Answer :
Final answer:
The K-T boundary at Gubbio indicates a discontinuity between the Cretaceous and Tertiary limestones due to a thin clay layer rich in iridium, marking a significant geological event and mass extinction at the K-Pg boundary.
Explanation:
The K-T boundary at Gubbio shows a discontinuity where Upper Cretaceous limestone is separated from Paleocene Tertiary limestone by a thin clay layer. This clay layer is significant because it marks a major geological event. At the K-Pg boundary, a mass extinction occurred that wiped out the dinosaurs and many other species; this is often delineated in the geologic record by a change in sediment type or a concentration of a rare element like iridium.
Geologists can observe these changes in stratigraphy by examining the differences in the way sedimentary rocks, such as limestone and shale, weather over time. Limestones typically resist weathering and form prominent layers, while shales are less resistant. The presence of iridium in the thin clay layer suggests a rapid influx of this element from space (possibly resulting from an asteroid impact), which supports the asteroid impact hypothesis for this extinction event.
