Marriage certificate - for widower Father GC

I am a US Citizen looking to file GC for my father. My mother passed away 3 years back . My parents were married in 1968 and do not have
a marriage certificate. They do have some B&W photos of the marriage. Is the marriage certificate a required document for filing GC for father?
We have my mother's unexpired passport that lists my father's name as the husband , and we have her Death certificate specifying his name
as the husband. Is this enough ?
Also, few years back when I applied for my employment based GC, my parents have given affidavits to me that they are my parents, can I use this
as a supporting document ?
Please suggest, thanks in advance.