Alan Wormington and Kirk Zufelt made the case for Thayer's Gull. Their comments may be of interest to others:
I need to see the spread tail and wing, but this could just as easily by a Thayer's Gull. In fact, it looks identical to a Thayer's Gull that was at Point Pelee about 10 days ago. Could be the same bird! Of all the large gulls, first-year Thayer's and Lesser Black-backed retain an all-black bill the longest. This appears to be the situation here. By now Iceland could / should have some paleness starting at the base of the bill.
If the bird was an Iceland, then the wingtips would be immacualte white. First-year birds can have varying amounts of light brown in the wingtips, but since the bird is sun-bleached any such darkish markings would have disappeared long ago. But brown coloration is still present in the wingtips, which also suggests Thayer's since juveniles have very extensive brownish wingtips and this would be slow to vanish even if severely sun-bleached. Also the darkness on the chest also looks more like Thayer's.
The bird is certainly a first going into second cycle Iceland or Thayer's or intergrade. I concur with Alan on the bill issue but would add that the head shape (Herring Gull-like kinda blocky looking) and bill size seems much more Thayer's Gull like- while Iceland tends to have a rounder "dove-like" head. It would be rather difficult to be absolutely definitive in a very worn bird like this but I would definitely be leaning fairly strongly towards Thayer's.Thanks, Alan and Kirk, for sharing your expertise.