Page 231 - Sedimentology and Stratigraphy

218 Shallow Sandy Seas

high, a few kilometres wide and tens of kilometres in
length, occurring spaced about 10 km apart. The sedi-

ments are typically well-sorted sands with a basal lag
of gravel (Hart & Plint 1995).

Offshore transition zone
In the offshore transition zone, between the fair-
weather and storm wave bases on storm-dominated
Fig. 14.2 Hummocky–swaley cross-stratification, a
shelves, sands are deposited and reworked by storms.
sedimentary structure that is thought to be characteristic
A storm creates conditions for the formation of bed- of storm conditions on a shelf.
forms and sedimentary structures that seem to be
exclusive to storm-influenced environments (Dott &
Bourgeois 1982; Cheel & Leckie 1993). Hummocky
cross-stratification (often abbreviated to HCS)is
distinctive in form, consisting of rounded mounds of
sand on the sea floor a few centimetres high and tens
of centimetres across. The crests of the hummocks
are tens of centimetres to a metre apart. Internal
stratification of these hummocks is convex upwards,
dips in all directions at angles of up to 108 or 208, and
thickens laterally: these features are not seen in any
other form of cross-stratification (Figs 14.2 & 14.3).
Between the hummocks lie swales and where concave
layers in them are preserved this is sometimes called
swaley cross-stratification (abbreviated to SCS).
Hummocky and swaley cross-stratification are
Fig. 14.3 An example of hummocky cross-stratified
believed to form as a result of combined flow, that sandstone with very well-defined, undulating laminae.
is, the action of both waves and a current. This occurs The bed is 30 cm thick.
when a current is generated by a storm at the same
time as high-amplitude waves reach deep below the Individual storm deposits, tempestites (11.3.1),
surface. The strong current takes sand out into the deposited by single storm events typically taper in
deeper water in temporary suspension and as it is thickness from a few tens of centimetres to milli-
deposited the oscillatory motion caused by the waves metre-thick beds in the outer parts of this zone several
results in deposition in the form of hummocks and tens of kilometres offshore (Aigner 1985). Proximal
swales. Swaley cross-stratification is mainly formed tempestites have erosive bases and are composed of
and preserved in shallow water where the hummocks coarse detritus, whereas the distal parts of the bed
have a lower preservation potential. One of the char- are finer-grained laminated sands: hummocky and
acteristics of HCS/SCS is that these structures are swaley cross-stratification occurs in the sandy parts
normally only seen in fine to medium grained sand, of tempestites (Walker & Plint 1992). An idealised
suggesting that there is some grain-size limitation tempestite bed (Fig. 14.4) will have a sharp, possibly
involved in this process. Storm conditions affect the erosive base, overlain by structureless coarse sedi-
water to depths of 20 to 50 m or more so HCS/SCS ment (coarse sand and/or gravel): the scouring and
may be expected in any sandy sediments on the shelf initial deposition occurs when the storm is at its peak
to depths of several tens of metres. These structures strength. As the storm wanes, hummocky–swaley
are not seen in shoreface deposits above fair-weather cross-stratification forms in finer sands and this is
wave base due to reworking of the sediment by ordi- overlain by fine sand and silt that shows horizontal
nary wave processes, so this characteristic form of and wave-ripple lamination formed as the strength of
cross-stratification is found only in sands deposited the oscillation decreases. At the top of the bed the
in the offshore transition zone (11.1). sediment grades into mud. The magnitude of the