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Heterolithic Macroforms in Amalgamated Fluvial Channel Belts: What Are They and How do We Predict Them?

Abstract

Outcrop-based models of amalgamated channel belts preserved in the Upper Triassic Chinle Formation in the vicinity of Moab, Utah comprise two primary architectural elements: an upper 4-14 m thick, trough cross-stratified, coarse- to fine-grained sand arranged into accretionary barforms, and lower 5-20 m tall, heterolithic macroforms composed of alternating sets of slumped, pedogenically modified muddy silt and massive to graded, conglomeratic, poorly sorted sandstone. Heterolithic macroforms generally underlie the clean, trough cross-stratified facies indicating that they are an early phase of channel belt development, followed by establishment of more stabilized flow regimes. Similar, but smaller heterolithic macroforms in Cretaceous strata (Wahweap, Masuk, and Farrer Fms.) have been interpreted as representing climatic forcing and resulting hydrologic modification of fluvial systems. Monsoonal climate during the Triassic is compatible with this hypothesis, but other causal mechanisms can be explored. Presence of salt diapirs in proximity to the Chinle Fm. channel belts allows for the possibility that episodic damming and breaching events played a role in modulating the fluvial system and controlling the type of macroform that was constructed. Specifically, emplacement of a topographic barrier to the fluvial fairway would result in diversion or damming of the flow. Seasonal increases in discharge would allow the rivers to overflow the barriers and gouge out new channels, leaving behind the chaotic, heterolithic accretionary packages. When the salt diapir was finally either breached or overridden, more typical fluvial deposition would occur. Another possibility is that they represent counter point bars in a sinuous fluvial channel, with alternating episodes of fine-grained sediment accumulation and over-the-point bar avalanching of coarse grained sedimentation. Determining which, or which combination of these mechanisms is responsible for the two part deposition of river deposits in these Triassic and Cretaceous deposits will help predict the occurrence of similar heterogeneity in analogous fluvial reservoirs.