DETERMINING THE PROVENANCE OF CLASTS WITHIN CONGLOMERATES OF THE PAYNE CLIFFS FORMATION (EOCENE), JACKSON COUNTY, OREGON

The Payne Cliffs Formation (Eocene) consists of 170 m thick basal conglomerate overlain by 2,200 m of sandstone with subordinate amounts of conglomerate and mudrock. The Payne Cliffs Formation is exposed along the northeast flank of the Klamath Mountains striking approximately N 30^oW and dipping 10^o to 15^o to the northeast in southwestern Oregon. Previous workers have interpreted the provenance of the Payne Cliffs Formation as the Klamath Mountains based upon paleocurrent data collected from trough cross-stratified sandstones and clast compositions from conglomerates (McKnight, 1984). In contrast, studies of detrital muscovites from sandstones of the Payne Cliffs Formation suggest a more distal source, the Idaho Batholith (Heller et al. 1992). The purpose of this study is to better determine the provenance of the Payne Cliffs Formation by pebble counts and studying the diagnostic lithologic characteristics of clasts within conglomerates.

Pebble counts were conducted at three stratigraphic horizons in the basal conglomeratic unit of the Payne Cliffs Formation near Phoenix, Oregon. Twenty clasts that exhibited unique lithology and/or were not identifiable using field criteria from these conglomerates were thin-sectioned and identified using petrographic techniques. The clasts in the basal conglomerate are dominated by metasedimentary and metavolcanic materials with subordinate amounts of quartzite, chert, vein quartz, granodiorite, slate-phyllite, and recrystallized carbonate. The occurrence of granodiorite, slate-phyllite, and recrystallized carbonate clasts in these conglomerates suggest a proximal source due to their poor durability.

In summary, the Payne Cliffs conglomerates appear to be dominated by parent materials derived from the Klamath Mountains, while sandstones exhibit a mixed provenance that includes the distal Idaho Batholith and proximal Klamath Mountains. The segregation of these source materials by grain size may be the result of proximity to the source area, with proximal sources yielding conglomeratic intervals, while sandstones represent a mixture of distal and proximal sources. Collaborative work is underway to study the age distributions of detrital zircons from the Payne Cliffs Formation to better resolve the provenance of this unit.