Geology Reference
In-Depth Information
Fig. 14.2.
Common variations in the lateral arrangement of facies zones
distinguished in the rimmed platform model (see
Fig. 14.1). The arrangement of the facies belts is controlled by the position of the sea level (A highstand, B stillstand,
C lowstand).
A:
The platform margin is characterized by a barrier reef zone (FZ 5) interfingering with an open-marine back-reef zone
(FZ 7) of the inner platform. Facies zones FZ 3 and FZ 4 of the Wilson model are not distinctly differentiated due to the
mixture of coarse reef debris and calcareous sand from the platform interior. Note the absence of the deep shelf facies zone
FZ 2.
B:
The reef zone (FZ 5) is part of a reef complex consisting of lagoonal back-reef sediments and a slope whose upper part is
supplied with talus forming a forereef zone.
C:
The sketch shows a common case whereby marginal sand shoals interfinger with slope sediments consisting of bioclastic
sand transported from the shoals to gentle slopes. Adapted from Schlager (2002).
-> Most studies combine the evaluation of Facies Zones
with the determination of Standard Microfacies
Types. The methods and potential of this approach
are described in Sect. 14.3.
ter outer ramp deposits. High-energy carbonates are
formed near the shoreline in inner-ramp settings or in
shoal areas of inner and mid-ramp settings, and not at
the shelf edge. Windward ramps are storm- and wave-
dominated and characterized by grainy sediment at the
shoreline. Leeward ramps are characterized by muddy
sediments with low grainstone content.
14.1.4 Carbonate Ramp Model
A carbonate ramp is a gently dipping sedimentary sur-
face on the sea floor. The facies belts are controlled
primarily by energy levels (fair-weather wave base and
storm wave base), variations in ramp topography, and
material transport by storms, waves and tides. The depo-
sitional slope gradient from the shallow-water shore-
line or lagoon to the basin floor is of the order of a few
meters per kilometer and usually less than 1°. Near-
shore, wave-agitated shallow-water carbonates pass
gradually offshore into deeper-water, low-energy de-
posits and then into basinal sediments.
Ramps were common during the Phanerozoic, par-
ticularly in times when frame- and reef-building organ-
isms were missing or rare. Most ramps have been de-
scribed from the Cambrian and Early Ordovician, Early
Devonian, Early Carboniferous, Early and Middle Tri-
assic, Middle and Late Jurassic, Early Cretaceous and
Early Tertiary.
Carbonate ramps can develop during the drowning
of shelves and during the early stages of platform for-
mation. Often they evolve into rimmed platforms. Con-
trasted with rimmed shelves, ramps lack the steep slope
at the shelf edge, show no continuous reef trends and
exhibit no shallow-water derived clasts in deeper-wa-
Dimension of ramps
The widths and lengths of ancient carbonate ramps
vary within a wide range. Maximum width ranges be-
tween less than 10 km to about 800 km. Most values
are below 200 km and many ramps have widths of only
<10 to about 20 km. The minimum and the maximum
lengths range between 10 km and 1600 km, but some
of the Early Paleozoic ramps extending beyond 1000
km are better classified as epeiric ramps. Most values
are below 500 km. Ramps with lengths of about 10 km
to 200 km are common. The dimensions of Cenozoic
ramps are usually within the range of some hundreds
of kilometers (West Florida, Cretaceous to present: width
200 km, length > 800 km; Trucial Coast, recent: width
200 km, length > 400 km; Shark Bay, Western Austra-
lia, recent: width 100 km, length 200 km; Yucatan Pen-
insula, Tertiary to present: width 200 km, length more
than 600 km). Yet smaller Holocene wave-dominated
ramps with a length of only about 50 km are known
(Kuwait, northern Arabian-Persian Gulf).
Characteristics of ramps
The principal criteria of carbonate ramps are
•
Gentle slopes on which updip nearshore shallow-
