I have a hard time believing plastics will persist as plastics. It doesn't take much heat to denature most plastic, and geologic sediments tend to experience a good bit of pressure and heat over time.

I doubt that they will be the same after a few million years, no. But what kind of rock will it make? Won't it be unlike anything else?

Aren't plastics hydrocarbons? Would the hydrogen be driven off (cracked) leaving something like coal.

I think that we might be more likely to find concentrations of metals left over from artifacts of civilization. If so, would these look different from naturally occuring metals?

Yes - I find it hard to imagine I-beams couldn’t be found embedded in a geological layer somewhere.

Very few large things make it into the fossil record ( https://news.ycombinator.com/item?id=16837795 ), and a small fraction of those are discovered. It's possible, but I-beams are rare compared to plastic particles, which are basically carpeting the seabeds, rivers and beaches now.

"microplastics everywhere". http://www.bbc.co.uk/news/science-environment-43363545

"Microplastic pollution in oceans is far worse than feared, say scientists", "pervasive and persistent nature" https://www.theguardian.com/environment/2018/mar/12/micropla...

"The deep sea is a major sink for microplastic debris" http://rsos.royalsocietypublishing.org/content/royopensci/1/...

We don't know what the effect of millions of years on this debris will be, but I am suggesting that due to the fact of the human species as a geological force, most of the rocks from this era will be a bit unusual in composition.

I-beams would dissolve into rust very quickly.

Your best bet for metals might be gold as it is not typically reactive with water or oxygen. And as a reminder, we do sometimes find solid blobs of gold... :-)

Agreed about the gold. That came to mind also.

I also agree that the I-beams would rust away, as would rebar embedded in concrete. Would they remain as identifiable shapes or would the material migrate?

"remain as identifiable shapes" is basically fossilisation, which does happen. And it likely applies to I-beams as much as it does to coke cans, plastic bottles and cigarette buts. Except that the latter are going to be much more numerous.

Maybe WE are the waste.... Former civilization created organic automatons for amusement. The civilization is long gone, but their relics continue on to this day in the form of 'life'.

So you're pretty sure the OP's geologist friend is mistaken?

I'm not certain of anything, but as for "OP's geologist friend" I would like to hear the comments of someone with the relevant domain knowledge on the long-term fate of all of these plastic particles.