ABSTRACT

This is a study to assess the paleogeographic significance of lateral variations in the turbidite sandstones of the middle Eocene Tyee Formation of the Oregon Coast Range. To establish the existence of such lateral variations away from the place of origin of turbidity currents one must first define a paleogeography of the area of deposition. This has been done in an earlier paper. (Lovell, 1969).

The clearest trends in the turbidites are a distal (northward) decrease in the ratio of sandstone to shale, and a distal increase in the proportions of sharp sandstone bases and relatively light, non-spherical heavy minerals such as biotite. Other characteristics discussed which show less clear lateral variations include regional and local thickness of beds, sole marks, internal organization of individual beds (Bouma divisions), mudflake conglomerates, slumped beds, carbonate concretions, trace fossils, and clay mineral type. None of these characteristics was used to establish the paleogeography; hence there is no circular reasoning involved in the conclusions concerning their value as paleogeographic indicators.

Some of these indicators may perhaps be used in turbidites in deformed and metamorphosed areas in which other evidence bearing on paleogeography is scarce.

Use of the specific values of the results from the Tyee Formation in turbidite sequences in general would be premature; one cannot yet say whether or not the indicators of "proximality" have an absolute value. It is suggested that a comparison of rates of change of the indicators away from the place of origin of turbidity currents in various turbidite sequences might yield significant results.