Geology Reference
In-Depth Information
The Standard Facies Zones (FZ) defined in Box 14.1
describe idealized facies belts along an abstract transect
from open-marine deep basins across a slope, a pro-
nounced platform marginal rim (characterized by reefs
or/and a zone with sand shoals), and an inner platform
to the coast.
The Facies Zones differ in setting, dominant sedi-
ments and prevailing biota, and common lithofacies.
Carbonates formed within these Facies Zones often ex-
hibit specific Standard Microfacies Types (SMF) as-
semblages that are used as additional criteria in recog-
nizing the major facies belts (see Sect. 14.3).
The definitions of the Facies Zones in Box 14.1 con-
sider Wilson (1970, 1975), recent suggestions by
Schlager (2002) and modifications regarding the diag-
nostic criteria. Facies Zone 9 is subdivided into 9A and
9B to distinguish the significant differences in arid
evaporitic and in humid climates. A new Facies Zone
10 is introduced for limestones affected by freshwater
dissolution and precipitation processes (e.g. in karstic
and pedogenic settings).
• The model does not show the effects of sea-level
fluctuations. It fits predominantly sedimentation pat-
terns formed during transgressive highstand and still-
stand phases.
• The model is limited to tropical and subtropical plat-
forms and does not include climatic controls, except
for the Facies Zones 9A and 9B. Facies distribution of
temperate and cool-water platforms can not be inter-
preted with the model and are better described using
models dealing with non-rimmed platforms or carbon-
ate ramps.
• The model does not sufficiently consider the changes
in the organic platform rims that took place over time
in the evolution of reefs. Facies Zone 5, comprising
platform-margin reefs and upper slope reefs of various
construction, composition and development stages, is
too generalized and must be analyzed independently
of the other Facies Zones. The specific methods are
discussed in Sect. 16.2.
• The model says nothing about windward-leeward
differentiation resulting in distinct geometrical asym-
metries that can be revealed by seismic surveys.
• The model neglects the wide facies spectrum of oce-
anic deep-water settings and gives a much too general-
ized picture of slope sedimentation.
-> Note that these facies belts are limited to tropical
platforms and do not consider platforms in cool-
water settings that often correspond better to non-
rimmed platforms or ramps. The Wilson model con-
tains more Facies Zones than can normally be found
on one platform. Rimmed platforms usually exhibit
a reduced number of Facies Zones and often a dif-
ferent lateral arrangement of facies belts (Fig. 14.2).
Application of the Standard Facies Zone concept
Despite the pitfalls listed above, the facies zonation
established by Wilson has been successfully used by
many authors involved in basin analyses and reservoir
studies, e.g. Hubmann (1992, Devonian), Herbig (1984,
Carboniferous), Schott (1984, Late Triassic and Early
Liassic), Fezer (1988, Late Jurassic), Morrow and
Webster (1991, Mississippian-Pennsylvanian), Mette
(1993, Jurassic), Steuber et al. (1993, Cretaceous),
Braun (1998, Triassic), Blomeier and Reijmer (1999,
Jurassic), and Scheibner and Reijmer (1999, Jurassic).
14.1.3.2 Discussion and Use of Standard
Facies Zones
The Wilson model has passed the test of time with fly-
ing colors despite its static approach and the broad gen-
eralizations of the original concept. However, one
should be aware that the model is just a snapshot illu-
minating potential depositional patterns and their lat-
eral relationships. Critical points which must be con-
sidered are:
The Wilson model assists in
• evaluating outcrops with regard to their paleogeo-
graphical position within a likely shelf-to-basin transect;
• recognizing potential depositional and environmen-
tal relationships between widely spaced stratigraphic
sections, particularly in remote areas;
• attributing core samples to depositional sites that
allow the evaluation of economically interesting facies
zones (e.g. within reef or shoal complexes or at plat-
form margins) and of depositional conditions that might
have created appropriate poroperm values;
• reconstructing 'lost' depositional sites by looking for
source areas of carbonate clasts found in slope and ba-
sinal sediments, and derived from the destruction of
carbonate platforms.
• The model is overcomplete. Platforms with a reduced
number of Facies Zones are the common case (Fig.
14.2).
• The model prefers sharp boundaries between Facies
Zones. Lateral facies boundaries, however, are often
very gradual (e.g. between the Facies Zones 7 and 8)
or can not be distinguished at all (e.g. FZ 3 and 4).
• The model underlines the occurrence of litho- and
biofacies within specific Facies Zones, but does not con-
sider the development of Facies Zones over time.
