The Miocene Eoil Basin, SE Korea, is a pull-apart basin formed by dextral crustal deformation during the backarc opening of the East Sea. The Gampo Conglomerate, the lowermost formation of the basin, provides significant information on how a pull-apart basin initiates and is filled by sedimentary strata. The formation consists of nonmarine conglomerates, sandstones, mudstones, and a variety of volcaniclastic deposits that were deposited in alluvial fans, delta slopes, braided streams, mouth bars, and lakes. These deposits are organized into several stratal packages (SP) with contrasting geometries. SP I at the base consists of mouth-bar to delta-front and volcaniclastic deposits, and shows rapid facies variations along the southeastern normal fault margins. SP I suggests that the early basin was bounded by multiple, incipiently developed normal faults with variable displacement and accommodation creation. SP II consists of thick alluvial-fan conglomerates and overlies SP I abruptly, suggesting sudden input of coarse-grained sediment from the uplifted footwalls. It is thickest near the central part of the southeastern normal fault margins, suggesting development of a large alluvial fan near a transfer zone. SP III consists mostly of mouth-bar sandstones, indicating abrupt subsidence of the basin and the transition from subaerial to subaqueous environments. SP IV consists of braided-stream conglomerates, suggesting gradual progradation of coarse-grained fluvial systems toward the basin when the extension and subsidence of the basin was slowed. SP V shows upward-deepening facies changes from braided-stream to mouth-bar deposits, suggesting resumption of basin subsidence. Notably, the cumulative thickness of SP III to SP V is thickest near the northeastern and southwestern strike-slip margins, suggesting migration of the loci of maximum subsidence from the normal fault margins towards the strike-slip margins after deposition of SP II. The change in stratal architecture suggests that the activity of the normal faults was dominant in the early stage of the basin evolution whereas the activity of the strike-slip faults became more pronounced afterwards. Overall, the stratal architecture of the Gampo Conglomerate appears to record the transition tectonics from rift to pull-apart during the incipient stage of a pull-apart basin development. The Eoil Basin is thus interpreted to have been hybrid in nature, influenced by the dynamic changes of the basin-margin faults, which were either normal or strike-slip. © 2011 Elsevier B.V.
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