Geobulletin alpha

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Author:
Date:	2015-11-26 00:57:36
Blog:	WATCH FOR ROCKS - Travels of a Sharp-Eyed Geologist
URL:	http://www.watchingforrocks.com/2015/11/living-dream-day-3-ruins-and-redwall.html

Summary:

I awaken slowly in the spring dawn and stretch inside my sleeping bag, feeling stiff and brittle as a sun-bleached boat paddle. Footsteps crunch gently across the sandy beach. Softly muffled chatter grows louder, pots and pans clang more frequently as our guides begin breakfast preparations at their lavish camp kitchen. It is time to extricate myself from my ground-dwelling nest. The coffee gong sounds brash and heavenly. It is the morning of our first full day on the Colorado River in Grand Canyon; on hands and knees I crawl out of my already sandy tent and shake myself awake, keen to continue the adventure. Just throw me in the river and I will limber up in no time.The cowboy coffee is perfect, nearly strong enough to stand a spoon upright in the middle of my cup. Breakfast is hot and plentiful, and apparently we have choices.Guide in gaudy flowered apron: How would you like your eggs?Me: How about eggs Benedict?Guide: Scrambled it is!Packing the j-rigsPerched in our camp chairs we enjoy a leisurely meal and a third cup of coffee, and watch the changing light brighten the rock layers as the sun crests the cliffs behind us. Soon it’s time to tote our dry bags to the boats, heave them and everything else to the guides, jump aboard, and push ourselves off downstream into the shimmering river and bright blue sky.A  beautiful morning to be on the riverGuide Buzz & Beyonce, the raven with the diamond choker & pink pfd.That rapid really looks like fun!Somewhere along this stretch of river, and in other places at the top of the Redwall, are outcrops of the Surprise Canyon Formation. These rocks are evidence of a major estuary system that once existed along an ancient western edge of the continent.  Deep valleys had been carved by rivers into the bedrock Redwall Limestone and then flooded again by intermittent seas during Late Mississippian times, 325 million years ago. It is the least visible rock unit in the Park, mostly because its outcrops are so remote and also because it does not display in unbroken layers but only as discontinuous lens-shaped areas that are remnants of the estuaries. I never do see any of it.Another rapid!Yucca in bloomThe Redwall LimestoneWe float down through strata of Redwall Limestone, massive and distinctive, a 500-foot to 800-foot-thick behemoth of a cliff-former that can be traced continuously over nearly the entire length of the Canyon. There are four units – from youngest to oldest (down section) these are the Horseshoe Mesa Member, the Mooney Falls Member, the Thunder Springs Member, and the Whitmore Wash Member. I have absolutely no idea which layer is which until I get home, gaze at my pictures lovingly and longingly, check my references until my brain begins to wilt, and try to figure out at least a smidgen of the stratigraphy. My best guess (six months later) is that we are within the Mooney Falls, the thickest member of the Redwall. It will become clear a little later downstream (Now I tell me!) as to why I best guess this.Meanwhile, at around river mile 30.5 the scene shifts. The layers are no longer horizontal, at least for the moment. We are crossing Fence Fault, a down-to-the-west, normal fault where younger Permian rocks have been down-dropped relative to older Mississippian rocks.Fence Fault - darker Permian strata is down-to-the-westFence FaultThree hundred and forty million years ago in Mississippian times the Grand Canyon area was a passive continental margin near the equator. Pretty much the entire western interior portion of North America was covered by a widespread, shallow tropical sea that extended from Canada to Arizona and all the way to Kansas. The rocks we see in the Redwall are basically pure limestone, deposited over millions of years as calcium carbonate (CaCO3) precipitated out of solution and as shelled organisms died and fell to the bottom of the sea, all in “an unbroken rain of carbonate debris” (Collier). Similar deposition occurs today in semiarid environments such as Florida and the Persian Gulf, each located on shallow ocean shelves where few rivers exist to bring in muddy, or clastic, sediments. These clastic-free expanses were (and still are) the perfect habitat for limestone-secreting organisms to not only survive but thrive. Along the Redwall, nearing South CanyonThe guides pilot the rigs onto a sandy beach at the mouth of South Canyon, near river mile 32. I day-hiked around here years ago, from the head of the canyon up near the Vermillion Cliffs (I never made it all the way to the river, though, but that is another story for another day). We disembark in the warm sunshine, and for an hour or so hike a short trail to view Native American petroglyphs and ruins. We also find fossils embedded in the rocks.Trail to South Canyon RuinsKris checks out the petroglyphs at South Canyon South Canyon Ruins I am so taken with the views up and down the river that I don’t hear a word of what the guide says about the ruins.  Fossils in the limestone also seize and hold my attention. The Mooney Falls Member of the Redwall is chock full of fossil life – sponges, corals, brachiopods, crinoids, straight and coiled nautiloids. The fact that fossils of these organisms arefound here tells us much about the conditions and depth of the seawater at the time of deposition.The sea must be shallow enough for photosynthesis to occur, around 100-200 feet depth. Photosynthesizing plants provide the food for many marine organisms such as our corals, brachiopods, and sponges. It follows, then, that shallow, clear, limy waters with little or no mud offer the best environment for shell-secreting sea critters to survive and thrive.Fossil coral - horn? rugose?  Anyone out there know?Fossil coral. See previous caption.Fossil spongeWe make our way down the rocky slope and hop ourselves back on to the boats. A cluster of dories rows up to shore as we push off downstream. It’s time for new discoveries for everyone and we haven't even had lunch yet!Dories row up as we are leavingDatura trailside*****************************************References:Abbott, L., and Cook, T., 2004, Hiking the Grand Canyon’s Geology, The Mountaineer BooksBeus, S., 2003, Redwall Limestone and Surprise Canyon Formation in Beus, S. and Morales, M., eds., Grand Canyon Geology, 2nd edition, Oxford University PressBlakey, R. and Ranney, W., 2008, Ancient Landscapes of the Colorado Plateau, Grand Canyon AssociationCollier, M., 1980, An Introduction to Grand Canyon Geology, Grand Canyon Natural History Association Stevens, L., 2013, The Colorado River in Grand Canyon – River Map & Guide, Grand Canyon Wildlands Council

Content analysis:

Stratigraphic context:

Recognized stratigraphic terms [n]:	Permian [2] Mississippian [3] Redwall Limestone [1] Surprise Canyon Formation [1] Horseshoe Mesa Member [1] Mooney Falls Member [2] Thunder Springs Member [1] Whitmore Wash Member [1]
Agenames chronostratigraphy [rating]:	Permian [0.33] Carboniferous [0.94] Paleozoic [0.95] Phanerozoic [0.95] Mississippian [0.94] Upper Mississippian [0.78] Lower Mississippian [0.78] [0.5]

Geographic context:

Location	Country	Latitude	Longitude
Arizona	US	34.1679	-111.931
Canada	CA	56.9547	-98.309
Colorado	US	38.9979	-105.551
Colorado Plateau	US	36.7871	-110.481
Florida	US	27.9758	-81.5411
Fossil	US	41.8154	-110.714
Grand Canyon National Park	US	36.2846	-112.808
Kansas	US	38.4981	-98.3202
North America		44.3308	-109.755
Oxford University	GB	51.7546	-1.25504
Persian Gulf		27.1638	52.1833
Sandy Beach	US	42.3392	-71.9441
United States	US	37.1679	-95.845
Washington	US	38.9048	-77.0163

Keywords:

Colorado River