I would like to put in my 1 cent worth. I think the bird will be just as confusing 
as an adult as it is a Juvenile. To me, these photos are fantastic. To be able to 
have this discussion in my living room with hundreds of other birds is so cool. Anyway.

I think that this bird markings, from tail to neck are thru and thru Juvenile Thayer's. 
I think the head shape and bill shape are that of a typical Iceland Gull. In fact the bill 
is even shorter than the recent Juvenile Iceland's I've had this week. But still comparable. 
The darkest Juvenile Iceland I have ever seen was at Conowongo Dam about 10 years 
ago and didn't compare to this bird. This Gulls primaries are perfect dark brown with 
typical pale edges. The bird is in the sun light with Ice reflecting on it and it still has a very dark head.
I have desperately been looking for something this close at the Cecil 
landfill with no such luck. My Vote/Guess, goes for Hybrid/Thayer's, with Thayer's being the prominent
gene.
Thanks.
Sean McCandless