The southeastern margin of the Cretaceous Yongdong Basin, Korea: a lacustrine fan-delta system

Abstract

The Cretaceous (Neocomian to Aptian—Albian) Yongdong Basin is a NE-SW trending strike-slip basin (8–10×50 km2), filled with alluvial, fluvial, and lacustrine deposits. The southeastern margin of the basin includes ca. 700-m-thick siliciclastic sequence, consisting of conglomerate, (gravelly) sandstone, and reddish or dark gray mudstone. A detailed facies analysis reveals that the sequence can be represented by 13 sedimentary facies which can be organized into 3 facies associations (FA) representing three distinctive depositional environments. The FA I (stream-dominated alluvial fan) comprises decimeter- to 1-m-thick, granule-to-cobble conglomerates, horizontally stratified gravelly sandstone, and reddish mudstone, deposited by hyperconcentrated flows, stream flows, and overbank suspension fallout. The FA II (fan-delta slope) consists of decimeter- to 2-m-thick, pebble-to-boulder conglomerates with wedge or lobate geometry and decimeter- to 2.5-m-thick, (crudely) stratified (laminated), graded or massive (gravelly) sandstones. They were deposited by debris flows and turbidity currents, respectively. The FA III (prodelta/basin plain) comprises a few cm to 1.2-m-thick, horizontally stratified (laminated), massive or graded (gravelly) sandstones with wedge or sheetform geometry, and decimeter- to 1.5-m-thick mudstone and sandstone-mudstone couplet. They were deposited by high- or low-concentration turbidity currents and settling from suspended sediments in buoyant plumes. The FA I is distributed along the basin margin and passes into the FA II toward the basin center, where FA III is in gradational contact with FA II, forming a radial distributional pattern from the basin margin. These facies associations form a mass-flow-dominated, lacustrine fan-delta system (Ibawisan fan-delta system) with a vertically-/laterally-stacked, fining-upward megasequence. The Ibawisan system probably resulted from continuous faulting along the basin margin and was associated with high rate of sediment supply, and thus formed thick, coarse-grained basin margin deposits abruptly wedging out toward the basin axis. The development of Ibawisan fan-delta system is related to locally different tectonic setting within the basin, and the recognition of lacustrine fan-delta system along the southeastern basin margin is important to understand the tectonic evolution and sequential development of the Yongdong Basin through time.