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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 37 (1967)No. 4. (December), Pages 1124-1138

Previous HitWaveNext Hit and Current Orientation of Shells

J. Stewart Nagle (2)


Shell orientation experiments in a flume, Previous HitwaveNext Hit tank, field current, and Previous HitwaveNext Hit environments have yielded a basic difference between Previous HitwaveNext Hit and current orientations. Rose diagrams of current oriented shells show one strong maximum; over two thirds of an assemblage of elongate conical or plate-form shells point into or away from the current, depending on the geometry and mass distribution of the shell. In shoaling, nonswash waves, long axes of the same forms are aligned parallel to the Previous HitwaveNext Hit crests or ripple marks. Because of currents associated with Previous HitwaveNext Hit progression, the maxima tend to be skewed, so that a Previous HitwaveNext Hit orientation pattern forms an obtuse arrow pointing in the direction of Previous HitwaveNext Hit progression. In low angle swash zones, orientations show the two maxima of Previous HitwaveNext Hit patterns, but the shells a e aligned perpendicular to the ripple marks by oncoming and outgoing swash.

Equant plate-form shells, rollers, and elongate plates and cones with protuberances may develop typical orientation patterns, but commonly show nondiagnostic orientations. Heavy shells fail to develop diagnostic orientations when they become entrapped in relatively soft sediment. Shells may become entangled on projections on gravel and shell hash, thus developing nondiagnostic orientations.

Shells are commonly oriented in a stable position by currents, then buried by scour around the shell margins. Under weak waves, cones are buried in Previous HitwaveNext Hit orientation by detritus on the bottom. Under ripple-forming waves, cones lie in the troughs of ripples, where they may be covered by orbitally scoured sand. Plates oscillate with the waves until the margins work into the sand or until they are buried by scoured sand. Shells are not buried in hard bottoms.

Application of these concepts to the marine Middle and Upper Devonian of Northeastern Pennsylvania showed that the Middle Devonian Mahantango Formation was deposited by waves moving from the southsoutheast. Apparently Previous HitwaveNext Hit action obliterated much of the bedding in this formation. The lower part of the Upper Devonian Fort Littleton Formation shows an abundance of current structures and shells in current orientation, probably because of current deposition below Previous HitwaveNext Hit base, as parts of the Upper Fort Littleton again show Previous HitwaveNext Hit orientations and obscure bedding, presumably because the basin was filled to above Previous HitwaveTop base.

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