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J. Stewart Nagle (2)
Shell orientation experiments in a flume, wave tank, field current, and wave environments have yielded a basic difference between wave 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 wave crests or ripple marks. Because of currents associated with wave progression, the maxima tend to be skewed, so that a wave orientation pattern forms an obtuse arrow pointing in the direction of wave progression. In low angle swash zones, orientations show the two maxima of wave 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 wave 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 wave 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 wave base, as parts of the Upper Fort Littleton again show wave orientations and obscure bedding, presumably because the basin was filled to above wave base.
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