Page 254 - Sedimentology and Stratigraphy

Types of Carbonate Platform 241

basin. The back-reef facies near to the barrier may
Rimmed carbonate shelf
experience relatively high wave energy resulting
in the formation of grainstones of carbonate sand
and skeletal debris reworked from the reef. Further Scale Lithology LIMESTONES bound Structures etc Notes
wacke
inshore the energy is lower and the deposits are mud pack grain rud &
mainly wackestones and mudstones. However, ooidal
and peloidal complexes may also occur in the shelf
lagoon and patch reefs can also form. In inner shelf
areas with very limited circulation and under condi- Reef boundstone
tions of raised salinities the fauna tends to be very
restricted. In arid regions evaporite precipitation may
become prominent in the shelf lagoon if the barrier
provides an effective restriction to the circulation of
seawater.

Rimmed carbonate shelf successions
Reef front rudstone
As deposition occurs on the rimmed shelf under breccia
conditions of static or slowly rising sea level the
whole complex progrades. The reef core builds out
over the fore reef and back-reef to lagoon facies overlie
the reef bioherm (Fig. 15.18). Distally the slope de-
posits of the fore reef prograde over deeper water
facies comprising pelagic carbonate mud and calcar-
eous turbidite deposits. The steep depositional slope
Fore-reef slope
of the fore reef creates a clinoform bedding geo- deposits
metry, which may be seen in exposures of rimmed
shelf carbonates. This distinctive geometry can also 10s metres
be recognised in seismic reflection profiles of the
subsurface (22.2) (Emery & Myers 1996). The asso-
ciation of reef-core boundstone facies overlying fore-
reef rudstone deposits and overlain by finer grained
sediments of the shelf lagoon forms a distinctive facies
association. Under conditions of sea-level fall the reef
core may be subaerially exposed and develop karstic
weathering, and a distinctive surface showing evi-
dence of erosion and solution may be preserved in Base of slope
pelagic mudstone
the stratigraphic succession if subsequent sea-level and carbonate
turbidites
rise results in further carbonate deposition on top
(Bosence & Wilson 2003).

15.4.4 Epicontinental (epeiric) platforms

There are no modern examples of large epiconti-
nental seas dominated by carbonate sedimen-
tation but facies distributions in limestones in the
stratigraphic record indicate that such conditions
Fig. 15.18 Schematic graphic log of a rimmed carbonate
have existed in the past, particularly during the shelf succession.
Jurassic and Cretaceous when large parts of the