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Early Cretaceous palynofloras from the Tanggula Mountains of the northern Qinghai-Xizang (Tibet) Plateau, China
Cretaceous Research 25 (2004) 531e542 www.elsevier.com/locate/CretRes
Early Cretaceous palynofloras from the Tanggula Mountains of the northern Qinghai-Xizang (Tibet) Plateau, China Jianguo Lia,), David J. Battenb,c a
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, PR China b Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK c Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK Received 14 November 2003; accepted in revised form 23 April 2004
Abstract Two sections of the Yanshiping Group in the Tanggula Mountains area of the Qinghai-Xizang (Tibet) Plateau, a remote part of western China, have yielded three types of palynomorph assemblages. Two of these clearly indicate that the Xueshan Formation and parts of the diachronous Suowa and Xiali formations were deposited during the early Cretaceous (Berriasiane?Barremian). Largely on negative evidence, because the assemblages are taxonomically impoverished, the rest of the succession is considered to range down into the Upper and possibly Middle Jurassic. The composition of the Cretaceous assemblages suggests that the climate during this period was warm and semi-arid. A Xizang-Tarim Subprovince of the palynofloral North Gondwanan Province is tentatively identified in the north-east Tethyan region. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Early Cretaceous; Qinghai-Xizang (Tibet) Plateau; Palynology; Floral provinces; Climate
1. Introduction The samples on which this paper is based were collected from the northern slope of the Tanggula Mountains of the tectonic Qiangtang Terrain, the boundaries of which are the Bangong Co-Siling Co Suture (Fig. 1) in the south and the Jinsha River Suture in the north. Mesozoic rocks are widely distributed and well exposed in this region. Since the 1960s, several geological and stratigraphic studies have been carried out, but most of these were of a reconnaissance nature. Recently geologists and palaeontologists have become aware that the region is crucial to studies on Mesozoic palaeobiogeography (Li, 1983; Li and Liu, 1994; Sha, 1998), and to the tectonic history and current geography of the Qinghai-Xizang (Tibet) Plateau (Sun and Zheng, 1998).
) Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (D.J. Batten). 0195-6671/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2004.04.005
Outcrops of the Yanshiping Group in the Tanggula Mountains are ideal for studying the JurassiceLower Cretaceous successions of this region. After they were first encountered in the 1960s, and named by Jiang in 1983, some palaeontological data have accumulated, mostly on ammonites, bivalves and brachiopods. Jiang (1983) listed the bivalves that had been found to date, and regarded the age of the uppermost part of this group, the Xueshan Formation, as early Cretaceous. A few years later, however, Yin (1988) suggested a Bajocian age for the bivalve fauna from the formation, whereas Bai (1989) considered the bivalves to indicate the Late Jurassic. A few poorly preserved ammonite specimens were recovered from exposures in Amdo County to the south of this area. These were considered by Wang Yigang and Chen Guolong (in Wang et al., 1979) and others (Westermann and Wang, 1988) to be MideLate Jurassic in age. However, not only are the specimens rather poorly preserved, which renders the determination questionable, but also no ammonites have been found in the northern Tanggula Mountains.
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Furthermore, during a recent field survey by Li Yong and others, a few beds containing foraminifera were encountered in the underlying Suowa Formation, and these indicate the Early Cretaceous (Li et al., 2000). Identifiable land plant megafossils have not been found so far in the interbedded marine and non-marine deposits of the Yanshiping Group. Palynomorphs have not previously received much attention in this region, and no palynologists have undertaken an investigation of the succession until now. As a result, little is currently known about the contemporaneous terrestrial flora, which in turn limits interpretations based on other geological studies. In this paper we present palynological data on two sections of the Yanshiping Group. Although the assemblages recovered probably range in age from mid Jurassic to early Cretaceous, the probable Jurassic assemblages are mostly impoverished and important only in the general geological context of the region. Hence, although these are considered herein, we concentrate on the Cretaceous assemblages, discussing their stratigraphic, palaeoenvironmental and phytogeographic significance.
2. Stratigraphic setting The Yanshiping Group in the Tanggula Mountains consists of, in ascending order, the Quem Co, Buqu,
Xiali, Suowa and Xueshan formations. Of the two sedimentary successions considered, the Quem Co Formation crops out only in the Yanshiping section, whereas the uppermost Xueshan Formation is exposed only in the Wenquan section (Figs. 1, 2). Both of these sections are named after near-by settlements. Their lithologies are briefly described below. 2.1. Quem Co Formation This lowest formation of the Yanshiping Group has an outcrop thickness of around 610 m in the Yanshiping section. It consists of varicoloured sandstones, siltstones, mudstones and some marls. Invertebrate fossils occur in a few beds: some gastropods but mainly bivalves in a fair to poor state of preservation. Spores and pollen grains (miospores) are scarce; so far only one sample has proved to be reasonably productive (Table 1). 2.2. Buqu Formation This formation is 1662.3 m thick in the Yanshiping section. It is also exposed at Wenquan but has not been measured and sampled there. It can be divided into two parts at Yanshiping. The lower 965 m consist of numerous grey to greenish grey sandy limestone, sandstone, and siltstone sequences, occasionally punctuated by
Fig. 1. Map showing the location of the two sections of the Yanshiping Group on the northern Qinghai-Xizang Plateau that are considered in this paper: Y, Yanshiping; W, Wenquan. Inset: general location map; the Bangong Co-Siling Co suture is indicated by the solid and dashed line.
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
533
Fig. 2. The Yanshiping (A) and Wenquan (B) sections.
purple sandstones in the upper part. The upper 697.4 m comprise mainly mediumedark grey, thick limestones, with some thin marl interbeds. The formation contains assemblages of both marine and non-marine invertebrate fossils. Marine bivalves, brachiopods, echinoids and foraminifera are sometimes sufficiently abundant to form a bioclastic limestone. Palynomorphs are common in the lower member but occur only sporadically in the upper section. Fragments of wood are also common in the lower member; these are usually carbonized and always poorly preserved.
2.3. Xiali Formation This formation has been measured in its entirety in both sections, with thicknesses of 572 m at Yanshiping and 595.6 m at Wenquan being recorded. The lithologies are mainly grey to greenish grey siltstones, sandstones and marls and dark purpleered siltstones, with occasional interbeds of grey mudstone, dark grey, thinlybedded limestone and greenish grey sandy limestone. Bivalves, ostracods, remains of charophyte algae and some poorly preserved (unidentifiable) plant fragments have been found, and a few miospore assemblages have been recovered.
2.4. Suowa Formation The Suowa Formation is also well exposed at both localities, but at Yanshiping the section terminates in the core of a syncline before the top of the formation is reached. The incomplete record of around 460 m contrasts with a total thickness of 838.7 m in the Wenquan section. The succession consists mainly of greenish grey and dark grey mudstones and siltstones, and mediumedark grey limestones and sandy limestones. There are also some marl interbeds and limestones. The latter occur more frequently in the lower part of the section; the upper part is dominated by siltstones and mudstones. Exposed surfaces of the sandstones frequently show large-scale ripples (Fig. 3A). Bivalves and miospores are comparatively abundant. Ostracods, charophyte gyrogonites and dinoflagellate cysts also occur. 2.5. Xueshan Formation This formation is more restricted in its distribution than the others. Its exposure is intermittent along a narrow range near the Tanggula Mountains Pass. It was named, and its type section at Wenquan measured, by Jiang (1983). Li Jianguo and colleagues re-examined
534
0.6
YSP67-1
1.3
Suowa
YSP65-1
0 .3
0.4 0.5
1 .8
YSP59-2 0 .6
0.6
YSP39-4
Buqu
YSP39-3
1.3
1.3 21.7
0.5
0.3
0.8 0.6 0.4
1.8
2.7 0.5
0.5
3.6
3.0
0.6
8.8
3.5
1.7
4.6
9.0 8.7
0.4
0.4
0.5
0.6
0 .6
0.5
0.6
0.6
0.6
0.6
1.1
3.8
1.3
1.3
8.7
8 .7 1.0
0.6
5.2 15.7
5.2 0.6
YSP34-2
37.7 41.1
7.2 0.3
YSP33-7
35.6
9.9 11.9
YSP33-4
1.1 10.6
2.7
YSP31-1
3.9 11.8
3.1
Quem YSP13-1 Co
24.4 36.6
5 .9
0.7
1.3
36.7 46.7
6.0
0.8
0.8
0.4 24.2 30.2
7.3
0.4
1.9 46.3 27.8
6.2
0.6
2.0 65.6 18.4
1.6
1.8
0.9
2.4
0.6
0.6
1.2
0.6
0.6
2.3
1.1
1.3
2.6
1.2
0.6
1.0
1.4 3.0
1.8 32.4 26.5 13.5
1.2
1.7 48.6 22.9
1.7
4.3 21.7
1.0
1.5
2.0 3 .2
4.6 6.0
4.6
2 .1
1.3
1.3
1.7
1.4
0.7
1.0 0.7
32.3 1.2
5.6
0.4
1.6
0.3
0.7
0.7
0.5
1.2
0.8
0.4
1.4
1.8
3.0
1.2
0.6
2.9
1.1
0.6
0.6
0.6
Total specimens
28
0.6
310 150
0.4
248
0.6
16 1
0.4
244
0.5
218
0.6
168 170
0.6 0.6
175
0.6
78
3.8 8.7
1.5
12.8
0.5
1.1 15.4
4.0
2.9
1.1
0.4
1.5
5.8
2.7
1.2 33.1
9.3
1.7 11.6
3.5
1.4
6.8
1.4
2.4
1.0
3.0
7.9
18.8
3.0
1.6 53.2 18.6
4.3
1.6
0.9 52.4 15.3
2.6
5.2
0.4
1.3
1.4 15.7
0.3
5.2
0.7
1.7
1.4
3.6
0.7
1.2 0.4
other pollen
Vitreisporites
Taxodiaceaepollenites
Ovoidites 0.3
0.4
8.2
9.3
Quadraeculina
Eucommiidites
Psophosphaera
Ephedripites (Spiralipites)
D. etruscus 7.7 3.3
1.8 47.9 29.7
1.3 57.7 15.4
4.7
1.6
1.8 47.6 20.8
8.7
0.8 5.2
Dicheiropollis
Cycadopites
9.6
0.4
0.3
C. classoides
37.0 37.0
1.0
1.2
0.6
C. annulatus
0.6
12.0 48.6 14.3 0.4
10.7
0.3
4.9
1.2
0.6
YSP35-1
Classopollis
Alisporites
75.0
0.4 0.5
0.6
15.4 50.2 10.0
Chasmatosporites s.l.
