3.5 Marine Delta Complexes
quences prevail in the lower delta plain, particularly
in the case of birdfoot deltas (see below). The seaward portions of these deposits are exposed to delta
destruction.
Facies Architecture of Birdfoot Deltas
The facies association of a birdfoot delta is in many
ways similar to that described in Figure 3.34, but
some important differences exist. During constructional phases the distributaries build, on top of
pro delta silty clays or clayey silts, elongate isolated
bodies of delta front and mouth bar sands out into the
sea (Fig. 3.35). The area in between is occupied by
bays accumulating muds with marine biota. If such
elongate delta lobes are abandoned, part of their delta
front sands retreat and form sand barriers which convert the former interdistributary marine bays into lagoons of varying salinity. Later, marsh and/or lake
deposits may follow, before the prograding fluvial
plain, its crevasse splays, and overbank fine-grained
muds take over. Therefore it is common for marine
prodelta sediments or bay muds to be directly overlain by lagoonal, intertidal, marsh, or lake sediments
in the interdistributary area of a birdfoot delta. The
interdistributary environment is a particularly favorable site for the accumulation and preservation of
organic matter and thus for the generation of coal
seams. Outcrops in this area may show similar sequences over fairly large distances and therefore give
little evidence of distributary channels and their specific facies association (see also below). In total, the
vertical and lateral facies successions display the
same variations as the classical lobate delta. However, they include a larger percentage of marine bay
sediments.
Prograding and Switching of Delta Lobes
Delta prograding. A delta lobe advancing over an
abandoned part of the former delta can pro grade
faster than a lobe which is built out into deeper water
(cf. Fig. 7.20). This is the case for the classicallobate
deltas as weH as for the birdfoot deltas. A "shallowwater delta" produces a thin delta front sequence and
tends to form continuous delta front sheet sands in
contrast to "deep-water deltas", which deposit
thicker, coarsening-upward delta front sequences as
well as thick prodelta sediments. In the latter case,
the distributaries tend to become widely spaced and
therefore generate lateraHy discontinuous mouth bar
sands (birdfoot delta).
Delta-lobe switching. Figure 3.35b demonstrates the
large-scale facies architecture of a predominantly
fluvial-dominated delta composed of aseries of delta
163
lobes similar to those known from the Holocene Mississippi delta (Frazier 1967). The upper (landward)
part of the individual delta lobes usually exhibits the
characteristics of shallow-water deltas, whereas the
lower part represents a deep-water delta and therefore a thick delta front sequence. As a result, the generalized and simplified sections perpendicular and
parallel to the coastline (Fig. 3.35b) show combined
delta front and delta plain deposits of rather varying
thicknesses.
In spite of the obvious imbrication 0/ delta lobes,
the overall, large-scale delta complex commonly renders the impression of a cyclic sequence in which
layers rich in organic matter (coal seams) are particu1arly arresting. More thorough investigations, however, frequently reveal that the coal seams switch
1aterally to deeper or higher positions in the total sequence, and the same is true of all the other facies
types.
The Scale of Marine Deltas and Depositional
Cycles
The problems associated with bed correlation and
cyclic sequences, mentioned above and described
later in more detail (Sects. 7.1, 7.6 and 7.9), underline the necessity to discuss the scale of delta facies
models.
The delta plains of present-day large marine deltas
cover areas of thousands of square kilometers. Their
delta fronts are several hundreds of kilometers wide,
and some of them have prograded seaward 100 to
200 km during the Quatemary or Neogene. We can
assurne that ancient delta plains were of the same
magnitude.
On the other hand, the thoroughly investigated
facies associations of deltaic coal fields usually cover
smaller areas and therefore can barely reveal the
complete architecture of geological bodies as large as
those of entire delta complexes shown in Fig. 3.35b.
Instead, the local studies usually give rise to the impression that individual beds can be traced over relatively long distances and that cyclicity is rather regular and possibly of regional or even global importance.
This is not true in those cases in which the vertical
sequences are caused solely by delta-lobe switching.
However, some deltaic coal cyclothems appear to be
controlled by eustatic sea-Ievel changes originating
from processes outside of the deltaic depositional
area (cf. Sects. 7.6 and 14.5).
quences prevail in the lower delta plain, particularly
in the case of birdfoot deltas (see below). The seaward portions of these deposits are exposed to delta
destruction.
