Research On Sedimentary Characteristics Of Stromatolites From Lower Ordovician,songzi City,Hubei Province

Since the 1960s,people study stromatolites made many achievements,but there are many deficiencies and doubts,such as stromatolites classification,nomenclature is not uniform,genetic mechanism of stromatolites is not completely unified.The cause of stromatolites decay is unknown.Through the investigation of the literature,the author discovered that from the perspective of the distribution of stromatolite geochronology,the Early Ordovician was an important period for the large-scale development of stromatolites.However,scholars at home and abroad have very little research on stromatolites during this period,and most of them are concentrated on the study of Proterozoic,a few Cenozoic and modern stromatolites.The sedimentary development of the Lower Carboniferous marine carbonates in Liujiachang area of Songzi is continuous,with good outcrops,and stromatolites and other microbial rocks are widely distributed.It is an ideal place for the study of the Early Ordovician stromatolites.The stromatolites were systematically divided into five categories:laminated stromatolites,wavy stromatolites,columnar stromatolites,mound-shaped stromatolites,and clot-like stromatolites by detailed observation of measured profiles and slice observation,and based on geometry of stromatolites.Among them,clot-like stromatolites are newly discovered and newly named types.The macroscopic and microscopic characteristics of various types of stromatolites are elaborated.Through the study of the division of lithofacies and the study of sedimentary facies characteristics in the study area,the sedimentary characteristics and sedimentary model of stromatolites in this area were discussed.From the distribution of stromatolites,stromatolites are mainly distributed in the restricted platform facies and open platform facies.And different types of stromatolites require different amounts of water energy.Columnar stromatolites require the most intense water energy,and are generally formed on the shoal subphase of open platforms.The water body energy required for mound-shaped,wavy,and clot-like stromatolites takes the second place,and is generally formed at the grain banks subphase of the restricted platform and the shoal subphase of the open platform.Laminated stromatolites require the lowest amount of water energy,and are generally formed in the middle and upper parts of the grain banks subphase of the restricted platform.The value of Lower Ordovician?¹³C_PDBDB varies from-3.02‰to 1.32‰in the study area,mainly concentrate between 0‰and-1.00‰;The value of?¹⁸O_PDBDB varies from-9.95‰to-8.11‰,mainly concentrate between-0.80‰to-0.90‰;The seawater temperature between 17.83?and 26.1?,and the average temperature is 21.56?.The main distribution range 20?to 25?.The study area basically belonged to the warm or hot tropical climate;From the trends of?¹³C_PDB values and 1000×?Sr/Ca?,it can be seen that during the Nanjinguan Period,sea level declined early and then rose for a short time,mid-sea level dropped again and then rose,and late sea level kept decreasing.Paleosalinity of the period in a downward trend.During the early part of the Fenxiang Period,sea level rises slowly and then declines.The mid-sea level rises rapidly and then declines.The late sea level falls rapidly.The paleo-salinity during this period may have experienced a relatively high salinity to a relatively low salinity to a relatively high salinity.During the Honghuayuan Period,sea levels are rising.The paleo-salinity during this period may have experienced a relatively high salinity to a relatively low salinity to a relatively high salinity.By systematically analyzing the geochemical elements of the stromatolites in the study area,it has been found that the Sr/Ba ratio of stromatolites is generally higher.And the laminated,wavy,clot-like,mound-shaped,and columnar stromatolites Sr/Ba were sequentially increased.It was speculated that the depth of the seawater in which the laminated stromatolites,wavy stromatolites,clot-like stromatolites,mound-shaped stromatolites,and columnar stromatolites are located deeper,and the farther away from the coast.The content of SiO₂,Al₂O₃ and Fe₂O₃ and the content of rare earth La in the stromatolites of various forms both appear and have low values,indicating that stromatolites are formed in relatively clean sedimentary bodies.The temperature for the development of stromatolites strata in the study area is generally between 21.54?and23.13?.And the temperature change is not large and stable,indicating that the warm shallow sea environment is conducive to the development of algae to form stromatolites.Therefore,a warm,clean,light-transmitting shallow sea environment is a favorable development environment for stromatolites.The stromatolite in the study area belong to the stage of mature-overmaturation of type III kerogen.Among them,the TOC content in the Nanjinguan Formation is 0.19%,and the organic matter abundance is high.It is a better source rock and has good hydrocarbon generation potential.The TOC values measured by the samples in Fenxiang Formation and Honghuayuan Formation are all low,only 0.11%and 0.08%,which can respectively be deemed to poor source rocks and non-hydrocarbon source rocks.Since the samples were basically taken from the field profile and subjected to long-term weathering,erosion,oxidation,and biodegradation,most of the hydrocarbons have suffered losses.So the measured TOC values were significantly lower.The actual TOC value should be greater than the measured value.The stromatolites in the study area are generally poor-better source rocks,and their hydrocarbon generation potential is general.