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1 正常海退与强制海退在Van Wagoner等(1987)的早期层序地层学概念中,体系域被定义为一连串同期的沉积体系。他们将层序划分为Ⅰ型层序(包括低水位、海进和高水位体系域)和Ⅱ型层序(包括陆架边缘、海进和高水位体系域)。但随着层序地层学的应用推广到被动边缘以外的地区,这些概念、术语和地层模式都有了变化,如Ⅱ型层序的概念已基本被废弃,而强制海退的沉积模式被提出来了。根据以往的概念及对现代和古代沉积物的研究,可识别出2种海退,即正常海退和强制海退。海平面的相对变化受到容存空间及沉积物供给量的影响,故海退发生于2种特定条件下:(1)输入海岸的沉积量超过容存空间的增加量,即(△Vs/△t)>(△Va/△t)式中Vs代表供给海岸的沉积物体积,Va代表沉积物作用的现有空间,t为时间;(2)相对海平面下降。这2种情况下海岸线均向海迁移(图1)。
1 Normal Retreat and Forced Retreat In the concept of early sequence stratigraphy of Van Wagoner et al. (1987), the system domain was defined as a series of sedimentary systems of the same period. They classify the sequence into type I sequences (including low-level, marine-derived and high-level system tract) and type II sequences (including continental margin, marine and highstand systems). However, as the application of sequence stratigraphy extends beyond the passive margin, these concepts, terminology and stratigraphic patterns have changed. For example, the concept of type Ⅱ sequence has been basically abandoned, while the forced regression model has been proposed Out. Based on past concepts and studies of modern and ancient sediments, two types of regression are identified: normal regression and forced regression. Recession occurs under two specific conditions: (1) the sediment input to the coast exceeds the increase of the storage space, ie, (ΔVs / △ (ΔVa / Δt) where Vs represents the volume of sediment supplied to the coast, Va represents the existing space for sediment action, t is the time, and (2) the relative sea level declines. In both cases, the coastline is moving towards the sea (Figure 1).