Facies Architecture of Birdfoot Deltas
The facies association of a birdfoot delta is in many
ways similar to that described in Figure 3.34, but
some important differences exist. During constructional phases the distributaries build, on top of
pro delta silty clays or clayey silts, elongate isolated
bodies of delta front and mouth bar sands out into the
sea (Fig. 3.35). The area in between is occupied by
bays accumulating muds with marine biota. If such
elongate delta lobes are abandoned, part of their delta
front sands retreat and form sand barriers which convert the former interdistributary marine bays into lagoons of varying salinity. Later, marsh and/or lake
deposits may follow, before the prograding fluvial
plain, its crevasse splays, and overbank fine-grained
muds take over. Therefore it is common for marine
prodelta sediments or bay muds to be directly overlain by lagoonal, intertidal, marsh, or lake sediments
in the interdistributary area of a birdfoot delta. The
interdistributary environment is a particularly favorable site for the accumulation and preservation of
organic matter and thus for the generation of coal
seams. Outcrops in this area may show similar sequences over fairly large distances and therefore give
little evidence of distributary channels and their specific facies association (see also below). In total, the
vertical and lateral facies successions display the
same variations as the classical lobate delta. However, they include a larger percentage of marine bay
sediments.
Prograding and Switching of Delta Lobes
Delta prograding. A delta lobe advancing over an
abandoned part of the former delta can pro grade
faster than a lobe which is built out into deeper water
(cf. Fig. 7.20). This is the case for the classicallobate
deltas as weH as for the birdfoot deltas. A "shallowwater delta" produces a thin delta front sequence and
tends to form continuous delta front sheet sands in
contrast to "deep-water deltas", which deposit
thicker, coarsening-upward delta front sequences as
well as thick prodelta sediments. In the latter case,
the distributaries tend to become widely spaced and
therefore generate lateraHy discontinuous mouth bar
sands (birdfoot delta).
Delta-lobe switching. Figure 3.35b demonstrates the
large-scale facies architecture of a predominantly
fluvial-dominated delta composed of aseries of delta
163
lobes similar to those known from the Holocene Mississippi delta (Frazier 1967). The upper (landward)
part of the individual delta lobes usually exhibits the
characteristics of shallow-water deltas, whereas the
lower part represents a deep-water delta and therefore a thick delta front sequence. As a result, the generalized and simplified sections perpendicular and
parallel to the coastline (Fig. 3.35b) show combined
delta front and delta plain deposits of rather varying
thicknesses.
In spite of the obvious imbrication 0/ delta lobes,
the overall, large-scale delta complex commonly renders the impression of a cyclic sequence in which
layers rich in organic matter (coal seams) are particu1arly arresting. More thorough investigations, however, frequently reveal that the coal seams switch
1aterally to deeper or higher positions in the total sequence, and the same is true of all the other facies
types.
The Scale of Marine Deltas and Depositional
Cycles
The problems associated with bed correlation and
cyclic sequences, mentioned above and described
later in more detail (Sects. 7.1, 7.6 and 7.9), underline the necessity to discuss the scale of delta facies
models.
The delta plains of present-day large marine deltas
cover areas of thousands of square kilometers. Their
delta fronts are several hundreds of kilometers wide,
and some of them have prograded seaward 100 to
200 km during the Quatemary or Neogene. We can
assurne that ancient delta plains were of the same
magnitude.
On the other hand, the thoroughly investigated
facies associations of deltaic coal fields usually cover
smaller areas and therefore can barely reveal the
complete architecture of geological bodies as large as
those of entire delta complexes shown in Fig. 3.35b.
Instead, the local studies usually give rise to the impression that individual beds can be traced over relatively long distances and that cyclicity is rather regular and possibly of regional or even global importance.
This is not true in those cases in which the vertical
sequences are caused solely by delta-lobe switching.
However, some deltaic coal cyclothems appear to be
controlled by eustatic sea-Ievel changes originating
from processes outside of the deltaic depositional
area (cf. Sects. 7.6 and 14.5).