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Comment and Reply Depositional history, tectonics, and provenance of the Cambrian-Ordovician boundary interval in the western margin of the North China block: Comment Jeong-Hyun Lee1 and Seung-bae Lee2,† 1Department of Geology and Earth Environmental Sciences, Chungnam National University, Daejeon 34134, Republic of Korea 2Geology Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea INTRODUCTION the Precambrian-Cambrian unconformity (Lee boundary in the western Sino-Korean block et al., 2016). Myrow et al. (2015) focused on (cf. Darby and Gehrels, 2006) have been sug- Myrow et al. (2015) detailed the sedimen- the upper part of this succession, consisting of gested to indicate tectonic uplift that might have tology, biostratigraphy, and chemostratigraphy fine-grained sedimentary facies such as lime formed this unconformity (Myrow et al., 2015; of the Cambrian-Ordovician succession in the mudstone and marlstone alternations, and they further discussed in next section). western North China block (or Sino-Korean reported that trilobites belonging to the Black- In addition to the unconformity in the west- block) at Wuhai, Inner Mongolia, China, and welderia zone, indicating the middle Guzhan- ern Sino-Korean block, Myrow et al. (2015) recorded an ~30 m.y. unconformity that formed gian Stage of Cambrian Series 3 (Park et al., reported three additional localities in the block during the upper Cambrian Series 3–upper 2013), occur 18.51 m below the Cambrian- with evidence of unconformities across the Middle Ordovician. They provided a valuable Ordo vician boundary. Lee et al. (2016) reported Cambrian-Ordovician boundary. These include description of the succession accompanied by trilobite Jiulongshania rotundata from strata the eastern (Taebaek, South Korea), north- excellent illustrations of sedimentary facies 16.4 m below the boundary, which also occurs eastern (Baishan, Jilin, China), and northern as well as detailed data on chemostratigraphy, within the Blackwelderia zone of Myrow et al. (Baotou, Inner Mongolia, China) margins of thus providing an important basis for further (2015). Sedimentary facies as well as trilobite the Sino-Korean block. These localities display geologic studies in the region. In addition, occurrences collectively suggest a relatively sedimentologic features that would suggest a they presented a discussion on the origin of the slow sedimentation rate, indicating that the break in sedimentation, including sandstone ~30 m.y. unconformity and suggested that the uppermost part of the Cambrian succession in overlying carbonate (Taebaek), subaerial expo- western Sino-Korean block would have been Wuhai area would not exceed the Guzhangian sure and an erosion surface (Baishan), and con- placed near the western Himalaya during the Stage (Myrow et al., 2015; Lee et al., 2016). glomerate and sandstone on a paleokarst sur- early Paleozoic, where a hiatus of similar mag- The lower part of the Sandaokan Formation face (Baotou; Myrow et al., 2015, their table 1). nitude has been reported (Myrow et al., 2006, unconformably overlying the Cambrian suc- However, some of these features are not related 2016). This result seems to support the theory cession was correlated to the conodont biozone to an unconformity (Taebaek) or are not cor- of McKenzie et al. (2011), who first suggested Histiodella holodentata of the middle to upper rectly quoted (Baishan). a close paleogeographic affinity between the middle Darriwilian Stage, Middle Ordovician In the Taebaek area of Korea (Taebaeksan Sino-Korean block and Himalaya based on (Myrow et al., 2015). Therefore, the Cambrian- Basin), an ~1200-m-thick mixed carbonate- detrital zircons and trilobites. However, the Ordovician unconformity developed in the siliciclastic sequence was deposited from the paleogeographic interpretation of Myrow et al. Wuhai area of the western Sino-Korean block Cambrian Series 2 to the Middle Ordovician (2015) requires further discussion because of indicates a hiatus of ~30 m.y. or more (Myrow (Kwon et al., 2006). The Cambrian-Ordovician the following reasons: (1) incorrect description et al., 2015). boundary lies in the basal part of the Dongjeom of the “unconformities” in other localities of the Myrow et al. (2015) compared another hiatus Formation, which conformably overlies the Sino-Korean block, and (2) presence of other in the western Himalaya with that of the west- Hwajeol Formation (Choi et al., 2016). Evi- potential options on the formative mechanism of ern Sino-Korean block, which lasted from the dence of a time gap or sedimentary hiatus across this unconformity, including tectonic tilting and latest Cambrian Series 3 or earliest Furongian the Cambrian-Ordovician boundary is not rec- eustatic sea-level change. In the following sec- to approximately the early Middle Ordovician. ognized in this area, which can be noted from tion, we describe each topic in detail and discuss Chronostratigraphic control on the western detailed sedimentological and paleontologi- the nature and implication