Geology of the salt and gypsum deposits of southwestern Virginia
Republished with revision from Contributions to economic geology, 1911, U.S. Geological survey. Bull. 530, pt. I
Public-domain full text preserved in the Mountain Man Mining Library. Original source: archive.org.
Virginia Geological Survey
University Of Virginia
Leonard Watson, Ph. D.
Director
n bulletin No. VIII
Biennial Report
On The
Mineral Production ot Virginia
During the Calendar Years
1911 and 1912
By Thomas L. Watson
With Chapters On
Zirconiferous Sandstone Near Ashland, Virginia
By Thomas L. Watson And Frank L, Hess
And
Geology of the Salt and Gypsum De posits of Southwestern Virginia
By
George W. Stose
Library
UNIVERSITY OP VcAUfOtNIA
Earth
sawcE
Library
Exchange
Geology Of The Salt And Gypsum Deposits Of Southwestern Virginia3
By George W. Stose.
Location.
Large deposits of salt and gypsum are known to occur along a belt of country 20 miles long running northeastward from the village of Plasterco, Va., and lying in Washington and Smyth counties. Much of this territory is in or near the valley of the North Fork of Holston River, and this
8I°30'
50 Miles
Fig. 4. — Index map of southwestern Virginia. The area descril>ed and mapped in this report is indicated by the shaded rectangle. Railroad connections for this area only are shown.
portion is made accessible to railroad transportation by the Saltvillc branch of the Xorfolk & Western Railway, which joins the main line at Glade Spring. The location and relations of this area are shown in figure 4. Two gypsum plants and one salt or alkali works are now in operation in
aRepublished with revision from Contributions to Economic Geology, 1911. U. S. Geological Survey Hull. 530. Part I.
52 Salt And Gypsum Deposits Of South Westekn Virginia.
this area. Numerous old gypsum workings and prospects indicate the extent of the deposits, some of which are at present not commercially workable because of lack of transportation facilities. The active mines, old workings, and prospects are shown on the geologic map in figure 5.
Topography.
The area represented on the accompanying map (fig. 5) comprises a mountain ridge 1,000 feet high separating parallel valleys and rising above an adjacent deeply dissected plateau. The ridge, named Pine Mountain at the southwest and Brushy Mountain at the northeast, trends in a general N. 70° E. direction and its elevation ranges from 2,500 to 3,000 feet. It is cut nearly at right angles by several deep water gaps through which pass the waters from Clinch Mountain that drain into the ]STorth Fork of Holston River. This stream flows southwestward, in general hugging the foot of Pine Mountain, and its valley descends from an altitude of 2,000 feet at the northeast to 1,500 feet in the southwestern part of the area. The plateau to the southeast ranges from 2,000 to 2,500 feet in altitude and its surface is dissected into narrow transverse ridges and rounded hills.
Geology.
Stratigraphy.
The rocks in which the deposits occur are of Mississippian ("Lower Carboniferous") age. A generalized section of the Carboniferous rocks derived from several detailed sections in the vicinity of the mines is as follows :
Generalized section of Carboniferous rocks in the vicinity of Saltville, Va.
Newman limestone: Feet.
Hard argillaceous limestojie or calcareous shale, with a
few beds of crystalline limestone 400+
Red calcareous sandstone and coarse crinoidal limestone,
with some beds of argillaceous limestone 75
Light-blue to gray argillaceous,, shaly limestone and cal- careous shale, with a few thicker fossiliferous lime- stones 1,150
Largely thick even-grained blue fossiliferous limestone,
with some beds of crystalline fossiliferous limestone. .. 250
1,875+
Geology. 53
Maccrady formation: Feet.
Earthy limestone and shale, dark gray, weathering lighter
and crumbly, abundantly fossiliferous 225
Gray sandstone, mostly calcareous and crumbly, and shaly
argillaceous or earthy limestone; fossiliferous at the top 120
Soft rocks, including shaly limestone and probably earthy
sandstone and red shale, largely concealed 300
Upper part red shale and shaly sandstone, with some gray shaly sandstone; lower part soft light-buff shale, with thin* black carbonaceous shale and coal seamlets, con- taining Mississippian plants 120
Price sandstone: Feet.
