Somebody told me about this, as she had been to a geological meeting with her son where they were showing off a sample of plastic in rock. I later put it into my search engine and up popped a number of stories from last year that I had failed to appreciate or the implications involved. I'm still not sure as sedimentary rocks are usually assigned long periods of time in order to accumulate – and we are talking about what sounds like instantaneous accretion (or so it would seem). Of course, academic geologists consulted put a bit of a damper on the issue, as they clearly felt not enough time had passed – but the general idea that bits of plastic could become one item among others within a conglomerate rock was possible, and almost certainly likely.
At www.livescience.com/46057-human-trash-becomes-new-plastiglomerate-rock.html …. http://news.sciencemag.org/earth/2014/06/rocks-made-plastic-found-hawaii… … and at www.newstatesman.com/print/node/204126 … the story is that plastic waste is changing the geology of the earth's rock. Tiny pieces of discarded plastic are forming a new layer of sedimentary rock according to researchers at the University of Western Ontario and the Algalita Marine Research Institute in California. It is actually combined with volcanic rock, sea shells, sand and corals, a nice conglomerate mixture. They were stumbled upon on the Big Island of Hawaii and may have been formed from melting plastic that had been burnt on camp fires or by fishermen using plastic or fibre glass tackle – see www.geosociety.org/gsatoday/archive/24/6/pdf/i1052-5173-24-6-4.pdf
Given the hype involved as it happens to suit the faction claiming an anthropocene geological period it is something that in all probability will happen in the future – or during a volcanic event (as heat is the apparent vector). In this case the plastic has been cemented with other sediments as a result of a camping fire – hardly the stuff geology is made from. Volcanoes, lava flows, landscape fires given available plastic debris, could potentially do the same thing. Melted plastic cements various fragments together. It can also fill in cracks in already formed rocks – providing an additional sedimentary feature to the rock.