The St. Peter Sandstone

Recent mapping adventures have reminded me just how much I enjoy studying the St. Peter Sandstone.  This sandstone was named by Arkansas’ first State Geologist, David Dale Owen, for exposures on the St. Peter River, now called the Minnesota River, in southern Minnesota.  The sandstone is Middle Ordovician in age (around 460 million years old) and during that time it extended all the way from Minnesota into Texas.

It is easy to recognize the St. Peter Sandstone whether you are in Minnesota or Arkansas – clean, sugary, white sandstone.  In fact, here is a photo taken from the place where it was first described, known as the type section, under a bridge near Fort Snelling, in St. Paul, Minnesota.

St. Peter Sandstone in its type area. 

In Minnesota the sandstone easily falls apart.  In Arkansas, the surface of the outcrop is case- hardened meaning there is a hard rind on the rock that forms due to iron-rich water percolating through it and depositing iron on the surface as the water evaporates.  Where this rind is broken, the sand grains fall apart more easily, as at the type area.

The St. Peter Sandstone cropping out in the parking area for the spring at Blanchard Springs Recreation Area.

The contact of the St. Peter Sandstone with the underlying Everton Formation is particularly interesting.  It is unconformable which means there was a period of non-deposition and erosion before the St. Peter was deposited.  It is also undulatory with as much as 20 feet of relief in Arkansas.  The relief is the difference between the top and bottom of an undulation.  Flint, 1956, reports that these undulations can reach up to 200 feet in Wisconsin.  Pretty amazing!

In the photo above, the relief at the contact is probably around 5-6 feet.  Note the rock hammer for scale.  The rock above the hammer is the St. Peter Sandstone while the rock the hammer is resting on is the Everton Formation.  Also notice the curvature of the contact.  The reason for the unconformable undulating contact is that the sand in the St. Peter was deposited upon the karsted Everton surface.  Karst forms when rock such as limestone is exposed to slightly acidic rainwater or groundwater and develops sinkholes, caves, and enlarged fractures. Since this karst surface has been buried by the St. Peter Sandstone, it is considered paleokarst.

The geologic story goes something like this.  After the sea that deposited the Everton Formation retreated, the limestone at the top of the formation was exposed.  Weathering and erosion lasted for up to tens of millions of years, during which time an extensive karst surface developed (Palmer and Palmer, 2011).   Sand was brought into the area from the source area to the north (Great Lakes region) by rivers and wind. Later, as the sea advanced again, it spread the sand over the area filling in the depressions and forming a thick deposit covering a large portion of the mid-continent.

In this photo the relief is approximately 18 feet.  Note the 6-foot-tall geologist for scale.  The red line indicates the contact between the St. Peter above and the Everton below.

The St. Peter Sandstone is relatively resistant to erosion compared to the surrounding rocks; therefore, it is usually a bluff-former.  The tallest bluff I have seen crops out at Blanchard Springs Recreation Area near the group camp and the amphitheater.

Enjoy these photos of the St. Peter Sandstone and hope to see you in the field!

Angela Chandler

Tall (approximately 70 feet tall) St. Peter bluff behind the amphitheater at Blanchard Springs Recreation Area.

    St. Peter Sandstone bluff near Blanchard Springs Recreation Area. 

The St. Peter Sandstone dipping to creek level at the swimming area in Blanchard Springs Campground.     

References and other sources on the St. Peter Sandstone:

Flint, A.E., 1956, Stratigraphic relations of the Shakopee Dolomite and the St. Peter Sandstone   in southwestern Wisconsin: Journal of Geology, vol. 64, no. 4, pp. 396-421.

Giles, A.W., 1930, St. Peter and older Ordovician sandstones of northern Arkansas:  Arkansas Geological Survey Bulletin 4, 187 p.

Palmer, A.N. and Palmer, M.V., 2011, Paleokarst of the USA:  A brief review; in U.S. Geological   Survey Karst Interest Group Proceedings, Fayetteville, Arkansas:  U.S. Geological Survey Scientific Investigations Report 2011-5031, pp. 7-16.

Geopic of the week: Skolithos

 

Skolithos is a common type of trace fossil that has been found in rocks as old as 541 million years.  Trace fossils are not the fossilized remains of organisms but rather the burrows, footprints, and other structures that resulted from the animal’s activities.

In the case of skolithos, it’s widely believed that a vermiform (resembling a worm) animal created the straight, vertical, tube structures.  These worm-like critters probably lived by filtering plankton from the turbulent water of a shallow marine environment.  The vertical tubes may have been a dwelling place to retreat to, though their specific purpose is not known.

In the above picture, captured in north central Arkansas, a sandstone has weathered to reveal skolithos traces permeating the approximately 460 million year old rock.  This example is from an exposure of the St. Peter Formation, Buffalo National River Park, Marion County, Arkansas.

To see more views of skolithos traces from Arkansas click here