Sung-Ja Choi

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

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.

Provenance of Creataceous sandstones in the southeastern Yongdong Basin, Korea: CHIME geochronology of detrital monazites

CHIME (chemical Th-U-total Pb isochron method) ages were determined on detrital monazites in Cretaceous sandstones, southeastern part of the Yongdong Basin and in granitic gneiss (?foliated granite) of the Yongnam Massif, northern part of the Kimchon area. Most detrital monazites in the sandstone samples are chronologically unzoned, and yield CHIME ages of about 180 and 250 Ma. Some detrital monazites show age signatures of middle Precambrian (ca. 1200 and 1750 Ma) and possible Silurian (420 Ma). Monazites in the granitic gneiss yield an unequivocal age of 251.2±3.0 Ma. The age spectrum of detrital monazites coupled with paleoflows and petrographic data suggests that clastic materials in the southeastern part of the Yongdong Basin were largely derived from the granitic and metamorphic rocks of the Yongnam Massif in the Sangju—Kimchon—Keochang area.

Review on the Chugaryeong Fault

추가령단층은 양산단층과 거의 평행하게 북동 주향 으로 한반도를 동에서 서로 가로 지르고 있으며, 지형 도와 위성영상 등에서도 명확한 선상구조를 보이고 있 다. 추가령단층에 대한 연구는 Kofuji(小文次郞, 1903) 에 의하여 시작되었으며, 그 후 지구물리학적 연구, 고 지자기 연구, 암석학적 연구 등이 있으며, 추가령 단층 의 운동시기, 운동 감각 등에 대해 여러 견해가 제시 되었다. 추가령단층을 따라서 삼첩기말-쥬라기 퇴적분 지, 쥬라기 화강암, 백악기 퇴적분지, 제4기 현무암 등 이 단층을 따라서 분포하고 있다. 그러므로 추가령단 층은 삼첩기 말부터 제4기 동안 지속적인 단층운동이 있었던 것으로 간주되고 있다. 추가령단층대는 1903년에 처음 언급되었으나, 수도 권을 통과함에도 불구하고 양산단층대 만큼은 연구가 활성화 되지 않았다. 최근, 지질학분야의 발전과 함께 추가령단층에 대한 새로운 많은 사실들이 드러나고 있 다. 이 단보에서는 추가령 용어, 추가령단층에 대한 기 존 문헌조사와 지질조사를 통하여 새롭게 규명된 추가 령단층의 범위, 운동시기, 운동감각 등을 재조명하였다.

Reactivated Timings of Some Major Faults in the Chugaryeong Fault Zone since the Cretaceous Period

Recently developed illite-age-analysis (IAA) approach has been applied to determine the multiple events for the Singal and Wangsukcheon faults in the Chugaryeong fault belt, Korea. Fault reactivated events during Late Cretaceous to Paleogene events(69.2±0.3 Ma and 27.2±0.5 Ma) for the Singal fault and of 75.4±0.8 Ma for the Wangsukcheon fault were determined by combined approach of the optimized illite-polytype quantification and the K-Ar age-dating of clay fractions separated from the fault clays. These absolute geochronological determinations of the multiple tectonic events recorded in the Chugaryeong fault belt are crucial to establish the tectonic evolution of the Korean Peninsula since Late Cretaceous.

Reactivated Timings of Inje Fault since the Mesozoic Era

Recently developed illite-age-analysis(IAA) approach was applied to determine the fault-reactivated events for the Inje fault that cut through Precambrian biotite granitic gneiss with NNE-SSW trend in the middle of Korean peninsula. Three distinct fault-reactivated events of shallow crustal regime were recognized using the combined approach of optimized illite-polytype quantification and K-Ar age-dating of clay fractions separated from 4 fault clay samples: 87.0±0.12 Ma, 65.5±0.05 and 66.6±1.38 Ma, 45.6±0.15 Ma, respectively. As well, 2M1 illite ages of 193~196 Ma and 254.3±6.96 were discernible, which may be related to the fault-activated time in the relatively deep crust. The study results suggest that the Inje fault would be firstly formed at 254.3°ae6.96 Ma and sporadically reactivated in shallow regime since about 87 Ma. These reactivation events in shallow regime might be due to the Bulguksa orogeny that would be strongly influenced in Korean peninsula at that time.

An Exploratory Study on the Geometrical Reconstruction of the Late Cretaceous Misiryeong Laccolith and the Lost Size of the Baekdam Basin

As a product of geological mapping of the Changamjeom Sheet (1:50,000) located around Misiryeong at a distance towards the west from Sokcho city, a small laccolith of hypabyssal rocks has been geometrically reconstructed to understand the original laccolith shape using structural contour lines of random spot sediments laid on the quartz feldspar porphyry and a schematic cross section. The laccolith age is constrained to ca. Ma (SHRIMP). The originated Baekdam sedimentary basin with the lost size has been yielded to 6 km wide and 16 km long.