Geology Reference
In-Depth Information
mites (Bell 1981; Hozman 1983). Some micritic lime-
stones exhibit exceptional purities and high intergranu-
lar porosities and therefore have a high aptitude for
disintegration and grainy destruction. This is a good
base for the production of high quality technical lime-
stones.
Rock color and color stability.
The colors of lime-
stones are influenced by environmental and diagenetic
factors. Interconnected environmental factors are wa-
ter energy, sedimentation rates, influx of non-carbon-
ate material, pore water flux and the position of the
oxidation/reduction boundary. The differences in these
factors govern the distribution of coloring agents such
as iron and its oxides, manganese and organic substance
and favor the development of specific colors in differ-
ent parts of marine environments. The iron content, rock
color and depositional facies can exhibit distinct cor-
relations as shown by Upper Jurassic carbonates from
Southern Germany (Grunenberg 1992; Engelbrecht
1992; Dimke 1997). The recognition of these relations
during an early phase of prospecting assist in a guided
exploitation of the carbonate rocks.
Facies-dependent controls on weathering connected
with fluid sorption, dissolution and freezing are:
•
depositional structures (bedded carbonates are gen-
erally more susceptible to weathering by freezing
than massive limestones) and inhomogenities of
carbonate bodies (variations in texture, mineralogy
as well as joints),
•
depositional texture (mud- or grain-support, grain-
or crystal fabric),
•
effective porosity and permeability,
•
the amount and distribution of non-carbonate mate-
rial (freezing potential can increase with increasing
clay mineral content or with the concentration of
clay minerals within layers).
These generalizations should be regarded with some
caution, because of the scarceness of studies on causal
relationships between the facies criteria of carbonate
rocks and those physical and chemical data that con-
trol weathering patterns.
Case studies.
Grimm (1990) provided a wealth of
information on the weathering potential of rocks com-
monly used for monuments in Germany. The data in-
clude geological characterization, fabric, microfacies
criteria (composition, texture, diagenetic criteria) and
SEM observations as well as physical parameters (spe-
cific gravity, porosity, water storage capacity and per-
meability). Grimm classifies weathering resistance as
very good, moderate and poor. Very good and good
weathering resistance are characterized by the absence
or rarity of abrasion criteria. Samples with moderate
resistance exhibit selective weathering of parts of the
rock as well as common chipping, exfoliation and crum-
bling. Poor resistance is marked by strong disintegra-
tion. Using these data, three categories (I: very good
and good; II: good to moderate, intermediate weather-
ing resistance and wide resistance range; III: moderate
and poor) can be differentiated, allowing the textural
composition of limestones and weathering to be com-
pared.
18.4 Weathering, Decay and
Preservation of Carbonate Rocks
18.4.1 Weathering of Carbonate Rocks
Physical, chemical and biological weathering of sub-
aerially exposed carbonate rocks lead to
•
abrasion connected with disintegration, chipping,
crumbling, exfoliation,
•
corrosion connected with the solution of carbonate
minerals, changes in the non-carbonate minerals and
organic matter, causing changes in the roughness of
the surfaces and in rock colors,
•
the formation of weathering crusts,
•
changes in the hardness, strength and stress patterns.
These alterations are evident in material used for
the construction of buildings, pavements or works of
art. Carbonate rocks used as outdoor building stones
(e.g. dimension stones forming walls or monuments)
are affected by natural and man-made weathering
agents, leading to stone decay caused by moisture and
salts, frost action, chemical weathering and biological
contributions (Winkler 1994). Weathering resistance
depends on many, partly interconnected factors. Po-
rosity and permeability and the fabric (expressed by
grain size, grain distribution, grain form, grain type,
depositional texture, mud-support and grain-support)
are of primary importance, because these factors con-
trol the storage capacities of water and gasses.
There is a common assumption that micritic, mud-
supported limestones are more heavily affected by
weathering than sparitic grain-supported carbonates,
because of the higher specific surface of fine-grained
limestones. This is only partly supported by the data.
Very good and good weathering resistance occurs with
about equal amounts in mud- and in grain-supported
rocks. Intermediate resistance seems to be more com-
mon in micritic limestones, whereas low weathering
resistance appears restricted to grain-supported carbon-
ates.
