Page 250 - Sedimentology and Stratigraphy

Types of Carbonate Platform 237

documented being carbonate ramps, which are
gently sloping platforms, and rimmed shelves,
which are flat-topped platforms bordered by a rim
formed by a reef or carbonate sand shoal. The tectonic
setting influences the characteristics of carbonate
platforms (Bosence 2005), with the largest occurring
on passive continental margins (24.2.4) while smaller
platforms form on localised submarine highs such as
fault blocks in extensional settings (24.2) and on salt
diapirs (18.1.4). The different types of carbonate plat-
form can sometimes occur associated with each other:
an isolated platform may be a carbonate ramp on one
side and a rimmed shelf on the other and one form
Fig. 15.14 Cliffs of Cretaceous Chalk. may evolve into another, for example, a ramp may
evolve into a rimmed shelf as a fringing reef develops
from the Mesozoic and Cenozoic, but are most com- (Bosence 2005).
monly found in the Late Cretaceous in the northern
hemisphere in a stratigraphic unit which is called
The Chalk (capitalised) (Fig. 15.14). 15.4.1 Carbonate ramps

15.3.5 Platform margins and slopes The bathymetric profile of a carbonate ramp
(Fig. 15.15) and the physical processes within the
The edge of a carbonate platform may be marked by sea and on the sea floor are very similar to an open
an abrupt change in slope or there may be a lower shelf with clastic deposition. The term ‘ramp’ may
angle transition to deeper water facies. The front of give the impression of a significant slope but in fact
a reef can form a vertical ‘wall’ and along with the slope is a gentle one of less than a degree in most
other slopes too steep for sediment accumulation are instances (Wright & Burchette 1996), in contrast to
by-pass margins. Sediment accumulates at the base slope environments associated with rimmed shelves,
of the slope, brought in by processes ranging from which are much steeper. Modern ramps are in places
large blocks fallen from the reef front to submarine where reefs are not developed, such as regions of
talus slopes, slumps, debris flows and turbidites cooler waters, increased salinity or relatively high
(Mullins & Cook 1986). The most proximal material input of terrigenous clastic material. However, in
forms rudstone deposits, which are sometimes called the past carbonate ramps formed in a wider range
megabreccias if they contain very large blocks, pas- of climatic and environmental settings, especially dur-
sing distally to redeposited packstones, to turbiditic ing periods when reef development was not so wide-
wackestones and mudstones. Depositional margins spread. In macro- to mesotidal regimes tidal currents
form on more gentle slopes with a continuous spec- distribute carbonate sediment and strongly influence
trum of sediments from the reef boundstones or shoal the coastal facies. Wave and storm processes are
grainstones of the shelf margin to packstones, wacke- dominant in microtidal shelves and seas. The effects
stones and mudstones further down the slope. Finer of tides, waves and storms are all depth-dependent
grained sediments tend to be unstable on slopes and and ramps can be divided into three depth-related
slumping of the mudstones and wackestones may zones: inner, mid- and outer ramp.
occur, resulting in contorted, redeposited beds.
Distribution of facies on a carbonate ramp
15.4 TYPES OF CARBONATE The inner ramp is the shallow zone that is most
PLATFORM affected by wave and/or tidal action. Coastal facies
along tidally influenced shorelines are characterised
A number of different morphologies of carbonate by deposition of coarser material in channels and
platform are recognised (Fig. 15.15), the most widely carbonate muds on tidal flats (Tucker & Wright