Hard irregular-bedded rusty-gray sandstone, with some
heavier beds 95
Largely shaly sandstone, with a few harder beds 275
Massive gray to reddish-gray sandstone, thin bedded toward top, and fine conglomerate with scattered white quartz pebbles generally at base 50
Devonian rocks:
Thin-bedded sandstone and sandy shale containing
brachiopods of Chemung age. Platy sandstone and slaty shale.
The Price sandstone is a hard ridge-making rock which forms the ridge known as Pine Mountain and Brushy Mountain. The southeastern face of this ridge is a dip slope of the hard rocks of this formation, which dip about 40° SE. The severed edges of the dipping strata are finely exposed in the gaps through the mountain and make picturesque ledges and cliffs. Less well exposed in the gaps and on the northwest slope of the ridge are the underlying shaly sandstones and shales which are spar- ingly fossiliferous and of Chemung age. The lithologic character of the Price sandstone, its general stratigraphic position, and the presence of coal seamlets near its top, indicate its equivalence to the Pocono, at the base of the Carboniferous system, and the invertebrate fossils obtained from it corroborate this opinion. In the adjacent region to the northeast the Price sandstone contains thick coal beds whose flora establishes its Mississippian age.
The Maccrady formation is composed of materials relatively so soft and easily disintegrated that it is deeply eroded and in general poorly exposed. It outcrops in the valley of the North Fork of the Holston but is largely covered by the terrace and flood-plain deposits of that stream.
54 Salt And Gypsum Deposits Of Southwestern Virginia.
The basal black shale and reddish sandy beds are not uncommonly exposed in the lower spurs of Pine Mountain,, but the earthy limestones and shales of the formation are seen in few places. A few fossils have been found in some of the thin calcareous beds, and certain dark shales near the middle are in places highly fossiliferous. At the base are coal seamlets and under- clays that carry plant remains. The invertebrates have been assigned by George II. Girty to the upper Mississippian, and he correlates the formation with the Moorefield shale of Arkansas. It also probably represents the lower part of the Mauch Chunk of Pennsylvania. In places plastic red and olive to bluish clays with gypsum deposits occur in the midst of the Maccrady formation. Their occurrence and relations are discussed under the heading "Origin of the Deposits/* on pages 64-73. This formation has been called the Pulaski shale in geologic reports describing adjacent areas to the northeast, and this name would be used here were it not that Pulaski has a prior established usage for an Ordovician formation in New York. The new name Maccrady is here given to the formation,, from the village of that name on the North Fork of Holston River, where the best section of the formation was measured.
The Newman limestone is calcareous throughout but contains shaly portions which weather readily to clay and soil. The limestone generally makes hills, which in most places assume rounded forms due to dissection by streams flowing across the trend of the beds into the larger longitudinal streams. The formation is highly fossiliferous and the fauna indicates its general equivalence with the Greenbrier limestone of West Virginia and Pennsylvania and the Batesville sandstone of Arkansas.
Pre-Carboniferous rocks are present in two tracts within the area presented on the map (fig. 5). Beneath the basal Carboniferous sandstone lie Devonian sediments, mostly shales and sandstones, about 2,700 feet thick, underlain in turn by Silurian sediments, also mostly shales and sandstones. These are not differentiated on the map, as they do not concern the problems here discussed. These rocks occupy the north- western portion of the mapped area and form the slopes of Clinch Mountain, which is capped by the basal Silurian formation, the Clinch sandstone, of Medina age.
In the southeastern part of the area mapped are Cambrian strata, mostly hard gray to blue magnesian limestone and dolomite, which are also undifferentiated on the map. The oldest of these Cambrian rocks are adjacent to the Carboniferous, with successivel}' younger beds to the southeast.
Structure. 55
Structure.
