GRAND CANYON GEOLOGY as seen from a trip down the Colorado River.

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FIELD TRIP STOPS - All the PreCambrian and Paleozoic Rocks that comprise the Grand Canyon as viewed from a 6-day rafting trip down the Colorado River.

 

LOCATION: Starting at Lees Ferry, Arizona and continuing west on the Colorado River for about 200 miles.

 

GEOLOGIC FEATURES: All the PreCambrian (Metamorphic, Igneous, and Sedimentary rocks, as old as 1.8 billion years) and Paleozoic Sedimentary Rocks of the Grand Canyon; Unconformities (Disconformity, Non-Conformity, and Angular Unconformity); Fault-block Mountains; Canyon formation; Dune Crossbeds; The Great Unconformity; Columnar Jointing (Columnar Basalts).

 

DESCRIPTION: Episode 1 - Over 2 billion years ago sediments were laid down in the area of the present-day Grand Canyon. These sediments were buried deeply by overlying sediments and eventually metamorphosed and uplifted during a mountain building event about 1.7 bya to form the Vishnu Schist and Zoroaster Granite.

 

Episode 2 - The highlands dominated by the Vishnu Schist and Zoroaster Granite were eventually eroded to sea-level, forming a peneplane. PreCambrian sediments were then deposited in the sedimentary basin. These PreCambrian sediments are referred to as the Grand Canyon Series. They range in age from 1.25 billion to 825 million years in age). Rock formations include the Bass Limestone (non-conformably lying on the metamorphic and igneous rocks), followed stratigraphically by the Hakatai Shale, Shinumo Quartzite and Dox Formation.  Subsequent uplifting resulted in Fault Block Mountains whose landscape was composed of tilted strata of the Grand Canyon Series.

 

Episode 3 - These highlands were eventually eroded to form another peneplain. Portions of the inclined PreCambrian sediments that were downdropped during fault-block formation were preserved from erosion.  Cambrian sediments of the Tapeats Sandstone were laid down either on the pockets of inclined PreCambrian starta (forming an angular unconformity) or directly on the Vishnu/Zoroaster crystalline rocks (a non-conformity) beginning about 570 million years ago. This gap between the PreCambrian crystalline and sedimentary rocks is referred to as The Great Unconformity. After the Tapeats came deposition of the Cambrian Bright Angel Shale and Muav Limestone.  There are no Ordovician or Silurian rocks in the Canyon, having either been eroded away or never deposited. Above the Cambrian Muav lies the Mississippian Redwall Limestone (or in places, the Devonian Temple Butte Formation) in a Disconformity. Above the Redwall lies the Supaii Formation (Pennsylvanian and Permian), Hermit Shale (Permian), Coconino Sandstone (Permian), Toroweap Fm. (Permian), and Kaibab Limestone (Permian).

 

Episode 4 - Uplift of the Colorado Plateau spurred the Colorado River to cut into the Canyon creating its present topography.  Evidence of recent basalt flows into the canyon from volcanoes on the rim can be seen.

 

STUDENT QUESTIONS:

(1) Name and define the three major types of unconformities seen in the Grand Canyon.

(2) From the photos provided, give a Figure number that illustrates each type of unconformity. You can also a photo from the field trip “Grand Canyon Geology as viewed from South Rim” for a good photo of an angular unconformity. 

(3) Using the photos of rock formations, name three major sedimentary environments that existed during the Paleozoic within the area of the Grand Canyon over time. Provide the Formation names and the evidence used to determine the environment.

(4) Some rock formations have faces (cliffs) that are near-vertical while others have faces with inclined slopes. From the photos, examine the rock types associated with each of these faces and suggest a reason for their topography.

(5) Who is John Wesley Powell and what was his role in the devrlopment of the Grand Canyon?

(6) CHALLENGE: What is the maximum gap in time that is missing across the Great Unconformity? Explain how you arrived at your answer.

(7) CHALLENGE: What is a proposed mechanism to account for the rise of the Colorado Plateau?

DIAGRAMS:

 

 

SELECTED  REFERENCES:

-NPS: Grand Canyon – Geologic Formations. Accessed on Jan.7, 2020:  

https://www.nps.gov/grca/learn/nature/geologicformations.htm

-Stevens, Larry. 1983. The Colorado River in Grand Canyon - A Guide. Red Lake Books, Flagstaff, AZ. 115 pp. [This waterproof book is a must for any river trip down the Canyon taking you down the river, mile by mile, from Lake Powell to Lake Mead (300 miles).

