东海陆架盆地西湖凹陷平湖组潮汐韵律与潮汐周期分析

Analysis of tidal rhythmites and tidal cycle of Pinghu Formation in Xihu Sag, East China Sea Shelf Basin

为了分析东海陆架盆地西湖凹陷平湖组潮汐韵律与潮汐周期,首先,通过精细岩心观察和描述,识别平湖组A井潮汐韵律层的沉积构造类型,明确沉积环境;其次,采用Matlab数字图像的灰度测量技术以及频谱分析等方法,研究平湖组A井潮汐韵律层的周期性;第三,根据统计的毫米级纹层厚度数据序列,将砂质纹层、泥质纹层以及层偶的厚度进行傅里叶变换,定量对比三者的厚度变化周期;最后,将厚度变化周期与潮汐周期相关联,探讨不同类型潮汐周期的识别依据。研究结果表明:平湖组A井岩心中共发育砂泥复合层理、双黏土层以及砂泥对偶层等3种沉积构造类型,沉积环境主要为半封闭的潮坪,沉积相带包括潮间带的泥坪、混合坪、砂坪以及潮下带的潮汐水道;砂质纹层、泥质纹层以及层偶厚度的周期性具有一定的差异,这种差异来自于沉积时的潮流周期性质及其大小等因素的影响;平湖组A井识别出18.52个和9.62个这2个主要反映潮流周期特征的纹层组厚度旋回,将平湖组A井划分为年级、十年级以及百年级3类潮汐周期韵律,各级次周期韵律对应的纹层组数表现为相邻2个纹层组(波峰)代表年级潮汐周期(3 a),18.52个和9.62个这2个层偶旋回均代表十年级潮汐周期(分别为30 a和15 a),平湖组A井整段代表百年级潮汐周期(120 a)。

In order to analyze tidal rhythmites and tidal cycle of Pinghu Formation in Xihu Sag, East China Sea Shelf Basin, firstly, the sedimentary structure type of tidal rhythmites in Well A of Pinghu Formation was identified through detailed observation and description of the core, and the sedimentary environment was defined.Secondly, the ancient cyclic tidal rhythmites of Well A were studied by grayscale measurement technology of Matlab digital image and spectral analysis. Thirdly, according to the statistical millimeter-level layer thickness data sequence, Fourier transform was performed on sandy layer thickness, muddy layer thickness and couplet thickness. Finally, the cycles of thickness variation with tidal cycles were quantitatively compared and analyzed,and the identification basis of different types of tidal cycles was discussed. The results show that the sedimentary structures include interbedded sand and mud, double clay layer and couplet in the core of Well A of Pinghu Formation. The sedimentary environment is mainly manifested as semi-closed tidal flat deposition, and the sedimentary facies zone include mud flat, mixed flat, sand flat in intertidal zone and tidal channel in subtidal zone.The cycles of sandy laminae, muddy laminae and couplet thickness are certain different, which comes from the influence of factors such as the nature and size of tidal cycles during deposition. Two main laminar group thickness cycles of 18.52 and 9.62 that reflect tidal cycle characteristics are identified in Well A of Pinghu Formation. Well A of Pinghu Formation is divided into three tidal cycle types, including Grade year, Grade decade and Grade century. The number of laminar groups corresponding to each level of tidal cycle is the two adjacent laminar groups(wave crests), which represents a Grade year tide cycle(3 a). 18.52 and 9.62 layer even cycles both represent Grade decade tide cycle(30 a and 15 a, respectively), and the entire section of Well A in the Pinghu Formation represents the Grade century tide cycle(120 a).