Geology Reference
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level fluctuations, small-scale sequences to sea-level
fluctuations in the Milankovitch band (e.g. Read and
Goldhammer 1988; Goldhammer and Harris 1989;
Strasser 1994). However, autocyclic processes such as
migration of bars and islands, delta-switching and lo-
cal progradation may also create shallowing-upward
sequences in shallow-marine environments (Strasser
1991; Goldhammer et al. 1993). These processes are
local and can not be followed over large lateral dis-
tances.
and space. Platforms may expand as their margins grow
outward; grow upward while their margins remain sta-
tionary; or retreat at their margins backward (e.g. Jansa
1981; Blendinger 1986). Platforms are born, grow and
die. Growth is caused by aggradation and/or prograd-
ation. Demise is connected with the decrease in and
cessation of carbonate production due to (1) drowning
caused by rapid eustatic sea-level rise or tectonic sub-
sidence, (2) subaerial exposure caused by sea-level fall
or tectonic uplift, (3) high siliciclastic input, or (4) paleo-
oceanographic effects causing changes in water circu-
lation, temperature and salinity.
Major variables influencing the evolution of plat-
forms are tectonic setting and subsidence, sea-level fluc-
tuations, carbonate productivity and sediment transport,
the nature of the sedimentation at platform margins,
the evolution of reef-building organisms through time,
and variations in diagenetic processes.
14.1.2.3 Different Depositional Settings
Require Different Facies Models
The basic differences of carbonate depositional settings
were discussed in Chap. 2 of this topic. Fig. 2.6 gives a
brief survey of some of these settings. The discussion
in the present chapter is focused on facies zones and
models forming the base for the interpretation of an-
cient carbonate environments.
The discussion starts with facies zones occurring on
rimmed shelves (platforms) and adjacent slopes and ba-
sins (Sect. 14.1.3). This Wilson model is contrasted with
the ramp model (Sect. 14.1.4) describing sedimenta-
tion on a gently dipping slope, and with the non-rimmed
platform model (Sect. 14.1.5). The specific conditions
of isolated platforms and atolls are examined in Sect.
14.1.6. The sedimentation on extremely extended
epeiric platforms and epeiric ramps is the topic of Sects.
14.1.7 and 14.1.8.
Many platforms are 'rimmed' at their platform-ocean
margins (e.g. South Florida Shelf, the Belize Shelf, or
the Queensland Shelf with the Great Barrier Reef) or
they are 'non-rimmed' (e.g. Western Florida Shelf).
Both rimmed and non-rimmed platforms may be at-
tached to the continent or occur detached from the con-
tinent as isolated platforms and atolls.
Rimmed carbon-
ate shelves and platforms
are characterized by
•
a nearly flat platform top,
•
a landward low-energy lagoon,
•
an outer wave-agitated depositional, bypass or ero-
sional margin, marked by
•
a continuous to semicontinuous reef-dominated or
sand-shoal dominated rim along the margin that re-
stricts circulation and wave action,
-> Note that these facies models are limited with re-
gard to their predictive capacity owing to their static
view of time and of the relative sea-level changes
that control facies shifts. These difficulties can be
partly overcome by models based on sequence
stratigraphy demonstrating how depositional se-
quences respond to lowstand, transgressive and
highstand conditions of relative sea level under hu-
mid and arid conditions (Handford and Loucks
1993). Sequence stratigraphy models can not replace
the static models which have their own merits, but
they complete our understanding of facies develop-
ment through time (see Sect. 16.1.2).
•
a pronounced increase in slope, ranging from a few
degrees to 60° and more, and
•
abundant mass-flow deposits (e.g. debris flows, mega-
breccias, turbidites, slumps) downslope from the
shelf edge containing platform-derived material.
14.1.3.1 Standard Facies Zones and the
Modified Wilson Model
The succession of major facies belts on rimmed tropi-
cal carbonate platforms was used by Wilson (1975) to
establish a Standard Facies Model depicted as a basin-
to-shore transect and comprising Standard Facies Zones
(Fig. 14.1). The basis of the model is the recognition of
consistently recurrent patterns of carbonate facies in
the Phanerozoic record and the environmental inter-
pretation of these patterns by using characteristics of
Holocene sedimentation patterns.
14.1.3 Facies Zones of Rimmed Carbonate
Platforms: The Wilson Model
The most frequently used facies models are those hy-
pothesizing on platforms and ramps. Carbonate plat-
forms are dynamic systems that change through time
