Lecture 5 - Mesozoic and Cenozoic History of the Colorado Plateau- Zion and Bryce National Parks

 

Mesozoic and Cenozoic Eras: The Geology of Zion and Bryce National Parks

Although most of the Precambrian and Paleozoic rocks that we examined in the Grand Canyon are present in the region of Zion and Bryce National Parks, they are buried below a thick sequence of younger rocks (Mesozoic in Zion and Mesozoic and Cenozoic in Bryce)and not exposed at the surface of the Earth. Exposed at the surface are a thick sequence of Mesozoic and Cenozoic sedimentary rocks.

Sedimentary Deposits of Zion and Bryce National Parks

  1. Zion National Park
    1. early Triassic Moenkopi Formation- shales, siltstones, sandstones, and mudstones deposited primarily by streams that traveled from mountains to the northeast (the ancestral Rocky Mtns.) across a coastal plain region leading to the ocean off western North America. Seawater left in lagoons on the coastal plain deposited some limestone and salt deposits as this water evaporated.
    2. middle Triassic Chinle Formation- primarily mudstones and thin bedded limestones with abundant amphibian and reptile fossils and petrified wood suggest a swamp environment.
    3. late Triassic Moenave and Kayenta Formations- sandstones, siltstones and mudstones deposited in a continental flood plain environment with occasional lakes forming. These two formations together make up what has been called the "Vermilion Cliffs" of the Grand Staircase which represents the step like changes in elevation as one moves to younger and younger sedimentary units progressing north from the Grand Canyon to Zion and Bryce National Parks.
    4. early Jurassic Navajo Sandstone- Pure quartz sandstone with very well rounded polished grains and large cross-bedding. North America had been slowly moving northward through the Mesozoic and had reached sub-tropical latitudes by this time where major deserts form. In this environment sand dunes migrated across a very large desert that is recorded in the rock record over thousands of square miles in the western US. The Navajo Sandstone forms the most impressive cliffs in Zion National Park. This formation makes up what has been called the "White Cliffs" of the Grand Staircase. When massive, strong rock units like the Navajo sandstone are buried and subsequently unroofed by erosion the decompression causes them to develop concentric fractures similar to the layers of an onion. Water percolates through these fractures eroding the rock and froming "arches". This kind of fracturing and weathering of rock is known as EXFOLIATION.
    5. middle to late Jurassic Temple Cap and Carmel Formations- siltstones and sandstones of fluvial origin and shallow marine limestones indicate that the ocean returned for a short time to this region at this time.
    6. late Jurassic -early Cretaceous rocks are missing- probably eroded
    7. late Cretaceous Dakota Sandstone- marks more permanent return of the ocean to this region with deposition of beach sands.
  2. Bryce National Park
    1. late Cretaceous Dakota Sandstone- beach deposit
    2. late Cretaceous Tropic Shale- shallow marine shale
    3. other late Cretaceous sandstone and shale deposits which represent regression of the shallow sea where mostly eroded away leaving only small remnants in the Park
    4. Eocene (Tertiary) Claron (sometimes called the Wasatch) Formation- most prominent formation in Bryce Canyon made up of limestones, mudstones and sandstones deposited in a continually expanding and then shrinking lake environment. This formation makes up the "Pink Cliffs" , the highest step in the Grand Staircase.


Unlike the Paleozoic Era when shallow ocean deposition dominated in the Colorado Plateau region, the Mesozoic was a time of primarily continental deposition. Like the rocks of the Grand Canyon, the Mesozoic sedimentary rocks in Zion were also mildly affected by tectonic forces that began to build the Rocky Mountains during the latest Mesozoic- early Tertiary time. (See notes from Lecture 4). Rocks that have been folded and faulted during this activity are most evident in the northwestern part of Zion Park.

Cenozoic History of Zion and Bryce

The major rifting event that is responsible for creating the San Andreas Fault and opening of the Gulf of California began in the Cenozoic and still continues today as the ocean between Baja California and Mexico widens. The major effects of this event in theGrand Canyon where elevation of the Colorado Plateau so that theColorado River could carve the Canyon. This uplift between about 5-10million years ago is also responsible for the Virgin River cutting Zion Canyon. The very recent volcanism (1-2 million years) that we saw in the Grand Canyon (see Lecture 4 notes) also took place in Zion and can be seen in the very northern part of the Park. Additional features resulting from this rifting, are large faults that have dissected this region of the Colorado Plateau into smaller plateaus that are separated by valleys. Three of the most conspicuous of these fault are from west to east, the Hurricane fault (west of Zion), the Sevier Fault (between Zion and Bryce) and the Paunsagunt Fault (east of Bryce). (Figure 3.2, p. 44).

The last chapter of the geologic history of these parks is achapter that we see in all the parks of the Colorado Plateau and willbe discussed in detail in the next lecture. This chapter contains thestory of how erosion has shaped (and continues to shape) thespectacular landscapes evident in the parks.

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This page was last reviewed on 1/12/04.