Responses of River Deltas to Sea-Level and Supply Forcing: Autostratigraphic View

Abstract

A long-standing geological notion that dates back to Huttonian theory of the late 18th century (Schlager, 1993) suggests that (1) there can exist a balanced state between the effect of relative sea level rise and the effect of sediment supply to the depositional system, for example evidenced by coastal aggradation and a vertical shoreline trajectory, and that (2) regression and transgression reflect imbalances between the two primary drivers: i.e. regression when sediment supply dominates sea level rise, and transgression when sea level rise dominates sediment supply (Fig. 1A). More specifically, it has been taken as axiomatic that given steady external forcing by constant sediment supply (rate QS) and constant relative sea level rise (rate Rslr), a river delta grows to achieve an equilibrium configuration, produces a particular sediment-stacking pattern and maintains a constant rate of shoreline migration in a particular direction (Weller, 1960; Van Andel & Curray, 1960; Sloss, 1962; Curray, 1964; Swift, 1968; Swift et al., 1971; Curtis, 1970; Vail et al., 1977; Mitchum et al. 1977; Brown & Fisher, 1977; Posamentier et al., 1988; Galloway, 1989; Swift & Thorne, 1991; Shanley & McCabe, 1994; Stanley & Warne, 1994; Myers & Milton, 1996; Neal & Abreu, 2009). We refer to this mode of stratigraphic response as equilibrium response, by which steady external forcing results in steady stratigraphic pattern of deposition. Autostratigraphy, a fairly new arrival in the field of geology, suggests that this presumed mode of stratigraphic response does not hold true in general, but instead that (1) even with steady forcing, river deltas generally fail to sustain a constant and uniform stratigraphic pattern of deposition (Fig. 1B), and (2) unsteady forcing can result in uniform stratigraphic configuration. Exploring such nonequilibrium responses (see below) is essential if we are to elucidate the complex stratigraphy that river deltas produce at different time scales. Introducing principles of autostratigraphy and related basic notions, the present chapter outlines these recent discoveries and gives a synthetic understanding of the origin of regression and transgression and of aggradation and degradation in deltaic settings.