The USGS has a distinguished reputation as a national scientific organisation; pretty much by definition, what it says is going to reflect the mainstream scientific viewpoint. As you say, Nbahn, it is hardly going to be a bastion of fringe theories. Regarding Plimer being wrong: I haven't seen a single other source supporting his views - have you? As far as I've been able to determine he is literally the only person making this claim. It seems to be a "minority of one" viewpoint.