Sandstone Paleokarst

If you have spent any time on Beaver Lake in northwestern Arkansas, then you have probably seen sandstone paleokarst features.  Some stand tall like towers while others appear to be irregular to rounded masses.  It is common to see only the tops of these features when the lake level is low to normal.

     Top of sandstone mass in Beaver Lake.  Photo taken in October, 2016.

  Sandstone mass along Beaver Lake.  Photo taken in October, 2016.

These features have been in geology literature since 1858 when David Dale Owen made his first geological reconnaissance of the northern counties.  He described a mass of isolated sandstone within adjacent magnesian limestone (now called dolostone) which stands out forming a conspicuous feature in the landscape.  Purdue, 1907, called them cave-sandstone deposits and was the first to consider them paleokarst.  Purdue and Miser, 1916, noted many of these deposits and concluded several were ancient sinkholes that had been filled with sand.  Two theses that pre-date the construction of Beaver Lake (Arrington, 1962, and Staley, 1962) mention numerous sandstone bodies within the Powell.  One very large sandstone mass was seen in the White River (Arrington, 1962).  It is approximately 45 feet tall!  Unfortunately, it is now covered with water.

       Sandstone mass in Carroll County.  From Owen, 1858

Sandstone mass in the White River near Hwy 12 access to Beaver Lake.  From  Arrington, 1962.

So how did these features form?  First, let’s define paleokarst.  Paleokarst consists of karst features that formed in the geologic past and were preserved in the rock record.  Karst features include sinkholes, springs, and caves.  These features form when acidic rain and ground water dissolves carbonate rocks (mainly rocks that contain calcium carbonate – calcite, or calcium-magnesium carbonate – dolomite).

The majority of sandstone masses are surrounded by dolostone, composed of dolomite, in the Powell Formation.  The Powell is Lower Ordovician in age, meaning it formed around 470 million years ago (mya).  It is likely that this formation was exposed to weathering at that time.  Depressions of various size, called sinkholes, developed on the exposed land surface.  Later, sand filled the depressions and eventually became rock called sandstone.  The age of the sandstone masses ranges from Middle Ordovician (approx. 450 mya) to Middle Devonian (approx. 390 mya).  Therefore, there is a gap in the rock sequence, between dolostone in the Powell and the sandstone, called an unconformity, lasting from 20-80 million years.

Sandstone mass (center) surrounded by Powell dolostone along Beaver Lake.  Photo taken in September, 2016.

Why is paleokarst important, other than being interesting features to observe?  Paleokarst provides clues to former geologic conditions and changes in climate and sea level (Palmer and Palmer, 2011).  We know that sea level was high in the Lower Ordovician and shallow seas covered all of northern Arkansas.  But, in the Middle Ordovician, sea level lowered and the sandstone paleokarst features provide additional evidence supporting this change.

Many sandstone paleokarst features were located while mapping the War Eagle quadrangle.  Fifty-two sandstone masses were located around Beaver Lake.  This is not a complete list, however, since the main focus of mapping was not a paleokarst inventory.

    Sandstone masses (yellow) located from recent geologic mapping around Beaver Lake.

The War Eagle quadrangle was mapped in preparation for State Park Series 4 – Geology of Hobbs State Park.  Follow the link below to see the geologic map of the War Eagle quadrangle:  http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/War%20Eagle.pdf.

Until next time,

Angela Chandler

 

References:

Arrington, J., 1962, The geology of the Rogers quadrangle:  University of Arkansas M.S. Thesis, 61 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, p. 7-16.

Owen, D.D, 1858, First report of a geological reconnaissance of the northern counties of Arkansas made during the years 1857 and 1858:  Little Rock, 256 p.

Purdue, A.H., 1907, Cave-sandstone deposits of the southern Ozarks:  Geological Society of America Bulletin, vol. 17, pp. 251-256.

Purdue, A.H., and Miser, H.D., 1916, Geologic Atlas of the United States, Eureka Spring-Harrison Folio, Arkansas-Missouri:  U.S. Geological Survey Folio No. 202, 82 p.

Staley, G.G., 1962, The geology of the War Eagle quadrangle, Benton County, Arkansas:   University of Arkansas M.T. Thesis, 56 p.

 

 

 

Geo-pic of the week: Graptolites

 

This is a picture of shale, collected from the Womble Formation, near Lake Ouachita State Park, Arkansas.  The photo shows examples of the, now extinct, Graptolites: fossilized colonies of tiny marine animals.

There were many types of Graptolites.  Some were attached to the sea floor, like corals, while others floated in the water, like plankton.  The feather-shaped fossils pictured here are actually the nests in which the animals lived.  Each tooth-like tube, on the edges of the nests, housed a tiny animal.  Several of these nests would be linked together into a larger colony.

At one time the oceans were full of Graptolites, but by about 300 million years ago they died out for unknown reasons.  Because they were abundant, widespread, and continually evolving, Graptolites are important fossils for dating ancient marine rocks. 

To download a copy of our self-guided tour of Lake Ouachita geology, click here http://www.geology.ar.gov/pdf/Lake%20Ouachita%20Geologic%20Float.pdf