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When a mudstone was actually a “sand”: Results of a sedimentological investigation of the bituminous marl formation (Oligocene), Eastern Carpathians of Romania

Schieber, Juergen, Miclăuș, Crina, Seserman, Anca, Liu, Bei, Teng, Juan
Sedimentary geology 2019 v.384 pp. 12-28
Oligocene epoch, basins, bedload, calcite, cement, deformation, fabrics, feldspar, marl, quartz, sand, sandstone, Carpathian region, Romania
The bituminous marl formation (BMF) is an important source rock in Romania. At the study location, in the Eastern Carpathians, the BMF composition is dominated by calcite (coccolith debris, cement) and diagenetic silica, with the remainder consisting largely of detrital clays, quartz and minor feldspar. Abundant intervals with soft sediment deformation, likely slump deposits, alternate with thinly layered marls that may locally contain layers and lenses of rippled and cross-laminated sandstone. The latter suggest intermittent action of traction currents. Although the fine grained nature and abundant nanoplankton content of the marls suggest that they originated via pelagic settling through the water column, upon close inspection, they consist of flattened sand-size yet fine-grained aggregates (soft clasts). Experimental studies suggest that these were likely eroded from the seabed and transported in bedload by bottom currents. Inclined fabric elements within marls suggest that soft clasts formed ripples, and that marl layers are the depositional consequence of ripple migration. Whereas typical marl layers probably reflect migration of small ripples of a few cm’s height, there are also cross-bedded marly beforms with ca. 20 cm pre-compaction relief that by size are muddy megaripples. Unlike in sand, where bedform hierarchy reflects increasing flow velocity, muddy megaripples more likely are a reflection of bottom current systems of substantial duration.With marl layers as well as interbedded sands recording bottom current activity, the BMF represents a combination of slope processes and bottom current activity. The likely environment of deposition is a lower slope to basin setting with contour currents reworking pelagic sedimentation and intermittent sediment supply by slumping.