3.1 Beach and Shoreface Sediments
This discrepancy between modem observations and
the limited number of preserved tempestites in the
ancient record suggests that even many prominent
storm beds are wiped out or obscured by subsequent
very rare, extremely large storm events. The action of
earlier weaker storms may be inferred indirectly from
the occurrence of mechanically abraded, coarse partieIes and/or the presence of materials from different
sources in the ultimately preserved tempestite bed.
Numerous successive storm events can build up a
more or less rhythmic tempestite-shale sequence over
a certain geologie time period (see below).
Proximal-Distal Trends and Tempestite
Sequences
Common proximal-distal trends of tempestites are
summarized in the facies model of Fig. 3.8a. The
nearshore zone of swaley and large-scale hummocky
cross-stratification is usually devoid of muddy
interbeds. Basinward, with the presence of muddy
intercalations, either a zone of relatively thick and
often amalgamated tempestites follows, or there is a
zone of sediment bypassing (Fig. 3.8b, see below),
apart from sediment-filled scours. In any case,
tempestites tend to change laterally in thickness and
frequently pinch out. Further seaward, i.e., more distaIly, the number of individual tempestites over a
certain time span first tends to increase, because
amalgamation becomes rare. Then it decreases due to
the limited travel distance of the storm-induced suspension currents of relatively weak and medium
storms. Distal tempestites are thin and fine-grained
and show the same inorganic sedimentary structures
as distal turbidites, but they differ from turbidites in
their faunal characteristics and vertical facies trends
(Einseie and Seilacher 1991). Thin and intensely
bioturbated mud tempestites are difficult to identify
and therefore frequently overlooked.
The facies model of Figure 3.8a, showing continuous tempestite beds from the foreshore to deeper water, cannot be applied everywhere. The modified
model of Fig. 3.8b takes into account that tempestite
deposition may occur only on the inner and outer
shelf. In shallower water, the material stirred up by
storms is either frequently reworked in place (amalgamation), or it is transported basinward through an
intermediate zone of nondeposition (bypass zone;
107
bypass model after Myrow 1992). The typical bypass
zone is characterized by isolated pot and gutter casts
and the scarceness of continuous tempestite beds. In
some basins, both tempestite models may have been
successively realized. The bypass model appears to
be weIl suited for settings with subsidence rates increasing from the margin toward the center of the
basin.
With or without sediment bypassing, numerous
successive storm events can build up more or less
rhythmic tempestite-shale sequences in subsiding
basins. These sequences may reflect three different
trends ofbasin evolution:
- Steady-state conditions in a foreshore-shelf environment, i.e., the average sedimentation rate more or
less compensates for subsidence. In this case, comparatively thick sequences of tempestites altemating
with shelf muds can develop, and the palaeo-water
depth at a certain location within the basin will remain about constant.
- Deepening basin, i.e., the average sedimentation
rate is lower than subsidence. Then the vertical sequence will displayatrend from thick, relatively
coarse-grained proximal tempestites to thin, finegrained distal tempestites, and finally end up with
indistinct mud tempestites or purely autochthonous
shelf muds (Fig. 3.8c). The transition zone from
proximal to distal storm beds may be on the order of
20 to 50 m in vertical seetion.
- Shallowing basin, with a sedimentation rate higher
than subsidence. The resulting tempestite sequence
coarsens (thickens) upward (Fig. 3.8d), and the transition zone from shelf muds to a silicieIastic or
bioeIastic foreshore and coastal environment will be
also of limited thickness. Such a regressive trend favors amalgamation of storm beds.
Hence, tempestites can be distinguished from
turbidites not only on the basis of certain sedimentary
structures (particularly hummocky cross-stratification
and wave ripples) and faunal characteristics of individual beds, but often also by rapid facies changes in
their vertical sequences.
Tempestite sequences displaying repeated
coarsening-upward sections have been described
from various locations and sediments of different
ages. These tempestite cyeIes are usually explained
by relative sea-Ievel changes (cf. Sect. 7.9).
