Geology Reference
In-Depth Information
•
Clasts are supported by fine-grained sediment con-
sisting of micrite, calcisiltite of argillaceous material.
Calcareous debris flow sediments are predominantly
deposited on deep-marine slopes or at the base of slopes,
in contrast to turbidites that are deposited in proximal
or distal positions depending on the position of the
source areas. Grain flow deposits may pass rapidly into
pelagic and hemipelagic sediments.
•
Larger boulders or pebbles may be embedded within
debris flow deposits.
A specific type of debris flow deposits are
disorga-
nized limestone conglomerates,
consisting of shallow-
and/or deep-water clasts forming extended sheets and
channels.
Common criteria are:
15.7.2.5 Turbidites
•
Sand- to boulder sized clasts; very poor sorting.
Turbidites are sedimentary deposits laid down by tur-
bidity currents and intercalated in fine-grained sedi-
ments (Edwards 1993).
Turbidite successions vary greatly in sedimentary
structures, bed thickness, and textural features. Varia-
tions are controlled by distance between the source and
of the depositional area, point or line source, composi-
tion of the sediment available, topography of the depo-
sitional area and density of turbidity currents (low or
high density suspensions).
Common criteria of turbidites are:
•
Angular, subrounded or rounded clasts.
•
Randomly distributed, often densely packed clasts.
•
Monomict or polymict clasts, possibly derived from
shallow-marine, slope or deep-marine sediments.
•
Lime mud or argillaceous mud matrix.
•
Internal structures within the beds rare or absent.
•
Normal grading absent; inverse grading in the basal
zone of the unit.
Examples of disorganized limestone conglomerates
and breccias have been described from the Cambrian
(Keith and Friedman 1977; McIlreath 1977; Cook and
Taylor 1977), Devonian (Cook et al. 1972), Cretaceous
(Cook and Enos 1977), Pleistocene (Crevello and
Schlager 1980).
•
Regular vertical sequence of units characterized by
specific sedimentary structures (Bouma Sequence
respectively Meischner Sequence).
•
Graded bedding and lamination common.
•
Bottom surface sharply defined, frequently with sole
marks. Upper bed surface usually not well defined,
showing transitions into the overlying beds.
15.7.2.4 GrainFlow Deposits
•
Biota consisting of allochthonous, redeposited fos-
sils derived from shallow-water and slope environ-
ments. Autochthonous fossils rare, apart from trace
fossils. Pelagic and deep-water benthic fossils oc-
cur in intercalated shales and may form a small part
of the fossils occurring within the turbidites.
Grain flows are gravity flows in which material of dif-
ferent grain sizes is supported within the flow, mainly
by the strength of a fluid matrix consisting of water
and clay minerals.
Identification of grain flow deposits is difficult be-
cause most of the features are also produced by other
transport processes. Very often grain flow deposits are
associated with turbidites (e.g. Stauffer 1967; Eberli
1987).
Common criteria are:
Bouma Sequence:
An ideal turbidite sequence con-
sists of a vertical succession of internal sedimentary
structures. The Bouma Sequence (Bouma 1962), first
studied in siliciclastic deposits, exhibits five intervals,
from base to top:
• graded or massive unit (division) A,
• lower parallel laminated unit B consisting of thin
laminae,
• unit C characterized by current ripple lamination
with several sets of unidirectional foreset bedding
and/or convolute bedding,
• upper parallel laminated unit D consisting of very thin,
often obscure laminae within fine-grained sediment,
• very fine-grained unit E without primary sedimen-
tary structures.
As a rule, complete Bouma Sequences should char-
acterize proximal turbidites, incomplete Bouma Se-
quences distal turbidites (Walker 1967).
•
Thick massive beds.
•
Deposits draping over irregularities and thinning up
flanks of channels.
•
Flat tops and flat base. Sometimes flute marks.
•
Scours and injection structures at the base.
•
Partial to complete clast support.
•
Clasts floating in sandy or muddy matrix.
•
Poor sorting.
•
Grain orientation parallel to flow; faint, dish-shaped
laminae.
•
No parallel lamination or cross lamination.
•
Normal grading absent or rare. Sometimes reverse
grading near the base.
