[Next Section] [Previous Section] [Table of Contents]

Introduction

This study describes folds exposed in an outcrop of the late Paleocene to early Eocene Umpqua Group in the Umpqua River valley, northwest of Roseburg, southwestern Oregon (Figure 1). Within a sequence of continental slope, shelf, and deltaic deposits, the folds are part of a larger fold and thrust belt on the Klamath Mountain borderland stretching from the Oregon Cascade Range to the Pacific Ocean.

Approximately 360 meters long and normal to the strike of the folds, the outcrop consists of rhythmically bedded sandstone and shale layers up to 70 cm thick. Wavelengths are 9 to 28 m and amplitudes, 3 to 7 m. Plunge of the folds ranges from 2° to 34°, W and E, giving much of the outcrop close to a down plunge view, a relatively undistorted view of the folds. The folds are predominantly asymmetric chevron-like folds with a concentric-like fold and kink-like folds. The many faults and joints are evidence of a complex interplay of folding and fracturing, creating ramps, wedges, and shear zones within the sequence of fold forms.

The objective of this study is to examine and describe the exposed folds, mapping the fold forms and faults in general and detail of the best exposures. Multilayer fold theory is used to address two questions: a) why did folds form in these layers, and b) what conditions in the multilayer contributed to the present fold form. Any constraint on the conditions of folding will enhance the understanding of the formation of the outcrop and provide a glimpse into the Paleogene setting of the region.

Figure 1. Location of study area. The outcrop is along Garden Valley Road, 4.2 km SSE of Umpqua. Base map is from USGS Garden Valley 7.5' Quadrangle. The geology is from Wells (1998).