青海省北祁连中三叠统孢粉组合及其沉积相意义

MIDDLE TRIASSIC SPORE-POLLEN ASSEMBLAGE FROM NORTHERN QILIAN,QINGHAI AND ITS SIGNIFICANCE IN SEDIMENTOLOGY

青海省祁连县俄博乡大擦汗沟、土圈沟、柯柯里等三叠系剖面的粉砂岩、砂质页岩、泥质页岩中首次发现较丰富的孢粉和疑源类化石,计57属98种。孢粉中的蕨类植物孢子占总组合的44.6%,裸子植物花粉占33.9%;疑源类有8属13种,占21.5%,称Verrucosisporites-Triadispora-Trematosp haeridium孢粉组合。该组合面貌与国内外中三叠世不同相区的孢粉组合特征基本相似并可对比。北祁连山南坡本组合及上下其它组合中海相疑源类的存在,表明该区在三叠纪受到海水影响,海水完全退出是在三叠纪之后。文章还初步探讨其沉积相意义。

More than 30 rock macerated from a set of samples were collected and sediments comprising siltstone, arenaceous shale and muddy shale beds outcropping in Dacahangou, Tuquangou and Kekeli of the Qilian County, southern slope of the northern Qilian Mountain Ranges, northern Qinghai. These outcrops composed of 5 beds （no. 4 - 8 in ascending order） constitute a comprehensive section （listed in Chinese text） measured about 243 m in thickness, lithostratigraphically, the section is correlated with the lower part of the Middle Triassic Dingjiayao Formation typically developed in Baiying, a city in Gansu Province. Among the macerated samples, four have been proven to yield plant microfossils, this is the first discovery in the area concerned. In the sequence of Kekeli （previously assigned to the Upper Permian） two samples are palyniferous [-9021BF2-5 and 9021BF2-6, the latter yielding an association predominated （98%） by acritarchs]. While the sequence outcropped at Dacahangou and Tuquangou with two palyniferous samples （8624BH69 and 8624BH70 originally dated as Upper Triassic） was assigned to the so called ＂ Lower-Middle Triassic Daxigou Group＂. The present research reveals that the 4 samples yield abundant but not very well preserved miospores and acritarchs, collectively they are named the Verrucosisporites （8. 3% on average）- Triad is pora （≤4. 2%）- Trematos phaerid ium （7.2%） Assemblage, including 98 species （forms） referred to 57 genera. Among the assemblage, pteridophytic spores stands first, occupying 44.6% of the total, gymnospermous pollen comes second, reaching 33.9 %, and acritarchs reaches 21.5 % on average. This assemblage can roughly be correlated with the known Middle Triassic ones at home and abroad. It contains such important taxa as Aratrisporites （5.2%）, and Parataeniaesporites pseudostriatus （Kopytova） Liu 1980 [= Colpectospollis pseudostriatus （Kopytova） Qu, 1986] that seems confined to Middle-Late Triassic and has not been found from Lower Triassic so far in Urasia. Another point deserves of attention is the absence of Lundbladispora and Taeniaesporites which played important roles in the Early Triassic assemblages, especially in North China on the one hand, and of Duplexisporites, Ovalipollis, Riccisporites, Canalizonospora, Corollisporites and Zebraaporites, etc. characteristic of the Late Triassic on the other. Therefore, we conclude that the spores-bearing strata should be Middle Triassic and possibly Anisian in age. The region dealt with in this paper belongs to the ＂North Qilianshan Stratigraphic Small Area＂ of the ＂Qilian- northern Qinling Stratigraphical Regime＂, the strata of the Daxigou Group distributed across here along a NW-SE direction as a narrow and long belt- manner has been correlated with the relevant strata in N. China and considered to be continental deposits of Triassic. However, due to the discovery of rather abundant acritarchs （spinate and non-spinate associations appeared alternately till the Late Triassic）, indicating a littoral environment represented by marine-continental in- terbedded facies. In other words, the sea water induced by a deep- big fracture （＂Fz＂ nearly in NE direction） then existing and reached this region （ southern slope of the North Qilianshan Mountains）, and did not retreat until post-Triassic. Thus the present data provide some evidence in favor of reconstructing the palaeogeography of this region and interpreting the tectonic background of the Qilianshan Orogenic Belts.