-USGS: Geology and Ecology of National Parks – Grand Canyon Geology. Accessed on Jan. 7, 2020: 

https://www.usgs.gov/science-support/osqi/yes/national-parks/grand-canyon-geology

 

PHOTOS:

Figure 1 - Flying from Las Vegas to Lee's Ferry, AZ we get the first view of the Grand Canyon. The flat rock formation in the foreground the the top of the Kaibab Limestone (Permian), the youngest rock formation in the Canyon.(A4)

Figure 2 - MILE 0 - Marble Canyon. Lee's Ferry, AZ. Before entering the Canyon, equipment and provisions are loaded into the raft. Rocks in the background are Triassic in age (Moenkopi Fm.), slightly younger than the sedimentary rocks found in the Grand Canyon itself. (A9)

VERMILLION CLIFFS (Triassic)

Figure 3 - Leaving Lee's Ferry we see the red Triassic sediments of the Moenkopi Fm. that lie on top of the Permian Kaibab Limestone that caps the rocks present in the Grand Canyon which lies further downstream. Notive the clear water since naturally flowing sediment had been trapped upstream by the Glen Canyon Dam.(A10)

Figure 4 - MILE 4.5 - Coconino Sandstone (Permian) lies below the Toroweap Fm. and Kaibab Limestone and exhibits large-scale crossbedding indicative of a desert dune environment. (A13)

Figure 5 - In front of the CoconIno crossbeds, seen in the center of the photo, is a bank of sand that was used as an area to disembark from the raft for lunch. At the time of this photo, daily fluctuations in the river level due to regulated variations in flow rates upstream at the Glen Canyon Dam to meet hydroelectric needs had caused the gradual erosion of the sandbanks. This sand was not replenished as it was captured by the dam upstream. (A14)

COCONINO SANDSTONE

TOROWEAP FM.

KAIBAB LIMESTONE

Figure 6 - The Kaibab and Coconino are more resistant to erosion than the Toroweap and as such tend to form steep cliffs rather than the more gentle slopes of the Toroweap Fm. The rocks we see here are the ones we see capping the deep Canyon as viewed from the heavily visited south rim further downstream. (A15)

COCONINO SANDSTONE

HERMIT SHALE

Figure 7 - The Hermit Shale (Permian) underlies the Coconono Sandstone. The shale weathers as a slope whereas the resistant sandstone weathers near-vertical. (A16)

COCONINO SANDSTONE

HERMIT SHALE

TOROWEAP FM.

KAIBAB LIMESTONE

Figure 8  - Permian strata.  The Kaibab represents a fully marine environment; The Coconino is an ancient Desert; The Hermit and Toroweap represent Flood Plain and River deposits. (A18)

Figure 9 - MILE 18.5. "Boulder Narrows." A piece of the Supai Fm. (Penn. &  Permian) lies in the river. The rubbly Hermit Shale overlies the Supai. (A20)

Figure 10 - MILE 20.5. Campsite (Day 1). North Canyon. Redbeds of the Supai Fm. which represent Flood Plain and River deposits.(A22)

Figure 11 - Campsite (Day 1) (A24)

SUPAI FORMATION

REDWALL LIMESTONE

Figure 12 - MILE 24 (Day 2). Supai Fm and Redwall Limestone. (A28)

MILE 49.5. Unconformity

REDWALL LS (Miss)

MUAV LS (Camb)

Figure 13MILE 49.5 - Unconformity (Disconformity). Notice that the Cambrian Muav Limestone is directly adjacent to the Mississippian Redwall Limestone. That is to say, there are no Ordovician, Silurian, or Devonian rocks here. In places within the Grand Canyon there are Devonian rocks (Temple Butte Formation). Here, the unconformity represents about 145 million years of missing time. The upper limit of the Redwall is marked by the beginning of the Supai red shales. (B13)

BRIGHT ANGEL SHALE (Camb.)

REDWALL L.S. (Miss.)

Figure 14 - MILE 52 - The Cambrian Bright Angel Shale weathers crumbly. The Muav overlies the Bright Angel. Note the steep reddish cliffs of the Redwall Limestone. The actual color of the Redwall is gray.  It appears red due to iron staining from the weathering of the overlying red sediments of the Supai Formation.(B14)

BRIGHT ANGEL SHALE (Camb.)

TAPEATS SS (Camb.

Figure 15 - MILE 60.5 - Cambrian Sandstone and Shale. The Tapeats is typically tabular. The Bright Angel is finely laminated, subsequently weathering into fine pieces. (B15)

Figure 16 - MILE 62 - Campsite (Day 2).  Setting up for dinner. It should be noted that the rafts were pushed away from the shoreline in the evening because with the reduction in demand for hydroelectric power at night, the river level drops. Had the rafts not been moved from their position seen above, they would be on land in the morning and unable to move.(B20)

BASALT (PreCambrian)

Figure 17 - MILE 70. Basalt Canyon. Within the PreCambrian sediments of the Grand Canyon and found directly above the Dox Fm. there are Basalt flows of the Cardenas Fm. (B25)

SHINUMO QUARTZITE (PreCamb.)

Figure 18 - MILE 74.5  Shinumo Quartzite of the PreCambrian Grand Canyon Series. (B27)

BASS LS (PreCamb.)

