Biostratigraphy and depositional anatomy of a large olistostrome in the Eocene Hieroglyphic Formation of the Silesian Nappe, Polish Outer Carpathians

Abstract

The study focuses on a large olistostrome unit (~200 m thick and 4 km in strike-parallel extent) embedded in the Mid-Eocene shaly Hieroglyphic Formation of the Silesian Nappe, exposed in the Rożnów Lake area. Foraminifer biostratigraphy and petrographic comparisons are used to identify the provenance of olistoliths. The olistostrome is tripartite with respect of its olistolith composition. The lower part of the olistostrome abounds in olistoliths of sandstones derived from the Early Eocene turbiditic Ciężkowice Formation, whereas the middle part is dominated by olistoliths of Early Eocene bathyal mudshales. The upper part contains olistoliths of Middle Eocene turbiditic “banded sandstones”, known from the Hieroglyphic Formation and deposited in the bathyal zone above the CCD. The bathyal provenance of the olistostrome contrasts with the abyssal origin of the hosting green shales. The olistostrome unit is inferred to be composite, emplaced in the earliest Bartonian or at the Lutetian/ Bartonian transition by a series of at least three large debris flows that closely followed one another. Biostratigraphical data and slump-fold vergence suggest resedimentation from the bathyal northern slope of the Silesian Cordillera that bounded the abyssal Silesian Basin to the south. Northward movement of the thrust-formed cordillera must have warped up the base-of-slope deposits of the Ciężkowice Formation, causing their gravitational collapse. This event destabilized the former lower-slope muddy deposits, resulting in a second phase of resedimentation by retrogressive slumping, which led to the collapse of mid-slope sandy turbidites. The slope failures involved contemporaneous Mid-Eocene sediment with an admixture of foraminifers derived from the upper slope or shelf margin and with exotic bedrock debris shed from the eroded cordillera crest. The catastrophic multi-phase emplacement of the olistostrome marked the last major thrusting pulse of the second (Late Cretaceous–Late Eocene) stage of tectonic evolution of the Outer Carpathian accretionary prism.