Page 196 - Sedimentology and Stratigraphy

Delta Environments and Successions 183

delta environment in which these combinations of first, in shallow water close to the river mouth
processes occur. where it may be extensively reworked by wave and
tide action. The current from the river is dissipated
away from the channel mouth and wave energy
12.3.1 Delta-top subenvironments decreases with depth, leading to a pattern of progres-
sively finer material being deposited further away
Deltas are fed by a river or an alluvial fan and there is from the river mouth. This area, the delta slope, is
a transition between the area that is considered part often shown as a steep incline away from the delta
of the fluvial/alluvial environment and the region top, but the slope varies from only 18 or 28 in many
that is considered to be the delta top or delta plain fine-grained deltas to as much as 308 in some coarse-
(Fig. 12.5). Delta channels can be as variable in form grained deltas.
as a river and may be meandering or braided, single River-borne suspended load enters the relatively
or divided channels (9.2). Branching of the river still water of the lake or sea to form a sediment
channel into multiple courses is common, to create a plume in front of the delta. Fresh river water with a
distributary pattern of channels across the delta top. suspended load may have a lower density than saline
The coarsest delta-top facies are found in the chan- seawater and the plume of suspended fine particles
nels, where the flow is strong enough to transport and will be buoyant, spreading out away from the river
deposit bedload material. Adjacent to the channels are mouth. As mixing occurs deposition out of suspension
subaerial overbank areas (9.3), which are sites of occurs, with the finest, more buoyant particles travel-
sedimentation of suspended load when the channels ling furthest away from the delta front before being
flood. These may be vegetated under appropriate cli- deposited in the prodelta region. Gravity currents
matic conditions and in wet tropical regions large, may also bring coarser sediment down the delta
vegetated swamps may form on the delta top. These front and deposit material as turbidites (4.5.2).
may be sites for the accumulation of peat (3.6.1),
although if there is frequent overbank flow from the
channel the deposit will be a mixture of organic and 12.3.3 Deltaic successions
clastic material to form a carbonaceous mud. Cre-
vasse splays (9.3) may result in lens-shaped sandy The definition of a delta includes the concept of pro-
deposits on the delta top. gradation, that is, deposition results in the sediment
On deltas where the channels build out elongate body building out into the lake or sea. The sedimen-
lobes of sediment, sheltered areas of shallow water tary succession formed will therefore consist of pro-
may be protected from strong waves and currents. gressively shallower facies as the prodelta is overlain
These sheltered areas along the edge of the delta top by the delta front, which is in turn superposed by
are called interdistributary bays (Fig. 12.5) and mouth-bar and delta-top sediments. The succession
they are regions of low-energy sedimentation between formed by the progradation of a delta therefore has a
the lobes. The water may be brackish if there is suffi- shallowing-up pattern, a series of strata that consis-
cient influx of fresh water from the channel and over- tently shows evidence of the younger beds being
bank areas and the boundary between the floodplain deposited in shallower water than the older beds
and the interdistributary bays may be indistinct, espe- they overly (Fig. 12.6). In the delta-front subenviron-
cially if the delta top is swampy. ment the deepest water facies, the prodelta deposits,
are the finest grained as they are deposited in the
lowest energy setting. In a shallowing-up succession
12.3.2 Delta-front subenvironments they will be overlain by sediments of the delta slope,
which will tend to be a little coarser, and the shallow-
At the mouth of the channels the flow velocity is est facies will be those of the mouth bars, which are
abruptly reduced as the water enters the standing typically sandy or even gravelly sediment. The beds
water of the lake or sea. The delta front immediately formed by delta progradation will therefore show a
forward of the channel mouth is the site of deposi- coarsening-up pattern (4.2.5).
tion of bedload material as a subaqueous mouth The shallowing-up, coarsening-up pattern is one of
bar (Fig. 12.5). The coarsest sediment is deposited the distinctive characteristics of a deltaic succession,