Geology Reference
In-Depth Information
Fig. 12.3.
Storm processes and storm beds.
Major storms will interact with the seafloor down to the storm wave base occurring
in depths of several tens of meters. Storms directed onshore lead to a buildup of water masses and, when the storm abates, to
the generation of offshore-directed return flows causing bottom currents that transport sediment down below the storm wave
base into low-energy depositional environments forming tempestite beds. Shoreface sediment put into suspension can be
transported offshore by density currents down to beyond storm wave base. The figure displays storm processes affecting
ramps and platforms. Reefs are different. After Tucker and Wright (1990).
formation of lag deposits, and change the distribution
patterns of benthic organisms. The effect of storms on
sedimentation is controlled by water depths and the lo-
cations of the fair-weather wave base and the storm-
weather wave base. Storms set up waves and currents
below the fair-weather wave base, cause reworking of
sediment and develop characteristic bed forms. On-
shore-directed storms generate currents leading to the
buildup of water in nearshore regions or in lagoons.
During the decline of storm intensity, offshore-directed
bottom currents transport shallow shoreface sediment
to deeper offshore settings (Fig. 12.3).
commonly mud-dominated. The base of the distal tem-
pestite beds is plane. Proximal tempestites often con-
tain many shell layers, distal tempestites only few.
Criteria of ancient tempestites:
The following text
summarizes field and laboratory criteria denoting the
storm-related deposition of carbonate sediments. The
facies analysis of ancient storm deposits requires the
integration of lithofacies, biofacies and microfacies.
Diagnostic features of carbonate tempestites
can be
derived from observations of modern storm sediments.
Some of the criteria listed in the following text are of-
ten difficult to recognize in ancient calcareous storm
deposits, e.g. erosive soles and rippled tops of the tem-
pestite beds, because of bioturbation and diagenetic
overprint.
Tempestites
Limestone and sandstone beds deposited by storms
are called
tempestites
(Kelling in Ager 1974). The base
of these storm beds is sharp and erosional, and exhibits
a variety of sole marks. Common internal structures
include accumulations of shells, graded bedding, flat
bedding, parallel and cross lamination. Ripple bedding
and burrowing may be common at the top of the layer.
Sedimentary structures and sequences of tempestites:
•
Tempestite beds are
intercalated in marly, shaly or
carbonate beds.
They are common in shallowing-up
sequences. Storm beds range in thickness between a
few millimeters and several tens of centimeter. Thick-
ness often differs markedly from that of underlying and
overlying beds.
• Storm waves and currents deposit layers of pack-
stones and grainstones often exhibiting characteristic
undulated bedding. This
hummocky cross stratification
(HCS; Harms et al. 1975; Swift et al. 1983; Duke 1987)
shows gently curved, low-angle cross lamination and
laminae with convex-upward (hummock) and concave-
upward (swale) developed curvatures. Hummocky
cross-stratified pack- and grainstones occur as 0.1 to
about 2 m thick units interbedded with lime mudstones
and wackestones or shales. Many storm sediments form
amalgamated beds. Hummocky limestone beds exhib-
iting packstone and grainstone textures are common in
Proximal and distal tempestites:
Storm beds exhibit
much variation in thickness, grain size and internal
structures, depending on the proximity and the inten-
sity of the storm (Aigner 1985; Myrow and Southerd
1996). Similar to turbidites, tempestite bed thickness
varies proportionally with the maximum grain size of
the bed.
Proximal
storm beds are thick-bedded, bio-
clast-dominated and coarse grained, and include many
amalgamated and composite beds containing ecologi-
cally and stratigraphically mixed biota. The base of the
beds is an erosion surface.
Distal
tempestite
beds tend
to become thinner, finer,
rarer, better preserved and more biogenic in composi-
tion toward deeper depositional environments. They are
