The deep red colors of the Hakatai Shale Moving up the Grand Canyon, on top of the Bass Formation limestone we find this unit, the Hakatai Shale, named for a type location in Hakatai Canyon. The Hakatai shale is up to about 200 meters thick at its thickest location and consists of alternating layers of sandy mudstone, sandstones, and true mudstones. The contact between the Hakatai shale and the Bass Limestone is the first conformable contact we’ve found while moving up the Grand Canyon; the units grade into each other over a period of several meters in some locations. The bright red colors of this unit are a feature we will find in several Grand Canyon units, but nowhere are red colors are more well expressed than here. This unit is a shale, consisting of fine-grained sediments collected in quiet waters protected from the actions of waves. Those sediments contained enough iron that when this rock is exposed to water and air it turns a deep red color and can even paint the limestone below it a reddish color when there is runoff. Shale layers tend to not be very strong, so this unit doesn’t outcrop very well. Some of the more sand-rich layers do stand out but generally this unit appears as eroded slopes in-between the more resistant layers above and below it. The Hakatai is divided into 3 sub-members; the lowest member consists of fine-grained, intermixed sandstones and mudstones, the middle member is the most pure shale of the sequence, and the upper layer grades into more quartz and feldspar rich sandstones. Features like the mudcracks shown in the 2nd image suggest that although there was little wave action, these rocks were deposited in fairly shallow water, as found in tidal flats or estuaries today. Ages have been measured for layers of ash in the Bass formation below this unit and in the intrusive igneous rocks that occasionally cross-cut it, as seen here, bracketing the deposition of this unit between 1250 and 1140 million years ago. Keep in mind though – that’s over 100 million years of time. We know it falls somewhere in that range, but 100 million years is a very long time. Particularly with sedimentary rocks that don’t grow their own grains, sometimes that’s the best we can do. -JBB Image credits: commons.wikimedia.org/wiki/File:Grand_Canyon_Supergroup_Hakatai_Shale_with_Basalt_Dike.jpg commons.wikimedia.org/wiki/File:Grand_Canyon_Supergroup_Hakatai_Shale_0002_-_Flickr_-_Grand_Canyon_NPS.jpg Read more: 3dparks.wr.usgs.gov/coloradoplateau/lexicon/hakatai.htm bobspixels/kaibab.org/geology/gc_layer.htm bulletin.geoscienceworld.org/content/117/11-12/1573.full Previous articles: https://facebook/photo.php?fbid=71718732167564 https://facebook/photo.php?fbid=717596974968016 https://facebook/photo.php?fbid=718487278212319 https://facebook/TheEarthStory/posts/718917208169326 https://facebook/TheEarthStory/posts/719035941490786 https://facebook/TheEarthStory/posts/719534524774261 https://facebook/photo.php?fbid=720485404679173 https://facebook/photo.php?fbid=720916891302691 Dear Readers, Most of our posts are not reaching you in your news feed due to fbs filtering system. If you wish to enjoy our posts more often, use the following for information on how to go about it: tinyurl/qgwac8k.
Posted on: Sat, 19 Jul 2014 21:25:01 +0000
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