Geology Reference
In-Depth Information
• Relationships between siliciclastic grains and car-
bonate grains. Are the siliciclastic grains incorporated
within carbonate grains (e.g. in oncoids or as nuclei of
ooids, Fig. 16.18)?
crop is caused by different weathering as a result of
differential diagenesis. Tightly cemented limestone
beds alternate with less resistant, less cemented marl
beds. The non-compacted nature of the limestones im-
plies an early diagenetic cementation and an import of
carbonate from an external source occluding the pri-
mary porosity (Bathurst 1987).
Classification.
The classification of mixed carbonate-
siliciclastic rocks suggested by Mount (1985; Sect. 8.5)
or the differentiation of arenite types (Fig. 16.19) are
recommended.
A common rock type formed in siliciclastic-carbon-
ate environments is characterized by a framework sup-
ported by sand-sized grains. The term
arenite
(Zuffa
1985) is recommended for a sample containing more
than 50% of siliciclastic sand and/or carbonate rock
fragments. Arenites consisting of subequal amounts of
carbonate rock fragments, bioclastic material and si-
liciclastic sand are termed bioclastic hybrid arenites.
Arenites composed of carbonate rock fragments and
siliciclastic sand are designated as calcilithic hybrid
arenites. Arenites consisting mainly of carbonate rock
fragments are called calcilithic arenites. The termino-
logical classification assists in the genetic interpreta-
tion of mixed shelf sediments deposited in front of a
mainland with a relatively high, articulated relief. Ex-
amples are the Late Cretaceous (Turonian to Santon-
ian) sediments of the Lower Gosau Subgroup in the
Tyrolian Northern Calcareous Alps in Austria (Sand-
ers 1998).
Controversial discussions continue on whether the
limestone-marl alternations are caused by primary
variations in the composition of the sediment - often
explained as climatically forced differences; e.g. in pe-
lagic sequences see Mount and Ward 1986; Bellanca
et al. 1996), or/and by a diagenetic enhancement of prim-
ary rhythmic sedimentation patterns (Ricken 1994).
Important points are:
• For some rhythmic successions a primary, sedimen-
tary nature of limestone-marl alternations is proven by
differences between the fossil content of limestones and
marls.
• Diagenesis strongly enhances differences in the ini-
tial sediment composition (Ricken 1986). Initial dif-
ferences may involve fluctuations in carbonate supply
(productivity cycles; Seibold 1952) or periodic in-
creases in clay supply (dilution cycles; Einsele 1982).
• Munnecke (1997) and Munnecke et al. (2001) pro-
posed a model that explains the development of lime-
stone-marl alternations by selective dissolution of ara-
gonite in marl beds and reprecipitation of calcite ce-
ment in limestone beds. Estimating the primary miner-
alogy of skeletal grains both in limestones and marls is
essential in evaluating this model.
• Because the primary sedimentary composition (fos-
sil content, carbonate mineralogy) may be changed by
selective dissolution (e.g. of aragonite), redistribution
of calcium carbonate and reprecipitation, differences
in carbonate content, sedimentary components or trace
elements do not necessarily reflect primary differences
in the sedimentary succession.
• For successions lacking indications of systematic
initial differences, a diagenetic origin of rhythmic al-
ternations from homogeneous precursor sediments has
been assumed (diagenetic overprint by selective disso-
lution of aragonite; Munnecke and Samtleben 1996;
Munnecke 1997).
• Computer modeling shows that diagenetic self-or-
ganization alone is not sufficient to produce limestone-
marl alternations that can be followed laterally over
hundreds of meters or even kilometers (Böhm et al.
2003). The simulation model indicates that the inter-
action of diagenetic self-organization, an external trig-
ger (e.g. paleoclimate) and minor differences in pri-
16.6.2 Limestone-Marl Sequences: Primary
or/and Diagenetic Origin?
Classical definitions differentiate limestone from marl
using the ranges of 95-100% CaCO
3
and 35-65%
CaCO
3
(Correns 1949), but the definitions of marl vary
strongly (Bausch 1997). In rhythmic successions the
terms limestone and marl are used in a descriptive sense.
Layers that are more resistant to weathering are called
limestones; the intercalated less-resistant layers are
commonly termed marls. The limestones of limestone-
marl alternations are not or only slightly compacted.
The highest carbonate content occurs in the central part
of the bed or nodule. Marls are strongly compacted and
have lower carbonate contents than the adjacent lime-
stone beds.
Phanerozoic limestone-marl alternations varying
from nodular (mostly marl-dominated) to well-bedded
(limestone-dominated) sequences are widespread in
ancient shelf and deep-sea environments. These se-
quences occur from shallow to deeper marine to hemi-
pelagic settings. The striking appearance in the out-
