The Jurassic (Toarcian) Posidonienschiefer of southern Germany is famous for its well preserved vertebrate fossils and its high organic content. The
majority of the Posidonienschiefer (10 meters thick in the study area) consists of
the Bituminous Shale, a fossiliferous laminated illite claystone. Two thin (30-40 mm) clayey pyritic biomicrosparites, the Upper and Lower Schlacken, interrupt
the Bituminous Shale. Geologists who have studied the Posidonienschiefer disagree
about the exact nature of its depositional environment. The argument centers on
the interpretation of an impoverished benthic fauna and whether or not the water
column directly above the sediment-water interface was anoxic or normally
oxygenated. Kauffman (1981) proposed that an algal/fungal mat located at or near
the sediment/water interface marked the boundary between aerobic and anaerobic
conditions during deposition. The purpose of my research was to investigate the
geologic conditions during deposition and early diagenesis of the Bituminous Shale
and the Schlacken and to search for evidence of microbial activity. A detailed
petrologic investigation of these two lithologies found no evidence of an
algal/fungal mat, but did reveal the important contribution of microbial activity
in the formation of pyrite and calcite cement. The Bituminous Shale was deposited in a low-energy tropical seaway. The
upper water-column supported a diverse marine fauna. The aerobic/anaerobic
boundary in the water column may have been located several millimeters above the
sediment/water interface. Pore waters of the ocean-floor mud were dysaerobic to anaerobic. Occasional oxygenation events allowed opportunistic benthic organisms
to colonize the sea-floor. Compaction of the Bituminous Shale occured prior to cementation of
original porosity. Framboidal pyrite was formed during sulfidic diagenesis under
anaerobic, but open, sediment/pore water conditions. Euhedral pyrite formed
later as communication between pores became restricted during sediment
compaction. The skeletal grains of the Schlacken formed as a winnowed lag deposit of
Bituminous Shale sediment. During the early stages of sulfidic diagenesis the winnowed beds were rapidly cemented in a concretion-like sheet. Early
cementation preserved delicate algal spores and clay fabric. Fossil bacteria were discovered in the calcite cement of the Schlacken by
modified petrographic techniques, and confirmed with the scanning electron
microscope. Experiments in which live bacteria were gradually entrapped in
halite produced a crystal fabric identical to that of the fossiliferous calcite cement
of the Schlacken. The microbial production of bicarbonate and ammonia during sulfidic
diagenesis played a significant role in altering local geochemical conditions in the
Schlacken sediment and initiated the precipitation of calcite cements. Fossil
bacteria in the cements of the Schlacken are direct evidence of the presence and
entrapment of bacteria during cementation, but do not conclusively prove their
active role in the formation of calcite. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/6770 |
| Date | 08 December 2009 |
| Creators | Hiebert, Franz Kunkel |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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