Ozark Mountain geology- an ancient sea, volcanic islands, continental collision and erosion

Dave,
   We traveled through the Ozark Mountains in Missouri and Arkansas last summer. What is the geology of this beautiful wooded hill country? What are some geologic sites to visit when we return?
Arliss Clark
Detroit

Arliss,


The Ozark region

   I visited the Ozarks when I was 7 on a family vacation. One of my earliest geologic memories is finding weapons-grade flint in the Ozarks, and watching a guy make a small arrowhead out of a piece I found.  It is wonderful country for a vacation with the kids. Campgrounds and RV parks abound. Most of what follows is from a US Geological webpage and Charles Spencer's useful books, Roadside Geology of Missouri, published by Mountain Press Publishing Company.

PLACES OF GEOLOGIC INTEREST:
Arkansas is famous for its caves, many in Ozark limestone.
A guide to Arkansas geologic sites.
Johnson's Shut-ins State Park (in the Saint Francois Mountains, Missouri): refreshing swimming holes eroded in rocks erupted on ancient volcanic islands.
Elephant Rocks State Park also in the Saint Francois Mountains, Missouri: rounded rounded boulders of 1.5-billion-year-old granite.

   The Ozarks are a part of the Ouachita-Ozark Highlands and are actually an eroded plateau. Most of the high points are of similar elevations, separated by valleys. The rocks are mainly limestone (with interbedded chert) and sandstone deposited on an ancient sea floor during the Paleozoic era (from around 540 to 250 million years ago). At the time, only volcanic islands rose above the sea anywhere around here. These islands are represented by volcanic rock, mostly rhyolite from violently explosive eruptions and the granite that cooled beneath the volcanoes. The volcanics are really ancient: 1.5 to 1.4 billion years old! Not quite as old as the 3.6 billion year old Morton Gneiss I wrote about a month or so ago, but they are the oldest rocks in the southeastern USA. The island rocks are preserved in the Saint Francois Mountains at the northeast corner of the plateau, up against the Mississippi River, but they extend a great deal further, forming the continental crust beneath the sedimentary rocks in much of the region.

Volcanic ash from explosive eruptions 1.5 billion years ago is
now rhyolite rock at Johnson's Shut-ins State Park

   Thousands of feet of sea floor sediment was deposited all around and on top of the older volcanics. The flat-lying sediment became the Paleozoic sedimentary rocks that typify the Ozarks. These rocks are seen throughout the highlands. The limestone was once coral reefs; the sandstone was eroded from the volcanic islands and deposited on the floor of the now-disappeared Rheic Ocean.

The Ozark rocks (lower center) were uplifted in a plate collision.
Click photo to enlarge and see modern state boundaries.

   But then, a gradual but huge change occurred here. During the Mississippian Period (340 million years ago) two tectonic plates were converging. What was at the time the southeast margin of North America ran up against a plate that included much of South America and western Africa. These plates were coming together to form a supercontinent called Pangea, which included all the continental landmasses until its break up around 200 million years ago. The seafloor rocks were folded and faulted as the ocean basin contracted. The collision of the plates raised a huge rugged mountain chain, much as the Himalaya-Karakoram-Hindo Kush mountains are today forming during the collision of the Indian and Asian plates. These mountains stretched across the seam (the "suture zone") marking the area of collision, which ran more or less along the equator. Remnants are the Marathon Hills of east Texas, the Ouichitas in central Arkansas, and the Appalachians. The sedimentary rocks just to the north were uplifted and tilted about a degree or two towards the south to form a plateau. These are the rocks that underlie the Ozarks.

'Dumbo' is the largest eroded boulder at Elephant Rocks State Park.
It is eroded from granite that was the source of the
volcanic rocks in the Saint Francois Mountains.

   Pangea eventually broke apart to form the modern continental and oceanic plates. The Ozark rocks have been above sea level for the past 340 million years and subjected to the attack of erosion.
   The Ozarks are famed for their limestone caves and 'shut ins'. The latter are wonderful natural swimming pools eroded in the volcanic rocks at the eastern edge of the Ozarks in Missouri.

How do we know how old a rock or fossil is?

Dear Dave,

How do geologists determine the age of an ancient volcanic eruption or a glacial advance? How do we know how many years ago Tyrannosaurus lived?
Bertina H.
Saint Louis, MO

Bertina,

Geologists, paleontologists and archaeologists often want to know the age of a soil deposit, a rock layer (such as a lava flow), or the fossils in the rocks. We speak of two types of 'ages'. 1. Relative age is the relation in time between rock layers (and the fossils they may contain). Relative age can tell us that a particular rock layer is older or younger than another. It CAN'T tell us if a rock or fossil is 3 million or 300 million years old. 2. Absolute age is the time in years before the present time. If the rock can be dated at all, the absolute age can be determined. An example: a lava flow is determined to be 32.4 million years old (almost always with some imprecision, expressed. for example, as " ± 0.2 million years").

This week I'll deal with relative age. I'll tackle absolute age in a future blog post. That will help us understand how long ago Tyrannosaurus lived.

There are some basic laws that state the principles of relative ages of rocks. Geologists apply these to the rock record to determine sequences of time.

The "Law of Superposition" says that any undisturbed sedimentary rock layer that lies above another is the younger of the two. This principal was formalized in the mid-1600s by a Dane, Nicolas Steno, who is among the founders of modern geology.

Steno's Law of Superposition says that the rocks higher in the stack are younger than those below. Sounds like common
sense now, but this was a radical thought in the 1650s, when all sedimentary rocks were held to be the same age: deposited by Noah's Flood over a period of 40 days and 40 nights.

The "Law of Cross-cutting relationships": An intrusion of magma that cuts across other rocks is younger than the rocks it intrudes. Scotsman James Hutton (1726 - 1797)



Diagram illustrating cross-cutting relations in geology. These relations can be used to give structures a relative age. Relative ages are, from oldest to youngest: A - folded rock strata offset by a thrust fault. The folding is younger than the rocks, and the fault is younger than both rock A and the folding; B - large granitic intrusion (cutting through A); C - erosional angular unconformity (cutting off A & B) on which brownish yellow rock strata were deposited; D - volcanic intrusion (a dike), cuts through A, B & C; E - even younger rock strata (overlying C & D); F - normal fault (movement down on the right side) that cuts A, B, C & E. Diagram by Woudloper.

The "Law of Inclusions": Fragments of one rock layer that are enclosed in another rock layer are older than the enclosing layer. This was also recognized by Hutton.

The Law of Inclusions: if Rock B contains fragments of Rock A, then B must be younger than the fragments of rock it contains. The intruding rock (Rock A) must have been there first to provide the fragments.    

The "Law of Faunal Succession": Englishman William Smith (1790) recognized that fossils occur in a definite, invariable sequence in the geologic record.

Law of Faunal succession: fossil remains of living things are present in rock layers at definite intervals, and exist within a discrete period of time. In this instance, using the Law of Superposition, would the age Rock Unit A be older or younger than the age of Rock Unit B?     

If you can see these relationships, you can begin to understand the sequence over time. In the case of Bertina's hypothetical volcanic eruption and glaciation, apply these methods if the deposits are in contact with each other. If they aren't both present in the same rock outcrop, it may be far more difficult to determine relative age. In a future post I'll deal with absolute dating. That involves radiocarbon and other isotopic methods. Stay tuned!
Dave