Andrew C. Phillips

Tidal drawdown: A mechanism for producing cyclic sediment laminations in glaciomarine deltas

A process for generating sediment laminations by tide-controlled storage and release of delta-plain sediment is described, using data from two glacier-fed deltas in Glacier Bay, Alaska. Intertidal mudflats and lower channel reaches store fluvial bed load and suspended load when the lower delta plain is flooded, which occurs periodically during spring intervals and continuously during neap intervals. Spring lower low tides expose the channel mouths at the delta lip and the entire delta plain, which causes abrupt transport pulses of stored sediment into the basin. Each pulse and ensuing flood tide forms a coarse and fine laminated couplet in the fjord-head basin. Neap-generated deposits are either structureless or faintly laminated.

Pitfalls in interpretation of shallow seismic data

High-resolution shallow seismic methods are the most widely used geophysical methods in near surface characterization. However, in many cases interpreting the seismic images can be misleading. In this article, we present three case studies where results from P-wave seismic reflection, SH-wave seismic reflection, and multi-channel analysis of surface wave (MASW) surveys were incorrectly interpreted because of inadequate constraints on either the surveyed sites surface or subsurface conditions. A P-wave reflection survey feature was first interpreted as a shallow fault zone but it was later determined to result from a high level of background noise as the acquisition passed through a road intersection. A SH-wave seismic reflection survey feature was interpreted to be a reverse dip-slip fault but targeted drilling showed it was deep local erosion into the bedrock surface. Finally, in an MASW survey, a steeply dipping feature was first interpreted as a bedrock valley. However, later exploratory drilling showed the feature to be a shallow layer of very soft lake sediment that severely damped most of the applied surface wave frequency band. Although initial interpretations were incorrect, they stimulated discussions among geophysicists and geologists and underscored the need for meaningful cooperation and discourse between the scientists before, during, and after geophysical data acquisition.