Thursday, December 2, 2010

In Between The Rocks

Low sandstone cliffs near trailhead
Trail drops through ledgy slopes of Kayenta Formation; Red Mtn. in distance
Travel opportunities to far-away geologically exotic destinations do not seem to be looming on my personal adventure horizon this week.  Luckily, I happen to live in a place where people come from all over the world to experience the incredibly scenic red-rock landscape; more often than not I don’t even have to go very far to be watching for various lovely rocks.  My “travels” on this particular day trip involved tossing my camera, a windbreaker and a bottle of water into a backpack, shifting my car into gear, driving 20 miles to get out of my neighborhood in the snow, and then meeting a friend at the Chuckwalla trailhead parking lot in the Red Cliffs Desert Reserve near St. George, Utah.  

Tilted sandstone beds appear to be ledgy cliffs of upper  Kayenta Formation
The Reserve is a hiker’s and geologist’s paradise but there were only a few other people out and about on this balmy 42°F brilliant-blue-sky early December day.  For years I have wondered about these rocks – are they the top of the Kayenta Formation?   Or are they the base of the overlying Navajo Sandstone?  They show up very distinctively and prominently in the cliffs of Zion National Park, barely 50 miles away.  But near St. George, where millions of years of plate tectonics have pushed, stretched, and tilted the crust much more so than in the Zion area, the boundary between the Kayenta and Navajo can be less clear.  At least to me it can be less clear.  And since I am usually willing to exchange confusion for clarity I decided to check out the Chuckwalla. 

Approximately 1.0-1.2 million year old basalt flow eroding onto sandstone
Basalt eroding down sandstone cliff;snow-covered  Pine Valley Mtn. in distance
My local geologic map shows that the rocks on this particular patch of ground are mainly Early Jurassic (around 190 million years old) Kayenta Formation with assorted more recent volcanic basalt flows plastered across the hillsides.  The rocks of the Kayenta are siltstones and sandstones, outcropping as alternating reddish slopes and ledges, and record an environment of shallow lakes and low to moderate-energy streams coming off ancient mountains to the west.  Fossilized mud cracks tell of drying conditions.  Thin limestone layers and trails of aquatic snails and worms reveal the existence of ancient ponds and lakes.  Commonly occurring in the siltstones are tracks of upright-walking three-toed dinosaurs. 

This was the time of the break-up of Pangaea, when the supercontinent was centered more or less on the equator.  It was a time of seasonal climate with wet summers and cold winters at the edge of a vast desert, the influence of which was soon to predominate as plate tectonics drove prehistoric North America to drift incrementally northward into an arid desert belt.  Here the lakes and streams of the Kayenta would eventually give way to the immense dune fields of the future Navajo Sandstone.
Chuckwalla trail winds through upper Kayenta Formation sandstones

Sue ponders rocks and birdlife
Across highway 18 from the trailhead the younger Navajo Sandstone (Early Jurassic Period, about 180 million years ago) outcrops as massive hills.  It dawned upon me that the trail most likely cuts through the very upper layers of the Kayenta and possibly the very lower part of the Navajo – the map description states that these ledgy slopes of the Kayenta grade up to ledgy sandstone cliffs at the top of the formation.  And that is exactly what I was seeing.  

As I moseyed along the trail in between the rocks I contemplated the almost inconceivable changes that had occurred those hundreds of millions of years ago when the earth’s plates were shifting, its climate was changing, and these ancient sediments were being deposited so that, two hundred million years in the future, I could look at these rocks and appreciate the wonder to be found in them. 

 
Probably not the best location to build a house!
Tilted sandstone beds appear to be upper Kayenta Formation
Arch or window???

4 comments:

  1. My local geologic map shows that the rocks on this particular patch of ground are mainly the Late Triassic (around 210 million years old) Kayenta Formation

    Hmmmm...interesting map. The Kayenta Formation is Early Jurassic in age (~190 Myo); it's the Chinle that is Late Triassic. Similarly...