other spores
Stereisporites
Pterisisporites
Polypodiaceaesporites
Pilosisporites
Osmundacidites
Neoraistrickia
Lygodiumsporites
0.3
14.4 41.0 13.3 0.6
Lygodioisporites
Lycopodiacidites
Leptolepidites
Impardecispora
Klukisporites
Hymenophyllumsporites
Dictyophyllidites harrisii
Foraminisporis
Deltoidospora 1.2
3.4
0.3
1.2
0.8
0.6
0.3 0.7
1.2
0.6
YSP54-1
0.6
0.8 0.5 0.6
YSP56-1
YSP39-5
C. minor
0.4 0 .6
YSP61-1
0.6 0.7
0 .4
3.6
3.6
0.3
YSP64-2
YSP59-1
Cyathidites
3 .6
YSP67-2
YSP59-3
Converrucosisporites
Concavissimisporites
YSP67-3
YSP46-1 Cyathidites – Classopollis – Cycadopites
Cicatricosisporites
Calamospora
Asseretospora
Baculatisporites
Annulispora
Fm Samples
Xiali
Dicheiropollis peak Dicheiropollis-ClassopollisCicatricosisporites
Taxa
0.5
8.7
23
1.0
0.5
1.0
195
0.4
0.4
1.2
259
175 1.2
172
1.2
291
0.7 4.0
2.0
101
1.0 0.5
0.5
0.3
1.0
319
0.4 0.7
0.3
188
287
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Palynomorph assemblages
Table 1 Distribution of palynomorphs through the Yanshiping section
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(Figs. 4, 5), the bulk of the organic residues were subjected to a little oxidation with hydrogen peroxide. At least 200 specimens were counted from productive samples whenever possible; a minimum of 100 palynomorphs was recorded from all of the low-yield samples listed in Tables 1 and 2 apart from three in the Yanshiping and six in the Wenquan sections, respectively. Other, more impoverished, samples are not tabulated.
4. Palynological assemblages
Fig. 3. A, ripple marks on the surface of a sandstone bed in the Suowa Formation, Wenquan section. B, the red and yellow deposits of the Xueshan Formation of which the hill in this photograph is composed, make it easy to recognize in the field.
the section in 1999 and measured a total thickness of approximately 229 m, but the top of the formation has yet to be reached, the steep and dangerous terrain rendering it inaccessible. It is distinguishable in the field by its varicoloured appearance (Fig. 3B), consisting as it does of reddish, yellowish and off-white siltstones, sandstones and mudstones, interbedded with layers of grey to black mudstone and siltstone. Miospores are fairly numerous. Bivalves occur in the lowest beds, as well as a few dinoflagellate cysts. Unidentifiable plant fragments are common in the black mudstones and siltstones.
3. Material and methods One hundred and fifty-six samples were collected for palynological examination from the two sections. They were processed in the laboratory by standard techniques using hydrochloric and hydrofluoric acids, and the residues were sieved through a screen of 10 mm mesh. Since most of the palynomorphs were found to have been blackened as a result of regional metamorphism
Sixty-two samples yielded small to fairly large assemblages of palynomorphs: 21 from the Yanshiping section and 41 from the Wenquan section. Most of these are from the Suowa and Xueshan formations, some are from the Buqu Formation, a few from the Xiali Formation, and one from the middle part of the Quem Co Formation. Forty-two genera of spores and gymnosperm pollen grains were recorded. Most specimens are very dark in colour (brownish blackeblack) and poorly preserved to the extent that the majority could not be identified to species. A few may have algal origins. No angiosperm pollen grains were found. Examples of the components of the palynofloras are illustrated in Figs. 4 and 5. All of the palynomorphs encountered and their quantitative distribution in the moderately productive samples through the succession are noted in Tables 1 and 2. In general the palynofloral data from the two localities are similar. Three palynomorph assemblagetypes have been recognized. Their characteristics are recorded below in ascending order. 4.1. Cyathidites-Classopollis-Cycadopites assemblage This assemblage is recognizable from the middle of the Quem Co Formation (a single sample) to the upper middle part of the Xiali Formation (units 13e54) in the Yanshiping section, and from around the middle of the Xiali Formation to the lowest part of the Suowa Formation (unit 14 to the near the bottom of unit 21) in the Wenquan section. Spores recorded include Asseretospora sp. (0e0.5%), Baculatisporites sp. (0e1.5%), Calamospora sp. (0e1.3%), Cyathidites minor (0e50.2%), C. spp. (0e37.7%), Deltoidospora spp. (0e14.3%), Klukisporites sp. (0e8.7%), Lycopodiacidites sp. (0e1.2%), Neoraistrickia sp. (0e1.3%), Osmundacidites sp. (0e0.3%), Pterisisporites sp. (0e1.3%) and Stereisporites sp. (0e5.2%). One specimen of Cicatricosisporites sp. was also encountered in the Xiali Formation at Yanshiping. Gymnosperm pollen grains include Alisporites sp. (0e8.7%), Chasmatosporites sensu lato (0e4.3%), Classopollis annulatus (0e65.5%), C. classoides (0e23.6%), C. sp. (0e71.4%), Cycadopites sp. (0e18.8%), Eucommiidites sp. (0e1.0%),
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Fig. 4. Some of the palynomorphs recovered from the Yanshiping Group; all !800. For each specimen, the numbers prefixed by YSP (Yanshiping section) or WQ (Wenquan section) give the stratigraphic unit (first two numbers), slide number (second two) and specimen number on slide (last two). A, Stereisporites antiquasporites (Wilson and Webster) Dettmann, 1963, YSP130105. B, Deltoidospora regularis (Pflug) Song and Zheng, 1981, WQ390318. C, Asseretospora parva (Li and Shang) Pu and Wu, 1985, YSP650106. D, Pterisisporites medirhaptes Li, 1979, YSP560103. E, Neoraistrickia taylorii Playford and Dettmann, 1965, WQ360120. F, Converrucosisporites sp., WQ350206. G, cf. Dictyophyllidites harrisii Couper, 1958, YSP350105. H, Calamospora nathorstii (Halle) Klaus, 1960, WQ360107. I, J, Cicatricosisporites amalocostriatus Zhang, 1965: I, WQ390317; J, WQ390218. K, Cyathidites minor Couper, 1953, YSP350104. L, Leptolepidites sp. cf. L. verrucatus Couper, 1953, WQ180401. M, Cyathidites sp., YSP390305. N, Lygodiumsporites sp. cf. L. subsimplex (Bolchovitina) Gao and Zhao, 1976, YSP650109. O, Converrucosisporites sp., YSP330401. P, Cicatricosisporites australiensis (Cookson) Potonie´, 1956, WQ360304. Q, Osmundacidites elegans (Verbitskaya) Xu and Zhang, 1980, WQ350201. R, S, Classopollis sp.: R, WQ390212; S, WQ390215. T, Klukisporites sp., WQ210309.
Quadraeculina sp. (0e4.0%), Taxodiaceaepollenites hiatus (0e2.0%) and Vitreisporites sp. (0e1.0%). Palynomorphs attributed to Ovoidites sp. (0e1.9%) and Psophosphaera sp. (0e8.7%) and, hence, of possible algal origin, were also noted. Spores proved to be more abundant and diverse in the Yanshiping section, and Classopollis occurs stratigraphically lower there than at Wenquan (Fig. 6). Classopollis and Cyathidites are the dominant components of the assemblage, together often amounting to 70e90% of the specimens recorded. Cycadopites is also an important element, occurring relatively more consistently and often in larger numbers than other taxa. Spores, in particular forms attributable to Cyathidites,
are more numerous in the Quem Co and Buqu formations than in the Xiali Formation; by contrast, the occurrence of Classopollis is more consistent in the Xiali Formation than in the two older formations. 4.2. Dicheiropollis-Classopollis-Cicatricosisporites assemblage This assemblage occurs in the uppermost part of the Xiali Formation and the lower Suowa Formation (units 56e61) in the Yanshiping section and in part of the lower Suowa Formation (from the near bottom of unit 21 to unit 23) at Wenquan. The spores recovered include Annulispora sp. (0e0.6%), Asseretospora sp. (0e0.8%),
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
537
Fig. 5. Some of the palynomorphs recovered from the Yanshiping Group, continued; all !800. A, Alisporites rotundus Rouse, 1959, YSP390304. B, Alisporites parvus De Jersey, 1962, YSP390502. C, Protopodocarpus sp., YSP330702. D, Cycadopites nitidus (Balme) Pocock, 1970. E, Vitreisporites sp. YSP560102. F, Classopollis annulatus (Verbitskaya) Li, 1974; G, Dicheiropollis sp., WQ210605. H, Cycadopites elongatus (Bolkhovitina) Zhang, 1978, WQ390202. I, J, Classopollis classoides (Pflug) Pocock and Jansonius, 1961: I, WQ390205; J, WQ360303. K, L, Dicheiropollis etruscus Trevisan, 1971: K, YSP650105; L, WQ380710. M, Ovoidites sp., WQ360113.
