102
with a basal lag deposit and subsequent parallellamination and current ripple cross-stratification. In ideal
cases, the HCS division is overlain by wave ripple
cross-stratification and oscillatory ripple marks reflecting the final stage of a waning storm. However,
rippled tops mayaiso result from subsequent large
waves.
- On the inner shelf and parts of the outer shelf, either hummocky cross-stratified sand layers or thinner
graded sandy and silty beds (distal tempestites) with
cross-stratified and sometimes rippled tops are typical (Fig. 3.4e). Muddy interbeds are produced either
by storms which have moved inland and eroded finegrained material, or by the slow and repeated deposition of suspended river load.
- At greater water depths on the outer shelf, the current component of the combined storm flow becomes
dominant, leading to current-rippled fine sand and
silt beds (cf. Fig. 3.8b). Storm events affecting this
zone are extremely rare followed by long periods of
quiescence. In high-energy shelf seas, distal
tempestites occur at water depths up to and in excess
of 50 m. They appear to be more discontinuous than
proximal ones.
In the North Sea, modem distal tempestites are traceable
over tens ofkilometers in water depths up to 30 m (Aigner
and Reineck 1982). Sandy and muddy tempestites studied
in the Gulf of Mexico can be ascribed to specific tropical
storms recorded during the past 40 years (Fig. 3.6a; Keen
and Slingeriand 1993). They have been found in box cores
at water depths of 10 to 20 m and as far as -60 km seaward
from the coastline (Fig. 3.6b). The thicknesses of individual tempestites vary considerably (Fig. 3.6c); many beds
pinch out proximally or distally; several thin beds combine
locally to form one thick bed; some beds have been destroyed by subsequent erosion.
A complete, idealized tempestite of intermediate distance from the coastline shows the following sedimentary structures (from top to bottom, Figs. 3.4d
and 3.5a and c):
- Redeposited shelf mud (muddy tail of combined
flow).
- Wave ripples and wave-ripple cross-stratification.
- Low-angle hummocky cross-stratification.
- Parallellamination and current-ripple cross-stratification.
- Graded layer with basal lag deposit.
- Erosional base with sole marks (in places bipolar
or multi-directional) and casts of animal burrows.
N orrnal shelf mud, intensely bioturbated.
In shallower water, the graded subdivision is often
reduced or missing. Instead, large-scale hummocky
cross-stratification, swaley stratification, and coarsegrained infillings of scours are most characteristic.
Chapter 3 Coastal and Shallow Sea Sediments
At greater water depths, hummocky cross-stratification becomes less distinct and is more or less replaced by parallellamination, wave-ripple cross-bedding, and lenticular-wavy bedding with wave ripple
marks (Fig. 3.8b). The latter structures are partially
produced by subsequent storms affecting previously
deposited fine sand and silt.
These structures are described in detail, e.g., by Craft
and Bridge (1987) and Krassay (1994). They should
not be confused with lenticular and flaser bedding
observed in mixed sandy and muddy tidal flats. Calcareous tempestites with wave-rippled tops have
been also described from pelagic facies in the Jurassic of southem Spain (Molina et al. 1997). In this
case, the water depth of the basin cannot have been
much deeper than the storm-wave base, and the
tempestites probably formed during relative sea-Ievel
lowstand.
Lags often consist of gravel, small rock fragments,
broken shells of molluscs and other organisrns. In
Proterozoic and Cambrian strata, tempestites frequently consist mainly of reworked muddy intraclasts
or reworked microbial mats (flat pebble conglomerates, Fig. 3.7c; e.g., Sepkoski et al. 1991; Mount and
Kidder 1993).
Other Characteristics ojTempestites
Grain size distribution and composition of tempestites vary greatly. They range from coarse grained
(sand and gravel) to silty and muddy types. Pure
siliciclastic types with hardly any fossil remains contrast with calcareous bioclastic sandstones,
wackestones or packstones (Fig. 3.5a). The components of the latter type are derived either from preevent epibenthic or shallow infaunal populations.
Amalgamation and/or cannibalism are common
phenomenona in the proximal zone (Fig. 3.4d).
These terms signify that either pre-existing thick
tempestites are truncated, or thin tempestites are
completely reworked by subsequent storms and their
material is incorporated into new tempestites. This
process may occur repeatedly, until an extremely big
storm event ultimately produces a bed, the base of
which can be preserved.
Multiple reworking prornotes abrasion and break up of
mechanically unstable particles and possibly also accelerated dissolution of carbonate and other minerals. Thus,
amalgamation leads to increasingly "mature" lag sediments,
including placer deposits. In places, skeletal remains of
vertebrates, particularly teeth but also coprolites are concentrated in the basallayer of storm beds (bone beds). The
latter have been phosphatized prior to reworking,
The ichnofacies of the muddy host sediments of intermediate to distal tempestites is norrnally character-
with a basal lag deposit and subsequent parallellamination and current ripple cross-stratification. In ideal
cases, the HCS division is overlain by wave ripple
cross-stratification and oscillatory ripple marks reflecting the final stage of a waning storm. However,
rippled tops mayaiso result from subsequent large
waves.
