Geology Reference
In-Depth Information
Category I includes predominantly bioclastic wacke-
stones followed by grainstones, floatstones, rudstones
and mudstones.
of <1
m. High amounts of very small pores within a
range of between 100 and >1
m may impede the infill
of free pores with water and therefore, the disintegra-
tion of the limestones by freezing. However, these dif-
ferences are modified by pore geometry. A high water
saturation potential of grainstones (depending on pri-
mary texture, selective dissolution, incomplete cemen-
tation) will negatively affect frost resistance.
Although these examples can not be regarded as sta-
tistically significant for specific limestone or dolomite
types, the importance of texture and diagenetic char-
acteristics in causing weak or strong physical, chemi-
cal and biochemical weathering of carbonate rocks is
evident.
Further studies must focus on data leading to quan-
titative interpretations (using defined weathering-po-
tential indices, e.g. the 'microfracture index' describ-
ing the number of microcracks in a 10 mm traverse of
the thin section) and on the study of the weathering
and decay of carbonate rocks by dissolution and bio-
chemical actions (bacteria, algae, fungi, lichens, macro-
plants, borings). Biological weathering and the inter-
action of carbonate rock surfaces with bacteria, cyano-
bacteria, fungi and lichens play an enormous role in
the weathering of buildings and works of art. It is esti-
mated that about 70% of the biodeterioration of lime-
stones and marbles is caused by microbes forming
crusts and biofilms (Danin 1982; Saiz-Jimenez 2002).
Category II also includes a broad textural spectrum
due to
•
irregularly distributed stylolites that enhance weath-
ering,
•
differences in interparticle porosities and the degree
of cementation in bioturbated and non-bioturbated
parts of limestone,
•
selective dissolution of fossils and ooids, and
•
selective dissolution of the micritic matrix.
Category III includes limestones used as building
stones in monuments, which are highly susceptible to
rapid weathering in humid climates. Primary controls
of the poor weathering resistance are:
•
differences in the lithological composition (differ-
ential weathering of carbonate and non-carbonate
or hard and soft particles),
•
differences in grain sizes and primary inter- and
intragranular porosities within various depositional
layers,
•
differences in the dissolution of bioclasts due to dif-
ferent primary skeletal mineralogies,
•
strong solution along circum- and intragranular sty-
lolites.
In summary, depositional texture is an important but
not the only control on weathering resistance, as shown
by a comparison of microfacies criteria and physical
data:
Effective porosity and permeability, but more im-
portant, the size and size distribution of the pores (de-
termining water saturation), act as important controls
on soundness and slake durability, which in turn may
influence the degree of weathering. The effective po-
rosity of the building stones studied by Grimm (1990)
varies between 0.20 and 24 vol.% for category I lime-
stones, 0.20 and about 9% for category II, and 0.8 and
35% for category III. Most mudstones, wackestones
and floatstones have porosity values <5%; values of
<1% are most frequent. Most grainstone and rudstone
samples exhibit porosities of >3%, often within a range
between about 5 and 25%.
The size of intragranular pores and solution voids
can strongly affect a susceptibility to freezing. Because
larger pores, e.g. within calcareous tufa or travertine,
become only partly filled with water, freezing may not
destroy their fabric. The pore size distribution of many
mudstone and wackestone samples with low porosi-
ties is characterized by a high percentage (about 55%,
up to nearly 100%) of intercrystalline pores with a size
Another topic is a better understanding of the time-
span involved in destructive weathering. Well-dated
monuments (e.g. archaeological sites, churches or
tombstones on churchyards) offer an elegant possibil-
ity for this approach (Marinos and Koukis 1988; Aires-
Barros et al. 1990; Düppenbecker and Fitzner 1991;
May 1994).
Microfacies, texture and weathering of building
stones.
Relations between depositional texture and
weathering are evident for bioclastic and oolitic grain-
stones (Koch et al. 1999; Andriani and Walsh 2002).
Microporosity and hygroscopic porosity are responsible
for atmospheric moisture condensation in fine and
medium calcarenites. Coarse calcarenite with larger
pores enables rapid fluid transfer into pores, causing
rapid cementation and a reduced weathering potential.
Different weathering types, intensity and distribution
can be also related to variations in the grain composi-
tion and clay content as demonstrated by Tertiary pe-
lagic
Globigerina
limestones (Fitzner et al. 1997).
Weathering indices.
Precise diagnoses of weather-
ing damage are required for evaluating and preserving
