Tag Archives: Unconformity

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.

type section st. peter

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.

st. peter at spring parking (2)

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!

st pete everton unconformity 3

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.

inked739 contact 2_li

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

st. peter ss amphitheater

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

590 st. peter bluff-2

    St. Peter Sandstone bluff near Blanchard Springs Recreation Area. 

dipping st. pete swimming 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.

Geo-pic of the week: Fracture-Fill at Shine-Eye

DSCN5369

The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.

Geopic of the week: Unconformity

 

brownstown and jackfork contact

Pictured above is the contact between two very different rocks: the Brownstown gravel above, and the Jackfork Sandstone below.  This is what geologists call an angular unconformity.  The contact represents a vast period of time.

The lower sandstone was deposited in the Pennsylvanian period, around 300 million years ago, when Arkansas was at the bottom of an abyss.  It was deposited in horizontal layers, but later, when the South American continent collided with the North American continent, it was deformed into mountains and raised above sea level.  For about the next 200 million years it eroded and subsided, until eventually, a shallow sea covered it again.  Rivers washed gravel into that sea, which became the Brownstown gravel.

This picture was taken near De Gray Lake State Park in south central Arkansas.