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 degrees upward or ca. 50 degrees 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 patterns that indicate a prevailing orientation of gravity flows towards the SSW and SSE.

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