Sedimentary geology of the zeolitic volcanic lacustrine Pliocene Rome Beds, Oregon, 1

Abstract

Stratigraphic and sedimentological information is given concerning the Pliocene Rome Beds of eastern Oregon which are composed of tuffaceous mudstones, and volcanic and lithic sandstones and conglomerates formed under a mid-latitude, high-plateau, arid climate. Diagenesis produced various types of zeolites, phyllosilicates, carbonates, iron oxides, gypsum, and other authigenic minerals. Four sedimentary facies have been recognized: ( 1) beach and associated shoreline; ( 2) lacustrine with near-shore (e.g., beach) and local fluvial plus flood-plain; ( 3) dominantly lacustrine and fluvial plus flood-plain; and ( 4) dominantly lacustrine with rare fluvial deposits. These environmental facies have been distinguished by differences in regional and local stratigraphic relationships, sedimentary structures, fabrics and textures. Sedimentary differentiation, being a function of wave and current energy applied over a sufficiently long time interval, was incomplete so that the four facies are gradational. Consequently, some features observed in the Rome lacustrine complex differ from other terrestrial and marine sediments described in the literature. A number of sources of the sedimentary particles have been recognized: much older sedimentary, metamorphic, plutonic and volcanic terrains, and approximately contemporaneous volcanic provenances. The directions of supply and/or distribution of the clastics have been inferred by measurements of cross-bedding, channel directions, and elongation of conglomerate lenses to be north and northwest in the northwestern and west and southwest in the southwestern localities.