Pannóniai üledékképződés és szerkezeti mozgások az Északi-pikkely (Kelet-Mecsek) területén

Abstract

The ’Northern Imbricates’ along the northern edge of the Eastern Mecsek Mts., SW Hungary, are a complicated, narrow, uplifted band of basement, partly covered by Neogene sediments. The area is heavily deformed, with young, post-Miocene tectonic movements. Pannonian (Upper Miocene) lacustrine sediments are most widely distributed around the village of Nagymanyok. Through their investigation it is possible to reveal the Late Miocene and subsequent evolution history. The present study concentrates on the temporary outcrops of Lake Pannon deposits created in 2015 south of Nagymanyok, accompanied with fossil collection from the individual strata. This ~120 m thick sedimentary successsion is atypical compared to other parts of the Mecsek Mts., with relatively fine-grained, fossil-rich clays to fine sands above the thin (<3 m), gravelly base sands of local provenance. The fauna is dominated by bivalves; however, gastropods, ostracods and fish and plant remains have also been found. The base sands belong to the Prosodacnomya dainellii - P. vutskitsi mollusc biozones, the finer sediments to the Congeria rhomboidea biozone. Through the joint interpretation of the litho- and biofacies the sedimentary environments and the age of deposition of the sediments were identified, while the evolution of the wider surroundings was delineated by adding the tectonic observations, borehole data and seismic sections to the previous information. Lacustrine sedimentation first started only in the northern foreland of the mountains, accummulating offshore marls, claymarls and silts (Szak Claymarl Fm.). The time of flooding is unknown in the trough directly at the foot of the mountains due to the lack of wells and paleontological data; the wide platform north of it became flooded no sooner than 8.9 Ma based on mollusc data. This was followed by the fine sands to clays of the Alpine-Carpathian delta system (Ujfalu Fm.), approx. 7.3 Ma ago, as indicated by molluscs belonging to the Prosodacnomya dainellii and P. vutskitsi littoral biozones. As no clinoforms are present on seismic profiles, the northern foreland must have been a shallow underwater high at that time.The Northern Imbricates in the mountains were dryland during most of the Late Miocene. Sedimentation started here approximately coevally with the arrival of the delta system at the foot of the mountains. Flooding ensued on a dissected topography. The delta sediments with dominantly sublittoral fauna above the thin littoral base sands indicate rapid transgression. Late Miocene sedimentation was accompanied by small-scale movements of strike-slip character. Stronger tectonic deformations followed afterwards: strike-slip to north-vergent reverse displacements happened along the northern master fault of the Mecsek and roughly parallel planes.