The Cambrian rocks on the southeast are part of a great overthrust mass which rode on a flat fault plane over the Carboniferous strata on the northwest, as shown in the structure sections in figures 6 and 7. The Cambrian strata dip rather uniformly 30° -40° SE., successively older Cambrian strata appearing at the northwest. Massive gray dolomite and magnesian limestone of Cambrian age are adjacent to the fault throughout most of its course in the mapped area and probably form the competent strata that carried the thrust. There is no indication of an anticlinal axis in these lower limestones southwest of Saltville, where this formation lias a narrow outcrop, but northeast of Saltville there is close folding in the broad belt of this formation adjacent to the fault, with all dips over- turned to the southeast. A still lower Cambrian formation of red argillaceous shale and sandstone is exposed over part of this area. This folded portion of the Cambrian may represent the axis of an overturned anticline, the breaking and overthrusting of which initiated the faulting. This is no local or minor fault, however, for it has been traced throughout the southern Appalachians into the Eome fault, which has been demon- strated to have a horizontal displacement of at least 5 miles in the vicinity of Eome, Ga. A thrust fault of such magnitude and length must have a deep-seated origin and its plane may be a shear plane cutting diagonally across the strata, without folding except that produced by friction or drag.
The fault plane is exposed at several places in the area, dipping south- east, and its inclination varies from 20° to 60°. Figure 8 is a sketch of the faulted rocks in the cliff southwest of Maccrady. !N"ext to the fault piano the dolomite of the overthrust mass is hardened and the bedding obliterated, and the vertical beds farther from the plane of movement are jointed parallel to the plane. The softer shaly limestones beneath are mashed and altered by circulating waters to clay adjacent to the fault.
Another section of the fault laid bare by old gypsum workings 2 miles east of Broad Ford shows the Cambrian dolomite resting on red and green clay containing gypsum, with 1 foot of black banded carbonized calcareous clay gouge directly beneath the fault plane, which dips 20°-40° SE. In places a dolomite breccia of large and small masses marks the fault contact. In the railroad cut at Plasterco the cemented breccia is freshly exposed and its components are seen to be largely dolomite, with minor fragments of chert, limestone, and shale.
Opposite Maccrady Gap a mass of Clinch sandstone of Silurian age and associated rocks of sufficient size to make a hill 250 feet high and nearly 1 mile long was caught up along the fault and is shown on the map (fig. 5) by the fault dividing west of North Holston.
Cmc Cn
A -A'
Fig. G. — Structure sections across Holston Valley along lines indicated by letters on the margins of the geologic map (fig. 5). Cn, Newman limestone; Cmc, Maccrady formation; Cp, Price sandstone; SB, undifferentiated Devonian and Silurian rocks; f , undifferentiated Cambrian rooks, mostly dolomite. Scale, double the scale of figure 5.
Salt And Gypsum Deposits Of Southwestern Virginia.
H-H
Fig. 7. — Structure sections across Holston Valley along lines indicated by letters on the margins of the geologic map (fig. 5). Cn, Newman limestone; Cmc, Maccrady formation; Cp, Price sandstone; SD, undifferentiated Devonian and Silurian rocks; £ . undifferentiated Cambrian rocks, mostly dolomite. Scale, double the scale of figure 5.
Structure Sections.
The outcrop of the fault plane is very crooked in the northeastern part of the area, owing to the facts that the plane is very flat in most places and is probably somewhat folded or wavy along the strike. Where the fault lies between the Cambrian dolomite and the shale of the Maccrady formation, it affords favorable channels for circulating underground water, from which springs issue at many places, and large solution channels are formed that may have aided in breaking down and removing the overlying dolomite at their out- lets along the fault and may have assisted the formation of deep Fig- 8.— Section of the faulted rocks in the
cliff southwest of Maccrady, Va. a, Massive Cambrian dolomite, bedding vertical but indistinct, jointing parallel to fault; b, zone of altered dolomite, bedding entirely obliterated; c, zone of altered and crushed argillaceous material, banded parallel to the fault; d, earthy limestone and calcareous shale ( Carboniferous ) .
reentrants in the trace of the
plane. These reentrants are
invariably underlain by soft
clays of the Maccrady formation,
which form low flats generally
without rock exposures. The
reentrant at Saltville is one of the largest, and is entirely barren of rock
exposures. Another reentrant is at Broad Ford, where there are only a
few outcrops of the lower harder beds in the Maccrady. Northwest of
Chatham Hill is a still larger reentrant, due to the flattening of the
general structure and a corresponding wider exposure of the softer rocks
after being stripped of the overthrust Cambrian dolomite. These reentrant
areas are the chief places where salt and gypsum deposits have been found
and are of especial interest in the study of the distribution and origin of
these products.
