question archive Describe the Chuckanut Formation
Describe the Chuckanut Formation
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Describe the Chuckanut Formation. Include in your answer age, rock types, depositional environment, fossils and any other important and interesting information!
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- The Eocene Chuckanut Formation consists of a phenomenally thick sequence (9000 m) of alluvial sandstone, conglomerate, mudstone, and coal, originally deposited in flood plains in subsiding basins near the coast of Washington- or at least, where the coast was.
- It is a fluvial sedimentary formations of Eocene age, deposited from about 54 Ma to around 35 Ma.
- The original Chuckanut formation appears to have been deposited as a single unit in a large basin, and subsequently separated by faulting.
- It predominantly consists of fine- to medium-grained sandstones with lesser amounts of interbedded shale, conglomerate, and coal which were laid over a metamorphic suite of rocks, those under the Chuckanut now known as the Shuksan, and its correlate under the Swauk known as the Easton.
- Eocene fossil trackways are found in the Chuckanut Formation, composed of birds,mammals, and some turtle tracks.
- It is thought that the Chuckanut Formation shoreline was of riverine environments combined with a large scale bedding plane of material deposition.
- The diversity and abundance of bird and animal tracks preserved in Eocene strata of the Chuckanut Formation in Washington contrasts to the scarcity of body fossils. These ichnofossils were made by vertebrates that inhabited river margins, the only depositional environment favorable for track preservation.
- The discovery of tracks only at Chuckanut Formation sites that expose large bedding planes indicates the importance of considering outcrop architecture during the search for vertebrate ichnofossils, and inspires the hope that similar fossils may eventually be found in correlative formations in the Pacific Northwest.
