ARKANSAS: VARIETY OF GEOLOGY

At the Tucson 2015 Show I met a dealer from Arkansas who had “Crowley’s Ridge Agates” for sale.  I had noticed these agates at a few other shows but they are not common, at least out here in the west.  In thinking about Arkansas minerals two items always pop into my head—diamonds and quartz.  I am certain these two minerals bring more rockhounds to the state than all of the others added together.  I have prospected for both, but have been successful in acquiring specimens of only one.  Care to guess which mineral?  But agates from Arkansas--they were unfamiliar.
 

.  Crowley’s Ridge Agate.  Photo lifted from EBAY.

Back in my teaching days I offered a course in stratigraphy, including landforms and historical geology of North America, so I sort of remembered something about this rather anomalous ridge in northeast Arkansas.  However, I thought Crowley’s Ridge was composed of wind-blown sediments sitting on top of a few Tertiary rocks; therefore, my curiosity asked where were the agates being collected?  What is the source of the agates?  And then the big question—what do I really know about the geology of Arkansas?  

Sketch map showing physiographic regions of Arkansas.  Map from Arkansas Geological Survey.

Arkansas has some fascinating geology and has land situated in two major physiographic regions, the Atlantic Plain and the Interior Highlands; each has several subdivisions.  The Atlantic Plain includes the Mississippi River Alluvial Plain (term of the Arkansas Geological Survey as I am used to calling it the Mississippi Embayment) that occupies the eastern one-third part of the state along the River; locals refer to it as “The Delta.”.  The Plain is a lowland without much relief and represents deposition in a basin extending from Cairo, Illinois, on the north (where the Ohio River joins the Mississippi) to the Mississippi River Delta.  The oldest exposed rocks/sediments are Cretaceous marine while the youngest are being deposited by the River today.  As far as I can tell, there are not many minerals to collect in the generally featureless Plain, but there may be invertebrate fossils—and agates!  Crowley’s Ridge is about a 250--500 feet high, 200 mile long “ridge” sticking up in the alluvial plain and extending from southeastern Missouri into northeastern Arkansas.  The origin of the ridge seems uncertain (seismic? wind accumulation? erosional remnant?); however, the sediments are late Cenozoic in age.  The surficial flora and fauna are more closely related to states east across the river than to the rest of Arkansas.  At any rate, the gravel pits along the ridge produce agates known as Crowley’s Ridge Agates, a somewhat bland colored (in many/most cases) fortification agate.  They occur as nodules, many are fist-size, in mostly tan to cream to brown colors.  The source of the agates is unknown but they must come from Paleozoic rocks to the north and were deposited by the ancestral Mississippi River.  The Cambrian Potosi Formation cropping out in Missouri seems a possibility as I have collected similar looking specimens from that state.

Northeast Arkansas showing location of Crowley’s Ridge. Map from Arkansas Geological Survey.

I also wish to point out that some dealers and agate sellers on the internet dye the agates various colors and term them “Fairburn-like” or “Lake Superior-like”.  Beware of bright pink Crowley’s Ridge Agates! 

Diagram of the Reelfoot Rift shown in cross-section.  Sketch from USGS.

Location of the New Madrid Seismic Zone in the Reelfoot Rift.  The Zone is the source of the numerous (somewhere over 4000 in the last40 years) earthquakes in the area.  Map from USGS.  

The most interesting aspect of the Mississippi Embayment is in the subsurface; a feature termed the Reelfoot Rift or Reelfoot Failed Rift.  About 500 Ma. (Cambrian), or perhaps earlier, the bedrock in the continent begin to rift apart, much like the rifting geologists see today in the Red Sea in the Middle East.  At the north end, near Cairo, Illinois, the rift takes a dog leg to the east where the basin is termed the Rough Creek Graben.  Both features are bounded by lateral faults.  Had the rifting continued, then two (or more) new continental plates would have been created and Arkansas would be a “foreign country”!  In fact an acquaintance of mine from near Toad Suck, Arkansas, believes they are a different country!  However, for some reason the rifting stopped after the center had down-dropped, and the basin begin filling with perhaps a mile or more of marine clastic and limestone rocks.  Beginning in the late Paleozoic, and continuing into the Cretaceous, a series of igneous rocks were intruded into the area, probably along the boundary faults.  By the end of the Cretaceous, and lasting through the Eocene, the rifting was reactivated and the Mississippi Embayment formed and received sediments, some marine, throughout the remainder of the Tertiary.  By the Pleistocene, glacial meltwater flooded the area depositing sands and gravels in a braided stream complex.  It was not until the Holocene (last 10 k years) that the modern meander system of the River formed. (above history from Crone and Schweig, 1994). 

Today the Reelfoot Rift Zone is being laterally compressed (the opposite of rifting) in an east-west direction and reactivating the old faults with the result being earthquakes!  The New Madrid Seismic Zone is the most active tectonic zone east of the Rocky Mountains and is responsible for the 1811-12 earthquakes (remember the story about the Mississippi River running backward) that literally were felt across the eastern U.S. and perhaps the strongest in modern history.  In addition, a more recent earthquake with a magnitude 7.7 rattled the country in May 2011 (Anonymous, 2011). 

West of the Mississippi River Alluvial Plain is the West Gulf Coastal Plain, also part of the Atlantic Coastal Plain Physiographic Region.  Most of the Coastal Plain outcrops in Arkansas are poorly consolidated, flat-lying, off-lapping rocks of Cretaceous, Tertiary and Quaternary age. Cretaceous rocks are mostly shallow marine and represent poorly consolidated sediments of the proto Gulf of Mexico. The Tertiary rocks are alluvial, flood plain, swamp, and shallow water marine.  Lignitic coal beds are common (Fig. 4).  Minerals present include gypsum, chalk, marl, barite, celestite, greensand, and ilmenite; however, collectable outcrops are tough to locate (Howard, 2007 revised).  Perhaps the most interesting items noted from the Cretaceous rocks are a few scattered dinosaur bones and several thousand dinosaur tracks. 

