With the continued uplift of the Rocky Mountains in the west, the Tertiary (from 65-1.65 million years ago) of Oklahoma is marked by a large volume of non-marine gravel, sand, silt, and clay that was eroded off of these highlands and deposited along ancient river valleys flowing eastward across most of the state. These ancient rivers that flowed off the young Rocky Mountains during the Tertiary formed the general pattern of east-flowing river drainages we see today (Johnson, 1996). Along with river deposits, much of the Tertiary is characterized by lake, as well as windblown sand deposits. Remnants of these ancient rivers, lakes and windblown sands are exposed across the greater part of the High Plains of western Oklahoma, and are represented by deposits of the Ogallala Formation. This formation can reach over 500 feet in thickness, and may have extended as far east as Tulsa. Post-Tertiary erosion, however, has limited the formation to the western extreames of the state.
Coincident with uplift of the Rocky Mountains and the formation of the Ogallala Formation, the western parts of the Great Plains States saw an increase in volcanism during the middle part of the Tertiary. The most famous volcanic deposits that formed due to the uplift of the Rocky Mountains are those of Yellowstone National Park. Closer to home, volcanism was occurring in northern New Mexico and south-central Colorado. One such volcano in Colorado erupted, and formed an extensive lava flow of basalt. This basalt flow covered much of the region west of the Oklahoma Panhandle, and a part of it caps Black Mesa in Cimarron County today.
One major characteristic that separates Tertiary-age rocks from those of the Mesozoic is the absence of dinosaurs. Gone are Tyrannosaurus rex, Stegosaurus, Triceratops, and all of the other behemoths that we know and love. A great cataclysm occurred at the end of the Cretaceous that killed off all dinosaur species, called a mass extinction. This extinction may have been caused by a meteorite or comet impact, or was due to a worldwide regression of the ocean, or to a severe disease that infected the great ceatures, or it was due to any combination of the above. Whatever the cause, by the beginning of the Tertiary no dinosaur ever roamed the Earth again, and once the dinosaurs became extinct a new type of animal, the mammal, would dominate the surface of the Earth. It is for this reason that the Cenozoic Era (which includes the older Tertiary Period, and the younger Quaternary Period) is known as the ‘Age of Mammals'.
Tertiary rocks of Oklahoma formed from rivers and river terraces can contain abundant fossils forms of these new mammals, including the remains of horses, camel, mastodons, and rhinoceros, as well as petrified wood. Lake sediments from the Tertiary contain more familiar and simple forms such as snails, clams, and fossil algae (Johnson, 1996).
To glimpse what Oklahoma may have looked like during this time click on Paleogeography of the Tertiary.
Reference: Johnson, K.S. 1996. Geology of Oklahoma, p. 1-9. In, K.S. Johnson and N.H. Suneson (eds.), Rockhounding and Earth-Science Activities in Oklahoma. Oklahoma Geological Survey Special Publication, 96-5.