3.3 Shallow Seas
therefore frequently pers ist for much longer time periods. This point will be discussed below. The following basin models take into account variations in
terrigenous sediment influx as well as changes in the
hydrodynamic regime of the basin. The sediment is
mainly derived from land sources but mayaiso be
transported by currents from point sources over considerable distances parallel to the coast. With decreasing accumulation rates of clastics the percentage
of autochthonous biogenic material in the shelf sediment normally increases.
High Terrigenous Input
(1) Low-energy waves, mud input. Figure 3.20b demonstrates shallow seas with high terrigenous input
dominated by mud (silt and clay-size particles). Under prevailing low-energy wave conditions, the
shoreface sediments are rich in silty muds overlain
by a thin cap layer of beach and foreshore sands. In
deeper water, muds of different nature are encountered. One portion of the mud is carried directly from
the mouths of rivers as flocs or aggregates to its location of final deposition, whereas the other portion is
reworked by moderate storms along the lower
shoreface and redeposited as thin mud tempestites in
deeper water (cf. Sect. 3.1.2). Such an alternation of
autochthonous and allochthonous mud layers is frequently masked or completely obliterated by intensive burrowing on the sea floor. It is mostly overlooked in field exposures. Muddy shorelines and
shoreface zones commonly occur downdrift from
deltas prograding into low-gradient shallow seas
(e.g., Fraser 1989).
Modem examples of mud-dominated shelves are the
Louisiana-Texas coast west of the Mississippi delta and the
Surinam coast north of the Amazon delta (Wells and
Coleman 1981; Nittrouer and Wright 1994). The Amazon
shelf is affected by the Guiana current carrying mud northward which accumulates on the inner and middle shelf.
Another prominent example is the Yellow Sea fed by the
muddy Yellow River (cf. Fig. 11.3; Sect. 11.2.2; Alexander
et al. 1991). The sedimentation rates in these setting can
reach values of several m!ka.
(2) Low-energy waves and high input 01 sand and
mud may lead to a situation as shown in Figure
3.20c. The thickness of shoreface sands increases
considerably; it may include offshore sand bars and
proximal sandy tempestites (cf. Sect. 3.1.2) in the
transition zone to deeper water. Farther offshore,
bioturbated mud tempestites alternating with silty and
clayey autochthonous host sediments are again a
characteristic facies. As in the example in Figure
3.20b, some shell beds may be present, but biogenic
127
production is generally very diluted by the high input
of terrestrial material.
(3) Storm-dominated sea, high input 01 sand and
mud (Fig. 3.20d). The beach-shelf profile becomes
more gentle. The sandy, in places gravelly facies belt
near the co ast, comprising shoreface sands, sands
with swaley and hummocky cross stratification (cf.
Sect. 3.1.2) and sandy tempestites, expands seaward
into deeper water. In such cases, sands represent a
great portion of vertical sections through the entire
basin fill (e.g., Howard and Reineck 1981; Walker
and Plint 1992). Part of the sands transported by
storrns into deeper water can be reworked by subsequent storm-induced currents, direct storm wave action, or oceanic currents (see below). Far offshore,
mud tempestites should be the primary dominant bed
type which is later overprinted by bioturbation.
(4) Tide-dominated seas. As known from present-day
tidal- and current-influenced shallow seas, such as
the North Sea, the Irish Sea, and the Atlantic shelf of
North America, tidal currents mayaiso affect the
sedimentary processes at water depths up to 50 m
and more (Fig. 3.20e). Both high-energy models (Fig.
3.20d and e) are characterized by widely extended
sand bodies. Tidal currents generate characteristic
sand waves, which may show mud draping. In addition, they form and slowly move flat, large sand
ridges. These can attain tens of kilometers in length,
several hundred meters to some kilometers in width,
and up to several tens of meters in height.
Prograding shorelines and their associated thickening and
coarsening-upward shelf sediments have been described
from many locations, such as the Paleozoic Appalachian
and the Mesozoic (Western Interior) foreland basins in
North America or the mid-Cretaceous epeiric seaway of
Australia (e.g. Plint 1988; Krassay 1994; Prave et al.
1996). However, the overall trends are overprinted by
short-period sea-Ievel changes creating stacked shoaling
cycles (cf. Sects. 7.2 and 7.9).
Prominent modem examples of large sand bodies and
sand ridges are the shelves of the western North Atlantic
and the southern North Sea, where flat sand-ridges cover
large areas (Reineck and Singh 1980; Stubblefield and
McGrail 1984; Swift 1985; Saito 1989b; Johnson and
Baldwin 1996). However, at least part of these large sand
ridges originally represented coastal sand bodies formed
during the last low sea-Ievel stand (cf. Sect. 7.3). One has
to be cautious to use these examples as analogs of ancient
shelf sands. Middle Jurassic crinoidal limestone bars, observed in the Jura Mountains of eastern France, have been
interpreted as tidal dunes and sand waves deposited at a
water depth of 100-150 m (Neumeier 1998).
