What are the key elements defining the separation between Jurassic and Cretaceous?
Short answer, there isn't one. 

The J-K boundary does not have a well defined fingerprint or marker, because of this it is in flux with different studies constantly offering to tweek or redefine its date based on their own criteria.  

https://www.sciencedirect.com/science/article/pii/019566719190001S

https://ui.adsabs.harvard.edu/abs/2017EGUGA..1912252L/abstract

https://www.researchgate.net/publication/236003223_Fixing_a_basal_Berriasian_and_Jurassic-Cretaceous_J-K_boundary-_perhaps_there_is_some_light_at_the_end_of_the_tunnel