南襄盆地泌阳凹陷深水湖底扇厘定及碎屑锆石U-Pb年代学物源追踪

Sub-Lacustrine Fan of Chengdian and Zircon U-Pb Ages and Constraint on Its Provenance in Biyang Depression,Nanxiang Basin,China

针对南襄盆地泌阳凹陷程店深水区湖底扇沉积体,进行了岩心、粒度、薄片、重矿等测试资料、砂体结构及地球物理响应的综合分析.结果发现,湖底扇砂体横向与平面结构呈现孤立特征,岩性总体偏细,具粗粒岩性,以块状岩相为主体,显示出砂质碎屑流流态特点,含波状层理和交错层理的层理发育的岩相,2类岩相具分异粒度特征.湖底扇沉积体显示出明显重力流性质,兼具牵引流特点,为复合式搬运机制,其发育受控阵发性洪水作用、盆地幕式旋转掀斜作用及沉积缓倾斜坡因素,是深水背景下洪水成因湖底扇沉积.为追踪其物源,分别从侯庄三角洲与湖底扇区选取砂岩样品,进行碎屑锆石U-Pb定年分析,结果显示:程店湖底扇与侯庄三角洲区砂岩样碎屑锆石测点年龄分布具相似特征,碎屑锆石表面年龄分布显示单峰特征,集中在166～115Ma,该年龄段锆石类型以岩浆成因为绝对主体,含少量变质锆石;同时在247～210Ma、475～412Ma、813～663Ma、3 489～1 692Ma几个主要年龄区间亦具一定数量碎屑锆石分布.综合多方面资料,结合区域地质背景,初步得出如下结论:程店湖底扇砂体与北部侯庄三角洲砂体为同物源沉积组合,源岩主体形成时代为中侏罗世-早白垩,对应燕山构造阶段,主体物源岩系为宽坪-陶湾群构造岩相带中南侧发育的燕山期岩浆岩系,同时,前晋宁期、晋宁期、加里东期及印支期热事件形成的变质岩系或岩浆岩亦可能具不同程度的贡献.

Aiming at Chengdian deep water sandstone in Biyang depression of Nanxiang basin,we analyzed the core,the particle size,thin section,heavy minerals testing materials,structure of sandbody and the geophysical response of sedimentary sandbody,and found that the sub-lacustrine fan is an isolated body with fine and coarse lithology which consists of mainly massive lithologic facies with the characteristics of sandy debris flow sediments,but wavy bedding and cross bedding lithologic facies,two lithologic facies have its own particle-size feature,which shows the feature of turbidity current and tractive current.It＇s controlled by the paroxysmal flood,the revolving tilted effect and a gentle slope,we define it as a flood cause sub-lacustrine fan of composite transportation.In order to trace its provenance using clastic zircons dating analysis of sandstone samples,we selected samples from Chengdian sub-lacustrine and the Houzhuang delta area.The result shows：zircons microarea apparent ages of Chengdian sub-lacustrine and Houzhuang delta mainly similarly distribute between 115Ma and 166Ma with single peak and the magmatic zircons occupy the main part,with a few metamorphic zircons.Also,there are some different type zircons distributing in the age populations of 247-210Ma,475-412Ma,813-663Ma and 3489-1692Ma.In combination with regional geological settings and detritus zircon ages and heavy minerals composition,the authors come to the following preliminary conclusions： the sub-lacustrine sandbody derived from Houzhuang delta and its source rocks formation period is early Cretaceous to middle Jurassic,corresponding to Yanshan tectonic stage.Moreover,the source area is mainly Yanshanian granite in Kuanping-taowan tecnoic rock facies belt,including Yanshanian metamorphic rocks and the metamorphic rocks or magmatic rocks of Prejinningian period,Jinningian period,Caledonian period and Indosinian period may offer contributions to different degree.