What to wear to an outdoor winter wedding?

I'm 18 and attending my first wedding since I was 8. It's for my cousin Leigh (29). I would like to wear something that doesn't makes my family remember me as a 12 year old. I'm 5'7 with long wavy blonde hair and I have an amazing ruched aquamarine blue dress that is mid thigh with a square neck that I look amazing in. The only problem is that my cousins are outdoorsy people and are having their wedding themed after that. I mean the wedding is outdoors on grass, and the reception is outdoors on dirt. In October. On the top of a mountain in Virginia. Where the average temp. is 52 degrees but colder with the wind. See the problem I'm having? I also have some nice black pumps that I would love to wear with it. I don't have that big of a budget seeing as I'm in college and most of my cash is going towards tuition. So I would really like to use what I have. I was thinking maybe some black tights with my leather jacket but I think I would still be cold. Since I will be outside for quite a long period of time. Any suggestions?