Keweenaw Michigan field trip

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VIRTUALFIELD TRIP TO THE KEWEENAW PENINSULA, MICHIGAN - PRECAMBRIAN GEOLOGIC HISTORY

The mid-continent rift formed about 1100 Ma (millionyears ago) in the Precambrian. It extends from northern Kansas to Lake Superior,where it sharply turns south to the lower peninsula of Michigan. Toward the center ofthe rift, the thickness of the volcanic rocks reaches 25 kilometers. The following are the rock types from the oldest to youngest in age.

The Portage Lake Volcanics consistof 10 km of these sub aerially

Geologic section in the Keweenaw peninsula. All rocks are of Keweenawan age (Pre Cambrian)

erupted rocks, but only about 5 km is exposed on the Keweenawpeninsula. The period of rift volcanism extended from 1109 to 1089 Ma, with thePortage Lake Volcanics erupting over a two or three million year span around1095 Ma. There are more than 200 individual flows thatcomprise the Portage Lake Volcanics. The basalts vary from a magnesium -aluminum rich olivine thoelite with minor amounts of basaltic andesite,andesite, and dacite. These rocks were derived from magma generated in the upper mantle and becameincreasingly more primitive (chemical composition like the mantle) as the rocks evolved. There are minor amounts ofconglomerate in the section ranging in thickness from a few cm to 40 m thick,with increasing amounts as one goes higher in the section. These sedimentaryrocks comprise less than 5 % of the total volume, but contained about 40 % of thecopper production from the district and act as important marker beds in theotherwise similar appearing volcanic rocks.

The Porcupine volcanics are similar to the PortageLake volcanics, but have been erupted in the southwest portion of the map in thePorcupine Mountains.

The Copper Harbor Conglomerate varies between 100and 1800 meters thick. It overlays and interfingers with the top of the PortageLake Volcanics. There are up to 31 lava flows in the Lake Shore Traps, which arenear the top of the Copper Harbor Conglomerate. The Lake Shore traps have an ageof about 1087 ± 1.6 Ma. They exhibit more variability in composition and rangefrom Fe-rich olivine thoelites to thoelitic andesites at the top of the section.These sediments were deposited in an arid climate with seasonally high rainfalls.

The Nonesuch Shale is a mixture of siltstones,shales, carbonates, and minor sandstone. These were normally oxidized, but nearthe White Pine Mine south of the district, the shales becomeblack and reducing. This area has produced about equal amounts of copper sulfide and native copper ores.

The Freda Sandstone is a red/brown ferruginoussandstone, siltstone, and mudstone. This formation is at least 3700m thick (thetop of the section is not exposed).

The Jacobsville Sandstone (over 3000m thick) is ared to white coarse to fine-grained feldspathic quartzose sandstone deposited ina rift flanking basin to the east of the Keweenaw Fault. The Keweenaw faultformsthe contact between the Jacobsville and the Portage Lake Volcanics. TheJacobsville is unconformably in contact with early Proterozoicand Archean rocks further to the east. The Keweenaw fault was originally a normal faultbordering the eastern edge of the rift. Later compression of the rift, changed thethrow of the fault (fault movement) and resulted in a high angle reverse fault.

Photographs of rocks andoutcrops of rocks from the Keweenaw peninsula.

Geology east of 89º from U.S. Geological Survey Open-File Report 99-149By William F. Cannon, Michele E. McRae, and Suzanne W. Nicholson 1999. Geologywest of 89º after Bornhorst (1994).

View from west end of Mount Brockway toward Lake Bailey (center, in thedistance) and LakeSuperior (right) - Stop 25. The ridges are made of the more erosion resistantconglomerates while the valleys consist of more easily eroded sandstones andshales. To the south (left) of Lake Bailey is Lookout Ridge, which marks thecontact between the Copper Harbor Conglomerate and the Portage Lake Volcanics.Lake Bailey is on the same stratigraphic horizon as Lake FannyHooe.

Eagle River Falls (Stop 21 - N47° 24.07' W88° 17.67') Contact between top of Portage Lake Volcanics andbase of Copper Harbor conglomerates. Several potholes have developed to theleft. Ropy surface on the volcanics are pahoehoe type of lava. A dam ( on theleft) wasconstructed to provide water power for a manufacturing plant.

Horseshoe Harbor Stop C1. Conglomerate clasts consist of rocks such as thishighly vesicular basalt (5cm long). Minor crystals in the vesicles are of calcite orzeolites.

Copper Harbor conglomerate at Horseshoe Harbor. Most of the beach is composedof cobbles and pebbles of the Portage Lake Volcanics. If one walks theshoreline, you can see the size differentiation caused by the wave energy ofLake Superior. There are large pebbles just below the waterline, while smallerparticles are found at the beach water interface. Slightly further on shore, onecan find sand sized particles. The severe storms on Lake Superior cause therounding of the rocks and often bring up newly eroded rocks from the lake bed.This constant replenishment can result in good hunting for agates and otheramygdaloidal minerals found in the volcanic rocks. Some collecting has takenplace in the waters surrounding the Keweenaw peninsula. Datolite nodules have beenfound in offshore rocks and a large piece of copper has been recovered by scubadiving.

Central portion of a volcanic flow from the Lake Shore Traps within the CopperHarbor Conglomerate. These rocks are a fine grained and massive (without anymineralization) iron rich olivine thoelite basalt. The rocks parallel theshoreline and dip about 20° toward the lake. These rocks were not as severelymetamorphosed (only zeolite grade as compared to the prehnite/pumpellyite gradeinthe Portage Lake volcanics). Esrey Park Stop 24.

This is the Copper Harbor Conglomerate. It consists of clasts of the PortageLake Volcanics. Largest clast is 6cm in size. Horseshoe Harbor ( MichiganChapter - Nature Conservancy) Stop C1( N47° 28.37' W87° 48.03'). The white speckling in some of theclasts are amygdaloidal fillings of calcite or zeolites.

Stomatolites (white) in clastic rocks - either arkosic sandstones gradinginto fine grained conglomerates. Stromatolites (Collenia undosa) arecalcareous growths of algae and bacteria. These were deposited in stream channelswithin the Copper Harbor Conglomerate. The contortion may be due to softsediment deformation at the time of deposition or early compaction.Stromatolites are 3cm in the vertical direction. HorseshoeHarbor ( Michigan Chapter - Nature Conservancy) Stop C1.

AdditionalPhotographs of outcrops and rocks

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