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(India) and Yangtze Block (South China): Paleogeographic Implications

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(India) and Yangtze Block (South China): Paleogeographic Implications Neoproterozoic stratigraphic comparison of the Lesser Himalaya (India) and Yangtze block (south China): Paleogeographic implications Ganqing Jiang* Department of Earth Sciences, University of California, Riverside, California 92521, USA Linda E. Sohl Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory of Nicholas Christie-Blick Columbia University, Palisades, New York 10964-8000, USA ABSTRACT terpreted to correspond with a level within the Recent studies of terminal Neoproterozoic rocks (ca. 590±543 Ma) in the Lesser Him- glaciogenic interval (Fig. 2). A passive- alaya of northwestern India and the Yangtze block (south China) reveal remarkably sim- margin setting is inferred with con®dence for ilar facies assemblages and carbonate platform architecture, with distinctive karstic un- postglacial carbonate rocks on the basis of conformities at comparable stratigraphic levels. These similarities suggest that south platform scale, comparatively simple physical China may have been located close to northwestern India during late Neoproterozoic time, stratigraphic and facies architecture, and the an interpretation permitted by the available, yet sparse paleomagnetic data. Additional thickness of successions, with no evidence for parallels in older rocks of both blocksÐsimilar rift-related siliciclastic-volcanic successions either syndepositional tectonism or igneous overlying metamorphic basement, and comparable glaciogenic intervals of possibly Stur- activity. Figure 2 summarizes pertinent litho- tian and Marinoan or Varanger ageÐsuggest that this spatial relationship may have de- stratigraphy and lateral facies variations. veloped earlier in the Neoproterozoic. With the exception of basal Cambrian phosphorite and comparable small shelly fossils, stratigraphic contrasts between northern India and Preglacial Successions (Early south China and increasing biogeographic af®nity between south China and northwestern Neoproterozoic) Australia suggest that south China may have migrated toward northwestern Australia In the Lesser Himalaya, the preglacial suc- during the Cambrian. cession consists of locally tuffaceous silici- clastic rocks and associated minor ma®c Keywords: Neoproterozoic, Lesser Himalaya, India, south China, stratigraphic comparison, volcanic rocks, possibly rift-related, uncon- tectonic reconstruction. formably overlying Mesoproterozoic meta- morphic rocks (Kumar, 1985; interval A1 in INTRODUCTION evidence places Australia directly against Lau- Fig. 2A). Facies become ®ner overall toward The hypothesized Proterozoic superconti- rentia, although details of the con®guration the northwest, from nonmarine sandstone and nent of Rodinia has attracted considerable in- and timing of breakup remain elusive (e.g., conglomerate (Nagthat Formation) at the terest recently (e.g., Bartley and Kah, 2001; Wingate and Giddings, 2000; Karlstrom et al., Nainital syncline to nearshore siltstone and Powell and Meert, 2001). One point of debate 2001); (2) the synrift to early passive-margin sandstone (Chandpur and Nagthat Formations; concerns the position of south China within succession of south China (ca. 820±750 Ma) Jaunsar Group) in the region between the Rodinia. Paleomagnetic constraints for south resembles coeval strata of the eastern Austra- Garhwal and Nigalidhar synclines, to shale, China from 1100 to 500 Ma are few, and lian craton and western Laurentia only grossly siltstone, and sandstone (Simla Group) at Sim- available data permit several possible recon- (Drexel et al., 1993; Narbonne and Aitken, la (Fig. 1B). An erosional unconformity is structions (Evans et al., 2000). In one recon- 1995; Wang and Li, 2003), and the orientation present at or near the Chandpur-Nagthat con- struction, Li et al. (1995) suggested that south of sedimentary basins was not taken into ac- tact. The age of the rocks is constrained in- China was between the eastern side of the count; (3) synrift magmatism from ca. 860± directly by U-Pb dates of 771 6 2 and 751 6 Australian craton and the western side of Lau- 730 Ma in Australia and south China is not 3 Ma for volcanic rocks (Malani igneous rentia. This interpretation was based in part unique (e.g., Singh et al., 2002); (4) early suite) west of the study area, in Rajasthan upon the presence of 1.9±1.4 Ga crust in both Neoproterozoic (ca. 950±850 Ma) collision- (Torsvik et al., 2001). Cathaysia (southeastern China; Fig. 1C) and related volcanic rocks in south China (Ling et In south China, correlative strata (interval the Priest River Complex (southwestern Can- al., 2003) have no correlatives in southeastern B1 in Fig. 2B) unconformably