Geology Reference
In-Depth Information
Significance of the benthic carbonate factory con-
cept.
The concept links properties expressed by a sys-
tematic combination of environmental parameters, sedi-
ment composition, depositional architecture and growth
potential, and thus aids in predicting yet unknown at-
tributes (e.g. porosity development) of carbonate de-
posits. Some of the properties appear prominently in
seismic data, thus facilitating subsurface prediction.
Sediment accumulations of the factories differ in
composition, geometry and facies patterns (Schlager
2003). The tropical factory is characterized by flat plat-
forms, sharp shelf breaks and steep slopes. Raised rim
and empty lagoon ('empty bucket') are characteristic
of a stress-dominated tropical system. Cool-water ac-
cumulations exhibit seaward-sloping shelves and rela-
tively gentle slopes. The geometries of the mud-mound
factory are highly variable. Groups of convex mounds
are common in deeper-water settings, whereas flat-
topped mounds can develop in wave-swept shoal-wa-
ter environments.
Basic prerequisites for using the concept as a pre-
dictive tool and for interpreting vertical and lateral fa-
cies changes are qualitative and quantitative data, most
of which can be derived from microfacies analysis, in-
cluding
•
in the Mid-Paleozoic, Jurassic and Cretaceous (Bates
and Brand 1990). Fluctuations in marine carbonate min-
eralogy are generally acknowledged, but different opin-
ions exist with regard to the length and the boundaries
of the time intervals characterized by particular miner-
alogies. The oscillating trend (Sandberg cycle) is usu-
ally interpreted as an indication of major variations in
paleoceanographic parameters. The temporal distribu-
tion of once aragonitic non-skeletal constituents appears
to be discontinuous in the Phanerozoic (Sandberg
1991). Controls of nonskeletal mineralogy by pCO
2
and
covariant factors (e.g. oceanic Ca/Mg) are suggested
by the close agreement between sea-level curves and
the presence-absence curves for non-skeletal aragonite.
Van de Poel and Schlager (1994) studied the miner-
alogy of Mesozoic to Cenozoic skeletal grains contrib-
uting to the formation of carbonate platforms and found
that the biogenic aragonite-calcite intervals parallel
those observed in nonskeletal constituents.
Large-scale patterns in the skeletal mineralogy of
major carbonate producers (Wilkinson 1979; Stanley
and Hardie 1998) have been linked with plate tectonic
evolution and changing Mg/Ca ratios in seawater
(Stanley and Hardie 1999; Steuber 2002). The link be-
tween the reconstructed chemical composition of sea-
water and the skeletal mineralogy of reef-building or-
ganisms (Stanley and Hardie 1998) seems to be less
than originally suggested (see Kiessling 2002 and col-
lected papers in Kiessling et al. 2002). The Stanley-
Hardie model may work better for off-reef carbonate
platforms, as indicated by the coincidence of foramin-
iferal skeletal mineralogy with aragonite-calcite inter-
vals (Martin 1995) and the more frequent occurrence
of calcareous algae with High-Mg-calcite and arago-
nite skeletons in aragonitic intervals (Kiessling et al.
2003).
Micritic matrix, grains and fossils, and pore-filling
cement of limestones are usually preserved as calcite.
An understanding of the primary mineralogy requires
a thorough study of microfacies criteria allowing pre-
cursor mineralogy (Sect. 7.1.5.2) and skeletal diagen-
esis (Sect. 4.2.1) to be evaluated.
sediment composition (Sect. 6.2),
•
matrix and grain types (Chapter 4),
•
automicrite (Sect. 4.1.1),
•
indications of microbial contributions (Box 4.2,
Pl. 8., Sect. 9.1, Box 9.1),
•
environment- and facies-diagnostic fossils (Chap-
ter 10),
•
dominating skeletal grains and grain associations
(Sect. 12.2),
•
paleoenvironmental constraints including paleo-
water temperature (Tab. 2.3, Sect. 2.4.4), differen-
tiation of photic, dysphotic and aphotic zones (Sect.
12.1.4.1), paleo-water depth (Sect. 12.3.1), and nu-
trients (Box 12.8).
16.7.3 Temporal Changes in Non-Skeletal
and Skeletal Mineralogy
Secular variations in the relative abundance of arago-
nite (and High-Mg calcite) and Low-Mg calcite non-
skeletal constituents (ooids, see Sect. 4.2.5; cement;
micrite) of carbonate platforms were recognized by
Sandberg (1983; see Sect. 7.1.5; Fig. 7.1). Aragonite
(and High-Mg Calcite) were considered to dominate
in the Late Precambrian to the Cambrian, the Mid-Car-
boniferous to the Triassic, and the Tertiary to recent
intervals. Low-Mg calcite mineralogy was abundant
16.7.4 Temporal Changes in the Abundance
and Significance of Microfacies Criteria
Although Standard Microfacies Types are similar (com-
pare Fig. 14.28) and the main grain types differenti-
ated in microfacies studies can be observed all over
the Phanerozoic, the abundance and significance of
some microfacies criteria used in interpreting deposi-
tional environments vary through time. The consider-
