The Copper Harbor Conglomerate is a thick unit consisting of coarse-grained siliciclastics and some basalt flood lavas.  The underlying Portage Lake Volcanic Series is an extremely thick, 1.093-1.097 billion year old flood basalt deposit that fills up an ancient continental rift valley.  Once flood basalt volcanism had largely ceased, weathering & erosion of rocks flanking the rift valley produced large volumes of coarse-grained sediments.  These gravels and sands filled up what was left of the rift valley.  Occasional episodes of flood basalt lava eruptions interrupted the sedimentation.


So, the Copper Harbor Conglomerate ended having a mixture of conglomerates, sandstones, and basalt lava flows.  The basalt lava intervals are referred to as the Lake Shore Traps Member.  The pebbles in the conglomeratic intervals are mostly rhyolite/felsite clasts.


Stratigraphy & Age: Copper Harbor Conglomerate, lower Oronto Group, upper Mesoproterozoic, 1.085-1.093 billion years.


Copper Harbor Conglomerate - roadcut along Old Mill Hill Road, just S of Canal Road, west of the town of Houghton, UP of Michigan, USA.


Copper Harbor Conglomerate - reddish, fluvial arkosic sandstone (13.0 cm across) from the Old Mill Hill roadcut shown above.



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