The rocks northwest of the fault, except those immediately adjacent to it, lie in a monocline, dipping 25°-40° SE., which culminates in Clinch Mountain, northwest of the area mapped. The soft Carboniferous rocks near the fault are bent into an overturned syncline. The sections in figures 6 and 7 illustrate the progressive rise in this syncline from southwest to northeast. As the Newman limestone rises northward in the shallowing syncline, erosion has removed its upper portion and its remnant gradually diminishes in thickness from 3,300 feet in the most southern section until northeast of Saltville it is entirely absent. The
60 Salt And Gypsum Deposits Of Southwestern Virginia.
soft underlying Maccrady formation does not extend all the way along the southeast side of the syncline, but is faulted out in the southwestern part of the area. Where present on the southeast side it is vertical or overturned.
No outcrops are visible in the broad flat at Saltville, but the absence of hard outcropping strata and the record of only soft rocks of Maccrady type in the deep wells at this place indicate that the syncline is followed on the east by an anticline whose east limb carries the Price sandstone below the points reached by the drill but apparently not deep enough to bring the Newman limestone down to the surface, so on the sections the rocks are shown to be undulating in the portions under cover of the overthrust fault.
Northeast of Saltville the beds of the Maccrady formation are so poorly exposed that their attitude is not generally shown. At the cliff west of Maccrady the last clear exposure of the syncline is preserved in the ledges of shale and sandstone. Just east of North Holston a small anticlinal roll of thin limestone in the Maccrady is an indication of the undulations probably existing throughout this band of soft rocks. East of Broad Ford a similar gentle fold is exposed in the small stream gully crossing the lowland.
Farther northeast the structure flattens more and more, and in the reentrant northwest of Chatham Hill a thin limestone in the Maccrady formation indicates a very gentle syncline, followed on the southeast by a gentle anticline and another syncline, which is sharply turned up at the fault. The gentle syncline is also shown in the southward swing of the Price sandstone outcrops forming Brushy Mountain at the northeast end of the area mapped.'
From the overturned syncline of Newman limestone at the southwest it might at first be concluded that this was a syncline associated with an overturned anticline on the southeast, which broke and was thrust over upon the syncline. However, it is concluded from a wider study of the structure that the fault did not originate in a broken fold but is of deeper- seated origin, being manifest by a shear plane cutting diagonally across the strata and folding and crumpling those at the overridden contact by reason of friction and drag.
Salt And Gypsum Industries. 61
Salt And Gypsum Industeies.
Salt.
Salt seepages were known to exist in the vicinity of Saltville in pioneer days,, for this swampy flat was one of the salt licks frequented by wild animals and was sought by hunters and trappers and before them by the Indians. The early settlers dug shallow wells and extracted the salt from the brine that flowed from the springs. As early as 1836 two wells were reported in operation. During the Civil War the wells at Saltville were the main source of salt for the Confederacy. A brief history of the salt and gypsum industries of this region is given by T. L. Watson, Mineral Eesources of Virginia.a
Present development. — The salt industry is now conducted by the Mathieson Alkali Works, with offices at Saltville. The Saltville Valley and surrounding country are owned and controlled by the company.
Since 1895, when the Mathieson Co. came into control of the property, the brine has not been evaporated into salt, but is converted by a modified and improved ammonia-soda process into soda products, chiefly sodium bicarbonate, or baking soda, which is the basis of all baking powders and is used also to some extent in making soda water. A large part of the production is in the form of soda ash, used extensively in the manufacture of glass, pottery, etc. Sal soda is also made for this purpose. Caustic soda, put up in large hermetically sealed cans, is prepared for medicinal and other purposes.
