Steeply dipping bedding - possible unconformity?

[TheBitterEnd]

In a surface dig we have come across an interesting feature, basically one wall of the dig is a steeply dipping (>45?) bed but only a few metres higher there are obvious near horizontal beds. My first hunch was to think that this was either a big block that was just on it's side or something like a slickenside. But having exposed more of it, it is now much more likely to be a steepl dipping bed.

The limestones are probably of Holkerian age (but could be  Arundian or Asbian) and near by there are extensive near vertical beds of a similar age. So the question is could these beds have been tipped up* and then further beds layed down unconformably on top? Or are we looking at somehting else?

Here's a picture looking down, the bed is on the LHS

*I'm sure there is a correct geological term for that?

 

[Brains]

While not impossible I consider the possibility of uncomformity as described unlikely.  I strongly suspect faulting to be a much more likely explanation for the major discrepencies in dip in a small geographical area and within a particular formation. Another possibilty is cross bedding but this is uncommon in limestones. If your dig is near a major fault zone I would imagine this to be the simplest explanation?

 

[graham]

Brains has a point. An unconformity, as described, would be a major feature in the area and thus pretty well known. Like the one at Vallis Vale on Eastern Mendip

 

[TheBitterEnd]

I quite agree, it was more a case of ruling it out as an option. If someone can state with reasonable certainty that there is no evidence for significant orogeny during the  Arundian/Holkerian/Asbian then it must be something else.

 

[susie]

Here's an example of Visean limestone which has apparently horizontal beds above apparently almost vertical beds. This is at Loup Scar between Burnsall and Hebden, and it is simply a fold feature created during the Variscan Orogeny when the rocks on the edge of the Craven Basin were compressed against the Askrigg Block. See page 62 of Tony Waltham's "The Yorkshire Dales. Landscape and Geology".

 

[TheBitterEnd]

Thanks for that Susie, very informative. The exposure of our feature is really poor but I'll have a scratch around at some point.

 

[gus horsley]

I agree with the possible scenarios above.  It could also be a fault having such a pronounced vertical heave that it has bent the beds by dragging them down (or up), which is a combination of the replies above.  Or it could also be a very acute fold which has "broken", which also happens frequently in brittle rocks such as limestones.

 

[Andy Farrant]

Looks like the steeply dipping surface on the LHS is a fault plane (check for slickensides, striations and grooves), whilst the bouldery stuff on the far wall (middle of image) appears to be fault gouge.
Andy

 

[TheBitterEnd]

Thanks Andy, I had initially thought it was a slickenside but above this the beds seem to be may be up to 1m thick whereas there is about 4m of this exposed and no obvious bedding plane to be seen. I agree that it is very likely to be some kind of fault related feature though