Lecture 3 - Sedimentary Rocks and Environments of theGrand Canyon

Outline

I. Classification of Sedimentary Rocks

II. Reconstructing Past Sedimentary Environments

III. Sedimentary Environments of the Grand Canyon

Although the Earth is composed of only a small fraction of sedimentary rocks, about 5% of the outer layer of the Earth we call the crust, 75% of exposed continental material is sedimentary rock. Because this rock forms at the surface of the Earth, it is essential for understanding conditions that existed at the surface throughout geologic time. We can examine sedimentary rocks and interpret past conditions at particular locations on the Earth. Was the region under a great ocean? Was the climate hot and arid producing a desert or was it humid and warm resulting in a swamp? Where glaciers present, or was the region more topical with coral reefs similar to the Bahamas? Let's study sedimentary rocks and find out what the sediments of the Grand Canyon tell us about past conditions of the Colorado Plateau region.

I. Classification of Sedimentary Rocks

  1. Clastic- composed of fragments of existing rock produced by mechanical breakdown
  2. Chemical-sediment precipitated from solution
    Inorganic-(salt from evaporating seawater)
    Biogenic- chemical sediment precipitated from living organism usually composed of shells or body parts, limestone)

Clastic rocks are given different names depending on the size oftheir particles:
Cobbles/Gravel---------------Sand---------------Silt/mud--------clay/mud
Conglomerate----------------Sandstone(quartz)--Siltstone--------Shale
Breccia (angular particles)---Arkose (feldspar too) Graywacke (q,f,rockfragments)

II. Reconstructing Past Sedimentary Environments

  1. Composition- limestone was probably precipitated in a shallow ocean, sandstone made of all quartz, a mineral very resistant to erosion probably traveled a great distance or under very turbulent conditions, stream or beach deposit
  2. Texture- large, well rounded particles imply very energetic environment like a large stream while large angular particles might have been transported by ice which would not bounce the particles around and round them.
  3. Fossils- certain organisms only lived in damp swamp environments while others lived in deep oceans
  4. Sedimentary Structures-mudcracks preserved in mudstones would indicate an environment that was water covered but periodically dried out like a river floodplain or desert playa lake.

Based on composition, texture, fossils and sedimentary featuresand knowledge of present sedimentary environments, geologists can decipher what past environments where like from examining the rockrecord. Let's learn about what sediments are deposited in some modern environments first so that we can interpret past environments in the Canyon.

  1. Near-shore marine- Beach consists of well sorted, rounded quartz sand from reworking by waves, oceanward on the continental rise, silt and clay size particles settle out, still farther oceanward, but still on the rise, carbonate sediments form from the collection of shells of marine organisms. Abundant marine fossils and small cross-bedding in the beach sands that was created by wave action.
  2. Stream- Deposits will vary depending on size and power of stream but typically consist of well sorted layers of coarse to fine particles which reflects stream channel deposit overlain by fine flood deposit as channel shifts course. Land fossils may be present.
  3. Desert- fine grained sands carried by wind (eolian deposits, after Aeolus, the Greek God of wind) form dunes with large cross-bedding created in same way as in beach sands but the wind is more powerful making larger structures. May find salt deposits and mudcracks from playa lakes, desert lakes that exist only temporarily due to flooding and have filled and evaporated many times. May find fossil footprints

III. Sedimentary environments of the Grand Canyon

  1. Cambrian Rocks (500-570 Million years Ago)
    1. Tapeats Sandstone-medium to coarse grain sandstone consisting of quartz and feldspar with conglomerate at the base and finer particles as you proceed up the section, percentage of quartz also increases up section. Trilobite fossil fragments which were a common shallow marine organism in Cambrian time. Small scale cross-bedding occurs. Environmental Interpretation- Near shore marine (beach)
    2. Bright Angel Shale-siltstone and shale, oxidized Fe minerals impart red color, abundant marine fossils, trilobites and brachiopods. Environmental Interpretation- near shore continental shelf
    3. Muav Limestone- thick limestone, marine fossils present. Environmental Interpretation- distant continental shelf

The superposition of these three marine environments on top of one another suggests that sea-level was rising such that beach deposits get overlain by deeper water shale and then even deeper water limestone. When the sea advances on to the land it is called a transgression, when it retreats it is called a regression. The sequence of marine sandstone/shale/limestone is diagnostic of sea-level rise or a transgression of the sea and is known as a transgressive sequence. The Tapeats, Bright Angel, and Muav formations record a transgression of the Cambrian sea.

  1. No Ordovician -Silurian Rocks -no deposition or deposition and erosion so don't know what area was like during this time
  2. Devonian -Mississippian (320-400 Million years ago)
    1. Temple Butte and Redwall Limestones- marine fossils suggest shallow tropical ocean setting
  3. Pennsylvannian - Permian (250- 320 Mya)
    1. Supai Sandstone and Hermit Shale-red siltstones, sandstones, and shales with non-marine fossils (amphibian footprints and land plants) suggest coastal plain environment with streams emptying into a shallow sea.
    2. Coconino Sandstone- small, well rounded and sorted quartz sandstone with large cross-bedding indicate eolian origin, reptile tracks common Environmental Interpretation- Arid desert environment
    3. Toroweap- obvious formation from the Park since it comprises the tree-covered slope between the prominent light cliffs of the Coconino sandstone and the cliffs of the Kaibab limestone that rim the Canyon, Much varied formation consisting of sandstone, limestone and evaporites. Interpretation is a record of several transgression and regressions of the sea, more frequent than in the Cambrian so many transgressive and regressive sequences are present.
    4. Kaibab Formation-caprock of the Grand Canyon- primarily limestone with fossils of marine organisms that lived in shallow warm sea.

Continued History of this area will be told in the youngersedimentary rocks seen at Zion and Bryce Canyons.

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This page was last reviewed on 1/05/04.