Geology of Hot Springs National Park

Hot Springs National Park lies in the Zigzag range, which forms the southern limb of the Ouachita Mountains. The Ouachita Mountains are part of an ancient chain of folded and faulted rocks that extend from Central Mississippi to Texas. The Ouachitas began their formation during the Carboniferous Period (286 to 360 million years ago) as plates collided along what is now the eastern seaboard and the gulf coast of North America. The orogenic events that shaped these mountains are estimated to have occurred in the Late Pennsylvanian Period or during the Early Permian Period (about 260 million years ago).

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While the sedimentary rocks were originally deposited in accordance to the Law of Original Horizontality (sediment is deposited horizontally under the action of gravity), mountain-building processes arranged the strata into what we see today. Folding occurred when the stress of geologic forces created rumples in the rock strata, and normal and reverse faulting occurred as well. The Ouachita Mountains are largely composed of folds and thrust faults, which are created when a reverse fault breaks in a low angle. In the Hot Springs area, small thrusts typically displace strata towards the Southeast, with the highest ridges of the Zigzag range being composed of Arkansas Novaculite. These forces create anticlines and synclines, which can be found throughout the Ouachitas.

Stratigraphy

Hot Springs National Park now lies in the heart of a northern bulge of the Ouachita Thrust Belt, which was once a reentrant in an ocean basin. Before the collision of plates in the Carboniferous Period caused the folding and faulting of what we now call the Ouachita Mountains, ancient seas ruled Central and South Arkansas. Due to the abundant supply of various types of marine sediments, the Ouachitas are composed of sedimentary rocks, with some thermal alteration.

The oldest of the sedimentary rocks date from the Late Cambrian Period (500 million years ago) to the Early Mississippian Period (about 350 million years ago). Exceeding 8,000 feet of thickness, the strata are largely composed of deep marine shales, limestones, sandstones, and cherts. These strata include Hot Springs Sandstone and our famous Arkansas Novaculite. Overlying these strata are the Stanley Shale, Jackfork Sandstone, Johns Valley Shale, and the Atoka Formation, all formed in the Carboniferous Period. Due to the folding and faulting of the Ouachita Mountains, thermal spring water escapes through the Hot Springs Sandstone. In addition to the sandstone and shale one may see in Hot Springs National Park, visitors will also see tufa- a precipitated carbonate rock that forms around the mouths of the hot springs.