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Interesting outcome of the DePalma et al #PNAS paper will be implications for study & interpretation of K-Pg 'Event Deposits' close to the #Chicxulub crater. These are fascinating outcrops which *could* record the same processes as 'Tanis' but have proved controversial.. (1/)
Many marine K-Pg boundary sites around the Gulf of Mexico contain evidence for high-energy deposition by processes linked to the immediate aftermath of the impact event such as earthquakes, tsunamis/seiches. But a 'catastrophist' vs. 'non-catastrophist' debate has raged.. (2/)
The 'non-catastrophist' argument has centered on biostratigraphic evidence - i.e. that diagnostic Cretaceous #fossils are found stratigraphically *above* these deposits - and suggestions they are sequence boundaries/lags/hiatuses linked simply to sea level change. (3/)
But could dynamic sedimentary processes operating over extraordinarily short timescales (<1 day) & rapid reworking easily create complex deposits? Timing of processes associated with #Chicxulub can be calculated based on impact models (e.g. impact.ese.ic.ac.uk/ImpactEarth/Im…) (4/)
Seismic waves from large earthquakes, tsunamis/seiche, and ejecta fallout could affect surrounding coastlines within <1 hour of the impact event. In the deepwater Gulf, evidence exists for enormous sedimentary deposits associated with the impact pubs.geoscienceworld.org/gsa/geology/ar… (5/)
Seismic deformation of #Cretaceous sediments immediately underneath those containing in-situe impact ejecta have also been described from deepwater sediments in Colombia, consistent with widespread seismicity from Chicxulub doi.org/10.1130/G40224… (6/)
In 2018, @AMNH colleagues and I described a temporarily exposed K-Pg boundary site from #Mississippi, which like DePalma et al., could contain a record of the initial aftermath of the impact event in the ocean.. doi.org/10.1016/j.cret… [OA here: doi.org/10.31223/osf.i…] 7/
Although not much to look at, and only 4.5 m-thick. The '4th Street' site contains abundantly fossiliferous uppermost #Cretaceous sediments overlain by a ~30 cm-thick K-Pg unit full of impact ejecta (above the resting pickaxe to the right).. 8/
The #Cretaceous rocks contain #ammonites and other mollusks, as well as microfossils consistent with deposition right before (<500 kyrs was our conservative estimate) the K-Pg boundary #gratuitousammoniteshotincoming 9/
The boundary rocks overlies these, in-fills burrows in the uppermost #Cretaceous, and is made up of different units - we ID'd some sedimentary structures (loads/flames, parallell lamination) that looked like evidence for rapid deposition. It is also full of impact ejecta.. 10/
These ejecta spherules are what link this bed to #Chicxulub - little pieces of what were once the Yucatan Peninsula that ended up in northern Mississippi! They are altered to clay, but despite being delicate, beautifully preserved and scattered throughout the bed. 11/
The 'Spherule bed' also contains a #Cretaceous marine fauna - incl. ammonites, bivalves, gastropods, sharks, even crustacea.. No #Paleocene fossils to be seen. Crucially some #ammonites are in-filled with ejecta suggesting they were hollow immediately prior to deposition.. 12/
You can see some impact spherules (circled) associated with this #ammonite on the left (the long white object) - broken open to show it was hollow. On the right is a large chunk of reworked #Cretaceous sediment which suggests energy levels high enough to erode the seafloor. 13/
How did this deposit form? There was a range of opinion among my co-authors as to how conservative our interpretation should be.. In the end, we suggested this unit does record high-energy, rapid deposition associated with #Chicxulub - but couldn't say much about timescale.
..but it doesn't look like a 'lag' from sea-level changes. Interestingly, there was evidence for sea level change *above* the spherule bed. Which suggested to us that in places, the evidence of these 2 things could get mixed up & complicate the record. 15/
I hope the DePalma et al. paper will stimulate further study - as a student I was taught to be skeptical of 'catastrophes' in the rock record. But working on this site (& others) I think when the depositional stars align, evidence of pretty remarkable things can be preserved.
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