Baculatisporites sp. (0e0.8%), Calamospora sp. (0e1.8%), Cicatricosisporites sp. (0e0.9%), Converrucosisporites sp. (0e0.8%), Cyathidites minor (0e8.8%), C. sp. (0e5.3%), Deltoidospora (0.8e7.7%), Dictyophylliidites sp. (0e0.8%), Impardecispora sp. (0e1.5%), Klukisporites sp. (0e1.5%), Leptolepidites sp. (0e0.6%), Lycopodiacidites sp. (0e0.4%), Lygodioisporites sp. (0e0.6%), Neoraistrickia sp. (0e0.6%), Osmundacidites sp. (0e1.9%), Pilosisporites sp. (0e0.9%), Polypodiaceaesporites sp. (0e0.6%), Pterisisporites sp. (0e1.0%) and Stereisporites sp. (0e5.3%). Among the gymnosperm pollen taxa are Alisporites sp. (0e1.2%), Cerebropollenites sp. (0e7.2%), Chasmatosporites sensu lato (0e 3.8%), Classopollis annulatus (0e29.7%), C. classoides (0e13.5%), C. sp. (32.4e65.6%), Cycadopites sp. (0e17.3%), Dicheiropollis etruscus (0e5.4%), Eucommiidites sp. (0e1.9%), Quadraeculina sp. (0e0.9%), Taxodiaceaepollenites hiatus (0e2.9%) and Vitreisporites sp. (0e0.6%). Both Ovoidites sp. (0e0.8%) and Psophosphaera sp. (0e4.8%) are again also present. Gymnosperm pollen grains are clearly the dominant component, with Classopollis the most abundant of all, usually comprising 60e80% of the miospore association. Spores are greatly reduced in number by comparison with the previous assemblage, but there are also some new additions to the palynoflora. Of these, the most notable is Dicheiropollis etruscus, which occurs in
low numbers (!2%) but fairly consistently at Yanshiping, and in percentages ranging between 0.9 and 5.4 at Wenquan. The other new elements are Impardecispora, Lygodioisporites and Pilosisporites, all of which occur sporadically; and whereas only one specimen of Cicatricosisporites was encountered low down in the Xiali Formation at Yanshiping, representatives of the genus are more common in this assemblage. There do not appear to be any significant differences between the palynofloras from the two localities. 4.3. Dicheiropollis peak assemblage This assemblage occurs in the upper part of the Suowa Formation (units 64e67) at Yanshiping and in the middleeupper Suowa Formation and the Xueshan Formation (units 25e39) at Wenquan. The palynomorphs are somewhat better preserved than in the previous two assemblages. The spores recovered comprise Asseretospora sp. (0e1.7%), Baculatisporites sp. (0e1.3%), Calamospora sp. (0e0.6%), Cicatricosisporites spp. (0e1.7%), Converrucosisporites sp. (0e3.6%), Cyathidites minor (0e6.3%), C. sp. (0e7.6%), Deltoidospora sp. (0e5.9%), Dictyophyllidites sp. (0e0.6%), Foraminisporis sp. (0e0.7%), Impardecispora sp. (0e4.2%), Klukisporites sp. (0e3.6%), Leptolepidites sp. (0e1.7%), Lygodioisporites sp. (0e0.9%), Neoraistrickia
538
Suowa
Dicheiropollis peak Xiali
Dicheiropollis-ClassopollisCicatricosisporites
Cyathidites – Classopollis – Cycadopites
WQ38-5 WQ38-3 WQ36-4 WQ36-3 WQ36-2 WQ36-1 WQ35-2 WQ35-1 WQ34-8 WQ34-7 WQ34-6 WQ34-2 WQ33-3 WQ33-2 WQ33-1 WQ32-3 WQ31-3 WQ29-4 WQ27-1 WQ25-1 WQ23-1 WQ22-5 WQ22-4 WQ21-7 WQ21-6 WQ21-4 WQ21-3 WQ19-2 WQ18-4 WQ18-3 WQ17-9 WQ17-8 WQ17-7 WQ14-3
0.3
1.6
0.4
0.4
0.4
1.6
0.8
0.6
0 .7
0.6
0.3
0.5
1 .4 1.3
0. 6
0.5
0.5
0.9
0.9
0.5
1.6
0. 5
1.1
2. 2
2.7
2.3
0.7
0.4
0. 9
0.5
1.4
0. 5
0.5
0.6
1.2
1.2
0.6
0.5
0.6
0.6
0.6
0.6
2.5
0.8
0.8
0.8 1.7
1.4
6.3
7.6
2.5
0.8
2.7
0.5
0.9
4.5
1.8
3. 7 1.9 0.8
0.8
0.8
0.8
1.3
0.6
5.9
4.2
1.7
1.7
0.6
0 .8
3.2 3.5
2.4
49.7
0.7
55.3
1.4
2.3
67.9
2.2
1.3
54.1
1.2 32.7 23.8
2. 4
6.0
26.8
26.8 12.8
1. 8
6.7
29.3
48.4 21.0
1. 2
0.9
0.8
1.5
1.5
0.4
0.9
0.5
0. 5
1.4
0.9
0.6
1.4 0.9
0.4 0.6
0.6
2. 4
1.2
4.9
1.1
0.5
0.5
1 .1
1.1
0.5 14.5 11.8
0.5
1.1
64.5
2.2
1.1
1.0
87.5
0.5
0.5 14.0
2.8
73.0
5.0
3.3
90.0
0.6 12.7
3.8
3.2
62.7
0.6
2.5 19.5
2. 5
4.2
38.1
1.7
24.4
88.1
0.9 45.9 13.5
14.4
5.4
60.2 18.5 2.9
46.2
1.9
1. 9
4.6
1.0
4.8
3.8 40.9 19.7
3. 0
3.8
0.7 54.3 23.6
2.9
7.2
1.4
2.5
0. 9
0.9
0.5
54.8 25.1
7.3
1.0
3.0 34.2 23.1
16.1
0.9
0.9 71.4 17.9
0.9
1.4
1.7
1.7
1.7
3. 4
2.2 0.4
0.8 0.9
0.9
3.6
0.9 0 .9
0.9 2.8 1.9
0.9
4.8
1.5 1.8
2. 9 0.8
0.8
0.7
1.1
0.5
4.1
0.5
1.0
0.5
1.0
0.4
1.0
2.7
4.5
0.4 67.2 11.9 11.9
3. 6
0. 4
0.8
0.8
63.3 20.3
1.9
1.3
1.9
0. 6
2.1
6.4
7.0
66.7 18.1
8.3
2.3 68.2 21.2
5. 3
5.5
65.5 23.6
2.8 0.8
1.6
1.3
6. 7
17.3
1.8
0.9
0.5
0.8
1.1
0.5
2.1
1.8 21.6
0.6
0.5
6.3
1.5
0. 4 0.6
2.4
1.1
2.7
2.3
3.0
1.4 1.5 0. 9
0.9
Total specimens
other pollen 1.4
35.8
1.5
0.6
1.4
2.1
2.2
Vitreisporites
2.3
1.7
2.1
2.1 68.1 19.1
1.8
0.6
0.8
0.8
0.8
37.9 14.7
1.4
0.9
0.4
4.1
0.6
2.1
0.8
4.1
48.6 10.8
0.4
0.8
3.6
0 .9
0.9
0.4
58.8 19.0
4.4
4.0
0.4
0.9
10.2
71.1
1.5
0.4
0.9 0.4
37.2
8.9
5.3
1.2
3.8
0.8
0.8
0.6
1. 6
0.9
1.9
0.9 0.3
0.3
1.1 10.8
50.4
1.0
0.5
1.1 28.4 19.1
3. 1
1.9
1. 2
9.9
Taxodiaceaepollenites
79.4
Ovoidites
1. 6
Quadraeculina
3.2
1.0
Psophosphaera
10.3
Jugella
67.8
Exesipollenites
4.0
Eucommiidites
0.8
Ephedripites (Spiralipites)
8.5
D. etruscus
1.3 12.7
C. annulatus
80.7
Classopollis
2.8
7.7 11.3
0.8
Dicheiropollis
Chasmatosporites s.l.
Cerebropollenites
0.8
1.3 15.2 19.5
7.2
0.4
1.4
5.5
0.4
1.0
1.3
3.1
5.0
1.8
2.5
1.2 1.3
0.8
0.9
7 .7
0.9 0.4
0.4
0.9
1.4
79.3
0.4
1.3
0.8
2.7
3.8
0.5 1.5
1.2
5.9
0.4
2.2
1.8
1.0
1.7
2.7
5.3
17.0
0. 9
0.7 18.4
0 .5
1.9 4. 8
4.3
4.5
1.4
0. 5
1. 4
1.5
2.2
5.4
2.1
2.2 1.5
0.9
0.9 60.9 10.9
0.9
2.2 0.4
51.1
1.1
0. 5
1.4
1.6
42 . 5
2.2
1.2 23.1 13.9
1.4
1.0
1.7
Alisporites
0. 6
0.5
0.5
6.5
1.5
2. 0
1.2
1.1
1.1
3.7
other spores
Stereisporites
Pterisisporites
Polycingulatisporites
Osmundacidites
Pilosisporites
0.6 2.8
1.2
2.1
3. 0
29.2 14.6
0.6 0.4
0.6
0.7 0.5
1.3 1.2
1.5 22.5 17.5
0.8
0. 6 0. 5
0. 3
0.8
1.4
0.4
0.4
0.3
0.4
0.6 0.5
Neoraistrickia
0.3 0.4
0.7
1.2
0. 5
Lygodiumsporites
0.4 0 .9
1.2 0.7
Lygodioisporites
0.9
0.6
Cycadopites
0.4
C. classoides
0.4
Lycopodiacidites
1.0 0.7
0.7
0.3
Leptolepidites
1.5
Klukisporites
Deltoidospora
1.0
Impardecispora
C. minor
1.0
Dictyophyllidites
Cyathidites
Converrucosisporites
Cicatricosisporites
Calamospora
Baculatisporites
0.5
200 137 230 334 254 236 126 173 141 221 231 168 164 186 183 221 95 186 96 215 60 161 1 18 1 19 45 261 111 111 108 104 132 276 219 197 112 252 158 47 72 132 110
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
WQ39-5 WQ39-4 WQ39-3 WQ39-2 WQ38-9 WQ38-7 WQ38-6
Asseretospora
Fm Samples
Annulispora
Taxa
Xueshan
Palynomorph assemblages
Table 2 Distribution of palynomorphs through the Wenquan section
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
539
Fig. 6. Fluctuations in abundance of three major components of the palynomorph assemblages, Cyathidites, Classopollis and Dicheiropollis, through the sections examined. Some local differences between the two localities are suggested. The Xiali Formation yields more specimens of Cyathidites and fewer Classopollis in the Yanshiping section than in the Wenquan section.
sp. (0e0.9%), Osmundacidites sp. (0e2.2%), Pilosisporites sp. (0e2.8%) and Stereisporites sp. (0e8.9%). Gymnosperm pollen taxa include Alisporites sp. (0e1.4%), Cerebropollenites sp. (0e0.8%), Chasmatosporites sensu lato (0e2.5%), Classopollis annulatus (0e75.0%), C. classoides (0e9.6%), C. sp. (0e71.1%), Cycadopites sp. (0e10.8%), Dicheiropollis etruscus (3.3e90.0%), Ephedripites (Spiralipites) sp. (0e1.1%), Eucommiidites sp. (0e1.0%), Exesipollenites sp. (0e0.6%), Jugella sp. (0e0.5%), Monosulcites sp. (0e2.6%), Quadraeculina sp. (0e1.4%), Taxodiaceaepollenites hiatus (0e2.4%) and Vitreisporites sp. (0e0.7%). Ovoidites (0e4.9%) and Psophosphaera sp. (0e2.4%) were also recorded once more. Gymnosperm pollen again dominate the assemblage; spores are very much subordinate components. Classopollis and Dicheiropollis etruscus are most numerous, usually amounting to 80e90% or more of the palynoflora, the latter being much more common than in the previous assemblage. Two taxa, Ephedripites and Jugella, occur for the first time. A major difference between preparations from the two localities is that Dicheiropollis is generally less common at Yanshiping (3.3e32.3%) than at Wenquan (up to 90.0%). The reverse is partly true with respect to Classopollis,
percentage ranges being 61.7e89.4% and a much more variable 4.6e81.4% respectively. Another variation is that gymnosperm grains are a little more diverse at Wenquan than at Yanshiping, which lacks Alisporites, Eucommiidites, Exesipollenites, Jugella, and Quadraeculina.
