LOOKING THROUGH LOESS: UNRAVELING THE VOLCANIC HISTORY AT THE TRANSITION TO THE SNAKE RIVER PLAIN NEAR REXBURG, IDAHO

The southern margin of the Eastern Snake River Plain near Rexburg, Idaho records two mappable periods of basaltic volcanism: a period between the eruptions of the Kilgore Tuff and Huckleberry Ridge Tuff, and a period following the eruption of the Huckleberry Ridge Tuff. The first period includes at least three eruptive events; the second at least seven events. Both periods contain eruptions along rifts that we interpret as extensions of the Grand Valley normal fault. The first detailed maps of the area were created in the 1970s (by Prostka H, Embree G, & Doherty D). On the elevated benches in this area loess deposits (1 to >10 m thick) cover more than 99 % of the bedrock. As a result, the extent of individual basalt units and their temporal relationships are difficult to determine by mapping alone. Our attempts to correlate the scattered outcrops using petrography, remanent magnetization, composition, and Ar/Ar ages were mostly inconclusive. Recently, we have successfully used vertical exaggeration and oriented hill shades of high-resolution digital elevation models to map the extent of individual flow fields, improve flow-vent correlations, and determine relative ages. The improved map relationships have illuminated the volcanic history of the area, including how basalt eruptions have altered drainage patterns.