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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 38 (1968)No. 1. (March), Pages 164-174

Dimensional Grain Orientation of Ordovician Turbidite Greywackes

Diane Onions, G. V. Middleton


Orientations of quartz grains in the bedding plane have been examined in 44 beds of graded turbidite greywackes in the Normanskill formation (Ordovician) of the Hudson Valley, New York. Detailed studies of vertical variation were made for 14 beds. The results show that although the grain orientation is locally consistent in some beds, in many it varies (apparently at random). Three beds were sampled both laterally and vertically; the results showed at least some vertical and lateral consistency for two of the three beds. There is no consistent relationship over the area studied between the direction of preferred orientation of the grains and the sole marks of the same bed. In general, the grain orientation may vary as much as 90° to either side of the sole lineation, and the distr bution of grain-sole deviations is close to uniform, without any general preference for clockwise or counter-clockwise rotation of the grain orientation relative to the sole marks. In one bed which showed divergent flute and groove casts, orientation of the grains in the flute was parallel to the flute and orientation of grains higher in the bed was parallel to the grooves. Grain orientation is least well defined at the base of the bed; there is no general tendency for the deviation from the sole marks to increase with height above the base of the bed.

Maximum grain size and minimum grain breadth/length ratios decrease upwards in the bed. The most elongate grains are those which most clearly show the preferred orientation.

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