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"What we are seeing here in the KT boundary is not evidence of a massive meteor impact, but rather ____________"

Evidence for a flat Earth.

You see, as the disc moves through the universe with a continuous acceleration of 1G (hence gravity), it collects dust from the ether (hence geological layers). It just turns out that a little after the water from the flood escaped through the borders a few millennia years ago, Earth went through a nebula, which made a very definite layer above the remains of the creatures that didn't make it into the ark.

Rather than alternative science, I'm going to play with alternative scientific history: Your world's scientists just made one mistake in the opposite direction we did. Without the evidence from the Chicxulub crater itself (which you suggest we don't have to worry about), it's all highly plausible.

First, the actual science: As of our current models, a the Deccan Traps vulcanism events took about 800-900Ky, likely spanning the KT boundary, and it's still unclear whether those events could have been sufficient to trigger the mass extinctions.

Now, the history.

In our world, early models mistakenly showed that the events would have taken at least 2My, and started at least 1My too early. This meant the Deccan Traps theory was tightly connected to the gradual extinction theory, and fell along with it.¹ With all of the traditional explanations going by the wayside, people started paying serious attention to the impact theory, even before the discovery and dating of the Chicxulub crater.

In your world, early models instead mistakenly showed that the events took only 200Ky. This meant the Deccan Traps theory was tightly connected to the rapid extinction theory, and rose along with it. It soon became the mainstream consensus, and the impact theory, while not crank pseudoscience, was only explored by a small minority.²

_{1. As it turns out, the rapid extinction theory was probably too far in the opposite direction. But not wrong enough to invalidate the impact theory, and almost certainly not wrong enough to invalidate your world's rapid vulcanism theory.}

_{2. And most likely, like the Deccan backers in our world, the impact backers in your world are spending most of their time on hybrid theories—an asteroid impact triggered the Deccan traps, there were actually two extinction events less than 1My apart, etc.—with about the same effect on mainstream opinion.}

One plausible explanation would be a series of major volcanic eruptions, such as the 2 million year long eruption period at the Permian–Triassic boundary (the Great Dying) 250 million years ago in what is now called the Siberian Traps.

The Iridium signature could be explained as a peculiarity of the mantle plume that created the long eruption period; evidence that something not "on" the world, but possibly from "in" is what caused the extinction event.

Of course, we have to wonder where the basalt field from the K–T boundary has gotten off to. Perhaps it happened on an oceanic plate and subducted under a continent.

The easiest explanation would be a period of increased volcanism. Several volcanoes eject iridium-rich lavas and ashes. The Wikipedia entry for Piton de la Fournaise states,

Many craters and spatter cones can be found inside the caldera and on
the higher flanks of the volcano. Lavas with high concentrations of
iridium are routinely ejected through these vents

A series of mega-eruptions on the scale of the Toba event could well have coated the whole planet with iridium-rich ash, and the epicenter would not coincidentally appear to be in the same general area of the impactor. The signs of the impact itself could then be mistaken for signs of a Yellowstone-scale supervolcano eruption.

My answer will be based on an assumption that your civilisation is super-optimistic and as a result the majority believes nothing really bad can happen. No wars, extensive volcanic activity or anything that could threaten dinosaurs or humans. Not to mention disastrous asteroids. After all asteroids are nice bits of space visiting us sometimes and bringing civilisation advancement, aren't they?

So what really happened?

According to scientists the life on Earth is a chain of evolving civilisations that at some point decide to leave Earth. Every time the civilisation takes as much of the life as possible with them to bring them to some beter future across the skies. We call such events Mass Earth Leave (MEL) and count them from the last (so the newest one is the First MEL or FMEL, next one is the Second MEL or SMEL and so on; if a letter is already used you should use additional lowercase letter, so Fourth MEL will use acronym FoMEL). What are the exact reasons for that are unclear but there are few theories considered. Yet the evidence for MELs is strong, and especially the FMEL has enough traces to give us a rough understanding of the events in the past. It makes us thrive to see the Zero MEL or ZMEL, that is the Mass Earth Leave event of our own civilisation.

We also call the civilisation by the MEL they utilised (or will utilise) to leave Earth. So our civilisation is ZMEL, the one preceding us was FMEL civilisation, their predecessors were SMEL civilisation and so on.