of this unconformity. Hima layan succession is poor, and therefore the cal studies performed in the area over the past exact duration of this hiatus remains under ques- 30 yr (e.g., Woo and Park, 1989; Choi et al., CAMBRIAN-ORDOVICIAN tion (Myrow et al., 2006), although a duration 2003, 2016; Choi and Chough, 2005; Kwon UNCONFORMITIES ON THE SINO- of ~22–36 m.y. has been recently suggested for et al., 2006; Lee and Choi, 2007, 2011). There KOREAN BLOCK this hiatus (Myrow et al., 2016). Myrow et al. is an increase in input of sandstone around (2015) suggested that since the durations of this boundary, indicating a gradual transition At the Subaiyingou (or Subeigou) section the hiatuses in the western Sino-Korean block from a shale-carbonate–dominant environ- where the study was conducted, the Cambrian- and the western Himalaya are similar, these ment to a silici clastic-dominant environment Ordovician boundary occurs ~360 m above two regions could have been connected during without a significant hiatus (Choi et al., 2004; the early Paleozoic. Supposed sediment prove- Kwon et al., 2006). Late Furongian to earliest †Corresponding author: sblee@ kigam .re .kr. nance changes across the Cambrian-Ordovician Tremadocian trilobite biozones successively GSA Bulletin; July/August 2017; v. 129; no. 7/8; p. 1019–1021; https://doi.org/10.1130/B31806.1; published online 22 May 2017. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 129, no. 7/8 1019 © 2017 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/129/7-8/1019/1002234/1019.pdf by guest on 25 September 2021 Comment and Reply occur across the Hwajeol and Dongjeom for- reliable (Darby and Gehrels, 2006, their fig. 4). overprinting preexisting unconformities. For mations, which are well correlated to those of Such differences in age probability can even example, the Upper Ordovician Montoya North China and other parts of the world (Choi occur within the same lithologic units without Group in New Mexico and western Texas and Chough, 2005; Choi et al., 2016). any significant provenance changes (e.g., Lee of the United States unconformably over- The Cambrian-Ordovician boundary inter- et al., 2012). lies the Cambrian Series 3 Abrigo Limestone val at Dayangcha section, Jilin, northeastern Previous studies have noted a major shift in (Morgan, 2012). The duration of this uncon- China (Baishan, Jilin in table 1 of Myrow et al., the sedimentary succession on the Sino-Korean formity is similar to those of Wuhai and west- 2015), has been intensively studied in terms block during the late Cambrian Series 3, where ern Hima laya, but it does not mean that these of sedimentology, paleontology, and sequence a major drowning unconformity formed in the three localities would have been connected to stratigraphy (Chen et al., 1988, and references eastern to northeastern part of the block (Meng one another. Laurentia, a large paleocontinent therein; Ripperdan et al., 1993; Zhang et al., et al., 1997; Kwon et al., 2006; Chen et al., containing present-day North America, was 1999; Zhang and Erdtmann, 2004). Although 2011). Meng et al. (1997) noted the disparity in completely separated from Gondwana and the Myrow et al. (2015, their table 1) added the sedimentary environmental changes recognized Sino-Korean block during the early Paleozoic section as an unconformity-bearing locality in throughout the block during this time interval (Li and Powell, 2001). Instead, it is more likely North China with geological features of “sub- (late Cambrian Series 3–Furongian); there is a to infer that a combination of several sea-level aerial exposure,” the Cambrian-Ordovician transition from shallow- to deep-water facies fall events that occurred throughout this time succession in the Dayangcha section was in (drowning unconformities of Kwon et al., 2006; interval resulted in several large hiatuses of fact described as “rhythmical sequence of lime Chen et al., 2011) in the northern (to northeast- similar duration in various localities of the mudstone and shales” deposited along the outer ern) part of the platform, whereas supratidal to world. Durations and effects of unconformity- shelf of the platform (Chen et al., 1988, p. 415). subaerial facies occur in the southern (to south- forming sea-level fall events would have been Ripperdan et al. (1993) pointed out that the sec- western) part of the platform. Classically, this different in various localities, due to several tion shows several depositional breaks or “con- has been explained by a tectonic tilting event that factors, including topography, sedimentation densed sections,” which can be correlated with occurred across the Sino-Korean block, although pattern, climate, and local tectonics. Further global sea-level changes across the Cambrian- the causes for this event were only briefly pre- detailed sedimentologic as well as sequence Ordovician boundary interval. Zhang et al. sented as “the result of collision with a marginal stratigraphic studies throughout the Sino- (1999) also noted the glauconitic condensed basin to the north” (Meng et al., 1997, p.