Geology Reference
In-Depth Information
Fig. 7.1.
Secular variations in the mineralogical composition of non-skeletal carbonates (A) and skeletal grains (B, C), and
the frequency of reef cements over time.
A:
The
Sandberg model
(Sandberg 1983, 1985), based on the study of ancient ooids and carbonate cements, postulates
changes between times dominated by aragonite and High-Mg calcite (Late Precambrian to Cambrian, Late Mississippian to
Triassic, Mid-Cenozoic to Recent) and times dominated by calcite (Cambrian to Early Mississippian, Middle Jurassic to
Early Cretaceous). Note that the prevailing mineralogical composition of non-skeletal carbonates of several time intervals is
still a matter of discussion.
B:
The
Stanley-Hardie model
(Stanley and Hardie 1998) describes secular oscillations in the carbonate mineralogy of
hypercalcifying reef-building and sediment-producing organisms. The model relates Phanerozoic skeletal mineralogy to a
chain of causes that extended from mid-ocean ridge processes, via seawater chemistry to the mineralogical and biological
composition of reef communities and bioclastic carbonate deposits. Note slight differences in the temporal distribution of
aragonite- and calcite-dominated times as compared with the Sandberg model.
C:
The
Hardie model
(Hardie 1996) explains the oscillations in the prevailing mineralogical composition of non-skeletal
carbonates and synchronous oscillations in the mineralogy of marine potash evaporites by variations of the Mg/Ca ratio and
the Ca concentration in seawater. The boundary between the nucleation fields of Low-Mg calcite and aragonite + High-Mg
calcite is shown as a horizontal line at Mg/Ca = 2.
D:
The
amount of reef cement
and its importance in reefbuilding vary through time (Kiessling 2002). The plot is based on
the PaleoReef database (Kiessling and Flügel 2002) which considers low, medium, or high amounts of reef cements. In the
plot ordinal values have been transformed into approximate scale values. Vertical bars indicate 95% confidence intervals.
Times characterized by cement-rich reefs correspond roughly to phases of biocementstone reefs (Webb 1996). The abun-
dance of reef cements is apparently not related to specific cement mineralogies. Note the conspicuous difference in the
medium to high amounts of marine cement in Paleozoic and Triassic reefs as compared to the usually low mean cement
content in Jurassic to Cenozoic reefs. Chronostratigraphic time scale for all plots after Golonka and Kiessling (2002).
