鄂尔多斯盆地南部三叠系延长组孢粉相及其沉积古环境恢复

Palynofacies and sedimentary paleoenvironment reconstruction of Triassic Yanchang Formation in southern Ordos Basin

为揭示鄂尔多斯盆地南部三叠系优质烃源岩有机质来源,对三叠系延长组的岩心开展了孢粉相分析和沉积古环境恢复。①延长组9段—6段可识别出3种孢粉相:孢粉相Ⅰ以无定形有机质(AOM)为主;孢粉相Ⅱ以AOM和植物碎屑共同发育为特征;孢粉相Ⅲ以发育植物碎屑为主。②延长组可划分出3种孢粉相组合:组合A中以孢粉相Ⅲ为主,可见少量孢粉相Ⅰ和孢粉相Ⅱ,形成于河/湖三角洲和滨湖沉积,为近源富氧的高能环境产物;组合B中以孢粉相Ⅱ为主,可见少量孢粉相Ⅰ和孢粉相Ⅲ,是浅湖—半深湖亚相产物,反映低氧—缺氧的近源—远源环境;组合C中以孢粉相Ⅰ为主,仅出现于延长组7段3亚段,形成于半深湖—深湖亚相静水缺氧环境。③元素地球化学古环境分析显示:研究区在延长组7段3亚段沉积期为持续的高度湿润气候,在延长组9段1亚段、延长组7段1亚段和延长组6段3亚段沉积期可出现不连续的潮湿气候,其余时段为半干旱—半潮湿气候;湖平面在延长组7段3亚段沉积期达到最高;湖泊生产力在延长组8段1亚段沉积期开始升高,至延长组7段3亚段沉积期达到高峰;高度湿润气候、湖泊最大水深与最高生产力的同期出现,指示印支期盆地的快速沉降事件与卡尼期全球潮湿气候事件耦合是鄂尔多斯盆地南部延长组7段3亚段沉积期藻类勃发和降解AOM大规模聚集的关键因素,并导致发育优质烃源岩。

To reveal the organic matter sources of high-quality Triassic source rocks in southern Ordos Basin,palynofacies analysis and sedimentary paleoenvironment reconstruction were carried out on the core of the Triassic Yanchang Formation.The findings show as follows.(1)Three types of palynofacies can be identified from Member 9 to Member 6 of Yanchang Formation.Specifically,palynofacies TypeⅠis mainly composed of amorphous organic matter(AOM);palynofacies TypeⅡis characterized by co-development of AOM and plant debris;palynofacies TypeⅢis dominated by plant detritus.(2)Three types of palynofacies assemblage are classified.Assemblage A is mainly composed of palynofacies TypeⅢ,with a small amount of palynofacies TypeⅠandⅡ.It is formed in fluvial/lacustrine delta and lakefront facies,as being a near-source oxygen-rich product in high-energy environment.Assemblage B mainly contains palynofacies TypeⅡ,with a small amount of palynofacies TypeⅠandⅢ.It is a shallow to semi-deep lacustrine subfacies product,reflecting the hypoxic to anoxic near-source or far-source environment.Assemblage C is dominated by palynofacies TypeⅠ,which is only found in the 3rd submember of Member 7 of Yanchang Formation.It was formed in the semi-deep to deep lacustrine static water anoxic environment.(3)The element geochemical paleoenvironmental analysis shows that the study area had a continuous highly humid climate during the depositional period in the 3rd submember of Member 7 of Yanchang Formation,the discontinuous humid climate might occur during the depositional periods in the 1st submember of Member 9 and Member 7,and the 3rd submember of Member 6 of Yanchang Formation,and semi-arid to semi-humid climate occurred in the remaining periods.The lake level reached its highest during the depositional period in the 3rd submember of Member 7 of Yanchang Formation.The lake productivity began to increase in the depositional period of the first submember of Member 8 of Yanchang Formation,and reached its peak in the depositional period of the 3rd submember of Member 7 of Yanchang Formation.The simultaneous occurrence of highly humid climate,maximum lake depth,and maximum productivity indicates that the coupling of rapid subsidence in the Indosinian period and global humid climate events in the Carnian period is the key factor for the algal bloom and large-scale accumulation of degraded AOM in the depositional period of the 3rd submember of Member 7 of Yanchang Formation in southern Ordos Basin,leading to the development of high-quality source rocks.