Orbital Forcing and Sea-Level Changes in the Earliest Triassic of the Meishan Section, South China

Journal of Earth Science, Vol. 25, No. 1, p. 64–73, February 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0400-3 Orbital Forcing and Sea-Level Changes in the Earliest Triassic of the Meishan Section, South China Shifeng Tian*1, 2, 3, Zhong-Qiang Chen4, Chunju Huang5 1. Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China 2. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China 3. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China 4. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China 5. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China ABSTRACT: The Earliest Triassic Yinkeng Formation is exposed at the well-known Meishan Section, South China, which contains the Global Stratotype of Section and Point (GSSP) for the Permian- Triassic boundary (PTB). It records centimeter-scale rhythmic alternations comprised mainly by marlstone and limestone. Seven types of couplet embedded in five types of bundles were recognized based on occurrence and thickness of the lithologic units, suggesting that their formation was controlled by cyclic processes. The various orders of cycles observed correlate well with other Early Triassic counterparts recorded in South China. Here, we present new cyclostratigraphic results based on lithologic thickness and relative carbonate content of the Yinkeng Formation. Power spectra of carbonate content show that the ratio of major wavelengths recognized throughout the formation is similar to that of the 100 kyr short eccentricity, 33 kyr obliquity, and 21 kyr precession cycles, indicating that astronomical signals are recorded in the Earliest Triassic rhythmic succession. Consistence between pronounced lithologic rhythmicity and sea-level changes obtained from Fischer plots indicates that high-frequency climatic cycles may have driven sea-level changes immediately after the PTB mass extinction. Fur- thermore, the 4th-order sea-level changes interpreted from the sedimentary record match well with 100 kyr short eccentricity component of carbonate content, reflecting that the 100 kyr short eccentricity- induced climate changes may have likely controlled the deposition of 4th-order sequences recognized from rhythmic successions. KEY WORDS: orbital cycle, limestone-marl alternation, carbonate content, Fischer plot, Yinkeng Formation. 1 INTRODUCTION extreme climatic and oceanic conditions prevailed throughout As a consequence of the most devastated biotic crisis of the Early Triassic (Chen and Benton, 2012; Sun et al., 2012; earth life at the end of the Permian, the Earliest Triassic marine Algeo et al., 2011). faunas are very rare worldwide (Chen and Benton, 2012; Erwin, Detecting cyclic sedimentation patterns and temporal pe- 1994). The depauperate nature of marine faunas suggests a riodicities offer geoscientists better understanding the origin of rather low bioturbation intensity in marine sediment, which sedimentary sequences and their driving mechanisms enables the excellent preservation of very fine rhythmic cyclic- (Paulissen and Luthi, 2011). Milankovitch signals have been ity in the Early Triassic succession. The well-preserved, detected in Early Triassic marine records (Wu et al., 2012; centimeter-scale rhythmic sediments are characteristic of the Huang et al., 2011; Guo et al., 2008; Li et al., 2007; Tong et al., Early Triassic successions in South China (Wu et al., 2012; 2007; Yang and Lehrmann, 2003; Hansen et al., 2000; Rampino Guo et al., 2008; Tong et al., 2007; Yang and Lehrmann, 2003; et al., 2000). At the GSSP Meishan Section, the centimeter- Lehrmann et al., 2001). The Early Triassic was a critical period scale, rhythmic alternation is pronounced in the Early Triassic witnessing the much delayed recovery of ecosystems following succession (Chen et al., 2007, 2002; Yin et al., 2001). Moreover, the End-Permian mass extinction. Many studies show that the biostratigraphy, chronostratigraphy, event stratigraphy and ecostratigraphy of the Latest Permian to Early Triassic of *Corresponding author: [email protected] Meishan have been well studied, and thus offer high-resolution ©China University of Geosciences and Springer-Verlag Berlin time constraints for further high-resolution cyclostratigraphic Heidelberg 2014 analysis (Chen et al., 2010; Zhang et al., 2007; Yin et al., 2001). Whereas high-resolution cyclostratigraphy and sequence strati- Manuscript received April 22, 2013. graphy have received far less attention. Tong (1997) attempted Manuscript accepted July 11, 2013. to recognize Milankovitch cyclicity in the Early Triassic of Tian, S. F., Chen, Z. Q., Huang, C. J., 2014. Orbital