"Early admission" -technically- isn't a thing anymore. You don't apply for early admission or have any separate requirements. Basically, if the U of A thinks your final grade 11 marks are impressive, they'll take you in before first term diplomas. (So basically, you -are- being admitted early, but the U of A doesn't like to call it that anymore, so in case you hear them say "we don't do that anymore" then just know that that doesn't mean you can't get in before most people do.) People who get accepted early don't really have it THAT much better - they just have slightly more peace of mind knowing that all they need now is an overall 70% in the five courses they used to apply to remain admitted (this is called conditional acceptance, all faculties had the same 70% requirement iirc)
Now. While there may not be a target average for early admission, you still need to have an acceptable one to get in early. Like someone else on here said, it all depends on your program. When I applied for Arts, the average was about 72%-ish, and my grade 11 average was 81%. I applied mid-October and got accepted just a little less than a month later. I do know people that got in early for science and engineering - be warned, though. They had averages in the mid-90s. Basically, it's all relative to what the average ends up being for that year. To be safe, just assume the average will be higher than what it says on the website - most programs are in a rising pattern.
Lastly, if you don't get early admission, the good thing about the U of A "not having it anymore" is that you don't have to reapply for regular admission or feel bad about a rejection - because you won't get one! Instead, you won't hear back until April-ish, which is when you'll find most people get their acceptance letters on BearTracks. Worst case scenario is you get conditional acceptance very late, and you bomb your grade 12 year and don't fulfill the 70%. I've seen it once in my group of friends. My advice to you is to make sure you're not sitting on the pass-fail threshold for any course and that you stay above 70% in everything you used to apply.
Sorry for the long-winded reply - hope this helps!