Lecture 4 - Geologic History of the Grand Canyon

Outline

I. Precambrian Mountain Building

II. Paleozoic Sediment Accumulation

III. Mesozoic Uplift and Deformation

IV. Cenozoic Uplift and Evolution of the Colorado River

I. Precambrian Mountain Building

1. The Vishnu Schist and Zoroaster Granite
   The Precambrian rocks of the Grand Canyon reveal that nearly 2 Billion years ago large mountains dominated the Grand Canyon region. Although these mountains are long gone, their roots remain for us to examine in the Vishnu Schist and Zoroaster granites. The relationship between the schist and granites, the granites intrude the schist, resemble what we see in some present mountain belts. Large sequences of sedimentary and volcanic rocks collect as large plates of the Earth collide building mountains, the heat and pressure of mountain building metamorphoses the sediments into schist and melt some of the rock creating the granites that intrude into it.
2. The Grand Canyon Series
   These mountains were gradually eroded down to a nearly flat surface by 1250 Million years ago. At this time the ocean advanced and began to lay down the sediments of the Grand Canyon Series in a basin along what was the western margin of North America. Continued transgressions and regressions of the sea laid down a thick sequence of flat lying marine and continental sediments that are interspersed with lava.
3. Tilting and Faulting of the Precambrian Rocks
   Just over 1 billion years ago, the western margin of North America rifted, with an ocean opening up between pieces of the continent and the western continental fragment drifted away to possibly become part of present Asia. This is part of the plate tectonic process and we will talk more about this in about 2 weeks. This rifting event resulted in faulting or differential movement of large chunks of rock along a fault. The faulting caused the rocks to tilt as well as caused the rocks that were elevated during the faulting to be severely eroded. In the up thrown side of the faults all of the Grand Canyon Series are missing. In the down thrown fault blocks these tilted rocks were preserved.

II. Paleozoic Sediment Accumulation

This history is discussed in detail in lecture 3 notes under the heading Sedimentary Environments of the Grand Canyon

III. Mesozoic Uplift and Deformation

Although there are no Mesozoic aged rocks exposed in the Grand Canyon, study of adjacent regions reveal that a sequence of sedimentary rocks as thick as that exposed in the Grand Canyon was deposited on top of the Kaibab limestone during Mesozoic time. This thick sequence has since been eroded. Although study of the rocks in the Grand Canyon can tell us nothing about the sedimentary environments throughout the Mesozoic, (we will learn about this from the rock record at Zion, Bryce, and other parks of the Colorado Plateau) they do contain some record of the geologic history during this time. Gentle, asymmetric folds called monoclines are abundant in the Grand Canyon and were produced by the compressive forces that were responsible for creating the Rocky Mountains. The Colorado Plateau was more distant from these forces so rather than crumplingthe rocks as they did in the Rockies, they reactivated the Precambrian faults in the older deeper rocks of the Grand Canyon thatresulted in folding of the younger sediments. From detailed studies of this deformation event in other locations (we will study this later in the class) it is estimated to have taken place during the Cretaceous through Eocene time (latest Mesozoic- early Cenozoic time). The compressional forces uplifted the Plateau and allowed for erosion to remove the younger rocks.

IV. Late Cenozoic Faulting, Uplift, Volcanism and Evolution ofthe Colorado River

A major reorganization of tectonic plates off the western United States which is responsible for development of the San Andreas Fault and opening of the Gulf of California, produced tensional stretching forces in the Colorado Plateau region beginning about 20 million years ago. This resulted in abundant faulting in the rocks of the Grand Canyon. These tensional forces thinned the crust and allowed hot molten material (magma) from depth to rise close to the surface. The increased temperatures caused the Plateau to become elevated and this uplift allowed the Colorado River to start to efficiently carve the present Grand Canyon (about 6 million years ago). Some of the magma erupted at the surface of the Earth producing the young volcanic rocks (1-2 million years old) observed in the western portion of the Canyon. Periodically these volcanic flows dammed the Colorado River and created lakes in the present Canyon. The erosive power of the River eventually eroded the dam and this process repeated itself, leaving behind a record of several cycles of lava dams.

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