Palynostratigraphy of the Fengcheng Formation (Pennsylvanian to Cisuralian) from the Maye-1 borehole in the Mahu sag, northwestern Junggar Basin, Xinjiang, Northwest China
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Abstract:
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.
