NDLI: Middle Jurassic (Bathonian) encrusted oncoids from the Polish Jura, southern Poland

Content Provider	Springer Nature Link
Author	Zatoń, Michał Kremer, Barbara Marywski, Leszek Wilson, Mark A. Krawczyński, Wojciech
Copyright Year	2011
Abstract	Oncoids from two localities (Ogrodzieniec and Blanowice) of the Polish Jura, southern Poland, have been investigated with respect to their genesis and paleoecology. These oncoids occur within Middle Jurassic (Bathonian) deposits. Those from Ogrodzieniec are large, elliptical, and embedded within a presumably condensed carbonate bed. Those from Blanowice, on the contrary, are significantly smaller, irregular to box-like in shape, and occur within the ore-bearing clays. The oncoids from both localities consist of a distinct carbonate core and laminated cortex that is significantly thicker and better preserved in the Ogrodzieniec oncoids. SEM and optical microscopic investigation of the oncoid cortices revealed the presence of carbonate and silicate layers with web-like structures similar to those occurring in recent cyanobacterial microbialites. Thus, the oncoid cortices investigated may have formed in a photic zone environment with the aid of coccoid and filamentous cyanobacteria. Oxic conditions prevailed during oncoid cortex formation within the siliciclastic setting, which is manifested by low total organic carbon content, high pristane/phytane (Pr/Ph) ratio, and significant predomination of the C31 homohopanes. On the cortices’ surfaces, as well as between particular laminae, various encrusting organisms have been found. The encrusters, dominated by serpulids and bryozoans, are cryptic species that inhabited the undersides and recesses of the oncoids. Their presence on both the upper and lower surfaces of the oncoids indicates that the oncoids were episodically overturned on the seafloor. The much better developed cortex lamination and much higher diversity and abundance of encrusters in the Ogrodzieniec oncoids may point to better trophic conditions prevailing in a shallower marine environment characterized by transparent waters, as opposed to a deeper siliciclastic environment with less transparent waters and probably worse trophic conditions prevailing during formation of the Blanowice oncoids.
Starting Page	57
Ending Page	77
Page Count	21
File Format	PDF
ISSN	01729179
Journal	Facies
Volume Number	58
Issue Number	1
e-ISSN	16124820
Language	English
Publisher	Springer-Verlag
Publisher Date	2011-05-03
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Oncoids Cyanobacteria Encrusters Paleoecology Jurassic Poland Sedimentology Ecology Biogeosciences Paleontology Geochemistry
Content Type	Text
Resource Type	Article
Subject	Geology Stratigraphy Paleontology

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