Abstract

The upper Mesozoic sedimentary rocks of southwestern Oregon include turbidites, dark shales, and great thicknesses of resedimented conglomerates. At two localities, these lithologies are arranged in thinning- and fining-upward sequences that begin abruptly with graded conglomerates (individual beds as much as 50 m thick) and pass upward into massive sandstones, classical turbidites, and dark mudstones. Detailed study of the fabric of the conglomerates indicates paleoflow roughly toward the southeast; this direction is confirmed by sole marks on the associated turbidites. The highest angles of imbrication occur at the bases of beds; upward, the imbrication angle flattens, and the preferred alignment of clasts is better developed.

The angle of imbrication is not related to clast size (as expressed by D/10, the mean of the ten largest clasts), but D/10 is strongly related to conglomerate bed thickness, which indicates that the conglomerates (thickness, 1.85 to 50.0 m; mean, 17.5 m) are probably single flows.

Some of the thinner conglomerates belong to the graded-stratified model described by Walker. The thicker conglomerates are characterized only by normal grading (inverse grading is rare or poorly developed; no stratification), and a new model, the graded bed, is suggested as an intermediate form between the inverse-to-normally graded and graded-stratified models.

The thinning- and fining-upward sequences range from 3.40 to 85.0 m (mean, 31.5 m) and are tentatively interpreted to be the result of progressive channel abandonment on a submarine fan. In this interpretation, the conglomerates, although prominent in the section, are unusual events, transported by flows that were far too big for the scale of the channel. Thus, conglomerates plugged the channel bases, subsequently causing diversion of the normal flows into other channels.

First Page Preview

First page PDF preview
You do not currently have access to this article.