The Pliocene Flora from the Yushe Basin of Shanxi Province and its Environmental Significance

Transcription

1 The Pliocene Flora from the Yushe Basin of Shanxi Province and its Environmental Significance Jiaxin Cao and Haiting Cui (Department of Geography, Peking University) Scientia Geologica Sinica October, 1989 pp Translated by Will Downs Department of Geology Bilby Research Center Northern Arizona University September l990

2 Abstract Pliocene fluviolacustrine deposits within the Yushe Basin of Shanxi Province are extremely well developed, and contain abundant paleobotanical remains. Representative taxa from the early to middle Pliocene include: Magnolia, Liquidambar, Platycarya, Tsuga, and others, indicating a northern subtropical dense forest for the landscape of the Yushe basin during that period. There was, however, a trend toward a cold and dry climate. The Yushe region during the late stage of the Pliocene maintained a deciduous and broadleaf forest vegetation similar to the extant plant cover of the southern regions of North China. The extensive distribution of fluviolacustrine deposits in combination with these botanical characteristics indicate a seasonal climate for the Yushe region in the Pliocene. It is evident that annual and seasonal xeric to mesic and cold to warm fluctuations occurred. Introduction The Yushe Basin lies on the eastern Shanxi plateau as a synclinal, disc-shaped dendritic basin. The Pliocene floral collections came from localities in the Wuxiang and Yushe County boundary region, including Wangning, Gezuitou, Zhangcun, and other localities. Paleobotanical specimens came from lacustrine sediments in the lower, middle, and upper sections of the Zhangcun Formation (N 2 2 ). These paleobotanical localities produce copious taxonomically abundant assemblages, which is a rare phenomenon in China with the exception of the localities at Shanwang. Additionally, the region produces abundant fossil mammals, fish, turtles, and insects. Consequently, the Yushe basin is an ideal region for Late Cenozoic paleoenvironmental and paleogeographic research (Fig. 1). Figure 1. Paleobotanical Fossil Localities in the Yushe Basin The vast majority of plant macrofossils collected by the authors are leaves, with an additional small quantity of stems, seeds, fruits, and other elements. The flora is taxonomically rich and well preserved, consisting principally of broadleafed and deciduous trees, with small components from conifers and hydrophytes. I Late Cenozoic Stratigraphy and Paleoflora Yushe and Wuxiang are located in the Zhuozhanghe River Valley in southeastern Shanxi Province at an altitude of approximately 1,110 meters above sea level. This region constitutes a downwarped Cenozoic basin plateau. During the Pliocene, there was a suite of basins that formed lacustrine bodies where the well-developed fluviolacustrine sediments of the Yushe Group (N2-

3 2 Q1) were deposited. Fluviolacustrine sediments within the basin are older in the east becoming younger in the west. Flow direction was west or northwest. The basin is a monoclinal structure. The depositional sequence from east to west consists of: Lower Pliocene Renjianao Formation, Upper Pliocene Zhangcun Formation, and Lower Pleistocene Louzeyu Formation. These formations are recognized as the Yushe Group. * Paleobotanical specimens were derived from the Zhangcun Formation. The Renjianao Formation is composed of gray-purple and gray-yellow thick gravels grading to thinly bedded purple sandy mudstones and yellow sandstones predominantly concentrated along the eastern bank of the Zhuozhang River as nearly north-south trending depositional bands. It maintains a thickness of over 80 meters (Figure 2, Units 1-4), and overlies Triassic sandstones and shales with a noticeable angular unconformity. This formation may be divided into two members: The lower member is composed of gray-purple gravels derived from the Triassic sandstones with cobble diameters exceeding one meter but generally cm, maintain a low degree of rounding, but are intensely weathered. The upper member consists of gray-yellow gravels dominated by yellow Triassic sandstones and a small amount of quartz gravels with diameters slightly less than the lower member, but relatively highly rounded. A slight angular unconformity lies between the upper and lower members. The depositional environments reflected by these two units differ, with the lower member representing rapid fluvial sedimentation, semiheavy aggradation, and slumping. The second member represents large-scale fluvial sedimentation from large streams with distant sources. It is possible that quartz gravels were deposited secondarily in the upper member of the Renjianao Formation, derived from the Jurassic Heifeng Formation in the northwest corner of this region. Fossils have yet to be recovered from the lower member of the Renjianao Formation, but a few mammalian fossils have been recovered from the sandstones of the upper member. An unconformity between the Renjianao and Zhangcun Formation is recognized by the presence of massive yellow sands grading to thinly bedded purple clays and gray-green, grey-blue, gray-white, and gray-black sandy clays, clays, and calcareous muds. This is characteristic of lacustrine sedimentation, extending over 300 meters in thickness. Deposits dip northwest at angles from 5-15, and are extensively distributed in a series of subbasins of the Zhuozhang River Valley (or the Yushe Basin). The Zhangcun Basin is the most well developed of these subbasins containing the principal fossiliferous deposits of the Yushe Group. Three members may be recognized here based upon their sedimentological characteristics. Exposures are best in the Zhangcun, Louzeyu, and Wangning Tunnel region. Lower member of the Zhangcun Formation: Base is sandy gravels with a fining upward transition to lacustrine deposits of black and variegated organic silts and ponded muds grading again to a number of yellow massive sands. These sediments are distributed around the Wangning Tunnel and localities east of it (Figure 2, Units 5-12). Paleobotanical material is contained within Units 9-11, represented by stems, leaves, branches, fruits, and hydrophytes. Middle member of the Zhangcun Formation: Upper and lower sections of this member may be differentiated with the lower section characterized by large-scale planar, interbedded sandy clays, and thin sands representing shallow lakes. Botanical material has yet to be found here (Figure 2, Units 13-14). The upper section represents the middle of a lake with gray, gray-white, and white sandy clays, clays, calcareous muds, and limestones. Fish as well as a small amount of vertebrate fossils have been recovered from the eye-catching "white unit" in the Zhangcun section (Fig. 2, Unit 15). This section possesses extremely well-developed planar beds that are lamellated, and laminated. Within the laminations are numerous microripples characteristic of central lake deposits. The taxon Hemiptelea sp. is found within the fine yellow gypsiferous sands * Based on the Shanxi Province Stratigraphic Subdivision Chart.

