File:Glacially-grooved basalt (Scales Creek Flow, Portage Lake Volcanic Series, upper Mesoproterozoic, ~1.095-1.096 Ga; Surveyor's Arch outcrop, Houghton, Upper Peninsula of Michigan, USA) (8280979561).jpg
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Precambrian basalt lava flow with Pleistocene glacial grooves in northern Michigan, USA. The Portage Lake Volcanic Series is an extremely thick, Precambrian-aged, flood basalt deposit that fills up an ancient continental rift valley. This rift valley, analogous to the present-day East African Rift Valley, extends from Kansas to Minnesota to the Lake Superior area to southern Michigan. Unlike many flood basalts (e.g., Deccan Traps, Siberian Traps, Columbia River), the Portage Lake only filled up the rift valley. The unit is exposed throughout Michigan's Keweenaw Peninsula, in the vicinity of the towns of Houghton & Hancock. The Portage Lake succession thickens northward through the Keweenaw, up to >5.5 km worth of section in places. The dominant rock type is basalt - vesicular basalts, for the most part. Most of the original vesicles (gas bubbles) have since been filled up with a wide variety of different minerals. A vesicular basalt that has had its vesicles filled up with minerals is called an amygdaloidal basalt. Keweenaw amygdaloidal basalts have long had significant economic importance because native copper (Cu) is one of the more common vesicle-filling and fracture-filling minerals. Keweenaw has (had) the highest concentration of native copper anywhere on Earth. Numerous Keweenaw-area copper mines have exploited these cupriferous amygdaloidal basalts. Almost all of the copper mines have since shut down. Basalt is the not the only lithology in the Portage Lake succession - coarse-grained siliciclastics (conglomerates, sandstones) were occasionally deposited atop the basalts between lava flow events. These beds are fairly similar to the coarse-grained siliciclastics in the overyling Copper Harbor Conglomerate. Shown above is a basalt lava flow topped by glacial grooves, carved during the Pleistocene Ice age. Stratigraphy: Scales Creek Flow, Portage Lake Volcanic Series, Bergland Group, middle Keweenawan Supergroup, upper Mesoproterozoic, ~1.095 to 1.096 Ga Locality: Surveyor's Arch outcrop, across the road from the Houghton Gremlins' water tower, southern side of city of Houghton, Keweenaw Peninsula, Upper Peninsula of Michigan, USA |
| Date | |
| Source | Glacially-grooved basalt (Scales Creek Flow, Portage Lake Volcanic Series, upper Mesoproterozoic, ~1.095-1.096 Ga; Surveyor's Arch outcrop, Houghton, Upper Peninsula of Michigan, USA) |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/8280979561 (archive). It was reviewed on 10 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 00:14, 10 October 2019 | | 960 × 614 (425 KB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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