Paleontology of St. Croix National Scenic Riverway

Article by Justin Tweet, Paleontologist, American Geosciences Institute
for Park Paleontology Newsletter, Spring 2018

Trilobite head parts found in Cambrian sandstone at Marine on St. Croix, Minnesota

Photo taken by Justin Tweet.

In 2017, I undertook a project to document the fossils of St. Croix National Scenic Riverway (SACN). This park follows the St. Croix River and its tributary, the Namekagon River, from northwestern Wisconsin to the confluence of the St. Croix with the Mississippi River. Most of the fossiliferous rocks are found in a portion of the park from the Dalles of the St. Croix to the confluence, near Hastings, Minnesota. Beyond this, above the Dalles, the bedrock is almost entirely composed of Precambrian igneous rocks, which are infrequently exposed because of thick glacial deposits.

Schematic stratigraphic column of St. Croix Valley lower Paleozoic rocks, with common fossils. The dates are for the approximate ages of the rocks, not the geologic periods. The uneven right edge of the column approximates each unit’s resistance to erosion.

Lithologic patterns and symbols for burrows and stromatolites taken from the Federal Geographic Data Committee’s standards. Diagram prepared by Justin Tweet.

Although Minnesota and Wisconsin are not known for their tectonic activity, and are famous for their winter cold, the St. Croix Valley is within a failed 1.1-billion-year-old continental rift, and its sedimentary rocks were deposited in a shallow tropical continental sea when North America was near the equator. These sedimentary rocks were deposited from the Cambrian into the Ordovician, beginning about 500 million years ago. The youngest rocks within SACN are approximately 455 million years old, but they are very rare. Almost all of the rocks exposed within SACN had been deposited by 470 million years ago, in the Early Ordovician, and of these most had been deposited by 490 million years ago, before the end of the Cambrian. The Cambrian rocks are primarily sandstones, with some siltstones and shales between them. The Lower Ordovician rocks are primarily composed of dolomite, altered from the original limestone.

The geology of the St. Croix region was first documented by surveying expeditions conducted by David Dale Owen during the late 1840s. They made the first reports of fossils from the St. Croix Valley, and recognized them as perhaps the oldest fossils then known. They were followed by James Hall, one of the founding North American paleontologists. He described a number of trilobite species from the valley. Hall’s assistants and protégés continued to collect and describe fossils from the valley for decades, some for the Wisconsin Geological Survey, other fossils for the U.S. Geological Survey/Smithsonian Institution. Perhaps the most notable among these was Charles Walcott of the USGS and Smithsonian; he is better known for his work in the Burgess Shale, but his interest in the Cambrian led him to amass a significant collection from sites now within SACN. The names used for the formations and fossils became extremely complicated by the middle of the 20th century, and a series of graduate students from the University of Minnesota conducted research projects to disentangle the problems.

A handful of brachiopod shells, stained reddish, found in Cambrian sandstone (Wonewoc Sandstone) at Hudson, Wisconsin.

Photo taken by Justin Tweet.

The late Cambrian was a sort of “lull” in the history of life. Many of the organisms that had appeared during the Cambrian Explosion had gone extinct, and the next burst of diversification would not take place until the Ordovician. Occasional episodes of low oxygenation of the seas or unusually cold conditions have been suggested as possible causes for the relatively low diversity. The Cambrian fossils of SACN are dominated by brachiopods, snails and snail-like mollusks, trilobites, graptolites, conodonts, and the burrows and trails of unknown worm-like soft-bodied animals.

Brachiopods are shelled filter-feeding marine animals, something like clams, that are still around today. SACN brachiopods are mostly smaller than an inch and often have glossy shells with visible growth bands. The snails and snail-like mollusks that lived alongside them are known mostly from natural molds and casts of their shells, left behind when the original shell material was lost. A site just outside of SACN is famous in geological circles for its fossils of snail-like mollusks and other animals that lived among boulders surrounding an ancient island in the Cambrian sea.

Selected graptolite specimens from the St. Lawrence Formation of Afton, Minnesota, University of Minnesota collections. Black bar is 1 cm in each. A. Holotype (UMPC 4084) of Acanthograptus priscus. B. Holotype (UMPC 4090) of Dictyonema minnesotense.

C. Dendrograptus sparsus (UMPC 4097) and colonial stalk and basal disk of D. edwardsi (UMPC 4092). D. Dendrograptus hallianus (UMPC 4094). E. Holotype (UMPC 4088) of Callograptus staufferi (circled) NPS/Justin Tweet

Trilobites are extinct today, but in the Cambrian they were some of the most abundant and diverse animals. These animals shed their exoskeletons to grow, so most of their fossils are not their bodies, but resistant pieces of shed exoskeletons. Many species have been recorded from SACN, and can be used for relative dating of the beds. Graptolites are an unusual group of colonial animals. They are usually thought to be extinct, but modern pterobranch worms may be living graptolites. Individual graptolite animals lived in cup-like structures in their colonies. The St. Croix graptolites colonies were attached to the seafloor. Many were collected from Afton, but the site was lost to road work.

Pieces of Cambrian siltstone (Lone Rock Formation, Reno Member) with abundant and diverse invertebrate trace fossils, found at Marine on St. Croix, Minnesota.

Photo taken by Justin Tweet.

Conodonts are another extinct group. They were small eel-like animals with large eyes, and were cousins to vertebrates or very early vertebrates themselves. Almost all of their fossils are just their feeding elements, which consist of a variety of microscopic teeth. These are also useful for relative dating. Finally, soft-bodied animals left many burrows and trails in the sand, silt, and shale of the seafloor. Many of these animals will only ever be known by the traces they left, because they lacked hard parts to fossilize.

The Cambrian rocks of SACN have long been a source of notable fossils, documenting a past world that was much different than the present. Where today pine forests and towns overlook the St. Croix River on steep rocky bluffs, a shallow tropical sea populated by shelled animals, crawling trilobites, burrowing worms, and branching colonies of graptolites once existed. Although body fossils are often uncommon, trace fossils can be seen in many places in SACN. If you see any, remember to leave them be!