92
sediments, such as sulfates and carbonates, may have
been subjected to significant diagenetic changes
(e.g., Decima et al. 1988).
Fluid inclusions in salt minerals, e.g. in primary
halite crystals, can be used to deterrnine the original
water temperature during the crystallization of specific salt minerals (e.g. Lowenstein et al. 1999).
Subsurface Brines and Diagenesis
In studies on ancient lake sediments it should be
borne in mind that salts, which precipitated on or
directly below the surface of present-day sand and
mud flats, are mostly not preserved. Nodules of gypsum and thin salt layers may dissolve completely in
the subsurface and thus enhance the ion concentrati on of groundwater. Later, the cavities produced by
salt dissolution are frequently filled with secondary
calcite or quartz.
Salts are preserved in the subsurface if the surrounding groundwater has reached saturation for
these salts or they have been sealed by more or less
impermeable strata. Furthermore, most of the preserved salts undergo significant diagenetic changes
within the sediment.
After burial below 200-300 m, gypsum is replaced by
anhydrite (for more details see, e.g. Kasprzyk and Orti
1998). Similarly, other hydrous minerals are transformed to
new, more stable salt minerals (Eugster and Hardie 1978;
Chambre Syndicale 1980; Warren 1996, 1997). Pore waters rich in silica may lead to the formation of authigenic
silicate minerals such as zeolites; carbonates can be replaced by analcime and albite (EI Tabakh and Schreiber
1998). Some specific minerals such as Ca-borates mainly
form in the subsurface (e.g. Orti et al. 1998).
2.5.8 Economic Aspects of Lake Deposits
Residual Brines and Mineral Deposits
The enrichment of rare elements in residual brines of
closed lake basins (Sect. 2.5.4) is of economic interest. In several present-day or geologically young salt
Chapter 2 Continental Sediments
lakes, K, Li, B, F, Br and J have reached such high
concentrations that they can be mined. Preparations
to gain the light metal Mg from the unusually Mgrich brines of the Dead Sea are in progress.
As a result of mineral precipitation, oxidation of
pyrite, and decomposition of organic matter, some
brines become acidic. Then they can take up considerable amounts of heavy metals by dissolving oxide
coatings and/or by oxidation of metal-rich black
shale (Eugster 1985). Even alkaline brines may carry
heavy metals. These metals can precipitate as sulfides along a reducing front within or in coherence
with the lake sediments and thus form ore bodies
containing significant amounts of Cu, Zn, Pb, and
Co. Acidic groundwater brines rich in ferrous iron
precipitate iron hydroxides when they ase end to the
floor of well oxidized, shallow, or dry lakes (e.g. in
southern Australia).
Oil Shales and Hydrocarbons
Lacustrine black shales represent source rocks for
hydrocarbons and lake delta sands can act as reservoirs if they are capped by seal rocks such as lacustrine clay- and mudstones or evaporites. For this reasons, thick deposits of large ancient lake basins have
been and still are targets of hydrocarbon exploration
(cf. Sect. 14.3). The great volume of known immature lacustrine oil shales can be regarded as a reservoir which may be used in the future to extract oil by
the application of heat.
Of great economic interest is, for example, the occurrence
of widespread oil shales and trona beds in the Eocene
Green River Formation in North America (Eugster and
Hardie 1978; Eugster 1985).
The oil shales of the small Lake Messel in Southern
Germany were deposited at about the same time as the
Green River Formation. They are famous for their excellently preserved fauna, particularly vertebrates
(Koenigswald and Michaelis 1984; Schaal and Ziegler
1988). Here the black shales overbridge a time period
which is otherwise poorly represented in the
paleontological record.
sediments, such as sulfates and carbonates, may have
been subjected to significant diagenetic changes
(e.g., Decima et al. 1988).
Fluid inclusions in salt minerals, e.g. in primary
halite crystals, can be used to deterrnine the original
water temperature during the crystallization of specific salt minerals (e.g. Lowenstein et al. 1999).
Subsurface Brines and Diagenesis
In studies on ancient lake sediments it should be
borne in mind that salts, which precipitated on or
directly below the surface of present-day sand and
mud flats, are mostly not preserved. Nodules of gypsum and thin salt layers may dissolve completely in
the subsurface and thus enhance the ion concentrati on of groundwater. Later, the cavities produced by
salt dissolution are frequently filled with secondary
calcite or quartz.
Salts are preserved in the subsurface if the surrounding groundwater has reached saturation for
these salts or they have been sealed by more or less
impermeable strata. Furthermore, most of the preserved salts undergo significant diagenetic changes
within the sediment.
After burial below 200-300 m, gypsum is replaced by
anhydrite (for more details see, e.g. Kasprzyk and Orti
1998). Similarly, other hydrous minerals are transformed to
new, more stable salt minerals (Eugster and Hardie 1978;
Chambre Syndicale 1980; Warren 1996, 1997). Pore waters rich in silica may lead to the formation of authigenic
silicate minerals such as zeolites; carbonates can be replaced by analcime and albite (EI Tabakh and Schreiber
1998). Some specific minerals such as Ca-borates mainly
form in the subsurface (e.g. Orti et al. 1998).
2.5.8 Economic Aspects of Lake Deposits
Residual Brines and Mineral Deposits
The enrichment of rare elements in residual brines of
closed lake basins (Sect. 2.5.4) is of economic interest. In several present-day or geologically young salt
Chapter 2 Continental Sediments
lakes, K, Li, B, F, Br and J have reached such high
concentrations that they can be mined. Preparations
to gain the light metal Mg from the unusually Mgrich brines of the Dead Sea are in progress.
As a result of mineral precipitation, oxidation of
pyrite, and decomposition of organic matter, some
brines become acidic. Then they can take up considerable amounts of heavy metals by dissolving oxide
coatings and/or by oxidation of metal-rich black
shale (Eugster 1985). Even alkaline brines may carry
heavy metals. These metals can precipitate as sulfides along a reducing front within or in coherence
with the lake sediments and thus form ore bodies
containing significant amounts of Cu, Zn, Pb, and
Co. Acidic groundwater brines rich in ferrous iron
precipitate iron hydroxides when they ase end to the
floor of well oxidized, shallow, or dry lakes (e.g. in
southern Australia).
Oil Shales and Hydrocarbons
Lacustrine black shales represent source rocks for
hydrocarbons and lake delta sands can act as reservoirs if they are capped by seal rocks such as lacustrine clay- and mudstones or evaporites. For this reasons, thick deposits of large ancient lake basins have
been and still are targets of hydrocarbon exploration
(cf. Sect. 14.3). The great volume of known immature lacustrine oil shales can be regarded as a reservoir which may be used in the future to extract oil by
the application of heat.
Of great economic interest is, for example, the occurrence
of widespread oil shales and trona beds in the Eocene
Green River Formation in North America (Eugster and
Hardie 1978; Eugster 1985).
The oil shales of the small Lake Messel in Southern
Germany were deposited at about the same time as the
Green River Formation. They are famous for their excellently preserved fauna, particularly vertebrates
(Koenigswald and Michaelis 1984; Schaal and Ziegler
1988). Here the black shales overbridge a time period
which is otherwise poorly represented in the
paleontological record.