Nonelas tic Fades
Lithofa cies Fif: Claystone with Freshwater Molluscs.
This lithofacies class was proposed by McLean and
Jerzykiewiifz (1978). However, it is regarded here as
superfluous. Lithologically, the beds are identical to
Fm, and the molluscs constitute a lithologic accessory, which is insufficent to require definition of a
different lithof�cies class.
Lithofa cies Fr: Root Bed. These are common in vegetated floodplains (Fig. 5.31). This lithofacies represents soil development in a humid climate.
Carbonate nodules are typically absent, but silica
cementation may have occurred as a result of leaching. The host lithology may be sand,,silt, or mud,
with any primary stratification, such as cross�bedding, ob'scured or destroyed by root emplacement
and bioturbation, giving the beds a mottled appear"
ance. The roots may retain their original carbon, or
may be replaced by calcite or siderite.
Where associated with overlying coals (Fig.
5.31A), these beds have been termed seat earths or
underclays. As an alternative to the clastic coding
discussed here, seat earths may be discussed and
classified under the heading of paleosols, as described in Chap. 7.
5.5 Nonelastic Fades
A limited range of chemical sediments occurs in
fluvial deposits, mainly but not exclusively in the
floodplain. The most important of these are the soils,
which are best developed in semiarid climates. In
humid, tropical settings coal may occur, while in arid
climates minor evaporites may develop in inland
sabkhas.
Lithofacies P: Pedogenic Carbonates. A wide variety
of textures and structures develops where floodplains are exposed to surface weathering processes
for extended periods (thousands of years). Rain
infiltration leaches dissolvable ions downward,
whereas evaporation and capillary groundwater flow
during arid periods concentrate the same ions near
the surface. The result is the gradual development of
carbonate cements that coalesce into nodules and
these, in turn, into more or less continuous carbonate substrates, commonly with a blocky fracturing
pattern (Fig. 5.32). Modern calcitic soils are referred
to as caliche or calcrete, and the terms have been
127
adopted by sedimentologists for paleosols. Calcitic
paleosols are most typical of arid to semiarid, oxidizing climates, whereas siderite nodules occur in waterlogged, reducing settings, and are comm O nly
associated with coals.
The stratigraphy and fades of paleosols constitutes a specialized study that is touched on further in
Sect. 7.4.2, where additional petrographic and facies
classifications are provided.
Paleosols constitute useful local marker beds,
and can therefore yield valuable information on
basin stratigraphy (e.g., Allen and Williams 1982;
Behrensmeyer 1987; Eberth and Miall 1991).
Paleosol maturity can be deduced from color, soillayer differentiation, and nodule morphology, providing data on floodplain exposure times and clastic
influxes (Allen 1974b; Leeder 1975; Bown and Kraus
1987). These deposits are therefore of considerable
value in the study of large-scale basin architecture, a
subject that is considered at length in Chaps. 9 and
10.
Soil texture and geochemistry are sophisticated
subjects, yielding much information on local bio�
chemical processes, local climate, etc. (Johnson
1977; Bown and Kraus 1981, 1987; Retallack I981; see
also books referenced in Sect. 7.4.2). However, much
of this is beyond the purpose and scope of this book,
and is not discussed in detail.
Lithofa cies C: Coal. Coal is typically associated with
deltaic and fluvial floodplain environments (Fig.
5.33). McCabe (1984) has suggested that the presence
of thick coals, as opposed to carbonaceous mudstones, indicates the presence of raised peat swamps
undergoing rapid plant accumulation under humidtropical conditions.
Other Facies. Carbonates occur as tufas, and as bedded deposits) with stromatolites, oolites and
oncolites, in freshwater ponds and in some fluvial
channels (Ordonez and Garda del Cura 1983; Eberth
and Miall 1991). Algae form encrustations and replacements wherever suitable substrate material is
present1 such as dead plants or shells, and clasts.
Oolites and oncolites in the form of grains are moved
by fluvial transport (Fig. 5.34). Eberth and Miall
(1991) reported lacustrine marls interbedded with
fine-grained floodplain clastic units. They consisted
of laminated micrite, microspar, siltstone, and ferric
oxide.
