Didn't realise there was a thread on this, but the excavations at the Tanis site have been ongoing for over a decade, mostly under a veil of secrecy because they didn't want a frenzy of fossil hunters destroying parts of the deposit. There'd been rumblings and rumours about it in geological circles but nothing concrete until it hit the presses in 2019 or so.
en.wikipedia.org
The Hells Creek formation had already been under intensive study for at least sixty years since it's one of the key strata where the K-T boundary is evident and one of many deposited where the original "iridium anomaly" was discovered. Even ignoring the timing of the event, the site has already yielded a plethora of unique fossils and I think is one of the most recent lagerstatten sites (read: awesome fossils) discovered. Sadly there doesn't seem to be a huge amount of pictures of them available on t'interwebs yet and the Attenborough doco is probably the best presentation I've seen of them so far.
As noted, it was the seismic shock from the impact that indirectly formed the Tanis deposit from what was originally a sort of
inland sea. The exact height of the waves that would have hit the Tanis site are hard to predict but would have been at least 10-100m tall. The tsunamis that followed in the seas and oceans as a direct result from the impact were still relatively small at only 100m or so high (if the impact had landed in deep water the resulting waves could have been an entirely unsurfable 4km high).
Statistically speaking, given the amount of death and debris there was at the time, fossilisation around a catastrophic mass-extinction event like this is
more likely, but yes we're incredibly lucky to have found it (as noted above, the whole formation has been under intensive study since the 60s and it was still only found in 2008). The specifics of it are that the seismic waves from the impact caused big seiche waves* (the sort of big waves you can create in the bath if you move the water at the right frequency) in certain sites very quickly (within five or ten minutes of the asteroid impact) and buried the flora and fauna before anything else hit it - so as well as smooshing a whole bunch of land and water animals and mud together in a matter of minutes (speed is usually a key requirement for exceptional preservation), they were protected from subsequent damage from the rest of the events of the impact.
* Seiche waves end up producing some interesting geological features but a personal favourite of mine from a geological disaster/cautionary tale perspective that not a lot of people seem to know about is the Vajont Dam in Italy. To cut a long story short, corners were cut short in the geological survey of the site and an adjacent mountain essentially fell in to the lake, creating a 200m-high seiche wave that overtopped the dam and obliterated several towns and villages and about 2000 people. The dam is still there, more or less undamaged. If you find yourself in the NE of Italy it's well worth a visit if you like that sort of thing.
en.wikipedia.org
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