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constituents for the diagenetic potential should be thor-
oughly examined. Replacement of carbonate by silici-
fication and the formation of cherts can result in an
excess surplus of carbonate (Hobert and Wetzel 1989).
Stability of carbonate minerals: The stability of car-
bonate minerals is different under marine or meteoric
conditions. Textbooks favor a stability ranking for ma-
rine carbonates with calcite > Mg-calcite (<12 mol%
MgCO 3 ) > aragonite > very high Mg-calcite (>12 mol%
MgCO 3 ) and for meteorically influenced carbonates a
ranking with calcite > aragonite > High-Mg calcite.
However, laboratory experiments (e.g. Walter 1985) and
preservation patterns of fossils show that these rankings
describe only frequently observed stability successions
and do not allow for deviations. The commonly ac-
cepted sequence of diagenetic alteration (Mg-calcite >
aragonite > calcite) is observed only in solutions su-
persaturated with respect to both calcite and aragonite.
In solutions undersaturated with respect to calcite, ara-
gonite skeletal grains with complex microstructures dis-
solve more rapidly than Mg-calcite. In solution super-
saturated with respect to calcite but still undersaturated
with respect to aragonite, mineralogic controls become
more significant and Mg-calcites containing ≤ 12 mol%
MgCO 3 dissolve at rates similar to or greater than those
for aragonites.
Relations between carbonate minerals and organic
matter: The precipitation of carbonate minerals is con-
Plate 28 Integration of Depositional and Diagenetic Microfacies
Microfacies thin sections of carbonate rocks exhibit depositional criteria reflecting environmental constraints
acting during sedimentation, and diagenetic criteria brought about by processes affecting carbonate sediments
and rocks after initial deposition until after lithification. These processes take place in freshwater (meteoric),
marine and burial environments and are recorded in thin sections by criteria related to pore-filling cementation,
compaction and pressure solution, recrystallization, and dolomitization. The plate documents the integration of
depositional and diagenetic features.
1 Reef limestone. The bulk of many reef limestones consists of reef-derived detritus rather than the autochthonous organic
frameworks as documented by this sample. Depositional microfacies: The thin section shows skeletal grains, organic
crusts, sediment and carbonate cements. Bioclasts are represented by a gastropod (transverse section; black arrow points
to the inner layer, white arrows to the outer shell layer), corals (C) and a dasyclad green alga (D; transverse section of
Diplopora ). Biogenic crusts are microbial spongiostromate crusts (SPC) consisting of micritic and peloidal laminae of
different thickness. The crusts exhibit biogenic overgrowth (BO) made of foraminifera, tiny tubiform fossils, sponges and
microbes. Fine-grained sediment (S) within and between the skeletal grains consists of allomicrite with small shell frag-
ments (lower part of the graded internal sediment in the gastropod) and microbial automicrite (evidenced by peloid
microstructure and bacterial microfossils visible only at high magnifications). The sequence of depositional events com-
prises sedimentation of bioclasts, followed by biogenic overgrowth and microbial encrustation on skeletal grains result-
ing in a total occlusion of most of the interskeletal pores. Note the geopetal fabric formed by frozen spirit levels within the
gastropod and in other small voids. Diagenetic microfacies criteria are cements, neomorphic alterations, dissolution
features and fracturing. Calcite cements occur predominantly within the shell interior (marine radial fibrous cement, RFC,
and late diagenetic recrystallized blocky cement, BC) and in some tiny voids within the matrix. Aragonitic fossils (corals
and the inner layer of the gastropod shell, black arrow) were replaced by coarse sparry calcite. The original skeletal
mineralogy is reflected by a different type of preservation: The originally calcitic outer shell layer (white arrows) differs
in the finer crystal size and contains relicts of the microstructure. Note the different preservation of the primarily arago-
nitic gastropod shell and the originally High-Mg calcitic microbial automicrite crusts (SPC), both of which are now
preserved as Low-Mg calcite. The reef limestone was affected by near-surface and marine diagenesis (inversion of arago-
nite; radiaxial fibrous cement) and burial diagenesis (coarse blocky calcspar cement). Mechanical compaction was hin-
dered by rapid cementation of synsedimentary organic encrustation. Breakage and dissolution seams within the gastropod
shell and between the gastropod and the coral point to incipient pressure solution. The complex microcrack network is of
tectonic origin. Late Triassic (Dachstein reef limestone, Norian): Gosaukamm, Austria. Crossed nicols.
2 Lagoonal limestone. The sample represents a slightly dolomitized skeletal rudstone formed in a restricted lagoonal inner-
shelf environment. Depositional microfacies: The sediment consists of dasyclad green alga (star-shaped cross section of
Clypeina , CL), and various coated grains and pisoids characterized by closely spaced laminae (long arrow). Diagenetic
microfacies: Rare isopachous fibrous cements (short arrows) indicate diagenesis in a marine phreatic environment. Inter-
granular voids are filled by clear granular cements (GC) representing former drusy cements that underwent crystal coars-
ening (aggrading neomorphism) during burial. Dolomitization postdates cementation as shown by the occurrence of
idiomorphic dolomite rhombohedrons within micrite, grains and the cement of interparticle pores. Late Jurassic (Titho-
nian): Trnovo, southwestern Slovenia.
--> 2: Koch et al. 1989
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