COPPER HARBOR
CONGLOMERATE
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.