Happy Geologic Map Day!

Why are geologic maps important?

The answer cannot be stated any better than the following:

 “The first geology map was prepared to solve practical problems involving the distribution of different types of rocks at and near the Earth’s surface, and that is still the reason geologic maps are made today.  Uses of geology maps first expanded into exploration for natural resources, including minerals and petroleum.  As an example, during World War II, maintaining the supply of strategic minerals became so critical that geologists in the U.S. military were assigned to make geologic maps for mineral exploration.”

“Most building materials, except wood, are from various specific rocks and rock products. The suitability of rocks and unconsolidated surficial materials to support foundations is essential for construction of homes, skyscrapers, dams, bridges, or simply a road. Rocks and unconsolidated surficial materials fundamentally affect the nature of the soil cover and plant growth is related to soil type. “  (Thomas, 2004)

And don’t forget, we are all familiar with spectacular natural disasters, such as earthquakes, volcanoes, storms,  and floods!

What’s in a geologic map?

The map above is a detail from a larger map, the Geologic Map of the Murfreesboro quadrangle in southwest Arkansas.  This geologic map shows the rocks and alluvial deposits that are exposed at the surface.  Contrasting colors are used to indicate the different rock formations and stream terraces in this area.

The yellow color illustrates alluvium deposited by the Little Missouri River and its tributaries.  Alluvium consists of loose sand and gravel.  This material is an important resource for road construction and other purposes.

During the Cretaceous, a shallow marine ocean covered this area and lime mud was being deposited.  These deposits are represented by the greenish colors and the symbols Kt and Kto, which breaks down into K for Cretaceous, t for Trinity group, and to for Tokyo Formation. But an explosion rocked the area here during the Cretaceous.  An igneous dike pushed up from deep in the earth’s crust, bringing diamonds to the surface.  The blue areas represent exposures of that rare, diamond-bearing igneous rock. The largest blue circle highlights the location of Crater of Diamonds State Park.  Many areas in Arkansas are unique or geologically interesting and have become preserved in our State Parks for all to enjoy!  Folks from all over travel to our state to try their luck at finding diamonds just like the 4.38-carat diamond that was found recently.  Where else can you do that?

So don’t be afraid to look at a geologic map!  Start with a geologic map of the area around your home.  If you need help figuring it out, a friendly geologist is available to help you anytime. Reach out to us at (501)296-1877 or email us at ags@arkansas.gov.

If you are interested in geologic maps published by the Survey and want to learn more about our mapping program, then visit the links provided below:

The Arkansas Geological Survey (AGS) has conducted geologic mapping since the late 1800s.  The first Geologic Map of Arkansas was produced by H.D. Miser and published by the U.S. Geological Survey in 1929.  In the 1960s, the agency was involved with statewide field mapping, which resulted in the publication of the Geologic Map of Arkansas in 1976 and was revised in 1993.

https://www.geology.arkansas.gov/docs/pdf/maps-and-data/geologic_maps/geologic-map-of-arkansas-1993-(34×52).pdf.

The AGS continues to coordinate with other agencies in acquiring supplemental funds for mapping.   Starting in the early 1990s the AGS began geologic mapping at a 1:24,000 scale under the U. S. Geological Survey’s National Cooperative Geologic Mapping Program. The STATEMAP Program provides matching funds for completion of maps in various parts of the state.  Present mapping is focused on the Bethesda, Batesville, Sulphur Rock, and Charlotte quadrangles in  Independence County. 

http://ncgmp.usgs.gov.  

For a full listing of our geologic maps, please visit:

https://www.geology.arkansas.gov/maps-and-data/geologic-maps.html

Reference:  Thomas, W. 2004, Meeting challenges with geologic maps:  American Geological Institute Awareness Series.

 www.agiweb.org/pubs

Geo-pic of the week: Igneous Dike

 

100 million years ago, during the Late Cretaceous Period, a preponderance of igneous activity occurred in the continental region now known as Arkansas.  In fact, all of the igneous rocks discovered in the state were emplaced around that time.  Some of them are well known, such as Magnet Cove, located east of Hot Springs, or the diamond-bearing intrusion near Murfreesboro.    There are also lots of smaller igneous intrusions like the one shown in the picture above. 

Small igneous intrusions are found throughout the Ouachita Mountains.  There are so many small intrusions that new ones are regularly discovered.  Weathering at the earth’s surface has typically destroyed the original rock’s characteristics and what remains is mostly soft clay because the minerals that make up the intrusion are unstable under surface conditions. 

If you happen to notice an unusual-looking body of rock that cuts across the strata of a road cut or other rock outcrop when you’re exploring the Ouachita Mountains, it’s likely that you have seen a Cretaceous igneous dike.