『Abstract
There is evidence from the stratigraphic record of examples of
fluvial deposits that were the products of deposition from river
systems which had decreasing discharge down-flow and transitions
from proximal, channelised to distal, unconfined flow. These deposits
form fan-shaped bodies several tens of kilometres in radius, and
their stratigraphic architecture is aggradational, with no evidence
of deep incision driven by base-level fall. The fluvial systems
that generated these deposits formed under conditions for which
there is no complete analogue today: an endorheic basin with a
relatively arid climate adjacent to an uplifted area with higher
precipitation. A conceptual model for fluvial systems of this
type has therefore been built on the basis of outcrop examples
and a consideration of the controls on sedimentation. Proximal
areas are characterised by amalgamated coarse, pebbly and sandy
channel deposits with little preservation of overbank facies.
Channel dimensions are generally smaller in the medial areas,
but sizes are variable: deposits are of braided, meandering and
simple channels which show varying degrees of lateral migration.
The channel-fills may be mud or sand, with overbank flow processes
playing an important role in filling channels abandoned on the
floodplain after avulsion. The proportion of overbank deposits
increases distally with sheets of sand deposited as lateral and
terminal splays by unconfined flow. Interconnection of sandstone
bodies is poor in the distal areas because channel-fill bodies
are sparse, small and are not deeply incised. The radial pattern
of the sediment body forms by the repeated avulsion of channels:
active channels build up lobes on the alluvial plain and rivers
switch position to follow courses on lower lying areas. The term
‘fluvial distributary system’ is here used to describe a river
system which has a downstream decrease in discharge and has a
distal zone which is characterised either by terminal splays on
to a dry alluvial plain or a lake delta during periods of lake
highstand.
Keywords: Fluvial distributary system; Terminal fans; Fluvial
fans; Endorheic basins; Fluvial stratigraphic architecture』
Contents
1. Introduction
2. Characteristics of a fluvial distributary system
2.1. Dimensions of the sedimentary unit
2.2. Proximal facies
2.3. Medial facies
2.4. Distal facies
2.5. Distributary pattern
3. Fluvial channel and overbank processes in a distributary system
3.1. Trends in fluvial channels
3.2. Discharge variations
3.3. Floodplain deposition
3.4. Floodplain channel-fills
3.5. Distal zone sheets: terminal splays and floodouts
3.6. Formation of a fan of fluvial deposits
3.7. Bifurcation of channels
3.8. Proximal to distal extent of channels
4. Fluvial distributary systems and lakes
5. Conditions for the formation of a fluvial distributary system:
tectonic and climatic setting
6. Fluvial distributary systems, terminal fans, fluvial fans,
megafans and lake deltas
6.1. Alluvial fans, fluvial fans, megafans, humid fans
6.2. Subaerial and ‘losimean’ fans
6.3. Terminal fans
6.4. Lake deltas
6.5. Fluvial distributary systems
7. The stratigraphic architecture of fluvial distributary system
deposits
8. Conclusions
Acknowledgements
References