In the southwestern Yellow Sea a 200 km long and 90
km wide, radiating field of tidal current ridges was observed at water depths up to 20-30 m (cf. Fig. 11.3; Liu
Zhenxia et al. 1989). The sand ridges consist of fine sand
originating from the abandoned deltas of the Yellow River
and the ancient Yangtze River and were shaped as a result
therefore frequently pers ist for much longer time periods. This point will be discussed below. The following basin models take into account variations in
terrigenous sediment influx as well as changes in the
hydrodynamic regime of the basin. The sediment is
mainly derived from land sources but mayaiso be
transported by currents from point sources over considerable distances parallel to the coast. With decreasing accumulation rates of clastics the percentage
of autochthonous biogenic material in the shelf sediment normally increases.
High Terrigenous Input
(1) Low-energy waves, mud input. Figure 3.20b demonstrates shallow seas with high terrigenous input
dominated by mud (silt and clay-size particles). Under prevailing low-energy wave conditions, the
shoreface sediments are rich in silty muds overlain
by a thin cap layer of beach and foreshore sands. In
deeper water, muds of different nature are encountered. One portion of the mud is carried directly from
the mouths of rivers as flocs or aggregates to its location of final deposition, whereas the other portion is
reworked by moderate storms along the lower
shoreface and redeposited as thin mud tempestites in
deeper water (cf. Sect. 3.1.2). Such an alternation of
autochthonous and allochthonous mud layers is frequently masked or completely obliterated by intensive burrowing on the sea floor. It is mostly overlooked in field exposures. Muddy shorelines and
shoreface zones commonly occur downdrift from
deltas prograding into low-gradient shallow seas
(e.g., Fraser 1989).
Modem examples of mud-dominated shelves are the
Louisiana-Texas coast west of the Mississippi delta and the
Surinam coast north of the Amazon delta (Wells and
Coleman 1981; Nittrouer and Wright 1994). The Amazon
shelf is affected by the Guiana current carrying mud northward which accumulates on the inner and middle shelf.
Another prominent example is the Yellow Sea fed by the
muddy Yellow River (cf. Fig. 11.3; Sect. 11.2.2; Alexander
et al. 1991). The sedimentation rates in these setting can
reach values of several m!ka.
(2) Low-energy waves and high input 01 sand and
mud may lead to a situation as shown in Figure
3.20c. The thickness of shoreface sands increases
considerably; it may include offshore sand bars and
proximal sandy tempestites (cf. Sect. 3.1.2) in the
transition zone to deeper water. Farther offshore,
bioturbated mud tempestites alternating with silty and
clayey autochthonous host sediments are again a
characteristic facies. As in the example in Figure
3.20b, some shell beds may be present, but biogenic
127
production is generally very diluted by the high input
of terrestrial material.
(3) Storm-dominated sea, high input 01 sand and
mud (Fig. 3.20d). The beach-shelf profile becomes
more gentle. The sandy, in places gravelly facies belt
near the co ast, comprising shoreface sands, sands
with swaley and hummocky cross stratification (cf.
Sect. 3.1.2) and sandy tempestites, expands seaward
into deeper water. In such cases, sands represent a
great portion of vertical sections through the entire
basin fill (e.g., Howard and Reineck 1981; Walker
and Plint 1992). Part of the sands transported by
storrns into deeper water can be reworked by subsequent storm-induced currents, direct storm wave action, or oceanic currents (see below). Far offshore,
mud tempestites should be the primary dominant bed
type which is later overprinted by bioturbation.
(4) Tide-dominated seas. As known from present-day
tidal- and current-influenced shallow seas, such as
the North Sea, the Irish Sea, and the Atlantic shelf of
North America, tidal currents mayaiso affect the
sedimentary processes at water depths up to 50 m
and more (Fig. 3.20e). Both high-energy models (Fig.
3.20d and e) are characterized by widely extended
sand bodies. Tidal currents generate characteristic
sand waves, which may show mud draping. In addition, they form and slowly move flat, large sand
ridges. These can attain tens of kilometers in length,
several hundred meters to some kilometers in width,
and up to several tens of meters in height.
Prograding shorelines and their associated thickening and
coarsening-upward shelf sediments have been described
from many locations, such as the Paleozoic Appalachian
and the Mesozoic (Western Interior) foreland basins in
North America or the mid-Cretaceous epeiric seaway of
Australia (e.g. Plint 1988; Krassay 1994; Prave et al.
1996). However, the overall trends are overprinted by
short-period sea-Ievel changes creating stacked shoaling
cycles (cf. Sects. 7.2 and 7.9).
Prominent modem examples of large sand bodies and
sand ridges are the shelves of the western North Atlantic
and the southern North Sea, where flat sand-ridges cover
large areas (Reineck and Singh 1980; Stubblefield and
McGrail 1984; Swift 1985; Saito 1989b; Johnson and
Baldwin 1996). However, at least part of these large sand
ridges originally represented coastal sand bodies formed
during the last low sea-Ievel stand (cf. Sect. 7.3). One has
to be cautious to use these examples as analogs of ancient
shelf sands. Middle Jurassic crinoidal limestone bars, observed in the Jura Mountains of eastern France, have been
interpreted as tidal dunes and sand waves deposited at a
water depth of 100-150 m (Neumeier 1998).
In the southwestern Yellow Sea a 200 km long and 90
km wide, radiating field of tidal current ridges was observed at water depths up to 20-30 m (cf. Fig. 11.3; Liu
Zhenxia et al. 1989). The sand ridges consist of fine sand
originating from the abandoned deltas of the Yellow River
and the ancient Yangtze River and were shaped as a result