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sarracenia

Learning our Local Limestone

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sarracenia

Hi all y'all.

I'm curious about learning more about our local geography to learn more about the context for fossils I find around here.

I know a little bit, but not a ton and I'd love to learn more. I think I should probably start with what's exposed at the surface at my house.

We're in the Balcones Fault Zone, and I've seen this spot mapped as both undifferentiated Fredricksburg Group and as Edwards Group.

We've got a little intermittent creek with a deep enough valley/canyon that there's probably a 50' elevation difference between the highest and lowest areas.

Being in West Austin, we do have exposed limestone cropping out at the surface. There aren't really any big cliffs, but the terrain sort of stairsteps down from the top to the bottom, with small cliffs a few feet tall interrupting the slope down.

I haven't looked at all the layers closely, but it seems like there are a few layers of more porous limestone completely chock-full of shells. Would those be reef layers? This rock is a good example from one of those layers, but it seems to have more shells and bigger pieces than most of the similar layers (though it may just be less weathered...)

IMG_4077.thumb.JPG.0e26aab09b882222c46e8d6614523298.JPG

The hard limestone exposed in the bed of the creek has what appear to be burrows, I think?

IMG_4092.thumb.JPG.6cc85c76b89cc828032f563f676937e3.JPG

The same rock layer has a few of what I believe are rudist fossils. I'm still not 100% on what rudists look like, but is this one?

IMG_4064.thumb.JPG.abd9624d222f5a10ab3b432c82192a75.JPG

Higher up, there's some extensive chert/flint layers, but also some other more fossil-y layers.

 

I'm thinking it might be a good idea to try making a stratigraphic column so I can better figure out exactly what I'm looking at, but I'm not really sure how to do that. I've got no experience in geology and would love to learn. Do any of y'all have good advice for how to go about doing this? I'm still not 100% on how to classify different carbonate rocks, so any resources you can point to for that would be greatly appreciated!

 

I know a wee bit about our local geology, but I'd also appreciate any more in-depth resources y'all can recommend!

Thanks a lot!

 

 

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Fossildude19

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JohnJ

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erose

You are so far ahead of most newbies already just based on reading what you have observed so far. Undifferentiated Fredericksburg would include the Edwards. You mentioned seeing flint or chert. That is typical of the Edwards. 
 

There is a lot of reference material available on the geology of Austin. Go to the publications page of the Texas Bureau of Economic Geology and search “Austin”. Look for maps and guidebooks in particular. The stuff is cheap and they will mail it to you quickly. Of course I am assuming they are still operating as normal.

 

You may also want to try some of the online geology maps or download a geo map app like RockD. The trick with the maps is to read the descriptions included. See if they jive with what you actually see* Pay attention to not just descriptions of rock color but also “weathered” color.

 

if the geological terms seem confusing get a copy of a geological dictionary. 
 

Also try and get some photos of the fossils you see. That little bit does look like it could be a rudist which would also suggest Edwards strata.

 

good luck and have fun.

 

* the maps are not perfect and can be off by tens to hundreds of feet.

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Uncle Siphuncle

+1 for Edwards and rudist

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sarracenia

Thanks y’all for the help! 

In terms of what I’ve found a lot of, there’s been quite a few oysters (like most places around Austin). I’ve also found several scallops (especially fragments) and some snails of some sort (again, many fragments). I did notice that some of the snail fragments seemed to not just be steinkerns.

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I’ve definitely found quite a few more rudists, but they’re all embedded in the bedrock. I’ve also found lots of rocks with what appear to be impressions of fossils — including oysters and gastropods.

74678954-D944-4958-8123-322E3F08A21C.thumb.jpeg.639833dd518ef81f025babf6f60bd987.jpeg

 

I’m working on trying to piece together a stratigraphic column. It’s a bit tricky because of all the vegetation and the fact that not all the layers of bedrock are entirely exposed.

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grandpa

Hello and welcome to the forum.  I'm also in Austin.  Perhaps I've missed it, but I'm surprised that no one has mentioned the Austin Paleo Society as a local fossil club and great place to learn about the local fauna and stratigraphy and where it is exposed.  The club also hosts monthly meetings with presentations and monthly field trips.  [All of this is of course currently on hold of a sorts due to the Viddies.]  You would be a welcome addition for the club, should you decide to join.  Also, @erose who wrote you an informative note above is the president, and a fine one at that.;)

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erose
27 minutes ago, grandpa said:

Hello and welcome to the forum.  I'm also in Austin.  Perhaps I've missed it, but I'm surprised that no one has mentioned the Austin Paleo Society as a local fossil club and great place to learn about the local fauna and stratigraphy and where it is exposed.  The club also hosts monthly meetings with presentations and monthly field trips.  [All of this is of course currently on hold of a sorts due to the Viddies.]  You would be a welcome addition for the club, should you decide to join.  Also, @erose who wrote you an informative note above is the president, and a fine one at that.;)

Once again, thanks for the kind words grandpa.  The club is currently on a reduced schedule. We have suspended all in person meetings until this virus is no longer a threat. Last night we had a virtual board meeting and made some decisions regarding the rest of our schedule.  We will be starting online "Zoom" meetings in June. and we will be starting up field trips as well.  Members can easily keep a proper distance at the majority of our usual collecting sites. We will keep them close to home, day trips only.  To find out about all this stuff you can join our club and you will receive Paleo Footnotes, our newsletter.  Current members and new members will have their memberships extended by however many months we go without live meetings.  We also have a public Facebook page and, for members, a Group FB page.  Our web site: austinpaleo.org

 

 

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