4 3 of this section. Abundant paleobotanical material and fossil fish are contained in a set of extremely organic argillaceous fluvial silts and sands in the vicinity of Gezuitou where there are exposures of swamp facies in the middle member of the Zhangcun Formation. Figure 2. Stratigraphic Cross-section of the Yushe Group at Renjianao, Zhangcun, and Louzeyu (1) Gravel; (2) Sandy gravel; (3) Sands; (4) Sandy clay; (5) Clays; (6) Calcareous muds; (7) Swamp deposits; (8) "R" laterite; (9) Lishi Loess; (10) Malan Loess; (11) Fossiliferous rocks; Units 1-4 Renjianao Fm. (N 1 2 ); Unit 5 Basal sandy gravels of Zhangcun Fm. (N2 2 ); Units 6-12 Zhangcun lower member variegated lacustrine clays; Units Zhangcun Formation interbedded sands and clays; Unit 15 Zhangcun Formation middle and upper member central lacustrine calcareous muds; Units Zhangcun Formation upper member sands and sandy clays; Units Louzeyu Formation (Q 1 1 ) Upper Member of the Zhangcun Formation: The sediments are composed of yellow sands and silty-sandy mudstones, portions of which are calcareously cemented and grade to silty-sandy mudstones with well-developed lamellations. There is an approximately one-meter-thick layer that is very fossiliferous with paleobotanical material and fossil fish (Figure 2, Units 16-18). The Louzeyu Formation is characterized by massive yellow sandstones grading to thinly bedded gray-purple sandy clays and gray-green sandy clays, with an approximate total thickness of 100 meters, distributed in the upper western tributary basins of the Zhuozhang River, at the western edge of the Yushe Basin. The yellow massive sands are obliquely bedded as well as crossbedded and contain small siliceous gravels. Each sand body may reach up to meters thick. The upper member of this formation grades to three units of grey-green clays and marls. The deposits appear to be dispersed fluvial sediments within the bottom of a desiccated lacustrine basin. The formation dips 3-8 northwest, but where it occurs in the basin's western margins it is deformed and rotated southeast or east, such that there appears to be a small unconformable contact with the underlying Zhangcun Formation, apparently the result of an orogenic erosional surface. The formation is unfossiliferous in paleobotanical material but contains fossil mammals. After preliminary diagnosis of the macro-paleobotanical material, the following taxa have been identified from the lower member of the Zhangcun Formation: Picea sp., Pinus sp., Salix angusta Braun, and Ulmus sp. Hydrophytes include Potamogeton pectinatus L. sp. Pl. (Plate 3), Zannichellia cf. palusturis L. sp. Pl., and others. The middle member of the Zhangcun Formation contains Zelkova ungeri Kouats, Ulmus shansiensis Chaney, and others. The upper member of the Zhangcun Formation contains Quercus miovariablis Hu and Chaney (Plate 4), Q. cf. dentata (Plate 1), Q. cf. liaotungensis Koidz, Q. cf. mongolia Fish and Tarcz, Carpinus cf. miofangiana Hu and Chaney (Plate 4), Carpinus subcordata Nathorst, Acer florinii Hu and Chaney, Acer subpictum Saporta (Plate 2), Acer diabolicum Bl., Zelkova unger Kovat, Ulmus cf. Parvifolia Jacq. Pl., Juglans sp., Pterocarya sp., Cudrania sp., Broutsonetia sp., Populus latior, Salix cf. wallichiana, Albizza sp., Spiraea sp., Evodia sp., Koelreuteria cf. integrifolia, in addition to hydrophytes which include Typha sp.

