It preserves fossils dating back to the middle Pennsylvanian period.

The first, located primarily to the south, is mostly limestone with some interbedded shale and sandstone.

The second facies, located to the north and northwest, is mostly gray shale with some thick limestone and thin sandstone beds.

[1] However, in 2004, Kues and Giles recommended restricting the Madera Group to shelf and marginal basin beds of Desmoinean (upper Moscovian) to early Virgilian age, which excluded the Porvenir Formation.

[2] Spencer G. Lucas and coinvestigators also exclude the Porvenir Formation from the Madera Group.