[Next Section] [Previous Section] [Table of Contents]
Located in a fold and thrust belt in southwestern Oregon, the folded early Eocene Umpqua Group turbidites of this study have a N-S exposure approximately 360 m long and 20 m high. Nearly a down plunge view, the exposure of short wave trains, 1 to 1½ wavelengths of 10 to 28 m, is dominated by asymmetric chevron-like folds, but includes a symmetric concentric-like fold and kink-like folds.
A folding mechanism consistent with the fold and fault patterns of the outcrop is layer-parallel shortening. The form of theoretical multilayers produced in layer-parallel shortening is related to the conditions of folding: contact strength of the layers, relative strength of the layers, and layer thickness. The folding units within the outcrop exhibit contrast in layer strength. The concentric-like fold is bounded above by a unit of thicker beds having relatively less shale, acting as a stiff medium in a confined multilayer since the unit thickness is greater than the wavelengths of the folds. The package of chevron-like folds does not appear to have a comparable theoretical multilayer in form. Within the kink-like fold, nonlinear slip at layer contacts, necessary in forming ideal monoclinal kink folds, may be evident in localized tan siltstone and associated calcareous nodules.
Asymmetry in the outcrop, short limbs of chevron-like folds facing south and kink-like fold facing north, is consistent with the regional setting in a NNW verging thrust sheet and top-to-the-south layer-parallel shear. Top-to-the-south bedding thrusts on the long, north-dipping limbs of the chevron-like folds, and duplex-like structures in some sandstone beds are evidence of the sense of shear.
Minimum shortening in the outcrop is 35 to 40 percent attributable to folding, but not including bed thickening in layer-parallel shortening and shortening in bed repetition by faulting.
The fold package appears to be in a relatively thin unit near the base of a thrust fault within a larger NNW verging thrust sheet. As a relatively thin unit bounded by muddier units above and below, there is sufficient contrast in strength over the thickness, enabling shorter wavelength folds to develop.