6.3 Red Beds
The hydrous ferric iron oxides, such as goethite and the
less well defined limonite, are brown to ocher in color.
The aspects of diagenetic reddening have been discussed by Walker (1967, 1976, 1979), Füchtbauer (1988),
and Dubiel andSmoot (1994).
To generate the red staining of clays and silts less
than I % of hematite is sufficient. The total iron content of claystones, however, is comrnonly two to
three times higher than that needed by the staining
pigment, because clay minerals and other mineral
phases contain both ferric and ferrous iron. In alternating red and green claystones, the iron content of
red beds is, if at all, generally insignificantly higher
than that of green beds.
The red color of sands tones is caused by grain
coatings containing hematite; their hematite content
may be even less than that in claystones. Reddish
colored top layers of sandstones can be caused by
mechanical infiltration of detrital clay.
The presence of limonite or hematite indicates that
the sediment is in an oxidizing state. This is generally
better achieved and maintained in continental environments above the groundwater table than in aquatic
systems. Red beds are therefore particularly comrnon
in fluvial deposits. During the early history of the
Earth, when the atmosphere was poor in or free of
oxygen (cf. Sect. 6.5.1), the formation of red beds
was 1imited or impossible.
Oxidizing conditions within a sediment can be
maintained for a 10ng time when little or no organic
matter or reducing pore waters are available to reduce the ferric to ferrous iron. Then, iron cannot be
incorporated into iron-bearing carbonates, clay minerals, or form iron sulfide (pyrite). Furthermore, the
practically insoluble ferric iron cannot be carried
away by circulating pore water like dissolved ferrous
iron. However, if the buried sediments contain sufficient organic matter to reduce all the pre-existing
limonite and hematite, the brown or red pigment cannot survive. Hence, these sediments become drab
gray in color. Secondary reddening can occur if the
excess organic matter is destroyed by oxidizing pore
water or some other mechanism.
Having these basic rules in mind, the occurrence
of red beds in various depositional environments is
easy to understand.
Primary, Allochthonous Red Beds
Red, lateritic soils develop preferentially in tropical
regions with seasonally wet and dry climates (wetdry tropics). These soils contain hematite in addition
to hydrated iron oxide and other stable minerals such
as quartz and clay (cf. Sect. 9.1). Such soils can be
eroded and carried by rivers or wind into regions
with differing climates (Fig. 6.3a). In rare cases, red
dust and red-stained sand rnay reach deserts and form
255
primarily red sand dunes and clay dunes (cf. Sect.
2.3). However, most modem eolian dunes are yellowish and ocher in color.
The reddr-ning of sand dunes in Australia is only observed
in old eoJian sands where originally brown-coated sand
grains have become red by aging (W opfner and Twidale
1988, see below). Red soils and sand dunes have been described, e.g., by Millot (1964), Solle (1966), Valeton
(1983) and rnany others.
More frequently, river-transported red soils accumulate as fluvial deposits in semi-arid and arid
lowlands, or they are swept into lakes where they
may alternate with carbonates and playa evaporites.
If the redeposited soils contain sufficient organic
matter, hematite and ferric iron hydroxides are
slowly reduced. As a resu1t, the bed becomes drab
gray or greenish in color. Alternations of red and
green beds, as observed in many ancient lake deposits, may be caused by minor differences in primary
organic matter content. Red silts and sands, or
carbonate-rich muds, which originally contain little
hematite and ferric iron hydroxides, require less organic matter for decoloring than do silty clays and
clayey silts with relatively high ferric iron contents.
For that reason, sandstones, limestones, and
evaporites, intercalated into red claystones, frequently show drab gray colors.
Redeposited lateritic'soils on river floodplains can
maintain their original color when, as in semi-arid
regions, Gparse vegetation cannot provide sufficient
organic matter for ferric iron reduction. Ancient examples of this type of red beds frequently contain in
situ pedogenic calcrete (cf. Sects. 2.2.3 and 9.2), indicating a relatively dry paleoclimate. Similarly,
rapid deposition of carbon-poor prodelta sediments
in lakes and in the sea may allow the maintenance of
the primary red color in parts of the prodelta sequences.
Red prodelta deposits are known, for example, from the
marine deltas of the Orinoco, Amazon, and Yangtze
Rivers, i.e., rivers draining tropical and subtropical regions.
In shallow seas with limited inputs of lateritic soils, the red
pigment mostly vanishes at depths of a few centimeters to
decimeters below the sediment-water interface, as observed
in the Adriatic Sea (Hinze and Meischner 1968) or in the
western part of the Gulf of Aden (pers. observ. ).
If red soils are carried into lakes and swamps in humid regions, they are usually reduced and form gray
deposits alternating with organic-rich layers such as
peat. Occasionally, a thick red bed may be preserved
within a sequence with coal searns, as for example in
some Tertiary coals in Europe. Another instructive
example is the barren red Carboniferous coal measures of Britain. Here, the red beds occur adjacent to
the productive coal measures of the same age (Glennie 1970; Turner 1980). Conclusive evidence for the
The hydrous ferric iron oxides, such as goethite and the
less well defined limonite, are brown to ocher in color.
