From Depositional Systems to Sedimentary Successions on the Norwegian Continental Shelf, First Edition. Edited by A. W. Martinius, R. Ravnås, J. A. Howell, R. J. Steel, and J. P. Wonham. © 2014 International Association of Sedimentologists. Published 2014 by John Wiley & Sons, Ltd. 139 Int. Assoc. Sedimentol. Spec. Publ. (2014) 46, 139–148 The autostratigraphic view of responses of river deltas to external forcing: A review of the concepts T. MUTO* and R.J. STEEL † * Graduate School of Fisheries Science and Environmental Studies, Nagasaki University, 1-14 Bunkyomachi, Nagasaki 852-8521, Japan (E-mail: tmuto@nagasaki-u.ac.jp) † Department of Geological Sciences, University of Texas at Austin, 1 University Station, C-1100, Austin, Texas, 78712, USA INTRODUCTION A critical advance in our understanding of the geology of river deltas during the last 10 years comes partly from the clarification of the intrinsic stratigraphic responses resulting from external forcing (sea-level forcing, particularly) (Muto et al., 2007) and partly from rapid developments of experimental stratigraphy and geomorphology (Paola et al., 2009). In addition, the finding of deterministic autogenesis (e.g. Muto & Steel, 2002a) has been crucial because it clearly indi- cates the general lack of a quantitatively balanced state between the effect of relative sea-level rise and the effect of sediment supply to the deposi- tional system (Muto, 2001; Swenson & Muto, 2007). The opposite has been incorporated as a core hypothesis in most genetic stratigraphy models including sequence stratigraphy. Genetic stratigraphy that takes full account of deterministic autogenesis is referred to as auto- stratigraphy (Muto et al., 2007). The autostrati- graphic model of river deltas implies that (1) abrupt stratigraphic breaks are not necessarily associated with sudden changes in rate of base- level movement but can result from purely auto- genic responses of the system and (2) stratigraphic responses that fall on a break-free curve can result from an unsteady change in external forc- ing. These are expressions of non-equilibrium response. We review here some key aspects of the response of river deltas to external forcing (par- ticularly dynamic sea-level forcing). AUTOGENESIS AND EXTERNAL FORCING Autogenesis has conventionally been associated with responses that are local (a small part of the system), stochastic and cyclic, such as typically illustrated with river avulsion or delta-lobe switching (Beerbower, 1965; Edmonds et al., 2009; Straub et al., 2009). However, there is also another type of autogenesis that is global (i.e. the entire system), deterministic and non-cyclic, as has been noticed recently (Muto & Steel, 2002a; Muto et al., 2007; Paola et al., 2009). It is this latter type of ABSTRACT The geology of river deltas has long been based on the notion, here referred to as the ‘hypothesis of equilibrium response’, that steady external dynamic forcing gives rise to steady stratigraphic configuration. Though this equilibrium response is physically pos- sible, the hypothesis does not hold true in general and can cause serious misinterpreta- tion of the stratigraphic record. Autostratigraphy takes a totally different view which takes full account of non-equilibrium responses and accompanying deterministic autogenesis, suggesting that (1) steady dynamic external forcing generally results in unsteady stratigraphic response and (2) steady stratigraphic configuration can result from unsteady dynamic external forcing. Further exploration of non-equilibrium responses of river deltas to external forcing will bring more clarity to their stratigraphy. Keywords: Autogenesis, autostratigraphy, deterministic, equilibrium, forcing, non-equilibrium, river deltas, sea level, stratigraphic response. 0002112076.INDD 139 3/25/2014 7:40:55 PM UNCORRECTED PROOFS