What are the key elements defining the separation between Jurassic and Cretaceous? We have one mass extinction that seperates Trias from Jurassic, roughly huge volcanims over a long period of time, and Pangea starting to divide. 
Another mass extinction that marks the end of Cretaceous period, linked to the fall of an asteroid.


However there is no mass extinction between the two:
What are the crucial elements for defining the limit 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