LAKE OUACHITA UNIQUE GEOLOGY EarthCache
LAKE OUACHITA UNIQUE GEOLOGY
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The posted coordinates for this Earthcache are located on the Caddo Bend trail in the Lake Ouachita State Park.
More than 450 million years ago, an ocean covered the area now known as Lake Ouachita. Deep deposits of sediment later compressed into sandstone.
The Ouachita Mountains are a physiographic section of the larger Ouachita province, which in turn is part of the larger Interior Highlands physiographic division.
The Ouachita Mountains are fold mountains like the Appalachian Mountains to the east, and were originally part of that range. During the Pennsylvanian period, roughly 300 million years ago, the coastline of the Gulf of Mexico ran through the central parts of Arkansas. The South American Plate drifted northward, riding up over the denser North American continental crust. The way this uplifting occurred still baffles geologist. Geologists call this collision the Ouachita orogeny. The collision buckled the overriding plate, producing the fold mountains we call the Ouachitas. At one time the Ouachita Mountains were very similar in height to the current elevations of the Rocky Mountains. Because of the Ouachitas’ age, the craggy tops have eroded away, leaving the low formations that used to be the heart of the mountains.
Most of the mountain ranges in the United States run north and south. The Ouachitas are distinctive in that they run east and west. Also, the Ouachitas are distinctive in that volcanism, metamorphism, and intrustions are notable absent throughout most of the system. The Ouachitas tend to be clustered into distinct sub-ranges separated by relatively broad valleys.
The Ouachitas are noted for quartz crystal deposits and novaculite. This quartz was formed during the Ouachita orogeny, as folded rocks cracked and allowed fluids from deep in the earth to fill the cracks. Silica-rich groundwater seeped out of bedrock fractures and crystallized into quartz veins .
The Ouachita Mountain area of Arkansas is dominated by Cambrian through Pennsylvanian clastic sediments, with the lower Collier formation in the core of the range and the Atoka formation on the flanks. The Atoka formation, formed in the Pennsylvanian Period, is a sequence of marine, mostly tan to gray silty sandstones and grayish-black shales. Some rare calcareous beds and siliceous shales are known to have been found. The Collier sequence is composed of gray to black, lustrous shale containing occasional thin beds of dense, black, and intensely fractured chert. An interval of bluish-gray, dense to spary, thin-bedded limestone may be present. Near its top, the limestone is conglomeratic and pelletoidal, in part, with pebbles and cobbles of limestone, chert, meta-arkose, and quartz. It was formed during the Late Cambrian.
Recumbent Folds
Lake Ouachita is home to the Womble Shale. This Shale is mostly black shale that weathers to a brown color and contains some thin lenses and beds of limestone, sandstone and chert. In this area there is thin to medium-bedded fine-grained dense blue-gray limestone exposed in the Womble along the lake. Also present are beds of conglomerate that are believed to have been transported to their current position as debris flows from a higher shelf to the northeast. All of these beds have been tightly folded and faulted in their current position along the lake. Notice the faulted recumbent folds in the limestones of the Womble Shale.
Zebra Rock
Lake Ouachita is home to Blakely sandstone which contains quartz veins. Quartz veins are found in a wide belt in the central core area of the Ouachita Mountains. The quartz came from a solution of hot water formed during the Ouachita Mountain building event. There is a close association of quartz veins with fault zones. Faulting causes the adjacent rocks to become cracked and opens up fractures creating pathways for water to flow. The hot water solution was rich in silica and as it flowed through the open fractures quartz precipitated out of the solution. These rock layers dip to the north.
Submarine Slide
Lake Ouachita State Park is located on a small finger of land which is an anticline. On the northern edge or limb the rocks dip to the north. Interbedded sandstone and shale in the Blakely Formation dip approximately 15 degrees to the south. At the southern end of the point the rocks increase the dip to 50 degrees. The rocks are slightly folded on this southern limb and contain fractures that are filled with quartz. There are large gray masses of rock that sits on the hill above tilted rock which is at lake level. These gray rounded masses look very distinct from the underlying rocks. They weather and erode differently because they are a conglomerate made up of boulders, cobbles and pebbles of sedimentary rocks. These masses are called exotics because the pieces of rock that make up the mass were broken off the shelf and carried down the slope of the continental shelf to their current position. The exact mechanism that brought the material down the slope is unknown, however we can postulate that there was possibly an earthquake that broke the rocks off the shelf and caused them to slide down the slope.
In order to claim this Earthcache you must answer the following questions. Please send an email with your answers. Do not post your answers in your online logs. You can post pictures of the area if you like, but do not post pictures that may give the answers away.
1. From the posted coordinates what direction do you find the Recumbent Folds?
What is the prominent color of these folds?
2. From the posted coordinates which direction do you find the Zebra Rock?
What is the width of the quartz veins?
3. From the posted coordinates which direction do you find the Submarine Slide?
What type of layers do you see in this formation?
Lake Ouachita State Park Permit # EC28-15-02
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