Over 50 wells have been drilled in the vicinity of Saltville, about 25 of which are at present in operation. They range in depth from a few hundred feet to 2,280 feet, the average being about 1,000 feet. The shallower wells are on the northwest side of the flat and the deeper ones on the southeast side, near the fault. The former are dry wells and have to be flushed with water through the outer casing. The wells on the southeast side are wet and the brine flows in as fast as it is pumped out. In the wet wells the rocks become honeycombed and cave in, in some wells bending the pipe so as to cripple or entirely disable the well. The brine is raised by ordinary deep pumps each operated by a walking beam driven by an electric motor housed in a small shack at the well, and the brine is piped to an open reservoir in the town. From the reservoir it is piped to the company's plant covering several acres on the east bank
aWatson, T. L., Mineral Resources of Virginia, Jamestown Exposition Com., 1907, pp. 211-213.
62 Salt And Gypsum Deposits Of Southwesteen Virginia.
of the North Fork of Holston Kiver, about a mile distant, where it is converted into baking soda and the other sodium products.
For the conversion of salt to these compounds large quantities of pure calcium carbonate are used, and an aerial bucket tram carries crushed limestone from the company's quarry 3 miles southeast across the limestone hills. As the limestone must be free from magnesium and other impurities, satisfactory rock is difficult to obtain in quantity. Part of the present supply comes from quarries at Marion, Va., about 25 miles distant by rail on the main line of the Norfolk & "Western Eailway. About 600 tons is used daily.
Gypsum.
Gypsum has been used for fertilizer for many years, and as early as 1835 the great possibilities of this deposit as a source of supply for the agricultural lands of Virginia were recognized. Over a decade ago gypsum was converted to plaster of Paris by roasting on only a small scale, as the product did not then have wide usage, but the adoption of this kind of plaster for walls in buildings, especially as a finishing coating, because of its superior hardness and whiteness, has made its production a large and profitable industry. When mixed with cement it acts as a retarder, greatly increasing the value of that product, and for gypsum to be used in this way there is now a large demand. As land plaster or fertilizer the gypsum is simply ground and not roasted. It has proved very beneficial to certain soils and for certain crops, being highly recommended for peanut cultivation.
Present development. — Two gypsum companies are operating in the area at the present time. The United States Gypsum Co., with offices in Chicago, leased the Eobertson tract, adjoining the Mathieson Alkali Co.'s property on the southwest, from the Buena Vista Plaster Co. and has been operating for the last few years. This plant is located in a narrow exten- sion of the broad flat at Saltville, separated from it by a low divide. Two shafts furnish access to the workings, which are reported to be about 100 feet below the surface, each set of workings seeming to be in a distinct body of gypsum. A third abandoned shaft leads to another mass of the deposit, and as other new bodies are located by drilling over the bottom land additional shafts will be sunk. Large deposits of gypsum on the eastern edge of the tract directly adjoining the Mathieson property were previously worked out by the owners.
As just mentioned, the gypsum in this mine seems to be in detached masses of great size and not in continuous beds, as might be expected.
Salt And Gypsum Industries. 63
This will be referred to again later under the heading "Origin of the Deposits." The gypsum is mostly a white to gray granocrystalline rock inclosed in clay, the gray variety streaked with fine dark argillaceous material. Numerous small anhydrite crystals are scattered through some of the gypsum from the old southernmost shaft, and these appear more prominently on weathered specimens. The gypsum is brought to the surface by elevators and conveyed by tram cars to the company's mill, where it is roasted and pulverized. The molding of plaster bricks, tiles, and hollow blocks in the company's shop is a new branch of the industry in this region.
The Southern Gypsum Co.'s plant and office are at North Holston, reached by the company's branch railroad from Saltville. The mine is on the old Pier son plaster-bank farm, in one of the embayments of lowland adjoining the North Fork of Holston Eiver which is underlain by the soft shales of the Maccrady formation. The shaft in the lowland is connected by an aerial bucket tram with the main roasting and grinding plant at the railroad on the hillside. A large part of the crude product is ground for fertilizer at the lower mill near the shaft, much of the gypsiferous clay being of the right mixture to be used in this way for land plaster, effecting a great saving in the expense of mining. For wall and finishing plaster and cement retarder only the purer lump gypsum is employed.
