Permian Basin Dynamics

PART ONE:  The Paleohistory of the Permian Basin's Evaporite Deposits Including the Formation of Nash Draw


I was surprised, recently, while driving from Carlsbad, New Mexico, to the airport between Midland and Odessa, Texas, at how much the word Permian permeates the Permian Basin.  

There are numerous businesses and institutions with the word Permian in their names, and there is even a University of Texas-Permian Basin (UTPB)!

So what is this Permian thing?  If you go to the ever helpful University of California-Berkeley paleontology website (click here to go there) you can find a very nice list of geologic time periods: 

Phanerozoic Eon
(543 mya to present)
Cenozoic Era
(65 mya to today)
Quaternary (1.8 mya to today)
Holocene (10,000 years to today)
Pleistocene (1.8 mya to 10,000 yrs)
Tertiary (65 to 1.8 mya)
Pliocene (5.3 to 1.8 mya)
Miocene (23.8 to 5.3 mya)
Oligocene (33.7 to 23.8 mya)
Eocene (54.8 to 33.7 mya)
(65 to 54.8 mya)
Mesozoic Era
(248 to 65 mya)
Cretaceous (144 to 65 mya)
Jurassic (206 to 144 mya)
(248 to 206 mya)
Paleozoic Era
(543 to 248 mya)
Permian (290 to 248 mya)
(354 to 290 mya)
       Pennsylvanian (323 to 290 mya)
       Mississippian (354 to 323 mya)

I cut the list off at the Carboniferous because it illustrates the fortuitous timing of Mother Nature in providing the world with a huge abundance of plant matter which is then followed by a die-off and a time when that plant-life's remains would be capped, in some locales, by the deposition of deep layers of evaporite rocks.  

Note that we are talking about a time before the dinosaurs. Although there were big lizards in this time period, we'll likely never see a "Permian Park" movie (like we did the dinosaur-rich"Jurassic Park" movie--for those of you who are movie-illiterate).


So in the Permian Basin, which covers a chunk of far west Texas and eastern New Mexico, there are evaporites laid down over plant matter that, given time and increasing temperature and pressure, was changed to oil and gas (coal and tar in other places).  In some parts of the Permian Basin this material escaped with time, but in other places it was thoroughly trapped and subsequently squeezed by tectonic forces into distinct fields, hence the many, many well-defined oil fields in the Permian Basin today.

So where did these capping evaporites come from? From the evaporation of sea water.  And why was there a sea in what is now New Mexico and Texas?  Because.  

Look at the Panthallassic Ocean boundary on this map, taken from the PALEOMAP Project website by Christopher Scotese:

What is now the Permian Basin was then the land-ocean boundary of the Pangea supercontinent and the Panthallasic Ocean.  And there were some funky dynamics taking place at this boundary.

The best illustrations I have yet seen of some of these shore-dynamics are on a page created by Dr. Bruce Cornet of El Paso Community College, posted on the Sunstar-Solutions website.

The location of the evaporites in the Permian Basin is given in this schematic by Dr. Cornet:

The role of the reefs in setting up a basin where sea water can flow in and be evaporated, to be followed by yet more sea water coming in at a specific rate over and seeping through the reef.  If the land surface rises the deposition of salt will stop and there may be intermittent larger inflows from periodic sea level rises or local storm surges causing massive seawater inflow.  This would tend to lay down a cap of limestone over the salts.  Wetter periods may produce flooding on and runoff from the surrounding highlands, adding thin layers of clay between evaporite layers at any time during this sequential process.  The general setting for all these dynamics involving an upland, a reef and a sea is very well illustrated in a later figure on Dr. Cornet's page:

On a separate page, Dr. Cornet suggests that these dynamics can currently be observed at the Tetiaroa Atoll in the South Pacific.

