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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 66 (1996)No.
5. (September), Pages 888-899
Sedimentology and Orientation of Tentaculite Shells in Turbidite Lime Mudstone
to Packstone: Lower Devonian, Barrandian, Bohemia
Jindrich Hladil (1), Petr Cejchan (1), Ananda Gabasova
(2), Zdenek Tdeaborsky (2), Jana Hladikova (2)
ABSTRACT
A turbidite origin of upper Pragian lime mudstone is proposed on the
basis of imbrication and graded bedding in the lower parts of beds and
the abundance of calcisiltite components. Deposition of lime mud and calcisiltite
dominated the middle parts of the turbidite beds. This muddy part of turbulent
gravity surges settled on the submarine slope as a dense suspension. Downslope
sliding continued after deposition from sediment gravity flows as dewatering
occurred and the sediment underwent plastic deformation. The spatial orientation
of embedded tentaculite shells is unusual, but corroborates the sediment-gravity-flow
origin of the beds. The apices of narrow conical shells show preferred
positions of ca. 50° upward or ca. 50° downward, with upward directions
more common (63.5 ); the shells avoid both bedding-parallel and bedding-perpendicular
orientations. Double-cone envelopes arranged according to a subhorizontal
mirror plane are the basic orientation pattern. This double-cone pattern
of movement of narrow-cone particles was successfully simulated by simple
experiments in turbulent water. The steep inclination of shells reflects
rapid solidification of dense turbulent mud. Solidification was probably
intensified by later sliding and thrusting shears during the early plastic
stage of sediment deformation. Azimuth directions show only slight peaks
parallel and perpendicular to the direction of downslope propagation of
the gravity flow. The complex distribution of tentaculitid shells was analyzed
by factor analysis, which yielded four types of orientation patt rns that
indicate a prevailing orientation of gravity flows towards the SSW and
SSE.
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