Geology Reference
In-Depth Information
Turbidites
Tempestites
Top
Wave ripples and ripple
Absent
Common in proximal parts
cross lamination
Current ripples and
Common
Less common than in
Sedimentary
current ripple bedding
turbidites
Convolute lamination
Common
Rare
structures
in beds
Hummocky cross stratification
Absent
Common
Traction carpet with
Common in proximal
Absent
inverse grading
parts
Sole marks
Unidirectional
Often bipolar, irregular scouring,
Bottom
gutter cast channeling
Benthic background
Deepwater facies
Shallowwater facies
community
Displaced fossils within
Shallow and deepwater
Shallowwater species only
Biofacies
event beds
species
Autochthonous postevent
Episodic colonization by
Fauna similar to preevent
fauna and bioturbation
specific fauna preceding
fauna if similar substrata are
return to background
available
conditions
Amalgamation
Less common
Very common and pronounced
Stratigraphic
Lateral continuity of single beds
Often over long distances
Usually limited
context
Thickness of sequence
Often large, commonly but
Limited, associated
not always associated with
with shallowwater facies
deepwater facies
Fig. 15.20. Turbidite and tempestite criteria. High-energy events can produce similar or identical textures and sedimentary
structures. The vertical sequence of turbidites as well as that of storm deposits (see Sect. 12.1.2.1) is characterized graded
and laminated structures. Differentiating turbidite and tempestite beds requires consideration of various aspects of sedimen-
tary structures, biofacies and the stratigraphic context. See Fig. 15.19 for an example.
may act as a guide for sampling and studying lime-
stone turbidites. Box 15.3 lists papers containing meth-
odological advice on the advantages and disadvantages
of using calciturbidites as facies indicators.
Field aspects
• Determine the geometry, lateral extension and thick-
ness of the turbidite beds.
• Evaluate the frequency of turbidite beds within a
given section. Measure the distance between turbidite
beds. Do turbidites prevail over beds formed by nor-
mal pelagic sedimentation (common if the turbidites
have been formed in a position proximal to the source
area) or are turbidite beds limited to only a few beds
within a sequence consisting predominantly of basinal
sediments (common in distal position)?
• Look for indications of pressure solution that may
strongly alter appearance of bedding (see Sect. 7.5.2).
How to investigate limestone turbidites?
Use criteria displaying diagnostic features on dif-
ferent scales. Note similarities with tempestites (Fig.
15.19, Fig. 15.20).
Sampling
Sampling of turbidite beds must consider the great
variation in turbidite sequences. Densely spaced
samples should be taken from the base and the top of
the bed, and within the depositional units of the se-
quence. Equal distance sampling within the units is rec-
ommended.
Many calciturbidites are stylolitized. Take samples
from individual stylobeds for recognizing significant
changes in grain sizes. Additional samples must be
taken from the background sediment. Try to investi-
gate the total thickness of turbidite beds. This is easily
done because many beds are only a few tens of centi-
meters thick.
Bouma/Meischner Sequence units
• Try to recognize the Zones 1 to 3 of the Meischner
Sequence or the divisions A to E of the Bouma Se-
quence. Which zones are missing? Have a look at the
boundaries between the zones, which may be sharp,
gradual or faint.
• Describe the units by means of texture considering
the size of the dominant transported grains. Differenti-
ate between textures made by silt-sized and sand-sized
particles, or fine- and coarse-sized particles.
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