Monday, March 07, 2011

Tahoka Lake

The inaugural Llano Estacado Master Naturalist class trip was to Tahoka Lake.  Tahoka lake is a 1x2 mile salina just NE of the small town of Tahoka, Texas.  Tahoka is near the eastern edge of the Llano Estacado.  From Tahoka, the next town to the east on US 380 is Post, Texas, which I visited about 3 years ago on a long weekend trip with geologists from ConocoPhillips.

Tahoka is about 100 miles north of Midland.  The drive is mostly flat, through oil fields and cotton fields.  Here, north of Midland, there is a lot of center-pivot watering.  They get more rainfall up here, and the Ogallala aquifer is better developed.  On the way, we passed a water field where Midland gets about half of its municipal water.  The biggest surprise on the way was a small lake that US 87 takes a path right through/over.  It was full of water, even after ~6 months with no rainfall (at least in Midland).  This was a good sign for water in Tahoka Lake.

The group was quite large.  The trip was the first one, the budding master nats were all excited to get out and explore.  Also, the lake is on private land (now a non-profit trust).

Tahoka Lake

My first view of the lake was a shocker.  It is quite large.  I'm not used to seeing features with this much relief on the Llano.  From rim to lake the maximum relief is around 100'.  The lake is fairly flat-bottomed, and was probably less than 10' deep when we visited.  Of course with more rains, it fills up.  There is no surface outlet.  So how was the lake formed?  The lakebed rests on Triassic-age mudstones.  Around the edges are some exposures of Cretaceous-age carbonate rocks.  The rest of the rim is made up of Ogallala clastic rocks, capped by calieche caprock.  This stratigraphy should now be very familiar to regular readers of this blog: it is the basic stratigraphy of the exposed rocks of the Llano Estacado.  So why is this lake here?  It lines up with postulated faults visible on aerial photographs.  Many of the drainages in the Llano, and playas/salinas on the Llano, also tend to line up with these semi-linear fault features.  I'm sure I'll post more on this subject in the future.  How is material moved from the lake off the Llano?  The underlying Triassic beds are probably near impermeable.  That makes me think some of the movement must be through the Ogallala beds.  Gravel and sands are not very soluble in groundwater or surface water.  Carbonate rocks are quite soluble, however.  This adds up to make me think that Tahoka Lake is the site of a large remnant piece of Cretaceous limestone, sitting atop the Triassic and spared erosion by the rivers of Ogallala times.  This hypothesis is further supported by the outcropping of some Cretaceous carbonates along the edge of the lake.


To start off, our leader Burr gave a lecture on the area.  Two co-leader geologists/hydrologists also chimed in.  One interesting idea I picked up was the suggestion that the carbonate that forms the caprock was delivered via the air in the form of dust.  I'd always heard it was leached from below by groundwater being drawn toward the surface by evaporation.  I wonder if oxygen isotopes could help tell that story.  

A pretty good crowd for a Saturday

Evil sunflowers

After our lecture the group broke up and wandered down the hill toward some bluffs in the distance.  I was amazed how most people walked single-file, right behind the person in front of them.  These were budding master naturalists, but were acting more like lemmings.  Of course, many intrepid souls struck off on their own.  Also, it does make sense to stick around Burr for some of the walk, as he is always dispensing information in the form of entertaining yarns.  

The southern Llano had a wet summer, causing mass numbers of sunflowers to spring up and grow.  Now dead, these sunflower corpses are tall, pokey and scratchy -- and will probably stand for another year or two.  No rains yet this year, so our wildflower crop is looking to be poor. 

Ephedra

I didn't know what Ephedra looked like.  I guess that is the point of these field trips.  Now I know.  There are several varieties, and now I see it everywhere I go on the Llano.

Juniper hillside

Up on the hillside is an area with juniper trees.  The definitely have them at Big Spring, too, but I guess this is a little isolate population here.  

Juniper

The small bluff was capped by calieche caprock, 6-8' thick in places.  Below the caprock were a number of small shelters and "caves".  All around were animal bones, honeycomb, scat, and lichens.

Pretty lichen

Lichen close-up

Deserted honeycomb on the underside of the caprock

Wandering MatNats

Insect home on caprock

Thorns

Inundated Salt Cedar

?

Edge of Tahoka Lake

Detail of cactus with some type of infection

MatNats along the shore

Tahoka Lake

Lichen

The bluish lichen on this rock grew on the more siliceous area and not on the surrounding limestone.  I'm guessing this is due to chemical weathering of the limestone, not due to nutrients.

Is this lichen on the tree?



Tahoka Lake.  An interesting and beautiful place.  And you can go visit.

1 comment:

Plants Amaze Me said...

So the small lake on US 37 I see it on the map, do you mean you had to drive through water?
How do you know all this stuff, like "So how was the lake formed? The lakebed rests on Triassic-age mudstones"?
I'm looking up so many words, calieche, Ogallala clastic, playas, oh my brain, Ha Ha.
I'm likin' your lichen.
MatNats=Master Naturalist cute.
Can I visit Tahoka Lake I thought it was on private land?