Sketch map showing location of the Ozark Province and its subdivisions.  Map lifted from Wikipedia.

The Ozark Plateau Physiographic Province, part of the Interior Highlands, covers the northern part of Arkansas, north of the Arkansas River; however, the province is more widespread in Missouri and also extends into northeastern Oklahoma and southeastern Kansas. The St. Francis Mountains (exposures of Precambrian igneous and volcanic rocks) in the Missouri section are the tectonic center of this domal uplift and rocks dip gently away from the core (generally to the south in Arkansas). Tectonically, the Ozarks are an intracratonic uplift that seemed to fluctuate above and below sea level throughout the Paleozoic.  Geologists are uncertain as to the cause of this uplift as the Ozarks are not close to the edge of a tectonic plate (Brown, 2004).  Generally, Ordovician through Mississippian rocks are shallow water limestones and dolomites (the Precambrian and Cambrian rocks are in the subsurface) while Pennsylvanian rocks are sandstones and shales.

Although most refer to the Ozarks as “mountains”, the relief is caused, not by uplift, but by streams cutting downward through the plateau.  The Salem Plateau is the lowest part of the Ozarks (elevation ~1500 feet) and Ordovician carbonates predominant.  The Springfield Plateau, a few hundred feet higher than the Salem (~1800 feet) is held up by Mississippian cherty limestones; karst topography and caves are common. The most rugged part of the Ozarks is an area known as the Boston Mountains (~2600 feet) where Pennsylvanian clastics, shales and sandstones, form the surface (McFarland, 2004). 

The Ouachita Physiographic Region (note first figure) has two distinct subdivisions: 1) the Arkansas River Valley running east-west across the state; and 2) the Ouachita Mountains to the south. 

The Arkansas River Valley is an interesting area since it provides an outlet to the Mississippi River for the Arkansas River, a stream of 1469 miles of length that starts in the high mountains of Colorado near Leadville.  The river has meandered across the central valley leaving behind a wide flood plain deposited on top of folded, Ouachita-style rocks--Pennsylvanian sandstones and shales (originally deposited in deltas and near shore marine environments).  The really interesting landforms in the valley are the numerous synclines and anticlines that erode as positive features and project above the floodplain (monadnocks) (Foti, 2008).  In fact, I once visited the summit of Petit Jean Mountain in the Valley at an elevation of 2460 feet, not much lower than Mount Magazine, the highest point in Arkansas at 2753 feet, and also situated in the Valley.  

Monadnocks, curving ridges of resistant rocks that structurally are anticlines or synclines, in the Arkansas River Valley.  Photo from Google Earth©.

South of the Arkansas River Valley are the Ouachita Mountains, an area of folded ridges and valleys.  Clastic Paleozoic rocks predominant, as opposed to limey rocks in the Ozark Mountains (deposition in a shallow marine environment the continental shelf).  During most of the Paleozoic, in what is now the Ouachitas, a deep offshore abyssal plain, perhaps more than 3000 feet below sea level, was to be found (Arkansas Geological Survey, 2012).  The Ouachitas are actually related to the Appalachian Mountains, in fact a continuation of these Mountains separated on the surface by the Mississippi River Embayment, but very noticeable in the subsurface.

Folded Paleozoic rocks in the Ouachita province.  Gently dipping rocks of the Coastal Plain (southern part of photo) lap onto the folded rocks.  Photo from Google Earth.

So, these folded mountains have an interesting geological history--with deposition in a deep water basin during the Ordovician and Devonian but a wild change in the Mississippian and Pennsylvanian.  During this later Paleozoic “action” what is now South America collided with Laurentia (the geological name for what is now the North American continent).  The rocks and sediments in the marine basin were thrust up on the continent at this convergent plate boundary.  This collision, known as the Ouachita Orogeny, compressed and folded the rocks as they were thrust northward.  The structures are easily seen in the above figure.   Perhaps as much as 50,000 feet of Paleozoic rocks, much of it black shale, quartzose sandstone, or bedded chert rocks are present.  The rocks and structures are quite complex (see J. Calvert, 2007 at:  http://mysite.du.edu/~jcalvert/geol/ouach.htm for additional history).  Out here in Colorado geologists believe the formation of the Ancestral Rocky Mountains is related to this collisional event.

There is much more geologic history associated with Arkansas than presented in this short posting.  I intend a return visit in the near future if I can break away from my beloved Rocky Mountains.

REFERENCES CITED

Anonymous, 2011, Poster of the New Madrid Earthquake Scenario of 16 May 2011 - Magnitude 7.7: www.earthquake.usgs.gov/earthquakes/eqarchives/poster/2011/20110516.php

Brown, S. R., 2004, Process and Timing of Uplift in the Ozark Plateau, Missouri: Geological Society of America Abstracts with Programs, Vol. 36, No. 5.

Crone, A.J., and Schweig, E.S., compilers, 1994, Fault number 1023, Reelfoot Scarp and New Madrid Seismic Zone, in Quaternary Fault and Fold Database of the United States: http://earthquakes.usgs.gov/regional/qfaults

Howard, J. M., 1987, with revision 2007, Mineral Species of Arkansas, a Digest:  Arkansas Geological Survey Bulletin 123.

McFarland, J. D., 2004, Stratigraphic Summary of Arkansas: Arkansas Geological Commission, Information Circular 36.