258
Summary (Red Beds)
- Red beds contain small amounts (often less
than 1 %) of very fine-grained, uniformly dispersed hematite (Fe 2 0 3 ), or grain coatings containing hematite.
- Solely the color of red beds has no particular
paleoclimate significance.
.
..
- Red beds reflect either redeposlted latentlc
soils (allochthonous red beds derived from
wet-dry tropics) or sediments which were originally brownish in color and diagenetically altered (secondary red beds).
- Oxidizing conditions within a sediment and
thus the red color can be maintained for a long
time when little or no organic matter or reducing pore waters are available to reduce the ferric to ferrous iron.
- This is achieved in environments where sediments poor in organic matter are deposited rapidly (e.g. in some prodelta areas), or in settings
of very low or interrupted sedimentation where
the organic matter is mineralized before being
buried (e.g. on floodplains or in the deep sea).
- Sediments containing little ferric iron tend to
become gray.
6.4 Marine Evaporites
6.4.1 Introduction
Marine sea water-derived salt deposits are often more
uniform, thicker and wider extended than continental
evaporites (Sect. 2.5). Their large representatives, however, impose some difficult problems because modem
analogs do not exist. Marine salt deposits are kno~n
from the late Precambrium throughout PhanerozOlc
times, but there were some major epochs ofhalogenesis
(cf. Sect. 6.4.3). Due to their importance in geology and
economy, including the oil industry, salt deposits have
received much attention by earth scientists of different
specific disciplines.
Numerous publications have summarized our knowledge in
this field (e.g., Richter-Bemburg 1968; Braitsch 1971;
Kirkland and Evans 1973; Nissenbaum 1980; Zharkov 1981;
Sonnenfeld 1984; Dronkert 1985; Peryt 1987b; Kendall1988;
Müller 1988; Schreiber 1988a and b; Sonnenfeld and
Perthuisot 1989; Warren 1989, 1996, 1997). Publications
dealing with salt tectonics are mentioned below.
Because marine evaporites originate from sea water of
more or less constant composition (at least since the
Cambrian), their chemical characteristics can be predicted to some degree. Sodium carbonate salts typical
Chapter 6 Special Depositional Environments
of many lake evaporites are commonly absent in sea
water-derived salts. However, potash salts only indicate
sea-water origin when they are associated with salts
containing considerable amounts ofMgS0 4 • More problematic is the mode of mineral precipitation in various
types of salt basins. There is only a limited number of
cases in which the sequence and amount of marine salts
can be explained just by evaporation of a certain volume
of sea water.
Normal sea water has a density of 1.025 g/cm 3 and
contains about 35 g per liter dissolved constituents.
These can theoretically fonn the following (water -free )
salts (in percent by weight of the total salt content, Fig.
6.4a and b):
- 78% NaCl (halite),
18%potash salts, i.e., chlorides andsulfates ofKand
Mg (e.g., sylvite KCI, camallite MgC12KCI.6HzÜ,
kainite MgS0 4 KC13HzÜ, kieserite MgS0 4 HzÜ,
etc.),
- 3.5% CaS0 4 (gypsum and anhydrite),
0.3% carbonates, and some minor constituents such
as bromides, fluorides, borates.
Considering the order of precipitation from brines of
increasing concentration, the evaporite minerals can be
subdivided into the following two main groups (Warren
1996):
- Evaporitic alkaline-earth carbonates (aragonite, dolomite, low-Mg calcite and high-Mg calcite).
- Evaporite salts (gypsum, anhydrite, halite, trona, carnallite, etc.).
The evaporitic carbonates fonn in the initial stages of
brine concentration with salinities ranging from 351400/00 ( degree of evaporation 1-4 x in relation to nonnal
sea water, or water loss of 0-75%). The evaporite salts
are precipitated in the higher saline stages when the
salinities have reached ~ 140-250% (degree of evaporation ~7-11 x, or water loss ~75-85%).
The following models for the depositional environment of primary evaporites were developed from recent
examples and the study of ancient salt deposits. All of
them have in common that the depositional basin lies in
an arid to semi-arid zone and loses more water by evaporation than it receives by precipitation and inflow of
river water. Consequently, ancient salt deposits are important paleoclimatic markers, indicating arid, nonnally
low-latitude zones as opposed to wet tropical or temperate and cold high-latitude regions.
After burial and subsequent exhumation, primary
evaporites undergo significant changes. In addition,
subsurface brines can fonn evaporitic cements in the
matrix of nonnal sediments. F or these reasons, several
types of evaporites have to be distinguished (after Warren 1996, simplified):
Summary (Red Beds)
- Red beds contain small amounts (often less
than 1 %) of very fine-grained, uniformly dispersed hematite (Fe 2 0 3 ), or grain coatings containing hematite.