Since the FMEL the nature needed 65 mln years for the next (our) civilisation to evolve. Note that only about 13% of that time is marked with the existence of the Hominini tribe, Homo Sapiens exists for only 350k years and the expansion of a traceable civilisation actually happened in less than 15k years. Also the civilisation really jumped forward in the last 150 years or so and you may assume that it will keep speeding up in a logarithmic scale.

We have traces showing that the SMEL, directly preceding FMEL happened 250 mln years ago so our predecessors had roughly 185 mln years to evolve. Of course the evolution doesn't always happen with the same speed, the SMEL civilisation needed only 54 mln years to get to the point where they were able to perform MEL but there are probably some random factors that expedite the growth of civilisation. Fall of a large meteor is supposed to be one of them as they might bring some materials unknown to those living on Earth at the specific time and that can trigger a search for a ways to create such materials artificially, leading to new discoveries that can be utilised in various ways. There are probably others that supports the evolution of sentience.

The most accepted theory these days says that after each MEL the majority of well developed life forms leave Earth leaving space for creatures that so far occupied niches. Once the great dinosaurs left the Earth at FMEL mammals finally were able to evolve to sentience. Similarly after SMEL there was now space for the reptiles to evolve. We are aware there were plenty bipedal dinosaurs which marks the group similar to Primate. Even though we don't have strong evidence except the apparent MEL it seems that at some point, probably 0,5-1 mln years before MEL the species started to evolve in the direction of sentience and somewhere within probably last 100k years, maybe more a huge civilisation evolved. 65 mln years is enough to wipe all traces of it but the MEL.

At some point two things had to happen.

1. The civilisation discovered something that made them want leave the Earth.


2. They discovered a way to lift entire groups of species into the stars.

With the technology in place and the desired they introduced the plan of leaving Earth, taking as much as possible with them.

Based on the traces found in the so called KT boundary that geologically indicates the time of MEL we can assume that the star travel technology produced as a waste plenty of the Iridium-191 as this is the particle found in plenty in the KT boundary. It can be though a leftover of Mercury-191, Gold-191 or Platinum-191 (*) that were produced instead during lift-offs. There most probable initial launching platform was the Chicxulub crater, however potentially before all the ships launched there were other launchpads built as well, less recognisable at our current technology level for us since the technology also evolved making it possible for less prominent launchpads.

Obviously almost all large dinosaurs were taken with the civilisation as there are no traces of the dinosaurs after the FMEL. Some might argue that we don't have traces of the sentient dinosaurs but there are may be two main reasons for that, probably combined. One is that since the civilised form existed for a very short time we simply haven't found it yet. We do not find all traces, only those that were subject to fossilisation so it might also be that none of the graveyards happened to have the right conditions for it. Other reason is that we might simply not be able to recognise which dinosaurs were really the sapiens ones. All we find is bones, not brains.

There is a small possibility, that the effects of the MEL cause such changes to the climate that most of the remaining species on Earth went extinct. If that's the price we will have to cope with this cost. It can be also a reason why no great thinking dinosaurs decided to stay behind on their mother planet.

So why perform MEL in first place?

As already mentioned - we don't know for sure. This discovery is still before us. There are at least two concurring theories now though worth considering:

A better planet(s) for living had been found

We are only starting to discover planets outside of the Sun planetary system and we are not able yet to decide if a planet is worth getting there or not. But imagine there are multiple planets within pretty easy reach to each other (of course with the technology we do not posses yet as well), habitable for us but giving something extra - extra space, extra life support conditions etc. Alle the MEL civilisations could have found something like that. We might even find our ancestors out there in the deep space one day.

If the space travel technology was developed enough to enable all the habitats of Earth to safely travel to those planets, then why not?

The zero-G conditions are required for further development

This is even more thrilling. We constantly look for ways to extend our lifespan. What if we still miss one factor that makes things harder - gravity? Of course with current technology and evolution stage lack of gravity works against us. But maybe once we really get to the space travels we can evolve even further into life forms that are almost indestructible?

So now you know!

We found traces of 5 Mass Earth Leaves. We could have missed some. Maybe the details of them were different (especially technology invented). But at least we have an idea where to look for our future development and expansions. We can be sure of that - one day ZMEL will come and we will reach out to the stars!

What I missed in the existing answers was that the OP's civilisation doesn't want to believe in anything bad. Most of the answers say "something bad happened, it just wasn't the meteor that caused it.

(*) I was partially inspired by Ash's answer and the chain of isotope by-products (included 1 to 1 in my answer).

