Wednesday, November 28, 2012

Missing Time

What is the estimated span of a human lifetime in these early days of the 21st century – 80 years or so? By comparison, the massive cliffs of Navajo Sandstone in Zion National Park are the remnants of a vast dune field that existed for nearly ten million years. 

Navajo Sandstone of the East Rim


For millions of years before sand dunes swept across what would one day be southwestern Utah, shallow seas advanced and retreated over an immense flat, featureless coastline. Gradually these shallow marine conditions changed and Utah became part of a large fresh–water interior basin drained by rivers and streams flowing in a northwesterly direction. Braided streams flowed for hundreds of miles across vast lowlands. Sediments were deposited in flood plains, and lakes and sinuous streams drained low, forested basins. 

Around 190 million years ago, plate tectonics brought ancient North America and Utah near to the equator and an arid climatic belt. Much like conditions today, Utah was in a rain shadow created by mountains to the west. But unlike today (so far, anyway), vast dune fields much like the modern Sahara Desert encroached upon and overwhelmed the existing river, playa, and lake environments. 

I went for a hike on the east side of the Park recently, to consider the big picture of those rocks found on top of the Navajo Sandstone. There is a good bit of missing time between the two, in the form of a distinctly beveled surface called the J–1 unconformity

Temple Cap lies above Navajo Sandstone


Here, the lower part of the overlying Temple Cap Formation represents a brief respite from desert dune conditions as rising warm, shallow seas leveled the crest of these dunes and left behind red mudstones. Over time, however, new windblown dunes returned, only to be covered in their turn by another shallow  sea and another bit of missing time, the J–2 unconformity. Above this unconformity is the Carmel Formation, deposited in yet another shallow inland sea. 

The fossil dunes of the Navajo Sandstone


An unconformity is a gap, an absence of time in the geologic record. It usually results from some sort of change such as uplift and/or erosion which causes deposition to stop for a substantial period of time of at least several millions of years. 

Temple Cap Formation lies unconformably on top of Navajo Sandstone
 
And so, many of the changes in the geologic record that occurred above the dune fields of the Navajo Sandstone can be seen as a consequence of the advance and retreat of shallow seas. 

The seas come in, the seas go out.  Where does the time go?







4 comments:

  1. And we may be back to seas in. Gorgeous! Love Zion.

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    1. We'll just have to wait and see about those seas.

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  2. Nina

    Nice! I share your (and James Hutton's) fascination with unconformities -- so much time! Geology is such an evocative science: standing in front of certain outcrops, one's mind can't help but be blown.

    I enjoy reading your blog. Thanks for sharing your observations with us.

    -- Matthew

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    1. Glad you enjoy the posts, Matthew. One of these days I will get to Scotland and Siccar Point and pay homage to Hutton.

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