TECTONIC AND ROCK-TYPE INFLUENCES ON TERRACES AND CHANNEL MORPHOLOGY: BIG SHEEP CREEK, BEAVERHEAD AND TENDOY MOUNTAINS, SOUTHWEST MONTANA

EDMAP supported mapping of the Dixon Mountain and Caboose Canyon 7.5 minute quadrangles, SW Montana reveal a suite of Pleistocene and Holocene terraces along Big Sheep Creek influenced by both rock type and active tectonics of the Red Rock fault. Terraces were surveyed, correlated, and dated based on their cobble petrography, elevation above local base level, downstream interfingering with alluvial fans, and upstream projection into heads of glacial outwash in the headwaters. Straths are less conspicuous in the upper basin where the terrace deposits are in excess of 5m thick; however, straths are well-preserved in the lower two-thirds of the basin, especially where the channel meanders across wide valley reaches underlain by softer shale, siltstone, or mudstone. Narrow gorges and lack of straths coincide with resistant rock types such as carbonates or quartzite. The Quadrant Sandstone, in particular, acts as a local base level, separating the Big Sheep long profile into two segments and buffering the upstream segment from the active Red Rock fault. Straths are steep, narrow, discontinuous, and converge upstream from the fault to the Quadrant SS. Upstream, straths are gentle, wide, continuous, and progressively diverge away from the modern channel profile. Correspondence of terrace age to glacial-interglacial cycles suggests that the strath profiles reflect local base level fall and incision of the Beaverhead Mountains by Big Sheep Creek. Since the middle Pleistocene, base level fall downstream of the Quadrant SS reach is decidedly episodic, occurs at a rate more rapid than terrace genesis by climatic factors, and directly determines the rate of incision for this part of the valley. In contrast, climate change is the dominant influence on valley morphology, terrace genesis, and incision upstream of the Quadrant SS reach. The disparity of incision rates downstream and upstream of the Quadrant reach speaks to the recent activity on the Red Rock fault.