HAKATAI (PreCamb.)

SHINUMO (PreCamb.)

Figure 19 - MILE 76.5 - PreCambrian Strata of the Grand Canyon Series that are tilted due to the formation of Fault-Block Mountains (at the end of EPISODE 2 in the Description Section). (B28)

VISHNU SCHIST

BASS LS

Figure 20 - MILE 77  -Unconformity (Nonconformity) that formed as a result of the uplift and erosion of the Vishnu Schist (1.7 byo) and subsequent deposition of the sedimentary Bass Limestone (1.25 byo). There are about 450 million years missing at this unconformity. (E30)

Figure 21 -  MILE 77 - Inner Gorge. The Vishnu Schist and Zoroaster Granite. (B32)

Figure 22 -  MILE 77 - A sheet of resistant Zoroaster granite within the Inner Gorge. (B32)

Figure 23 - Pink Granite and Black Schist (B34)

Figure 24 - MILE 101 (C2)

TAPEATS SS

VISHU SCHIST

Figure 25 - MILE 104 - The Great Unconformity marks the division between the Cambrian and PreCambrian. Here the Vishnu Schist (1.7 myo) is overlain by the Cambrian Tapeats Sandstone (570 myo). The Nonconformity here represents a gap in time of over 1 billion years! (C4)

PALEOZOIC

Tilted PRECAMBRIAN

Figure 26 -   DAY 4 - MILE 107 - Note the tilted PreCambrian sediments of the Grand Canyon Series in the foreground and the horizontal Paleozoic strata in the background. The characteristic white band near the top of the Paleozoic is the Coconino Sandstone.(C6)

Figure 27 - MILE 113 - The Great Unconformity. Tapeats over Vishnu (C19)

Figure 28 - MILE 126 - Tapeats SS. Note the characteristic slabby appearance of the Tapeats. (C31)

Figure 29 - MILE 132 - Deubendorf Rapids. Note the two skilled kayakers. Rapids along the Colorado River are the result of the caving of rock walls into the river. (C35)

Figure 30 - MILE 135 - Unconformity - Bass LS over Vishnu Schist. Note the way that the sediments lap (feather) onto what must have been a hill on a rocky coastline composed of the Vishnu Schist about 1.25 billion years ago. (D4)

Figure 31 - MILE 136 - Campsite Day 4 across from Deer Creek Falls flowing through the Tapeats SS.(D9)

TAPEATS (570 MYA)

VISHNU SCHIST (1.7 BYA)

Figure 32 - Day 5 - MILE 137 - Outcropping of the Great Unconformity (1.1 billion years missing) (D11)

Figure 33 -  MILE 178 - Vulcan's Anvil - a Volcanic Neck (D26)

Figure 34 - MILE 179 - Lava Falls Rapid - according to our guide, "the fasted navigable body of water on the planet."  After the Grand Canyon was formed, a recent flow of lava (pictures upcoming) filled the canyou to form a dam.  After the dam collapsed from the force of water building up upstream of the dam, the lava wall collapsed to for the rapid.(D30)

Figure 35 -  MILE 179 - Lava Falls Rapid.(D32)

Figure 36 - MILE 179 - The recent lava that flowed down the Canyon walls can be seen draped over the preexisting sedimentary strata. (E3)

Figure 37 - Volcano on the distant plateau (E6)

Figure 38 - Recent Lava flows down the Canyon side (E7)

Figure 39 - Vulcan's Throne Cinder Cone Volcano as seen from the air with lava that once poured down the Canyon wall.(A6)

Figure 40 - MILE 186 - Columnar Basalt at Campsite (Day 5). As the lava cools relatively rapidly, it shrinks to form polygonal columns (E8)

Figure 41 - Polygonal Columnar Basalt as viewed on edge. (E10)

Figure 42 - Mile 186 - "Restroom" area at Campsite. Within the gap of the volcanic columnar basalts is a space where a restroom was located. The yellow life jacket to the bottom right was an indicator that nobody was currently using the "facility." (E11)

Figure 43 - Luxury Restroom - An ammo container lined with a heavy plastic bag and a toilet seat on top. Other ammo cans held Toilet Paper and disinfectants. Especially for a geologist, sitting here surrounded by columnar basalt, was a bit unreal. (E13)

Figure 44 - DAY 6 - MILE 187 - With basalt in the background, the helicopter swooped down take 5-6 passengers at a time to the Canyon rim where a light aircraft would take them to Las Vegas or Lee's Ferry. (E35)

Figure 45 - Back to Las Vegas.  Notice the red sediment that entered Colorado River from the Little Colorado further upstream. (F10)

Figure 46 - Hoover Dam means that Las Vegas is near, and freedom from the thermal updrafts that can cause absolutely miserable air sickness for those who had not taken precausions. (F11)

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