Geology Reference
In-Depth Information
15.7.3.4 Jurassic of the Northern Calcareous
Alps: Detailed Information from Limestone
Turbidites on Source and Deposition Patterns
mentation in basins with strong input from intrabasinal
platforms and reefs. The methodological approach of
the study and of this model is recommended.
Late Jurassic sedimentation in the Northern Calcare-
ous Alps is characterized by carbonate platforms with
reefs (Plassen Formation), and pelagic carbonates de-
posited in deep-marine basins (Oberalm Formation).
Slope carbonates are characterized by a mixture of al-
lochthonous and autochthonous sediments. Autochtho-
nous sediments are formed by benthic organisms grow-
ing on lithified surfaces. Allochthonous carbonates
consist of fine-grained and sand- and gravel-sized ma-
terial exported from the platform and platform mar-
gins to the slope and the basin. One of the best studied
examples of allochthonous carbonates is the Tithonian
Barmstein Formation
(Steiger 1981). The Barmstein
limestones are exposed between Hallein near Salzburg,
Austria, and Berchtesgaden, Bavaria. The thick-bed-
ded carbonates are predominantly turbidites, sometimes
with a fluxoturbidite at the base. Most of the coarse-
and fine-grained resedimented material was eroded
from the Plassen platforms that originated on isolated
highs.
The limestones were investigated by means of quali-
tative and quantitative microfacies analysis, texture
analysis describing the composition of the sedimen-
tary sequences, grain-size analysis including the sta-
tistical evaluation of grain-size parameters, and pale-
ontological analysis. The latter comprises a systematic
treatment and paleoenvironmental interpretation of
thin-section fossils. Microfacies data on rock fragments
and lithoclasts, diagenetic criteria and the abundance,
association and taphonomic features of fossils were
used in reconstructing the facies types of the Plassen
platforms acting as source areas. The microfacies of
Triassic and Jurassic extraclasts from the section be-
low the Late Jurassic platform carbonates reveals the
intensity and course of submarine erosion over time.
The destruction of the platforms and the increase in
slope sediments was triggered by diapirism and related
tectonics.
The study shows the great potential of the combined
study of microfacies, fossils and sedimentological cri-
teria. Of particular interest is the differentiation of plat-
form material that was lithified at the time of erosion
and redeposition, non-lithified platform sediments and
organisms living in various parts of the destroyed plat-
forms, and limestone clasts representing input from
subaerially exposed parts of the source area. The study
of field logs and the consideration of the geometry and
extension of the turbidites and their distribution within
the small depositional basins (a few tens of kilometers
in size) result in a conclusive model of deep-water sedi-
15.7.4 Slope Stability Reflected by Texture
and Microfacies
Carbonate platform flanks are almost exclusively made
up of sediments shed by the platform itself. The com-
position of this platform overproduction determines the
angle that the sediment can maintain (Kenter and
Fig. 15.26.
Slope angles of carbonate platforms are controlled
by the dominant depositional fabric.
Grainy, non-cohesive
mud-free buildups form steeper slopes than muddy, cohe-
sive sediments. Grain-supported fabrics with minor or no
matrix (grainstones, rudstones, clast-supported breccias; e.g.
Gosaukamm; Pl. 27/2) build slope angles varying up to about
40°. Grain-supported textures with matrix (packstones; e.g.
Late Jurassic mounds in Germany) and mud-supported fab-
rics exhibit slope angles up to 15°. Mudstones (e.g. modern
Little Bahama Bank or Late Paleozoic algal mounds) have
low slope angles up to 5°, mud-supported wackestones and
floatstones up to about 15°. Well-documented examples are
marked by circles; examples lacking precise control of ge-
ometry by squares; and examples whose flanks are stabilized
by microbial automicrite (e.g. Waulsortian mounds, or the
Sella Platform in the Dolomites) and/or synsedimentary car-
bonate cements or organic framework by triangles.
Early sta-
bilization results in medium to steep slope angles.
The range
of fabric and slope angles is shown by bars for each data
point. These data indicate that the physical behavior of the
sediments in the gravity field, angle of shearing and read-
justment processes are linked to the composition of the slope
sediment.
Textural composition appears to be the major con-
trol on slope angle and slope curvature
besides windward-
leeward setting, sea level and climate.
Based and slightly modified from Kenter (1990), with
additional data for the Gosaukamm (own observations) and
for the Sella (Keim and Schlager 1999). A list of localities
and references can be found in Kenter (1990).