The bulk of the gypsum here is much like that at the United States Co.'s plant, granular and crystalline. Some large sheets of pure selenite are encountered, and small veinlets of satin spar are common in the clay. Large masses of black argillaceous material called "black rock" occur in the midst of the gypsum, and apparent bedding of the gypsum is indicated by banding of black grains of the same material. The gypsum is reported to occur in beds of considerable thickness and extent and not in isolated masses, as at the United States Co.'s mine. The deposits have been tested by bore holes over all the river bottom of the embayment. The beds vary greatly in thickness, however, being somewhat lenticular in shape. The gypsum formerly outcropped at the river, where it was mined in open cuts in the early days for fertilizer. It is now mined from the shaft in the bottom land in all directions at a maximum depth of about 100 feet.
Deposits not at present utilized. — Old partly filled pits where gypsum, or "plaster," as it is commonly called, was mined from the surface in earlier days are visible all along this belt from a point a mile west of Plasterco to the vicinity of Chatham Hill. Large quantities of good gypsum still remain in these old workings. Near Plasterco large pits,
64 Salt And Gypsum Deposits Of Southwestern Virginia.
abandoned shafts,, and caved-in ground abound, marking the places where the Buena Vista Co. and the Eobertsons formerly operated extensively and removed much of the available gypsum that was close to the surface. Smaller openings were made in the embayment about 1 mile to the south- west, but the deposits there have been only slightly explored. They are all owned by the old Buena Vista Co. and are leased to the United States Gypsum Co. In the Saltville Valley thick deposits of gypsum are reported in all the wells drilled for salt, and some beds at the surface were formerly mined for the manufacture of a kind of cement. They are owned by the Mathieson Alkali Co. and are not now being worked.
At North Holston and in the embayment just east of it several old gypsum pits formerly worked on the Pierson and Miller farms are nearly obliterated. Several old pits are to be seen also near Broad Ford, some to the west but most of them in the broad embayment to the east. One is still open in the river bank on the Taylor farm, about a mile east of Broad Ford, where the gypsiferous shales have been dug out from beneath the overthrust Cambrian dolomite. Another pit on the Taylor farm is among the low hills to the northeast, beyond the point where the North Fork of Holston Eiver leaves the belt of the Maccrady formation. A shaft on the adjacent Barnes place opened a large deposit by drifts but is now abandoned and filled with water.
Northeast of the Taylor farm conditions continue to appear favorable for the occurrence of gypsum, except that the exposed area of the Maccrady formation is narrow, but gypsum is not known to have been reported in the next 3 miles. Beyond, however, on the Buchanan property, important deposits occur and were mined on a large scale and crushed in the com- pany's mill on the property. The smaller holes have fallen in and been filled up, but some of the larger ones are full of water and are reported to be very deep. Pits are scattered over the broad embayment in the Maccrady formation not only in the Locust Cove Creek bottom but also on the low divide and small valley to the west. Several pits were also located north of Chatham Hill, and the crude gypsum was crushed in a water- power mill on the river at Chatham Hill.
Origin Of The Deposits.
Former Views.
In his early description of these deposits W. B. Rogers correctly identifies the beds inclosing them as "Lower Carboniferous" and states further that they are at the fault contact between these beds and older
Origin Op The Deposits. 65
limestones. As to their origin he adopts the explanation that oxidizing iron pyrites in the shales produced sulphuric acid, which, acting on lime- stone, converted it into calcium sulphate. He says :a
In speculating upon the origin of the gypsum of this region, the readiest explanation that suggests itself is that which ascribes its production to similar causes with those which gave birth to the gypsum of the Tertiary strata of lower Virginia. It has been incidentally remarked above that pyritous slate occurs in fragments mingled with the gypsum and clay of the salt wells and other places. Supposing the valley to have once been filled with the debris of this slate and of the neighboring limestones, we would have all the materials brought together which are necessary for the production of the gypsum, while the slate after decomposition would become the clayey matrix in which the crystals would collect. This view is rendered more probable from the occurrence, even in the midst of the solid masses of plaster, of fragments of the siliceous rock which skirts the valley on the south. It is at least certain that the gypsum has not been deposited here, as in some other parts of the world, from the waters of thermal springs holding it in solution, since in that case it would be found disposed in layers as travertine and not in the irregular and scattered condition which has been described.
