DISTINGUISHING ALLUVIAL FAN AND FLUVIAL SYSTEM INTERACTIONS AT FORELAND BASIN MARGINS: CRITERIA AND SIGNIFICANCE

Distinguishing locally derived alluvial fan and distally sourced fluvial deposits is critical for establishing the kinematic evolution of foreland-propagating orogenic wedges and their related foreland basin systems. Interaction of fan and fluvial systems provides evidence of spatial and temporal proximity to surface uplift. Debris flow and sheetflood alluvial fan deposits are most commonly preserved along thrust-cored anticlines at the leading edge of the propagating orogenic wedge. Drainage basins in the anticlinal ridges are relatively small, poorly integrated, and source synthetic and antithetic debris flow fans; less commonly, erosion of source rocks producing large volumes of sand/fine gravel sized material leads to sheetflood fan development. Differences in debris-flow and sheetflood fan radii provide estimates of distance from the anticlinal ridge sediment source areas and bespeak the importance of distinguishing fan types. In both cases, fold-derived fan gravels possess labile-rich petrofacies and well-developed unroofing sequences, and are texturally immature.

Anticlinal ridges most commonly nucleate in foredeep settings characterized by high-energy, axial to transverse gravelly braided fluvial systems tapping well-intergrated drainage basins in the orogen interior. Braided fluvial gravels are both compositionally and texturally more mature; clast unroofing sequences are absent. Anticline uplift diverts the braided channels through surface uplift and alluvial fan growth leading to interaction of distinct coarse-grained depositional systems. Deformation commonly incorporates older braidplain deposits in the growing anticline, resulting in recycling of fluvial gravels in fan deposits. Fluvial erosion of fan toes conversely incorporates angular locally derived fan detritus into the braided channel system. Both processes lead to textural inversions and petrofacies mixing that serve as important tools for establishing interaction of coeval alluvial fan and fluvial deposystems and hence, proximity to loci of surface deformation due to folding.

Examples of alluvial fan/fluvial system interaction resulting from frontal fold growth are present in Upper Cretaceous strata of the Beaverhead Group of southwest Montana and Baseline Formation of southern Nevada.