Also comments by kingledion and Geronimo plus contradicting approaches of some others lead me to this approach. I want to thank all of you ;-)

Don't blame me if now a new MEL-believers move similar to Flat-Earth believers rise in the near future ;-)

The iridium layer is the product of gradual deposition and natural accumulation over millions of years.

Yes, meteorites have iridium. Iridium dust rains down on us all the time! Little meteors are always hitting the atmosphere and ablating. And what happens to that iridium? It lands on the ground.

And then what? Iridium is heavy! So the fine dust migrates downwards - sinks if you will - into the soil. It will continue to sink until it meets a barrier to sinking any further and there it will accumulate.

Saying that the iridium layer is from one big meteor is like saying the Chilean guano deposits are evidence of monstrous birds that took giant football-field sized dumps. What you see at this KT boundary is millions of years of iridium dust, gradually deposited on the earth meteorite by meteorite and then arrested in its migration to the core by a layer of less permeable minerals.

Still doubtful? Then consider - would one giant clump of iridium impacting the earth produce an even deposition of iridium dust all over the world? Of course not. But a gradual salting of the earth with tiny pieces of iridium coming from every direction would produce exactly that even distribution that is seen at the KT boundary.

I understand it is more exciting and newsworthy to have world killing asteroids crushing T Rexes, and Bruce Willis, and atomic bombs and all that. But not every thing needs to be an summer blockbuster. Walk out of the theater on that summer night and if you are lucky, you will see a colorful shooting star bringing our planet its daily dusting of precious metal.

The creatures of that time where huge and monstrous and a danger to anything alive.
God saw that his creation was corrupted and sent the great Deluge to wipe out the monsters and let only the benevolent and even-tempered creatures live.

When the angels closed the flood gates of heaven to end the Deluge, the water flowed over their shimmering wings, dissolving the iridium out of them. The receding waters spread the iridium evenly over the lands as a blessing of God. This prevented the anew corruption of creatures for a very long time. Therefore the iridium is highly valuable for anyone trying to safe their soul and gain Gods blessing.

If you don't want to recreate Christian myths, God could have set the entire world on fire to wipe out the monsters. When the angels saw that even the "good" creatures burned, they dived down from Heaven to save them. The feathers they lost in the rescue were burned to fine, silvery ashes and spread as iridium over the world.

Okay, I don't answer your question, but I do show that an alternate history is required to keep the other evidence for catastrophic asteroid impacts from being uncovered regardless of what geologists in your society think about the KT boundary. And if your story happens in an alternate universe, perhaps geology develops at a lesser pace and has not yet become precise and sophisticated enough to realize what the KT boundary means.

If the story happens on Earth in an alternate universe, then the probability that the humans will ignore the possibility of asteroids causing extinction level events will be lower than the probability of that happening in some other solar system specially designed to make the danger higher but the natives less aware of it. But the probability of them being oblivious to the danger can be made much higher than it is in our alternate universe.

So I suggest several historical changes that should be necessary to make your alternate universe scoff at the idea of dangerous extraterrestrial bodies and their impacts on Earth.

I think that your premise that discovery of the KT boundary is what made humans aware of the dangers of asteroid collisions is an oversimplification.

The Alvarez impact theory was formed in 1980.

The article "Giant Meteor impact" by J. E. Enever was published in Analog March 1966 and reprinted in a couple of anthologies with six editions between 1968 and 1976.

http://www.isfdb.org/cgi-bin/title.cgi?1154561

As i remember, the article started with a premise familiar to science fiction readers in 1966, that an asteroid or giant meteor was detected heading for Earth and the space force is scrambling to deflect or destroy it. Then it is calculated that the asteroid will land in an ocean and everyone relaxes, because there is no need to prevent the asteroid from landing harmlessly in the ocean, right?

And then the article says wrong, an asteroid landing in the oceans would be an even worse disaster than the same asteroid landing on land, and proceeds to calculate and demonstrate why.

I remember there was a list of suspected astroblemes, or "space scars" on Earth, some of impressively large size. I remember the name of Vredefort in South Africa, which is still listed as the largest diameter impact structure on Earth at 160 kilometers, larger than Chicxulub at 150 kilometers.

The possibility of an extraterrestrial object striking Earth with devastating impact has been the subject of a number of stories for a long time.

Examples include:

The Year 4338; Petersburg Letters (1835) by Vladimir Odoevsky is set a year before Biela's Comet is predicted to collide with Earth.

