Keweenaw Michigan field trip

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VIRTUAL FIELD TRIP TO THE KEWEENAW PENINSULA, MICHIGAN - PRECAMBRIAN STRATIGRAPHY - II

The Glacial deposits of the Keweenaw include large sand deposits that were dumped in earlier topographic lows (between Hancock and McLain State Park).


Specimen of Jacobsville sandstone showing bleaching of some layers, Jacobsville, Michigan. Seaman museum specimen 12 cm. across.	Outcrop of Jacobsville sandstone at L'Anse. Stop I1


Outcrop of Jacobsville sandstone. Note the red claystone inclusions. These were probably ripped up from some preexisting sediments and deposited within the sandstones. Brunette Park (field of view 50 cm.).	Outcrop of Jacobsville sandstone at Brunette Park. Note crossbedding of sandstone ( field of view 1.5 m.).

Jacobsville sandstone. Note the claystone clasts that have been included in this specimen. Brunette Park.


Subcrop of Jacobsville showing alternating oxidized (red) and reduced layers (white). NE of Brunette Park.	Subcrop of Jacobsville formation at Brunette park.


Specimen of Freda sandstone showing bleached spots. McLain State Park.	Piece of Freda sandstone showing crossbedding and mottling. McLain State Park. Stop H6 - on beach near entrance to park.


Specimen of Freda sandstone showing crossbedding and claystone clast. McLain State Park.	Specimen of Freda sandstone showing crossbedding. McLain State Park.


Nonesuch shale, White Pine, Ontonagon county. Seaman museum specimen 12 cm. across. At this locality, the Nonesuch is a black shale which was mined for it's chalcocite and native copper contents.	Nonesuch shale Stop H8 Hancock campground. These rocks are more siltly than the typical Nonesuch shale (the shale tends to weather more rapidly than the coarser grained material). Also note that this material is more oxidized than the shale found further to the southwest.


Specimen of Nonesuch shale from Hancock campground. Specimen is 10 cm. across.	Nonesuch shale Stop H8 Hancock campground.

The whitish outcrop in the central portion of the photo is the Bare Hill rhyolite. Although rhyolites form only a small portion of the igneous rocks (less than 1% of the igneous rocks) in the Keweenaw peninsula, they can be found throughout the area in small bodies.


Mount Bohemia diorite Seaman museum specimen 12 cm. across. Although basalts are the primary rock type in the Portage Lake volcanics, there are some more granitic intrusive rocks found in the area. The diorite has been extensively altered, and there are some copper sulfide minerals deposited in this rock.	Mount Houghton. One of the areas of rhyolitic intrusive rocks on the eastern end of the peninsula.

Specimen of pahoehoe lava from the 28th level of the Isle Royale mine. The ropey surface is result of cooling of the top of a very fluid basalt magma. The direction of magma movement can be determined from the structure of the ropey surface. Seaman museum specimen is 35 cm. across.

Allouez conglomerate, Seaman museum specimen 30 cm. across. Although conglomerates within the Portage Lake volcanics were of relatively minor volume, they have proven important in copper production and correlation of beds throughout the district. The extent of the volcanic beds could vary widely depending upon how close one was to the vents feeding the magma, while the sedimentary conglomerates developed over most of the area during times of volcanic quiescence. The volcanic rocks do not possess any distinguishing characteristics from preceding eruptions.


Although the majority of sedimentary rocks included within the Portage Lake volcanics were conglomerates, some finer grained material such as this sandstone were also deposited. This specimen from the National mine is 3 cm. high.	Boulder containing pebble sized clasts (boulder is 60cm across). This specimen is on the dump of the Minesota mine. Although the primary production of this mine was from fissures, some ore was mined from conglomerate beds.


Close-up of a columnar basalt. The development of columnar basalts is usually best in relatively thick flows. Stop F3 north of Ahmeek.	Columnar basalt developed in thin flow just above the Greenstone flow. The best examples (with columns 2 meters across) are found on Isle Royale. Stop F3.


Specimen of Michigamme formation. Some very minor mica is being developed in this slate. From beach at L'Anse stop I1, specimen is 15 cm. across.	Beach at L'Anse stop I1, showing pebbles of the early Proterozoic Michigamme Formation (cleaved slate).