Sedimentological implications of an unusual form of the trace fossil <Emphasis Type="Italic">Rhizocorallium</Emphasis> from the Lower Muschelkalk (Middle Triassic), S. Poland |
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Authors: | Monika Kowal-Linka Adam Bodzioch |
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Institution: | 1.Institute of Geology,Adam Mickiewicz University,Poznań,Poland;2.Department of Biosystematics,Opole University,Opole,Poland |
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Abstract: | Specimens of the trace fossil Rhizocorallium isp. with retrusive limbs occur in 10–20-cm-thick composite beds: the bottom layers contain abundant Planolites and Thalassinoides trace fossils, and the top layers are built of horizontally bedded pelitic limestone and contain less bioturbational structures.
The first specimens of Rhizocorallium isp. occur just 2–3 cm above the lower surfaces of the top layers. The horizontal protrusive sections are 1 cm high and 1 cm
wide. At some sites, the horizontal tunnels end, and the limbs become first obliquely retrusive (45°) at a distance of 1.5–2.5 cm,
and then vertically retrusive at a distance of 4–6 cm, or at once vertically retrusive at a similar distance. They reach to
the upper surfaces of the top layers. These composite beds with unusual Rhizocorallium mark successive stages of distal tempestite sedimentation. The erosional events are recorded as uneven bases and a lack of
Rhizocorallium, which suggests erosion to approx. 10 cm deep. The lower parts of the top layers record deposition of lime mud from a powerful
flow. The protrusive sections of Rhizocorallium record relatively quiet periods resulting from significantly decreased sedimentation. The successive retrusive sections provide
evidence that the quiet conditions suddenly came to an end, and that sedimentation was reactivated or accelerated. The last
retrusive limbs of Rhizocorallium isp. mark the border of the storm event and the next erosion event. The composition of the fecal pellets shows that these
burrowing animals were deposit-feeders, fast enough to find a sufficient amount of food, and also fast enough to escape being
buried. |
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