Mixed formations @ borders

[Rocky Stoner]

Hello friends, I am wondering how much "mixing" there would be of the material from neighboring formations as the transition from one to the other occurs at the surface. I suppose the gradient of the formation layers and fractures as well as the slope of the surface would play a large part in producing the "blended" area (if there actually is one).

Referencing the pics, For instance, my location is Mahantango indicated by the red arrow. Less than a mile to the west, the grade is fairly steep up through the Marcellus, then about 2 miles to the Oriskany Sandstone which I find quite a bit of in my area.

Could the map be off a bit ?

Is there typically a "mixed blend" where formations meet at the surface ?

Can one approximate the width of these blended zones ... if they do in fact occur ?

 

Just curious, as usual.

Thanks for your time.

 

 

[Pumpkinhead]

The Mahantango formation conformably overlies the Marcellus formation, meaning that the deposition of sediments is unbroken between the two. I don't know whether or not this conformable contact is gradational or abrupt, but nevertheless the Mahantango formation should contain Mahantango strata and the Marcellus formation Marcellus strata. If you happen to be finding material from the Marcellus formation or Oriskany sandstone in your area where the bedrock is from the Mahantango formation the most likely explanation for this in my opinion is that these other two formations are more elevated than your position and at some point material has been eroded downwards and been deposited where you live.

 

A situation in which you can get "mixing" at contacts is if the contact is unconformable. In this situation the base of the overlying formation can be a conglomerate containing material from the underlying formation. Near to where I live there is an unconformity like this where Precambrian quartzite is overlain by Upper Ordovician rocks. The Ordovician rocks at the contact contain large quartzite boulders. I hope this addresses your question.

[Fossildude19]

All of the material Rocky is finding looks like  typical Mahantango Formation fossils and matrix. 

I don't think there is any mixing, from what I have seen you post, Rocky.  

[Rocky Stoner]

2 hours ago, Pumpkinhead said:

......... I hope this addresses your question.

It most certainly does. The erosion you noted is what I had in mind when mentioning the upward surface gradient here.

Thank you much !

1 hour ago, Fossildude19 said:

All of the material Rocky is finding looks like  typical Mahantango Formation fossils and matrix. 

I don't think there is any mixing, from what I have seen you post, Rocky.  

Hi Tim, you are right. But, the reason I'm asking is because 300 yards west of the area from my recent postings, there is an upward gradient that has no shale at the surface. It's all oriskiny sandstone, limestone and chert. Just at the surface, I have not done any digging there. My neighbors field is along that same area and when plowed it is the same also. I walked up there today and brought back 2 samples after cracking many with my hammer. The sandstone has many fossils but are faint and weathered on the exposed surfaces. Cracking the open reveals some fossils but there is no relative layers and the coarse sand doesnt leave crisp impressions. The limestone and/or Huntersville chert is extremely hard but does have fossils in quite a bit of it, mainly where it is cemented to a layer of the sandstone. I brought back 2 samples. These are larger than most but have the best features. Gravity / erosion makes perfect sense if you see exactly where I am. Will take 2 posts for the pics. There is a strange fossil in the sandstone, maybe you recognize it ?

I hope the pics come through OK. Sandstone this post.

Thanks again guys,

regards

[Rocky Stoner]

Here is the other piece.

These two are much larger than the average there, next time I'll take the camera for a better representation.

[Rocky Stoner]

I found this interesting info searching "Geology of the Appalachians":

 

The birth of the Appalachian ranges marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent   Pangaea with the Appalachians and neighboring Little Atlas (now in Morocco) near the center. These mountain ranges likely once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded.

 

I can imagine quite a bit of mixing downslope of the convergence of the relatively thin formations at the surface.

A very diverse patch of geology here on this little ridge.

Attached is a small piece of the extremely hard limestone (?). When cracked open with a hammer, there was a surprise inside.

While waiting for rain (Saturday) to wash 2 tilled areas, I might pick around in the sand/limestone a bit.

Thanks for looking, I think this question has been answered.

Kind regards.

[Oxytropidoceras]

In steeply to moderately dipping strata, it is quite common for there to be displacement of strata by slumps, even landlsides of various scales. Along with downslope movement due to surface creep, these processes would tend to mix and displace material that is found as loose blocks on the surface. Because of either the scale or the type, surficial versus bedrock, of the geologic mapping, it is not practicle to either map them or show the effects of landslides, slumps, and surface creep.

 

Yours,

 

Paul H.

Edited July 22, 2017 by Oxytropidoceras
fine-tuned wording - added text for clarification

[Rocky Stoner]

Very well explained and clearly understood.

Just as one would envision.

Thank you Paul.