Geology Reference
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distribution processes, and early dissolution of grains
preceding cementation. Brachert et al. (1996) refer to
this particular type of microfacies as
microtaphofacies
.
These microfacies types reflects the taphonomic his-
tory of bioclast rather than original distribution patterns.
The microtaphofacies concept provides important in-
sights into the controls on microfacies of carbonate
ramps and platforms by sealevel fluctuations (Sect.
16.1.2.2).
calcite (Pingitore 1976), and by biogenic micritization.
Aragonitic bioclasts were likely prone to sea floor dis-
solution, but preservation of Quaternary aragonitic
pteropods is reported from the deep continental slope.
Generally, preservation of aragonitic fossils in carbon-
ate rocks requires very specific depositional and diage-
netic conditions (Füchtbauer and Goldschmidt 1964;
Scherer 1977).
The selective diagenetic changes in invertebrate skel-
etal grains were investigated by experimental dissolu-
tion of recent skeletal carbonates, particularly of mol-
lusks, bryozoans, echinoderms, corals and calcareous
algae (Walter 1985). These studies and the study of the
microarchitecture of hardparts show the dominating role
of the ultrastructure for the relative reactivity of skel-
etal carbonates during dissolution and preservation
(Dullo 1987).
The impact of the diagenetic loss of bioclasts on the
composition of carbonate rocks has been thoroughly
studied by comparison of modern and Pleistocene car-
bonates affected by meteoric diagenesis (Land 1967),
and is exemplified by the 'diagenetic sieve' (Dullo 1984;
Fig. 4.10).
Initial diagenetic alterations of the hardparts of ma-
rine organisms may occur within a short time (Flessa
and Brown 1983). The diagenetic thickening of skel-
etal elements of recent corals takes place within a time
range of twenty years, prior to the alteration of arago-
nite to calcite (Potthast 1992). Selective dissolution and
replacement of aragonitic bioclasts continues in shal-
low-marine burial environments (Brachert and Dullo
2000).
In addition to all the processes described above, do-
lomitization (Murray 1964), silicification, burial diagen-
esis and compaction can contribute strongly to the oblit-
eration of skeletal grains and to their complete loss in
the geological record (see Pl. 36/1, 3, 4; Pl. 37/7, Pl.
47/4, Pl. 109/8).
Major preservation biases are caused by dissolution
processes involved in
skeletal grain diagenesis
(Chave
1964): Skeletal diagenesis is a matter of multiple
choices in space and time, as exemplified by the diage-
netic sequence patterns of corals (Schroeder 1984; see
Pl. 35). Because skeletal durability relates strongly to
mineralogy and ultrastructure, relative diagenetic
preservational orders of skeletal structures are common,
and can be used to estimate diagenetic changes. The
relative order of preservation in meteoric environments
is calcite > aragonite > High-Mg calcite. High-Mg cal-
cite is unstable in freshwater and can easily transform
to Low-Mg calcite by incongruent leaching or only little
change in microstructures and microfabrics (Bathurst
1971).
The process of magnesium loss during stabilization
of Mg-rich calcites and the effect of that loss on the
resulting calcite microfabric are not well understood
(Towe and Hemleben 1976; Dreifuss 1977; Oti and
Müller 1985). Some of the altered HCM parts show
textural alterations, others do not. The characteristically
finer size distribution and the relic-free microfabrics
resulting from HCM stabilization are distinctly differ-
ent from the coarse, pitted, relic-bearing calcites which
replace originally aragonitic constituents. The interpre-
tation of diagenetic alterations in fine-grained calcitic
skeletal grains (porcelaneous foraminifera: Budd and
Hiatt 1993; echinoderms: Richter 1979, 1984) is also
complicated by the level of observation since the re-
sulting 'grain diminution' (Sect. 4.1.2) is difficult to
substantiate in thin sections.
Aragonite skeletons (mollusks, corals, green algae)
are replaced by calcite (Bathurst 1964; Talbot 1972),
and have a consistent alteration behavior (Sandberg
1984). The replacement crystals are one or more or-
ders of latitude larger than the crystals they replace.
The original microstructures are commonly not pre-
served. The transformation of aragonite skeletons into
calcite bioclasts takes place in meteoric and phreatic
environments by cementation after dissolution of ara-
gonite via a mold stage (Pl. 149/5), by in situ calcifica-
tion and micro- to macrorecrystallization connected
with a solution film between aragonite and replacing
(3) Different methods can lead to different results:
Thin sections of limestones do not document the total
record of skeletal grains. Some very small skeletal
grains may be missed. Agglutinated or silicified mi-
crofossils may be underrepresented. Toomey (1983)
while studying a Late Paleozoic shelf limestone in thin
sections and acid residue samples, demonstrated that
only about 80% of agglutinated foraminifera could be
recorded in the acid residue samples.
Some organism groups are almost always under-
represented or missing in thin sections when compared
with acid residues of limestones (e.g. agglutinated fora-
minifera, ostracods, echinoderms, scolecodonts, con-
odonts, fish remains). One example are holothurians
