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Anachronistic facies in the Lower Triassic of South China and their implications to the ecosystems during the recovery time ZHAO XiaoMing1,2, TONG JinNan1†, YAO HuaZhou2, ZHANG KeXin1 & ZQ CHEN3 1

Key Laboratory of Biogeology and Envionmental Geology of Ministry of Education, State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Wuhan 430074, China; 2 Yichang Institute of Geology and Mineral Resources, Yichang 443003, China; 3 School of Earth and Geographical Sciences, the University of Western Australia, Crawley, WA6009, Australia

The end-Permian mass extinction not only severely distressed the Paleozoic ecosystems but also dramatically changed the sedimentary systems, resulting in a peculiar Early Triassic ecosystem and submarine environment during the recovery time following the mass extinction. The Lower Triassic is characteristic of the wide occurrence of various distinctive sediments and related sedimentary structures, such as flat-pebble conglomerates, vermicular limestone, subtidal wrinkle structures, microbialite, carbonate seafloor fans, thin-bedded limestone and zebra limestone-mudstone. These sediments were common in the Precambrian to Early Ordovician marine settings, and then they occurred only in some extreme and unusual environments with the expansion of metazoan faunas. However, the Early Triassic witnessed an “anachronistic” reappearance of some distinctive sedimentary records in normal shallow marine settings. The study of these anachronistic facies should be of great importance for the understanding of the unique ecosystem and marine environment through the great Paleozoic-Mesozoic transition. The anachronistic facies characterized by vermicular limestone have been documented in many localities in South China and occur at various horizons of the Lower Triassic. Most types of reported distinctive sediments over the world have been observed in the Lower Triassic of South China. This provides an excellent opportunity for understanding the Early Triassic environment and its coevolution with the biotic recovery. Among the anachronistic facies the vermicular limestone is the most characteristic and common distinctive sediments in the Lower Triassic of South China but has received relatively few investigations. Taking it as a case study, we will detail the variation of vermicular limestone and its stratigraphic distribution in the Three Gorges area, Hubei Province. The investigation on the vermicular limestone and other distinctive sediments from the Lower Triassic of South China further indicates that the appearance of anachronistic facies immediately following the mass extinction and the elimination from normal shallow marine facies with the radiation of Mesozoic marine faunas imply the natural response of the sedimentary systems and ecosystems to the great Paleozoic-Mesozoic transitional events and their induced harsh environments. Therefore, the ups and downs of the anachronistic facies may act as a proxy for the evolution of ecosystems independent of fossil analyses. Early Triassic, anachronistic facies, biotic recovery, ecosystem, Three Gorges area, South China

Received June 16, 2008; accepted August 28, 2008 doi: 10.1007/s11430-008-0128-y † Corresponding author (email: [email protected]) Supported by NSFC Innovation Research Group Program (Grant No. 40621002), MOE Innovative Research Team Program (Grant No. IRT0546), China Geological Survey Project (Grant No. 1212010610709), and “111” Project (Grant No. B08030)

Sci China Ser D-Earth Sci | Nov. 2008 | vol. 51 | no. 11 | 1646-1657

The end-Permian mass extinction was the most devastating extinction in the history of metazoans. Nearly 49% of the marine families[1,2] became extinct, and an estimated 80% ― 90% of the marine species disappeared[2,3]. The biotic recovery following the endPermian mass extinction also was the longest one in the Phanerozoic and delayed until the Middle Triassic[4,5]. The depleted ecosystems were dominated by cosmo― politan members[6] and opportunists[7 9], and this ecological crisis spanned the entire Early Triassic. While the ecosystems were heavily attacked by the end-Permian mass extinction events, the sedimentary systems also had dramatically changed, resulting in the ― unique “reef gap”[10 12], “chert gap”[13] and “coal gap”[14] in the Early Triassic. Various distinctive sedimentary records exist in the shallow marine Lower Triassic, such ― as flat-pebble conglomerates[15 17], vermicular limest― one[15,18 20], subtidal wrinkle structures[21], microbial― ite[17,22 31], carbonate seafloor fans[17,32], and zebra limestone-mudstone[16,33]. These distinctive sediments in the Cambrian and Precambrian rocks are regarded as the unusual sediments[34] or anachronistic facies[35] when they occurred commonly in the post-Ordovician normal shallow marine facies, because they had disappeared from the normal shallow marine settings since the Ordovician when the metazoans occupied the ecosystems but occurred only in some high-pressure environments devoid of metazoans, such as restricted lagoons. This paper is to summarize these distinctive sediments, anachronistic facies, in the Lower Triassic of South China, and to evaluate the cause-effect between the sediments and environmental conditions during the recovery time.

