Page 30 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 2: PRINCIPLES OF CARBONATE PRODUCTION 21

PROTECTION AND RESTRICTION IN CARBONATE ENVIRONMENTS

RESTRICTION

PROTECTION NO RESTRICTION BIOTIC RESTRICTION EVAPORITES PRESENT

e.g. skeletal sand shoal e.g. sand shoals in Hamelin e.g. rubble and sand “sub-
AGITATED (no mud) “White Bank” in Florida Reef Basin (Shark Bay) littoral sheet” Hamelin Basin
Tract (Shark Bay)
CALM OR EPISODICALLY e.g. deep muddy lagoons of e.g. muddy sands of Florida e.g. muddy sands of tidal
AGITATED (muddy sedi- Pacific Atolls (Enewetak) Bay or Bahamas, W of An- flats Hamelin Basin (Shark
ments) dros Bay)

Fig. 2.12.— Protection against water turbulence and restriction of exchange with the open sea are two independent variables that can
be used to classify carbonate depositional environments in a protection-restriction matrix. This version is rather simple and meant for
immediate use in the field, refinement for specific case studies is easily possible.

abiotic category is not always free of subtle biotic influ- FROM PRECIPITATION MODES TO CARBONATE
ences (e.g. Webb, 2001). “Quasi-abiotic” may be an appro- FACTORIES
priate term for those who find the term abiotic too strin-
gent. Another point merits mention. The bio-induced cat- If one increases the scale of observation from samples and
egory includes carbonates precipitated by the action of liv- thin-sections to mappable formations and beyond, it turns
ing cells as well as material precipitated under the influence out that all large accumulations are mixtures of the three
of non-living organic matter. Trichet and Defargue (1995) precipitation modes described above (Fig. 2.13). These mix-
distinguished these two categories as “bio-mineralic” and tures are not random. They cluster into three preferred pro-
“organo-mineralic” respectively. The reason for lumping duction systems, or factories, that differ in dominant pre-
them here lies in the difficulty of distinguishing them at the cipitation mode (Fig. 2.14), mineral composition (Fig. 2.15),
scale of outcrops, formations or sequences. depth range of production (Fig. 2.16) as well as growth po-
Lowenstam and Weiner (1989) proposed the above classi- tential (see p. 24f). I introduced the classification by us-
fication to distinguish basic modes of mineral precipitation. ing the terms tropical, cool-water and mud-mound factory
It is comforting to see that the three classes can be recog- (Schlager, 2000, 2003). I think the terms T factory, C factory
nized in most instances by petrographic analysis. Nearly and M factory are preferable because there exist several defi-
all biotically controlled carbonate precipitates are organic nitions for each of the three key words and each letter stands
skeletons, endowed with characteristic shape, crystal fab- for at least two important properties of the respective factory
ric, mineralogy and, commonly, chemical signature. The (see below).
stunning success of carbonate microfacies analysis in the
1960’s and 70’s was largely due to the study of skeletal Tfactory
particles. Abiotic precipitates in the form of cements were
also recognized early by petrographic techniques and have T stands for “tropical” and “top-of-the-water-column”.
contributed important information on environmental con- Biotically controlled precipitates dominate. Most abun-
ditions in depositional and diagenetic environments. The dant among them are photo-autotrophic organisms, for in-
third group, the bio-induced precipitates, became clearly stance algae and animals with photosynthetic symbiotic al-
identifiable when techniques of organic chemistry were gae, such as hermatypic corals, certain foraminifers and cer-
combined with standard petrography during the past two tain molluscans. The other characteristic products are abi-
decades (e.g. Reitner et al., 1995b; Trichet and Defargue, otic precipitates in the form of marine cements and ooids.
1995; Reid et al., 2000). Petrographically, bio-induced pre- Clay-size precipitates, the “whitings”, are probably mix-
cipitates commonly occur as fine-grained carbonate (i.e. mi- tures of abiotic or biotically induced precipitates (Morse
crite) in the form of pellets, lumps or masses with variable, and Mackenzie, 1990; Yates and Robbins, 1999; Thomp-
often concentric lamination. Distinction from depositional son, 2001). Heterotrophs devoid of photo-symbionts are
mud is sometimes difficult but surprisingly often one can common, but non-diagnostic contributors. Construction of
establish that the material was hard or firm upon formation wave-resistant structures by organic framebuilding or rapid
and did not accumulate as soft, muddy sediment. Thus, the marine cementation is common, particularly at the shelf-
classification as automicrite (see above) can often be backed slope break.
by textural evidence.