Clarno Formation

The Clarno Formation

Outcropping widely throughout central Oregon, the Early to Middle Eocene aged Clarno Formation includes around 1800 m of deposits. It consists of both volcanic rocks and volcanogenic sedimentary rocks, ranging in age from approximately 54 to 39 Ma. The Clarno Formation includes lava flows, tuffs, lahars, mudstone, and conglomerates. These rocks formed within an extensional basin or series of basins near sea level in the Eocene.

 

Clarno Palisades

NPS Photo

From Palisades to Paleosols...

 

Red paleosols in the distance

NPS Photo

A small portion of these sequences were formed when massive walls of mud, ash, and debris came crashing down the slope of a volcano, engulfing the surrounding forest and its animal inhabitants. Over time, the mud, silt, soil, and rocks of these lahars, along with wood, nuts, seeds and leaves from the forest floor, were cemented together by silica. This cementing, or hardening of the rocks, was possibly aided by minerals from nearby hot springs. This combination left a solid cliff made up of sand, silt, and clay that entomb the jumbled remains of a forest, now called Clarno Palisades.


The source of the Clarno volcanic rocks is a broad arc of volcanoes formed by flat slab subduction (slab subducts horizontally or near horizontally) beneath western North America, from the Late Cretaceous through the Middle Eocene. These volcanoes produced a variety of rock that characterizes the Clarno Formation. Furthermore, ash falls from these volcanoes yielded the sediment that formed the repeated paleosols of the upper Clarno Formation.

 

Hancock Mammal Quarry with the Clarno Palisades in the background

NPS Photo

The Hancock Mammal Quarry


Today, fossils found in the Hancock Mammal Quarry (HMQ) are preserved in the siltstones and claystones of an ancient point bar. A point bar forms when sediments such as silt, clay, sand, and gravel drop out as water meandering steam rounds a bend and loses energy, building up a spit of land. Around 40 million years ago, with each successive flood, more sediment layers were added forming the quarry.

 

Fossils from the Clarno and Hancock Mammal Quarry Assemblages

 

The Clarno Formation contains 66 genera and 77 species of petrified wood are found in the Nut Beds, making it the most species rich fossil wood locality on Earth of any age. The Hancock Mammal Quarry represents the only vertebrate quarry deposit within John Day Fossil Beds National Monument.