|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?