5. Discussion 5.1. Geological age The age of the oldest (Cyathidites-Classopollis-Cycadopites) assemblage is difficult to determine because there are few data on which to base any conclusions. Classopollis is known from Late Triassic and younger Mesozoic deposits but no late Triassic markers have been recorded, nor have any taxa diagnostic of the Cretaceous Period been encountered. Assemblages that are dominated by Classopollis can be stratigraphically useful, but in the sections examined here, these do not occur in the lower part of the succession. The Early Jurassic can probably be ruled out, albeit on negative evidence (absence of taxa that typically occur in rocks of this age). The other major components of this
540
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
assemblage, such as Cyathidites, Alisporites, Cycadopites, and (often questionable) Chasmatosporites and Quadraeculina, are common or important elements of Jurassic palynofloras. Therefore, it is assigned an age range of MideLate Jurassic. By comparison with the above, the most significant difference in the composition of the younger Dicheiropollis-Classopollis-Cicatricosisporites assemblage is the occurrence of Dicheiropollis. Although it occurs in low numbers, it is usually present, generally within the range of about 2e6% of the palynoflora. This species has been regarded to indicate deposits of earliest Cretaceous age in North Gondwana and the Tethyan realm (e.g., Jardine´ et al., 1974a; Hochuli, 1981). It has also been encountered in lowermost Cretaceous beds in Yunnan and southern Xinjiang (south-west and north-west China, respectively: Zhang, 1995; Li, 2000). Many palynologists have suggested that the lowest occurrences of Cicatricosisporites, Impardecispora, Lygodioisporites and Pilosisporites, which are also sometimes present in association, are in basal Cretaceous deposits where they generally occur sporadically and in low numbers (e.g., Hughes, 1981; Wimbledon and Hunt, 1983; Norris, 1985; Allen and Wimbledon, 1991; Li and Liu, 1994; Batten, 1996). Hence we tentatively date this assemblage as earliest Cretaceous (Berriasiane?Valanginian). The remarkable abundance of Dicheiropollis etruscus in the upper part of the succession together with continuing low numbers of Cicatricosisporites, Concavissimisporites, Impardecispora, Lygodioisporites, and Pilosisporites, and two additional taxa, Ephedripites
Fig. 7. Stratigraphic ranges of major components of the palynofloras through the formations studied plotted alongside lithostratigraphic summary logs; the apparently diachronous relationship between the Suowa and Xiali formations is indicated. Qmc, Quem Co Formation; Bq, Buqu Formation; Xl, Xiali Formation; Sw, Suowa Formation; Xs, Xueshan Formation. A, Cyathidites-Classopollis-Cycadopites assemblage; B, Dicheiropollis-Classopollis-Cicatricosisporites assemblage; C, Dicheiropollis peak assemblage.
(S.) and Jugella (Fig. 7), suggest somewhat younger early Cretaceous, perhaps ranging from Valanginian to Hauterivian or Barremian. Our study therefore supports the Cretaceous age determination of Jiang (1983) for the Xueshan Formation based on bivalve occurrences and not the Bajocian and late Jurassic assessments of Wang et al. (1979), Westermann and Wang (1988) and Yin (1988), and Bai (1989) respectively. It also confirms the early Cretaceous age suggested by foraminifera (Li et al., 2000) in the underlying Suowa Formation. Furthermore, the uppermost Xiali Formation in the Yanshiping section is considered here to have been deposited during the early Cretaceous. It is assumed that the Jurassic/Cretaceous boundary is lower down in this formation or perhaps in the Buqu Formation. However, not only have the criteria for recognizing the boundary in other parts of the world yet to be agreed but also there are insufficient palynological data in either of the two sections examined for us even to suggest an approximate position for it. 5.2. Palaeoenvironment Although spores and pollen grains derived from land plants are fairly common in the Yanshiping Group, there is also fossil evidence of deposition in marine environments. Brachiopods, echinoids and marine bivalves are quite common, and both foraminiferal linings and poorly preserved dinoflagellate cysts are often encountered in the palynological preparations. Together with common ripple marks, these records suggest that much of the succession accumulated in alternating shallow marine, coastal and non-marine environments. The occurrence of Classopollis and Dicheiropollis in very large numbers has been interpreted by many authors to indicate deposition in warm to hot, arid or semi-arid, often coastal environments (e.g., Vakhrameev, 1970; Jardine´ et al., 1974a; Srivastava, 1976; Herngreen and Chlonova, 1981; Alvin, 1982; Zhang, 1995), although other habitats for the parent plants of Classopollis are also likely (Batten, 1975). We infer from our data that the climate of the Qinghai-Xizang Plateau area during the early Early Cretaceous (Berriasiane ?Hauterivian/Barremian) was probably warm and semiarid. Although the evidence is limited, the composition of the Cyathidites-Classopollis-Cycadopites assemblage (associated with the Quem Co, Buqu and lower part of the Xiali formations), in particular the generally lower numbers of Classopollis and greater abundance of spores, may indicate somewhat cooler and more humid conditions during the MideLate Jurassic. The variations in composition between coeval Wenquan and Yanshiping palynofloras that have been noted may reflect local environmental differences. It is possible
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
that the climate was generally more humid at Wenquan than at Yanshiping.
541
refer to it here as the Xizang-Tarim Subprovince of the North Gondwanan Province.
5.3. Palynofloral provincialism 6. Conclusions Palynofloral provinces have been delineated on a global scale and discussed for various ages of the Cretaceous Period on a number of occasions (e.g., Jardine´ et al., 1974b; Brenner, 1976; Herngreen and Chlonova, 1981; Hochuli, 1981, Batten, 1984, Batten and Li, 1987). The absence of saccate pollen and the occurrence of Dicheiropollis were considered by Jardine´ et al. (1974b) to be of major importance in distinguishing a Northern Gondwana Province from a Euro-North American Province. Brenner (1976) proposed four provinces for mid Cretaceous (BarremianeCenomanian) palynofloras of the world: Northern and Southern Laurasian, and Northern and Southern Gondwanan. These were also applied to the earlier Cretaceous by Hochuli (1981), who noted that Northern Gondwanan palynofloras differ from those of Southern Laurasia not only in consisting of much larger numbers of Classopollis, many monosulcate and araucariaceous pollen, and abundant and diverse species of Ephedripites, but also in containing Dicheiropollis and, in AlbianeCenomanian deposits, the Elaterosporites/Galeacornea group of palynomorphs. Apart from the scarcity of Ephedripites and absence of representatives of the last-named group, these features are also characteristic of the palynofloras from the sections examined in the Tanggula Mountains. They are, therefore, considered to show a closer affinity to the North(ern) Gondwanan Province than to the trilete-spore- and saccate-pollen-dominated assemblages that are typical of the South(ern) Laurasian Province (Herngreen and Chlonova, 1981; Hochuli, 1981). The geographic distribution of Dicheiropollis used to be regarded as restricted to Northern Gondwana, but its occurrence to the north of the Tethys Ocean (represented in Xizang by the Bangong Co-Siling Co Suture; Fig. 1), and its documentation from Xinjiang through Yunnan and in Thailand (Zhang, 1995; Li, 2000), suggest that this view should be modified. No record of the genus has been found in Cretaceous successions further north in China, and fossil megafloras in southern Xizang (south of the Yarlong Zangbo Suture) have proved to be comparable to those of Southern Gondwana (Zhou and Wu, 1994). We therefore consider that in eastern Asia the parent plants of Dicheiropollis were distributed around the Tethys Ocean. The reason for this may be that during the Early Cretaceous the ocean in this region was much reduced in areal extent and no longer a major barrier to the dispersal of plants. Pending further investigation, we suggest that it might be appropriate to distinguish this north Tethyan palynofloral region by name and, therefore, tentatively
The palynomorphs recovered from the Yanshiping Group have provided a basis for positively dating some of the succession as early Cretaceous (Berriasianepossibly Barremian) and for placing the Tanggula Mountains on the northern fringes of the palynofloral North Gondwanan Province where the climate is likely to have been warm and semi-arid. Similar palynofloras reported from Xinjiang and Yunnan confirm the extension of some of the floral elements of the North Gondwanan Province into these regions, perhaps because the Tethyan Ocean in eastern Asia was not as effective a barrier as it had been previously to the dispersal of plants whose spores and pollen grains characterize Gondwanan palynofloras. As a result, we have tentatively delineated a Xizang-Tarim Subprovince in the north-east Tethyan region.
Acknowledgements This research has been funded by the Major Basic Research Project of the Ministry of Science and Technology, China (G1998040800), and also supported financially by overseas student funds from the Chinese Academy of Sciences. We thank Miss He Cuiling, Miss Huang Fengbao and Mr. Fan Xiaoyi for technical help; Professors Zhou Zhiyan, Zhang Yiyong and Li Wenben for professional advice; and Dr. Eckart Schrank for comments on the manuscript.
