塔里木盆地下志留统遗迹组构及其环境解释

LOWER SILURIAN ICHNOFABRICS OF TARIM BASIN AND THEIR ENVIRONMENTARY INTERPRETATION

遗迹组构是由生物成因改造的沉积岩组构 ,是物理过程和生物过程相互作用的产物。文中运用生物扰动指数与遗迹组构组分图解综合分析方法在塔里木盆地下志留统塔塔埃尔塔格组中建立 4个遗迹组构 ,即反映下临滨和滨外环境的Helminthopsis Diplocraterion复合遗迹组构 ,反映下临滨环境的Palaeophycus遗迹组构和Skolithos遗迹组构 ,反映中上临滨环境的Skolithos Arenicolites遗迹组构。对遗迹化石和生物扰动构造进行阶层分析 ,从中识别出受固底底质控制的遗迹化石Diplocraterion 。

Ichnofabric is a sedimentary rock fabric resulted from biogenic reworking and is the product of the interaction of physical and biological processes. Thus ichnofabric records the total extent of preserved bioturbation, including discrete trace fossils as well as “mottled' bedding. Bioturbation index is defined by the semiquantitive measurement of bioturbation extent in the sediments. Taylor and Goldring(1993)put forward a classification scheme of bioturbation index(see table Ⅰ), in which each grade of bioturbation is clearly defined in terms of burrow density, amount of burrow overlap and the sharpness of the original sedimentary fabric. Each grade is allocated a numerical value and a descriptive term so that the index can be graphically plotted on a logging proforma and used in description. A descriptive scheme was also proposed by Taylor and Goldring(1993) , that can lead to a ichnofabric constituent diagram, which visually portrays the detail of an ichnofabric. The constituent diagram is recorded on the horizontal axis, the percentage area occupied by the primary and secondary sedimentary structures, plotted to a log scale. On the vertical axis the ichnotaxa are plotted, where possible, in the order in which they were emplaced, and with their cross section dimension, following an indication of the nature of the primary sedimentary structures. Based on the synthetic analysis on bioturbation index and ichnofabric constituent diagram , three ichnofabrics have been recognized from the Tataaiertage Formation (Lower Silurian) of Tarim Basin, they are: Helminthopsis Diplocraterion composite ichnofabric , Palaeophycus ichnofabric and Skolithos ichnofabric.