IMFUFA Seminar: The missing section in the geological record

– evidence of stability or of subsidence followed by uplift? How physical observable can help fill in the gaps in the geological record ved Peter Japsen (GEUS)
Start:kl. 14.00
Slut:kl. 16.00
Sted: Roskilde Universitet 27.1, lokale 1

The wonderful exposure of million-year old limestones at Stevns Klint is a UNESCO World Heritage site. The cliff records the devasting meteor impact 66 million years ago that caused mass extinction, and we can study the many forms of life in great details before and after the impact. But what happened after the deposition of the uppermost limestones that are visible on the cliff? But what happened during the sixty or so million years that separates the uppermost limestones from the trees that grow on top of the cliff? Did nothing happen? Or did the area subside so that younger sediments could accumulate and did the region at some point experience uplift so that the accumulated sediments could be removed by rivers? For many years, nobody asked these questions, but based on a study of sonic velocities in chalk, I was able to conclude that the limestones now exposed on the cliff, had been buried below some 750 m of sediments before they were eroded away after a phase of uplift most likely began 5 million years ago.

This example shows that the geological record is incomplete. This has huge implications for our understanding, not only for the geological history of the earth, but just as much for our understanding of the processes in the earth’s interior: do these processes keep the earth’s surface relatively stable or do they provide an unstable substrate for a wobbling surface? Where no geological record is present, we need insights from physics to unravel these past movements. Observables from geological units preserved below the ‘missing section’ can give us such information – have the rocks been hotter or have they experienced higher stress in the past? In the presentation I will briefly illustrate such methods.


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