remove CRON:Pam_unix entries from rsyslog

Question

PAM keeps filling up my syslog with this everytime CRON switches users.

CRON[18183]: pam_unix(cron:session): session opened for user root by (uid=0) 
CRON[18183]: pam_unix(cron:session): session closed for user root

I do want this written to its own log, but apparently this is using

auth,authpriv.*

and for obvious reasons I don't want to write both of these facils to a separate file.

Is there not a way to control what facility PAM is writing to?

Thanks