Abstract

Parting lineation is rare to common in medium to coarse siltstone at various localities of the Red Peak Formation (Early? Triassic) in Wyoming. Field studies indicate that the general orientation of paleocurrents can be determined reliably in siltstone beds through use of this structure. Both parting lineation and asymmetric ripple marks reflect the same current orientation (or direction) where the two structures occur in beds a few inches apart. The fabric of grain long axes in siltstone is anisotropic; the maximum preferred orientation is parallel with the surface expression of parting lineation in the same rocks. The fabrics of opaque grains (hematite, ilmenite) are essentially the same as the fabrics of non-opaque grains (quartz, feldspar) in the parting lineation and both are current controlled. We suggest that the parting lineation formed in the upper-flow regime, and, because of this, the magnetic field of the Early(?) Triassic did not affect small, unaltered, opaque grains. Rather, the fabrics of both the non-opaque and opaque grains are the reflection of the hydrodynamic conditions present during deposition.

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