- Solely the color of red beds has no particular
paleoclimate significance.
.
..
- Red beds reflect either redeposlted latentlc
soils (allochthonous red beds derived from
wet-dry tropics) or sediments which were originally brownish in color and diagenetically altered (secondary red beds).
- Oxidizing conditions within a sediment and
thus the red color can be maintained for a long
time when little or no organic matter or reducing pore waters are available to reduce the ferric to ferrous iron.
- This is achieved in environments where sediments poor in organic matter are deposited rapidly (e.g. in some prodelta areas), or in settings
of very low or interrupted sedimentation where
the organic matter is mineralized before being
buried (e.g. on floodplains or in the deep sea).
- Sediments containing little ferric iron tend to
become gray.
6.4 Marine Evaporites
6.4.1 Introduction
Marine sea water-derived salt deposits are often more
uniform, thicker and wider extended than continental
evaporites (Sect. 2.5). Their large representatives, however, impose some difficult problems because modem
analogs do not exist. Marine salt deposits are kno~n
from the late Precambrium throughout PhanerozOlc
times, but there were some major epochs ofhalogenesis
(cf. Sect. 6.4.3). Due to their importance in geology and
economy, including the oil industry, salt deposits have
received much attention by earth scientists of different
specific disciplines.
Numerous publications have summarized our knowledge in
this field (e.g., Richter-Bemburg 1968; Braitsch 1971;
Kirkland and Evans 1973; Nissenbaum 1980; Zharkov 1981;
Sonnenfeld 1984; Dronkert 1985; Peryt 1987b; Kendall1988;
Müller 1988; Schreiber 1988a and b; Sonnenfeld and
Perthuisot 1989; Warren 1989, 1996, 1997). Publications
dealing with salt tectonics are mentioned below.
Because marine evaporites originate from sea water of
more or less constant composition (at least since the
Cambrian), their chemical characteristics can be predicted to some degree. Sodium carbonate salts typical
Chapter 6 Special Depositional Environments
of many lake evaporites are commonly absent in sea
water-derived salts. However, potash salts only indicate
sea-water origin when they are associated with salts
containing considerable amounts ofMgS0 4 • More problematic is the mode of mineral precipitation in various
types of salt basins. There is only a limited number of
cases in which the sequence and amount of marine salts
can be explained just by evaporation of a certain volume
of sea water.
Normal sea water has a density of 1.025 g/cm 3 and
contains about 35 g per liter dissolved constituents.
These can theoretically fonn the following (water -free )
salts (in percent by weight of the total salt content, Fig.
6.4a and b):
- 78% NaCl (halite),
18%potash salts, i.e., chlorides andsulfates ofKand
Mg (e.g., sylvite KCI, camallite MgC12KCI.6HzÜ,
kainite MgS0 4 KC13HzÜ, kieserite MgS0 4 HzÜ,
etc.),
- 3.5% CaS0 4 (gypsum and anhydrite),
0.3% carbonates, and some minor constituents such
as bromides, fluorides, borates.
Considering the order of precipitation from brines of
increasing concentration, the evaporite minerals can be
subdivided into the following two main groups (Warren
1996):
- Evaporitic alkaline-earth carbonates (aragonite, dolomite, low-Mg calcite and high-Mg calcite).
- Evaporite salts (gypsum, anhydrite, halite, trona, carnallite, etc.).
The evaporitic carbonates fonn in the initial stages of
brine concentration with salinities ranging from 351400/00 ( degree of evaporation 1-4 x in relation to nonnal
sea water, or water loss of 0-75%). The evaporite salts
are precipitated in the higher saline stages when the
salinities have reached ~ 140-250% (degree of evaporation ~7-11 x, or water loss ~75-85%).
The following models for the depositional environment of primary evaporites were developed from recent
examples and the study of ancient salt deposits. All of
them have in common that the depositional basin lies in
an arid to semi-arid zone and loses more water by evaporation than it receives by precipitation and inflow of
river water. Consequently, ancient salt deposits are important paleoclimatic markers, indicating arid, nonnally
low-latitude zones as opposed to wet tropical or temperate and cold high-latitude regions.
After burial and subsequent exhumation, primary
evaporites undergo significant changes. In addition,
subsurface brines can fonn evaporitic cements in the
matrix of nonnal sediments. F or these reasons, several
types of evaporites have to be distinguished (after Warren 1996, simplified):