Provenance Modelling as a Technique for Analysing Source Terrane Evolution and Controls on Foreland Sedimentation

Tectonism and climate are widely viewed as principal controls on alluvial-fan sedimentation in foreland and other fault-bounded basins. For example, upward coarsening and thickening sedimentary sequences often are interpreted as responses to cratonward propagation of thrust-fault systems in foreland basins. Source-rock lithology is recognized as influencing gross fan morphology (e.g. sandy versus gravelly fans), but the impact of time-varying provenance has not been assessed. The Sphinx Conglomerate of south-western Montana clearly displays the controlling role of changing provenance in an eroding foreland thrust belt on sedimentary style in the adjacent foreland basin. The Maastrichtian Sphinx Conglomerate crops out atop the 3315 m high Sphinx Mountain in the Madison Range as an erosional remnant, over 1000 m thick, of the proximal realm of a Laramide foreland basin. Conglomerate clasts define an unroofing sequence: recognizable Cretaceous to Cambrian clasts appear progressively upward in the Sphinx. Conglomerate units increase upward in abundance and thickness, and the Sphinx Conglomerate is overridden by a foreland thrust fault, implying that conglomerate distribution is a progradational sedimentary response to faulting. However, the Mesozoic section exposed in the Madison Range is largely muddy and contains few units capable of producing conglomerate clasts. In contrast, the middle and lower Palaeozoic section consists largely of carbonates capable of generating great volumes of cobbles in a temperate climate. Thus, the inverted stratigraphy seen in clasts, apparent overall upward coarsening, and increasing abundance and thickness of conglomerate beds suggest that sedimentary style in the Sphinx is determined largely by the lithology of units exposed at specific times in the encroaching thrust plate. To test this hypothesis, we tabulated thicknesses of resistant lithologies capable of yielding gravel clasts for each Phanerozoic unit exposed in the Madison Range, as compared to total unit thickness. Conglomerate clast counts of the Sphinx Conglomerate permitted the identification of units eroding in the adjacent thrust plate at specific times. Comparison of the composition of hypothetical conglomerates eroded from particular suites of source units with the actual composition of Sphinx Conglomerate samples yielded striking results. Modelled compositions closely match actual compositions in several instances. These results suggest that changing provenance can produce bedding trends in foreland basins that are often otherwise attributed to tectonic or climatic controls. Provenance modelling can aid in evaluating these alternative interpretations.