Geology Reference
In-Depth Information
outer ramp settings (Markello and Read 1981; Kreisa
1981; Wu 1982; Aigner 1982; Wright 1986).
•
Base of storm beds:
The base is erosional and sharp.
Erosive contacts
of tempestites composed of packstones
with the underlying bed indicate erosion of the sea bot-
tom (commonly mudstone and wackestone; see Pl. 102/
1, 3).
Sharp and plane boundaries
between graded pack-
stone of tempestites and the underbeds point to rework-
ing by storms followed by rapid transport of reworked
material into muddy areas (Pl. 102/2).
•
Sole marks:
Channels and scours
cutting into fine-
grained limestones and filled with skeletal packstones
and grainstones are often associated with hummocky
beds. Infilling material is heterogeneous and unsorted.
Gutter casts
(U-shaped depressions) indicate paleo-
current directions.
•
Burrowing
is common at the top of storm beds.
Strong bioturbation indicates breaks in storm activity.
•
Internal structures:
Storm beds may exhibit charac-
teristic
vertical sequences
(Dott and Burgeois 1982;
Duke 1985; Aigner 1985; Monaco 1992). A complete
storm wave sequence consists of a basal lag unit char-
acterized by an erosional base and coarse-grained
resediments and skeletal grains, overlain by parallel-
laminated, hummocky, flat-laminated, cross-laminated
and mudstone units (Pl. 102/2). This sequence reflects
deposition during waning flows. Note that this sequence
is an ideal one that is by no means developed in all
tempestites and differs for proximal and distal parts.
Proximal storm layers comprise beds consisting from
bottom to top of an erosional base, parallel to low-angle
lamination with minor internal discordances, wave
ripples, and a mud layer. A common variation is an ero-
sive basal coarse-grained lag layer, a central well-sorted
and matrix-poor calcisiltite or fine-grained calcarenite
layer, and an upper fine-grained rippled calcilutite layer.
Distal storm layers exhibit erosive or non-erosive bases,
internal flat layers, rare grading, and parautochthonous
accumulations of shells.
the tempestite. Parallel lamination may indicate unidi-
rectional flows (Pl. 102/3).
• Millimeter- to decimeter-sized
micrite clasts
and in-
traclasts
(Pl. 6/1) in wackestones, packstones and grain-
stones may indicate redeposition of storm-derived ma-
terial.
•
Lithoclasts
comprising distinctly
different micro-
facies
, e.g. fenestral bindstones and coral boundstones,
pointing to high-energy events and erosion in peritidal
and reefal environments.
•
Reworked black pebbles
(Pl. 16/1) can indicate
storm-induced coastal erosion.
•
Non-carbonate extraclasts
may represent storm-de-
rived material (Pl. 16/4).
•
Flat-pebble conglomerate fabrics.
Clast-supported
calcirudites composed of coarse flat-shaped intraclasts
typically occur in subtidal environments together with
storm deposits, but also originate in storm-affected tidal
environments (Pl. 125/3). Many examples have been
described from Cambrian and Ordovician storm depos-
its. The preservation of flat-pebble structures is facili-
tated by early lithification enhanced by microbial ac-
tivity (Ito and Matsumoto 2001).
•
Edgewise conglomerate fabrics
(Pl. 58/1) may indi-
cate storm-influenced sedimentation.
Fossils in tempestites
• Distinct
quantitative differences
exist in the
compo-
sition
of tempestite beds and under- and overlying beds
(Stel 1975).
•
Skeletal concentrations
are common features of
many modern and ancient carbonate and siliciclastic
shelves. Diagnostic criteria of storm-controlled bivalve
shell layers are mixtures of epifaunal and endofaunal
elements, bimodal sorting of complete shells and com-
minuted shell debris (Pl. 102/5), random orientation,
packstone and grainstone matrix between the shells and
an erosive base of the millimeter- to decimeter-thick
shell layers (Pl. 102/1, 3). The complete or disarticu-
lated shells are well preserved (Pl. 17/4). Abrasion, bio-
erosion and encrustations are absent. Distal tempestites
often exhibit telescoped mollusk valves (Pl. 102/5).
Common fossil groups contributing to the formation
of storm-related skeletal concentrations are bivalves,
brachiopods and crinoids.
•
Trace fossils
and different types of
bioturbation
in
storm-induced beds provide clues to the estimation of
the depositional water depths of tempestites (Wu 1982;
Aigner 1985). Bioturbation is commonly strong in the
upper part of storm beds.
Microfacies criteria
• Distinct
differences in grain sizes
of tempestites and
of under- and overlying beds (grain- and packstones
intercalated within mudstones or wackestones). The
primary grain-size spectra may be obscured due to bio-
turbation.
•
Graded bedding
is often common. The graded parts
of the tempestite beds are commonly succeeded by
finely laminated bioclastic packstones and wackestones
(Pl. 102/3) which in turn are overlain by dark lime mud-
stones or marls.
•
Parallel lamination
associated with
ripple laminated
intervals
in beds with hummocky cross stratification
indicate a high current regime during the deposition of
Th
e microfacies of tempestites
is characterized by
packstone textures, densely packed grainstones and
rudstones. Abundant grains are reworked bioclasts and
