THE GEOLOGY OF BUFFALO MOUNTAIN PARK

JOHNSON CITY, TENNESSEE

 A BRIEF CHRONOLOGY

One billion years before present.  That’s a “one” followed by nine zeros.  In Europe, it would be a thousand million.  Knowing this, however, doesn’t make the immutable vector of time any more comprehensible, but that is how far we must stare back into its abyss to find the beginning of this story.

The North American continent and another continent -- we aren’t sure which one -- were firmly welded together, probably as part of the pre-Pangeaen supercontinent of Rodinia, in a collision and mountain-building event known as the Grenville Orogeny.  Together, these joined land masses drifted somewhere near the equator.  In the old geology books, these nearby continents were given the sobriquet of “Terra Incognita”.  With artistic license, they were shown covered by clouds and dragons.  A romantic notion, but there were no dragons back then; there were no sea monsters, no giant squids, no lizards, not even a worm.  But there was life; in fact, life had been around for perhaps 2.5 billion years, probably in the form of  one-celled organisms known as prokaryotes.  The more complex eukaryotes were relative newcomers.

These early continents were apparently very mobile.  Driven by heat rising from the Earth’s interior, they would first collide, than they would push apart, sometimes giving up territory and at other times gaining in size.  Besides this, curved rows of islands formed by oceanic volcanos (island arcs) would also collide, adding to the land mass. 

Then, 200 million years went by.  It was then 820 million years ago, and the North American continent had broken away from its neighbor to the east.  A new ocean flowed into the rift.  This ocean has been dubbed the Iapetus, or proto-Atlantic Ocean.  Not much was happening at Buffalo Mountain, but the action was not too far away.  As spreading continued and the crust rebounded, thick bodies of granite, called plutons, found their way into the near-surface of the crust.  One such pluton, the Beech Granite, occupies the tops of ridges farther to the east.

By 650 million years ago, an inland basin had developed to the west of the Blue Ridge.  Sediment derived from the Blue Ridge became thick sequences of clastic rocks (rocks composed of fragments of older rocks) that now outcrop along the crest of Iron Mountain, Holston Mountain, and of course, Buffalo Mountain. 

The inland basin probably began as a rift that occurred as two sections of the crust pulled away leaving the center section to sink.  Runoff from the adjacent land masses was prodigious, because there was no vegetation to stem the erosion.  Rivers cut rapidly through the barren landscape, washing all types and sizes of rocks downstream and into the basin.  Gravel and coarse sand settled out first as fans and delta deposits.  The finer silt was carried farther from shore before settling.  Finally, the clay was transported the farthest, being lighter and having a natural affinity for the water.  The clay settled into the deep, anoxic zone of the basin (deep water where there is a lack of free oxygen).

It seems as though the warranty on an ocean or a supercontinent is only good for about 200 million years, because by 450 million years ago, the Iapetus Ocean had begun to close.  Ocean-bottom crust was being subducted; which occurs when the denser (3.0), basaltic crust of the ocean floor is overridden by the less dense (2.7), granitic continental crust.  The subducted crust plunges slowly back into the earth’s viscous mantle region, about 25 miles below the surface.  The friction and increased heat typically give rise to a curved line of volcanos known as an island arc, as the lighter sediments become molten and rise like huge bubbles searching for neutral bouyancy.  The molton bodies that manage to breech the surface become volcanic islands.  The bodies that stall and solidify and are forced to wait for erosive forces to exhume them are known as plutons. 

About this time, a garnet-grade metamorphic event associated with the Taconic Orogeny occurred in this area, which means the rocks were exposed to a temperature of 600 to 800 degrees Celsius and a pressure of 7 to 10 kilobars.  Under those conditions, if the requisite minerals are present, garnets will form.  Garnets found in the Bakersville metagabbro (a dense, igneous intrusive rock that occurs around Roan Mountain and Bakersville) are attributed to this episode.  The Taconic Orogeny is the name given to the probable initial collision between land masses that later spelled doom for the Iapetus Ocean. 

In the inland sea to the west of what would become Buffalo Mountain, a long period of carbonate (limestone and similar rocks) deposition gave way to thick, black shales and occasional coarse conglomerates as the crust warped in response to the change in plate motion.  This warping created an upward bend, or anticline, which resulted in accelerated erosion. This, in turn, furnished the sediment for the shales and conglomerates to the west.

Conglomerates are rocks composed of clastic material coarser than sand, such as gravel and cobbles.  An astounding display of these remarkable rocks occurs on Masters Knob, the location of the huge blue water tank and antenna farm visible from most places in North Johnson City.  An even more accessible locality is along the north side of Boone Lake on Hyder Hill Road, a few miles east of Rocky Mount.  Hard quartz and flinty clasts are found alongside softer carbonate fragments, suggesting rapid deposition.

The Middle Ordovician shales deposited at this time in the Buffalo Mountain Park area are found only in the lower reaches of Hartsell Hollow, exposed by the down-cutting of the creek.  Close proximity to an overthrust fault has left the exposure full of bends, twists, and folds.  Also, the shale is highly fractured.  Vegetation grows readily on the weathered shale, further hastening the weathering process.

By 400 million years ago, volcanic islands began to collide with the North American Continent, and the Acadian Orogeny was in progress.  As the ocean closed and pressures increased, ductile thrusting (movement along a nearly horizontal fault) and retrograde metamorphism was occurring in the Blue Ridge.  As the closing continued, two major episodes of thrust faulting, a ductile one at 360 m.y., and a more brittle one at 250 m.y., lifted the crust to great heights so that erosive forces could begin to carve out the mountains we see today.

During these episodes of faulting, two thrust sheets overrode the area that is now Buffalo Mountain.  The amount of vertical displacement is a phenomenal 14,000 feet; possibly the greatest stratigraphic throw (amount of vertical movement) recorded anywhere in the Appalachians.

Robert E. Whittemore  (TN  3006)

Chief Geologist, General Shale Brick

May 14, 2000

Revised 9/28/02 -- thanks to Ingrid Luffman, ETSU, for the generous coaching.

[Back]