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An unconformity may be a very complicated surface. A few strike and dip measurements are not adequate to define such a surface, and simple solution of the three-point problem commonly does not improve the definition. However, the use of trend surface analysis permits (a) determination of the "best fit" plane which represents the strike and dip for the region studied; and (b) evaluation of local departures from that plane, in terms of paleotopography.
This procedure has been demonstrated in a study of the basal contact of the Tuscaloosa Group with an underlying variably weathered "gneiss" complex, which was sampled at 17 localities in Macon and Tallapoosa Counties (Notasulga Quadrangle), Alabama. At each locality the elevation and nature of the basal Tuscaloosa contact was recorded along with lithologic observations. A trend surface analysis was used to generate a first order regression plane. This plane represents mean strike and dip of the unconformity; it strikes N81°W and dips 0.61°SW. This result is highly significant F2,14 = 190.5; N = 17) and accounts for 96% of the variance in elevation (R2 = 0.965).
First order residuals plotted against map location reveal little regional trend. However, mapping of basal Tuscaloosa grain-size data onto the residuals reveals a preponderance of coarse material at localities with negative residuals. This indicates the expected preferences for coarse (gravel) deposition in paleotopographic "lows." Mapping the weathered state of the underlying gneiss onto the residuals reveals no pattern, suggesting that much of the alteration of this unit postdated the unconformity. These results, although on a simple data set, illustrate the utility of the technique to stratigraphic problems involving mean strike and dip.
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