Geology Reference
In-Depth Information
(e.g. U.S. National Research Council Rock-Color Chart
based on the Munsell Color System: Goddard et al.
1948).
The use of the
Rock Color Chart
requires (a) deter-
mination of the gradation of color (hue), followed by
(b) estimation of light to dark values, and (c) estima-
tion of the degree of saturation (chroma). Colors are
indicated by a code (e.g. 5 R 4/6) or by a descriptive
name (e.g. moderate red). Color names used in the Rock
Color Chart do not always correspond to the subjec-
tive color impression of geologists, which may be ad-
equate for general surveys but not, for example for the
characterization of carbonate rocks used in the arts or
as building stones. Colorimetric analysis is another
approach but relatively rarely used for limestones
(Engelbrecht 1992), except when determining techno-
logical criteria.
Colors of carbonate rocks are strongly controlled
by depositional conditions, diagenesis and recent weath-
ering. The amount and composition of the non-carbon-
ate fraction is crucial for the color aspect because the
adsorption of iron and manganese on the surface of
finely dispersed clay minerals is often responsible for
distinct colors. Fe and Mn content, rock color and depo-
sitional facies may exhibit distinct correlations, spe-
cifically in hemipelagic and pelagic sediments.
Of particular significance is the oxidation-reduction
balance: Yellow colors, for example, can be caused by
the oxidation of FeS as demonstrated by the famous
Jurassic Solnhofen Limestone of Southern Germany
which appears in fresh samples bluish gray. Compari-
sons of photometric color measurements with geo-
chemical data of limestones indicate correlations of
bluish and yellowish colors with pyrite and goethite
contents and dark or light rock color values with clay
and silt contents. Green and drab gray colors may be
caused by organically derived carbon and Fe
2+
com-
pounds such as sulfides in reduced sediments. Various
shades of red, yellow and brown colors are due to Fe
3+
compounds such as hematite and limonite in oxidized
sediments. Yellowish shades of marbles can be caused
by small quantities of siderite.
Dark colors are also induced by aromatic hydrocar-
bons (pure solvent-soluble organic matter composed
of carbon and hydrogen only). The amount of organic
matter and the degree of light reflection are related
(Patnode 1941). Black colors can also be caused by
thermal heating. Sedimentary limestones may undergo
considerable color change during metamorphism
(Winkler 1994).
Fig. 3.1.
Common lithologies of carbonate rocks and mixed
carbonate-siliciclastic rocks. The symbols used are those from
the standard Shell Legend.
in weak HCl stains limestone pink to mauve but leaves
dolomite unstained.
Mixed siliciclastic-carbonate lithologies include
marls (a sediment consisting of 65-35% carbonate and
35-65% clay), argillaceous limestones (a limestone
containing an appreciable amount of clay, less than
50%) and sandy limestones (a limestone containing an
appreciable amount of quartz sand).
Texture:
Attention should be focused on the deposi-
tional variability of limestone beds, on depositional pro-
cesses recorded by characteristic sedimentary patterns,
and on the control of microfacies-based paleoenviron-
mental interpretations using larger-scaled criteria. The
texture of major limestone types can be characterized
in the field using the classification proposed by Dun-
ham (1962); see Sect. 8.3.2.
Much information is provided by the size, shape and
sorting of grains, the ratio of matrix to grains, and the
fabric. Details are described in Chap. 4. The textural
composition of beds and laminae may be characterized
by distinct vertical gradations in grain size (graded bed-
ding). 'Normal grading' (coarser grains at the base, finer
grains at the top) and 'reverse or inverse grading'
(coarser particles at the top grading downwards to finer
particles) are important in the interpretation of deposi-
tional processes (Pl. 17/1,
Pl.
102/2,
Pl.
115/2,
Pl.
116/
1,
Pl.
126/2).
Rock color:
Many limestones are a shade of gray, but
limestones can also display distinctive rock colors
which need a more precisive terminology (Folk 1969).
Dolomites are often creamy yellow or brown in color.
Rock colors can be characterized by simple verbal de-
scriptions (e.g. medium to dark gray) or by comparing
fresh rock fracture surfaces with standard color schemes
Why is this limestone red?
Vivid rock colors gener-
ally provoke questions which are difficult to answer.
