San Jose Formation


The San Jose Formation is a Paleogene geologic formation in the San Juan Basin of New Mexico and Colorado.

Description

The San Jose Formation is mostly sandstone but with some mudstone beds. The formation was deposited by high-energy streams on a muddy floodplain that was the last preserved sedimentation episode in the San Juan Basin. Stream direction was from the northwest, north, and east towards the south.
The basal Cuba Mesa Member is found throughout the depositional basin and is a coarse-grained sheet sandstone. The Regina Member is floodplain mudstone and disconnected sheet sandstone that intertongues with the sandstone-dominated Ditch Canyon Member in the northwestern part of the basin and the LLaves Member on the east side of the basin. The uppermost member is the siltstone-dominated Tapicitos Member. The Cuba Mesa Member was deposited during subsidence in the basin center, while the later members were desposited during episodes of monoclinal folding along the Nacimiento Fault on the west escarpment of the Nacimiento Mountains.

Fossils

The mudstone beds of the San Jose Formation are locally rich in fossils. These include the Almagre and Largo faunas of the early Eocene.

History of investigation

In late 1874, Edward Drinker Cope was a member of the Wheeler Survey but ignored orders from Wheeler to proceed north from the Chama River to Colorado. Instead, he headed south to explore the area north of the San Pedro Mountains, where fossils had been reported. He found early Eocene fossils in the badlands north of Regina, New Mexico and spent about seven weeks collecting fishes, reptiles, and what was then the oldest known mammal fossils in North America. He also collected the fossil of a giant flightless bird. Cope later wrote his father that this was "the most important find in geology I have ever made".
In 1948, G.G. Simpson visited the same area and assigned the fossil beds to the San Jose Formation. In 1967, the formation was divided into the Cuba Mesa Member, Regina Member, Llaves Member, and Tapicitos Member.