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Batten, D.J., Li, Wenben, 1987. Aspects of palynomorph distribution, floral provinces and climate during the Cretaceous. Geologisches Jahrbuch A 96, 219e237. Brenner, G.J., 1976. Middle Cretaceous floral provinces and early migrations of angiosperms. In: Beck, C.B. (Ed.), Origin and Early Evolution of Angiosperms. Columbia University Press, New York, pp. 23e47. Herngreen, G.F.W., Chlonova, A.F., 1981. Cretaceous microfloral provinces. Pollen et Spores 23, 441e555. Hochuli, P.A., 1981. North Gondwanan floral elements in lower to middle Cretaceous sediments of the southern Alps (southern Switzerland, northern Italy). Review of Palaeobotany and Palynology 35, 337e358. Hughes, N.F., 1981. Jurassic-Cretaceous boundary palynology in Europe. The Palaeobotanist 28e29, 316e323. Jardine´, S., Biens, P., Doerenkamp, A., 1974a. Dicheiropollis etruscus, un pollen caracte´ristique du cre´tace´ infe´rieur Afro-Sudame´ricain. Conse´quences pour l’evaluation des unite´s climatiques et implications dans la de´rive des continents. Sciences Ge´ologiques, Bulletin 27, 87e100, 1 pl. Jardine´, S., Kieser, G., Reyre, Y., 1974b. L’individualisation progressive du continent africain vue a` travers les donne´es palynologiques de l’e`re secondaire. Sciences Ge´ologiques, Bulletin 27, 69e85. Jiang, Zhongti, 1983. Some problems on the Jurassic stratigraphy of Qiang-Tang Region. Contributions on the Geology of the Qinghai-Xizang (Tibet) Plateau 3, 87e112 (in Chinese). Li, Wenben, 1983. Microfloral areas of Early Cretaceous in China. In: Editorial Committee for Basic Palaeontological Theory Series (Ed.), Palaeobiogeographic Provinces of China. Science Press, Beijing, pp. 142e151 (in Chinese, English abstract). Li, Wenben, 2000. Early Cretaceous palynoflora from northern Tarim Basin. Acta Palaeontologica Sinica 39, 28e45 (in Chinese, English abstract). Li, Wenben, Liu, Zhaosheng, 1994. The Cretaceous palynofloras and their bearing on stratigraphic correlation in China. Cretaceous Research 15, 333e365. Li, Yong, Wu, Ruizhong, Shi, He, Zhu, Lidong, Yi, Haisheng, Wang, Chengshan, 2000. New development on stratigraphy in northern
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Early Cretaceous palynofloras from the Tanggula Mountains of the northern Qinghai-Xizang (Tibet) Plateau, China Jianguo Lia,), David J. Battenb,c a
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, PR China b Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK c Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK Received 14 November 2003; accepted in revised form 23 April 2004
Abstract Two sections of the Yanshiping Group in the Tanggula Mountains area of the Qinghai-Xizang (Tibet) Plateau, a remote part of western China, have yielded three types of palynomorph assemblages. Two of these clearly indicate that the Xueshan Formation and parts of the diachronous Suowa and Xiali formations were deposited during the early Cretaceous (Berriasiane?Barremian). Largely on negative evidence, because the assemblages are taxonomically impoverished, the rest of the succession is considered to range down into the Upper and possibly Middle Jurassic. The composition of the Cretaceous assemblages suggests that the climate during this period was warm and semi-arid. A Xizang-Tarim Subprovince of the palynofloral North Gondwanan Province is tentatively identified in the north-east Tethyan region. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Early Cretaceous; Qinghai-Xizang (Tibet) Plateau; Palynology; Floral provinces; Climate
1. Introduction The samples on which this paper is based were collected from the northern slope of the Tanggula Mountains of the tectonic Qiangtang Terrain, the boundaries of which are the Bangong Co-Siling Co Suture (Fig. 1) in the south and the Jinsha River Suture in the north. Mesozoic rocks are widely distributed and well exposed in this region. Since the 1960s, several geological and stratigraphic studies have been carried out, but most of these were of a reconnaissance nature. Recently geologists and palaeontologists have become aware that the region is crucial to studies on Mesozoic palaeobiogeography (Li, 1983; Li and Liu, 1994; Sha, 1998), and to the tectonic history and current geography of the Qinghai-Xizang (Tibet) Plateau (Sun and Zheng, 1998).
) Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (D.J. Batten). 0195-6671/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2004.04.005
Outcrops of the Yanshiping Group in the Tanggula Mountains are ideal for studying the JurassiceLower Cretaceous successions of this region. After they were first encountered in the 1960s, and named by Jiang in 1983, some palaeontological data have accumulated, mostly on ammonites, bivalves and brachiopods. Jiang (1983) listed the bivalves that had been found to date, and regarded the age of the uppermost part of this group, the Xueshan Formation, as early Cretaceous. A few years later, however, Yin (1988) suggested a Bajocian age for the bivalve fauna from the formation, whereas Bai (1989) considered the bivalves to indicate the Late Jurassic. A few poorly preserved ammonite specimens were recovered from exposures in Amdo County to the south of this area. These were considered by Wang Yigang and Chen Guolong (in Wang et al., 1979) and others (Westermann and Wang, 1988) to be MideLate Jurassic in age. However, not only are the specimens rather poorly preserved, which renders the determination questionable, but also no ammonites have been found in the northern Tanggula Mountains.
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J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
Furthermore, during a recent field survey by Li Yong and others, a few beds containing foraminifera were encountered in the underlying Suowa Formation, and these indicate the Early Cretaceous (Li et al., 2000). Identifiable land plant megafossils have not been found so far in the interbedded marine and non-marine deposits of the Yanshiping Group. Palynomorphs have not previously received much attention in this region, and no palynologists have undertaken an investigation of the succession until now. As a result, little is currently known about the contemporaneous terrestrial flora, which in turn limits interpretations based on other geological studies. In this paper we present palynological data on two sections of the Yanshiping Group. Although the assemblages recovered probably range in age from mid Jurassic to early Cretaceous, the probable Jurassic assemblages are mostly impoverished and important only in the general geological context of the region. Hence, although these are considered herein, we concentrate on the Cretaceous assemblages, discussing their stratigraphic, palaeoenvironmental and phytogeographic significance.
2. Stratigraphic setting The Yanshiping Group in the Tanggula Mountains consists of, in ascending order, the Quem Co, Buqu,
Xiali, Suowa and Xueshan formations. Of the two sedimentary successions considered, the Quem Co Formation crops out only in the Yanshiping section, whereas the uppermost Xueshan Formation is exposed only in the Wenquan section (Figs. 1, 2). Both of these sections are named after near-by settlements. Their lithologies are briefly described below. 2.1. Quem Co Formation This lowest formation of the Yanshiping Group has an outcrop thickness of around 610 m in the Yanshiping section. It consists of varicoloured sandstones, siltstones, mudstones and some marls. Invertebrate fossils occur in a few beds: some gastropods but mainly bivalves in a fair to poor state of preservation. Spores and pollen grains (miospores) are scarce; so far only one sample has proved to be reasonably productive (Table 1). 2.2. Buqu Formation This formation is 1662.3 m thick in the Yanshiping section. It is also exposed at Wenquan but has not been measured and sampled there. It can be divided into two parts at Yanshiping. The lower 965 m consist of numerous grey to greenish grey sandy limestone, sandstone, and siltstone sequences, occasionally punctuated by
Fig. 1. Map showing the location of the two sections of the Yanshiping Group on the northern Qinghai-Xizang Plateau that are considered in this paper: Y, Yanshiping; W, Wenquan. Inset: general location map; the Bangong Co-Siling Co suture is indicated by the solid and dashed line.
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
533
Fig. 2. The Yanshiping (A) and Wenquan (B) sections.
purple sandstones in the upper part. The upper 697.4 m comprise mainly mediumedark grey, thick limestones, with some thin marl interbeds. The formation contains assemblages of both marine and non-marine invertebrate fossils. Marine bivalves, brachiopods, echinoids and foraminifera are sometimes sufficiently abundant to form a bioclastic limestone. Palynomorphs are common in the lower member but occur only sporadically in the upper section. Fragments of wood are also common in the lower member; these are usually carbonized and always poorly preserved.
2.3. Xiali Formation This formation has been measured in its entirety in both sections, with thicknesses of 572 m at Yanshiping and 595.6 m at Wenquan being recorded. The lithologies are mainly grey to greenish grey siltstones, sandstones and marls and dark purpleered siltstones, with occasional interbeds of grey mudstone, dark grey, thinlybedded limestone and greenish grey sandy limestone. Bivalves, ostracods, remains of charophyte algae and some poorly preserved (unidentifiable) plant fragments have been found, and a few miospore assemblages have been recovered.
2.4. Suowa Formation The Suowa Formation is also well exposed at both localities, but at Yanshiping the section terminates in the core of a syncline before the top of the formation is reached. The incomplete record of around 460 m contrasts with a total thickness of 838.7 m in the Wenquan section. The succession consists mainly of greenish grey and dark grey mudstones and siltstones, and mediumedark grey limestones and sandy limestones. There are also some marl interbeds and limestones. The latter occur more frequently in the lower part of the section; the upper part is dominated by siltstones and mudstones. Exposed surfaces of the sandstones frequently show large-scale ripples (Fig. 3A). Bivalves and miospores are comparatively abundant. Ostracods, charophyte gyrogonites and dinoflagellate cysts also occur. 2.5. Xueshan Formation This formation is more restricted in its distribution than the others. Its exposure is intermittent along a narrow range near the Tanggula Mountains Pass. It was named, and its type section at Wenquan measured, by Jiang (1983). Li Jianguo and colleagues re-examined
534
0.6
YSP67-1
1.3
Suowa
YSP65-1
0 .3
0.4 0.5
1 .8
YSP59-2 0 .6
0.6
YSP39-4
Buqu
YSP39-3
1.3
1.3 21.7
0.5
0.3
0.8 0.6 0.4
1.8
2.7 0.5
0.5
3.6
3.0
0.6
8.8
3.5
1.7
4.6
9.0 8.7
0.4
0.4
0.5
0.6
0 .6
0.5
0.6
0.6
0.6
0.6
1.1
3.8
1.3
1.3
8.7
8 .7 1.0
0.6
5.2 15.7
5.2 0.6
YSP34-2
37.7 41.1
7.2 0.3
YSP33-7
35.6
9.9 11.9
YSP33-4
1.1 10.6
2.7
YSP31-1
3.9 11.8
3.1
Quem YSP13-1 Co
24.4 36.6
5 .9
0.7
1.3
36.7 46.7
6.0
0.8
0.8
0.4 24.2 30.2
7.3
0.4
1.9 46.3 27.8
6.2
0.6
2.0 65.6 18.4
1.6
1.8
0.9
2.4
0.6
0.6
1.2
0.6
0.6
2.3
1.1
1.3
2.6
1.2
0.6
1.0
1.4 3.0
1.8 32.4 26.5 13.5
1.2
1.7 48.6 22.9
1.7
4.3 21.7
1.0
1.5
2.0 3 .2
4.6 6.0
4.6
2 .1
1.3
1.3
1.7
1.4
0.7
1.0 0.7
32.3 1.2
5.6
0.4
1.6
0.3
0.7
0.7
0.5
1.2
0.8
0.4
1.4
1.8
3.0
1.2
0.6
2.9
1.1
0.6
0.6
0.6
Total specimens
28
0.6
310 150
0.4
248
0.6
16 1
0.4
244
0.5
218
0.6
168 170
0.6 0.6
175
0.6
78
3.8 8.7
1.5
12.8
0.5
1.1 15.4
4.0
2.9
1.1
0.4
1.5
5.8
2.7
1.2 33.1
9.3
1.7 11.6
3.5
1.4
6.8
1.4
2.4
1.0
3.0
7.9
18.8
3.0
1.6 53.2 18.6
4.3
1.6
0.9 52.4 15.3
2.6
5.2
0.4
1.3
1.4 15.7
0.3
5.2
0.7
1.7
1.4
3.6
0.7
1.2 0.4
other pollen
Vitreisporites
Taxodiaceaepollenites
Ovoidites 0.3
0.4
8.2
9.3
Quadraeculina
Eucommiidites
Psophosphaera
Ephedripites (Spiralipites)
D. etruscus 7.7 3.3
1.8 47.9 29.7
1.3 57.7 15.4
4.7
1.6
1.8 47.6 20.8
8.7
0.8 5.2
Dicheiropollis
Cycadopites
9.6
0.4
0.3
C. classoides
37.0 37.0
1.0
1.2
0.6
C. annulatus
0.6
12.0 48.6 14.3 0.4
10.7
0.3
4.9
1.2
0.6
YSP35-1
Classopollis
Alisporites
75.0
0.4 0.5
0.6
15.4 50.2 10.0
Chasmatosporites s.l.
