6.5 Nonactualistic (Precambrian) Depositional Environments
283
Economic Aspects of Evaporites
and Salt Structures
Mining of rock salt from young and ancient evaporites
is almost as old as mankind. Potash salts have become
increasingly important with the development of the
chemical industry and the use of fertilizers in agriculture. Because potash salts formed in limited areas
inrelatively small quantities, their exploration and mining is more difficult than that of rock salto Gypsum and
anhydrite are commonly gained in open quarries and
widely used in construction work and for other purposes.
6.4.9 Summary (Marine Evaporites)
- Thick and widespread marine evaporites are
known from the Neo-Proterozoic throughout the
Phanerozoic.
- The contribution of the different salts to marine
evaporite sequences varies greatly. This results
from the different modes of salt precipitation in
shallow and deep basins, with and without brine
reflux, often modified by see-water seepage and
loss of highly concentrated brine by leakage. In
addition, mixed-source brines receiving meteoric
water playa role.
- The facies of marine evaporites often changes
from marginal carbonates and sulfates to halite
in the central parts of the basin. Relative sealevel changes create high- and low-frequency
evaporite cycles.
- The thicknesses of varves indicate the high sedimentation rates of evaporites and mimic to some
6.5 Nonactualistic (Precambrian)
Depositional Environments
6.5.1 Introduction
This brief chapter is added to remind the reader that
most of our knowledge on depositional systems summarized in this book is deduced from observations in
modern environments. It is based on the frequently
quoted principle: "The Present is the key to the Past".
Furthermore, it is widely accepted that the depositional
environments did not change fundamentally during the
Phanerozoic. This view is derived from the fossil record and from the results of various geochemical investigations. Even long-term, profound climatic variations from an icehouse to a greenhouse state of the
Earth (cf. Sect. 7.8) can be explained by minor
changes in the composition of the atmosphere and the
configuration and salinity ofthe oceans. A major step
Diapirism creates stratigraphic traps for hydrocarbons. Subsiding depotroughs (Fig. 6.l6c,d) are favorable for the migration and storage of hydrocarbons in
sandy reservoirs (cf. Chap. 14). Due to their plastic
behavior at shallow burial depths, evaporites act as
seal rocks for upward migrating oil and gas. Since
some years, reservoirs even below allochthonous salt
sheets are being explored.
The possibility of disposing radioactive waste and
other problematic chemicals in salt diapirs has been
controversially discussed in several countries.
extent the solubility of carbonates, gypsum, and
halite.
- Synsedimentary and subsurface evaporite dissolution leaves behind distinct surfaces, microkarst phenomena, and may bring about modifications of the basin topography.
- Shallow groundwaterlbrine circulation and
deeper, more closed flow systems largely transform the primary evaporites into secondary
evaporites (e.g. by early cementation, dehydration, mineral phase change).
- Further modifications (tertiary evaporites) are
caused by uplift and erosion of buried salt
rocks.
- Large-scale salt structures in viscoelastic evaporites. are generated by sediment loading (buoyancy) , withdrawal of salt from the subsurface,
and downdip gravitational spreading.
in the evolution of sedimentary environments occurred
in the late Paleozoic when the continents were colonized by plants.
Prior to this stage of plant evolution, all continents
were more or less barren ofhigher life and represented
huge deserts. In contrast to the present-day situation, a
major part of these deserts must have received considerable amounts of rain and should therefore have undergone rapid mechanical and chemical erosion. N evertheless, we can understand these processes from
observations in modern, mountainous regions with
sufficient precipitation, but sparse vegetation.
Serious problems arise for the interpretation of the
Precambrian, that is the Archean and Proterozoic
spanning the times from ab out 4600 to 2500 Ma and
2500 to ~600 Ma before present, respectively. Since
weathering of rocks and sedimentary processes were
always closely related to the properties and circulation
of the atmosphere and the oceans, as well as to the
283
Economic Aspects of Evaporites
and Salt Structures
Mining of rock salt from young and ancient evaporites
is almost as old as mankind. Potash salts have become
increasingly important with the development of the
chemical industry and the use of fertilizers in agriculture. Because potash salts formed in limited areas
inrelatively small quantities, their exploration and mining is more difficult than that of rock salto Gypsum and
anhydrite are commonly gained in open quarries and
widely used in construction work and for other purposes.
6.4.9 Summary (Marine Evaporites)
- Thick and widespread marine evaporites are
known from the Neo-Proterozoic throughout the
Phanerozoic.
- The contribution of the different salts to marine
evaporite sequences varies greatly. This results
from the different modes of salt precipitation in
shallow and deep basins, with and without brine
reflux, often modified by see-water seepage and
loss of highly concentrated brine by leakage. In
addition, mixed-source brines receiving meteoric
water playa role.
- The facies of marine evaporites often changes
from marginal carbonates and sulfates to halite
in the central parts of the basin. Relative sealevel changes create high- and low-frequency
evaporite cycles.
- The thicknesses of varves indicate the high sedimentation rates of evaporites and mimic to some
6.5 Nonactualistic (Precambrian)
Depositional Environments
6.5.1 Introduction
This brief chapter is added to remind the reader that
most of our knowledge on depositional systems summarized in this book is deduced from observations in
modern environments. It is based on the frequently
quoted principle: "The Present is the key to the Past".
Furthermore, it is widely accepted that the depositional
environments did not change fundamentally during the
Phanerozoic. This view is derived from the fossil record and from the results of various geochemical investigations. Even long-term, profound climatic variations from an icehouse to a greenhouse state of the
Earth (cf. Sect. 7.8) can be explained by minor
changes in the composition of the atmosphere and the
configuration and salinity ofthe oceans. A major step
Diapirism creates stratigraphic traps for hydrocarbons. Subsiding depotroughs (Fig. 6.l6c,d) are favorable for the migration and storage of hydrocarbons in
sandy reservoirs (cf. Chap. 14). Due to their plastic
behavior at shallow burial depths, evaporites act as
seal rocks for upward migrating oil and gas. Since
some years, reservoirs even below allochthonous salt
sheets are being explored.
The possibility of disposing radioactive waste and
other problematic chemicals in salt diapirs has been
controversially discussed in several countries.
extent the solubility of carbonates, gypsum, and
halite.
- Synsedimentary and subsurface evaporite dissolution leaves behind distinct surfaces, microkarst phenomena, and may bring about modifications of the basin topography.
- Shallow groundwaterlbrine circulation and
deeper, more closed flow systems largely transform the primary evaporites into secondary
evaporites (e.g. by early cementation, dehydration, mineral phase change).
- Further modifications (tertiary evaporites) are
caused by uplift and erosion of buried salt
rocks.
- Large-scale salt structures in viscoelastic evaporites. are generated by sediment loading (buoyancy) , withdrawal of salt from the subsurface,
and downdip gravitational spreading.
in the evolution of sedimentary environments occurred
in the late Paleozoic when the continents were colonized by plants.
Prior to this stage of plant evolution, all continents
were more or less barren ofhigher life and represented
huge deserts. In contrast to the present-day situation, a
major part of these deserts must have received considerable amounts of rain and should therefore have undergone rapid mechanical and chemical erosion. N evertheless, we can understand these processes from
observations in modern, mountainous regions with
sufficient precipitation, but sparse vegetation.
Serious problems arise for the interpretation of the
Precambrian, that is the Archean and Proterozoic
spanning the times from ab out 4600 to 2500 Ma and
2500 to ~600 Ma before present, respectively. Since
weathering of rocks and sedimentary processes were
always closely related to the properties and circulation
of the atmosphere and the oceans, as well as to the