Page 202 - Sedimentology and Stratigraphy

Variations in Delta Morphology and Facies 189

*! $

$

$

Fig. 12.12 Gilbert-type deltas are coarse-grained deltas that prograde into deep water. They display a distinctive pattern of
steeply-dipping foreset beds sandwiched between horizontal topset and bottomset strata.

become remobilised to form debris flows and finer braided rivers and, in some cases, reworked by wave
sediment mixed with more water will generate tur- processes at the shoreline. In front of the topset lies
bidity currents. The lower part of the delta front the foreset (the delta front), which is very distinctive
may therefore be a site of deposition of these mass- because the beds are at a steep angle, typically up to
flow deposits, which may extend out on to the around 30 degrees and close to the angle of rest of
basin plain. granular material. Deposition on a foreset occurs by
two mechanisms (Nemec 1990b): debris flows of
poorly sorted gravel mixed with sand and mud, and
12.4.4 Coarse-grained deep-water deltas well-sorted gravels deposited by a grainflow (ava-
lanche) process (4.5.3). Slumping (18.1.1) is often
The combination of a supply of coarse sediment and a seen on the delta because the steep slopes of the
steep basin margin results in a particular delta form foresets can become unstable. At the base of the fore-
that is unlike all other deltas and therefore merits a set slope sediments are finer, comprising mud, sand
special mention (Fig. 12.11). They even have a special and some gravel, which lie approximately horizon-
name ‘Gilbert-type deltas’, named after the Amer- tally and are the products of turbidites and suspension
ican geologist G.K. Gilbert who first described deposits deposition in a prodelta setting, known in this context
of this type in 1895. Gilbert-type deltas have a char- as the bottomset.
acteristic three-part structure (Figs 12.12 & 12.13). As a Gilbert-type delta progrades, the foreset builds
The topset (the delta top) is a subaerial to shallow- out over the bottomset and in turn the foreset is
marine environment, with gravels deposited by overlain by topset facies: the resulting deposit is in