【作者】 杨荣军；

【导师】 刘树根；

【作者基本信息】 成都理工大学 ， 矿产普查与勘探， 2009， 博士

【摘要】 四川盆地西部上三叠统马鞍塘组硅质海绵礁是世界上出露规模最大的同类礁体,同时马鞍塘组是四川盆地海相碳酸盐岩最晚的层位,故对该生物礁及其地层的研究,不仅具有重要的理论意义,而且对探讨四川盆地的演化也有重要价值。论文对四川盆地西部22个上三叠统马鞍塘组生物礁进行了细致观察,并对其中10个生物礁进行了详细的剖面测制和取样。通过剖面测制、系统采样及薄片的分析,认为川西卡尼期生物礁是以六射海绵为主的硅质海绵动物与以凝块石为主的蓝藻细菌共同在同斜缓坡中的较深水海底(30-200m)营造起来的生物礁丘(生物丘),主要造礁方式为障积作用。四川盆地西部上三叠统马鞍塘组硅质海绵礁具有明显的地形突起,礁高变化较大,从2米到70米不等。整个礁群出露区整体沿龙门山带分布,北东向约65.6km,垂直于龙门山的宽度约为1-2km。礁中主要发育海绵、海百合、有孔虫、腕足、双壳等,中心可以见到近原地生长的海绵群体。该礁组合亚相带特征明显,下部礁基为灰白色层状鲕粒灰岩,有时顶部还发育厚度相对较薄的亮晶生屑灰岩,鲕滩厚度从30-60m不等;上部礁核、礁翼、礁间均为青灰色-黑灰色含生屑微晶灰岩,三者中生物种类区别不明显,但化石产出状态有差异;礁间具有成层性,厚度比礁体薄得多,礁核、礁翼为块状构造,风化后礁翼相对疏松,礁核发育较多群体海绵化石;礁顶部为黑色层状泥岩,主要发育双壳类,与生物礁生物群落差别较大。卡尼期三级层序界面之下的生物礁-滩组合的统一粉砂质泥岩盖层致使致四川盆地西部卡尼期生物礁消亡。马鞍塘组生物礁在时间和空间上生长、发育、绝灭可以反应四川盆地的演化过程:马鞍塘组生物礁礁基为浅水高能的鲕粒灰岩,极少受到陆源碎屑物质的干扰,随后水体变深,硅质海绵生物礁开始适应环境并发育在鲕粒灰岩之上,在生物礁发育过程中,随着陆源碎屑颗粒逐步增多并水体变深,水体亦变得浑浊、闭塞,生物礁随之绝灭,最终转变成小塘子组泥质粉砂岩沉积。这说明生物礁产生、绝灭的过程和盆地周围的造山带隆起密切相关。以生物礁为中心,沿龙门山展布的马鞍塘组剖面上看:川西马鞍塘组已不是统一的碳酸盐岩沉积,在川西北马鞍塘组上部碎屑岩开始增多,说明北面的山脉已经成为物源区;中部生物礁区下部是鲕粒灰岩,礁的中上部也可以见到碎屑条带;而在向南的龙深1井,鲕滩较薄,上部主要为黑色泥岩;三个区域的上覆地层均为小塘子组粉砂质泥岩。这个过程与北部秦岭和龙门山的逐步隆起相呼应。马鞍塘组出露的礁滩经历了浅海海底-深海海底-早成岩-中成岩-晚成岩-表生等成岩阶段,没有明显早期暴露的标志。礁滩中原生孔隙在早成岩阶段末期基本被压实充填,次生孔隙发育不明显,所以马鞍塘组礁滩孔渗性较差。但通过川西地震资料的解释,认为川西地腹马鞍塘组近尖灭处有丘状隆起,具有礁型隆起的特征。川西地区可能存在类似欧洲牛津期特提斯域中缓坡上生物礁序列,地震中的丘形隆起可能为高能生物礁体,可能为油气勘探的有利目标。更多还原

