大尺度自成因机制与自成因地层学

Large-scale autogenic stratigraphic mechanisms and autostratigraphy

在海平面升降、构造活动、沉积物供给等外部驱动作用下,沉积地层的叠置受自成因和他成因两种机制的控制。近年的认识表明,自成因机制可发生在沉积过程的各个时空尺度。不同于小尺度自成因机制仅发生在沉积系统的局部,大尺度自成因机制可在盆地范围内系统性地控制地层的发育,能够修饰甚至改变他成因过程。要想准确地探究地层的叠置样式与外部驱动条件之间的关系,必须有效地鉴别大尺度自成因与他成因过程。基于对大尺度自成因过程的认识,逐渐建立与发展了自成因地层学。它提出的自成因与他成因观、平衡响应与非平衡响应观为解析地层叠置样式与外部驱动条件之间的关系提供了新的视角。自成因地层学一方面对外部驱动条件分为稳定的(速率等作用方式保持不变)与不稳定的(速率等作用方式发生改变),由前者形成的地层叠加样式(或过程)称为自成因的,后者形成的地层叠加样式(或过程)称为他成因的。另一方面也将地层叠置样式分为稳定的(加积速率与进积速率的比值R_(agg)/R_(pro)保持不变)与不稳定的(R_(agg)/R_(pro)发生改变)。若稳定的外部驱动条件形成稳定的地层叠加样式,称为平衡响应,是自成因过程;稳定的外部驱动条件形成不稳定的地层叠加样式称为自成因的非平衡响应。此外,不稳定的外部驱动条件也可能形成稳定的地层叠加样式,称为他成因的非平衡响应。自成因地层学认为,由于自成因非平衡响应机制的普遍存在,地层的叠加过程通常表现得不稳定。相比而言,稳定的地层叠加样式仅在特殊情况下才能发生。传统成因地层学应当重视大尺度的自成因机制。

In response to external forcings such like eustatic changes,tectonic activities,and sediment supply,the stacking of strata is controlled by both autogenic and allogenic mechanisms.One of the main progresses in recent years is a realization that autogenic ones may take place in a wide variety of spatio-temporal scales during the deposition.Unlike small-scale autogenic mechanisms that happens on local positions of the depositional system,large-scale ones control the formation of strata systematically in regional scale,which might modify and even change the allogenic processes.To check the relationship between strata stacking patterns and external forcings,large-scale autogenic processes must be determined from allogenic ones.Based on the understanding of large-scale autogenesis,autostratigraphy has developed in recent years.Autostratigraphy provides new perspectives in analyzing the relationships between strata stacking patterns and external forcings,based on two pairs of coupled notions,i.e.autogenesis and allogenesis,equilibrium and non-equilibrium responses.Autostratigraphy defines external forcings of constant rates as steady,and those of changing rates as unsteady.Strata stacking patterns or processes controlled by steady external forcings are considered as autogenesis,while by unsteady external forcings are considered as allogenesis.Autostratigraphy also defines steady strata stacking patterns or processes where the ratio of aggradation rate and progradation rate(i.e.R agg/R pro)remains constant during the interested period.Otherwise,the strata stacking patterns or processes are considered as unsteady.Equilibrium response is then defined as a response where steady external forcings form steady strata stacking patterns and thus is autogenic.If steady external forcings form unsteady strata stacking patterns,this response is called non-equilibrium response and is also autogenic.Moreover,unsteady external forcings may also form steady strata stacking patterns,which is also a kind of non-equilibrium response but is allogenic.Under the framework of autostratigraphy,the stacking up of strata generally takes the large-scale autogenic non-equilibrium responses and thus usually exhibits an unsteady pattern.In contrast,steady stacking patterns of strata only takes places under very rare conditions.The acquiescence of equilibrium response between strata stacking patterns and external forcings and its corollary in conventional genetic stratigraphy should be revisited.