    Across highway 18 from the trailhead the younger Navajo Sandstone (Early Jurassic Period, about 190 million years ago)

    ...the Navajo is later Early Jurassic (~ 180 Myo). Must be a rather old map to show the Kayenta as Triassic!

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  2. Thanks, Dinogami! I made the corrections. So is the Glen Canyon Grp (Wingate-Kayenta-Navajo) Late Triassic to Early Jurassic? Is the Navajo SS considered Early Jurassic also? This was a real mind-bending experience and I spent HOURS sifting through it all!!! And while I have your attention - so these rocks on the Chuckwalla ARE the upper Kayenta, right? Interesting to see the Kayenta as such pronounced sandstone - for years I thought they were the Navajo.

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  3. The position of the Triassic-Jurassic boundary is not certain in southern Utah, and it's been the matter of no small amount of research! Biostratigraphic and magnetostratigraphic (the primary two means of trying to ascertain its position in the absence of anything radiometrically datable) studies don't really agree. For much of the last couple of decades, the Tr-J boundary was thought to be in the Dinosaur Canyon Member of the Moenave Formation here in southwestern Utah, and low in the Wingate Sandstone in the Four Corners area. The latter was primarily based on vertebrate biostratigraphic interpretations (some phytosaur skulls impressed on the underside of what was thought to be the bottom of the Wingate Sandstone). However, it's now being thought that the phytosaur-bearing sandstone is actually a channel fill in the uppermost Chinle, so the boundary may be in the unconformity between the two. As for the Moenave, the picture is more complex and even involved vertebrate track biostratigraphy! A recently published magnetostratigraphic analysis even put the boundary low in the Whitmore Point Member of the Moenave, but there is a lot of disagreement with this study (and, as you probably know, both biostratigraphy and magnetostratigraphy are fraught with perils in differing interpretations of the data!).

    Anyway, yes, the Navajo is nowadays considered Early Jurassic (albeit later in the Early Jurassic), and possibly just creeping into the earliest Middle Jurassic. The Navajo is even harder to date because it is almost entirely devoid of fossils, and what few are known are either (a) endemic, (b) widespread, stratigraphically, or (c) not really useful anyway (like burrows). But many of the units atop the Navajo in south-central and southeastern Utah, like the Page Sandstone, Romana Sandstone, Curtis Formation, Summerville Formation, etc. are all considered Middle Jurassic (and eking into the early Late Jurassic). I don't know the history in detail, but this may have something to do with pushing the Navajo down the time scale.

    For the most part, the Kayenta and Navajo have been considered disconformable, but around St. George, I've certainly not noticed any marked lithostratigraphic change between the two, and have some suspicion that, at least locally, the contact between the two is gradational and conformable, with the upper Kayenta becoming increasingly sandy. ...not that I've done a great deal of lithostrat in the area; this is just based on drive-by geology!

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  4. Thanks so much for your welcome comments, Dinogami. Lots to think about here.

    It would really be interesting to take a look at some thin sections - from the sandy upper Kayenta Fm on the Chuckwalla trail, and from the Navajo Sandstone on the hillsides across highway 18. I wonder how different the grains would appear under the microscope; what kind of grain shapes the Kayenta sandstone would have compared to the well-rounded Navajo grains, and what the matrix might look like in the Kayenta (and the Navajo, for that matter - I've never looked at either - I have always been enamored of the metamorphics, as you know - not much sedimentary experience so far).

    Rock-crawling at the Kayenta/Navajo contact could be a potential Dixie Geological Society field trip!

    "...fraught with perils" puts me in mind of Snidely Whiplash, tying the Whitmore Point member interpretation to the railroad tracks, sneering "Nyah-ha-ha!!! The Tr-J boundary is in the Dinosaur Canyon member! This interpretation will NEVER see the light of day!"

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