Geology Reference
In-Depth Information
a
b
Progradational: supply >> deficit
Aggradational: supply = deficit
barrier
island
barrier
island
back-barrier
tidal basin
salt
marsh
back-barrier
tidal basin
salt
marsh
MSL
MSL
c
d
Transgressive: supply < deficit
Transgressive: supply << deficit
barrier
island
barrier
island
back-barrier
tidal basin
back-barrier
tidal basin
salt
marsh
MSL
MSL
beach deposits
eolian dune deposits
overwash deposits
tidal deposits
salt marsh
Fig. 10.28
Four geological cross-sections of barrier island
depositional systems reflecting the stratigraphy produced by
particular sediment budget situations controlled by the inter-
action betwen relative sea-level rise and sediment supply
(Modified after Flemming
2002
). Note that the shoreward fining
sedimentary facies succession characterising the back-barrier
tidal flat deposits (
yellow
) are not shown. Legend to color
scheme includes beach, eolian dune, overwash, tidal flat (with
channels), is salt marsh facies, but excludes the upper and lower
shoreface
tectonic, isostatic and climatic processes acting either
together or independently of each other. The interac-
tion between the rate of relative sea-level rise and the
rate of sediment supply defines the stratigraphic
response of the system. Because sea-level fall leaves
barrier islands and back-barrier tidal flats stranded,
sustained accretion is usually associated with sea-level
rise. In this context three basic stratigraphic response
types can be distinguished, namely progradational,
aggradational and retrogradational ones. Each has a
characteristic and hence diagnostic stratigraphic
expression (Galloway and Hobday
1975
). The former
two occasionally act in conjunction to produce aggrad-
ing progradational systems. In the case of both pro-
grading and aggrading tidal flats, the entire depositional
sequence is conserved (total retention), while in the
retrogradational (transgressive) case varying parts of
the sequence are lost, depending on the overall sedi-
ment deficit. For this reason it is useful to distinguish
between partly conserved systems in which the deposi-
tional sequence is partially retained, and totally
reworked systems in which only the final high-stand
sequence is retained (Kraft
1971
).
The main four types of stratigraphic responses
outlined above are illustrated in the schematic cross-
sections of Fig.
10.28
. Each type represents a particular
sediment budget situation that reflects the stratigraphic
response resulting from the rate of sediment supply
from external sources relative to the deficit created by
sea-level rise over the same time interval. When keep-
ing one of the control parameters constant, a change in
the other will automatically affect the sediment budget,
as a consequence of which the stratigraphic response
changes from one state to another. It should be noted,
however, that the four examples presented here
represent time slices of particular sediment budget
situations and that, in nature, one may find transitional
systems reflecting budgets intermediate between any
two of these. Furthermore, in the course of time, the
sediment budget of a particular locality may change
and an existing depositional type will then grade into
another.
Cases a and b in Fig.
10.28
reflect the prograding
and aggrading stratigraphies that result from a positive
sediment budget where as much or more sediment is
imported from external sources than required for the
