Influence of Provenance on Detrital and Diagenetic Mineralogy of Small Tertiary Fans in Southwestern Montana: ABSTRACT

Abstract

In the North Boulder River basin in southwestern Montana, alluvial fans of the Renova (Oligocene-Miocene) and the Sixmile Creek (Miocene) Formations were deposited on the flanks of north-south-trending uplifts that also supplied the detritus. The Elkhorn Mountain volcanics (78 m.y.) overlying the Boulder batholith make up the western highlands, a small patch of Precambrian Belt Group rocks occur in the southwest and Paleozoic siliciclastic and carbonate rocks forming the eastern margin. The fan sediments thus allow adequate control for studying the influence of source rocks on detrital and diagenetic mineralogy. Modal analysis of 6228 grains in 31 thin sections shows a decrease of VRF away from the igneous sources (37% to 1% in a north-south transect; 37% to 7% in a west-east transect) along with an increase in quartz (8% to 24% and 3% to 13%) and plagioclase (2% to 16% and 2% to 10%); orthoclase abundance is low except in the southwest. Volcanic ash and glass shards are found in the younger sediments in the northern part of the basin. Their data show a positive correlation between the abundance of orthoclase and kaolinite (north-south transect); between SRF and carbonate cement (west-east transect) and between glass shards and smectite (bothmore » north-south and west-east transects). They infer that the diagenetic mineralogy of these sands was controlled essentially by the detrital particles, which were strongly controlled by source rocks in this area.« less