3.3 Shallow Seas
3.3.4 Modifications of the Fades Models
Relative Sea-Level Change and Other Factors
The generalized vertical sequences described above
can be modified significantly by a variety of factors
not mentioned so far:
(1) Subsidence and relative sea-level change. The
thicknesses of the facies sequences shown in Figures
3.20 and 3.21 correspond to the original water depths
of the sedimentary basins. Taking subsidence into
account, the thicknesses of the total basin fills, as
well as those of single facies types increase.
~ If subsidence is equal to the sedimentation rate for
a certain time period, the facies at each location
within the basin remain unchanged. As a result, each
particular facies zone, for example the shoreface or
the outer shelf sediments, can theoretically reach
very great thicknesses.
~ ]f the basin floor subsides faster than sediment accumulates, the water depth at a certain locality increases. Then the different facies zones migrate landward, and vertical sequences show the opposite tendency as described above, i.e., fining-upward or
thinning-upward trends. Similar results are brought
about by a rapid sea level rise leading to an increase
in water depth.
~ Conversely, tectonic uplift of the basin floor or sealevel fall cause a seaward migration of the facies
zones and thus a shortening of the vertical sequences.
These problems are discussed further in Sects. 7.2
and 7.3 (see also, e.g., Aigner and Dott 1990).
(2) The two-dimensional models of Figs. 3.20 and
3.21 do not take into account local control of sedimentary processes resulting from irregular basin
morphology, shifting and abandonment of river deltas, input of sediment from different sources, regional and temporal changes of c1imate, direction of
dominant winds and currents, etc. Coevel
depositional processes rnay strongly differ from one
location to the other in the same basin (Fig. 3.20a).
At one site, the coastline may retreat landward and at
the other pro grade seaward at the same time. Such
complications should be borne in mind, if the theoretical models are applied to observations in nature.
Sediment-Starved Shelves
Wide areas of the present-day continental shelves on
passive continental margins, e.g. along the east and
west coasts of the Atlantic Ocean, are sedimentstarved (Emery and Uchupi 1972; Milliman and
Syvitski 1992). This is not only true of the outer
shelf, where frequently relic sediments of earlier sea131
level lowstands are present (sea above), but also
found in mid-shelf and inner shelf regions. Such areas are characterized by a high-energy hydraulic regime, repeated erosion and intense biological proces ses on hardbottolDS. The resulting features may be
referred to as "condensed sections" produced during
multiple sea-level fluctuations throughout the Quaternary. During lowstands, these parts of the shelf
were exposed to sub aerial erosion, and during
highstands, as today, they accumulated sediment only
in limited, more or less isolated patches, whereas
c1ose-by erosional processes dominated. Similar phenomena are known from many ancient shallow-water
sediments commonly described as "reworked horizons" generated by storm-wave action during relative
sea-level fall (cf. Sect. 7.3, Fig. 7.16).
A carefully studied modem example of a sediment-starved
mid-shelf region is the Onslow Bay on the North Carolina
continental shelf of the Atlantic (Riggs et al. 1998). Here,
Neogene strata outcrop on the sea floor and are affected by
intense bioerosion producing considerable volumes of sand
and mud. The sand partially forms a thin veneer (0-1 m) on
top ofthe irregular relief ofthe hardbottom, but most ofthe
young sediment is exported by storms to the continental
slope. Hardbottom free of sand is settled by epilithic and
endolithic organisms. Blocks from small c\iffs, generated
by differential erosion, locally cover the hardbottom.
3.3.5 Large-Scale Shallow-Marine Sand Bodies
The processes of the be ach and shoreface zones inc1uding the inner and outer shelf regions described
above may generate widely extended, partially am algamated or stacked marine sand sheets. Prerequisites
for such a development are effective sediment
sources and a relatively fast subsiding basin. In this
case, even moderate sea level falls cannot cause
emergence and thus lead to subaerial erosion and
reworking of the marine sand sheets (cf. Sects. 7.2
and 7.3).
A well exposed and repeatedly studied ancient example of
sandy and muddy shelf-slope sediments is the foreland
basin of the Western Interior seaway in North America
(Fig. 3.22a; Shurr 1984; Swift and Rice 1984; Swift et al.
1987; cf. SecL 12.6). The western margin of this seaway
was bordered by the emerging Rocky Mountains which
shed large quantities ofmud and sand into the seaway (Fig.
3.22b). Currents generated by the dominant northwesterly
to northeasterly winds and deflected by Coriolis forces (cf.
SecL 3.1.2) caused storm tides along the western coastline
(Fig. 3.22c and d). The measured opposite paleoflow directions are explained by the special coastal morphology of
the study area situated in a large bight where setup ofwater
along the coasts has created geostrophic currents of varying
directions. The shelf sands are either linked to the
shoreface sands, or they form large, partially isolated, lenticular sand bodies in between shelf muds.
