四川龙门山区下泥盆统的介壳岩形成模式及其意义

四川龙门山区(前山)早泥盆世地层中,特别是在白柳坪、甘溪、谢家湾三组地层中,含有相当数量的介壳层。它包括呈两向伸展的介壳毯和三向伸展的介壳层(包括透镜体),据在地层中产出的层位,自身的特点及其与围岩的关系等方面综合分析,表明介壳毯形成于正常波基面向海一侧,介壳岩则是位于波基面以下,正常风暴浪基面附近向海一侧的部位;前者主要为簸扬作用和沉积速率快慢相间的幕式沉积过程的结果,后者则和风暴作用有关,属近积风暴岩,介壳岩的研究不仅对正确划分白柳坪、甘溪、谢家湾三组提供了充分的岩性依据,而且也为正确解释古环境及盆地沉积史提供大量可靠依据。

THE GENETIC MODELS AND IMPLICATION OF SHELL BEDS FROM THE LOWER DEVONIAN OF THE LONGMENSHAN AREA, SICHUAB, CHINA

There are a number of shell beds in the Devonian from the Longmenshan Area of Northwestern Sichuan, especial in the Bailuping, Ganxi, Xiejiawan Formations of Lower Devonian. It can be divided into two typies i. e. 1)Pavements:thin virtualy two dimensional bedding plane concentrations; 2)shell deds: Comcentrations with three-dimensional extension According to the characteristic features of the shell beds,host sediments and the relationship between each other. The implicatim for palaeoecology of pavement is the followng. Most species which dominate the pavement are oppertunistic species,the depositional process was discontinous,i. e. sedimentation prevailed;the mechanism of pavement formatiom by starvation and sediment bypassing; the substrate was silty of muddy with water-saturated, variable turbidity soupy-mudi. The water depth weer near but below fair water wavebase;the faunas clearly correspond to the concept of "timeaveraged" Palaeocommunity. The implication of Palaeoecology of shell beds is as follows. The shell beds are mostly strom-generated, some of them are typical tempestites; They are complex in origin and generated by more than one storm event; some were (?)nger,some are weaker. They affected the same area in different times; The Depth of water in which shell beds formed was near and below average storm wave base. According to some well preserved, commony articulated shells at the top of some shell beds,the short-term ecologic succession and taphonomic feedback can be defined. The Genetic model of Parements and shell beds are shown in the figures 1,2 of the text,respectively.