Geology Reference
In-Depth Information
12.1.5.3 Case Study: Black Shale Develop
ment on a Carbonate Platform
Box 12.6.
Criteria of paleo-oxygenation levels
(based
on Allison et al. 1995; Goldring 1991; Oschmann 1988,
1991; Rhoads and Morse 1971; Sageman et al. 1991;
Savrda and Bottjer 1986, 1989).
A specific Late Triassic facies type (Seefeld facies)
of the Northern Calcareous Alps is characterized by
organic-rich laminated marlstones and limestones in-
tercalated in a sequence consisting of dolomitic wacke-
stones and mudstones of the Hauptdolomit facies (Hopf
et al. 2001). This facies represents a tidal, partly re-
stricted depositional environment.
Differences in paleo-oxygenation conditions are in-
dicated by biotic composition and diversity, preserva-
tion of fossils, microfacies, bioturbation and micro-
lamination (Fig. 12.10).
Aerobic conditions are indicated by a gastropod-os-
tracod-foraminifera community and a monotypic bi-
valve assemblage. Small burrowing bivalves associated
with ostracods reflect dysaerobic conditions.
Anoxic conditions are characterized by the absence
of benthic biota, the presence of microlamination, and
the excellent preservation of a fish fauna within the
bitumen-rich beds. Organic geochemical data (pristane/
phytane ratio) provide evidence of cyclically occurring
anoxic conditions in morphological depressions within
the platform, and a stagnant, stratified water body.
Biota
• Presence or absence of benthic organisms. Complete
absence or lack of benthic organisms with shells.
• Absence of deposit feeders may indicate oxygen-de-
ficient conditions.
• Trace fossil assemblages. Abundance, type and size
of trace fossils.
• Diversity of benthic organisms and trace fossils.
Number of species.
• A benthic fauna of very low diversity, or lack of any
benthic elements point to oxygen-deficient condi-
tions.
• Specific morphological and physiological adaptions
of benthic organisms (e.g. mollusks, brachiopods) to
oxygen-deficient and H
2
S conditions.
• Morphology, test size, wall thickness and pore pat-
terns are used to differentiate a benthic foraminiferal-
oxygen index (Kaiho 1994). The index describes dis-
solved-oxygen levels in the modern ocean.
•
Exceptional preservation of soft parts of fossils po-
tentially indicating anoxic conditions.
Fabric
•
Lamination.
•
Fine-grained, microlaminated sediments with me-
dium to high amount of organic matter indicate oxy-
gen-deficient environments.
Basics: Oxygen
Allison, P.A., Wignall, P.B., Brett, C.E. (1995): Paleo-oxy-
genation: effects and recognition. - In: Bosence, D.W.J.,
Allison, P.A. (eds.): Marine paleoenvironmental analysis
from fossils. - Geological Society London, Special Pub-
lications,
83
, 97-112
Berner, R.A. (1984): Sedimentary pyrite formation: an up-
date. - Geochim. Cosmochim. Acta,
48
, 605-615, Oxford
Ekdale, A.A., Mason, T.R. (1988): Characteristic trace-fos-
sils associations in oxygen-poor sedimentary environ-
ments. - Geology,
16
, 720-723
Etter, W. (1995): Benthic diversity patterns in oxygenation
gradients: an example from the Middle Jurassic of Swit-
zerland. - Lethaia,
28
, 259-270
Hopf, H., Thiel, V., Reitner, J. (2001): An example of black
shale development on a carbonate platform (Late Trias-
sic, Seefeld, Austria). - Facies,
45
, 203-210
Jones, R.W. (1987): Organic facies. - Advanced Petrology
and Geochemistry,
2
, 1-90
Kukal, Z. (1990): Recent and ancient marine anoxic environ-
ments. - Casopis pro mineralogii a geologii,
35
, 287-300
Oschmann, W. (1991): Anaerobic-poikiloaerobic-aerobic: a
new zonation for modern and ancient neritic redox facies.
- In: Einsele, G., Riecken, W., Seilacher, W. (eds.): Events
and cycles in stratigraphy. - 565-571, Berlin (Springer)
Oschmann, W. (1993): Environmental oxygen fluctuations
and the adaptive response of marine benthic organisms. -
Journal of the Geological Society London,
150
, 187-191
Oschmann, W. (1994): Adaptive pathways of benthic organ-
isms in marine oxygen-controlled environments. - Neues
Jahrbuch für Geologie und Paläontologie, Abhandlungen,
191
, 393-444
•
Alternation of millimeter-laminated sediments to vari-
ously burrowed sediments may point to dysoxic con-
ditions.
•
Abundance and intensity of bioturbation. Size and
orientation of burrows.
•
A shift from bioturbation to discrete burrows indi-
cates a gradual decrease in the oxygen level.
Geochemistry
•
Preservation and composition (organic compound ra-
tios, e.g. pristane/phytane) of organic matter (Jones
1987).
•
Total organic carbon (Pedersen and Calvert 1990) and
hydrogen index (Tyson and Pearson 1991).
•
Organic carbon/sulfur ratios (Gautier 1986; Kajiwara
and Kaiho 1992).
•
Uranium enrichment (Wignall and Myers 1988).
•
Rare element anomalies (e.g. cerium; Wright et al.
1987) and concentrations (Anderson et al. 1989).
•
Pyritization of iron minerals (Raiswell et al. 1988).
fine lamination, high lateral persistence, absence or rare-
ness of benthic organisms and trace fossils are used to
suggest anoxic or dysoxic conditions. These features
occur in argillaceous sediments as well as in limestones
(e.g. Tyszka 1994). Box 12.6 lists criteria used to inter-
pret ancient oxygen-poor or oxygen-free sea bottom
environments.
