퇴적학/층서학/고생물학/석유지질학 10월29일(금)
Sectional traces of dinosaur footprints in the Cretaceous lacustrine deposits of Korea
Dinosaur tracks are very abundant in the Cretaceous lacustrinedeposits of Korea, and are recognized in both bedding surfaces and cross sections. Innumerable dinosaur tracks are exposed in the profiles of track underlayers as diverse modes of depressions according to the species of trampling dinosaurs, the substrate condition, and the angles of exposures to the full-length section. In this study diverse ways of underalyer deformation of the trampled deposits are described and deformation mechanism is delineated. The underlayer deformation generated by dinosaur trampling is grouped into three types in general: (1) asymmetrical deformation, (2) nearly symmetrical deformation, and (3) deformation with thrust faults.
Type 1 deformation is characterized by faulted walls with drag folding in steeper sides and downwarping in less steep sides. The central parts of the underlayers are sometimes normally faulted and have tension cracks. It represents full-length section of footprint. Type 2 deformation is characterized by nearly symmetrical synclinal deformation and may represent transverse section of footprint. Type 1 and 2 are interpreted to have been produced by the trampling when substrates beneath the trampled deposits were unconsolidated. It is likely that type 1 and 2 are plastic deformation produced in the weaker beds by differential movements of the relativelyresistant beds below due to vertical normal stress of dinosaur foot. The weaker beds, being unable to resist the frictional stress of effect induced by the movement of beds below, are dragged and downwarped in sides of footprints. Type 3 deformation is characterized by the development of thrust faults with wedge-shaped depressions under the folded underlayers. Type 3 is deemed to have been generated by the trampling when substrates beneath the trampled deposits were semi-consolidated. The gradation from drag folds into tight folds, thrust faults, and imbricated structure due to brittlenature of beds below the underlayers also indicates plastic deformation. Some of sectional traces occur as truncated traces, and only parts of depressions and deformation were preserved. The dinosaur trampling can not be thus recognized without the understanding of overall features of underlayer deformation. Consequently, this study may provide useful criteria to identify dinosaur tracks in sectional profiles of outcrop exposures and rock slabs, and enhance our understanding for behaviour and habitat of dinosaurs.
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