Stratigraphy of Deep-Water Cretaceous Deposits in Gyangze, Southern Tibet, China ) Xianghui Lia, , Chengshan Wangb, Xiumian Huc
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Cretaceous Research 26 (2005) 33–41 www.elsevier.com/locate/CretRes Stratigraphy of deep-water Cretaceous deposits in Gyangze, southern Tibet, China ) Xianghui Lia, , Chengshan Wangb, Xiumian Huc aState Key Laboratory of Oil-Gas Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, PR China bChina University of Geosciences, 29 Xueyuan Lu, Beijing 100083, PR China cDepartment of Earth Sciences, Nanjing University, Nanjing 210093, PR China Accepted in revised form 15 November 2004 Available online 12 January 2005 Abstract Extensive tectonic activity in southern Tibet (Tibetan Tethys Himalayas) resulted in overthrusting and tectonic deformation of Cretaceous strata, which in the study area was not recognized by earlier researchers. Field studies of three typical cross-sections in southern Tibet has led to the revision of previous stratigraphy. The thickness of the Cretaceous in Gyangze varies between 300 and 700 m, but is not over 2000 m as previously estimated at Gyabure and Weimei, where the strata are overturned and repeated owing to previously unrecognized thrusts. In upward stratigraphic order four lithological markers were used in the field study: belemnite and black marker, lenticular limestone and white marker, red marker, and olistolith marker. The first two of these are associated with the fine-grained clastic shelf sequence of the Gyabula Formation, which is of Berriasian–early Santonian age. The third is the deep-sea red-beds marker of the Chuangde Formation, dated as middle Santonian–early Campanian. The fourth depicts the slump facies and olistolith of the middle Campanian–Paleocene Zongzhuo Formation. The revised Cretaceous stratigraphy provides a framework for studies of the pelagic red beds of the Chuangde Formation, which is intermittently exposed in southern Tibet. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Stratigraphy; Lithostratigraphic markers; Pelagic red beds; Cretaceous; Gyangze; Southern Tibet 1. Introduction Xu et al., 1990; TBGMR, 1993, 1994, 1997) have studied the Cretaceous strata in the north Tethyan Himalayas, Mesozoic strata deposited on the distal shelf and in which Upper Cretaceous, Tethyan, pelagic red beds slope of the northern Indian continental margin are well are found, although they are less common than in the exposed in southern Tibet (central north Tethyan southern Himalayas in shelf sediments of the Tethyan Himalayas; Fig. 1), especially in the Gyangze area. The succession (e.g., Wan et al., 1993; Willems and Zhang, deep-water Cretaceous sequences crop out south of the 1993a,b; Willems et al., 1996). Our studies have shown Yarlung Zangbo suture zone (Fig. 1). It is a mountainous that several major errors were made by previous investi- area, with the chains having an elevation of 1200– gators, mainly as a result of the complex tectonics of 1500 m above the Tibetan plateau. As the mountains are the region. In this paper, we attempt to correct the not covered by vegetation, the sedimentary strata are general geological and stratigraphic interpretation of the well exposed on the slopes. Several groups of Chinese Cretaceous in the study area (Fig. 1). geologists (Yang and Wu, 1962; Wang et al., 1976; During field studies in 1998–2001 we examined three structural cross-sections and measured two new sections ) Corresponding author. (Fig. 2) in the Gyangze area where Cretaceous strata are E-mail addresses: [email protected], [email protected] (X. Li). exposed. For each of the cross-sections, we tried to 0195-6671/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2004.11.004 34 X. Li et al. / Cretaceous Research 26 (2005) 33–41 Fig. 1. Geological sketch map of Gyangze and Nagarze areas. The inset at the top left shows tectonic/stratigraphic zonations of the Tibetan Tethyan Himalayas: A, Higher (Crystalline) Himalayas; B, south Tethyan Himalayas; C, north Tethyan Himalayas; D, Indus-Yarlung Zangbo suture zone; E, Xigaze forearc basin; F, Gandese Arc. The Chuangde Formation does not appear on the map because it is less than 30 m thick. Both the Gyabula and Zongzhuo formations can be subdivided into members on a smaller-scale map (1:250,000). determine the stratigraphic sequence and thickness of 2. Results the Cretaceous by excluding strata that are repeated as a result of extensive tectonic deformation and late 2.1. Stratigraphic succession and thickness thrusting. The faults and folds were identified using a combination of sedimentary structures (upward Previous mapping by Chinese geologists reported the orientation), changes in dip angles, lithostratigraphic Cretaceous succession in the northern Himalayas