This discrepancy between modem observations and
the limited number of preserved tempestites in the
ancient record suggests that even many prominent
storm beds are wiped out or obscured by subsequent
very rare, extremely large storm events. The action of
earlier weaker storms may be inferred indirectly from
the occurrence of mechanically abraded, coarse partieIes and/or the presence of materials from different
sources in the ultimately preserved tempestite bed.
Numerous successive storm events can build up a
more or less rhythmic tempestite-shale sequence over
a certain geologie time period (see below).
Proximal-Distal Trends and Tempestite
Sequences
Common proximal-distal trends of tempestites are
summarized in the facies model of Fig. 3.8a. The
nearshore zone of swaley and large-scale hummocky
cross-stratification is usually devoid of muddy
interbeds. Basinward, with the presence of muddy
intercalations, either a zone of relatively thick and
often amalgamated tempestites follows, or there is a
zone of sediment bypassing (Fig. 3.8b, see below),
apart from sediment-filled scours. In any case,
tempestites tend to change laterally in thickness and
frequently pinch out. Further seaward, i.e., more distaIly, the number of individual tempestites over a
certain time span first tends to increase, because
amalgamation becomes rare. Then it decreases due to
the limited travel distance of the storm-induced suspension currents of relatively weak and medium
storms. Distal tempestites are thin and fine-grained
and show the same inorganic sedimentary structures
as distal turbidites, but they differ from turbidites in
their faunal characteristics and vertical facies trends
(Einseie and Seilacher 1991). Thin and intensely
bioturbated mud tempestites are difficult to identify
and therefore frequently overlooked.
The facies model of Figure 3.8a, showing continuous tempestite beds from the foreshore to deeper water, cannot be applied everywhere. The modified
model of Fig. 3.8b takes into account that tempestite
deposition may occur only on the inner and outer
shelf. In shallower water, the material stirred up by
storms is either frequently reworked in place (amalgamation), or it is transported basinward through an
intermediate zone of nondeposition (bypass zone;
107
bypass model after Myrow 1992). The typical bypass
zone is characterized by isolated pot and gutter casts
and the scarceness of continuous tempestite beds. In
some basins, both tempestite models may have been
successively realized. The bypass model appears to
be weIl suited for settings with subsidence rates increasing from the margin toward the center of the
basin.
With or without sediment bypassing, numerous
successive storm events can build up more or less
rhythmic tempestite-shale sequences in subsiding
basins. These sequences may reflect three different
trends ofbasin evolution:
- Steady-state conditions in a foreshore-shelf environment, i.e., the average sedimentation rate more or
less compensates for subsidence. In this case, comparatively thick sequences of tempestites altemating
with shelf muds can develop, and the palaeo-water
depth at a certain location within the basin will remain about constant.
- Deepening basin, i.e., the average sedimentation
rate is lower than subsidence. Then the vertical sequence will displayatrend from thick, relatively
coarse-grained proximal tempestites to thin, finegrained distal tempestites, and finally end up with
indistinct mud tempestites or purely autochthonous
shelf muds (Fig. 3.8c). The transition zone from
proximal to distal storm beds may be on the order of
20 to 50 m in vertical seetion.
- Shallowing basin, with a sedimentation rate higher
than subsidence. The resulting tempestite sequence
coarsens (thickens) upward (Fig. 3.8d), and the transition zone from shelf muds to a silicieIastic or
bioeIastic foreshore and coastal environment will be
also of limited thickness. Such a regressive trend favors amalgamation of storm beds.
Hence, tempestites can be distinguished from
turbidites not only on the basis of certain sedimentary
structures (particularly hummocky cross-stratification
and wave ripples) and faunal characteristics of individual beds, but often also by rapid facies changes in
their vertical sequences.
Tempestite sequences displaying repeated
coarsening-upward sections have been described
from various locations and sediments of different
ages. These tempestite cyeIes are usually explained
by relative sea-Ievel changes (cf. Sect. 7.9).