陆相煤系沼泽、陆源碎屑体系交替演化对古气候变化的响应

Compared with offshore basin，during the sedimentary history of stretching Terrestrial basin paleoclimate changes not only controls the absolute changes of lake level by rainfall in the basin, but also affects the rate of sediments supply through controlling of the development of plants, which have make the two kinds of contradiction sedimentary systems, namely swamp and terrigenous clastic system deposited alternately. To reveal the relationship between the alternating evolution of swamp and terrigenous system and paleoclimate changes, a early to middle Jurassic coal measure from Dameigou area in Qaidam basin will be studied. Some genetic geological parameters, such as ratio of rock /feldspar of clastic rock, sporopollen of argillaceous rock and coal seam, carbon isotope of inertinite in organic matter, et al. will be employed as the indication index for tectonic activities, paleoplants, paleoclimate(humidity and temperature). The relationship among tectonic activities, paleoplants, paleoclimate and sedimentary evolution will be analyzed based on two scientific issues, namely occurrence and termination cause of coal accumulation and abandon and resurrection mechanism of terrigenous clastic system, to find the relationship model between the alternating evolution of swamp and terrigenous system and paleoclimate changes in Terrestrial coal measures.

与近海型盆地相比，伸展构造背景下的陆相煤系沉积过程中，气候变化不仅通过流域内降雨量控制着绝对湖平面变化，还通过控制物源区植被发育影响沉积物供给速率，并使煤系表现为沼泽和陆源碎屑两种矛盾沉积体系的交替更迭。为了揭示陆相煤系沼泽、陆源碎屑体系交替沉积特征与古气候变化间的作用关系，以柴达木盆地北缘大煤沟剖面早、中侏罗世煤系为主要研究对象，应用碎屑岩石英/长石比，泥质岩和煤层孢粉、有机质惰质组分碳同位素等作为煤系沉积期高分辨率构造活动、古植物、古气候（湿度、温度）和沉积环境演化的指示参数。通过层序格架内构造活动，古植物、古气候和沉积环境间综合对比分析，围绕聚煤作用（全球碳存储）发生、终止原因和陆源碎屑体系废弃、复活机制两个科学问题，提出陆相煤系层序格架内沼泽、陆源碎屑体系交替演化与古气候变化关系模式。

为揭示陆相煤系沼泽、陆源碎屑体系交替沉积特征与古气候变化间的作用关系，以柴北缘早、中侏罗世煤系为研究对象，利用岩心和测井、碎屑岩岩矿鉴定、泥质岩有机碳同位素、矿物学和地球化学等数据进行了煤系沉积期环境演化及古气候和源区构造活动的恢复研究。结果表明：1）早侏罗世盆地断陷期发育一套三角洲-湖泊沉积，中侏罗世盆地进入断坳转换期，水体变浅，发育河流-三角洲沉积，自下而上发育#7-#1煤层，其中#7煤为区域性可采煤层，经频谱分析估算#7煤层泥炭沉积速率约为0.2mm/年。2）早侏罗世泥质岩有机碳同位素组成（δ13Corg）变化在-29.3‰—-21‰（x̄=-24.83‰），垂向上出现四次负偏移，分别对应四次全球海洋缺氧事件（OAE），可能与卡鲁大火成岩省活动有关。中侏罗世δ13Corg变化在-31.9‰-21.7‰（x̄=-24.05‰。），垂向上呈波动式缓慢负偏，出现了两次负偏移，响应于燕山运动。3）中侏罗阿连末-巴柔期构造活动指数（TI）由2个快速降低-缓慢升高旋回组成，指示源区构造活动具有缓慢增强-快速减弱的特征；矿物蚀变指数（MIA）由2个快速升高-缓慢降低旋回组成，指示气候呈现快速湿暖-缓慢干热的变化趋势；上部巴通-卡洛期TI由1个稳定降低-稳定升高旋回组，源区构造活动呈现稳定增强-稳定减弱的特征，MIA由2个稳定升高-稳定降低的旋回，具有缓慢湿暖-缓慢干热的变化趋势。源区构造活动和气候变化在阿连-巴通期具有较好的相关性，构造活动减弱与气候湿暖化相伴随。盆地断陷期（阿连末-巴柔期）陆源碎屑体系废弃和区域可采煤层发育的驱动机制为源区构造活动的减弱和气候的湿暖化，盆地河流回春作用及聚煤作用的终止与物源区构造活动的增强和气候的干热化密切相关。挤压坳陷期（巴通-卡洛期）整体聚煤作用变差，多发育不连续的薄煤层，卡洛夫中晚期湖泛事件与增强的物源区构造活动有关。