File:Allouez Conglomerate-flood basalt contact (Portage Lake Volcanic Series, upper Mesoproterozoic, 1.094 Ga; Delaware Copper Mine, Keweenaw Peninsula, Upper Peninsula of Michigan, USA) 2.jpg
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editDescriptionAllouez Conglomerate-flood basalt contact (Portage Lake Volcanic Series, upper Mesoproterozoic, 1.094 Ga; Delaware Copper Mine, Keweenaw Peninsula, Upper Peninsula of Michigan, USA) 2.jpg |
English: Conglomerate-basalt contact sample from the Precambrian of 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, reaching over 5.5 kilometers 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. Some of the coarse-grained siliciclastic interbeds of the Portage Lake Volcanic Series have also been impregnated with native copper. An example of this is the Allouez Conglomerate - it is sufficiently cupriferous that some copper mines were located on its outcrop belt. The Delaware Copper Mine, although long inactive, is still accessible underground as a tourist site. The mine targeted copper that occurs in the conglomerate as intergranular masses and as clast coatings ("skull copper"). The rock seen here is from the Delaware Copper Mine waste rock pile. The reddish-brown material at top is fluvial, lava-pebble conglomerate. The brownish material at bottom is amygdaloidal basalt. Stratigraphy: Allouez Conglomerate (formerly the No. 15 Conglomerate), just below the Greenstone Flow, upper Portage Lake Volcanic Series, Bergland Group, upper Mesoproterozoic, 1.094 Ga Locality: Delaware Copper Mine, just north of Rt. 41, near the town of Delaware, northern Keweenaw Peninsula, Upper Peninsula of Michigan, USA |
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Source | https://www.flickr.com/photos/47445767@N05/50725474686/ |
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@N05/50725474686. It was reviewed on 16 December 2020 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
16 December 2020
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current | 17:13, 16 December 2020 | ![]() | 3,862 × 2,560 (7.28 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/50725474686/ with UploadWizard |
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Camera manufacturer | Canon |
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Camera model | Canon PowerShot D10 |
Exposure time | 1/60 sec (0.016666666666667) |
F-number | f/11 |
ISO speed rating | 80 |
Date and time of data generation | 18:28, 11 December 2020 |
Lens focal length | 11.614 mm |
Width | 4,000 px |
Height | 3,000 px |
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Pixel composition | RGB |
Orientation | Normal |
Number of components | 3 |
Horizontal resolution | 180 dpi |
Vertical resolution | 180 dpi |
Software used | Adobe Photoshop Elements 18.0 (Macintosh) |
File change date and time | 01:53, 16 December 2020 |
Y and C positioning | Co-sited |
Exif version | 2.21 |
Date and time of digitizing | 18:28, 11 December 2020 |
Meaning of each component |
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Image compression mode | 3 |
APEX shutter speed | 5.90625 |
APEX aperture | 6.90625 |
APEX exposure bias | −0.66666666666667 |
Maximum land aperture | 4 APEX (f/4) |
Metering mode | Pattern |
Flash | Flash fired, compulsory flash firing, red-eye reduction mode |
Supported Flashpix version | 1 |
Color space | sRGB |
Focal plane X resolution | 16,460.905349794 |
Focal plane Y resolution | 16,483.516483516 |
Focal plane resolution unit | inches |
Sensing method | One-chip color area sensor |
File source | Digital still camera |
Custom image processing | Normal process |
Exposure mode | Manual exposure |
White balance | Auto white balance |
Digital zoom ratio | 1 |
Scene capture type | Standard |
Lens used | 6.2-18.6 mm |
Date metadata was last modified | 20:53, 15 December 2020 |
Unique ID of original document | DF25CED772B933E2303CC9EEC619940E |