We have used the word "evaporites" throughout this page without defining the term, except to say it is what you get when you evaporate seawater.  As seawater becomes more concentrated (taken from the Free Encyclopedia website):

So how do you get a very deep sequence of mostly limestone?  You regularly bring in fresh seawater and the precipitation sequence never gets beyond the calcite and dolomite stages, which are laid down layer upon layer (with very subtle differences in accessory minerals making for a layered rock, the layers are called "facies" and are studied to find out more about the local environment changes over time).

And how do you get deep sodium and potassium salt layers?  A steady influx of seawater into a nearly closed off area (as in the lagoon illustration above) at about the same rate as it is being removed by evaporation.  The calcium and magnesium carbonates and sulfates will fall out of solution (precipitate) around the edges of the lagoon behind the barrier reef, and in the center the chloride salts will build up relatively uninterrupted.

In the Permian Basin, after a period of halite type salt deposition, things changed again and a deep sequence of carbonate rocks was laid down over the salt beds.  In turn toward the end of the Permian there was uplift and deposition of a red mudstone brought in from the surrounding upland.  

This red mudstone is called the Dewey Lake Redbed in this area, and it is a formation found to run in a band of red sub or exposed soil all the way from New Mexico northeastward through Texas, ending in Oklahoma.  In the Nash Draw pages about the canyons of Livingston Ridge, this is the red material visible under the modern caliche cement and wind blown sand that form the surface of that ridge today.


So now we have brought this whole discussion toward Nash Draw.  Nash Draw, as explained on the first page on that location, is a downdropped area  where from one to a few hundred feet of a deeper very soluble salt layer were dissolved out about half a million years ago and the land mass above it settled down by approximately that depth.  Of course  subsequent erosion from surrounding lands keeps the basin filling up with sediments, so the current depth of the draw does not exactly match the depth of the salt that was removed so long ago and allowed the collapse.

So how did the upper formations of sediment and limestone fail to keep water away from the soluble salt deep below in this relatively small area:  the formation of sinkholes!  Sinkholes are features common in karst areas.  It is where surface waters were able to dissolve away some of the near-surface limestone, and create channels that allow rapid movement from the surface to some depth.  As this process continues, caves may form, hence the Carlsbad Caverns (and thousands of other caverns) for which this area is, justly, famous.  

But not every sinkhole finds itself turning into a huge underground opening, and many that do find the opening that has been created underground to be inadequately buttressed, and then there is a collapse, often all the way from the surface downward into the hole.  So a beautiful, intact colossal-sized cave system like Carlsbad Caverns is a rare thing. Hence its National Park status.

What may be more common is sinkholes that never amount to much more than a hole in the ground that water disappears into.  This is suspected to have happened in Nash Draw where the overlying sediments allowed water to access the limestone to form sinkholes.  The water in these sinkholes seems to have moved predominately downward until they reached the very soluble halite salt beds and began to dissolve and remove salt.  [The obvious question: where did the salt water discharge to as this process got started is still a mystery.  Today the answer is easier: about 60% of the salt water in the Salt Lake in the south of Nash Draw comes from underground.]

As soluble as this salt was, each sinkholes dissolved salt away radially, creating caves that spread laterally until they met, forming a cave system that in turn led to an insupportably long and wide cave that just had to collapse.  And so it did, and so we have Nash Draw today, and for the last half million years.

Another obvious question is whether or not this process continues today.  Are there active sinkholes around the edges of Nash Draw that are allowing water to reach the soluble salt below, so that Nash Draw will continue to spread out in all directions?  In geology one never says never, but one does invoke geologic time to indicate that in the context of your lifetime or mine, it means never.

After all, the formation of Nash Draw occurred 600,000 to 500,000 years ago, and it has spread relatively little since then.  At the southeastern margin of Nash Draw there is a small area with active sinkholes, and there may be other such areas that I am not yet aware of.

The next two pages will explore this small active sinkhole area.

 See a small Sinkhole Cave near Nash Draw

 See A Complex of Sinkholes near Nash Draw (in two sub-pages)

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