Geology Reference
In-Depth Information
Box 2.4.
Lateral classification of carbonate shelves (Wilson and Jordan 1983) and carbonate ramps (Burchette and
Wright 1992).
Carbonate shelves
Inner shelf
: Near-coast tide-dominated zone including peritidal and shallow subtidal environments varying and restricted
salinity; sluggish circulation; biota low-diverse.
Midshelf
: Extended shallow subtidal zone between the near-shore area and the shelf break; below fair-weather wave-
base, but above storm-wave base; mud-dominated but with grainy storm sediments; water depths between a few tens
of meters and 100 to 200 m; normal marine, but different conditions in local restricted areas; biota high-diverse.
Outer shelf
: Rimmed shelves: A narrow zone near the shelf break, with shoals and reefs. Non-rimmed shelves: A wide
zone below normal storm-wave base which may be affected by intruding ocean currents.
Carbonate ramps
Inner ramp
: Between upper shoreface (beach or lagoonal shoreline) and fair-weather wave base; seafloor more or less
constantly affected by wave agitation; includes shoreline deposits, sand-shoals, and back-barrier peritidal sediments.
Midramp
: Between fair-weather wave base and storm-wave base. The bottom is frequently reworked by storm waves
and swells. Sediment composition and textures reflect proximal-distal trends.
Outer ramp
: Below normal storm-wave base, down to the basin plain. Mud-dominated, but with few storm beds. In
deeper zones, restricted bottom conditions may develop.
Today the terms shelf, ramp and platform are used
in describing the setting and depositional geometry of
shallow-marine carbonates, but some confusion still
exists with regard to the definition of these terms:
Originally, the term
platform
described shallow-ma-
rine carbonate bodies with flat tops and steep flanks,
formed by the accumulation of sediment on the shelf
or in the ocean. Today the term often is used rather
loosely for thick, often flat-topped sequences of shal-
low-marine carbonates. Steep-sided flanks are not nec-
essarily definition criteria. Many authors regard car-
bonate platform as a very general term that includes
ramps, shelves and various types of flat-topped plat-
forms or they use the term if they cannot yet make an
assignment to any of these categories.
'Carbonate shelf'
characterizes a carbonate deposi-
tional system which develops a constructional relief
above the sea floor and is bordered on the shoreward
side by marginal-marine or continental sediments, and
offshore by slope and basinal sediments. The transi-
tion from shallow water to basin occurs over a rela-
tively short distance and is marked by a distinct break
in the slope. In this definition the term means the same
as 'attached platform' a term, sometimes used for plat-
form settings starting at the coast and continuing off-
shore.
One of the major tools of microfacies studies is the
understanding of ancient facies belts and depositional
areas related to particular marine settings (Chap. 14).
This aim requires the differentiation of well-defined
categories. Fig. 2.6 exhibits sketches and definitions
of broad categories of shallow-marine depositional set-
tings. The figure illustrates definitions used by Ahr
(1973, 1998), Ginsburg and James (1974), Schlager
(1981), Enos 1983), Wilson and Jordan (1983), Read
(1982, 1985), Harris et al. (1985), Wright and Burchette
(1998), and relies on the excellent reviews by Tucker
(1985), Tucker and Wright (1990), Jones and Des-
rochers (1992), and Burchette and Wright (1992).
Lateral subdivision of shelves and ramps
Carbonate shelves and homoclinal carbonate ramps
are laterally subdivided into at least three parts (Box 2.4).
Recognizing the various parts of ramps (Fig. 2.7) is
important for using the 'standard microfacies' approach
(Sect. 14.3). The differentiation of inner and mid ramp
environments can be strongly improved if the degree of
storm, wave, or tidal influence is considered. The iden-
tification of sediments, deposited in a 'basin' adjacent
to the outer ramp of homoclinal or distally steepened
ramps is difficult. The lack of coarse tempestites and
the absence of turbidites can be used as diagnostic cri-
teria. Sediments in shallow basins, but also in restricted
basins and outer ramp settings may be strongly biotur-
bated (and, therefore, be mistaken for lagoonal facies).
In distally steepened ramps the slope break is commonly
located around the mid- or outer ramp. The correspond-
ing 'outer slope' would form an appropriate additional
subdivision of ramps (Burchette and Wright 1992).
2.4.3.3 Shelf Margins
The boundary between the shelf and the continental
slope is marked by the 'shelf break', a point at the shelf
edge where a significant change in gradient occurs in
the outermost edge of the continental shelf (Stanley and
