Lithofacies and architecture of a basinwide tuff unit in the Miocene Eoil Basin, SE Korea: Modes of pyroclastic sedimentation, changes in eruption style, and implications for basin configuration

摘要

The Miocene Eoil Basin, SE Korea, is a small half-graben basin that was rifted by both extensional and dextral strike-slip deformations during backarc opening of the East Sea (Sea of Japan). The basin was filled by fluviolacustrine sediments and abundant basaltic and dacitic volcanic deposits. The Paljori Tuff is a 2–20-m-thick, basinwide dacitic volcaniclastic unit that is intercalated in the topmost part of a fluviolacustrine formation. The tuff consists of two pyroclastic units (units II and III), which are underlain and overlain by two resedimented volcaniclastic units (units I and IV). Unit I at the base is composed mainly of fine-grained resedimented tephra; unit II is a massive and stratified tuff produced by a dense pyroclastic density current that was ponded in the southwestern part of the basin; unit III is a massive lapilli tuff emplaced by a voluminous and turbulent pyroclastic density current that swept across the basin toward the northeast without ponding or blocking by intrabasinal highs and basin-margin relief; and unit IV at the top is composed of coarse-grained resedimented tephra. The general coarsening-up of these units, the lack of nonvolcaniclastic sedimentary interbeds, and the offset stacking of the units toward the northeast suggest an episodic, short-duration eruption from the southwest that waxed in mass-eruption rate and changed eruption style from phreatomagmatic to magmatic. These units show basin-scale variations in geometry, including: (1) thick accumulations of unit I in the northeastern and southwestern extremities of the basin; (2) ponding of unit II in the southwestern part of the basin; (3) truncation of unit III in the same area; and (4) thickening of unit IV toward the northeastern basin margin. These provide important clues to the distribution of and temporal changes in accommodation space and, hence, the configuration and structural evolution of the Eoil Basin. It is inferred that the greatest subsidence occurred at the southwestern and northeastern corners of the basin; the southwest-northeast–trending axes of the basin plunge gently toward the northeast, and the major intrabasinal faults of the basin were produced before the eruption of the Paljori Tuff. The basinwide preservation of the tuff and its fairly uniform thickness suggest that the basin was undergoing rapid extension and subsidence because of the onset of the rift climax. The Paljori Tuff shows that important clues to, and high-resolution records of, the large-scale configuration and structural evolution of a sedimentary basin can be obtained from the study of lithofacies variations and the architecture of a single basinwide volcaniclastic unit.