What makes good deck footing substrate on clay soil?
Alaska Man is right in that any deck attached to your structure must have the same type of frost footings that the structure does, otherwise it's likely to put strain on the building that it shouldn't.
Wooden deck posts are almost always set on concrete footings themselves--either poured or pre-cast. Only non-bearing fence posts should be on washed rock or gravel alone. For your purposes I'd probably put a few inches of washed rock down, then pour or place concrete pads on that to support the footing.
Your local inspection office will have specifics for you, even if you ask about it hypothetically. They'd know what works reliably in your area, and at what depth.