resulted in drastic facies changes over short distances, including frequent slumping. Slumps in the Promina Beds occur in marine deposits of different age and in different sub basins (Fig. B). They commonly display contorted strata (Fig. C–E) and evidence of sediment torsion, resulting from gravitational en masse movement of sediment along steep and unstable slopes. Their transport direction can be inferred from their geometry and in most cases it is perpendicular to the southeast- trending structural lineaments (Fig. B). Most of the documented slumps show transport direction towards the south-west (Fig. B, E). These observations indicate that the sub-basins were likely asymmetrical, i.e. steeper along their NE margin, which increased slope instability and its susceptibility to repeated slumping. Alternatively, tectonic perturbations associated with progressive nappe thrusting might have been more severe along the sub- basins’ NE margins. Slumps are locally associated with micro-faults formed in fairly coherent sediment, resembling the segmented zone of a typical seismite (SEILACHER, 1969), which corroborates the latter assumption and suggests that slumps were most likely triggered by earthquakes and enhanced by steep basin morphology. In places, however, slumped deposits were emplaced from other directions and could have been triggered by major regional earthquakes or locally oversteepened north- eastern anticline limbs. By assessing slump directions and age of the deposits in which they are encased, it is possible to reconstruct basin development dynamics, e.g., discern between in-sequence and out-of-sequence thrusts or identify possible blind-thrust anticlines that nowadays may be concealed by topography or sediment cover.

Dinaric Foreland Basin, Promina Beds, slumping direction, sub-basin geometry

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