During the Early Triassic, South China was located at low latitudes in the eastern Tethys[42]. Most parts of the South China were covered by shallow marine facies and deposited an association of limestone, mudstone and marl, while some shallow platforms received carbonate-dominated sediments. Because of the widespread marine sequences successive from the Upper Permian to Lower Triassic, South China provides unique conditions to study the Permian-Triassic boundary and Lower Triassic stratigraphy, as well as the great Paleozoic-Mesozoic transitional events. The anachronistic facies have been documented in many localities of South China, and are distributed at various horizons in the Lower Triassic. Nearly all types of the known distinctive sediments and sedimentary structures have been reported. This provides an excellent opportunity for understanding the Early Triassic environment and its coevolution with the biotic recovery.

1 General geology

The Lower Triassic in the Lower Yangtze region is a sequence deposited on a carbonate ramp and lithostratigraphically divided into Yinkeng, Helongshan and Nanlinghu Formations in ascending order. The Permian-Triassic boundary lies at the base of the Yinkeng Formation[43] whereas the Induan-Olenekian boundary is situated in the middle part of Yinkeng Formation[44,45] (Figure 2). Flat-pebble conglomerates are common in middleupper Yinkeng Formation, middle Helongshan Formation, lower and uppermost Nanlinghu Formation[15,41]. Vermicular limestone occurs in middle Helongshan Formation and Nanlinghu Formation[15,19,20,46]. Microbi-

The South China block was composed of the Yangtze platform and South China Fold System. It was bordered on the north by the Qinling-Dabie Orogenic Belt, a suture between the North China and South China ― blocks[36 38]. To the northwest, it bordered the SongpanGanzê Fold System, interpreted to be a remnant oceanic basin filled with Triassic flysch[39]. The South China block was bordered on the southwest by the Ailaoshan-Songma fault, which is interpreted as its suture zones with the Siamo-Sibumasu and Indochina plates, respectively[37] (Figure 1).

2 Distribution of the Lower Triassic anachronistic facies in South China Based on the tectonic setting and stratigraphic regionalism, the distribution of the Lower Triassic anachronistic facies is described in four regions respectively, i.e., Lower Yangtze region, including southern Anhui, southern Jiangsu, northwestern Zhejiang and northeastern Jiangxi provinces; Middle Yangtze region, including Hubei, northwestern Jiangxi, northern Hunan and eastern Chongqing; Upper Yangtze region, including Chongqing, Sichuan, Guizhou and eastern Yunnan; and Youjiang region, including Guangxi and southern Guizhou (Figure 1). 2.1 Lower Yangtze region

ZHAO XiaoMing et al. Sci China Ser D-Earth Sci | Nov. 2008 | vol. 51 | no. 11 | 1646-1657

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Figure 1 Early Triassic tectonic palaeogeography and lithofacies in South China (modified from refs. [40,41]), showing the distribution of anachronistic facies in South China. 1, Interbedded mudstone and siltstone in shore, shelf facies; 2, turbidite in deep, water basin facies; 3, shale, limestone and grainstone in ramp facies; 4, dolomite and limestone in restricted platform and evaporate facies; 5, tempestites and limestone in open platform facies; 6, dolomite, limestone and oolites in platform, marginal facies; 7, Emeishan basalt; 8, source area (old land).

alite occurs at the Permian-Triassic boundary and upper Yinkeng Formation[15,41,47,48]. Thin bedded limestone is the most abundant, occurring in lower-middle Yinkeng Formation to upper Nanlinghu Formation[15,41]. Zebra limestone is distributed only in uppermost Yinkeng Formation and Helongshan Formation[15,41,49] (Figure 2). 2.2 Middle Yangtze region During the Early Triassic, the Middle Yangtze region experienced a transgression-regression cycle, and deposited carbonate sediments interbedded with silici-clasts in a shelf to platform facies. The lithostratigraphic sequence is divided into Daye and Jialingjiang Formations in ascending order. The Permian-Triassic boundary lies at the base of Daye Formation and the Induan-Olenekian boundary is situated in the lower part of Daye Formation (Figure 2). Flat-pebble conglomerates are common in the middle-upper Daye Formation. Vermicular limestone occurs widely in the middle and upper Daye Formation and the 1648

