Thursday, March 5, 2015

What a Geologist Sees - Part 1

No, I don't need glasses... nor a therapist, just 'cause I see stuff like this when I look at Grand Canyon photographs.  We  geologists are afflicted in this manner.  So just politely say "that's nice" to humor us.  The photo is from the south rim of the Grand Canyon (maybe from Hopi Point or somewhere near there).

Normal people are not expected to remember all of these terms, so I will explain a few of them so next time you visit the Grand Canyon (or the first time), even if you can't remember the specifics, you can revel in what geologists call "deep time". [Seriously, if you spend a little time reading articles on the internet before visiting these National Parks, you will enjoy them more.] 

Above the Inner Gorge, virtually all of the rocks are sedimentary rocks, the earliest of which were deposited on a continental shelf and when the bulk of the continent was flooded by a shallow inland (epeiric) sea, the rest of them were deposited, though there is a 100 million-year-long gap in the layers (more on that later).

In the geologic science of Stratigraphy (the study of layered rocks), when there is an interruption of the geologic record, represented by an erosion surface, we call these "Unconformities, of which there are four types; 1) Nonconformities; 2) Angular Unconformities; 3) Disconformities; and 4) Paraconformities. Types 1, 2, & 3 are visible in this Grand Canyon photo.  On the left side of the photo, an arrow points to a "Nonconformity", where the Cambrian-age Tapeats Sandstone overlies the Proterozoic Brahma Schist (metamorphic rocks). To the right of the "Normal fault", the Brahma Schist is overlain by the Proterozoic Bass Limestone (also a Nonconformity here), the reddish-colored Hakatai Shale, and the cliff-forming Shinumo Sandstone, all of which are very old sedimentary rocks.  The Brahma Schist, the Vishnu Schist and the Zoroaster Granite represent the first of two mountain ranges that were formed, then eroded away before the Cambrian Tapeats Sandstone was deposited approximately 540 million years ago.

After these mountains were weathered and eroded, the rising seas covered the area and deposited a thick sequence of sedimentary and volcanic rocks, called the "Grand Canyon Supergroup".  These rock layers were then "block faulted" (tilted, but not folded), yielding the second mountain range, which was then eroded prior to the Cambrian Period sea-level rise. When horizontal sedimentary rocks overlie older, tilted sedimentary rocks, the erosion surface between them is called an "Angular Unconformity". Where that particular arrow is pointed, the Hakatai Shale (and the cliffs of the Shinumo Sandstone) represented an island surrounded by the Tapeats Sandstone. When sea level continued to rise and the shoreline moved toward the present-day southeast, the Bright Angel Shale was deposited over the exposed Shinumo Sandstone. If you enlarge the image, you may be able to see the layers of the Hakatai Shale tilting to the right (in relation to the horizontal Bright Angel Shale above). 

The Grand Canyon is one of the sites referred to as "The Great Unconformity" due to the gap between the Proterozoic Grand Canyon Supergroup and the Lower/Middle Cambrian Tonto Group (Tapeats Sandstone and Bright Angel Shale). The Franklin Mountains, within and north of El Paso, TX represent another exposure of this 500+ million year gap, between the underlying Proterozoic igneous, metamorphic, and sedimentary rocks and the overlying Middle Cambrian Bliss Sandstone and Silurian Fusselman Dolomite.

As sea level continued to rise (for millions of years) eventually flooding most of the continent, the Muav Limestone was deposited on top of the Bright Angel Shale. These three oldest sedimentary rocks units compose the "Tonto Group" and represent a continual rise of sea level that can be traced to 100 miles SE of El Paso, TX.  After the Muav Limestone was deposited, there was an interruption in the sequence, sea level dropped (and/or the land rose). The next time the sea covered the area and left layers behind was 100 million years later, with the Redwall Limestone, deposited during the Mississippian Period.  The eroded surface between the Muav Limestone and the Redwall Limestone is a "Disconformity" and rocks representing the entire Ordovician, Silurian, and Devonian Periods are missing. [There are some Devonian rocks elsewhere in the Grand Canyon.]

The coverage of the area by the sea that deposited the Redwall Limestone happened very rapidly. Limestones are generally deposited in shallow, tropical waters at a distance from any large landmasses. Normally, when sea level rises gradually, the sequence is sandstone overlain by shale overlain by limestone - telling you of the gradually deepening waters.

[The remaining layers of the Grand Canyon section of the Colorado Plateau (above the Redwall Limestone) can be described another time.]

I hope this little geology lesson hasn't put you to sleep. The Grand Canyon is a fascinating place as it is unusual to see so much geologic time represented in such a small area. The oldest metamorphic rocks in the Inner Gorge are about 1.7 billion years old. The oldest horizontal layer is about 540 million years old and the youngest horizontal layer (in this photo) is the Permian Kaibab Limestone, deposited about 240 million years ago, so the horizontal layers in the Grand Canyon represent about 300 million years (estimated) of geologic time, with 100 million years missing (remember the Disconformity).  The canyon itself is thought to be only about 5 to 10 million years old. Why the entire Colorado Plateau underwent this rapid uplift, but remained essentially horizontal and largely un-deformed and largely un-faulted is something of a mystery to geologists and the source of many lively discussions. [Yeah, I left out some details, otherwise this post would be ten paragraphs longer.]

[This "What a Geologist Sees" series is being presented in a chronologically-reverse order, the opposite of how it was originally posted in the retired blog.  There might be a gap or two due to an editing error.]

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