Geology Reference
In-Depth Information
5.3.1.2 Microfacies Analysis of Neptunian
Dikes
calcite cement (Fig. 5.13).
Common cements are early-
marine radiaxial-fibrous calcite and younger granular
calcite.
Age-diagnostic micro- and macrofossils:
Fissure in-
fillings record continuous or interrupted sedimentation
over very long time spans. These time spans can often
be evaluated with the help of conodonts, foraminifera,
and ammonites and by using 'microfacies zones' (see
Sect. 10.3).
Repeated rupture and filling:
Alternations of cement
and sediment infill indicate repeated rupture and fill-
ing of the fissures (Pl. 24/4, Wiedenmayer 1963). As
many as twenty generations of fillings have been ob-
served. Excellent examples are neptunian dikes cut-
ting through Upper Triassic lagoonal limestones and
filled with several generations of Liassic sediments at
the Loser in the Styrian Northern Alps.
Field data of neptunian dikes include rock color, ori-
entation, geometry, and size of the fissures as well as
the spatial relationships between fissures and breccias
(Füchtbauer and Richter 1983).
The color of the neptunian dikes can be distinctly
different from that of the host rock (e.g. red Liassic
fissure infillings in gray or white Triassic platform and
reef carbonates of the Alpine-Mediterranean region,
Figs. 5.12 and 5.13), but many synsedimentary fissures
are filled with carbonates displaying the same color as
the host rocks. Sharp boundaries between the fissure
infillings and the host rock indicate a tectonic origin of
the fissures (Fig. 5.12), wavy and undulated bound-
aries can be caused by subaerial or submarine dissolu-
tion and erosion (Pl. 24/1). Of major importance is the
comparison of microfacies types of fissure sediments
and host rocks, the evaluation of fossils with regard to
taphonomic and ecologic criteria as well as original
biotopes and age, and the investigation of carbonate
cements using thin sections, cathodoluminescence pe-
trography, and isotope data.
5.3.1.3 Case Studies of Neptunian Dikes in
Carbonates
Neptunian dikes are particularly associated with car-
bonate buildups because
• lithified carbonate responds in a brittle manner to
stress and develops fractures at an early stage of lithi-
fication,
• steep carbonate slopes tend to develop fractures due
to unloading and mass movements along margins of
active and drowned platforms, and
• the solubility of carbonate favors percolation of un-
dersaturated waters, assisting in void and fissure for-
mation.
Microfacies data which should be evaluated in thin
sections are:
Composition and texture of internal sediment:
Com-
mon microfacies of submarine fillings are red and gray
mudstones and bioclastic wackestones and packstones
with foraminifera, mollusk shells (ammonites, gastro-
pods, accumulations of pelagic pelecypods, Pl. 24/6)
and echinoderms (accumulations of crinoids, Pl. 24/4,
Figs. 5.12 and 5.13). In addition, brachiopods and vari-
ous pelagic biota, e.g. radiolarians and globochaete al-
gae may occur (Pl. 24/3). Lithoclasts, derived from fis-
sure walls, consisting of reworked calcite cements, or
resulting from the erosion of host rocks and stratigraphi-
cally overlying rocks, may be common. Lithoclasts can
represent reworked fragments of hardgrounds (Seyfried
1981; Wendt 1976).
Differences in the sorting of particles reflect fissure
systems that are more or less open and closed (Winterer
et al. 1991). S-parallel fissure infills are commonly fine-
bedded (Fig. 5.13). The individual beds can differ in
color shades, fossil content and stratigraphic age of the
fossils included. Depending on the environment the 'in-
ternal sediments' are of shallow-marine to pelagic ori-
gin. Microfacies of neptunian dikes were described by
Gonzalez-Donoso et al. (1983), Vachard et al. (1987)
and Vera et al. (1988).
Cements:
Fissures that were inaccessible to infill-
ing sediments are lined and partly or totally filled with
Box 5.7.
Selected case studies of neptunian dikes.
Tertiary
: Premoli Silva et al. 1998; Vachard et al. 1987
Cretaceous:
Andrieux 1967; Company et al. 1982,
Garcia-Hernandez et al. 1989, Greber 1988
Jurassic
:Böhm et al. 1999; Bouillin and Bellono 1990;
Fabricius 1968; Fels and Seyfried 1993; Jurgan 1969;
Lantos and Mallarino 2000; Lehner 1991; Martire
1996; Molina et al. 1985; Schöll and Wendt 1971;
Seyfried 1978, 1980; Vera et al. 1984; Wendt 1969;
Wiedenmayer 1963; Winterer and Sarti 1994
Triassic
: Blendinger 1986; Cozzi 2000; Fabricius 1966;
Fischer 1964; Füchtbauer and Richter 1983; Krystyn
et al. 1971; Schlager 1969; Schöll and Wendt 1971;
Wendt 1973; Wiedenmayer 1963
Permian:
Pray and Stanton 1992; Stanton and Pray 1993
Devonian
: Belka 1989; Gischler 1992; Krebs 1968;
Macdonald et al; 1994, Playford et al. 1989;
Szulczewski 1973; Tucker 1973; Wendt and Belka
1991; Wendt et al. 1984
