Geology Reference
In-Depth Information
ation of these temporal changes is crucial in using
microfacies analysis as an essential tool in understand-
ing the formation and properties of carbonate rocks.
Oncoids.
The composition and depositional settings
of oncoids have changed over time (Sect. 4.2.4.1). Mi-
crobial oncoids and algal oncoids have contributed to
subtidal and intertidal platform carbonates since the Pre-
cambrian. Spongiostromate oncoids are common con-
stituents of Proterozoic platforms. The first oncoids
formed by cyanobacteria (cyanoids) appeared in the
Late Vendian. Spongiostromate and porostromate on-
coids differ in their temporal distribution. Paleozoic
spongiostromate oncoids occurred predominantly in
lacustrine and transitional marine environments, where-
as porostromate oncoids flourished in marine subtidal
environments from the Cambrian to the Jurassic. These
oncoids became more frequent in the Late Paleozoic,
and peaked in the Late Jurassic (see Box 4.10). A ma-
jor change in the environmental setting of porostro-
mate oncoids during the Jurassic must be considered,
using Late Mesozoic and Cenozoic oncoids as paleo-
environmental indicators: The last normal-marine cyan-
oids are known from the Cretaceous. Spongiostromate
oncoids became the only type in marine settings, but
continued to be abundant in lacustrine environments
until today. Starting in the Late Cretaceous and Early
Tertiary, spongiostromate oncoids were increasingly re-
placed by coralline algal rhodoids (Sect. 4.2.4.2).
Rhodoids occurred as early as the Late Paleozoic, but
did not become important constituents of carbonate
platforms until the Tertiary.
Micrite.
Different possibilities for the
origin of mi-
crite were discussed in Sect. 4.1.1 and summarized in
Fig. 4.1. Biologically induced automicrite formed in
place at the sea bottom or within the sediment and trig-
gered by organomineralization or microbial processes
has been known since the Proterozoic, whereby the for-
mation by metabolic processes of cyanobacteria started
near the Precambrian-Cambrian boundary and was a
common process in the Early Paleozoic (Pratt 2001).
The formation of allomicrites through the disintegra-
tion of the skeletons of benthic biota started in the Cam-
brian with micrites formed by disintegrated invertebrate
skeleton material. This mode of micrite formation pre-
vailed throughout the Paleozoic. Micrite formation
through abrasion of shells increased from the Late Pa-
leozoic (Gischler and Zingeler 2002). Allomicrites
originating from the disintegration of calcareous algae
(
Halimeda
model; Fig. 4.3) became important in the
Mesozoic and increased in abundance during the Ceno-
zoic (Fig. 10.10). Allomicrite formation caused by bio-
erosion by microborers was low in the Paleozoic, but
became an important process from the Mesozoic (Fig.
9.15). The greatest change in allomicrite formation took
place in the Jurassic with the appearance and radiation
of calcareous plankton (foraminifera; coccolithophorids
and other nannofossils), forming pelagic carbonates
throughout the Mesozoic and Cenozoic.
Skeletal grains.
The association and abundance of
skeletal grain types contributing to the formation of
carbonate rocks varied through the Phanerozoic (Box
10.1; Fig. 10.3). The widespread deposition of carbon-
ate rocks coincides with the development and diversi-
fication of invertebrates at the beginning of the Cam-
brian. Common skeletal grains in Early Paleozoic car-
bonates are bryozoans, brachiopods, echinoderms and
ostracods. A long-term increase in the contribution of
calcareous algae, foraminifera and mollusks to the for-
mation of shelf carbonates started in the Late Paleo-
zoic and continued during the Mesozoic and Cenozoic.
The abundance and carbonate production by calcare-
ous algae varied considerably (Sect. 10.2.1). Different
major algal groups flourished at different time inter-
vals (Fig. 10.4) as did benthic foraminifera (Sect.
10.2.2). Late Paleozoic, Late Mesozoic and Cenozoic
larger foraminifera exhibit specific environmental dis-
tributional patterns that must be considered, using the
fossils interpret depositional settings and evaluate stan-
dard facies models (Fig. 14.7). Using shell beds as pa-
leoenvironmental proxies must take into account the
differences in life habits, taphonomic behavior and skel-
etal mineralogy of brachiopods and bivalves (Sect.
Peloids
can be formed by various processes (Sect.
4.2.2; Fig. 4.11). In-situ formation (precipitated peloids
and microbial peloids) and mud peloids occurred as
early as the Precambrian. Peloids of biotic origin (al-
gal peloids, bioerosional peloids and fecal pellets) and
bahamite peloids appeared in the Cambrian. Algal pe-
loids are common in Early Paleozoic carbonates, fecal
pellets are abundant in Mesozoic limestones. The abun-
dance of peloids in platform carbonates varied. Peloids
are usually more common than ooids and were abun-
dant in the Devonian, Early Carboniferous, Middle Tri-
assic, Late Jurassic and Early Cretaceous.
Ooids.
Ooid abundance varied substantially through-
out the Phanerozoic. Times of high ooid contributions
to platforms (Late Cambrian-Early Ordovician, Early
Carboniferous, Jurassic) do not coincide with times
characterized by specific mineralogical compositions
or specific geographical distribution patterns of plat-
forms. Giant centimeter-sized ooids are a particular
feature of Precambrian carbonates.