- On the inner shelf and parts of the outer shelf, either hummocky cross-stratified sand layers or thinner
graded sandy and silty beds (distal tempestites) with
cross-stratified and sometimes rippled tops are typical (Fig. 3.4e). Muddy interbeds are produced either
by storms which have moved inland and eroded finegrained material, or by the slow and repeated deposition of suspended river load.
- At greater water depths on the outer shelf, the current component of the combined storm flow becomes
dominant, leading to current-rippled fine sand and
silt beds (cf. Fig. 3.8b). Storm events affecting this
zone are extremely rare followed by long periods of
quiescence. In high-energy shelf seas, distal
tempestites occur at water depths up to and in excess
of 50 m. They appear to be more discontinuous than
proximal ones.
In the North Sea, modem distal tempestites are traceable
over tens ofkilometers in water depths up to 30 m (Aigner
and Reineck 1982). Sandy and muddy tempestites studied
in the Gulf of Mexico can be ascribed to specific tropical
storms recorded during the past 40 years (Fig. 3.6a; Keen
and Slingeriand 1993). They have been found in box cores
at water depths of 10 to 20 m and as far as -60 km seaward
from the coastline (Fig. 3.6b). The thicknesses of individual tempestites vary considerably (Fig. 3.6c); many beds
pinch out proximally or distally; several thin beds combine
locally to form one thick bed; some beds have been destroyed by subsequent erosion.
A complete, idealized tempestite of intermediate distance from the coastline shows the following sedimentary structures (from top to bottom, Figs. 3.4d
and 3.5a and c):
- Redeposited shelf mud (muddy tail of combined
flow).
- Wave ripples and wave-ripple cross-stratification.
- Low-angle hummocky cross-stratification.
- Parallellamination and current-ripple cross-stratification.
- Graded layer with basal lag deposit.
- Erosional base with sole marks (in places bipolar
or multi-directional) and casts of animal burrows.
N orrnal shelf mud, intensely bioturbated.
In shallower water, the graded subdivision is often
reduced or missing. Instead, large-scale hummocky
cross-stratification, swaley stratification, and coarsegrained infillings of scours are most characteristic.
Chapter 3 Coastal and Shallow Sea Sediments
At greater water depths, hummocky cross-stratification becomes less distinct and is more or less replaced by parallellamination, wave-ripple cross-bedding, and lenticular-wavy bedding with wave ripple
marks (Fig. 3.8b). The latter structures are partially
produced by subsequent storms affecting previously
deposited fine sand and silt.
These structures are described in detail, e.g., by Craft
and Bridge (1987) and Krassay (1994). They should
not be confused with lenticular and flaser bedding
observed in mixed sandy and muddy tidal flats. Calcareous tempestites with wave-rippled tops have
been also described from pelagic facies in the Jurassic of southem Spain (Molina et al. 1997). In this
case, the water depth of the basin cannot have been
much deeper than the storm-wave base, and the
tempestites probably formed during relative sea-Ievel
lowstand.
Lags often consist of gravel, small rock fragments,
broken shells of molluscs and other organisrns. In
Proterozoic and Cambrian strata, tempestites frequently consist mainly of reworked muddy intraclasts
or reworked microbial mats (flat pebble conglomerates, Fig. 3.7c; e.g., Sepkoski et al. 1991; Mount and
Kidder 1993).
Other Characteristics ojTempestites
Grain size distribution and composition of tempestites vary greatly. They range from coarse grained
(sand and gravel) to silty and muddy types. Pure
siliciclastic types with hardly any fossil remains contrast with calcareous bioclastic sandstones,
wackestones or packstones (Fig. 3.5a). The components of the latter type are derived either from preevent epibenthic or shallow infaunal populations.
Amalgamation and/or cannibalism are common
phenomenona in the proximal zone (Fig. 3.4d).
These terms signify that either pre-existing thick
tempestites are truncated, or thin tempestites are
completely reworked by subsequent storms and their
material is incorporated into new tempestites. This
process may occur repeatedly, until an extremely big
storm event ultimately produces a bed, the base of
which can be preserved.
Multiple reworking prornotes abrasion and break up of
mechanically unstable particles and possibly also accelerated dissolution of carbonate and other minerals. Thus,
amalgamation leads to increasingly "mature" lag sediments,
including placer deposits. In places, skeletal remains of
vertebrates, particularly teeth but also coprolites are concentrated in the basallayer of storm beds (bone beds). The
latter have been phosphatized prior to reworking,
The ichnofacies of the muddy host sediments of intermediate to distal tempestites is norrnally character-