珠江口盆地白云凹陷珠江组深水海域海绿石赋存状态、成因及研究意义

The glauconite is one of the most typical facies indicators in sedimentary rocks, and has great sedimentary and stratigraphic significance. The glauconites are very rich in both the submarine fan sandstone reservoirs and the globigerina limestone reservoirs in the Miocene Zhujiang Formation, Baiyun Sag, Pearl River Mouth Basin. But they have not been paid enough attention to for a very long time. Based on the stratigraphy, sedimentology, petrography, mineralogy, geochemistry and 40Ar/39Ar age, the occurrence, maturity and genetic types of the glauconites in the Miocene Zhujiang Formation, Baiyun Sag, Pearl River Mouth Basin will be studied systematically. The origin and their controlling factors of the glauconites will be considered in detail. The 40Ar/39Ar dating data of the glauconites in the globigerina limestones could provide the basic information for the accurate dating of the bottom boundary of the Zhujiang Formation and the starting time of the deep-water traction currents. The sedimentological and sequence stratigraphic significane of different genetic types of glauconites will be sum up. The research results will provide a new method for the dabates of the deep-water Zhujiang depositon in Baiyun sag, Pearl River Mouth basin, and provide absolutely new basic data for the depositonal characteristics of the deep-water traction currents and gravity flows in the northern South China Sea area, and rebuilding the history of ancient oceanic current circulation in the Pearl River Mouth Basin and the northern South China Sea.

海绿石是沉积岩中最典型的指相矿物之一，具有极其重要的沉积学和地层学研究意义。珠江口盆地白云凹陷中新统珠江组深水相储层砂岩和抱球虫灰岩中都含有丰富的海绿石，但长期以来并没有得到足够的重视。在地层学、沉积学、岩石学、矿物学、地球化学、40Ar/39Ar年代学等研究的基础上，系统地研究珠江口盆地白云凹陷中新统珠江组砂岩和抱球虫灰岩中海绿石的赋存状态、成熟度及其成因类型，深入地揭示珠江组海绿石的成因及演化的控制机制，利用抱球虫灰岩中的海绿石40Ar/39Ar年龄为珠江组底界年龄和深水牵引流发育时间提供直接的地质年代学证据，归纳总结不同成因类型海绿石所具有的沉积学和层序地层学意义，为进一步理解目前珠江口盆地白云凹陷中新统珠江组深水沉积作用研究中一些仍存有争议的沉积学和地层学问题提供一个新的研究思路，并为珠江口盆地新近纪深水牵引流和重力流沉积过程乃至南海北部古洋流循环历史的重建研究提供崭新的基础资料。

海绿石以独特的绿色自生色和球粒形状与围岩形成明显反差，常被用作典型的指相矿物。首先，详细地研究了珠江口盆地中新统珠江组沉积学特征，富含海绿石的珠江组取芯段除发育深水扇沉积，还发育丰富的、与远洋低密度浊流和半远洋-远洋悬移沉积有明显区别的深水牵引流沉积作用，其发现具有重要地质意义。其次，详细研究了海绿石赋存的部分围岩深水灰岩的岩石学及成岩作用特征，深水灰岩埋藏成岩作用期间经历的所有明显变化都是破坏性的过程，较早进入超压旋回且后期缺乏构造破裂，使其最终成为中-高孔、低-特低渗孔隙型潜在储层。再次，通过珠江组26个细粒沉积物样品的稀土元素数据来研究其目前仍存在较大争议的物源问题，主要物源为长英质岩石。最后，在沉积相、岩石学的基础上，通过岩石薄片、X射线衍射和电子探针等手段，详细地研究了珠江组海绿石的矿物学特征。在盆地相泥岩和粉砂质泥岩中富含海绿石矿物。海绿石常见浅绿色、绿色、浅黄绿色、浅棕褐色等四种颜色，形态主要为初生状态的完整颗粒状和低成熟状态的具裂纹颗粒状，磨圆度多呈棱角状至次棱角状，分选性较差，海绿石与石英碎屑的粒径大小相差悬殊竟有10倍以上，为深海原地自生型海绿石。这是目前报道的为数不多的深海海绿石的案例。因此，海绿石既可以形成于温暖的浅海环境，还可以形成于深海环境，利用海绿石作为指相矿物时，需要结合其它的指相标志。