湖北宜昌下白垩统五龙组风成砂的发现及其物源分析

Discovery of the Lower Cretaceous aeolian sandstones in the Wulong Formation in Yichang,Hubei Province,and its provenance

白垩纪时期,我国中纬度地区广泛发育风成砂岩,指示了副热带沙漠带的存在。对白垩纪风成砂岩的发育年代、分布范围、物质来源、所记录的古风向等方面的研究,有助于重建东亚地区白垩纪不同时期的古地理和古气候特征。为此,对湖北宜昌地区下白垩统五龙组(K_(1)w)中发育的一套巨厚的白垩纪砂岩开展了系统的野外沉积特征调查、室内粒度分析、石英颗粒表面微观特征的扫描电镜观察,以及碎屑锆石U-Pb年代学研究,探讨了其成因和物源特征。结果表明:①该套砂岩分布于江汉盆地西缘白垩纪洪积扇的扇缘部位,出露厚度超过40 m,发育大型高角度交错层理,其粒度组成十分均一,其野外特征和粒度组成符合风成沙的特征;②扫描电镜观察显示,石英颗粒普遍磨圆较好,表面发育碟形和新月形撞击坑,以及溶蚀作用和沉淀作用导致的微观特征,与风成砂的特征一致;③该套砂岩的碎屑锆石年龄存在4个主要的峰值,分别约为2.46,1.85 Ga,780,170 Ma,这些年龄峰值与秦岭-大别造山带及黄陵隆起之上沉积盖层的碎屑锆石年龄组成一致,指示其物源可能主要来自黄陵隆起形成后其上覆盖的南华系-侏罗系的侵蚀物,同时可能也混入了部分秦岭-大别造山带的物质;④该套砂岩中最年轻的锆石年龄为(119.9±2.8)Ma,指示该段地层的年龄不早于早白垩世晚期,与湖北省地质调查院将五龙组划归下白垩统一致。上述结果指示着该套砂岩为早白垩世时期干旱气候条件下发育于干旱盆地洪积扇边缘的沙漠沉积,指示着湖北宜昌地区在早白垩世副热高压的影响下已出现了地带性沙漠。

[Objective]Eolian sandstones were widely developed in the middle latitudes of China during the Cretaceous era,which indicated the existence of subtropical desert belts.Studies on the age,distribution,provenance,and paleowind directions of Cretaceous aeolian sandstones would be helpful for the reconstruction of paleogeography and paleoclimate in East Asia during different periods of the Cretaceous era.[Methods]Here,we report the origin and provenance of a>40 m-thick sandstone layer within the Lower Cretaceous Wulong Formation(K_(1)w)in Yichang,Hubei Province by systematically investigating its sedimentary facies in the field,particle size distribution,quartz grain surface textures,and detrital zircon U-Pb chronology.[Results]The results show that,①The Cretaceous sandstones were developed distal to an alluvial fan system that formed on the western margin of the Jianghan Basin.The thickness of the sandstones outcropped over 40 meters,with large scale high angle cross-bedding,and its grain size is extremely uniform.These sedimentary characteristics are in accordance with modern aeolian sandstones.②Scanning election microscope images show that the quartz grains are generally well rounded,with disk-shaped and crescent-shaped impact scars and microfeatures caused by dissolution and precipitation on their surface,and these features are consistent with aeolian sands.③Four major age peaks were shown at~2.46 Ga,~1.85 Ga,~780 Ma and~170 Ma in the detrital zircon spectrum of the sandstones in the Wulong Formation.These peaks are consistent with the zircon ages of the Qinling-Dabie Orogen Belt and the sedimentary cover of the Huangling Dome,indicating that the sandstones may be mainly derived from the erosion of the Nanhua-Jurassic strata after the uplift of the Huangling Dome,as well as erosions from the Qinling-Dabie Orogen.④The youngest zircon grain in the sandstone was(119.9±2.8)Ma,indicating that the age of the Wulong Formation was not earlier than the late Early Cretaceous and was consistent with the conclusion from geological survey which considered the Lower Cretaceous Wulong Formation.[Conclusion]These results show that the sandstones were desert deposits that developed at the margin of diluvial fan of arid basin under the late Early Cretaceous arid climate,indicating that the Yichang area of Hubei Province was within the zonal deserts since the Early Cretaceous period under the influence of the subtropical high.