PRESQUE ISLE RIVER
The Porcupine Mountains in
the northwestern UP have fantastic scenery and interesting geology. The
lower stretches of the Presque Isle River have riverbank exposures of upper
Mesoproterozoic Oronto Group sedimentary rocks that include the Freda Sandstone
and the Nonesuch Shale. The photo below shows some great pothole
development in the Nonesuch Shale (~1.07-1.08 billion years) along the
Presque Isle River, near the southern shore of Lake Superior.
Nonesuch Shale and huge potholes - lower
stretches of the Presque Isle River, Porcupine Mountains, UP of Michigan, USA.
The Nonesuch Shale is famous
for being a source of rare Precambrian oil. It also hosts economic
concentrations of disseminated copper sulfides and native copper in
nearby areas (see pic below).
Native copper (Cu) sheet that occupied a
vein or bedding plane in the Nonesuch Shale at White Pine Mine, east of the
Porcupine Mountains, UP of Michigan, USA. Copper mineralization occurred
at about 1.05-1.06 billion years. Seaman Mineral Museum public display (Michigan
Technological University, Houghton, Michigan, USA).
Pothole in the upper Nonesuch Shale
at Manido Falls, a little upstream from the first photo above. Potholes
enlarge by abrasive grinding action as debris gets swirled around by rushing
river water.
Pothole in the upper Nonesuch Shale
at Manido Falls, Presque Isle River, Porcupine Mountains, UP of Michigan, USA.
Manido Falls (Presque Isle River).
This waterfalls occurs at the contact between the Nonesuch Shale (below)
and the Freda Sandstone (above). The Nonesuch here is mudshale and
siltstone, while the Freda here is only slightly coarser-grained.
Stratigraphically upward, the Freda is a nice sandstone.