All of my trips except for 2006 have been to Western Australia, where I’ve done field work as a grad student and in my part-time career as a geologist. I completed my Ph.D. research on a suite of volcano-produced underwater landslide deposits (fine-grained turbidites and other density flows comprised of hydrovolcanic material for those of you who want to know). Since 1993, my colleague Bruce Simonson and I have worked on a set of meteorite impact deposits.
Let me explain this briefly. The Hamersley Basin strata – the sedimentary rocks – that we work in were deposited on the ocean floor, between about 2.7 and 2.4 billion years ago. Yes, that’s billion: they’re a little more than 50% of the Earth’s age. This was long before multicellular life evolved, although there was plenty of slime such as cyanobacteria around. These rocks have subsequently been uplifted and exposed on land in Western Australia. They are in exceptionally pristine condition. Unlike most rocks of their age, they’ve experienced essentially no mountain building, and remain flat-lying, as they were formed. It’s possible to trace individual layers for tens of kilometers and sedimentary features haven’t been destroyed. Most sedimentary geologists would kill for this kind of availability. The rocks are also largely unaltered by post-depositional chemical processes. Many close to primary textures and minerals are preserved. Finally, the lack of advanced life forms means that the physical structures that formed when the sediments were deposited remain undisturbed. Such features in more recent rocks are commonly erased by burrowing marine organisms – worms and the like. The point of this – the Hamersley Basin is a great place to study sedimentary rocks.
On our first trip we found a distinctive sedimentary layer that we called the Spherule Marker Bed: the SMB. Unlike all the other Hamersley Basin strata we’d seen, it contained lots of sand-sized spherical particles. Within a year, Bruce figured out that these were best interpreted as material that was deposited in the Basin after a very large meteorite impact. When an asteroid a few kilometers or so in diameter hits the Earth, much of the asteroid and the target area it hits are vaporized or converted to molten rock. This material blasts up into the atmosphere, and probably into space around the earth. Somewhere in the process, the vapor condenses into spherical droplets, which together with the molten rock rain back onto the Earth’ surface. The SMB is one of these deposits. In this case the impact debris ended up on what was then the ocean floor. Bruce deserves full credit for sorting out this interpretation, which is pretty firm. The spherules don’t look like normal terrestrial particles. Geochemical work also points to an extraterrestrial impact.
My focus has been on what happened after the impact. The SMB isn’t a simple layer. It shows much evidence of catastrophic reworking by both tsunami and underwater landslides (high and low density turbidites). My best interpretation of what happened is that the impact took place in the ocean. After the layer formed on the seafloor, tsunami generated by the impact arrived, eroded and then redeposited this bed. The tsunami continued on, and struck relatively nearby land areas. Tsunami run-up and backwash triggered or contributed to the subsequent underwater landslides, which again eroded the original layer and deposited a new one – the SMB we see today.
A final exciting part of this work has been discovery of more than one impact deposit. We’ve found at least three more, each of which represents a different impact. They’re both above and below the SMB, separated from it by hundreds of meters of rock. All of them show evidence of extreme reworking like the SMB, which has strengthened my interpretations. Moreover, all of these impact deposits have been found in South Africa, in rocks very similar to the Hamersley Basin. This is important as it lets us correlate time. The impacts occurred essentially instantaneously on a geologic scale, so, they are like time lines that tie the two areas together.
All of these discoveries; the SMB and the other layers, their sedimentary histories, and the intercontinental correlation are important to understanding Earth history. These layers are arguably the best-preserved impact layers of their age. They're in much better shape than most younger layers. The depositional history I’ve worked out is important in understanding what might happen in the future. There are plenty of meteors, asteroids, and comets in the Solar System that might strike the Earth someday. If you want to worry about this, visit www.spaceweather.com.
