Looks like a Common Snapping Turtle.
There are two Snapping Turtles in North America, one is the Common Snapper and the other is the Alligator Snapping Turtle. I'd keep my distance from either one. They quicker than you can imagine and they mean business.
My experience with Snapping Turtles is that if you leave them alone, they just want to go about their lives peacefully.
Incredible animals who have been on the planet for an incredible amount of years and are very worthwhile to man as they scavenge a lot in freshwater bodies of water.
A Loggerhead is a completely different turtle. They are Sea Turtles...ocean going.
BTW, nice pix and using a telephotos like a Sigma 170-500 was a great choice...you can get a good picture of a wild animal from quite a distance. I have a Sigma 150-500 and do the same as you.