Geology Reference
In-Depth Information
2 Carbonate Depositional Environments
Knowing where modern carbonates occur, what they
are composed of, and what their controls are is essen-
tial for evaluating microfacies data. The objectives of
this chapter are to summarize the settings and environ-
ments in which carbonate sediments are formed and to
document which classifications are used for differenti-
ating these environments. Emphasis is placed on the
definitions of terms.
The potential of microfacies analysis in recognizing
and interpreting the environments discussed in this
chapter will be demonstrated later. If you want to know
how to describe paleoenvironmental criteria, and which
microfacies criteria can be used in deciphering the bio-
logical and depositional constraints on ancient lime-
stones, have a look at Chap. 12 and 15.
Shallow-marine carbonate depositional systems dif-
fer fundamentally from siliciclastic depositional sys-
tems in their intrinsic controls, i.e. the growth potential
including the 'aggregation potential' (the ability to grow
vertically and to track sea level) and the 'production
potential' (the ability to produce and export sediment).
The aggradation potential is critical for the history of
carbonate platforms and reefs, the production potential
for the progradation and retreat of platforms. These dif-
ferences are of central importance in the application of
sequence stratigraphy to carbonates (Schlager 1992).
2.1.2 The 'Sorby Principle': Limestones are
Predominantly Biogenic Sediments
Well over 90% or more of the carbonates found in mod-
ern marine environments are biological in origin, i.e.
the sediments are biotically induced (by an organic trig-
ger, e.g. microbial micrites) or biotically controlled
(skeletal autotrophic and heterotrophic organisms de-
termine the composition, location and timing of car-
bonate production). Some of the 'abiotic' carbonate pre-
cipitation (represented by marine cements) is also trig-
gered by organics or the activity of organisms. The
dominant role of organisms in the formation of lime-
stones was recognized as early as 1879 by Henry Clifton
Sorby. Studying Paleozoic, Mesozoic and Tertiary car-
bonate rocks in thin sections, he recognized the over-
whelming abundance of fossils and their importance in
the composition of carbonate sands and muds. The dis-
tribution and frequency of carbonate-producing organ-
isms depend strongly on environmental factors, such
as light, water temperature and sedimentary influx.
These controls as well as the paleoenvironmental set-
tings are reflected by microfacies criteria and paleon-
tological data.
2.1 Introduction
Microfacies analysis of carbonate rocks requires a
knowledge of modern carbonates and an understand-
ing of biological and geological changes during earth
history. R.N. Ginsburg's eloquent statement 'carbon-
ate buildups are like Shakespeare; the plays go on -
only the actors change' emphasizes the remarkable simi-
larities and analogues in the criteria of modern and an-
cient carbonates. The aphorism, however, ignores the
guiding role of the stage-managers who often decide
the basic mode of the play, similar to the changing geo-
logical and biological earth system parameters which
shape the appearance of carbonate buildups.
2.1.1 Carbonates are Born not Made
This simple phrase by Noel James (1979) highlights
the main theme of carbonate sedimentation and the dif-
ferences between carbonate and siliciclastic sediments.
Carbonates are 'born'. They originate as skeletal grains
or precipitates within the depositional environment. By
contrast, terrigenous clastic sediments are formed pri-
marily by the disintegration of parent rocks and are
transported to the depositional environment.
2.1.3 Modern Carbonates: Obligatory
Reading
There is a wealth of well-written reviews and concise
updated syntheses of modern carbonate sediments (see