3.3.4 Modifications of the Fades Models
Relative Sea-Level Change and Other Factors
The generalized vertical sequences described above
can be modified significantly by a variety of factors
not mentioned so far:
(1) Subsidence and relative sea-level change. The
thicknesses of the facies sequences shown in Figures
3.20 and 3.21 correspond to the original water depths
of the sedimentary basins. Taking subsidence into
account, the thicknesses of the total basin fills, as
well as those of single facies types increase.
~ If subsidence is equal to the sedimentation rate for
a certain time period, the facies at each location
within the basin remain unchanged. As a result, each
particular facies zone, for example the shoreface or
the outer shelf sediments, can theoretically reach
very great thicknesses.
~ ]f the basin floor subsides faster than sediment accumulates, the water depth at a certain locality increases. Then the different facies zones migrate landward, and vertical sequences show the opposite tendency as described above, i.e., fining-upward or
thinning-upward trends. Similar results are brought
about by a rapid sea level rise leading to an increase
in water depth.
~ Conversely, tectonic uplift of the basin floor or sealevel fall cause a seaward migration of the facies
zones and thus a shortening of the vertical sequences.
These problems are discussed further in Sects. 7.2
and 7.3 (see also, e.g., Aigner and Dott 1990).
(2) The two-dimensional models of Figs. 3.20 and
3.21 do not take into account local control of sedimentary processes resulting from irregular basin
morphology, shifting and abandonment of river deltas, input of sediment from different sources, regional and temporal changes of c1imate, direction of
dominant winds and currents, etc. Coevel
depositional processes rnay strongly differ from one
location to the other in the same basin (Fig. 3.20a).
At one site, the coastline may retreat landward and at
the other pro grade seaward at the same time. Such
complications should be borne in mind, if the theoretical models are applied to observations in nature.
Sediment-Starved Shelves
Wide areas of the present-day continental shelves on
passive continental margins, e.g. along the east and
west coasts of the Atlantic Ocean, are sedimentstarved (Emery and Uchupi 1972; Milliman and
Syvitski 1992). This is not only true of the outer
shelf, where frequently relic sediments of earlier sea131
level lowstands are present (sea above), but also
found in mid-shelf and inner shelf regions. Such areas are characterized by a high-energy hydraulic regime, repeated erosion and intense biological proces ses on hardbottolDS. The resulting features may be
referred to as "condensed sections" produced during
multiple sea-level fluctuations throughout the Quaternary. During lowstands, these parts of the shelf
were exposed to sub aerial erosion, and during
highstands, as today, they accumulated sediment only
in limited, more or less isolated patches, whereas
c1ose-by erosional processes dominated. Similar phenomena are known from many ancient shallow-water
sediments commonly described as "reworked horizons" generated by storm-wave action during relative
sea-level fall (cf. Sect. 7.3, Fig. 7.16).
A carefully studied modem example of a sediment-starved
mid-shelf region is the Onslow Bay on the North Carolina
continental shelf of the Atlantic (Riggs et al. 1998). Here,
Neogene strata outcrop on the sea floor and are affected by
intense bioerosion producing considerable volumes of sand
and mud. The sand partially forms a thin veneer (0-1 m) on
top ofthe irregular relief ofthe hardbottom, but most ofthe
young sediment is exported by storms to the continental
slope. Hardbottom free of sand is settled by epilithic and
endolithic organisms. Blocks from small c\iffs, generated
by differential erosion, locally cover the hardbottom.
3.3.5 Large-Scale Shallow-Marine Sand Bodies
The processes of the be ach and shoreface zones inc1uding the inner and outer shelf regions described
above may generate widely extended, partially am algamated or stacked marine sand sheets. Prerequisites
for such a development are effective sediment
sources and a relatively fast subsiding basin. In this
case, even moderate sea level falls cannot cause
emergence and thus lead to subaerial erosion and
reworking of the marine sand sheets (cf. Sects. 7.2
and 7.3).
A well exposed and repeatedly studied ancient example of
sandy and muddy shelf-slope sediments is the foreland
basin of the Western Interior seaway in North America
(Fig. 3.22a; Shurr 1984; Swift and Rice 1984; Swift et al.
1987; cf. SecL 12.6). The western margin of this seaway
was bordered by the emerging Rocky Mountains which
shed large quantities ofmud and sand into the seaway (Fig.
3.22b). Currents generated by the dominant northwesterly
to northeasterly winds and deflected by Coriolis forces (cf.
SecL 3.1.2) caused storm tides along the western coastline
(Fig. 3.22c and d). The measured opposite paleoflow directions are explained by the special coastal morphology of
the study area situated in a large bight where setup ofwater
along the coasts has created geostrophic currents of varying
directions. The shelf sands are either linked to the
shoreface sands, or they form large, partially isolated, lenticular sand bodies in between shelf muds.