5 4 Characteristics and Environmental Significance of the Pliocene Yushe Botanical Complex The Pliocene Yushe floral community is dominated by angiosperms with additional small amounts of ferns and gymnosperms. Principal elements include the Fagaceae, Aceraceae, Ulmaceae, and Betulaceae, with secondary occurrences of the Moraceae, Juglandaceae, Salicaceae, Rutaceae, Sapindaceae, Rosaceae, and the Leguminosae. The most abundant taxa are Quercus, Acer, Fagus, and Carpinus. Warm temperate taxa dominate the assemblage, although there are small amounts of sub-tropical species. The Pliocene lakes of the Yushe basin were extensive with flourishing hydrophytes, hygrophytes, Potamogetonaceae, Typhaceae, and Equisetinae. The presence of these fossil plants represents an important foundation for the interpretation of the paleoclimate and the paleogeographic environment. 1. The Climate and Environment as Interpreted from the Paleobotanical Community Based on their stratigraphic distribution, the Pliocene floral communities of the Yushe Basin may be divided into two phases: the early to middle Pliocene phase, and the late Pliocene phase. The characteristics of these floral complexes reflect climatic conditions during these two phases that were basically the same, both being a semi-mesic monsoonal climate under a southern warm temperate regime. There are also, however, slight discrepancies between the two. The early to middle Pliocene phase botanical fossils found in the black pond and lacustrine mudstones of the lower and middle members of the Zhangcun Formation in the regions around the Wangning Tunnel and the Gezuitou regions are dominated by the deciduous and broadleaf leaves and branches of such genera as Ulmus, Fagus, and Salix, in addition to the Cupressaceae and Pinaceae. Additionally, there are abundant hydrophytes such as Potamogeton. According to additional results of pollen analysis, abundant samples of which were recovered from the grayblack mudstones around the Wangning Tunnel, angiosperm pollen constitutes 82% of the assemblage, while non-arboreal taxa and shrubs constitute 17.5%. Arboreal connifers such as Picea, and the Pinaceae are dominant. The broadleaf Ulmus vastly dominates the assemblage with secondary occurrences of Quercus. Large amounts of the non-arboreal pollen such as the Graminae are also present. Worth noting is the extreme abundance of Pediastrum boryanum in these deposits, reflecting even more the tranquil and clear shallow lake and pond environment during the time of deposition of these sediments. Macrobotanical analysis of a single sample of rock collected by the initial Shanxi Petroleum Brigade from the fluvial muds at the Wangning Tunnel (1973) produced different results, with most abundant taxa being Ulmus, Fagus, and Quercus, and secondarily Celtis, Carya, Liquidambar Platycarya, and Magnolia. In pollen counts, conifers such as the Cupressaceae were most abundant, with secondary occurrences including Pinus and Picea. It may be deduced from the characteristics of the pollen and plant macro-fossils that the early to middle Pliocene floral landscape consisted of mountainous coniferous forests; deciduous broadleaved forests on low mountains or undulating hills; deciduous broadleaf mixed arboreal forests under mesic fluviolacustrine riparian habitats (or mixed forests containing conifers adapted to warm conditions); and hydrophyte communities in shallow lacustrine embayments. These form a single ecological system of clinal habitats. The early to middle Pliocene macro- and microfossils may be divided into the following complexes based upon floral distribution and climatic relationship: 1. Cold semi-mesic to a cold mesic climate typically represented by Picea and Abies: Larix pollen occurs in the lacustrine and pond sediments at Wangning Tunnel indicating the presence of cold winter seasons. Coniferous forests represent the highest altitude habitats.

