Page 197 - Sedimentology and Stratigraphy

184 Deltas

Fig. 12.6 A cross-section across a delta
lobe: progradation results in a coarsening-
up succession.

but can be considered to be more diagnostic of a delta river dominated, the Rhone Delta is mixed sand and
only if the top of the succession shows a transition mud and is wave-dominated, the Skeidarasandur is
from deposition in subaqueous to subaerial environ- mainly gravelly with river and wave influence, and
ments. Evidence of deposition on the delta top may be so on. Even with all the possible positions within that
the recognition of a river channel, signs of plant plot, there is also the additional variable of water
growth and soil formation or other exclusively sub- depth to be added. Every modern delta will have
aerial physical, chemical or biological processes, such individual characteristics due to the different factors
as desiccation cracks or tracks of land animals. The controlling its form, and it may be expected that the
sedimentary logs of different deltas illustrated in this deposits of ancient deltas will be similarly variable.
chapter all show this same, basic pattern, despite A simple, neat classification of deltas into a small
differences in the processes and setting in each case. number of types will represent only a small propor-
One caveat must be added: a coarsening-up marine tion of the possible forms, so it is more instructive to
succession capped by continental facies can be formed consider the effects of different factors on the delta
at a coastline where sediment is supplied by marine morphology and consequently on delta facies. An
processes (13.6.3), so it is important to establish that individual modern or ancient delta is likely to display
there is evidence of a river or alluvial fan supplying a combination of the features shown in the following
sediment from the land if the succession is to be sections.
reliably interpreted as a delta deposit.

12.4.1 Effects of grain size:
12.4 VARIATIONS IN DELTA fine-grained deltas
MORPHOLOGY AND FACIES
The deposits on a delta will include a high proportion
The combinations of factors that control delta of fine-grained material if the fluvial system supplying
morphologies give rise to a wide spectrum of possible it is a mixed-load river (9.2.2). Low gradient, mixed-
delta characteristics. Modern deltas provide examples load river channels characterise the lower tracts of
of a number of positions within the ‘toblerone plot’ in large river systems. Large rivers like these carry sedi-
Fig. 12.3: the Mississippi Delta is fine-grained and ment that is delivered to the delta as sandy bedload