四川盆地安岳地区须家河组--近海潮汐沉积

PARALIC TIDAL DEPOSITS IN THE UPPER TRIASSIC XUJIAHE FORMATION IN ANYUE AREA,THE SICHUAN BASIN

多年来,大家都公认四川盆地须家河组除底部须一段为海相沉积以外,其上各段均属湖泊—三角洲沉积相。可是通过2007年对多个露头剖面和钻井岩心的研究,发现四川盆地安岳地区须二段—须六段均为碎屑浅海产物。为此,在介绍潮汐沉积作用原理的基础上,详细分析了须家河组岩性构造特征,从双向交错层和交错纹层、砂泥岩薄互层层理、双黏土层构造、水流突变特征等4个方面论证了该组地层为潮汐成因的事实,露头和岩心特征也有力的证明了这一点。在此基础上,分析了须家河组的沉积相、沉积微相以及沉积模式。结果表明,研究区须家河组除边缘分布有三角洲沉积相以外,其余广大面积都属于浅海相大潮差河口湾沉积亚相,包括潮下、潮间和潮上带等多种沉积微相。

It has been generally accepted for many years that the Xujiahe Formation(T3x)in the Sichuan basin is of lacustrine-delta deposits in origin,except for the first member at its basal part which is attributed to marine deposits.However,studies on several outcrop profiles and core data in 2007 revealed that the 2nd-6th members of Xujiahe Fm in the study area are all clastic deposits of shallow marine facies,dominated by estuary deposits developed in an environment with a large tidal range.Beginning with introducing the basic principles of tidal deposition,this paper analyzes in detail the lithologic and structural features of the Xujiahe Fm.Its tidal origin is evidenced by bimodal cross bedding and cross lamination,interlayered bedding of thin sand and shale,dual clay ground structure,abrupt change of flow,as well as outcrop and core data.Analyses of the sedimentary facies,microfacies and models of the Xujiahe Fm show that most of the Xujiahe Fm is of shallow marine estuary subfacies with a large tidal range consisting of various microfacies such as supratidal,intertidal,subtidal zones,except for the margin where delta facies occur.