other spores
Stereisporites
Pterisisporites
Polypodiaceaesporites
Pilosisporites
Osmundacidites
Neoraistrickia
Lygodiumsporites
0.3
14.4 41.0 13.3 0.6
Lygodioisporites
Lycopodiacidites
Leptolepidites
Impardecispora
Klukisporites
Hymenophyllumsporites
Dictyophyllidites harrisii
Foraminisporis
Deltoidospora 1.2
3.4
0.3
1.2
0.8
0.6
0.3 0.7
1.2
0.6
YSP54-1
0.6
0.8 0.5 0.6
YSP56-1
YSP39-5
C. minor
0.4 0 .6
YSP61-1
0.6 0.7
0 .4
3.6
3.6
0.3
YSP64-2
YSP59-1
Cyathidites
3 .6
YSP67-2
YSP59-3
Converrucosisporites
Concavissimisporites
YSP67-3
YSP46-1 Cyathidites – Classopollis – Cycadopites
Cicatricosisporites
Calamospora
Asseretospora
Baculatisporites
Annulispora
Fm Samples
Xiali
Dicheiropollis peak Dicheiropollis-ClassopollisCicatricosisporites
Taxa
0.5
8.7
23
1.0
0.5
1.0
195
0.4
0.4
1.2
259
175 1.2
172
1.2
291
0.7 4.0
2.0
101
1.0 0.5
0.5
0.3
1.0
319
0.4 0.7
0.3
188
287
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
Palynomorph assemblages
Table 1 Distribution of palynomorphs through the Yanshiping section
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
535
(Figs. 4, 5), the bulk of the organic residues were subjected to a little oxidation with hydrogen peroxide. At least 200 specimens were counted from productive samples whenever possible; a minimum of 100 palynomorphs was recorded from all of the low-yield samples listed in Tables 1 and 2 apart from three in the Yanshiping and six in the Wenquan sections, respectively. Other, more impoverished, samples are not tabulated.
4. Palynological assemblages
Fig. 3. A, ripple marks on the surface of a sandstone bed in the Suowa Formation, Wenquan section. B, the red and yellow deposits of the Xueshan Formation of which the hill in this photograph is composed, make it easy to recognize in the field.
the section in 1999 and measured a total thickness of approximately 229 m, but the top of the formation has yet to be reached, the steep and dangerous terrain rendering it inaccessible. It is distinguishable in the field by its varicoloured appearance (Fig. 3B), consisting as it does of reddish, yellowish and off-white siltstones, sandstones and mudstones, interbedded with layers of grey to black mudstone and siltstone. Miospores are fairly numerous. Bivalves occur in the lowest beds, as well as a few dinoflagellate cysts. Unidentifiable plant fragments are common in the black mudstones and siltstones.
3. Material and methods One hundred and fifty-six samples were collected for palynological examination from the two sections. They were processed in the laboratory by standard techniques using hydrochloric and hydrofluoric acids, and the residues were sieved through a screen of 10 mm mesh. Since most of the palynomorphs were found to have been blackened as a result of regional metamorphism
Sixty-two samples yielded small to fairly large assemblages of palynomorphs: 21 from the Yanshiping section and 41 from the Wenquan section. Most of these are from the Suowa and Xueshan formations, some are from the Buqu Formation, a few from the Xiali Formation, and one from the middle part of the Quem Co Formation. Forty-two genera of spores and gymnosperm pollen grains were recorded. Most specimens are very dark in colour (brownish blackeblack) and poorly preserved to the extent that the majority could not be identified to species. A few may have algal origins. No angiosperm pollen grains were found. Examples of the components of the palynofloras are illustrated in Figs. 4 and 5. All of the palynomorphs encountered and their quantitative distribution in the moderately productive samples through the succession are noted in Tables 1 and 2. In general the palynofloral data from the two localities are similar. Three palynomorph assemblagetypes have been recognized. Their characteristics are recorded below in ascending order. 4.1. Cyathidites-Classopollis-Cycadopites assemblage This assemblage is recognizable from the middle of the Quem Co Formation (a single sample) to the upper middle part of the Xiali Formation (units 13e54) in the Yanshiping section, and from around the middle of the Xiali Formation to the lowest part of the Suowa Formation (unit 14 to the near the bottom of unit 21) in the Wenquan section. Spores recorded include Asseretospora sp. (0e0.5%), Baculatisporites sp. (0e1.5%), Calamospora sp. (0e1.3%), Cyathidites minor (0e50.2%), C. spp. (0e37.7%), Deltoidospora spp. (0e14.3%), Klukisporites sp. (0e8.7%), Lycopodiacidites sp. (0e1.2%), Neoraistrickia sp. (0e1.3%), Osmundacidites sp. (0e0.3%), Pterisisporites sp. (0e1.3%) and Stereisporites sp. (0e5.2%). One specimen of Cicatricosisporites sp. was also encountered in the Xiali Formation at Yanshiping. Gymnosperm pollen grains include Alisporites sp. (0e8.7%), Chasmatosporites sensu lato (0e4.3%), Classopollis annulatus (0e65.5%), C. classoides (0e23.6%), C. sp. (0e71.4%), Cycadopites sp. (0e18.8%), Eucommiidites sp. (0e1.0%),
536
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
Fig. 4. Some of the palynomorphs recovered from the Yanshiping Group; all !800. For each specimen, the numbers prefixed by YSP (Yanshiping section) or WQ (Wenquan section) give the stratigraphic unit (first two numbers), slide number (second two) and specimen number on slide (last two). A, Stereisporites antiquasporites (Wilson and Webster) Dettmann, 1963, YSP130105. B, Deltoidospora regularis (Pflug) Song and Zheng, 1981, WQ390318. C, Asseretospora parva (Li and Shang) Pu and Wu, 1985, YSP650106. D, Pterisisporites medirhaptes Li, 1979, YSP560103. E, Neoraistrickia taylorii Playford and Dettmann, 1965, WQ360120. F, Converrucosisporites sp., WQ350206. G, cf. Dictyophyllidites harrisii Couper, 1958, YSP350105. H, Calamospora nathorstii (Halle) Klaus, 1960, WQ360107. I, J, Cicatricosisporites amalocostriatus Zhang, 1965: I, WQ390317; J, WQ390218. K, Cyathidites minor Couper, 1953, YSP350104. L, Leptolepidites sp. cf. L. verrucatus Couper, 1953, WQ180401. M, Cyathidites sp., YSP390305. N, Lygodiumsporites sp. cf. L. subsimplex (Bolchovitina) Gao and Zhao, 1976, YSP650109. O, Converrucosisporites sp., YSP330401. P, Cicatricosisporites australiensis (Cookson) Potonie´, 1956, WQ360304. Q, Osmundacidites elegans (Verbitskaya) Xu and Zhang, 1980, WQ350201. R, S, Classopollis sp.: R, WQ390212; S, WQ390215. T, Klukisporites sp., WQ210309.
Quadraeculina sp. (0e4.0%), Taxodiaceaepollenites hiatus (0e2.0%) and Vitreisporites sp. (0e1.0%). Palynomorphs attributed to Ovoidites sp. (0e1.9%) and Psophosphaera sp. (0e8.7%) and, hence, of possible algal origin, were also noted. Spores proved to be more abundant and diverse in the Yanshiping section, and Classopollis occurs stratigraphically lower there than at Wenquan (Fig. 6). Classopollis and Cyathidites are the dominant components of the assemblage, together often amounting to 70e90% of the specimens recorded. Cycadopites is also an important element, occurring relatively more consistently and often in larger numbers than other taxa. Spores, in particular forms attributable to Cyathidites,
are more numerous in the Quem Co and Buqu formations than in the Xiali Formation; by contrast, the occurrence of Classopollis is more consistent in the Xiali Formation than in the two older formations. 4.2. Dicheiropollis-Classopollis-Cicatricosisporites assemblage This assemblage occurs in the uppermost part of the Xiali Formation and the lower Suowa Formation (units 56e61) in the Yanshiping section and in part of the lower Suowa Formation (from the near bottom of unit 21 to unit 23) at Wenquan. The spores recovered include Annulispora sp. (0e0.6%), Asseretospora sp. (0e0.8%),
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
537
Fig. 5. Some of the palynomorphs recovered from the Yanshiping Group, continued; all !800. A, Alisporites rotundus Rouse, 1959, YSP390304. B, Alisporites parvus De Jersey, 1962, YSP390502. C, Protopodocarpus sp., YSP330702. D, Cycadopites nitidus (Balme) Pocock, 1970. E, Vitreisporites sp. YSP560102. F, Classopollis annulatus (Verbitskaya) Li, 1974; G, Dicheiropollis sp., WQ210605. H, Cycadopites elongatus (Bolkhovitina) Zhang, 1978, WQ390202. I, J, Classopollis classoides (Pflug) Pocock and Jansonius, 1961: I, WQ390205; J, WQ360303. K, L, Dicheiropollis etruscus Trevisan, 1971: K, YSP650105; L, WQ380710. M, Ovoidites sp., WQ360113.
