Malcom--
I think you've answered your own question....as you say, the eroded surface (or peneplain) is an 'UNCONFORMITY'. More specifically, it sounds to be an angular unconformity. I suggest clearly describing the nature of the unconformity (e.g. planar vs. undulatory, when the unconformity formed, the rock units above and below the unconformity, the age of these units, the typical angular difference across the unconformity). Then, when you describe the 'DIP' of the 'UNCONFORMITY', your reader will know what you mean. This is a very common technique in tectono-stratigraphic studies. You can still also use this 'UNCONFORMITY' as a 'STRAIN MARKER' to help quantify fault offset or some other aspect about the observed deformation.
Good luck!
-Scott
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Scott Bennett
Research Geologist
U.S. Geological Survey
Seattle Field Office
Affiliate Assistant Professor
University of Washington
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www.scottekbennett.com
profile.usgs.gov/sekbennett
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