First step will be for you to get your building code requirements from your township code department. They may not have any specific requirements for such a deck but you should ask them first. They may only specify footer depth.
A better plan might be to abandon traditional deck construction techniques.
See if this will fly with local building department...
Level the area and fill with 4" deep crushed stone
lay 4x4 frame
in other words, no footers. Build it right on the ground on the stone. This has multiple benefits.
1) Its not attached to house or footers and may not be considered permanent. (maybe no permit)
2) You wont need to worry about sag or spans due to limited 4x4 strength
3) You wont need to dig to frost depth for footers
You will want your 4x4s spaced at 16" for 2x decking, 12" for 5/4 decking.
Use pressure treated.
One new technique I have seen is to put a 'cover' on the joist to prevent rot. if you use a 4x4 this could be more important as there will be more places for water to hold against the wood. Grace (the company name) has a product called Vycor - its a self stick membrane. Stick that to the tops of each joist of for long lasting protection.
The Handyguys Podcast [url]http://www.handyguyspodcast.com[/url]