Thermal maturity assessment of the Upper Triassic to Lower Jurassic Nampo Group, mid‐west Korea: Reconstruction of thermal history

Abstract

Thermal maturity of the Upper Triassic to Lower Jurassic Nampo Group, a sediment‐fill of the Chungnam Basin located in the central western part of South Korea, was assessed by illite crystallinity measurement and sandstone microtexture analysis. The Nampo Group consists of a fluvio‐lacustrine deposit bearing meta‐anthracitic coals and was over‐thrusted by the basement rocks. Sandstones are characterized by down sequence increasing illite crystallinity, from anchizone to epizone, which is strongly suggestive of burial heating. Deep‐burial diagenesis and deformation are evidenced by well‐developed pressure solution textures, whose intensity tends to increase down sequence, and by ductile deformation in the lowermost strata. On the basis of the result of illite crystallinity measurement, the maximum paleo‐temperature and total burial depth of the Nampo Group are estimated to be ca 340°C and 10 km, respectively; these conditions are in good agreement with the observed ductile deformation features. The absence of strata younger than the Nampo Group in and around the Chungnam Basin suggests that deep burial of the Nampo Group was caused by tectonic crustal loading. The tectonic overload was because of basement over‐thrusting that occurred during the Jurassic Daebo orogeny, which is closely related to the orthogonal subduction of the Izanagi Plate beneath the East Asian continent. Subsequent hydrothermal alteration disturbed the thermal maturity pattern, resulting in anomalously high illite crystallinity and meta‐anthracitization.

K-Ar dating of illites for time constraint on tectonic burial metamorphism of the Jurassic Nampo Group (West Korea)

“…In this sense, it is more likely that the Amisan Formation originally experienced thermal heating not in epizonal but in anchizonal conditions, and that mixing of 1M d illite with detrital 2M 1 illite having relatively low KI value consequently resulted in enhancing apparently high illite crystallinity of the measured samples. Thus, the maximum burial temperature of the Amisan Formation is thought to be below 280°C, which is in agreement with the sandstone textural observation suggesting the absence of ductile deformation in the middle part of the Amisan Formation (Egawa and Lee, 2008).…”

“…The depositional age of the Baegunsa Formation, the uppermost unit of the Nampo Group, is after Koh (2006). seem to be applicable to this study by the following lines of evidence: (i) younging radiometric ages of illites with decreasing clay size (Table 1), and (ii) no post-Jurassic granite intrusion in and around the study area (Egawa and Lee, 2008). Given that the Nampo Group in the Ocheon area escaped from hydrothermal overprinting, we conclude that the timing of illitization probably corresponds to that of the maximum tectonic burial metamorphism.…”

“…It is known that the Daebo orogeny ended at ca. 135 Ma ago (Kim, 1996), just 5 Ma later than the latest illite ages we dated, and thus it appears that the calculated illitic age corresponds to the age of lowgrade metamorphism attributable to basement overthrusting in the late stage of the Daebo orogeny (Egawa and Lee, 2008). It is very unlikely that the analyzed epizonal shales were hydrothermally altered by post-Jurassic granite intrusion.…”

“…Petrographic observation of sandstones and X-ray diffraction (XRD) analysis on less than 2 µm fraction reveal that the Nampo Group underwent significant compaction and low-grade metamorphism, respectively, and the evidence of a down-sequence increasing degree of thermal alteration is well preserved in the Ocheon area compared with the other two areas where the Nampo Group occurs (Fig. 1a) (Egawa and Lee, 2008).…”

“…This peninsulawide orogeny eventually facilitated (i) Jurassic basins formation and their filling and (ii) subsequent tectonic burial metamorphism attributable to basement overthrusting and loading on the basin fills Lee, 2008, 2009). It was suggested that the overthrusting was initiated in early Middle Jurassic time just after the deposition of the basin fills (Koh, 2006;Egawa and Lee, 2008), but the timing of tectonic burial metamorphism due to crustal loading remains poorly constrained. This study deals with K-Ar illite dating of the Nampo Group, a Lower to Middle Jurassic nonmarine basin-fill of the Chungnam Basin located in the central western Korean Peninsula (Fig.…”