Analysis of slump slip lines and deformation fabric in slumped Pleistocene lake beds

Slumped glacial delta sands and silts exhibit flexural slip folds and low-angle thrust faults where the beds remained coherent during slump deformation. Alternating cross-cutting relationships between f (sub l ) and f (sub r ) fold axial planes indicate that these fold groups are conjugate sets, related to the same slump movement. Analysis of slump fold axis orientations by the separation-angle method yields the fold slip plane and fold slip line. A deformation fabric reference diagram, deduced from fold and thrust fault data, shows a vertical deformation plane that is defined by the pole to the f (sub l ) and f (sub r ) fold slip planes, pole to rotated bedding, pole to thrust faults, and the fold slip lines. Central tendency estimators, Theta , Theta , and P ₅₀ , of distributions of fold azimuths show close agreement, as do dispersion indicators sigma , derived from L values, and (P ₈₄ -P ₁₆ )/2. Fold axis orientations in the slumped mass are normally distributed, but the perpendiculars to deduced slip lines have consistently higher azimuth values than other central tendency values. These results may indicate that the sampling plain biased the populations. In this case, the vector mean is not perpendicular to the deduced slip line. Silt intraclast breccias, containing slump overfolds, appear to have been deposited as noncoherent grain flows in the slump depression subsequent to slump deformation. The distribution of overfold axis orientations is not significantly different from random distributions. Separation-angle analysis of the same lower breccia data yields a valid slip line solution.