Scratch circles associated with the large foraminifer Bathysiphon from deep-sea turbiditic sediments of the Pagliaro Formation (Palaeocene), Northern Apennines, Italy
2013
Uchman, Alfred | Rattazzi, Bruno
Scratch circles, which are rare sedimentary structures (and particularly so in deep-sea sediments), occur as erosional forms on the lower surface of a turbiditic sandstone bed in the Palaeocene Pagliaro Formation (Italy, Northern Apennines, San Nazzaro section). It was produced by rotation of a large, tubular, agglutinated test of the foraminifer Bathysiphon. Originally, the tests of Bathysiphon stood vertically or obliquely in the sediment in life position. They were partly exhumed and broken by bottom currents prior to deposition of the overlying turbiditic bed, but remained articulated and anchored in their original position due to elastic organic parts. The broken but still articulated parts were rotated by the current and scratched the muddy sediment surface producing concentric furrows. The furrows were cast by sand of the turbiditic bed. This structure occurs in two morphological variants. Type A is a set of ridges arranged within a sector of a circle with central angle (arc segment's angular distance) from 15 to 35°, occasionally with a ridge or furrow (gnomon structure) running along the radius of the circle. Type B is an almost flat disc with poorly developed concentric ridges, within a wide sector of a circle in which the central angle ranges from 100 to 140°. The scratch circles, determined as flag scratch circles (in opposition to full scratch circles; new terms), are oriented concordantly with small flute casts formed mostly around broken tests of Bathysiphon. Both of them point to the same direction of current flow, but the flag scratch circles record flows over longer time periods.
اظهر المزيد [+] اقل [-]الكلمات المفتاحية الخاصة بالمكنز الزراعي (أجروفوك)
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