粤北丹霞盆地晚白垩世丹霞组锦石岩段砂岩沉积特征及成因

丹霞山是“中国丹霞”世界自然遗产的典型代表，目前对其成景地层（上白垩统丹霞组）沉积环境的研究程度不高，尤其对丹霞组锦石岩段沉积环境还存在争议。锦石岩段以厚层—巨厚层状中-细砂岩为主，发育大型交错层理，夹薄层—极薄层状泥岩、粉砂质泥岩以及钙质层。本文通过对丹霞盆地丹霞组锦石岩段砂岩进行扫描电镜、粒度分析以及岩石地球化学等方面的研究，探讨该套砂岩的沉积特征及成因。研究表明：该套砂岩以长石石英砂岩为主，显微镜下石英颗粒常见港湾状撞击凹坑，颗粒边缘发育铁、锰及泥质边框，胶结物主要为蒙脱石和方解石。扫描电镜下石英颗粒表面可见碟形撞击坑，具有风成沉积特征。该段砂岩粒度范围窄，分选性好，其稀土总量ΣREE = 40.2×10^-6～215.0×10^-6，LREE/HREE = 5.54～12.46，轻稀土相对富集，重稀土相对亏损。δEu负异常明显，Ce无异常，具有与中国大陆上地壳相似的高场强元素（Pb、Zr、Hf）和大离子亲石元素（Rb、Ba）组成，化学蚀变指数（CIA）值介于56.99～75.87之间，反映了较低的古风化程度，指示了干旱、炎热的的古气候条件。La/Yb-ΣREE图解和F1-F2判别函数图解研究表明，丹霞组锦石岩段砂岩的母岩物质成分以古老上地壳的长英质为主，具有被动大陆边缘属性的陆内裂谷特征。丹霞盆地晚白垩世锦石岩段沉积时期，具有干燥的地表沉积环境，可能与晚白垩世中期Campanian期东亚中纬度地区的干旱古气候条件相关。

Sedimentary characteristics and genesis of Late Cretaceous sandstone of Jinshiyan member of Danxia Formation,in Danxia Basin,northern Guangdong

Danxiashan is a typical representative of the world natural heritage of “China Danxia”. At present, the study degree?of the sedimentary environment of Later Cretaceous Danxia Formation is low, especially the sedimentary environment of the Jinshiyan member of Danxia Formation is still controversial. The Jinshiyan member is mainly composed of thick medium?fine sandstone, with large cross?bedding, intercalated with thin and very thin bedded mudstone, silty mudstone and calcareous layer. This paper discussed the sedimentary characteristics and genesis of sandstones of the Jinshiyan member by means of SEM, particle size analysis and petrogeochemistry. The results show that this set of sandstone is mainly feldspar quartz sandstone. Under the microscope, quartz particles are often harbor impact pits, and the edges of the particles are developed with iron, manganese and argillaceous borders. The cementings are mainly montmorillonite and calcite. Under SEM, the surface of quartz particles is characterized by disc impact crater and aeolian deposition. Separation of sandstones is good, ΣREE = 40.2×10^-6～215.0×10^-6, LREE/HREE = 5.54～12.46, LREE is rich, HREE is relatively deficient. The negative value of δEu is obvious, while the Ce is normal. It is composed of elements with high field intensity （Pb, Zr, Hf） and large ionic lithophile elements （Rb, Ba） similar to the crust of mainland China. The chemical alteration index （CIA） is between 56.99 and 75.87, which reflects the low degree of ancient weathering and indicates the dry and hot climate conditions. La/Yb-ΣREE diagrams and F1-F2 discriminant function diagram indicate that the sandstone of the Jinshiyan member of Danxia Formation is mainly from the ancient upper crust, has the characteristics of intracontinental rift properties of passive continental margin. During the depositional period of the Late Cretaceous Jinshian member in Danxia Basin, there was a dry surface depositional environment, which may be related to the arid paleoclimatic conditions of the Late Cretaceous Campanian in middle latitudes of East Asia.