The autostratigraphic view of responses of river deltas to external forcing: a review of the concepts - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 139

The autostratigraphic view of responses of river deltas to external
forcing: a review of the concepts

TETSUJI MUTO* and RONALD J. STEEL †
* Graduate School of Fisheries Science and Environmental Studies, Nagasaki University, 1-14 Bunkyomachi,
Nagasaki 852-8521, Japan (E-mail: [e-mail])
† Department of Geological Sciences, University of Texas at Austin, 1 University Station, C-1100, Austin,
Texas, 78712, USA

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.

INTRODUCTION 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).

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

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