Hyperpycnal flows and deltaic clinoforms - implications for sedimentological interpretations of late Middle Miocene fill in the Carpathian Foredeep Basin

The Middle to early Upper Miocene Machów Formation represents the youngest and little to non-deformed increment in the Carpathian Foredeep Basin that was initiated during the Paleocene in response to thrust loading of the North European craton. The onset of the Machów Formation was associated with the late Badenian (lateMiddle Miocene) flexural subsidence (ca 12.5 Ma BP) that resulted in underfilled-basin conditions recorded in the emplacement of submarine fans and shelf-margin deltas, up to 400 m in relief. These systems were fed from the south by a net of small, suspended-load dominated rivers, which resulted in the strongly heterolithic nature of the basin infill. There is a striking absence of large-scale traction structures, and thick sandstone/mudstone packages intercalated with thick-bedded massive sandstones provide evidence of deposition mainly from sustained turbidity currents typified by oscillations in velocity and sediment concentration within a flow. The evidence in both submarine-fan and delta-front deposits includes the abundance of climbing ripples, thick flat-laminated intervals within the otherwise massive thick-bedded sandstones, and inverse to pensymmetrical grading associated with a succession of structural divisions, which departs from that generated by harmonic energy dissipation typical for surge-type (semi-constant volume) turbidity currents. This evidence is interpreted to reflect deposition mainly from hyperpycnal flows, whose oscillatory behaviour reflected chiefly changing flood stages in the sourcing fluvial flow. The inferred semi-permanent flow feeding is also supported by the very high sedimentation rate (locally up to 24 cm/a) of the Machów Formation. Four clinoform types have been distinguished in the formation. Type 1 is most common and corresponds to platform (“shallow-water”) deltas of subseismic scale. The remaining three are shelf-margin (“deepwater”) deltas. The latter types differ one form another in the height and internal structure of clinoforms, as well as in the degree of downdip segregation of sand that either dominates in slope increments (type 2), forms shingled clinoform toes (type 3), or is concentrated in basin-floor fans in the front of a muddy clinoform slope. The basinward sand segregation is facilitated by the dissection of a clinoform’s top during the maximum fall in relative sea level and by long slopes that ease the transformation of hyperpycnal flows into high-efficiency turbidity currents. Numerous internal onlap unconformities make the shelf-margin deltas interesting targets for hydrocarbon search in stratigraphic traps. The identified type 4 clinoforms, up to 200 m high, in the Quinqueloculina reusii zone point to a major deepening in the upper part of the Machów Formation. This suggests a rise in flexural accommodation at ca 11 Ma BP, and contradicts the common view on uniform upward-shallowing throughout the formation