Forcing and Sea-Level Changes in the Earliest Triassic of the Meishan Section, South China. Journal of Earth Science, 25(1): 64–73, doi:10.1007/s12583-014-0400-3 Orbital Forcing and Sea-Level Changes in the Earliest Triassic of the Meishan Section, South China 65 Meishan but facies homogeneity at outcrop exposure in the tion differentiated by lithology. Two to three units can usually lower Yangtze region prevents a high-resolution sedimen- be grouped into a couplet. These couplets are interpreted as the tological analysis (Tong and Yin, 1998; Zhang et al., 1997; Yin depositional response to one cycle of sea-level change. The and Tong, 1996). This means that it is difficult to verify orbital couplets stack into lower-order cycles, here termed bundle, forcing mechanism without quantitative analysis. Whether or- based on the pattern of upward facies change. In general, black bital forcing was a major climatic driver during this critical shale, claystone, greenish gray mudstone, and calcareous mud- period is crucial for the understanding of the controlling factors stone characterize the lower part of cycles whereas greenish and depositional mechanisms during the period of biotic recov- gray mudstone, muddy limestone/marlstone, and limestone the ery following the End-Permian environmental crisis. upper part. Hence, we interpret the couplets and bundles to Here, we present a case study of the Early Triassic represent shallowing-upward sequences on the basis of greater Yinkeng Formation exposed at the GSSP Meishan Section, argillaceous content and thinner beds in their lower parts, and South China. We aim to (1) distinguish the different orders of progressively increasing bedding thickness and decrease in embedded cycles and verify the origin of the high-frequency argillaceous content in the upper parts of the units. Of the iden- cyclicity; (2) employ a modified version of Fischer plots to tified 332 units, seven basic couplet types, 5 bundle types and define the high-frequency sea-level changes and compare them 158 couplets have been recognized. with the global eustatic sea-level changes; and (3) explore the potential causes of sea-level changes in the Earliest Triassic. 3.1.1 Basic couplets Couplets 1-3 (BC1-3) are all thickening-upward couplets 2 GEOLOGICAL SETTING which consist of two portions. The lower part is black shale, The GSSP Meishan Section is located at Meishan Town of capped by pale muddy limestone or gray marlstone, gray lime- Changxing County, Zhejiang Province, South China. The stone, and gray calcareous mudstone as the upper part, respec- Meishan area is situated in the eastern part of the South China tively. The shale is interpreted to be deposited in a quiet Block, which was a giant island in the eastern Paleo-Tethys deep-water environment with thickness ranging from 1–5 cm, Ocean near the tropics during the Permian-Triassic transition while the three lithologic types of the upper part of couplet (Ziegler et al., 1998). Unlike the rather heightened paleo- indicate relative shallow-water environments with thickness geographic contoural difference prior to the PTB biocrisis, the ranging 2–18, 4–11, and 3–24 cm, respectively. Among these Earliest Triassic paleogeography in South China was relatively three basic couplets, BC1 is very common across the entire uniform: massive carbonate ramp bounded with numerous plat- Yinkeng Formation while BC2 is rare and only develops in the forms and shelf basins mainly in the western part of the South uppermost of Yinkeng Formation. BC3 occurs mainly in the China Block (Zhang et al., 2007; Yin and Tong, 1996). The Early Triassic succession comprises the Yinkeng and Helongshan formations. The former conformably overlies above the platform carbonate rocks of the Latest Permian Changxing Formation. This formation (Bed 25 to Bed 60) is characterized by interbedded black shale, greenish gray mudstone, gray calcareous mudstones, gray marlstone, pale muddy limestone, and limestone (Figs. 1, 2). The overlying Helong- shan Formation is dominated by gray thin-bedded limestone. The Yinkeng Formation and the lower Helongshan Formation are of Griesbachian age (Chen et al., 2007). The Yinkeng For- mation is much better exposed than the Helongshan Formation and thus is studied in detail from the PTB to Bed 60 herein. Centimeter to decimeter-scale limestone-marl alternations are recognized throughout the Yinkeng Formation (Figs. 1b, 1c, 2). A total of 13.34 m stratigraphic thickness which is composed of 33 beds (332 smaller units) was examined vertically. The data collected consist of the lithological description and thickness of each unit. 3 DATA AND METHODS 3.1 Cycle Recognition Assigning high-frequency

Load more