Chris White

Deformed bedding in meta#sedimentary #rocks

The horizontal bedding of the #Squantum Member of the #Roxbury Conglomerate (#Neoproterozoic, #Boston Bay Group) has been disturbed locally.

In the #glacial #tillite interpretation of the Squantum (favored through at least the 1970s), the distortion might be caused by glaciotectonic drag at the ice base. The current interpretation as a #debrite or #turbidite would probably attribute the deformations to synsedimentary slumping. My #photo. #fieldTrip

Jun 1, 2023 at 6:11pmWeb
Show thread*_jߍyropeJun 2, 2023
@Chris White Just out of curiosity: How long does it typically take to deform/fold a sediment? Would that be a very slow process, or could some big animal (or any other being/process) stir a sediment quickly?
Show threadChris WhiteJun 3, 2023
@jrp It…depends…and that’s part of the problem. Big structural folds may evolve over millions of years, and the metamorphic filiation in the Squantum Member probably developed in a timeframe of many millions of years, like hundreds of millions. *My* interpretation of these features is that they occurred during or very shortly after deposition, on a timescale of days to maybe centuries at most. That is, *I think* they are *syndepositional*, not structural or metamorphic.
Show threadChris WhiteJun 3, 2023
@jrp …and these are almost surely not biological, because these rocks predate big organisms. Either slumps/landslides (my favored explanation) or a glacier dragging soft sediments beneath it (common explanation before 1970, for these rocks).