Alternate document for Marriage certificate for family base immigration

[nilpa.patil]

Hi,

I am a US citizen. I am planning to file family base immigration for my parents. My parent got married in 1969. They  don't have any marriage registration or certificate at this point. They were few month short on marriage age required by Indian marriage act ( 21 year & 18 year). I don't think so they can get marriage register at this time due to above age issue. What are the other alternative document which can accept by USCIS in absence of marriage certificate?   How should I proceed in this situation? 

Thanks again for your help and guidance. 

Thanks 

[directory0818]

affidavit from any person who attended the marriage would suffice.