"The Star" (1897) by H.G. Wells in which a star collides with the Sun.

When Worlds Collide (1933) by Philip Wylie and Edwin Balmer. A rogue planet collides with and destroys Earth.

In the Flash Gordon (1934) comic strip the rogue planet Mongo is on a collision course with Earth, but Emperor Ming says he will change the course to avoid collision.

In the Flash Gordon (1936) movie serial the rogue planet Mongo is on a collision course with Earth, but Emperor Ming says he will change the course to avoid collision.

In the When Worlds Collide (1951) move, a rogue planet collides with and destroys Earth.

In E.E. Smith's Lensman series entire planets were moved as weapons and smashed into enemy planets beginning with Grey Lensman (1939).

Astronomers first discovered lunar craters over 400 years ago, in 1609. For centuries astronomers debated the origin of lunar craters. The main theories were volcanic action similar to the volcanoes on Earth and giant impacts. The impact theory was proposed by Gilbert in 1893, Baldwin in 1949, and Eugene Shoemaker about 1960. Lunar probes and expeditions in the era of Project Apollo proved the impact theory about 1970.

The Mariner 4 probe discovered craters on Mars in 1965, Mariner 10 discovered craters on Mercury in 1974 & 1975, and eventually craters were discovered on all solar system bodies that were examined, except for those with strong geological forces and erosion from gases and liquids resurfacing them.

Meteor Crater, or Barringer Crater, in Arizona has had its origin disputed between volcanic and impact since 1891 and until the meteor impact theory was finally proven by Eugene M. Shoemaker in 1960. And as "Giant Meteor impact" shows, by 1966 there was a list of possible impact structures on Earth.

Early in the 19th century, scientists accepted that rocks sometimes fall from outer space onto Earth. The Wold Cottage or Wold Newton meteorite of 13 December 1795, so famous in fiction, was one of the events that convinced scientists that meteorites are real. And then of course it would be obvious to wonder what damage a vast meteorite the size of an asteroid could do if it hit.

On 30 June 1908 there was a vast explosion - equal to about 3 to 5 megatons of TNT - in the Tunguska region in Siberia, little noted in the outside world. There were seismic and atmospheric effects around the world, and the skies glowed at night for several days. But there was little association with the few reports of an explosion in Siberia. The first suggestion that it was extraterrestrial was in 1921, and the first expedition to the site was in 1927. The asteroid/comet theory gained acceptance within a few decades.

On 12 February 1947 a spectacular meteorite fall was observed in the Sikhote-Alin Mountains in Russia. Many tons of fragments were recovered, and the largest crater was about 26 meters (85 feet) in diameter.

In the Star Trek episode "The Paradise Syndrome" 4 October 1968, an alien planet is threatened with collision with a giant asteroid.

In the Star Trek episode "That Which Survives", 24 January 1969, Sulu says:

Once in Siberia there was a meteor so great that it flattened whole forests and was felt as far away as

http://www.chakoteya.net/StarTrek/69.htm2

Which shows how famous the Tunguska Event had become by 1968. Of course by the future era of Star Trek there could easily have been one or more later massive impacts.

On August 10, 1972, the Great Daylight Fireball was seen over the western USA. An object about 3 to 14 meters, or 10 to 45 feet, in diameter passed through the Earth's atmosphere and then returned to space, reaching as low as 57 kilometers or 35 miles.

On 15 February 2013 the Chelyabinsk Meteor exploded over Russia at a height of 29.7 kilometers with a blast estimated to be equal to 400 to 500 kilotons of TNT, 26 to 33 times the yield of the Hiroshima bomb. The object was approximately 20 meters (65.6 feet) in diameter and weighed about 13,000-14,000 short tons.

So in order for your world's scientist to believe that the KT iridium layer was terrestrial in origin, a lot of things will have to go differently in your alternate universe.

Discovery of the reality of meteorites must happen as late as possible.

Either the Tunguska Event, the Sikhote-Alin event, the Great Daylight Fireball, and the Chelyabrinsk Meteor don't happen, or they happen, but the areas they happen in are totally uninhabited by civilized people so no reports of them reach the outside world of science, causing people to know that city devastating impacts are possible.

The Euro centric scientists of your world will discount rumors from people they consider too backward to report data accurately. This probably means that the Russian expansion into Siberia starting in the 16th century and the American expansion into the trans-Mississippi west in the 19th century never happen.