Baculatisporites sp. (0e0.8%), Calamospora sp. (0e1.8%), Cicatricosisporites sp. (0e0.9%), Converrucosisporites sp. (0e0.8%), Cyathidites minor (0e8.8%), C. sp. (0e5.3%), Deltoidospora (0.8e7.7%), Dictyophylliidites sp. (0e0.8%), Impardecispora sp. (0e1.5%), Klukisporites sp. (0e1.5%), Leptolepidites sp. (0e0.6%), Lycopodiacidites sp. (0e0.4%), Lygodioisporites sp. (0e0.6%), Neoraistrickia sp. (0e0.6%), Osmundacidites sp. (0e1.9%), Pilosisporites sp. (0e0.9%), Polypodiaceaesporites sp. (0e0.6%), Pterisisporites sp. (0e1.0%) and Stereisporites sp. (0e5.3%). Among the gymnosperm pollen taxa are Alisporites sp. (0e1.2%), Cerebropollenites sp. (0e7.2%), Chasmatosporites sensu lato (0e 3.8%), Classopollis annulatus (0e29.7%), C. classoides (0e13.5%), C. sp. (32.4e65.6%), Cycadopites sp. (0e17.3%), Dicheiropollis etruscus (0e5.4%), Eucommiidites sp. (0e1.9%), Quadraeculina sp. (0e0.9%), Taxodiaceaepollenites hiatus (0e2.9%) and Vitreisporites sp. (0e0.6%). Both Ovoidites sp. (0e0.8%) and Psophosphaera sp. (0e4.8%) are again also present. Gymnosperm pollen grains are clearly the dominant component, with Classopollis the most abundant of all, usually comprising 60e80% of the miospore association. Spores are greatly reduced in number by comparison with the previous assemblage, but there are also some new additions to the palynoflora. Of these, the most notable is Dicheiropollis etruscus, which occurs in
low numbers (!2%) but fairly consistently at Yanshiping, and in percentages ranging between 0.9 and 5.4 at Wenquan. The other new elements are Impardecispora, Lygodioisporites and Pilosisporites, all of which occur sporadically; and whereas only one specimen of Cicatricosisporites was encountered low down in the Xiali Formation at Yanshiping, representatives of the genus are more common in this assemblage. There do not appear to be any significant differences between the palynofloras from the two localities. 4.3. Dicheiropollis peak assemblage This assemblage occurs in the upper part of the Suowa Formation (units 64e67) at Yanshiping and in the middleeupper Suowa Formation and the Xueshan Formation (units 25e39) at Wenquan. The palynomorphs are somewhat better preserved than in the previous two assemblages. The spores recovered comprise Asseretospora sp. (0e1.7%), Baculatisporites sp. (0e1.3%), Calamospora sp. (0e0.6%), Cicatricosisporites spp. (0e1.7%), Converrucosisporites sp. (0e3.6%), Cyathidites minor (0e6.3%), C. sp. (0e7.6%), Deltoidospora sp. (0e5.9%), Dictyophyllidites sp. (0e0.6%), Foraminisporis sp. (0e0.7%), Impardecispora sp. (0e4.2%), Klukisporites sp. (0e3.6%), Leptolepidites sp. (0e1.7%), Lygodioisporites sp. (0e0.9%), Neoraistrickia
538
Suowa
Dicheiropollis peak Xiali
Dicheiropollis-ClassopollisCicatricosisporites
Cyathidites – Classopollis – Cycadopites
WQ38-5 WQ38-3 WQ36-4 WQ36-3 WQ36-2 WQ36-1 WQ35-2 WQ35-1 WQ34-8 WQ34-7 WQ34-6 WQ34-2 WQ33-3 WQ33-2 WQ33-1 WQ32-3 WQ31-3 WQ29-4 WQ27-1 WQ25-1 WQ23-1 WQ22-5 WQ22-4 WQ21-7 WQ21-6 WQ21-4 WQ21-3 WQ19-2 WQ18-4 WQ18-3 WQ17-9 WQ17-8 WQ17-7 WQ14-3
0.3
1.6
0.4
0.4
0.4
1.6
0.8
0.6
0 .7
0.6
0.3
0.5
1 .4 1.3
0. 6
0.5
0.5
0.9
0.9
0.5
1.6
0. 5
1.1
2. 2
2.7
2.3
0.7
0.4
0. 9
0.5
1.4
0. 5
0.5
0.6
1.2
1.2
0.6
0.5
0.6
0.6
0.6
0.6
2.5
0.8
0.8
0.8 1.7
1.4
6.3
7.6
2.5
0.8
2.7
0.5
0.9
4.5
1.8
3. 7 1.9 0.8
0.8
0.8
0.8
1.3
0.6
5.9
4.2
1.7
1.7
0.6
0 .8
3.2 3.5
2.4
49.7
0.7
55.3
1.4
2.3
67.9
2.2
1.3
54.1
1.2 32.7 23.8
2. 4
6.0
26.8
26.8 12.8
1. 8
6.7
29.3
48.4 21.0
1. 2
0.9
0.8
1.5
1.5
0.4
0.9
0.5
0. 5
1.4
0.9
0.6
1.4 0.9
0.4 0.6
0.6
2. 4
1.2
4.9
1.1
0.5
0.5
1 .1
1.1
0.5 14.5 11.8
0.5
1.1
64.5
2.2
1.1
1.0
87.5
0.5
0.5 14.0
2.8
73.0
5.0
3.3
90.0
0.6 12.7
3.8
3.2
62.7
0.6
2.5 19.5
2. 5
4.2
38.1
1.7
24.4
88.1
0.9 45.9 13.5
14.4
5.4
60.2 18.5 2.9
46.2
1.9
1. 9
4.6
1.0
4.8
3.8 40.9 19.7
3. 0
3.8
0.7 54.3 23.6
2.9
7.2
1.4
2.5
0. 9
0.9
0.5
54.8 25.1
7.3
1.0
3.0 34.2 23.1
16.1
0.9
0.9 71.4 17.9
0.9
1.4
1.7
1.7
1.7
3. 4
2.2 0.4
0.8 0.9
0.9
3.6
0.9 0 .9
0.9 2.8 1.9
0.9
4.8
1.5 1.8
2. 9 0.8
0.8
0.7
1.1
0.5
4.1
0.5
1.0
0.5
1.0
0.4
1.0
2.7
4.5
0.4 67.2 11.9 11.9
3. 6
0. 4
0.8
0.8
63.3 20.3
1.9
1.3
1.9
0. 6
2.1
6.4
7.0
66.7 18.1
8.3
2.3 68.2 21.2
5. 3
5.5
65.5 23.6
2.8 0.8
1.6
1.3
6. 7
17.3
1.8
0.9
0.5
0.8
1.1
0.5
2.1
1.8 21.6
0.6
0.5
6.3
1.5
0. 4 0.6
2.4
1.1
2.7
2.3
3.0
1.4 1.5 0. 9
0.9
Total specimens
other pollen 1.4
35.8
1.5
0.6
1.4
2.1
2.2
Vitreisporites
2.3
1.7
2.1
2.1 68.1 19.1
1.8
0.6
0.8
0.8
0.8
37.9 14.7
1.4
0.9
0.4
4.1
0.6
2.1
0.8
4.1
48.6 10.8
0.4
0.8
3.6
0 .9
0.9
0.4
58.8 19.0
4.4
4.0
0.4
0.9
10.2
71.1
1.5
0.4
0.9 0.4
37.2
8.9
5.3
1.2
3.8
0.8
0.8
0.6
1. 6
0.9
1.9
0.9 0.3
0.3
1.1 10.8
50.4
1.0
0.5
1.1 28.4 19.1
3. 1
1.9
1. 2
9.9
Taxodiaceaepollenites
79.4
Ovoidites
1. 6
Quadraeculina
3.2
1.0
Psophosphaera
10.3
Jugella
67.8
Exesipollenites
4.0
Eucommiidites
0.8
Ephedripites (Spiralipites)
8.5
D. etruscus
1.3 12.7
C. annulatus
80.7
Classopollis
2.8
7.7 11.3
0.8
Dicheiropollis
Chasmatosporites s.l.
Cerebropollenites
0.8
1.3 15.2 19.5
7.2
0.4
1.4
5.5
0.4
1.0
1.3
3.1
5.0
1.8
2.5
1.2 1.3
0.8
0.9
7 .7
0.9 0.4
0.4
0.9
1.4
79.3
0.4
1.3
0.8
2.7
3.8
0.5 1.5
1.2
5.9
0.4
2.2
1.8
1.0
1.7
2.7
5.3
17.0
0. 9
0.7 18.4
0 .5
1.9 4. 8
4.3
4.5
1.4
0. 5
1. 4
1.5
2.2
5.4
2.1
2.2 1.5
0.9
0.9 60.9 10.9
0.9
2.2 0.4
51.1
1.1
0. 5
1.4
1.6
42 . 5
2.2
1.2 23.1 13.9
1.4
1.0
1.7
Alisporites
0. 6
0.5
0.5
6.5
1.5
2. 0
1.2
1.1
1.1
3.7
other spores
Stereisporites
Pterisisporites
Polycingulatisporites
Osmundacidites
Pilosisporites
0.6 2.8
1.2
2.1
3. 0
29.2 14.6
0.6 0.4
0.6
0.7 0.5
1.3 1.2
1.5 22.5 17.5
0.8
0. 6 0. 5
0. 3
0.8
1.4
0.4
0.4
0.3
0.4
0.6 0.5
Neoraistrickia
0.3 0.4
0.7
1.2
0. 5
Lygodiumsporites
0.4 0 .9
1.2 0.7
Lygodioisporites
0.9
0.6
Cycadopites
0.4
C. classoides
0.4
Lycopodiacidites
1.0 0.7
0.7
0.3
Leptolepidites
1.5
Klukisporites
Deltoidospora
1.0
Impardecispora
C. minor
1.0
Dictyophyllidites
Cyathidites
Converrucosisporites
Cicatricosisporites
Calamospora
Baculatisporites
0.5
200 137 230 334 254 236 126 173 141 221 231 168 164 186 183 221 95 186 96 215 60 161 1 18 1 19 45 261 111 111 108 104 132 276 219 197 112 252 158 47 72 132 110
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
WQ39-5 WQ39-4 WQ39-3 WQ39-2 WQ38-9 WQ38-7 WQ38-6
Asseretospora
Fm Samples
Annulispora
Taxa
Xueshan
Palynomorph assemblages
Table 2 Distribution of palynomorphs through the Wenquan section
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
539
Fig. 6. Fluctuations in abundance of three major components of the palynomorph assemblages, Cyathidites, Classopollis and Dicheiropollis, through the sections examined. Some local differences between the two localities are suggested. The Xiali Formation yields more specimens of Cyathidites and fewer Classopollis in the Yanshiping section than in the Wenquan section.
