准噶尔盆地西北缘玛湖凹陷风城组主要形成于碱湖环境, 含有优质生烃母岩。对玛页1井风城组岩心样品的孢粉分析建立了Protohaploxypinus perfectus–Lunatisporites tersus (PT)孢粉组合。该组合包括20属29种孢粉化石。PT组合以双气囊具肋花粉占主导, 蕨类孢子含量很低为特征; 孢粉母体植物类群以裸子植物门种子蕨盾籽目为主 , 其次为松柏纲松柏目。该组合与准噶尔盆地南缘塔什库拉组上部至乌拉泊组的 Crustaesporites–Protohaploxypinus–Hamiapollenites孢粉组合可以对比, 均以双气囊具肋花粉为主要特征, 又同时出现重要的属种Gardenasporites bilabiatus, Triangulisaccites boleensis和Hamiapollenites saccatus。孢粉地层学和同位素年代学资料表明, 玛页1井风城组PT组合的时代很可能属于石炭纪宾夕法尼亚亚纪卡西莫夫期至二叠纪乌拉尔世阿瑟尔期, 玛湖凹陷区整个风城组沉积时代晚于宾夕法尼亚亚纪巴什基尔期, 其上部可能包含部分乌拉尔世阿瑟尔期沉积。风城组黑色页岩中产出的双气囊具肋花粉占绝对优势的孢粉组合和冷水古鳕类化石, 与互层状盐碱层中自生矿物碳镁钠石和碳酸钠钙石共同表明风城组页岩和盐碱韵律沉积很可能形成于冷干和暖干频繁交替的古环境中。

The Fengcheng Formation in the Mahu sag in northwestern Junggar Basin, Xinjiang, was deposited in an alkaline saline lake and contains large volumes of high-quality source rocks. PetroChina Xinjiang Oilfield Company conducted the Maye-1 borehole project and collected the entire sequence of the Fengcheng Formation in the Mahu sag to evaluate and explore the oil and gas potential in this area. In order to better constrain the age of the Fengcheng Formation, a palynostratigraphic project has been conducted since 2019. A total of fifty palynological samples, mainly of shale, claystone and siltstone, were collected from the Maye-1 core. All samples were cleaned, crushed, weighed (30-50 g for each sample), and macerated following the standard HCl-HF-HCl method. The organic residues were sieved through 180 and 10 μm meshes, respectively. The residues were then mounted on microscope slides using glycerin jelly. Palynomorphs were observed and photographed using a LEICA DM 2500 microscope equipped with a D800E camera. However, a considerable number of palynomorphs are not well preserved, usually with various sizes of secondary cavities or perforations. All slides are stored in the PetroChina Xinjiang Oilfield Company, Karamay, Xinjiang, China. Positions of the spores and pollen illustrated in the text figures are located using an England Finder Slide. Based on systematic palynological identification and statistics, a new fossil palynological assemblage, Protohaploxypinus perfectus-Lunatisporites tersus (PT) assemblage, is established for the Fengcheng Formation. Twenty-nine species in twenty genera of spores and pollen were recognized in the PT Assemblage from fifty macerated samples. In the PT assemblage, the Disaccites striatiti pollen is dominant, while the fern spores are relatively rare. The comparison of the dispersed and in-situ spores and pollen associated with their reliable parent plants shows that the palynoflora consists of dominant species of Peltaspermales and species of Coniferales as the second major component. The PT assemblage is correlatable with the Crustaesporites–Protohaploxypinus–Hamiapollenites assemblage from the upper part of the Tashenkula Formation to the Wulabo Formation at the southern margin of the Junggar basin due to the high portion of the Disaccites striatiti pollen in the palynoflora and the crucial co-occurring species, including Gardenasporites bilabiatus, Triangulisaccites boleensis and Hamiapollenites saccatus. In addition, the PT Assemblage, mainly consisting of the Disaccites striatiti pollen (＞ 70%), differs from the Bashkirian Calamospora breviradiata–Lunatisporites tersus assemblage from the age-constrained Jiamuhe Formation in the nearby Zhongguai uplift. The latter assemblage bears a slightly less Disaccites striatiti pollen (ca. 60%) and an equal amount of Calamospora spores (ca. 40%), indicating that the PT assemblage is reliably dated as post-Bashkirian. Based on the precise U-Pb zircon dates from two tuff layers (304.9 ± 0.69 Ma and 299.9 ± 0.64 Ma) in the lower part and a tuff bed (296.9 ± 0.8 Ma) in the upper part of the Fengcheng Formation in the Mahu sag, the age of Fengcheng Formation is constrained to the Kasimovian–Asselian. In summary, careful palynological comparisons and precise geochronological data suggest that the PT assemblage from the Fengcheng Formation of the Maye-1 borehole was likely deposited from the Kasimovian to the Asselian age. Moreover, the age of the entire Fengcheng Formation in the Mahu sag should be younger than the Bashkirian, and the upper part of the Fengcheng Formation may contain some Asselin sediments. Abundant Disaccites striatiti pollen and the intriguing emergence of the cold-water Gadus in the shales and the warm-phase authigenic alkali minerals of eitelite and shortite from the interbedded saline layers indicate that the rhythmic sedimentary strata of the Fengcheng Formation in the Mahu sag were probably formed in paleoenvironments with frequent fluctuations of cold-dry and warm-dry paleoclimate conditions.

中国科学院战略性先导科技专项(B 类) (XDB26010501)、中国科学院前沿科学重点研究项目(QYZDY-SSW-DQC023)、第二次青藏高原综合科学考察研究(2019QZKK0706)、国家自然科学基金委地区基金重点项目(U1702242)和新疆油田公司项目(2020-C4011)联合资助