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Interpreting the rocks of Coahuila - a 13 million year story

The remains of the Coahuilan dinosaurs are in the rock layers of the Difunta Group, of the Parras sedimentary basin. Originally, the Parras basin was an area of low elevation, partially covered by sea and coastal lagoons. It extended north of the present cities of Saltillo and Monterrey. Rivers carried mud and sand from deep in the continental interior, south and west of the basin. Over millions of years, the basin filled with sediment. Geologic processes converted the sediment to the rocks we see today with 4,000 meters of thickness, and raised them far above sea level.

The abundant fossils and type of rocks in the Difunta Group suggest there were muddy marshes, shallow seas, land and rivers, with a geography similar to the present Mississippi River delta. These conditions lasted for about 13 million years, from the Maastrichtian age of the upper Cretaceous Period (74 million years ago), until the middle Paleocene (61 million years ago).

Record of the extinction of the dinosaur

One of the most important characteristics of the Difunta Group is that these rocks record the effect of one of the most catastrophic events in the history of the Earth: the meteor impact and consequent extinction of hundreds of plant and animal species, including all the dinosaurs.

It is exciting to confirm this fact in the field by observing a rock layer from which many dinosaur bones and teeth can be collected, along with ammonites and other organisms. And in rock layers above this none of these fossils can be found, only those of the small organisms that survived the great extinction.