The fossils below are what I believe are Grammysioidea sp. pelecypod. It is smaller than it's cousin Grammysia and also lacks the longitudinal groove that is the telltale mark of that genera. As you can see the shell is slightly distorted. This is common in parts of the Mahantango formation and is likely due to post burial deformation by burrowing worms or trilobites.
These next two are the same shell but both valves are preserved.
Another complete shell.
Curious??? Re: deformation. Why organic and not tectonic?
ReplyDeleteWell, Tectonic can be the cause for many of the distortions that I find in the Mahantango but the amount of tectonic force varies from location to location. Were this tectonic then the distortion would be more exaggerated in my opinion and tend to squish or stretch the fossil in the direction of the slipping bedding layers. Also, not all the fossils at this location, or in any particular layer or zone at the site, display the same level or direction of distortion which you would expect if it were Tectonic. Finally the layers are near horizontal with only a slight dip to the northwest. They sit along the south facing slope of a gentle (for the Appalachians) anticline. No evidence that fault lines are present nor slickensides can be found within the rocks either.
ReplyDeleteTo be honest I'm not 100% sure that it's biologically caused but as the formation is rife with the rooster tail like impressions of Zoophycos (and there is a lack of tectonic cues) I tend to lean organic.
I'd be interested to hear your opinion though. Once can't learn until they are told why they are wrong. :)
Sounds like a good explanation to me... especially, the horizontal and not all fossils deformed part of it. Really, I am unfamiliar with the Mahantango, and after some quick reading, realize that it is a very diverse suite of rocks.
ReplyDeleteI'm working on an overall picture of the Mahantango but it's tough since it such a diverse grouping of rocks. It covers nearly the entire Hamilton sequence of rocks (which in New York is represented by five or six separate formations). The sediments grade from thick sandstone to calcareous shale and it can be difficult to correlate different exposures across a wide area due to variation in deposition. It basically records the advance of the Catskill Delta from southeast to northwest. Sadly there are very few good resources that I can draw on to better understand the formation so much of this I'm making up as I go along.
ReplyDeleteIt sounds as if it is as eat up as the Lexington... travel a few km and the facies have a different lithology. Though, fortunately for me, the Lex has been intensively studied for well over 100 years. Some of it is fairly consistent, but some members pinch out, form lenses, or just grade into something else... within a few kilometers!
ReplyDeleteI'll be traveling to Up State Ny sometime this spring. If I make it to the city, my return will be through your part of the country. I don't have a set itinerary, yet, and I am unsure of time constraints, but I would to hit some outcrops up there.