Nacogdoches, Tx Banita Creek

[Nate]

I made a stop today at a site I've been curious about for awhile. Most of the year, it's underwater. This time of year when things are dryer, these rocks in the creekbed get exposed. I have long thought about putting an Earthcache here because it's at a city park right behind the farmer's market, and it's just off Main St.

Today I went there specifically because the local geologic maps suggest that the Weches formation should be exposed here.

I did find what appeared to be a fossiliferous layer, but it looks so strikingly similar to asphalt, I want to put it up to some more experienced eyes than mine.

The Weches is described thusly:

Includes from top down Therrill, Viesca, and Tyus Members. Not separately mapped, glauconite, glauconitic marl, quartz sand, and clay, grayish green; weathers to yellowish brown and reddish brown, limonitic, banded, clay-ironstone; marine megafossils abundant; forms scarp; thickness 50-90ft; lower part equivalent to upper part of Cane River Formation in Louisiana.

Weathered surface (top)

Unweathered surface, with small shells visible

Side view, with more small shells visible

This stuff looks strikingly like asphalt or even possibly coal. The small bivalves almost look like they were recently deposited, too.

My questions:

Is this native rock, or is it possibly asphalt? There's a roadcut nearby (I don't have photos of it yet), but the roadcut is in sandstone (which would sit above this layer).

If it's native rock, is this part of the Weches formation, or possibly something else?

[MikeD]

I would say yes on the Weches. I can't tell exactly from the pictures, but I believe the matrix is weathered layers of banded glauconite, clay, silt, sand, limonite, blah, blah, blah. Looks like you are in the right place or close to it. The shells are Eocene in age and the original shell material is preserved, which is why they look "recent". It looks similar to some of the Stone City material.

[Nate]

I would say yes on the Weches. I can't tell exactly from the pictures, but I believe the matrix is weathered layers of banded glauconite, clay, silt, sand, limonite, blah, blah, blah. Looks like you are in the right place or close to it. The shells are Eocene in age and the original shell material is preserved, which is why they look "recent". It looks similar to some of the Stone City material.

What threw me is that the dark material looked black-organic. I tried searching for pictures of some of the material, and nothing I could find for glauconite or limonite really looked like what I had in my hand. What I could find for clay-ironstone looked similar, but I couldn't find any close-ups to really be certain. The texture is pretty crumbly. I pried this piece off of the bed with my bare hands. There is some banding, but the dark material is dominant by far and the other stuff is in such small quantities, it's hard to tell what exactly the banded material is.

All of the strata I've seen elsewhere in the area is very high in quartz sand content.

It's in the right place for Weches based on maps I've seen, but I was curious about asphalt because it's in an area that's been developed for close to 200yrs and has had plenty of opportunity to have asphalt dumped.

[tracer]

what mike said. glauconitic matrix is not at all similar to asphalt nor coal. it basically was like medium-depth marine shelf deposition of stuff like mollusc poop and other organic stuff, etc and low-oxygen conditions that led to it chemically changing through diagenesis into what it is today. there's usually quite a bit of variation in the layers of it, as mike mentioned, with iron-nodules/concretions, "mudstones", "lenses" of storm deposits, etc. some degree of infilled burrows, etc. the shell material is different from a lot of fossils found in texas, in that it is original material, but somewhat leached over time, so it's fairly fragile. trying to get nice fossils out of such matrix can be frustrating at times. by the way, exposures of many formations can look *very* different when first exposed versus when weathered a bit. i've only visited the weches a tiny bit, but it's similar to the stone city formation that i've been to quite a bit, and that can be very dark, dark green in the newly-exposed areas of almost pure glauconite, but then it weathers out to a light brown almost tan.

so anyway, probably more than you wanted to hear. by the way, there are elevated levels of naturally-occuring arsenic in the weches over there so don't be breathing the dust a lot.

oh, one other point. i've seen east texas "dirt" roads that are covered in mined "greensand", which is glauconite. if you ever go down a greenish-looking road, you might stop and look a bit at it. found a fairly cool little fossil on one.

[Nate]

what mike said. glauconitic matrix is not at all similar to asphalt nor coal. it basically was like medium-depth marine shelf deposition of stuff like mollusc poop and other organic stuff, etc and low-oxygen conditions that led to it chemically changing through diagenesis into what it is today.

That explains why this sample looks the way it does. It absolutely looks organic in its origins, and that was where I was drawing my comparisons with other organic materials I'm more familiar with.

by the way, exposures of many formations can look *very* different when first exposed versus when weathered a bit. i've only visited the weches a tiny bit, but it's similar to the stone city formation that i've been to quite a bit, and that can be very dark, dark green in the newly-exposed areas of almost pure glauconite, but then it weathers out to a light brown almost tan.

No need to explain that part to me. I have a little bit of formal education in geology (undergrad minor in environmental studies, which had a significant geology portion, and working on a graduate major in environmental science, which has a little bit of geology). this material, btw, does not look the slightest bit greenish.

so anyway, probably more than you wanted to hear.

not at all. the more technical, the better.

by the way, there are elevated levels of naturally-occuring arsenic in the weches over there so don't be breathing the dust a lot.

I'm aware, thanks.

oh, one other point. i've seen east texas "dirt" roads that are covered in mined "greensand", which is glauconite. if you ever go down a greenish-looking road, you might stop and look a bit at it. found a fairly cool little fossil on one.

I haven't seen anything like that.

[MikeD]

Well, there's two things I need to remember. Green roads and arsenic.

[elcaminojohn]

I stopped by there yesterday and looked. It is Weches glauconite. The water turns the iron ore a dark color. The fossils are in the 55mgy range and well imbedded, so with them being saturated, they would be difficult to remove intatct. I found a decomposed Athleta and a Polinices along with a number of "venus" type clams. All are common in the Weches formation. There are better places to hunt for them in the area.

FYI--The Weches Formation was named for the town of Weches on Highway 21 about 40 miles west of Nacogdoches. The outcrop that was first ID is at the corner of 21 and McLain Creek just below Mission Tejas State Park. Nice park with camping, etc...except now. Athleta They just had a perscribed burn and it would be like camping in an ashtray.