6 5 2. Warm semi-mesic complex, or a warm conifer forest represented by Cupressus and Pinus: These taxa suggest a warm to slightly xeric low mountain habitat. Additionally, Quercus- Carpinus forests appear within this type of habitat. Under a xeric and hot habitat the proportion of Ulmaceae such as Ulmus and Celtis increases. 3. Warm mesic complex: A complex dense forest community distributed along low mesic riparian regions dominated by tropically adapted deciduous broad leaf taxa including Carya, Platycarya, Rhus, Liquidambar, Zelkova, and others. Additional taxa found within this complex include a small amount of evergreen trees such as Magnolia, and tropically adapted conifers such as Podocarpus and Tsuga. Within the non-arboreal ground cover are the humidity and shadowadapted ferns. 4. Specialized habitats of concealed regions: These include hydrophyte communities represented by Potamogeton and Zannichellia, in addition to pond and hygrophyte plant communities with components such as Typha, Sparganium and Polygonum. Also included are drought-resistant shrub and nonarboreal communities inhabiting sandy beaches including Ephedra, the Gramineae, Artemisia, and the Chenopodiaceae. From the taxonomically rich dicotyledon components, the dominant non-arboreal plant communities of that time should represent a marshland or forested marsh such as found in the Mesozoic era. The upper Pliocene, represented by fossil plants in the Zhangcun region from the upper part of the Zhangcun Formation, is dominated by Quercus, Acer, Carpinus, Fagus, Populus, Salix, and hydrophytes, with secondary occurrences including Broussonetia, Cudrania, Juglans, Albizzia, Euodia, Koelreuteria, and Spiraea. A gradual alteration to a more xeric and cool climate occurred in the late Pliocene, with a decrease of the initial subtropical elements, and a gradual replacement by typical temperate deciduous broadleaf trees. Analysis of the fossil plants collected indicates that the vegetation was dominated by a typical Quercus-Acer-Carpinus forest. The montane conifer forests were principally Pinus-Cupressus. A taxonomically complex Populus- Salix forest was distributed along the riparian regions. Currently, related species with similar ecological characteristics and habitats may be found in modern plant communities allowing paleoecological reconstruction through modern analogue. The structure of the late Pliocene deciduous broadleaf forests was very similar to present models, with numerous taxa within the arboreal canopy maintaining small leaves or deciduous winter leaves. Underneath this canopy grew deciduous shrubs and other elements. In summary, although climate of the late Pliocene gradually trended to cool xeric conditions, the floral cover was still dominated by a semi-xeric summertime-green broadleaf Mesozoic-type forest, with a climatic model basically similar to the temperate seasonal conditions of modern times. 2. A Comparison of the Yushe Floral Community to Similar Floral Communities A comparison between the Pliocene Yushe flora and the Miocene Shanwang flora suggests that aside from sharing numerous dominant elements such as Quercus, Acer, Fagus, Carpinus, Ulmus, and others, both are botanical assemblages typical of a temperate zone. Subtropical evergreen taxa and subtropical broadleaf deciduous taxa found in the Shanwang community diminish noticeably in the Yushe community, including Cinnamomum, Litsea, Ficus, Carya, Sapindus, and Hamamelis, indicating the waning of a subtropical climate in the Yushe region. Yushe climate did not attain the sweltering humidity of the Miocene Shanwang region. However, some subtropical taxa were present at Yushe, reflecting a certain degree of transition between temperate and subtropical climates. The Pliocene Taiyu floral community compares relatively well with the late Pliocene floral complex from the Zhangcun vicinity which contains such genera as Acer, Fagus, and Ulmus, among others. In addition, another collection was made by the current authors from green-yellow silty mudstones in the lower section of lacustrine deposits at Bandaocun Village that contained