Lithofa cies Fif: Claystone with Freshwater Molluscs.
This lithofacies class was proposed by McLean and
Jerzykiewiifz (1978). However, it is regarded here as
superfluous. Lithologically, the beds are identical to
Fm, and the molluscs constitute a lithologic accessory, which is insufficent to require definition of a
different lithof�cies class.
Lithofa cies Fr: Root Bed. These are common in vegetated floodplains (Fig. 5.31). This lithofacies represents soil development in a humid climate.
Carbonate nodules are typically absent, but silica
cementation may have occurred as a result of leaching. The host lithology may be sand,,silt, or mud,
with any primary stratification, such as cross�bedding, ob'scured or destroyed by root emplacement
and bioturbation, giving the beds a mottled appear"
ance. The roots may retain their original carbon, or
may be replaced by calcite or siderite.
Where associated with overlying coals (Fig.
5.31A), these beds have been termed seat earths or
underclays. As an alternative to the clastic coding
discussed here, seat earths may be discussed and
classified under the heading of paleosols, as described in Chap. 7.
5.5 Nonelastic Fades
A limited range of chemical sediments occurs in
fluvial deposits, mainly but not exclusively in the
floodplain. The most important of these are the soils,
which are best developed in semiarid climates. In
humid, tropical settings coal may occur, while in arid
climates minor evaporites may develop in inland
sabkhas.
Lithofacies P: Pedogenic Carbonates. A wide variety
of textures and structures develops where floodplains are exposed to surface weathering processes
for extended periods (thousands of years). Rain
infiltration leaches dissolvable ions downward,
whereas evaporation and capillary groundwater flow
during arid periods concentrate the same ions near
the surface. The result is the gradual development of
carbonate cements that coalesce into nodules and
these, in turn, into more or less continuous carbonate substrates, commonly with a blocky fracturing
pattern (Fig. 5.32). Modern calcitic soils are referred
to as caliche or calcrete, and the terms have been
127
adopted by sedimentologists for paleosols. Calcitic
paleosols are most typical of arid to semiarid, oxidizing climates, whereas siderite nodules occur in waterlogged, reducing settings, and are comm O nly
associated with coals.
The stratigraphy and fades of paleosols constitutes a specialized study that is touched on further in
Sect. 7.4.2, where additional petrographic and facies
classifications are provided.
Paleosols constitute useful local marker beds,
and can therefore yield valuable information on
basin stratigraphy (e.g., Allen and Williams 1982;
Behrensmeyer 1987; Eberth and Miall 1991).
Paleosol maturity can be deduced from color, soillayer differentiation, and nodule morphology, providing data on floodplain exposure times and clastic
influxes (Allen 1974b; Leeder 1975; Bown and Kraus
1987). These deposits are therefore of considerable
value in the study of large-scale basin architecture, a
subject that is considered at length in Chaps. 9 and
10.
Soil texture and geochemistry are sophisticated
subjects, yielding much information on local bio�
chemical processes, local climate, etc. (Johnson
1977; Bown and Kraus 1981, 1987; Retallack I981; see
also books referenced in Sect. 7.4.2). However, much
of this is beyond the purpose and scope of this book,
and is not discussed in detail.
Lithofa cies C: Coal. Coal is typically associated with
deltaic and fluvial floodplain environments (Fig.
5.33). McCabe (1984) has suggested that the presence
of thick coals, as opposed to carbonaceous mudstones, indicates the presence of raised peat swamps
undergoing rapid plant accumulation under humidtropical conditions.
Other Facies. Carbonates occur as tufas, and as bedded deposits) with stromatolites, oolites and
oncolites, in freshwater ponds and in some fluvial
channels (Ordonez and Garda del Cura 1983; Eberth
and Miall 1991). Algae form encrustations and replacements wherever suitable substrate material is
present1 such as dead plants or shells, and clasts.
Oolites and oncolites in the form of grains are moved
by fluvial transport (Fig. 5.34). Eberth and Miall
(1991) reported lacustrine marls interbedded with
fine-grained floodplain clastic units. They consisted
of laminated micrite, microspar, siltstone, and ferric
oxide.