Abstract

The Keweenawan Copper Harbor Conglomerate, exposed in northern Michigan, is part of a thick succession of volcanics and terrigenous clastics deposited in the Keweenawan Trough, a failed intracontinental rift. It is a fining-upward wedge of volcanogenic conglomerate and sandstone deposited as proximal to distal braided-stream and sheet-flood facies on coalesced alluvial fans and sand flats. This unit, along with the lower portion of the overlying Nonesuch Shale, consists of an alluvial fan–shallow lake sequence that filled the rift during and following volcanic activity. The fining-upward nature of the sequence reflects a waning sediment supply probably due to lowering of the source area following faulting.

The Copper Harbor Conglomerate exhibits attributes of both arid-fan and humid-fan alluvial models. Coarse-grained facies are lithologically similar to alluvial-fan units deposited in modern humid regions. Conversely, finer distal Copper Harbor facies exhibit numerous features indicating flashy discharges and desiccation, processes typical of deposition in modern arid regions. Ambiguity of sedimentary features useful for climatic inference probably reflects the limited applicability of Holocene models to more ancient alluvial sequences, because Precambrian hydraulic regimes developed under various climatic settings may have differed from their Holocene counterparts.

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