230 / 2019-06-28 16:06:35

冀北坳陷宽城地区中元古界串岭沟组海相砂岩条带的岩石学和地球化学特征（Petrology and Geochemistry of Marine Sandstone Strips of Middle Proterozoic Chuanlinggou Formation in Kuancheng Area, Northern Hebei Depression）

砂岩条带;沉积环境;自生白云石;自生黄铁矿;串岭沟组;宽城地区,Sandstone strip; Sedimentary environment; Authigenic dolomite; Authigenic pyrite; Chuanlinggou Formation; Kuancheng District

冀北坳陷宽城地区中元古界串岭沟组页岩中普遍发育砂岩条带。通过岩心、薄片观察、碳氧同位素、有机碳含量(TOC)、总硫含量(TS)等分析手段，明确了砂岩条带的岩石学和地球化学特征，研究了碳酸盐矿物、黄铁矿的成因。结果表明：砂岩条带主要由较均一的细砂-粉砂级石英颗粒组成，碳酸盐矿物以微晶白云石为主。TOC在0.02~0.07％间，远低于同深度的黑色页岩，且存在小型球枕构造，表明砂岩条带是砂质碎屑在地震等机制触发下形成重力流随后长期埋藏压实形成的。黄铁矿以球状、草莓状黄铁矿为主，与白云石存在共生关系，TS相对高值显示二者形成于缺氧环境。因此，砂岩条带中的自生黄铁矿应是细菌硫酸盐还原(BSR)作用形成的HS^-与Fe2+反应形成的，这与甲烷渗漏区发育的ACP中均有丰富的黄铁矿有良好的可比性。自生碳酸盐的碳同位素(δ13C)均值为-4.28‰，较同时期碳酸盐岩δ13C相比有负偏，与甲烷厌氧氧化作用(AOM)成因的碳酸盐岩δ13C相比明显偏重，且随着与暗色页岩的距离增加δ13C出现正偏的趋势。因此，自生碳酸盐主要是孔隙水中碳酸盐沉淀形成，但明显受到AOM作用的影响。综合分析，宽城地区串岭沟组砂岩条带形成于富铁、缺氧的潮间泥坪。
Sandstone belts are generally developed in the shale of the Middle Proterozoic Cunlinggou formation in Kuancheng area of northern Hebei depression. The petrological and geochemical characteristics of sandstone belt are clarified, and the genesis of carbonate minerals and pyrite is studied by analytical methods of the observation to core and slice, carbon and oxygen isotopes, organic carbon content (TOC), total sulfur content (TS). The results show that the sandstone belt is mainly composed of homogeneous fine sand-silty quartz particles, and the carbonate minerals are mainly microcrystal dolomite. TOC is between 0.02 and 0.07%, which is much lower than the black shale of the same depth, and there are small ball-and-pillow structures, which indicate that It shows that sandstone strips are caused by sand debris triggered by earthquakes and other mechanisms to form gravity flow, and then formed by long-term buried compaction. Pyrite is mainly spherical and strawberry pyrite, and it has a symbiotic relationship with dolomite. The relative high value of TS shows that the two are formed in anoxic environment. Therefore, the authigenic pyrite in sandstone band should be formed by the reaction of HS^- with Fe2+ formed by the reaction of bacterial sulfate reduction (BSR), which is comparable to the abundant pyrite in ACP developed in methane leakage area. The mean value of carbon isotope (δ13C) of authigenic carbonate is - 4.28‰, which is negative compared with δ13C of carbonate rock during the same period, and is obviously heavier than that of carbonate rock formed by AOM, and the δ13C tends to be positive with the increase of distance from dark shale. Therefore, authigenic carbonates are mainly formed by carbonate precipitation in pore water, but are obviously influenced by AOM action. According to the comprehensive analysis, the sandstone strips of the Shunlinggou Formation in Guangcheng area are formed in the intertidal mud flat with iron rich and oxygen deficiency.