The aspects of diagenetic reddening have been discussed by Walker (1967, 1976, 1979), Füchtbauer (1988),
and Dubiel andSmoot (1994).
To generate the red staining of clays and silts less
than I % of hematite is sufficient. The total iron content of claystones, however, is comrnonly two to
three times higher than that needed by the staining
pigment, because clay minerals and other mineral
phases contain both ferric and ferrous iron. In alternating red and green claystones, the iron content of
red beds is, if at all, generally insignificantly higher
than that of green beds.
The red color of sands tones is caused by grain
coatings containing hematite; their hematite content
may be even less than that in claystones. Reddish
colored top layers of sandstones can be caused by
mechanical infiltration of detrital clay.
The presence of limonite or hematite indicates that
the sediment is in an oxidizing state. This is generally
better achieved and maintained in continental environments above the groundwater table than in aquatic
systems. Red beds are therefore particularly comrnon
in fluvial deposits. During the early history of the
Earth, when the atmosphere was poor in or free of
oxygen (cf. Sect. 6.5.1), the formation of red beds
was 1imited or impossible.
Oxidizing conditions within a sediment can be
maintained for a 10ng time when little or no organic
matter or reducing pore waters are available to reduce the ferric to ferrous iron. Then, iron cannot be
incorporated into iron-bearing carbonates, clay minerals, or form iron sulfide (pyrite). Furthermore, the
practically insoluble ferric iron cannot be carried
away by circulating pore water like dissolved ferrous
iron. However, if the buried sediments contain sufficient organic matter to reduce all the pre-existing
limonite and hematite, the brown or red pigment cannot survive. Hence, these sediments become drab
gray in color. Secondary reddening can occur if the
excess organic matter is destroyed by oxidizing pore
water or some other mechanism.
Having these basic rules in mind, the occurrence
of red beds in various depositional environments is
easy to understand.
Primary, Allochthonous Red Beds
Red, lateritic soils develop preferentially in tropical
regions with seasonally wet and dry climates (wetdry tropics). These soils contain hematite in addition
to hydrated iron oxide and other stable minerals such
as quartz and clay (cf. Sect. 9.1). Such soils can be
eroded and carried by rivers or wind into regions
with differing climates (Fig. 6.3a). In rare cases, red
dust and red-stained sand rnay reach deserts and form
255
primarily red sand dunes and clay dunes (cf. Sect.
2.3). However, most modem eolian dunes are yellowish and ocher in color.
The reddr-ning of sand dunes in Australia is only observed
in old eoJian sands where originally brown-coated sand
grains have become red by aging (W opfner and Twidale
1988, see below). Red soils and sand dunes have been described, e.g., by Millot (1964), Solle (1966), Valeton
(1983) and rnany others.
More frequently, river-transported red soils accumulate as fluvial deposits in semi-arid and arid
lowlands, or they are swept into lakes where they
may alternate with carbonates and playa evaporites.
If the redeposited soils contain sufficient organic
matter, hematite and ferric iron hydroxides are
slowly reduced. As a resu1t, the bed becomes drab
gray or greenish in color. Alternations of red and
green beds, as observed in many ancient lake deposits, may be caused by minor differences in primary
organic matter content. Red silts and sands, or
carbonate-rich muds, which originally contain little
hematite and ferric iron hydroxides, require less organic matter for decoloring than do silty clays and
clayey silts with relatively high ferric iron contents.
For that reason, sandstones, limestones, and
evaporites, intercalated into red claystones, frequently show drab gray colors.
Redeposited lateritic'soils on river floodplains can
maintain their original color when, as in semi-arid
regions, Gparse vegetation cannot provide sufficient
organic matter for ferric iron reduction. Ancient examples of this type of red beds frequently contain in
situ pedogenic calcrete (cf. Sects. 2.2.3 and 9.2), indicating a relatively dry paleoclimate. Similarly,
rapid deposition of carbon-poor prodelta sediments
in lakes and in the sea may allow the maintenance of
the primary red color in parts of the prodelta sequences.
Red prodelta deposits are known, for example, from the
marine deltas of the Orinoco, Amazon, and Yangtze
Rivers, i.e., rivers draining tropical and subtropical regions.
In shallow seas with limited inputs of lateritic soils, the red
pigment mostly vanishes at depths of a few centimeters to
decimeters below the sediment-water interface, as observed
in the Adriatic Sea (Hinze and Meischner 1968) or in the
western part of the Gulf of Aden (pers. observ. ).
If red soils are carried into lakes and swamps in humid regions, they are usually reduced and form gray
deposits alternating with organic-rich layers such as
peat. Occasionally, a thick red bed may be preserved
within a sequence with coal searns, as for example in
some Tertiary coals in Europe. Another instructive
example is the barren red Carboniferous coal measures of Britain. Here, the red beds occur adjacent to
the productive coal measures of the same age (Glennie 1970; Turner 1980). Conclusive evidence for the