Reconciling the roles of tectonism and climate in Quaternary alluvial fan evolution

Abstract

Tectonism and climate are the primary variables considered in conceptual models of alluvial fan evolution and basin-fill architecture, yet their roles and relative importance are far from resolved. The Madison Range and Madison River valley, southwestern Montana, were affected during late Quaternary time by varying degrees of tectonic activity as well as by climate change through multiple glaciations; we are thus able to evaluate the impact of tectonism and climate on fan evolution. Two primary fan deposits were correlated along the range front. They were deposited as proglacial outwash fans during glacial periods of increased sediment and water discharge. During the present interglacial period, these deposits were entrenched and secondary fans formed on lower fan areas. Temporal and stratigraphic relations indicate that climate was largely responsible for driving both fan aggradation and entrenchment. In contrast, stratigraphic relations between fan deposits and surface faults do not support a direct relation, either spatial or temporal, between local faulting and fan deposition. Because tectonism produces and maintains the relief necessary for fans to form, its primary role is long term, controlling the duration over which fan deposition may occur along a mountain front. If Quaternary alluvial fan evolution is representative of ancient alluvial fan evolution, this study has important implications for interpreting primary controls on deposition of the ancient alluvial fan sequences.