to be markers, stratigraphic correlation and biostratigraphic 2000–3000 m thick. Their measurements did not take information. Samples were collected and processed for into account tectonic repetition of strata by thrust foraminifera and radiolaria, which were studied sub- faults, probably because the original determination of sequently for their biostratigraphic implications. thickness was derived from the type section at Gyabure (also named the Gyabula cross-section) described by Wang et al. (1976) and Xu et al. (1990). The Gyabure N Chuangde Pu 4837.5 Tuoma section lies in a narrow valley north of Gyabure village, Jibuli ca. 20 km east-south-east of the town of Gyangze (Figs. 5582.6 Suojin Chuangde 1, 2). Wang et al. (1976) and Xu et al. (1990) considered the entire section to be a monocline and inferred that the Weimei strata become younger northwards (Fig. 3A, B). Beijia However, the presence of at least eight thrust faults Gyangze Rhenla Puqu (F1–8), three anticlines, and two synclines (Fig. 3B) To Ringbung 5427.7 (Dangerous) 5296.9 result in duplication of the strata. 4992.8 According to our interpretation, we consider that the first anticline is composed of sandstones and shales of To Nagarze Gyabure Valley the Upper Jurassic Weimei Formation. Following the fault F1, the first syncline incompletely crops out To Kangmar Gyabure between faults F1 and F3, and the second anticline is Nairi 0 2 4 km constrained by faults F3 and F5 (Fig. 3B). All of the strata in the second anticline are the counterpart of bed Fig. 2. Locations of measured Cretaceous sections in the Gyangze 6ofWang et al. (1976) and beds 8, 9 and lower part of area. Elevations of peaks are in metres. 10 of Xu et al. (1990). Thus, it is a composite syncline in X. Li et al. / Cretaceous Research 26 (2005) 33–41 35 NNE 0 200 400 m (Silty) shale Gyabure Siliceous shale valley or banded chert 10-13 9 Siltstone 7 8 1 6 K1 / K2 2 34 5 Sandstone J3 / K1 A Mudstone / Wackestone NNE 0 200 400 m Marlstone Lenticular / Gyabure F8 F pillow limstone valley 7 21 F1 Slump / F 17-20 6 15-16 olistolith 4 F F5 1 2 F3 F4 13 12 14 Slope cover 2 3 11 J 10 10 K 3 7 8 9 2z F Fault and number B 5 6 8 1 K / K K1j 1 2 Inferred fold 343 77 Dip and angle G19 Numbered bed NNE 0 50 100 m F8 J3 Upper Jurassic Gyabure F K Gyabula Fm F1 valley 7 1j (Previous) 0 50 100 m G19 G20 Gyabula Fm F2 G21 K(1-2)j K K2c (this paper) J3 2z G22 G1-3 K2z Zongzhuo Fm G4 G18 G5 G10-12 G16 G17 Chuangde Fm G6 G7-9 G13-15 K2c C K(1-2)j 343 77 354 82 K /K Estimated boundary 350 69 1 2 of system Fig. 3. Re-interpretation and comparison of the Gyabure section as measured by different authors: A, Wang et al. (1976);B,Xu et al. (1990) with our structural interpretation; C, this paper. which the strata involved are equivalent to beds 7–13 of interpreted it as becoming younger northward (Fig. 4), Wang et al. (1976) and beds 10–21 of Xu et al. (1990). and estimated it to be more than 1000 m thick. We have Fossils indicate that the strata between faults F1 and revised the interpretation of beds 9–4 of Wu et al. F2 are of Early Cretaceous age, and the sedimentary (1977), using sole mark orientations and renumbered structures (sole marks) show a normal succession north- beds W1–6 (Fig. 4), correlating them with strata in the wards, i.e., it becomes younger from bed 5 to 6 (Fig. 3B). Gyangze area. As a result, we obtained a thickness of We found Late Cretaceous strata in the upper part of less than 300 m for the Cretaceous of the Weimei the cross-section that lies between faults F7 and F8 section. (Fig. 3B), which contradicts the interpretation (Fig. 3A, The stratigraphic succession at Chuangde was inter- B) of Wang et al. (1976) and Xu et al. (1990). We also preted by Wu (1987) to be overturned, and has fre- carried out more detailed measurements of the succes- quently been cited as a key reference section for the sion between faults F1 and F2 and F7 and F8 (Fig. 3B, deeper water Cretaceous exposed in the north Tethyan C). By adding the thicknesses we obtained, and allowing Himalayas (southern Tibet). The section is near the for corrections for duplication of strata by thrust fault- village of Chuangde, about 15 km east-north-east of ing, we arrived at an average thickness of 300–500 m for Gyangze (Figs. 1, 2). This area is much easier to study the Cretaceous strata, although at several sections we than the Gyabure cross-section. Wu (1987) considered measured almost 700 m. the north-westward (from right to left in Fig. 5A) Another erroneous interpretation of the thickness sequence to be progressively younger and dated most of and sequence of the Cretaceous succession was also the strata as Late Cretaceous–Eocene, although there is identified at the locality of Weimei, which is not far no fossil evidence for the Palaeogene.