Study Of Miocene Lacustrine Stromatolites Characters And Palaeoenvironment In The Hoh Xil Basin

There were a series of Miocene lakes in the Hoh Xil Basin, while lacustrine sediments in the plateau were vulnerable to climate fluctuation. The paleoclimatic informations were recorded effectively in the typical light and shade folds of the Miocene stromatolites. Thus the study of the lacustrine stromatolite layers is of great significance.Based on sedimentological, stratigraphical, palaeontological analysis and related theories, the macroscopic and microcosmic structural characteristics and sedimentary environment of stromatolites were deeply researched. Combining outdoor and indoor, macro and micro investigation, in turn according to the petrological, microbiological characters of Miocene lacustrine stromatolites of Wudaoliang Group in Hoh Xil Basin , high resolution climatic informations in research area were obtained. Thin-section observation, scanning electron microscopy (SEM), major elements/trace element analyses, table C/O isotopeand,etc. were used to study the genetic theory of stromatolites and to discuss the uplift of Qinghai-tibet Plateau.Firstly, the petrological features of a major drilling-coring profile and four minor profiles previous measured in Hoh Xil Basin were described in detail according to field strata description, thin-section observation and SEM. The outcrops of stromatolites were especially paid attention. The main lithological types with muddy limestone, micritic limestone, stromatolites, grainstone limestone and anointed dissolve breccia in Wudaoliang Group were also summarized. The comparison of south-north strata showed the strata thickness was extremely uneven. There was a higher thickness in the north and a thinner one in the south. The bottom of Wudaoling Group overlied on the Oligocene Yaxicuo Group with a tiny angles or parallel unconformity, which indicated that the toppest layers of Yaxicuo Group was eroded to a certain degree.Secondly, combining the macro and micro structural characteristics, it was found that the stromatolite types were extremely abundant, including wave shape, cauliflower shape, directed asphere shape, cone shape, granlar columnar shape as well as their combination form. While the major three types were seen as vault shape stromatolite, dome shape stromatolite and layer (wavy) stromatolite. The stromatolite layers were well-preserved and were mainly trip-shaped, ripple-shaped and column-shaped. Their combination form were commonly showed in the longitudinal profile. The grains were mainly intraclast, spherulite and a few terrigenous elastic minerals in the grain-rich layers (light layers) , while the algae-rich layers were made of high content of organic matters, displaying micrite structure. The stromatolites in Wudaoliang area might be classified as skeleton, agglutinated and fine grained stromatolites which also implied different genesis.Thirdly, based on detailed observation of the drilling core, three zones of stromatolite could be found. Upwards, the corresponding depth of the stromatolites were respectively 72-103m, 22-37m and 9-13m, and the sedimentary facies of each stromatolitelayer was mainly shallow lake or lake shoreline. The first layer of the stromatolites was developed in a stable tectonic setting, and the microbes which made stromatolites lived underwater 45cm to nearly water surface. The stromatolites of second layer grew in the extremely shallow lake and were likely to appear in periodically shallow waters. While there were more terrigenous minerals in the third layer of stromatolites, which was speculated that it developed in the shoreline lake area closer to shore.In addition, accorrding to the geochemical analyses including the major elements/trace element,C/O isotope of stromatolites sample, the following character s were showed: the Sr/Ba ratios ranged from 1 to above 2.5 and stromatolites maybe grew in the brackish water– salt water. The Ca/Mg ratios could reach above 50 due to strong biological action and thriving plankton urge carbonate precipitation, while if the Fe/Mn ratio and Fe²⁺ were higher, the stromatolites developed in the shallow water and reducibility was stronger. The C isotope values of the stromatolites had similar characters to the O isotope values. And theδ(13)^C values of grain-rich and algae-rich layers were positive, but theδ(18)^O were negative. Generally, the values of grain-rich layers were higher than the algae-rich layers. These values of C and O isotope indicated that algae-rich layer was developed in drouthy winter of strong evaporation and high salinity. In relatively wetter summer, algae-rich layer was formed due to the storms which brought a lot of terrestrial detritus. Based on the above research, the mode chart about stromatolite layer was built.Finally, it was concluded that the climate background was generally of drought during the early Miocene interval in the Hoh Xil region, while it was moist vs each period of stromatolite deposited. The rainfall far exceeded the evaporation so as to a "Wudaoliang Ancient Great Lake" formed. The altitude of Hoh Xil region might have exceeded 2000m in early-middle Miocene. Thus the higher altitude influenced the planetary system of winds, which in turn prevented the moist air into the inland from the ocean and caused the dry climate. But it was relative wet when stromatolite deposited, possibly resulted form the regional climatic circulate.