254
sediment is reworked, and the various pre-existing
iron compounds are partially oxidized, dissolved, and
replaced by iron ooids, possibly with the aid of microorganisms. Alternating exposure to aerated waters
and reducing conditions, after shallow burial below
mud, may lead to oolite growth under changing redox
conditions (Fig. 6.2b).
During repeated transgressions and regressions,
the sand-sized ooids are sorted out and concentrated
in ooid bars in the foreshore zone or on submarine
swells. This zone alternatively migrates landward and
seaward with rising and falling sea level. If the sea
level becomes particularly low, the ooids move far
basinward and tend to become stable and buried under the subsequent finer grained transgressive sediments. In fact, many iron oolites represent the roof
beds of transgressive-regressive, shallowing-upward
sedimentary cyc1es. These typically show the following sequence (Fig. 6.2c, from top to bottom):
- Oolitic ironstone, distinctly cross-bedded (sometimes bipolar, indicating subtidal environments).
Summary (Iron Oolites)
- Oolitic ironstones occur predominantly in clastic
shallow-marine sequences and represent a kind
of condensed lag sediment.
- The iron of large ore deposits appears to have
been supplied as c1astic particles by rivers draining deeply weathered lateritic soils. In some
cases, volcanic ash may have delivered iron and
silica.
- The ooids and other iron-rich partic1es formed in
coastal waters, probably with the aid of benthic
microbial communities and under repeatedly
6.3 Red Beds
General Aspects
Red beds inc1ude claystones, sandstones, arkoses,
radiolarites, some limestones, and occasionally other
rock types such as tephra layers. The striking reddish
color of red beds, in contrast to the drab gray color of
most other ancient sedimentary rocks, has always
roused the interest of both lay persons and professional geologists alike. Most authors assurne that the
staining pigment of red beds is very fine-grained,
uniformly dispersed hematite (Fe 2 0 3 ), whereas hematite concentrated in larger crystals or at certain spots
does not cause red color.
It was frequently pointed out that red beds indicate
ancient arid environments, because they are often
associated with evaporites. Indeed, this type of red
Chapter 6 Special Depositional Environments
The oolites often alternate with shell beds and
other lag deposits and are sometimes overlain by
thin layers containing phosphorite nodules and
glauconitic minerals.
- Quartz sand, predominantly fine-grained.
- Greenish shales with some siderite concretions.
- Dark pyritic shales (peak oftransgression).
Oolitic ironstones are frequently excellent marker
beds.
Most Phanerozoic ironstones formed in epicontinental seas during periods of long-term high sea level
(Ordovician to Devonian and Jurassic to Paleogene),
superimposed by short-term high-frequency sea-Ievel
fluctuations (Van Houten and Arthur 1989). During
these times, the c1imate was warm and humid,
terrigenous sediment influx into the sea was low, and
deeper water masses tended to become poody oxygenated. Thus, black shale deposition, coeval with, or
shortly before or after iron oolite formation was common.
changing environmental conditions.
- In tidal-dominated regimes, the iron oolites
mostly represent sediments of the subtidal to
lower intertidal zones.
Phanerozoic ironstones formed preferentially
during long-term sea-Ievel highstands, but minor transgressive-regressive cycles led to winnowing, repeated migration, and final deposition of oolitic sand bodies (offshore bars or
more widely extended sheets of iron oolite).
bed is particularly common and occurs in regions of
low paleolatitude. In the Perrnian and Triassie, thick,
widespread red beds accumulated on all continents.
However, red sands or soils are rare and not typical
in present-day deserts. In addition, red beds were
identified in a variety of other depositional environments, inc1uding lake and marine sediments. In principle, they can occur even in glacial sequences.
These findings led to much controversy over the origin of
red beds. Summaries on the current state of knowledge
were published by Glennie (1970), Van Houten (1973),
Turner (1980) and in several textbooks on sedimentology
and stratigraphy (e.g., Dunbar and Rodgers 1957; Leeder
1982).
