c# Get all enum values greater than a given value?

I have the following enumeration of membership roles:

public enum RoleName
{
    RegisteredUser,
    Moderator,
    Administrator,
    Owner
}

I want to be able to fetch all roles greater than or equal to a given role.

For instance I input Administrator and I get an IEnumerable with RoleName.Administration and RoleName.Owner

Something of this sort:

public static void AddUserToRole(string username, RoleName level)
{
    var roles = Enum.GetValues(typeof(RoleName)).Cast<R>().ToList().Where(role => level > role);

    foreach (var role in roles)
    {
        Roles.AddUserToRole(username, role);
    }
}