John Day Fossil Beds National Monument - Geology

John Day Fossil Beds National Monument Within the boundaries of the John Day Fossil Beds are four major geological groups and over a dozen formations; most of these are highly fossiliferous. The Clarno Group (54-37 Ma) is composed mostly of mud and sandstones. The most prominent feature is the Clarno Palisades, which represent an ancient massive mudflow (called a lahar). Fossilized logs, sticks, and leaves are entombed in the tall cliffs of the palisades. The Clarno Group is also the home of two separate, but equally notable, paleontological quarries, the Clarno Nut beds, and the Hancock Mammal Quarry. The John Day Group (39-20 Ma) is subdivided into three distinct formations: from bottom to top the Big Basin, the Turtle Cove, and the Haystack Valley Formations. The Big Basin Form is composed of clay and mudstones and has both coal and lake (lacustrine) deposits interspersed throughout it. The world-famous Bridge Creek Fossil Flora is found in these ancient lake deposits. The Turtle Cove Member Form is composed of green clay and siltstone and is interspersed with volcanic tuffs that represent "pulses" of volcanism from ancient volcanoes. These tuffs are traceable all over eastern Oregon and can be scientifically dated with great accuracy, thus the many fossils that are excavated in the Turtle Cove Form may also be correlated accurately. The Haystack Valley Form is composed of conglomerate and sandstone and have also provided a great variety of unusual fossil specimens. The Picture Gorge Basalt Group (16 Ma) is part of the larger Columbia River Basalts that once covered much of eastern Oregon and Washington. The Picture Gorge basalts represent a near-vent environment and are composed of layer upon layer of columnar basalt. Between the basalt layers are sedimentary layers of silt and limestone, telling us that there was often a hiatus of time between successive volcanic flows. The Mascall Formation (15-12 Ma) is composed mostly of siltstone and entombs an extremely important fauna of extinct plants and animals. The Rattlesnake Group (8-6 Ma) lies directly above the Mascall Formation and is composed mostly of conglomerates and fossil-rich siltstones. A large volcanic layer, called the Rattlesnake Ash Flow Tuff, separates the upper and lower portions of this formation. In all, the geological formations of the John Day encompass nearly over 40 Ma of depositional sequence that is highly fossiliferous and can be scientifically dated with great accuracy.