细粒浊积岩特征、分布及发育机制:以川北唐家河剖面寒武系郭家坝组为例*

细粒浊积岩是一类较为特殊的浊积岩类型,与可用鲍马序列解释的完整经典浊积岩相比,其颗粒粒径明显偏细,小于63μm的颗粒含量超过50%。通过对四川盆地北部唐家河剖面寒武系第二统郭家坝组上部地层的系统研究发现,Stow细粒浊积岩序列发育广泛。该序列下部(T0-T3)主要为极细砂、粉砂与泥质纹层互层,发育微弱冲刷构造和泄水构造等软沉积物变形构造;中部(T4-T6)为泥质层段夹断续粉砂质纹层;上部(T7-T8)转变为均质泥岩段,生物扰动构造发育,整体序列具有正粒序特征。浊积岩序列中碎屑颗粒主要为石英、岩屑和云母,长石少见,杂基含量高;粒度频率分布曲线(直方图)显示颗粒分选性比传统浊积岩好,且自下而上各砂质纹层内颗粒粒径变细;概率值累计曲线表现为“一段式”和“上凸的折线式”2种,且悬浮组分占据了绝大优势;C-M图以平行于“C=M”基线为特征,但受碎屑颗粒粒级偏小的影响,投点整体沿C=M基线向左下偏移,显示该序列具有浊流递变悬浮沉积的特点。结合筇竹寺阶(第二统)沉积背景,认为靠近研究区西北侧的摩天岭古陆为浊积岩发育的主要物源区,台内地形的遮挡为细粒浊积岩的发育提供了良好的地形条件,悬浮搬运和阵发性环境事件(如风暴、洪水等)搬运为细粒沉积物主要的搬运机制。以唐家河剖面为代表的寒武系郭家坝组细粒浊积岩的广泛发育,对于认识四川盆地北缘古地理格局和非常规油气勘探具有重要意义。

Characteristics,distribution and mechanisms of fine-grained turbidite: A case study from the Cambrian Guojiaba Formation in Tangjiahe Section,Northern Sichuan Basin

The fine-grained turbidites are different from traditional turbidites described by A. H. Bouma in light of their much finer particle sizes(more than 50% lower than 63μm)and diagnostic features in sedimentary sequences. Study on the upper Guojiaba Formation of lower Cambrian at Tangjiahe Section shows that the fine-grained turbidite system containing typical Stow fine-grained turbidite sequences are widely developed at the Northern Sichuan Basin. The lower sequence(T0-T3)of fine-grained turbidite is stacked by fine-sand,silty and muddy laminae in ascending order,and featured by scour structures and some soft-sediment deformation structures(e.g., dish-like water escape structure). The middle sequence(T4-T6)is comprised of muddy laminae embedded with several discontinuous silty laminae. The upper sequence(T7)is consisted of homogeneous muddy laminae,and overlain by abundant bioturbation structures(T8). Meanwhile,the fine-grained sequences show clear normal grading. Therein,the siliciclastic grains are mainly composed of quartzes,micas and lithic fragments,with few feldspars. The grain size frequency distribution curves(histograms)exhibit that the grain sorting is better than traditional sand-rich turbidite,and grain sizes decreasing gradually from sandy laminae from lower to upper sequence. In addition,the frequency grain sizes show that the types of “one line”and “convex broken line”with the suspended components are dominant. The characteristics of C and M values are parallel with the “C=M”baseline suggesting a feature of suspended sediments in the system of turbidity. Moreover,both C and M values shift to the lower left zone(but also along with the “C=M”line)due to the relatively small grain sizes. In consideration of the sedimentary background of the Qiongzhusi Stage(Series 2),the Motianling Old Land located in northwest direction is probably the main sediment source. The interior and marginal topographies of neighboring mixed siliciclastic-carbonate shallow deposits may provide a favorable topographic condition for the transportation of fine-grained sediments to the shelf. Suspended transportation and paroxysmal events,such as storms and floods,are possibly the main influencing factors for the deposition of fine-grained sediments. The study of fine-grained turbidite from the Guojiaba Formation would be in favor of understanding the lower Cambrian paleogeographic condition,and would have potential value for unconventional oil and gas exploration on the Northern Sichuan Basin.