Mesilla Valley Shale facts for kids

The Mesilla Valley Shale is a special type of rock layer found in southern New Mexico, northern Chihuahua (a state in Mexico), and far western Texas. This rock layer holds amazing fossils that tell us about life from a long, long time ago, specifically from the Albian age of the early Cretaceous period. That's about 100 million years ago!

This formation is really easy to see at a place called Cerro de Cristo Rey. Here, it's part of many thick rock layers from the Cretaceous period that have been pushed up. These layers show how the Western Interior Seaway (a huge ancient sea) moved back and forth across the land during the middle of the Cretaceous period. When the sea advanced, it's called a transgression, and when it retreated, it's called a regression.

What is the Mesilla Valley Shale Made Of?

This rock formation is mostly made of dark green to olive-colored shale. Shale is a type of rock formed from mud or clay. It also has some thin layers of siltstone (rock made from silt, which is finer than sand but coarser than clay). You can also find highly fossil-rich layers that contain a lot of calcium, called calcareous beds.

The Mesilla Valley Shale sits directly on top of the Muleros Formation and is covered by the Mojado Formation. These layers are "conformable," meaning they were laid down one after another without any major breaks or erosion between them. The total thickness of the Mesilla Valley Shale is about 210 feet (64 m).

Scientists like Lucas and his team have divided this formation into three main parts. They believe the bottom two parts show a complete cycle of how sediments were deposited, and the top part is the beginning of a new cycle. This formation was formed in the upper to middle part of a continental shelf, which is the edge of a continent that is covered by shallow ocean water.

Fossils Found Here

The Mesilla Valley Shale is packed with fossils! Scientists have found at least 20 different groups (genera) and 23 different kinds (species) of fossils here.

Ancient Sea Creatures

Many of the fossils are molluscs, which are creatures like clams and snails. Some examples include Lima wacoensis, Lima mexicana, Pecten texanus var. elongatus, Pecten subalpinus, Plicatula incongrua, Ostrea quadriplicata, Texigryphaea washitaensis, Trigonia emery, Helicocryptus mexicanus, and Turritella granulata Sowerby var. cenomanensis.

You can also find echinoderms, like the sea urchin Heteraster bravoensis. These fossils are typical for the Albian age.

Other Amazing Fossils

Besides molluscs and echinoderms, the formation also contains:

• Ammonoids: These are extinct sea creatures with coiled shells, related to modern squid.
• Brachiopods: These are small, shelled marine animals.
• Foraminiferans: Tiny, single-celled organisms, especially Cribratina texana.
• Dinoflagellates: Microscopic organisms, some of which are plant-like.
• Serpulid worms: Tube-dwelling marine worms.
• Corals: Small marine animals that often form colonies.
• Ostracods: Tiny crustaceans with a bivalve-like shell.
• Calcareous algae: Algae that produce calcium carbonate.
• Some pieces of ancient land plants.

Trace Fossils: Clues from the Past

The formation also has trace fossils, which are not the actual remains of creatures but rather signs of their activity. Think of them as tracks or burrows left in the mud that turned into rock. These include Ancorichnus, Arenicolites, Bergueria, Bichordites, Cardioichnus, Chondrites, Cochlichnus, Coprulus, Gordia, Helicodromites, Lockeia, Ophiomorpha, Palaeophycus, Planolites, Protovirgularia, Rhizocorallium, Skolithos, Spongeliomorpha, Taenidium, Thalassinoides, Treptichnus, and even a biofilm called Rugalichnus.

This wide variety of trace fossils was preserved in layers formed by storms (called tempestites) in water deep enough that waves didn't usually reach the bottom. All these trace fossils are typical of the Albian age. The presence of Chondrites and other trace fossils that suggest low oxygen levels tells us that these marks were made during an ocean anoxic event. This was a time when large parts of the ocean had very little or no oxygen, which happened about 100.6 to 100.2 million years ago (Ma).

How We Learned About It

The rock layers that make up the Mesilla Valley Shale were first described by a scientist named E. Bose in 1906. He called them "subdivision 6" in his study of rock layers. Later, in 1976, another scientist named W.S. Strain officially gave this subdivision the name "Mesilla Valley Shale."