Pages: 119 - 136
Abstract: Upper Badenian-lower Sarmatian (Miocene) strata along the active (southern) margin in the Carpathian Foreland Basin reveal seismic-scale deltaic clinoforms that grew from the south and developed a shelf-to-basin floor relief of over 300 m. Two architectural types and six 4th-order sequences were distinguished along the clinoforms on the basis of correlation of an extensive network of seismic lines and geophysical well logs. A Type A clinoform consists of steeply dipping (3–4°), planar-oblique strata that occur in narrow (2–4 km wide) belts composed chiefly of aggrading thick-bedded massive sandstones with onlap upper terminations. Type B clinoforms occur in wide (8–12 km) belts of less steep (<=2°), strongly tangential strata composed of mouth-bar/prodeltaic increments that lack thick turbidites and show downward-stepping to retrogradational stratal arrangements. Both clinoform types document shelf-margin accretion, chiefly during periods of relative sea-level fall and early rise. None of these types predicts a coeval basin-floor fan development, either because the slope was too narrow to ignite delta-fed hyperpycnal flows into high-efficiency turbidity currents (type A), or the bulk of the sand delivered to the shelf edge was trapped within slope-perched mouth bars shifting 6–10 km around the freshly formed shelf margin (Type B) during relative-sea level oscillations. The lower three sequences (upper Badenian) reveal a strong aggradational component, whereas starting from the angular unconformity at base of the Anomalinoides dividens Zone (lower Sarmatian) the offlap break assumes an increasingly flat trajectory basinwards. This change is thought to reflect a decreased rate of addition of accommodation due to cessation of thrust loading, and faster progradation of the clinoform. It is concluded that large-scale clinoformed shelf margins are not limited to rifted continental shelf margins, but can also be present in foreland basins. In such an environment, flexural and fault-induced subsidence promotes long-term relative sea-level rise in the hangingwall and, consequently, the generation of long and high deltaic clinoforms on the accreting shelf margin, whereas the actively rising footwall precludes the preservation of paralic facies and provides an abundant sediment supply for delta growth.
Keywords: Western Carpathians, Carpathian Foreland Basin, clinoform, shelf-margin deltaDownload PDF document Show Text Version