Geology Reference
In-Depth Information
Tab. 15.2.
Criteria of proximal and distal limestone turbidites.
The mean maximum grain size (decreasing in a downstream
direction), the ABC-Index (percent of beds beginning with Division A plus half the percent of beds beginning with Division
B), and the ratio of the thickness of Division A to the mean maximum grain size may also be used. Grading curves and
proximality curves exhibiting the differences in maximum grain sizes at different vertical positions of the turbidite bed have
also proved useful (Engel 1974; Steiger 1981).
Proximal
Distal
Field Aspects
Distribution of turbidite beds
Densely spaced; isolated or amalgamated;
Scattered, isolated; back-
background sedimentation limited
ground sedimentation high
Ratio of turbidite and pelagic sediment
High
Low
Geometry of turbidites
Lenticular beds of varying thickness;
Regular, planar parallel
medium- to thick-bedded
beds; thin-bedded (thinner
than a few tens of cms)
Associated sedimentary structures
Slumping structures; breccias; conglomerates
Breccias rare
Base of turbidite beds
Sharp; sometimes scours, tool marks,wash-
Sharp
outs, reworked pebbles, sometimes erosive
Fluxoturbidite at the base of the bed
Abundant
Absent or rare
Top of turbidite beds
Gradual
Sharp and gradual
Internal Sequence
Complete
Relatively common
Rare
Bouma/Meischner Sequence
Graded part (Zone 1a)
Well-developed, thick
Decrease in thickness
Grading
Graded, poorly graded or not graded
Grading common
Graded zone thick
Inverse grading
Common
Absent
Coarse-tail grading
Common
Absent
Base of basal detrital parts
Generally sharp
Sharp
Top of basal detrital parts
Often sharp
Grades into finer sediment
Lamination (Zone 1c and 2a)
Less common, often restricted to
More common
thicker beds
Ripple and convolute lamination
Restricted to thicker beds
Less common to absent
Bedding (Zone 2b)
Micritic upper parts of turbidite beds
Thin or absent
Well-developed
Lowermost units of turbidite beds
Zone 1a (Division A)
Zone 1b or Zone 1c
Grain size
Average grain size
Rudite and arenite
Arenite and smaller
Significant vertical breaks in
Common
Absent
dominant grain sizes
Matrix
Matrix between grains in detrital
Sparry calcite
Micrite
parts of the turbidite
Ratio of turbidite and pelagic sediment
High
Low
Microfacies
Clasts
Lithoclasts, extraclasts, fossils, ooids,
Lithoclasts, fossils, ooids,
peloids
peloids
Fossils
Benthic shallow-water fossils derived from
Benthic platform- and
platforms and platform margins; slope-
slope-derived fossils; autoch-
derived fossils, rare pelagic fossils
thonous deep-water fossils
Microfacies of clasts
Variable to highly variable
Relatively uniform
tion in ecological types of fossils (Tucker 1969; Remane
1970; Steiger 1981). Crinoidal turbidites are charac-
terized by the dominance of horizontal lamination,
scarceness of the graded parts, the lack of the upper-
most fine-grained unit, and a sharp contact with the
shales above. Deep-water calciturbidites can originate
from the transport of pelagic sediment, primarily de-
posited on the tops or flanks of a sea-floor relief, to
intrabasinal depressions. A well-described example are
the Late Jurassic
Saccocoma
calciturbidites from Po-
land (Matyszkiewicz 1996).
Practical advice for studying limestone turbidites
The investigation of allodapic limestones requires
that field observations and data extracted from thin sec-
tions and fossils be well integrated. The following text
