Geology Reference
In-Depth Information
placement of aragonite fossils and microbial fabrics by
calcite produces identical neomorphic structures
(Chafetz et al. 1993). Aragonitic skeletons (e.g. mol-
lusks, corals, green algae) have a consistent alteration
behavior (Sandberg 1984). The replacement crystals are
one or more orders of latitude larger than the aragonite
crystals they replace. The crystals generally show pit-
ted surfaces, usually fluid inclusions, and occasionally
aragonite relicts on etched sections. Elevated Sr con-
tents may be present.
Clues to the lime-mud precursor mineralogy of fine-
grained limestones
: Paleozoic and Mesozoic micro-
crystalline limestones show differences with regard to
common crystal sizes (Honjo 1969; Boss and Wilkinson
1991). The matrix of many Paleozoic fine-grained lime-
stones consists of microspar in contrast to Mesozoic
limestones with a micrite-sized matrix (Sect. 4.1). This
difference has been explained by differences in the origi-
nal mineralogy of carbonate muds. The dominant min-
eralogy of mud precursors of micrites has a strong in-
fluence on the resulting microfabric (Lasemi and Sand-
berg 1993). A distinction between ADP (Aragonite-
Dominated Precursors) and CDP (Calcite-Dominated-
Precursors) micrites is therefore necessary. ADP micrites
are characterized by aragonite relicts, pitted crystal sur-
faces, common microspar fabrics and high amounts of
strontium. CDP micrites are inferred by the absence of
aragonite relicts, non-pitted crystal surfaces, the domi-
nance of very small crystals (< 4
m) and low amounts
of strontium.
Recognizing former aragonite in cements
: Arago-
nite relicts can be verified by X-ray diffraction, elevated
Sr (EDX), electron diffraction pattern (TEM) and simi-
larities with known aragonite relicts based on morphol-
ogy, etching behavior and brightness in SEM. Diag-
nostic criteria, arranged in a hierarchy of decreasing
reliability (Sandberg 1983), are (1) a relative coarse
calcite mosaic crosscutting original structures with in-
clusions of original aragonite; (2) calcite mosaics as
for 1, no aragonite relicts, but elevated amounts of Sr;
(3) calcite mosaics as for 1, and (4) molds or subse-
quently filled molds.
7.2 Major Diagenetic Environments
Recognizing former Mg-calcite in fossils
(Lohmann
and Meyers 1977; Leutloff and Meyers 1984; Yoo and
Lee 1993; Dickson 1995): Indications of former High-
Mg calcite in skeletons are the replacement of skel-
etons by ferroan calcite with preservation of the origi-
nal structures, inclusions of microdolomite, low Sr/Mg
ratios, and fossils that have modern analogs known to
consist of Mg-calcite.
Diagenetic environments are surface or subsurface
zones affected by specific diagenetic processes and typi-
fied by assemblages of characteristic criteria that can
be studied in thin sections and by geochemical meth-
ods. The concept was developed by Matthews (1967,
1971), Purdy (1968), and Land (1970). Diagnostic cri-
teria are cement types and cement fabrics (Sect. 7.4.3).
Box 7.1.
Selected case studies on processes occurring in different diagenetic environments.
Meteoric environment:
Buchbinder and Friedman 1980; Budd and Land 1990; Budd and Vacher 1991; Budd et al.
1993; Buddemeyer and Holladay 1977; Burns and Swart 1992; Dravis 1996; Dunham 1969a, 1969b, 1971, 1976,
1979; Evans and Ginsburg 1987; Friedman and Kolesar 1971; Humphrey et al. 1986; James and Choquette 1990;
Keupp 1978; Koch and Rothe 1985; Land 1970; Longman 1980; Matthews 1968; Quinn 1991; Reeckmann and Gill
1981; Richter 1979; Schroeder 1973; Steinen 1979; Steinen and Matthews 1973; Thorstenson et al. 1972; Vollbrecht
1990
Beachrock and marine vadose zone:
Alexandersson 1973; Amieux et al. 1989; Beier 1985; Binkley et al. 1980;
Cooper 1991; David and Kinsey 1973; Gischler and Lomando 1997; Gray and Adams 1995; Hollail and Rashed
1992; Kneale and Viles 2000; Meyers 1987; Neumeier 1999; Pigott and Trumbley 1985; Richter 1979; Schillings
1994; Scoffin and Stoddart 1983
Mixing zone and freshwater lenses:
Back et al. 1986; Chafetz et al. 1988; Kimbell and Humphrey 1994; Richter et al.
1990; Steinen et al. 1978; Stoessel et al. 1989; Stoessel and Ward 1990
Shallow marine environment:
Burns and Swart 1992; Evans and Ginsburg 1987; Ginsburg 1957; Ginsburg and James
1976; Ginsburg and Schroeder 1973; Longman 1980; Mullins et al. 1985; Vollbrecht 1990; Vollbrecht and Meischner
1996
Deep marine environment:
Adelseck 1978; Archer 1991; Backerand Mill 1986; Berger 1975; 1978; Broecker and
Takahashi 1978; Cook and Egbert 1983; Lingen 1975; Milliman 1971; Schlager and James 1978; Schlanger and
Douglas 1974; Scholle 1977
Burial environment:
Halley 1984; Hemleben and Auras 1984; Hobert and Wetzel 1989; Hutcheon 1989; James and
Choquette 1990; Wanless 1983
