Characteristics and gravel provenance of the Quaternary Baekuiri Formation

초록

The Quaternary Baekuiri Formation, which is the only unconsolidated sedimentary layer observed inland in Korea, covers the Devonian Misan Formation of the Paleozoic with unconformity and is overlaid by Quaternary basalt of the Cenozoic. Recently, interest in its academic value has been growing as it is being featured as a landmark of the Hantan River UNESCO Global Geopark. In this present study of the Baekuiri Formation distributed in the lower part of the sheeting joint in Eundae-ri, Jeongok, a total of 758 gravels in a horizontal section of 7.5 m were individually observed, and their components, size, roundness, and sphericity were classified. In the Baekuiri Formation, gravel with good support roundness is distributed mainly as various rock types, and the matrix-supported gravel layer and the sand layer with developed lamination are distributed depending on the location. Schist, granite, and gneiss account for more than 80% of the gravel components, and tuff, quartz vein, and quartzite are also distributed in the studied area. Because granites that intruded at various geological periods of the Mesozoic are distributed in the surroundings of the Baekuiri Formation, dating was performed to track them. As a result, the granite gravels were dated to approximately 111 Ma of the Cretaceous, 171 Ma and 179 Ma of the Jurassic, and 236 Ma of the Triassic periods. By estimating the origin of each gravel type, the schist, gneiss, quartzite, quartz vein, and other rock types seem to have originated from the Misan Formation or Daegwangni Layer of the Paleozoic and the Jangrak Formation of the Paleoproterozoic located in the northern area. Moreover, it is interpreted that the granite originated from the mountains in the eastern and northeastern regions and flowed in through the ancient Hantangang River over a radius of up to 30 km. Additionally, the tuff and volcanic breccia originated from volcanic rocks of the relatively close-by eastern Cheorwon Basin in the Cretaceous period and are characterized by their low production frequency. The deposition period of the Baekuiri Formation is estimated to be approximately 0.5 Ma ago, and its clear definition is needed in the future.

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