Page 232 - Sedimentology and Stratigraphy

Storm-dominated Shallow Clastic Seas 219

14.2.2 Characteristics of a storm-dominated
shallow-marine succession

If there is a constant sediment supply to the shelf,
continued deposition builds up the layers on the sea
bed and the water becomes shallower. Shelf areas that
were formerly below storm wave base experience the
effects of storms and become part of the offshore tran-
sition zone. Similarly addition of sediment to the sea
floor in the offshore transition zone brings the sea
bed up into the shoreface zone above fair-weather
wave base and a vertical succession of facies that
progressively shallow upwards is constructed
(Figs 14.5 & 14.6) (Walker & Plint 1992). The off-
Fig. 14.4 A bed deposited by storm processes. The base
(bottom of the photograph) of the bed has a sharp erosional shore facies mainly consists of mudstone beds with
contact with underlying mudrocks. Planar lamination is some bioturbation. This is overlain by offshore transi-
overlain by hummocky cross-stratification and capped tion facies made up of sandy tempestite beds inter-
by wave-rippled sandstone and mudstone (just below the bedded with bioturbated mudstone. The tempestite
adhesive tape roll, 8 cm across). beds have erosional bases, are normally graded and
show some hummocky–swaley cross-stratification.
storms that deposit beds tens of centimetres thick is The thickness of the sandstone beds generally
not easy to estimate, because the availability of sand is increases up through the succession, and the deposits
probably of equal importance to the storm energy in of the shallower part of this zone show more SCS than
determining the thickness of the bed. HCS. The shoreface is characterised by sandy beds
In the periods between storm events this part of the with symmetrical (wave) ripple lamination, horizon-
shelf is an area of deposition of mud from suspension. tal stratification and SCS, although sedimentary struc-
This fine-grained clastic material is sourced from river tures may be obscured by intense bioturbation.
mouths and is carried in suspension by geostrophic Sandstone beds in the shoreface may show a broad
and wind-driven currents, and storms also rework lens shape if they were deposited as localised ridges on
a lot of fine sediment from the sea floor and carry it the shallow sea floor. The top of the succession may
in suspension across the shelf. Storm deposits are be capped by foreshore facies (13.2).
therefore separated by layers of mud, except in cases
where the mud is eroded away by the subsequent
storm. The proportion of mud in the sediments 14.2.3 Mud-dominated shelves
increases offshore as the amount of sand deposited
by storms decreases. Some shelf areas are wave- and storm-dominated, but
receive large quantities of mud and relatively little
sand. They occur close to rivers that have a high
Offshore
suspended load: the plumes of suspended sediment
The outer shelf area below storm wave base, the off- from the mouths of major rivers may extend for
shore zone, is predominantly a region of mud deposi- tens or hundreds of kilometres out to sea and then
tion. Exceptional storms may have some effect on this are reworked by wind-driven and geostrophic cur-
deeper part of the shelf, and will be represented by rents across the shelf (McCave 1984). Muddy deposits
thin, fine sand deposits interbedded with the mud- on the inner parts of the shelf are normally intensely
stone. Ichnofauna are typically less diverse and abun- bioturbated, except in cases where the rates of sedi-
dant than the associations found in the shoreface and mentation of mud are so high that accumulation out-
offshore transition zone. The sediments are commonly paces the rate at which the organisms can rework the
grey because this part of the sea floor is relatively sediment. High concentrations of organic matter may
poorly oxygenated allowing some preservation of make these shelf muds very dark grey or black
organic matter within the mud. in colour.