6.3 SEDIMENTARY MODELS
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6.3.2.4 Lakes and Lacustrine Models
Lakes are bodies of water that are restricted from communication with the open sea.
They vary widely in water depth, size, and salinity, ranging from freshwater to hypersaline. Accounts of modern lakes have been given by Hawarth and Lund (1984) and Burgis and Morris (1987). Ancient lake sediments have been documented by Fouch and
Dean (1982), Dean and Fouch (1983), Anadon et al. (1991), Gierlowski-Kordesch and
Kelts (1994), and Talbot and Allen (1996). These studies show that lakes contain a similar range of subenvironments to those found in the marine realm. They range from deltas and barrier island coasts, to subaqueous fans deposited by turbidity currents. Lakes
deposit terrigenous and carbonate sediment, including oolite shoals and even what may
be loosely termed "reefs" (of algal origin). In arid climates hypersaline lakes deposit
evaporites in both marginal and median sabkhas. Lake deposits only differ from marine
sediments in their paleontology, and perhaps in some aspects of their geochemistry.
Attempts to define lacustrine sedimentary models have been made by Visher (1965),
Kukal (1971), and Picard and High (1972a, 1979). These attempts reach only a broad
consensus. Visher, and Picard and High worked essentially from the evidence of ancient
lacustrine deposits. They defined an ideal model showing an upward-coarsening grainsize profile. Picard and High write (1972b, p. 115): "Inasmuch as all lakes are ultimately
filled, regression dominates the history of a lake." Visher reached a similar conclusion
comparing the lacustrine sedimentary sequence to that produced by a regressive marine shoreline. According to these concepts an ideal lake sedimentary model generates
a sequence that commences with laminated fine sediments deposited in the deep lake
center. As the lake is infilled, marginal fluvial, deltaic, and swamp environments encroach inward. The laminated fines may grade up through turbidite sands, such as are
known from Lake Zug and Lake Meade, into beach sands, cross-bedded fluviodeltaic
channel sands, and marsh peats.
By contrast to this simple model, Kukal (1971) defined four different lake types,
based on the areal distribution of different kinds of sediment in modern lakes. By combining the classic geological approach of looking at lakes vertically with the bird's-eye
view of a recent sedimentologist, it is possible to define the sedimentary models shown
in Fig. 6.26. In addition to Kukal's four classes of lakes, this scheme introduces two
models for the ephemeral lakes of arid regions. The six models have been arranged according to the degree of aridity and of topographic relief that may be expected to generate the various lacustrine types.
6.3.2.4.1 Terrigenous permanent lakes
Kukal's first type of lake is the permanent variety which is infilled by terrigenous sediment (Fig. 6.26,1). These are commonly found today in mountainous terrain where high
precipitation and sediment runoff are combined. Numerous examples may be cited,
such as Lake Constance and Lake Zug in the Alps, and Lake Titicaca in the Andes.
Such lakes correspond to the typical lake model of Visher in that marginal fluviodeltaic
sands prograde into the lake to bury finer sediment settled out from suspension. Because of their location in mountainous terrains, the preservation of such lakes in the
215
6.3.2.4 Lakes and Lacustrine Models
Lakes are bodies of water that are restricted from communication with the open sea.
They vary widely in water depth, size, and salinity, ranging from freshwater to hypersaline. Accounts of modern lakes have been given by Hawarth and Lund (1984) and Burgis and Morris (1987). Ancient lake sediments have been documented by Fouch and
Dean (1982), Dean and Fouch (1983), Anadon et al. (1991), Gierlowski-Kordesch and
Kelts (1994), and Talbot and Allen (1996). These studies show that lakes contain a similar range of subenvironments to those found in the marine realm. They range from deltas and barrier island coasts, to subaqueous fans deposited by turbidity currents. Lakes
deposit terrigenous and carbonate sediment, including oolite shoals and even what may
be loosely termed "reefs" (of algal origin). In arid climates hypersaline lakes deposit
evaporites in both marginal and median sabkhas. Lake deposits only differ from marine
sediments in their paleontology, and perhaps in some aspects of their geochemistry.
Attempts to define lacustrine sedimentary models have been made by Visher (1965),
Kukal (1971), and Picard and High (1972a, 1979). These attempts reach only a broad
consensus. Visher, and Picard and High worked essentially from the evidence of ancient
lacustrine deposits. They defined an ideal model showing an upward-coarsening grainsize profile. Picard and High write (1972b, p. 115): "Inasmuch as all lakes are ultimately
filled, regression dominates the history of a lake." Visher reached a similar conclusion
comparing the lacustrine sedimentary sequence to that produced by a regressive marine shoreline. According to these concepts an ideal lake sedimentary model generates
a sequence that commences with laminated fine sediments deposited in the deep lake
center. As the lake is infilled, marginal fluvial, deltaic, and swamp environments encroach inward. The laminated fines may grade up through turbidite sands, such as are
known from Lake Zug and Lake Meade, into beach sands, cross-bedded fluviodeltaic
channel sands, and marsh peats.
By contrast to this simple model, Kukal (1971) defined four different lake types,
based on the areal distribution of different kinds of sediment in modern lakes. By combining the classic geological approach of looking at lakes vertically with the bird's-eye
view of a recent sedimentologist, it is possible to define the sedimentary models shown
in Fig. 6.26. In addition to Kukal's four classes of lakes, this scheme introduces two
models for the ephemeral lakes of arid regions. The six models have been arranged according to the degree of aridity and of topographic relief that may be expected to generate the various lacustrine types.
6.3.2.4.1 Terrigenous permanent lakes
Kukal's first type of lake is the permanent variety which is infilled by terrigenous sediment (Fig. 6.26,1). These are commonly found today in mountainous terrain where high
precipitation and sediment runoff are combined. Numerous examples may be cited,
such as Lake Constance and Lake Zug in the Alps, and Lake Titicaca in the Andes.
Such lakes correspond to the typical lake model of Visher in that marginal fluviodeltaic
sands prograde into the lake to bury finer sediment settled out from suspension. Because of their location in mountainous terrains, the preservation of such lakes in the