re: finally, chemical variation...
This is what I've been waiting for, some chemical variation to the sediments. If this were the classic evaporite sequence, and these rocks are increasing in chlorine down-section, then it is upside-down. Or is it actually a lateral variation such as a sabkha might produce? (I'm working on a chart on too, but I've been consumed with other things.) Or is it a function of upwelling brines? 
The contact with the basalt dunes is going to be interesting too. Is it a sharp contact? Is it gradational? Is it inter-fingered?
Finally, real stratigraphy.
~jms~