河北阳原东目连第四纪叠层石古环境分析

Analysis of the Palaeoenvironment of Quaternary Stromatolites at Dongmulian, Yangyuan County of Hebei Province

组成阳原县东目连湖积台地的第四纪“泥河湾层”地层中,发育多层藻迹层和一层“菜花形”、“锥体状”大叠层石(个体高20 cm),大叠层石层延伸1 km,构成古藻坪微地貌。经OSL和古地磁测试,大叠层石年龄推测为220 ka左右。镜下叠层石片子中见蓝藻门的纤细席藻种Phormidium tenue(Menegh)Genegh和湖泊鞘丝藻种Lyngbya Limnetica Lemmermann等。不同形态叠层石反映不同发育阶段,藻迹发育于高水位边缘部位,大叠层石发育于高水位与低水位之间边缘部位。亮层和暗层交替出现的纹层构造是组成叠层石的基本结构构造,它们是由蓝藻丝体生长代谢活动所致。据同位素、磁化率、粒度、地球化学元素、X衍射、孢粉和生物化石等分析,认为气候暖干、盐度高及大量苔藓虫、介形虫、龙介科蠕虫等生物存在的环境有利于大叠层石形成。叠层石可能属半咸水湖、滨湖浅水环境,但据首次在第四纪泥河湾层中发现的苔藓虫和龙介科蠕虫管分析,其成因是否与海水有过联系尚待进一步研究。

The Dongmulian Section, 15. 06 m in thickness, is located at Dongmulian Village of Yangyuan County, Hebei Province. The geographic coordinate of the section is 40°5'N and 114°12'E, and its elevation is 880 m a. s. 1. Three algae imprint layers are discovered between 0. 11 and 0. 9 m from the top of the section. One big stromatolite bed with one algae imprint layer appearing occasionally at the bottom is discovered between 9. 9 m and 10. 1 m. Big cabbage-like and cone-like stromatolites, 20 cm high, construct ancient algae mats, which ex-tend about 1 km. Stromatolites, dated 220 ka BP according to the OSL and Palaeomagnestism results, were discovered in lacustrine deposits, Nihewan Bed. Multiplicate proxies for the stromatolites are tested below: The δ18O values of carbonate in lacustrine sediments change from 30. 7‰ to 31. 3‰ (SMOW) , δ13C from - 1. 8‰ to 0. 9‰, magnetic susceptibility values from 40 × 10-6 SI to 48×10-6 SI, medium grade from 20 to 25 μ m and the Sr/Ca ratio from 5. 7 to 7. 2. The content of dolomite is more than 90% and quartz less than 5% in cabbage-like stromatolites by X-ray diffraction analysis, whereas dolomite 65% , Quartz 20% , feldspar 15% and minerals with iron less than 5% in cone-like stromatolites. Sporopollen analysis yields 88. 6%~89. 0% ligneous plant pollen, 7. 3%~10. 2% herb and shrubs pollen, and 4. 1%~0. 8% fern spore. The ostra-code shells are dominated by Limnocythere dubiosa companying with Eucypris inflata. Vermes have been dis-covered in stromatolite specimen, and lots of Bryozoa fossil discovered in silt of the base of stromatolites. Warm-dry climate, strandline lake, violent evaporation and high salinity are best conditions for stromatolites to form; or the basin was joined with sea during the time when the stromatolites were growing according to com-prehensive analysis. The stromatolites are formed by microbial activities of blue algae. Phormidium tenue (Menegh) Gomont Lyngbya limnetica lemmermann and Chroocoocaceae or Nostocaceae of blue algae are discov-ered in stromatolite slices. The basic structure of the stromatolite columns is characterized by alternations of light and dark laminae, which are formed by metabolism of blue algae. Light laminae constituted by quartz and fragmental products of carbonate, is formed in spring or summer when blue algae are suitable to live and grow. Dark laminae constituted by blue algae, organic and calcareous guard or crust outside algae is formed in autumn or winter when algae are dead to deposit. Calcareous guard or crust is made of calcium carbonate that is excreted or captured or felted by blue algae.