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New data on the trace fossil, Cruziana semiplicata (Furongian, Wiśniówka Sandstone Formation, Poland) : origin, ethology and producer

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Sadlok G.

tom Vol. 84, No. 1

35--50

Cruziana semiplicata Salter is a well known ichnospecies, ascribed to trilobites. Despite that, there are still contradicting views on its origin (infaunal vs. epifaunal), the ethology represented (crawling vs. feeding) and the identity of its producer (a few trilobite genera were proposed). In this paper, new data are presented and com- bined to create a coherent interpretative model for Cruziana semiplicata. According to this new model, Cruziana semiplicata is a fossilized version of an epifaunal, pascichnial (feeding) trace produced by an organism, positioned above the seabed with only a few frontal appendages touching the depositional surface and processing the sedi- ment below. This model is based on observations of trace fossil morphology and neoichnological observations of the feeding behaviour of Triops australiensis. Also, a short geometrical comparison with the co-occurring Rusophycus polonicus is made to show that these trace fossils most likely had different trace makers.

100%

Sadlok G.

2013

tom Vol. 83, No. 4

317--327

In general, the trace fossil Rusophycus, preserved as a concave-upward structure on the top of a bed, is considered to be a fossilized marking, made by a trace maker. The structures described from the Cambrian (Furongian) of central Poland are genetically related to Rusophycus. However, despite their occurrence on the tops of beds, they are not fossilized traces, but compaction-related features, resulting from differential sandstone and mudstone compaction with possible mediation by organic-rich, heterolithic sediments. The preservation of these structures probably was influenced by biofilms or biomats.

Slide origin of breccia lenses in the Cambrian of the North China Platform: new insight into mass transport in an epeiric sea

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Van L. A. J. T. , Han Z. , Han Y.

tom Vol. 18, No. 4

223--235

An oolite in the Furongian (Late Cambrian) Chaomidian Formation in Shandong Province, China, which was deposited on the North China Platform in an epeiric sea, contains several limestone breccia lenses of various dimensions (centimetres to decimetres thick and decimetres to more than 10 metres in length) in an E-trending section. The oolite, which is approximately 40 cm thick, was originally thicker, as indicated by a planar truncation surface that formed by wave abrasion. The breccia lenses in this oolite are generally mound-shaped with a flat base and a convex top. The western margin of the lenses is commonly rounded whereas the eastern margin commonly has a tail (consisting of a rapidly eastwards thinning breccia horizon that gradually ends in a horizon of isolated clasts). Some of the breccia lenses are underlain by a shear zone. The formation of the breccia lenses cannot be easily explained by normal depositional or deformational processes. It is concluded that the lenses represent fragments of a partly consolidated layer, consisting of both rounded and angular platy clasts, which slid down over a very gently inclined sedimentary surface which acted - possibly together with a water film - as a lubricant layer. During transport, the layer broke up into several discrete bodies that formed small ‘highs’ at the sedimentary surface of the shallow epeiric sea. Subsequently, waves partially eroded the lenses, mostly at their margins, producing their mound-shaped form. Sliding of blocks is known from a wide variety of environments in the sedimentary record; however, this is the first description of the sliding of blocks in an epeiric sea. This indicates that such a low-relief submarine carbonate setting is, like its siliciclastic counterparts, susceptible to this process.