Interactions of pāhoehoe and ‘a‘ā lavas and fluvial sediments on an alluvial plain (the Cretaceous Gyeongsang Basin, Republic of Korea)

Abstract

Two lava units of basaltic composition named the Cheongryongsa and Hakbong basalts are intercalated in the Cretaceous nonmarine backarc basinfill of Korea. The Cheongryongsa Basalt is 5 to 6 m thick and laterally ex-tends for about 12 km. It is underlain by massive sandy mudstone and overlain erosionally by a basaltic conglomerate, suggesting lava emplacement on a floodplain, followed by fluvial erosion and deposition. The basalt comprises flow-base breccia throughout the 12 km-long outcrop extent and is overlain by flow-top breccia at one locality. These breccias are composed mainly of irregular/fluidally shaped clasts in a massive mudstone matrix, and are interpreted as fluidal peperite produced by molten lava and wet sediment mingling. The Hakbong Basalt comprises three lava units, a few meters to several tens of meters thick, and intervening stratified sand-stones. Three breccia types occur beneath or above the basalt: 1) irregular-clast breccia, 2) angular-clast breccia, and 3) fluidal-clast breccia. They are interpreted to be clinker, blocky peperite, and fluidal peperite, respectively, associated with 'a'a over line lavas. The clinker could be distinguished from the peperite based on undeformed sedi-mentary structures in the interstitial sediment and the contrast in matrix grain size with the substrate sediment. Peperite formation was possible when the lava was emplaced upon very fine sandy to muddy sediment, which was probably under-consilidated and susceptible to liquefaction. The liquefied sediment needed to penetrate the carapace of clinker to come into contact with the coherent lava and generate peperite. Blocky peperite was produced by the interaction of semi-solidified lava with very fine sand, whereas fluidal peperite was produced by the mingling of molten lava and mud. These basalts show that various breccia types can be produced along subaerial lavas, involving autobrecciation, quench fragmentation, and peperitic fragmentation of lava. Many factors are interpreted to have been involved in forming these breccias, including the sediment type and consolidation state, the viscosity and emplacement setting of lava, and even a coincidental depositional event in the basin.