sp. (0e0.9%), Osmundacidites sp. (0e2.2%), Pilosisporites sp. (0e2.8%) and Stereisporites sp. (0e8.9%). Gymnosperm pollen taxa include Alisporites sp. (0e1.4%), Cerebropollenites sp. (0e0.8%), Chasmatosporites sensu lato (0e2.5%), Classopollis annulatus (0e75.0%), C. classoides (0e9.6%), C. sp. (0e71.1%), Cycadopites sp. (0e10.8%), Dicheiropollis etruscus (3.3e90.0%), Ephedripites (Spiralipites) sp. (0e1.1%), Eucommiidites sp. (0e1.0%), Exesipollenites sp. (0e0.6%), Jugella sp. (0e0.5%), Monosulcites sp. (0e2.6%), Quadraeculina sp. (0e1.4%), Taxodiaceaepollenites hiatus (0e2.4%) and Vitreisporites sp. (0e0.7%). Ovoidites (0e4.9%) and Psophosphaera sp. (0e2.4%) were also recorded once more. Gymnosperm pollen again dominate the assemblage; spores are very much subordinate components. Classopollis and Dicheiropollis etruscus are most numerous, usually amounting to 80e90% or more of the palynoflora, the latter being much more common than in the previous assemblage. Two taxa, Ephedripites and Jugella, occur for the first time. A major difference between preparations from the two localities is that Dicheiropollis is generally less common at Yanshiping (3.3e32.3%) than at Wenquan (up to 90.0%). The reverse is partly true with respect to Classopollis,
percentage ranges being 61.7e89.4% and a much more variable 4.6e81.4% respectively. Another variation is that gymnosperm grains are a little more diverse at Wenquan than at Yanshiping, which lacks Alisporites, Eucommiidites, Exesipollenites, Jugella, and Quadraeculina.
5. Discussion 5.1. Geological age The age of the oldest (Cyathidites-Classopollis-Cycadopites) assemblage is difficult to determine because there are few data on which to base any conclusions. Classopollis is known from Late Triassic and younger Mesozoic deposits but no late Triassic markers have been recorded, nor have any taxa diagnostic of the Cretaceous Period been encountered. Assemblages that are dominated by Classopollis can be stratigraphically useful, but in the sections examined here, these do not occur in the lower part of the succession. The Early Jurassic can probably be ruled out, albeit on negative evidence (absence of taxa that typically occur in rocks of this age). The other major components of this
540
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
assemblage, such as Cyathidites, Alisporites, Cycadopites, and (often questionable) Chasmatosporites and Quadraeculina, are common or important elements of Jurassic palynofloras. Therefore, it is assigned an age range of MideLate Jurassic. By comparison with the above, the most significant difference in the composition of the younger Dicheiropollis-Classopollis-Cicatricosisporites assemblage is the occurrence of Dicheiropollis. Although it occurs in low numbers, it is usually present, generally within the range of about 2e6% of the palynoflora. This species has been regarded to indicate deposits of earliest Cretaceous age in North Gondwana and the Tethyan realm (e.g., Jardine´ et al., 1974a; Hochuli, 1981). It has also been encountered in lowermost Cretaceous beds in Yunnan and southern Xinjiang (south-west and north-west China, respectively: Zhang, 1995; Li, 2000). Many palynologists have suggested that the lowest occurrences of Cicatricosisporites, Impardecispora, Lygodioisporites and Pilosisporites, which are also sometimes present in association, are in basal Cretaceous deposits where they generally occur sporadically and in low numbers (e.g., Hughes, 1981; Wimbledon and Hunt, 1983; Norris, 1985; Allen and Wimbledon, 1991; Li and Liu, 1994; Batten, 1996). Hence we tentatively date this assemblage as earliest Cretaceous (Berriasiane?Valanginian). The remarkable abundance of Dicheiropollis etruscus in the upper part of the succession together with continuing low numbers of Cicatricosisporites, Concavissimisporites, Impardecispora, Lygodioisporites, and Pilosisporites, and two additional taxa, Ephedripites
Fig. 7. Stratigraphic ranges of major components of the palynofloras through the formations studied plotted alongside lithostratigraphic summary logs; the apparently diachronous relationship between the Suowa and Xiali formations is indicated. Qmc, Quem Co Formation; Bq, Buqu Formation; Xl, Xiali Formation; Sw, Suowa Formation; Xs, Xueshan Formation. A, Cyathidites-Classopollis-Cycadopites assemblage; B, Dicheiropollis-Classopollis-Cicatricosisporites assemblage; C, Dicheiropollis peak assemblage.
(S.) and Jugella (Fig. 7), suggest somewhat younger early Cretaceous, perhaps ranging from Valanginian to Hauterivian or Barremian. Our study therefore supports the Cretaceous age determination of Jiang (1983) for the Xueshan Formation based on bivalve occurrences and not the Bajocian and late Jurassic assessments of Wang et al. (1979), Westermann and Wang (1988) and Yin (1988), and Bai (1989) respectively. It also confirms the early Cretaceous age suggested by foraminifera (Li et al., 2000) in the underlying Suowa Formation. Furthermore, the uppermost Xiali Formation in the Yanshiping section is considered here to have been deposited during the early Cretaceous. It is assumed that the Jurassic/Cretaceous boundary is lower down in this formation or perhaps in the Buqu Formation. However, not only have the criteria for recognizing the boundary in other parts of the world yet to be agreed but also there are insufficient palynological data in either of the two sections examined for us even to suggest an approximate position for it. 5.2. Palaeoenvironment Although spores and pollen grains derived from land plants are fairly common in the Yanshiping Group, there is also fossil evidence of deposition in marine environments. Brachiopods, echinoids and marine bivalves are quite common, and both foraminiferal linings and poorly preserved dinoflagellate cysts are often encountered in the palynological preparations. Together with common ripple marks, these records suggest that much of the succession accumulated in alternating shallow marine, coastal and non-marine environments. The occurrence of Classopollis and Dicheiropollis in very large numbers has been interpreted by many authors to indicate deposition in warm to hot, arid or semi-arid, often coastal environments (e.g., Vakhrameev, 1970; Jardine´ et al., 1974a; Srivastava, 1976; Herngreen and Chlonova, 1981; Alvin, 1982; Zhang, 1995), although other habitats for the parent plants of Classopollis are also likely (Batten, 1975). We infer from our data that the climate of the Qinghai-Xizang Plateau area during the early Early Cretaceous (Berriasiane ?Hauterivian/Barremian) was probably warm and semiarid. Although the evidence is limited, the composition of the Cyathidites-Classopollis-Cycadopites assemblage (associated with the Quem Co, Buqu and lower part of the Xiali formations), in particular the generally lower numbers of Classopollis and greater abundance of spores, may indicate somewhat cooler and more humid conditions during the MideLate Jurassic. The variations in composition between coeval Wenquan and Yanshiping palynofloras that have been noted may reflect local environmental differences. It is possible
J. Li, D.J. Batten / Cretaceous Research 25 (2004) 531e542
that the climate was generally more humid at Wenquan than at Yanshiping.
541
refer to it here as the Xizang-Tarim Subprovince of the North Gondwanan Province.
5.3. Palynofloral provincialism 6. Conclusions Palynofloral provinces have been delineated on a global scale and discussed for various ages of the Cretaceous Period on a number of occasions (e.g., Jardine´ et al., 1974b; Brenner, 1976; Herngreen and Chlonova, 1981; Hochuli, 1981, Batten, 1984, Batten and Li, 1987). The absence of saccate pollen and the occurrence of Dicheiropollis were considered by Jardine´ et al. (1974b) to be of major importance in distinguishing a Northern Gondwana Province from a Euro-North American Province. Brenner (1976) proposed four provinces for mid Cretaceous (BarremianeCenomanian) palynofloras of the world: Northern and Southern Laurasian, and Northern and Southern Gondwanan. These were also applied to the earlier Cretaceous by Hochuli (1981), who noted that Northern Gondwanan palynofloras differ from those of Southern Laurasia not only in consisting of much larger numbers of Classopollis, many monosulcate and araucariaceous pollen, and abundant and diverse species of Ephedripites, but also in containing Dicheiropollis and, in AlbianeCenomanian deposits, the Elaterosporites/Galeacornea group of palynomorphs. Apart from the scarcity of Ephedripites and absence of representatives of the last-named group, these features are also characteristic of the palynofloras from the sections examined in the Tanggula Mountains. They are, therefore, considered to show a closer affinity to the North(ern) Gondwanan Province than to the trilete-spore- and saccate-pollen-dominated assemblages that are typical of the South(ern) Laurasian Province (Herngreen and Chlonova, 1981; Hochuli, 1981). The geographic distribution of Dicheiropollis used to be regarded as restricted to Northern Gondwana, but its occurrence to the north of the Tethys Ocean (represented in Xizang by the Bangong Co-Siling Co Suture; Fig. 1), and its documentation from Xinjiang through Yunnan and in Thailand (Zhang, 1995; Li, 2000), suggest that this view should be modified. No record of the genus has been found in Cretaceous successions further north in China, and fossil megafloras in southern Xizang (south of the Yarlong Zangbo Suture) have proved to be comparable to those of Southern Gondwana (Zhou and Wu, 1994). We therefore consider that in eastern Asia the parent plants of Dicheiropollis were distributed around the Tethys Ocean. The reason for this may be that during the Early Cretaceous the ocean in this region was much reduced in areal extent and no longer a major barrier to the dispersal of plants. Pending further investigation, we suggest that it might be appropriate to distinguish this north Tethyan palynofloral region by name and, therefore, tentatively
The palynomorphs recovered from the Yanshiping Group have provided a basis for positively dating some of the succession as early Cretaceous (Berriasianepossibly Barremian) and for placing the Tanggula Mountains on the northern fringes of the palynofloral North Gondwanan Province where the climate is likely to have been warm and semi-arid. Similar palynofloras reported from Xinjiang and Yunnan confirm the extension of some of the floral elements of the North Gondwanan Province into these regions, perhaps because the Tethyan Ocean in eastern Asia was not as effective a barrier as it had been previously to the dispersal of plants whose spores and pollen grains characterize Gondwanan palynofloras. As a result, we have tentatively delineated a Xizang-Tarim Subprovince in the north-east Tethyan region.
Acknowledgements This research has been funded by the Major Basic Research Project of the Ministry of Science and Technology, China (G1998040800), and also supported financially by overseas student funds from the Chinese Academy of Sciences. We thank Miss He Cuiling, Miss Huang Fengbao and Mr. Fan Xiaoyi for technical help; Professors Zhou Zhiyan, Zhang Yiyong and Li Wenben for professional advice; and Dr. Eckart Schrank for comments on the manuscript.
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