Wave-generated structures in the Devonian lacustrine sediments of south-east Shetland and ancient wave conditions

Authors

  • PHILIP A. ALLEN

    1. Department of Geology, University of Cambridge, U.K.
    Search for more papers by this author
    • *

      Geologisches Institut, Universitat Bern, Sahlistrasse 6, CH-3012 Bern, Switzerland.


ABSTRACT

The most common wave-generated structures in the nearshore lacustrine sediments of the south-east Shetland basin are cosets of undulatory and unidirectional ripple cross-lamination. The undulatory lamination was produced at relatively high oscillatory flow strengths by accretion of rolling grain (post-vortex) ripples, and the unidirectional cross-sets were formed by the migration of vortex (orbital) ripples at lower strengths.

Unidirectional solitary lenses were generated under moderate but discontinuous wave activity on a partly sand-starved substrate. Some lenses were reworked during periods of more prolonged wave activity.

The Inman-Komar plot of near-bottom orbital diameter versus ripple spacing (λ= 0.80d0 for small d0, or λ= 0.65d0 as modified by Miller & Komar) may only be used in estimating ancient wave conditions for vortex ripples with low Vertical Form Indices and small wavelengths. This laboratory based relationship (minimum d0 conditions) is utilized in this study since wave periods in lakes are small.

The estimation of ancient wave conditions suggests that the ripples were produced in water depths of up to 10 m and in most cases less than 5 m. The formative waves possessed periods of up to 3.4 sec and suggest that the lake was relatively small, perhaps of the order of 20 km wide.

Ancillary