【Abstract】 The siliceous sponge reefs in the upper Traissic Maantang Formation in the western Sichuan basin are the largest ones in the world. Meanwhile the Maantang Formation is also the latest marine carbonate layer in the Sichuan basin. So the study on the Maantang Fm., especially on the reefs within the formation, is of great significance to the characteristics of siliceous sponge reefs in the world and the evolution of the Sichuan basin.The dissertation has the investigations on 22 reefs in the upper Triassic Maantang Fm., 10 ones of which are put detailed description, section measure and sampling. Through measured sections, systematic sampling and analysis of the thin sections, it is concluded the carnian reefs is mainly constructed by the hexactinellid sponge and the cyanobacteria in thrombolites, which based on the homoclinal carbonate ramp from 30m to 200m below the sea level and whose primary reef-building process is baffling. The fossils are commonly observed, such as sponge, crinoid, foraminifer, brachiopod, bivalve, bryozoan, worm, in these reefs, in which essentially in place sponge groups are revealed. The subfacies in the reef complex could be easily distinguished. The reef base is ca. 30-60m in thickness and mostly composed of gray bedded oolitic limestone, sometimes relatively thin sparry bioclastic limestone in the top of it. The reef core, the reef flank and the interreef are composed of caesious or black gray bioclastic limestone, and the kind of their fossils is not obviously different but their occurrence state is different each other. The interreef is bedded and their thickness is thinner than the reef core’s and the reef flank’s, which are block. The rock of the reef flank is looser the one of the reef core after they are weathered enough. The reef top is composed of black bedded mudstone, in which bivalves are common seen and the fauna in which varied from the one in the reef core & flank. The silty mudstone under the Carnian third-sequence boundary induced the extinction of the reefs in the Maantang Fm.The reefs have a relative topographic relief comparing the contemporaneous sea floor, and the height of every reef is from 2m to 70m. The outcropping area of the reefs is along the Longmenshan Belt and the length of the area is ca. 65.6km in northeast direction, and is 1-2km in width.The growing, developing and extinction of the reefs in the Maantang Fm. may indicate the evolution process of the Sichuan basin and the adjacent area. The reef base (i.e., the oolitic bank) develops in shallow water and high energy without terrigenous material. Then the increasing terrigenous material is deposited and the depth of the water became deeper. The surroundings around reefs turned into be feculent and be closed so that the reefs died away. Consequently the silty mudstone in the Xiaotangzi Fm. is deposited. The process could coincide with the movement of the orogenic belt on the edge of the Sichuan basin. The sediment is not exclusively carbonate during Carnian in the western Sichuan basin. The northwestern Sichuan basin has more and more terrigenous material up to the top of the Maantang Fm. That suggested the mountain of the northern Sichuan basin has uplifted and become the provenance. The upper part of the Maantang Fm. is also increasingly influenced by terrigenous material. And the Maantang Fm. in the Longshen Well 1 includes the lower part (the thin layer oolitic bank) and the upper part (a large suite of mudstone). The overlying formation of all the sections of the Maantang Fm. is the Xiaotangzi Fm., main including silty mudstone. The sedimentary process should be related to the gradual raise of the Qinlin old land and the Longmenshan orogen belt.The Bank/Reef of the Maantang Formation underwent shallow sea, deep sea, early diagenesis, middle diagenesis, late diagenesis, epidagenetic stage. There is not obviously early exposed in the Maantang Fm. The primary pores of the Bank & reef have been almost filled and the secondary pores do not develop well. So the porosity & permeability of the Maantang Fm. is considerably low. For the analysis of the samples from the outcrops, the Bank/Reef only reaches the level of non-reservoir or low yield-reservoir. However it is similar between the siliceous reefs in the western Sicuan basin and the Oxfordian siliceous reefs in Europe. Comparing with the serials of the Oxfordian reef, the shallow-sea, high-energy reef may be discovered near the central Sichuan Basin in that depression. They should be a target for oil/gas exploration.更多还原