218
6 DEPOSITIONAL SYSTEMS
as Lake Schonau and the Great Ploner Lake, and in the lakes of southern Canada
(Fig. 6.26, 2).
In Kukal's third type of lake, sapropelites in the center are ringed by carbonate shoreline sediments of algal and molluscan origin (Fig. 6.26, 3). Good examples of permanent lakes with extensive autochthonous sedimentation occurred in the Tertiary basins
of the Rocky Mountain foothills. One of the best known is Lake Uinta, which, during
the Eocene, covered some 23,000 km 2 of Utah and Colorado, depositing over 2000 m of
diverse sediment types. These included marginal deltaic sands and coal swamps, which
prograded basinward over varved oil shales of the Green River Formation. These regressive phases alternated cyclically with transgressions when carbonate marls in the
basin center passed into skeletal sands and algal oolites around the margin. Intensively
studied, because of oil fields in the marginal facies, key papers on Lake Uinta include
those by Bradley (1948), Picard (1967), Picard and High (1968,1972b), Eugster and Surdam (1973), Dean and Fouch (1983), and Fischer and Roberts (1991).
Oil shales, such as occur in the Green River Formation, are found also in PermoCarboniferous lakes of South Africa, South America, and New South Wales and in the
Carboniferous Lake Cadell in the Midland Valley of Scotland (Greensmith, 1968).
Kukal's fourth type of lake consists of marginal marshes which prograde centripetally to overlie organic muds (sapropelites) deposited in the central part of the lake
basin (Fig. 6.26, 4). Examples of this type of sedimentation occur in the cold wet gytta
lake of northern Canada and northern Europe and Asia. Although sapropelites and oil
shales are common lacustrine deposits, they seldom occur unassociated with other rock
types in ancient lake sediments.
6.3.2.4.3 Ephemeral lakes
Modern lakes that only exist for intermittent spans of time include the "playas" of the
North American desert, the Kavirs of Iran, and the great inland drainage basins of
the Sahara, Arabia, and the Australian interior. In such desert climates precipitation
is very erratic. Though rain may only come once every few years, when it does fall it
pours down. In the 1967 rainstorms of the Australian Lake Eyre basin, overnight falls
of 150 mm were recorded (Williams, 1971). Downpours such as these lead to the formation of ephemeral inland lakes (Fig. 6.26, 5).
Intermontane playa lakes are rimmed by piedmont alluvial fans, whose sedimentology has already been described (Fig. 6.28). The width of this marginal facies can be very
variable. In playas such as Qa Saleb and Qa Disi in the Southern desert of Jordan, the
mud flats locally impinge on sheer cliffs hundreds of meters high.
The typical deposit of modern playa lakes is a red-brown mudstone containing varying amounts of clay, silt, and disseminated carbonate. Scattered windblown sand grains
are common, and are often trapped in desiccation cracks (Fig. 6.29). Eolian mud rocks
can be diagnosed from the presence of scattered well-rounded sand grains. Because
of their tendency to evaporation, ephemeral lakes can also deposit evaporite minerals.
Hence the soda fiats of the North American desert and the inland sabkhas of the Saharan and Arabian deserts (Fig. 6.26, 6).
Ancient lakes analogous to the playas of modern arid climates have been well documented, notably from Triassic rocks of Europe and North America. Specific examples
include the Popo Agie member of the Chugwater Formation in Wyoming (Picard and
6 DEPOSITIONAL SYSTEMS
as Lake Schonau and the Great Ploner Lake, and in the lakes of southern Canada
(Fig. 6.26, 2).
In Kukal's third type of lake, sapropelites in the center are ringed by carbonate shoreline sediments of algal and molluscan origin (Fig. 6.26, 3). Good examples of permanent lakes with extensive autochthonous sedimentation occurred in the Tertiary basins
of the Rocky Mountain foothills. One of the best known is Lake Uinta, which, during
the Eocene, covered some 23,000 km 2 of Utah and Colorado, depositing over 2000 m of
diverse sediment types. These included marginal deltaic sands and coal swamps, which
prograded basinward over varved oil shales of the Green River Formation. These regressive phases alternated cyclically with transgressions when carbonate marls in the
basin center passed into skeletal sands and algal oolites around the margin. Intensively
studied, because of oil fields in the marginal facies, key papers on Lake Uinta include
those by Bradley (1948), Picard (1967), Picard and High (1968,1972b), Eugster and Surdam (1973), Dean and Fouch (1983), and Fischer and Roberts (1991).
Oil shales, such as occur in the Green River Formation, are found also in PermoCarboniferous lakes of South Africa, South America, and New South Wales and in the
Carboniferous Lake Cadell in the Midland Valley of Scotland (Greensmith, 1968).
Kukal's fourth type of lake consists of marginal marshes which prograde centripetally to overlie organic muds (sapropelites) deposited in the central part of the lake
basin (Fig. 6.26, 4). Examples of this type of sedimentation occur in the cold wet gytta
lake of northern Canada and northern Europe and Asia. Although sapropelites and oil
shales are common lacustrine deposits, they seldom occur unassociated with other rock
types in ancient lake sediments.
6.3.2.4.3 Ephemeral lakes
Modern lakes that only exist for intermittent spans of time include the "playas" of the
North American desert, the Kavirs of Iran, and the great inland drainage basins of
the Sahara, Arabia, and the Australian interior. In such desert climates precipitation
is very erratic. Though rain may only come once every few years, when it does fall it
pours down. In the 1967 rainstorms of the Australian Lake Eyre basin, overnight falls
of 150 mm were recorded (Williams, 1971). Downpours such as these lead to the formation of ephemeral inland lakes (Fig. 6.26, 5).
Intermontane playa lakes are rimmed by piedmont alluvial fans, whose sedimentology has already been described (Fig. 6.28). The width of this marginal facies can be very
variable. In playas such as Qa Saleb and Qa Disi in the Southern desert of Jordan, the
mud flats locally impinge on sheer cliffs hundreds of meters high.
The typical deposit of modern playa lakes is a red-brown mudstone containing varying amounts of clay, silt, and disseminated carbonate. Scattered windblown sand grains
are common, and are often trapped in desiccation cracks (Fig. 6.29). Eolian mud rocks
can be diagnosed from the presence of scattered well-rounded sand grains. Because
of their tendency to evaporation, ephemeral lakes can also deposit evaporite minerals.
Hence the soda fiats of the North American desert and the inland sabkhas of the Saharan and Arabian deserts (Fig. 6.26, 6).
Ancient lakes analogous to the playas of modern arid climates have been well documented, notably from Triassic rocks of Europe and North America. Specific examples
include the Popo Agie member of the Chugwater Formation in Wyoming (Picard and