Geology Reference
In-Depth Information
Very generally, the coloration of sediments is regarded
as synsedimentary (caused by the input of detrital col-
oring material or by contributions of ferric bacteria;
Mamet et al. 1997) or postsedimentary (caused by di-
agenetic changes within the sediment or coloration by
overlying beds). Red limestones may serve as an ex-
ample for the problems involved:
Colored paleosols (e.g.
terra rossa
, a residual soil,
over limestone bedrock, typically in karst areas) can
impact on rock colors. Reddening of paleosol horizons
is caused by (a) in situ alterations of iron-rich clays,
(b) reddened detritus brought from the exposed surface
downward through the soil by percolating waters,
(c) dehydration of goethite to limonite and hematite dur-
ing subaerial exposure in a well-drained environment
(Retallack 1981), or (d) concentration of reddish ma-
trix by eluviation within the paleosol profile during al-
ternating wet and dry climate periods (Goldhammer and
Elmore 1984).
Terra rossa
, brought to the sea will not
directly cause red colors (Hinze and Meischner 1968).
Near-surface cavities in karstified reef limestones may
contain red internal sediments, derived from an over-
lying soil zone, with deeper cavities being filled with
green and gray internal sediments (Satterley et al. 1994).
Changes of rock colors with depth below the surface
indicate changes in the oxidation state within the va-
dose and phreatic zones (Esteban 1991).
The red color of marine carbonates is primarily
caused by small amounts of Fe
3+
(about 2%: Franke
and Paul 1980). Volcanic emanations provide a direct
source of iron in seawater. But the weathering of land
masses associated with an input of iron to the sea (as
colloids or absorbed on clay minerals) and organic
matter provide the major source. Low sedimentation
rates, near-bottom waters rich in oxygen, the absence
of sulfate-reducing bacteria as well as the local Eh/pH
potential within the sediment and at the sediment/wa-
ter interface favor the development of red colors (e.g.
by dehydration of goethite to hematite: Rech-Frollo
1971).The intensity of the red coloration may depend
on the amount of detrital material and iron (Hallam
1967; Flügel and Tietz 1971). Because iron is trans-
ported from land to sea via several solution/precipita-
tion-phases, the importance of climatic controls for the
formation of red limestones must also be considered.
The following diagenetic factors promote rock col-
oration: (a) Compaction facilitates the flow of oxydized
pore water through the sediment. (b) Pressure solution
and freeing of iron from clay minerals may be respon-
sible for red-colored stylolite seams, often occurring
together with burial dolomites (Mattes and Mountjoy
1980). (c) During dedolomitization, Fe originally in-
corporated in dolomite can be precipitated as FeO(OH)
and oxidized to Fe
2
O
3
, producing reddish rock colors.
3.1.1.2 Bedding and Stratification, Sedimen
tary Structures and Diagenetic Features
Many of the structures described in this subchapter can
also be recognized at a microfacies scale, particularly
sedimentary fabrics and diagenetic textures (Fig. 3.4).
Bedding and stratification:
Carbonate rocks are strati-
fied or non-stratified.
Non-stratification
is a result of
(a) primary lack of bedding (e.g. in reefs), (b) bed-de-
structing processes (e.g. intensive burrowing), or
(c) diagenetic processes (e.g. dolomitization or strong
recrystallization of limestones).
cm
Shell Standard Legend 1995
Boggs 1995
Demicco and Hardie 1994
Meter bedded
Very thickly bedded
100
Layers greater than 100 mm,
Thickly bedded
thick to very thick beds
30
Decimeter bedded
Medium bedded
10
Thinly bedded
Layers 5 mm to 100 mm,
3
Centimeter bedded
Laminae >5 mm
Very thinly bedded
Thin beds <100 mm
1
Thickly laminated
0.3
Millimeter bedded
Layers <5 mm, fine laminae
Thinly laminated
Fig. 3.2.
Common field classifications of stratified sediments
. The Shell classification is based on scales. Many textbooks on
sedimentology, e.g. Boggs (1995), recommend a classification based on thickness ranges. Demicco and Hardie use only
three thickness categories, whose limits were determined by the experience of the authors studying predominantly Upper
Proterozoic and Lower Paleozoic carbonates. In choosing 5 mm as the upper boundary of the thinnest category, the importance
of thin microbial mats in tidal flats has been considered. Bed thicknesses between 0.5 and 10 cm and between 10 and 30 cm
are common in shelf carbonates.