overlie 819 6 ada) that could account for detrital zircons of Australia; and (5) the similarities in Cambrian 7 Ma (U-Pb) granites (Ma et al., 1984) in appropriate age in the lower Belt Supergroup faunal provinces between south China and the Yangtze Gorge area and Mesoproterozoic of Montana and adjacent Canada. Additional northwestern Australia (e.g., Burrett et al., ($1000 Ma) metamorphic rocks assigned to support for this hypothesis includes (1) the 1990) require south China to have migrated a the Lengjiaxi and Sibao Groups. Nonmarine similarity in age and chemistry of ma®c dikes great distance during late Neoproterozoic sandstone in the Liantuo Formation is equiv- and sills in southern Australia and south China time. In light of these dif®culties, we explore alent to the upper part of the Banxi Group (Li et al., 1999, 2003), (2) the existence of an an alternative arrangement. (Wuqiangxi Formation), which is separated erosional surface above ca. 825 Ma rocks in from shale and siltstone of the underlying Ma- both regions (Li et al., 1999), and (3) similar NEOPROTEROZOIC STRATIGRAPHY diyi Formation by an erosional unconformity. rifting history between south China and south- OF INDIA AND SOUTH CHINA At Huaihua (location 10 in Fig. 1D), the Banxi eastern Australia from ca. 820±750 Ma (Ling Neoproterozoic rifted margins developed Group is composed mainly of basinal shale et al., 2003; Wang and Li, 2003). along the northern edge of India (Fig. 1A; Ku- and siltstone with abundant turbidites. Asso- However, this reconstruction presents sev- mar, 1985) and along the southeastern and ciated rift-related ma®c volcanic rocks and eral dif®culties: (1) other zircon provenance western sides of the Yangtze block (Fig. 1C; tuff (Li et al., 1999, and references therein) Li et al., 1999). In both areas, the timing of have been dated to 748 6 12 Ma (U-Pb; Ma *E-mail: [email protected]. the rift to postrift transition is tentatively in- et al., 1984). q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; October 2003; v. 31; no. 10; p. 917±920; 3 ®gures. 917 and Krol Group in India (interval A3 in Fig. 2A) and to the Doushantuo and Dengying For- mations in China (interval B3 in Fig. 2B). The lower part of the Krol Group (Krol A) and the middle to upper part of the Doushantuo For- mation contain phosphorite-chert nodules with distinctive acanthomorph acritarchs and the cyanobacteria Salome hubeiensis, which is known only from these two successions (AC in Figs. 2A and 2B; Tiwari and Knoll, 1994; Shuhai Xiao, 2003, personal commun.). Sim- ilar Ediacaran impressions have been reported from the Krol D and E in India and from the upper part of the Dengying Formation in Chi- na, although this fossil evidence is as yet in- conclusive. The Lower Cambrian is marked by stratigraphic condensation, additional phosphorite, and comparable small shelly fos- sils in the basal Tal Group of India and in the Shuijintuo and Niutitang Formations of China. In each area, the carbonate platform exhib- its remarkably similar facies and stratigraphic details, including an evolution from mixed siliciclastic-carbonate ramp in the lower part, to rimmed shelf with marginal stromatolite- rich complex in the middle, to open shelf in the upper part. Moreover, each succession is divisible into seven intervals bounded by re- gional stratigraphic discontinuities, distinctive unconformities termed sequence boundaries appearing at the same stratigraphic level. Figure 1. A: Neoproterozoic tectonic framework for India, emphasizing sedimentary basins Prominent unconformities in the middle part ca. 800 Ma (modi®ed from Kumar, 1985). Southern limit of passive margin is approximated of the Krol and lower part of the Dengying as Main Boundary thrust, and is not palinspastically reconstructed. B: Neoproterozoic and (surfaces 4 and 5 in Figs. 2A and 2B) show Cambrian outcrop in Lesser Himalaya, India, and sections used to construct Figure 2A. C: similar karstic features, including depressions Neoproterozoic tectonic framework for south China, emphasizing inferred continental rift systems ca. 800 Ma (modi®ed from Li et al., 1999). D: Geologic map of study area, and with mappable relief, and breccias and cal- sections used to construct Figure 2B. crete ®lling dikes and smaller cavities (Jiang et al., 2002). Synglacial Successions (Sturtian and upper diamictite (Nantuo Formation) overlies Marinoan Glacial Intervals) an erosional unconformity (Jiang et al., 1996). PALEOGEOGRAPHIC IMPLICATIONS The Jaunsar and Simla Groups in north- The age of these rocks is still debated. Some These stratigraphic similarities suggest a western India are unconformably overlain by researchers suggest that both Chang'an and close paleogeographic relationship between glaciogenic diamictite, siltstone, and sand- Nantuo are Sturtian (e.g., Wang
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