Geology Reference
In-Depth Information
tion of nodular fabrics by shear processes affecting lime-
stone-marl/clay alternations. Reviews of genetic inter-
pretations were given by Gründel and Rösler (1963)
and Böhm (1992). Most models have been developed
for Ordovician and Silurian (Möller and Kringan 1988),
Devonian (Wendt 1982) and Jurassic (Elsmy 1981)
nodular limestones.
(e) Local cementation of burrows, partly induced by
the organic matter within the burrows (Fürsich 1973;
Abed and Schneider 1980). Bioturbation is regarded
as a major process in forming hydroplastic pseudo-
nodules (Elmi and Ameur 1984) and distributing
nodules (Möller and Kvingan 1988).
3
Formation of nodules through mechanical processes
including
(a) Early diagenetic down-slope creeping of lime-
stone/marl sequences and tearing up of partially lithi-
fied limestone beds (Seyfried 1978).
(b) Tectonic processes such as sedimentary boudinage
(Varol and Gokter 1994) or shearing of limestone/
clay sequences due to tectonic (Bourbon 1982;
Burkhard 1990; Schweigl and Neubauer 1997).
The models can be classified into three groups:
1
Nodule formation caused by solution processes and
explained as
(a) Relicts of an intensive submarine dissolution of
carbonate at the sea bottom (subsolution: Hollmann
1962, 1964), or
(b) Result of the different solubility of clay-rich and
carbonate-rich sediments during late diagenetic pro-
cesses connected with pressure solution (Bathurst
1987).
No single mechanism or sequence of mechanisms
can explain how all nodular limestones are formed.
Therefore, each case should be considered individu-
ally. Erosion and redeposition of hardened nodules
formed only a few decimeters below the sediment-wa-
ter interface seem to be a reliable interpretation for the
genesis of many ancient nodular limestones. Many au-
thors describe nodular fabrics in limestones as result-
ing from early diagenetic concretionary cementation.
However, submarine dissolution on the seafloor, a com-
bination of bioturbation, cementation and redeposition
as well as pressure-solution should also be taken into
account as possible factors.
2
Nodule formation caused by localized cementation
within the sediment: Explanations of cementation
seeds and the sources of carbonate necessary for ce-
mentation include:
(a) Bacterially-induced nodule growth (Jeans 1980)
(b) Formation of synsedimentary concretions by con-
centration of carbonate caused by decaying remains
of organisms (Kukal 1975)
(c) The term
pseudonodular
limestone. This refers
to Upper Jurassic sponge limestones composed of
densely-packed, rounded-oval to subangular carbon-
ate nodules up to 5 cm in size. The formation of
pseudonodules is explained as selective lithification
of carbonate crusts and associated siliceous sponges,
followed by fragmentation of already lithified frag-
ments caused by an increase in sedimentary load
(Matyszkiewicz 1994, 1997).
(d) Early diagenetic dissolution of carbonate within
the reduction zone caused by H
2
S- and NH
4
-rich pore
waters, followed by an ascendent flow of the pore
water into the oxidation zone due to compaction,
and leading to the precipitation of carbonate around
crystallization centers (Gründel and Rösler 1963).
Jenkyns (1974) modified this model by explaining
the carbonate-rich solution as aragonite dissolution
at the sedimentary surface, followed by a descen-
dent flow of calcite-supersaturated pore water, caus-
ing calcite precipitation a few centimeters below the
sediment surface. The model is supported by the ob-
servation of patchy cementation by High-Mg cal-
cite in subrecent carbonate mud in the Ionian Sea
and on Bahamian slopes, and is regarded as a pos-
sible precursor to nodular fabrics (Müller and Fab-
ricius 1974; Mullins et al. 1980).
Significance of nodular limestone fabrics
Evaluation of paleowater depth and paleotopo-
graphy
: Nodular limestones are bound to particular en-
vironments which developed in specific episodes of rift-
ing of furrows and basins and in specific paleogeo-
graphic situations (Wald et al. 1983; Elmi and Ameur
1984).
Settings include flat seamounts, steep slopes and flat
bottoms of basins and narrow furrows. Möller and
Kvingan (1988) drew attention to the relationships be-
tween paleotopography, paleodepth, depositional en-
vironment and the potential of calcareous sediments
for forming nodular limestones during diagenesis. Many
Paleozoic and Mesozoic nodular limestones were
formed on subtidal deep swells. Spectacular examples
are the nodular limestones of the 'Ammonitico rosso'
facies in the Jurassic of the Alpine-Mediterranean re-
gion (see Sect. 15.8.2.2, Pl. 139).
Most nodular limestones occur in deeper-marine
depositional settings. Some nodular fabrics, produced
in connection with burrowing, however, are formed in
