Geology Reference
In-Depth Information
Fig. 4.10.
The 'diagenetic
sieve'
(after Dullo 1990). The
study of modern submergent
reefs and Pleistocene emer-
gent reefs allows the stages
and timing of the diagenetic
alterations of skeletal grains
to be recognized (e.g. James
1974; Scherer 1975; Gvirtz-
man and Friedman 1977;
Vollbrecht 1990). Compari-
son of the biotic composition
of recent reefs off the Red
Sea Coast with a strongly al-
tered reef on the Pleistocene
terraces, occurring in differ-
ent levels of elevation above
the present sea, exhibits pro-
gressive diagenetic alter-
ations of bioclasts. These al-
terations result in the partial
to complete leaching of pri-
mary aragonitic biota, a gradual decrease in Mg-calcitic hardparts, concommitent processes of replacement and recrystalliza-
tion, and the precipitation of carbonate cements. Aragonitic bioclasts (scleractinians, hydrozoans, aragonitic mollusks) and
Mg-calcitic bioclasts (echinoderms, foraminifera, coralline calcareous algae) are drastically reduced in the geological record
by meteoric diagenesis. The Pleistocene reef limestone yields a higher amount of unidentified bioclasts, peloids, carbonate
cement and pores than modern reefs. The examples demonstrate the significant modification of biotic composition and the
frequency of skeletal grains in limestones by selective fossil diagenesis. The degree of progressive diagenesis within the reef
terraces is proportional to the age of the Pleistocene reefs, which varies between 86 000 and about 340 000 years.
Basics: Preservation of skeletal grains (bioclasts)
Adams, A.E., MacKenzie, W.S. (1998): A colour atlas of car-
bonate sediments under the microscope. - 180 pp., Lon-
don (Manson)
Majewske, O.P. (1974): Recognition of invertebrate fossil
fragments in rocks and thin sections. - International Sedi-
mentary Petrographical Series,
13
, 101 pp., Leiden (Brill)
Peryt, T.M. (ed., 1983): Coated grains.- 655 pp., Berlin
(Springer)
Richter, D. (1983): Classification of coated grains: discus-
sion. - In: Peryt, T. (ed.): Coated grains. - 7-8, Berlin
(Springer)
Steinhoff, I., Strohmenger, C. (1996): Zechstein 2 carbonate
platform subfacies and grain-type distribution (Upper Per-
mian, Northwest Germany). - Facies,
35
, 105-132
which occur in high abundance in acid residues of ba-
sinal limestones, but are extremely rarely observed in
thin sections. A combination of thin section studies and
the investigation of acid residues is strongly recom-
mended. These differences are important for the evalu-
ation of 'fossil diversity in thin sections' (Sect. 6.2.1.4).
The advantages and disadvantages of two-dimen-
sional versus three-dimensional aspects of skeletal
grains are illustrated in Pl. 9/1-6.
Significance of bioclasts
Skeletal grains are the most valuable grain types in
the context of determining the age of limestone samples.
The type and composition of skeletal grains is highly
sensitive to the depositional environment and offer sig-
nificant proxies for paleoenvironmental controls on car-
bonate depositionand paleoclimatic conditions. Under
favorable conditions, the diagenesis of skeletal grains
can document sealevel fluctuations as exemplified by
the study of Pleistocene beach sediments in Bermuda
Island (Vollbrecht and Meischner 1993).
The
diagenesis of sediments is reflected in the di-
agenesis of fossils. Because the original mineralogy of
most calcareous fossils is known or can be inferred,
fossil diagenesis is a perfect tool for reconstructing di-
agenetic pathways.
Biomineralization
Carter, J.G. (ed., 1990): Skeletal biomineralization patterns,
processes and evolutionary trends. Vol. I. - 832 pp., New
York (Van Nostrand)
Degens, E.T. (1979): Why do organisms calcify? - Chem.
Geol.,
25
, 257-269
Degens, E.T. (1989): Perspectives in biogeochemistry. -
423 pp., Berlin (Springer)
Leadbeater, B.S.C, Riding, R. (eds., 1986): Biomineraliza-
tion in lower plants and animals. - Syst. Ass. Spec. Vol.,
30
, 308 pp., Oxford (Clarendon Press)
Kempe, S., Kazmierczak, J. (1994): The role of alkalinity in
the evolution of ocean chemistry, organization of living
systems and biocalcification prozesses. - In: Doumenge,
F. (ed.): Past and present biomineralization processes. -
