Wednesday, June 27, 2012

After the Cretaceous: The Lincoln Creek Formation

Written May 7, 2012, published June 2014

Following the end-Cretaceous asteroid impact and subsequent dying off of dinosaurs and many large reptiles, 65.5 ma (million years ago), this area was a large shallow warm sea, dotted with volcanic islands, and filled with coral and oyster reefs. Along the east side of the sea, swamps grew on low slopes near the water, near present-day Centralia and Chehalis, WA. Plants grew in these swamps that later formed layers of coal.  In fossil-speak these are called coal swamps. This sea persisted for 50 my (million years), to around 20 ma, in the early Miocene. 

Many marine fossils are found in rocks from this period, including: snails, clams, corals, crinoids, brachiopods, barnacles, sharks’ teeth, fish, whales, seals and turtles. Burrowing shrimp from 45 ma were found in marine sediments; similar shrimp species live in Willapa Bay today.  

These geologic periods had wet warm climates and considerable volcanic activity due to a nearby subduction zone. Water-washed ash mixed with marine silts and sands makes a very good fossil-preserving combination. 

Three concretions and a fossil crab (inside a fourth concretion), were loaned by Karla Nelson for this article. She found these several decades ago while camping on Lincoln Creek in the east Willapa Hills with her family. Photo by Kathleen Sayce
A distinctive round rock called a ‘concretion’ often forms in marine sediments, where as fossilization proceeds, sediments cement together to make round rocks, with the fossil at the center. Concretions form easily with small shells and crustaceans, such as shrimp, barnacles and crabs. 

An outstanding sedimentary rock formation, the Lincoln Creek Formation, is from this period. The Lincoln Creek Formation is 2,000 to 9,000 feet thick, composed of tuffaceous (ashy) siltstone to fine-grained sandstone, and formed 37 ma.  It was originally described from a site on Lincoln Creek, off the Chehalis River in the Grays River Basin, Lewis County, WA, and covers about 1500 square miles in southwest Washington, including areas of Pacific and Wahkiakum Counties. This formation has a good exposure along the Willapa River east of Raymond.  

Mollusks and crustaceans are common in the Lincoln Creek Formation, as are microscopic foraminifera. Crabs are particularly common. Karla Nelson, Time Enough Books, and her family often camped on Lincoln Creek when she was a child, and collected concretions. When opened, these concretions typically contain fossilized crabs. 

Swampy shorelines persisted in lowlands along the west side of the Cascades during the Paleocene to early Miocene Period.  Trees in these swamps included palms and many conifers, mallows, species in the rose family (hawthorn, spiraea, amelanchier, sorbus, prunus, rubus), also gingko, banana, magnolia, and grasses. Specimens of many plant and animal fossils from this period can be seen at the Burke Museum ( HYPERLINK "" ), Seattle, WA. 

The most similar modern analog to those ancient coal swamps is mangrove thickets in the tropics. For an analog of that ancient tropical shallow sea, the most similar area today is Indonesia, including earthquakes, tsunamis and active volcanoes. 

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