lower-middle part Jialingjiang Formation[41]. Microbialite is found only at the Permian-Triassic boundary and in the uppermost Daye Formation[31,41,50]. Thin-bedded limestone occurs in the middle-upper Daye Formation and middle Jialingjiang Formation. Zebra limestone is distributed in the lower Daye Formation and middle Jialingjiang Formation (Figure 2). 2.3 Upper Yangtze region The Upper Yangtze region witnessed a major marine transgression in the Early Triassic[51], and accumulated a mixed carbonate and argillaceous sedimentary sequence in a shelf to platform facies. Lithostratigraphically the Lower Triassic is divided into Feixianguan (or Daye) and Jialingjiang Formations in ascending order. The Permian-Triassic boundary lies at the base of the Feixianguan (or Daye) Formation, and the InduanOlenekian boundary is situated in the lower part of Feixianguan (or Daye) Formation (Figure 2). Flat-pebble conglomerates are common in the middle-


Figure 2

Lower Triassic stratigraphic sequences and distribution of anachronistic facies in various regions of South China.

upper Feixianguan Formation and lower Jialingjiang Formation[16,52]. Vermicular limestone occurs widely in the Lower Triassic from middle Feixianguan Formation to middle Jialingjiang Formation[19,52]. Microbialite is found at the Permian-Triassic boundary and in mid― dle-upper Jialingjiang Formation[25 28,50,52,53]. Thin-bedded limestone occurs in middle and upper Feixianguan (or Daye) Formation and middle Jialingjiang Formation[52]. Zebra limestone is distributed in the upper Feixianguan Formation[52] (Figure 2). 2.4 Youjiang region The Youjiang region in this paper includes the main part of Guangxi and southern Guizhou. The anachronistic facies are common on the isolated carbonate platforms

in the Youjiang (Nanpanjiang) Basin of deep water facies. During the Early Triassic, the Youjiang Basin was rimmed on the northwest by the Upper Yangtze Platform, a vast shallow marine carbonate shelf. Some isolated carbonate buildups in the Youjiang Basin were the shallow platforms, rimmed with oolite shoals and marginal slopes. The lithostratigraphic sequence at the margins of the basin is divided into Luolou and Ziyun Formations in ascending order. A conformable Permian-Triassic boundary is situated at the base of Luolou Formation, and the Induan-Olenekian boundary in the lower-middle part of Luolou Formation (Figure 2). Flat-pebble conglomerates occur in the middle Luolou and middle Ziyun Formation[40,41]. Microbialite is at


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the Permian-Triassic boundary and in the Ziyun Formation[23,29,40,50]. Thin-bedded limestone occurs in the lower Luolou Formation, at the boundary between Luolou and Ziyun Formations, and in the upper Ziyun Formation[40,41]. Zebra limestone almost spans the whole Ziyun Formation[40,41,54] (Figure 2). Remarkably, microbialite and zebra limestone are most abundant and persistent in this region.

3 Summary on the Lower Triassic anachronistic facies in South China The Lower Triassic in South China contains abundant distinctive sediments and related sedimentary structures, including flat-pebble conglomerates, vermicular limestone, microbialite, zebra limestone, and thin-bedded limestone, among which the vermicular limestone is especially common and characteristic in South China but no wrinkle structures and carbonate seafloor fans have yet been found. 3.1 Flat-pebble conglomerates Flat-pebble conglomerates are the limestone composed of tabular micritic intraclasts and lime-mud matrix. Generally, pebbles are of a long axis and an aspect ratio of >3:1 in outcrop[15,40]. The content of pebbles exceeds 60%, filled by lime-mud matrix, and they were poorly sorted. The pebbles are in angular or subangular shape, and spread in imbricate, bunchy, radiate and irregular arrangement (Figure 3(a)). The conglomerate beds sharply contact with the overlying and underlying limestone beds. In thin-sections, the flat-pebble clasts are tabular with rounded edges in a coarse-grained matrix containing fossil debris and smaller intraclasts. The clasts themselves are, however, devoid of fossils. Notably, the small clasts do not appear to show any orientation and the edges of the clasts are generally smooth. The flat-pebbles are probably formed in a subtidal environment during the early stage of diagenesis when the unconsolidated thin-bedded limestone was rolled up by storm or gravity flow and then re-deposited[15,16]. The flat-pebble conglomerates were abundant in the marine sediments before the Early Ordovician but became rare in subsequent normal shallow marine settings. They are valued documents for the nature of sedimentary stratification in certain geological time, and thus Sepkoski et al. called them “anachronistic facies” when they reappeared in the post-Ordovician normal marine rocks[35]. 1650