Such historical changes will probably also help delay the popular science fiction idea of cataclysmic interplanetary collisions for decades or centuries.

There must not be any space race or any space exploration, which means that history has to be changed starting in the 1950s or earlier, which is no problem since the lack of exploration of Siberia and the American west must begin much earlier anyway. Thus there will be no space probes that prove that most objects in the solar system have many craters. And no proof that the Lunar craters are impact structures. And no great flourishing of planetary sciences in the later 20th century.

And above all, there can be no career of Eugene M. Shoemaker (1928-1997), who proved that Meteor Crater is a meteor crater and identified other impact structures on Earth, who proposed that Lunar craters are impact structures, who started searches for Earth-crossing asteroids that might potentially impact on Earth, discovering many, and who co-discovered Comet Shoemaker-Levy 9 in 1993, which was predicted to impact on Jupiter in July, 1994, an impact which demonstrated the almost unimaginable power of impacts and thus made a big impact on our way of thinking about impacts.

I suppose that those historical changes, plus some changes in geological theory, will be enough to keep the society in your alternate universes blissfully unaware of the dangers from cosmic rocks and their impacts until about AD 2018.

Sept. 6, 2018 I may add that in the Young Sheldon episode "Killer Asteroids, Oklahoma, and a Frizzy Hair Machine" March 29, 2018, 9-year-old genius Sheldon Cooper does a science fair project about detecting asteroids and avoid impacts, in the fictional year of 1989.

Of course that is fictional.

It is evidence of alien space-faring life. When the dinosaurs became extinct, an alien mothership went to Earth and hunted the fauna that went extinct with weapons that were produced using iridium. The alien population was so large, they were "quickly" able to wipe out all the dinosaurs by for fun, just as the Sound of Thunder suggests that humans would want to do.

They would have wiped out more life on Earth, but they did not because they did not want the wrath of a potential space-faring civilization that discovers what they did and they wanted to be able to come back again sooner than later to have fun hunting on Earth again.

The aliens used iridium for weapon manufacturing. Some of the factories for the alien's ammunition were left behind on Earth, leaving iridium behind. The homeworld of the aliens had a lot of iridium and the near-light-speed travel of the aliens meant that the iridium would decay a minimal amount. The iridium was most likely stored so that the aliens could get a small amount of energy from the decaying iridium.

Since your human  are blissfully unaware of their own vulnerability, I suppose that a not too violent answer is preferable:

"What we are seeing here in the KT boundary is not evidence of a massive meteor impact, but rather the proof of the existence of a super intelligent and curious alien race.

Indeed my dear colleagues, these extraterrestrial people travel the Universe to find the rarest species created.
Due to the perfect climate conditions during the Cretaceous, Earth permits the creation of one of the most rarest species possible: the dinosaurs.

When they discovered the precious fauna in our planet, the aliens deploy a great fleet to collect the animals.
Due to the use of iridium as fuel, and the size of the fleet, we can still find some traces of their operation. But none of the dinosaurs after that, since they were practically all taken.

And that, my dear colleagues, is the proof that we are not alone in this Universe, and one day, soon, we will meet our intelligent friends."

Your main problem is that there's much evidence that doesn't involve the K-Pg boundary or iridium about large-scale meteorite impacts on Earth and it's not going to take much of a leap to contemplate what the potential results of such an impact might be.

Lunar craters were theorized as being due to impacts as early as the Robert Hooke in 1665, given more detail in the late 19th century, was more firmly developed in the 1920s when people realized what Barringer (Meteor) Crater was and extrapolated from that. While there was still a loud group as late as the mid 1960s who argued for a volcanic origin of lunar craters, it was considered general knowledge they were probably due to impacts. And once people recognized them on the moon, and with Barringer proving they also existed on Earth, people really started looking. The Earth Impact Database, which lists all the known impact structures on the planet, was started in 1955. Charlevoix, as an example, was identified in 1965, and that same year a bunch of other likely impact craters were identified across the Canadian Shield. By 1970, the consensus was that the Sudbury Basin was the result of a massive impact.

Moreover, the idea of things crashing into the planet causing environmental issues already occurred to people long before the iridium anomaly was discovered and the Alvarez's published their theory in 1980. Kelly and Dachille published a paper in 1953 suggesting an impact was responsible for the extinction of the dinosaurs, and there were others over the years that suggested an impact for a cause, but what they were lacking was evidence. That was what the Alvarez team provided with the discovery of the iridium anomaly.