7 6 Ulmus shansiensis, Zelkova ungeri, Gleditsia sp., Pterocarya sp., and others, that in combination with the Ulmaceae represent a mixed deciduous broadleaf forest. According to Chaney (1935) and Song (1958), there are large portions of taxonomic similarities between the Pliocene Yushe flora and the Early Pleistocene Sanmen floral community. However, the Sanmen flora contains several elements that are noticeably different, with tropical adapted elements absent, and drought resistant elements noticeably more abundant, such as Ziziphus, Gramineae, and others. It is apparent from the nature of the several floral communities described above that there is a general developmental tendency toward a xeric and cold environment from the Miocene Shanwang flora to the Pliocene Yushe flora, and continuing with the Early Pleistocene Sanmen flora. Within these complexes, the drought-resistant taxa, cold-resistant taxa, and non-arboreal taxa increase proportionally. However, they predominate with other taxa typical of warm temperate zones. Additionally, all these floras share similar geologic backgrounds, regardless of whether discussing the Miocene Shanwang region, the Pliocene Yushe region, or the Early Pleistocene Sanmen Gorge region. These floras are recovered from rocks representing an extensive distribution of fluviolacustrine waters, swamps and marshes, and lacustrine basins surrounded by low rolling hills with few high mountains. Consequently, the deciduous nature of each of the aforementioned three diachronous regions have unique geographical and ecological characteristics, but also maintain similarities. 3. Fossil Plants and Depositional Environment Within the downwarped Pliocene synclinal Yushe Basin were extensively distributed fluviolacustrine river systems with irregularly patterned shorelines in a larger lacustrine basin. These systems also composed a dense network of tributaries flowing into relatively shallow lakes. The lacustrine depositional model is complex, encompassing shore facies, deltaic facies, shallow water, central lake facies, and marsh or swamp deposits. An intimate relationship exists between the taphonomy of the paleobotanical material and the depositional environments, as the flora is principally preserved in well-laminated silty mudstones where fossil leaves are concentrated in deposits representing shallow water facies, swamp facies, and proximal to central lake facies. The plants growing in the vicinity of the lakes at that time were fossilized after transportation by wind or water into the lakes to be entombed within the muddy sands. Fossils of large branches are only recovered from gray-black mudstones representing swamp facies. Several densely compacted leaf units are present, indicating the luxuriant forests surrounding the lakes of that time. These units originated due to depositional conditions favorable to dense concentrations of material where surface waters of the lakes and ponds were tranquil. Based upon analysis of the characteristics reflecting depositional sequence and taphonomic position, the pond and lake environments constituted ideal paleobotanical setting. Contributing to the factors of preservation were the mesic climate around the lakes and ponds, gentle surface relief, fertile soil, shallow lakes with calm surface waters, an abundant source of muddy sands with a rapid rate of deposition, fine and evenly distributed grain size, and well-developed bedding. Consequently, after material such as deciduous leaves dropped into the lacustrine waters they were rapidly buried by muddy sands and preserved as complete paleobotanical specimens that were not readily damaged. The extensive exposures of lacustrine sediments in the Yushe Basin that contain abundant paleontological resources are sufficient to indicate that the Yushe region during the Pliocene was a broad, scenic, vital, and exuberant natural mountain landscape. This is similar to the southeast Shanxi Province region that has a warm temperate, semi-mesic to slightly xeric climate, with an average temperature of 8-10 C, and annual rainfall of mm, dominated by a landscape of deciduous broadleaf forests and thickets of deciduous shrubs.

8 7 Several Points of Conclusion 1. Numerous indicative floral elements occur in the Yushe plant community during the early and middle Pliocene. Taxa including Magnolia, Platycarya, Liquidambar,and Tsuga indicate a certain degree of influence from a persistent subtropical climate, such that a dense forest existed as a northern subtropical transition phase with a large taxonomic proportion of deciduous trees. The presence of drought-resistant herbaceous taxa and small shrubs indicate drrying and cooling climatic trends. This is a noticeably different subtropical forest landscape from that of the Miocene Shanwang region. 2. The late Pliocene Yushe floral community reflects a warm temperate deciduous broadleaf forest landscape generally similar to the current flora of the southern regions of North China. This community reflects distinct altitude differentiation regardless of its even distribution. 3. The deciduous nature of the flora in addition to the extensive fluviolacustrine network in the Pliocene Yushe region indicates that atmospheric circulation conditions were similar to current conditions, with features of a seasonal climate. It is evident that xeric to mesic and large temperature fluctuations occurred within a single year. Acknowledgements Guidance and assistance were provided by colleagues Zhaochen Kong and Minghong Chen from the Chinese Academy of Sciences Institute of Botany who reviewed the text. Sincere gratitude is hereby expressed. Bibliography Chaney, R. W., 1935b: An Upper Pliocene Flora from the Sanmenian Series of Shansi Province. Bull. Geol. Soc. China, 14(3), Chaney, R. W., 1933, A Pliocene Flora from Shansi Province. Bull. Geol. Soc China. 14(3), He, Shiyuan et al., 1984: Annals of Peking Botany. Peking Press (in Chinese). Institute of Botany, Chinese Academy of Sciences et al.: 1978, The Fossil Plants of China (Vol. II) Science Press (in Chinese). Sao, Jiaxin, 1980: Research upon the Late Cenozoic Stratigraphy and Depositional Environments in the Wuxiang, Taiyu, Yushe Regions of Shanxi. Quaternary Research in China, 5(2), (in Chinese). Sao, Jiaxin, 1985: Depositional Characteristics and Evolution of Late Cenozoic Basin Subsidence at Yushe, Shanxi. Quaternary Research in China, 6(2), (in Chinese). Song, Zhichen, 1958: Research on the Fossil Plants, Spore, and Pollen Complexes from the Sanmen System. Quaternary Research in China 1(1), (in Chinese)