Thursday, July 24, 2014

The Great Unconformity is greater here.

The juniper-covered hillside above doesn’t look terribly spectacular, but don't be fooled!  A really long and puzzling story is told by these rocks -- the ones that are present, and especially the ones that aren’t.
Portion of stratigraphic column from Knittel et al. (2004), modified.
This diagram is a stratigraphic column corresponding to the hillside -- a schematic representation of the rock layers with the oldest at the base.  Note the box with nothing but question marks.  This is an unconformity, a hiatus, a missing interval, a gap in the rock record.  And it's huge.

The rock record is often referred to as a book, because rocks tell the story of the Earth if we know how to read them.  Following this analogy, an unconformity means pages are missing ... or in this case, whole chapters.  But were the pages really removed?  Perhaps they were never written.
Unconformity (Geol.): a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous.
Unconformities aren’t rare.  For example there are 14 major unconformities in the Grand Canyon of the Colorado River (USA; source).  One of these gets almost all the attention -- the Great Unconformity, where the Cambrian Tapeats sandstone overlies the Precambrian Vishnu schist, a gap in the rock record spanning more than a billion years.
View into the Grand Canyon from the South Rim.  From the Colorado River (below left of center), the gray Vishnu schist rises steeply to cliffs of brown Tapeats sandstone.  The contact between the two is the Great Unconformity.  Photo courtesy Jack Share of Written in Stone.
The Great Unconformity isn’t unique to the Grand Canyon.  It’s exposed at scattered locations across North America and around the world.  Why is so much of the rock record missing over such a large area?  What does this say about that time on Earth?  This is where the book analogy breaks down.  While there is a missing part to the story -- a really long one, maybe equivalent to several chapters -- the gap isn't silent.  Its existence tells something of what was going on at that time.

The Great Unconformity began back when there was probably just one continent, Rodinia. [If you’re unfamiliar with how continents shift, grow, join, split and jostle each other, see this introduction to plate tectonics by the US Geological Survey.]  Supercontinent Rodinia stood above sea level for a long time, perhaps 350 million years.  Not being underwater, deposition was minimal.  Instead, erosion appears to have reduced Rodinia to a low relatively-flat surface of igneous and metamorphic rocks (not much protective vegetation in those days).
These paleo-reconstructions of Rodinia are teaching slides from
Then Rodinia began to come apart, as supercontinents do.  Continent-sized and smaller pieces spread far and wide.  One of the biggest ones was Laurentia, or ancestral North America.  As Rodinia broke up, much of Laurentia/North America was covered by shallow advancing seas, and sandy sediments were laid over the ancient rocks, now on the seafloor.  With deposition underway once again, the gap in the rock record was brought to a close and the Great Unconformity ended.

The Great Unconformity in the Inner Gorge of the Grand Canyon (USA) is probably the best known and most visited ...
Geologist Wayne Ranney embraces more than one billion years of Earth history in the Grand Canyon.  Ages are approximate.  Photo courtesy Jack Share of Written in Stone (labels added).
... but the Great Unconformity in south central Wyoming is greater!  That’s because the underlying older rocks are older, and the overlying younger rocks are younger.
The Greater Great Unconformity near Fremont Canyon, Wyoming.  Ages are approximate.
Why are the older rocks older?  Because Laurentia/North America was not uniform in age -- it was an assemblage of older continents and other crustal pieces.  The Wyoming Craton was one of the oldest, with rock ages ranging from 1.7 to 3.6 billion years.  The granite at Fremont Canyon is roughly 2.4 billion years old.  In contrast, the Vishnu schist in the Grand Canyon is “only” about 1.7 billion years old.
Modern-day Wyoming is in one of the oldest parts of North America; source
2.4-billion-year-old granite in Fremont Canyon, cut by the North Platte River.
And why are the younger rocks younger?  That story’s a little more complicated.  After Rodinia broke up during the Cambrian period (roughly 500 million years ago), much of Laurentia was covered by a great shallow sea -- the Sauk Sea.  Lots of sand was deposited, so the Great Unconformity usually consists of Cambrian sandstone atop Precambrian igneous or metamorphic rocks, as in the Grand Canyon.  But this isn’t always the case.

Not all that long ago, the overlying sandstone of the Great Unconformity in south central Wyoming was assumed to be a typical Cambrian sandstone.  Then Sando and Sandberg (1987) went and looked at it.  Not Cambrian, they concluded.  Instead, they recognized a new formation, the Fremont Canyon sandstone, which was younger by a hundred million years or so (late Devonian).  Thus the Great Unconformity in south central Wyoming grew to two billion years.

But their discovery created still more questions.  Where are the Cambrian sands of the great Sauk Sea?  Why are they absent?  Were they removed by erosion?  Perhaps they were never deposited.  Indeed, parts of Laurentia remained above sea level during the Cambrian period -- the Canadian Shield and possibly a Transcontinental Arch.
Paleo-reconstruction of Laurentia during late Cambrian time, with highlands labeled.  Modified from the EarthViewer App, free from the Howard Hughes Medical Institute.
Like others, Sando and Sandberg assumed that this part of Wyoming had been on the Transcontinental Arch and therefore was not submerged during Cambrian time, explaining the "missing" sediments.  But Myrow et al. (2003) looked at a similar situation to the south in Colorado -- also supposedly involving the Transcontinental Arch -- and reached a different conclusion.  They found evidence that Cambrian sediments were deposited, but that subsequent uplift caused complete removal.  They recommended that
“... regional reconstructions of earliest Paleozoic paleogeography along the entire length of the purported Transcontinental Arch should be reevaluated ... paleogeographic reconstructions can be [seriously] biased by the presumption that missing strata represent periods of non-deposition rather than subsequent episodes of erosion”  [as we learned in Geology 101]
So the story told by the Great Unconformity remains incomplete.  At minimum we're missing much of the chapter on south central Wyoming, and probably more.  But scattered pages will continue to be found, and though they may be really hard to read, we'll learn a bit more about Earth's deep past.  How amazing and wonderful, especially for geo-trippers!

The Greater Great Unconformity is exposed southwest of Casper, Wyoming, thanks to uplift during the Laramide Orogeny and erosion by the North Platte River.  It can be conveniently viewed northwest of Natrona County Road 408 about a half mile east of the Fremont Canyon Bridge.  Other viewpoints are described in Knittel et al. (2004), pp 26-27 and 64-69.
Fremont Canyon is in the Heartland of Laramide Tectonics.  Click on image for a better view.


Jack Share's blog Written in Stone, Seen through my Lens includes very informative posts about the Great Unconformity, available here.

Knittel, P, Van Burgh, Jr., DP, Logue, TJ, Strube, BE, and Jones, RW.  2004.  Field guide for the Alcova area, Natrona County, Wyoming.

Myrow, PM, Taylor, JF, Miller, JF, Ethington, RL, Ripperdan, RL, and Allen, J.  2003.  Fallen arches: Dispelling myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain region.  Geological Society of America Bulletin 115:695-713.

Sando, WJ and Sandberg, CA.  1987.  New interpretations of Paleozoic stratigraphy and history in the northern Laramie Range and vicinity, Southeast Wyoming.  US Geological Survey Professional Paper 1450, 39pp.

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