Based on the occurrence of the flat-pebble conglomerates, Sepkoski[55] and Wignall and Twitchett[16] suggested that there should be broad synsedimentary submarine cementation and lower bioturbation during the Early Triassic, and the marine ecosystems after the end-Permian mass extinction had returned to the levels before the Ordovician metazoan radiation. This marine condition was a natural response to a global oceanic anoxic event resulted from the ocean overturn at that time. The intermittent flooding of the anoxic deep-sea water with oversaturation of calcium carbonate and the increase of alkalinity for the degradation of anaerobic bacteria had considerably promoted synsedimentary submarine cementation[32]. The broad synsedimentary submarine cementation and lower bioturbation favored the preservation of primal stratification of sediments, and finally made for the formation of the flat-pebble conglomerates. 3.2 Vermicular limestone Vermicular limestone, the most common distinctive sedimentary structure in the Lower Triassic of South China, is named because of its containing a large amount of Vermes-like texture[19]. It is made of darker-colored vermicular bodies and surrounding light-colored matrix. The crystallinity of calcites in the vermicular bodies exceeds the counterpart in matrix though the major components in both vermicular bodies and matrix are of no big difference except that the matrix contains a few more clay minerals. So far vermicular limestone has been reported only from the Lower Triassic of South China. As a case study, we will present some more details of the vermicular limestone based on its occurrence in the Lower Triassic of the Three Gorge area later in this paper. 3.3 Microbialite Various microbialites have been described from the Lower Triassic of South China, including stromatolite, ― oncolite, thrombolite[15,23,25 31,40,41,50,53], microbial mound, and biostrome and bioherm with calci-microbial texture[23,47,48] (Figure 3(h)). Microbialite appears spotted, branched or laminated on weathered outcrops at the Permian-Triassic boundary in some shallow carbonate facies (Figure 3(f)), whereas they are mainly laminated in the middle and upper parts of the Lower Triassic in South China. The microbialite at the Permian-Triassic boundary has received more


Figure 3 Photos showing the anachronistic facies from the Lower Triassic of South China. (a) Flat pebble conglomerate from Helongshan Formation in Guangde, Anhui; (b) stratiform vermicular limestone from Daye Formation in Wufeng, Hubei; (c) granular vermicular limestone from Daye Formation in Xingshan, Hubei; (d) columnar vermicular limestone from Jialingjiang Formation in Wufeng, Hubei; (e) ellipsoidal vermicular limestone from Jialingjiang Formation in Wufeng, Hubei; (f) thin-bedded limestone from Daye Formation in Xingshan, Hubei; (g) zebra limestone from Daye Formation in Changyang, Hubei; (h) microbialite from Feixianguan Formation in Huayingshan, Sichuan.


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researches in recent years[25 31,50] but the laminated Lower Triassic microbialite, including stromatolite, was seldom studied[15,47,48]. The microbialite is believed to be actively constructed by some benthic microbial communities dominated by cyanobacteria in the environment devoid of the metazoans that consume the microbes and disturb the sediments. The end-Permian mass extinction is regarded as the cause of such an environment at the Permian-Triassic boundary, which resulted in the microbialite spread into some normal shallow marine facies[11,22,56]. The persistence of some microbialites in the late time of the Early Triassic should be more or less related to the delayed recovery from the mass extinction. －

3.4 Thin-bedded limestone and zebra limestone Thin-bedded limestone is quite common in the Lower Triassic of South China. These thin micritic beds differ from the general thin-bedded carbonate rocks in their unique characters that the horizontal bioturbation and small or thin-shelled fossils characteristic of disasters are more or less common on the bedding surfaces but no vertical bioturbation and fossils exist in the inner part of the rocks[57,58] (Figure 3(f)). Zebra limestone termed in this paper defines a facies showing lamination of