Torrent and Schwertmann (1987) have pointed out that
the red color is produced by the special optical behavior of
tiny hematite clusters; the color of synthetic hernatite-clay
mixtures varies with the grain size of the hernatite crystals.
sediment is reworked, and the various pre-existing
iron compounds are partially oxidized, dissolved, and
replaced by iron ooids, possibly with the aid of microorganisms. Alternating exposure to aerated waters
and reducing conditions, after shallow burial below
mud, may lead to oolite growth under changing redox
conditions (Fig. 6.2b).
During repeated transgressions and regressions,
the sand-sized ooids are sorted out and concentrated
in ooid bars in the foreshore zone or on submarine
swells. This zone alternatively migrates landward and
seaward with rising and falling sea level. If the sea
level becomes particularly low, the ooids move far
basinward and tend to become stable and buried under the subsequent finer grained transgressive sediments. In fact, many iron oolites represent the roof
beds of transgressive-regressive, shallowing-upward
sedimentary cyc1es. These typically show the following sequence (Fig. 6.2c, from top to bottom):
- Oolitic ironstone, distinctly cross-bedded (sometimes bipolar, indicating subtidal environments).
Summary (Iron Oolites)
- Oolitic ironstones occur predominantly in clastic
shallow-marine sequences and represent a kind
of condensed lag sediment.
- The iron of large ore deposits appears to have
been supplied as c1astic particles by rivers draining deeply weathered lateritic soils. In some
cases, volcanic ash may have delivered iron and
silica.
- The ooids and other iron-rich partic1es formed in
coastal waters, probably with the aid of benthic
microbial communities and under repeatedly
6.3 Red Beds
General Aspects
Red beds inc1ude claystones, sandstones, arkoses,
radiolarites, some limestones, and occasionally other
rock types such as tephra layers. The striking reddish
color of red beds, in contrast to the drab gray color of
most other ancient sedimentary rocks, has always
roused the interest of both lay persons and professional geologists alike. Most authors assurne that the
staining pigment of red beds is very fine-grained,
uniformly dispersed hematite (Fe 2 0 3 ), whereas hematite concentrated in larger crystals or at certain spots
does not cause red color.
It was frequently pointed out that red beds indicate
ancient arid environments, because they are often
associated with evaporites. Indeed, this type of red
Chapter 6 Special Depositional Environments
The oolites often alternate with shell beds and
other lag deposits and are sometimes overlain by
thin layers containing phosphorite nodules and
glauconitic minerals.
- Quartz sand, predominantly fine-grained.
- Greenish shales with some siderite concretions.
- Dark pyritic shales (peak oftransgression).
Oolitic ironstones are frequently excellent marker
beds.
Most Phanerozoic ironstones formed in epicontinental seas during periods of long-term high sea level
(Ordovician to Devonian and Jurassic to Paleogene),
superimposed by short-term high-frequency sea-Ievel
fluctuations (Van Houten and Arthur 1989). During
these times, the c1imate was warm and humid,
terrigenous sediment influx into the sea was low, and
deeper water masses tended to become poody oxygenated. Thus, black shale deposition, coeval with, or
shortly before or after iron oolite formation was common.
changing environmental conditions.
- In tidal-dominated regimes, the iron oolites
mostly represent sediments of the subtidal to
lower intertidal zones.
Phanerozoic ironstones formed preferentially
during long-term sea-Ievel highstands, but minor transgressive-regressive cycles led to winnowing, repeated migration, and final deposition of oolitic sand bodies (offshore bars or
more widely extended sheets of iron oolite).
bed is particularly common and occurs in regions of
low paleolatitude. In the Perrnian and Triassie, thick,
widespread red beds accumulated on all continents.
However, red sands or soils are rare and not typical
in present-day deserts. In addition, red beds were
identified in a variety of other depositional environments, inc1uding lake and marine sediments. In principle, they can occur even in glacial sequences.
These findings led to much controversy over the origin of
red beds. Summaries on the current state of knowledge
were published by Glennie (1970), Van Houten (1973),
Turner (1980) and in several textbooks on sedimentology
and stratigraphy (e.g., Dunbar and Rodgers 1957; Leeder
1982).
Torrent and Schwertmann (1987) have pointed out that
the red color is produced by the special optical behavior of
tiny hematite clusters; the color of synthetic hernatite-clay
mixtures varies with the grain size of the hernatite crystals.