Autogenic process change in modern deltas: lessons for the ancient - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 149

Autogenic process change in modern deltas: lessons
for the ancient

CORNEL OLARIU
The University of Texas at Austin, Department of Geological Sciences, 1 University Station,
C1100, Austin, TX 78712 (E-mail: [e-mail]); National Institute for Marine Geology and
Geoecology - GeoEcoMar, 23-25 Dimitrie Onciul Street, Bucharest 024053, Romania

ABSTRACT

River deltas are usually interpreted in terms of fluvial-dominated, wave-dominated or
tidal-dominated process regimes and are thought to undergo change which is mainly by
unsteady external forcing conditions. However, modern deltas have shown significant
autogenic process changes over relatively short temporal and spatial scales (hundreds to
thousands of years; km to 10s of km) during their Holocene progradation. These deltaic
stratigraphic responses were autogenic because late Holocene external forcing was rela-
tively steady (constant rates of relative sea-level rise and sediment supply). Responses
over longer time intervals would probably be allogenic due to the increased probability
of unsteady external forcing over longer time scales. Nevertheless, the short temporal
and spatial scales by themselves are not a condition for the autogenic response of deltaic
environments, but the steady external forcing is. The dominant depositional process can
change down depositional dip as the delta progrades across the shelf and into slightly
deeper water, or can change laterally because of shifting fluvial discharge or because of
oceanographic differences in front of distributaries. The three most common types of
process change seen in Holocene deltas are from fluvial-dominance to wave-dominance
(Mississippi and Danube examples) from tidal-dominance to wave-dominance (Mekong
example) or from fluvial-dominance to tidal-dominance (Mahakam example). Particular
segments of large delta complexes can also show these changes. The pervasive and rapid
process changes seen in Holocene deltas suggest that such changes were also common in
ancient deltas, expressed by changes in the character of the deltaic succession, espe-
cially on the delta front; which is the regime-defining segment of the delta. Such changes
should be considered more as a rule than an exception. Campanian and Maastrichtian
delta deposits from the Western Interior Seaway and Laramide Washakie Basin show
clear evidence of such changing process regimes. A reasonably good time framework in
the rock record is needed to aid discussion of whether observed stratigraphic responses
are autogenic or allogenic and it is important to make this distinction because of local or
regional implications for stratigraphy. A common theme seen in shelf-delta stratigraphy
is spatially extensive flooding surfaces bounding a deltaic complex of 1 ka to 300 ka
duration but with great lateral, between-lobe variability within such units. The main
flooding-surface bounded interval, the allogenic response, is the overall cross-shelf
regression and transgression of the delta complex, whereas the great internal variability
reflects autogenic spreading of the lobes during the cross-shelf transits.

Keywords: Autogenic, river deltas, stratigraphy, process change, fluvial-dominated
delta, tidal-dominated delta, wave-dominated delta.

INTRODUCTION Garrison 2004; Petter & Steel, 2006; Uroza & Steel

2008; Olariu et al ., 2010), wave-dominated (Plink-
Bjorklund, 2008; Charvin et al ., 2010) or tide-domi-
nated deltas (Mellere & Steel 2000; Willis et al ., 1999;
Bhattacharya & Willis, 2001; Willis & Gabel, 2001;

Ancient delta deposits are usually interpreted as
fluvial-dominated (Bhattacharya & Walker, 1991;
Barton, 1994; Mellere et al ., 2002; van den Berg &