J. J. Stevenson,5 in 1885, after describing the mining development, occurrence, and distribution of the gypsum and salt, arrives at somewhat similar conclusions, as follows:
1. The gypsum deposits are not beds of Carboniferous or Cambro-Silurian limestones changed into gypsum.
2. These deposits occupy deep basins, which have been eroded in Lower Carbon- iferous shale or limestone or in the hard, slightly calcareous sandstones of the Knox group. In at least two localities branches protrude from the main body into drains or ravines, so that the horizontal plan resembles somewhat the splash made by throwing soft mud against a wall.
3. The character of the deposit is wholly independent of the rocks on which it rests.
4. The gypsum occurs in irregular masses, incased in red marly clay, which penetrates the gypsum to a variable distance; there is less of this clay in the eastern basins than at Saltville.
5. At a variable depth salt occurs with the gypsum, and this salt contains very little of iodides or bromides.
6. Blue clay overlies the gypsum at all localities yet examined.
7. No fossils of any sort have been found 'thus far in the gypsum, its incasing red clay, or in the overlying blue clay; but just west from Saltville a conglomerate cemented by gypsum occurs, in which remains of Mastodon have been found ; this overlies the blue clay and incloses many fragments of both blue and red clay.
8. These gypsiferous deposits occur in the vicinity of the Saltville fault.
But the amount of the erosion and ,the general relation of the gypsum to the blue clay, with the relation of the latter to the Quaternary conglomerate, suggest that the gypsum is not older than the Tertiary; until some fossils have been discovered, however, the question of age must be regarded as undetermined.
Capellini ascribes the formation of this gypsum [at Castellina Marittima] to the action of sulphur springs on calcium carbonate held in solution ; so that the
aRogers, W. B., A reprint of annual reports ... on the geology of the Virginias, 1884, pp. 141-142.
bProc. Am. Philos. Soc., vol. xxii, 1885, pp. 157-160.
66 Salt And Gypsum Deposits Of Southwestern Virginia.
carbonate was changed into sulphate and deposited as such in the littoral lakes of the middle Miocene. . . . The origin of the Holston gypsum is to be accounted for in some similar way. Several deep basins were occupied by lakes; that of the Saltville basin received not a little calcareous matter from the Lower Carboniferous beds forming its northerly shore, and some doubtless was received from the wash of the Knox beds on the southerly shore; in the basins farther east the calcareous matter derived from the wash should be far inferior to argillaceous matter. But the composition of the gypsum shows less of the red clay at Buchanan's than at Saltville. The principal source of the calcareous matter must be looked for not in the wash from the shores but in springs. That calcareous springs can produce deposits as extensive as those of this region is sufficiently shown by the extensive deposits around many of the springs at the far West. The calcium carbonate in solution would be converted into calcium sulphate by the sulphurous springs also issuing from the fault, and the gypsum would be deposited as such.
The red marly clays were derived from the wash and are more abundant at Saltville, where the soft red shales at the top of the Lower Carboniferous are fully exposed on the northerly side of the basin.
E. C. Eckel,a in 1902, concluded that the deposits were interbedded as original sediments in the "Lower Carboniferous" :
Though the salt and gypsum deposits have been long known and worked and have been examined by many geologists, a wide range of opinion exists as to their age and origin, as will be seen on comparing the literature of the subject. It is sufficient in this place to note that, as to age, the deposits have been variously referred to the Silurian, Carboniferous, Triassic, Tertiary, and Pleistocene, while different authorities have considered them as originating from deposition from sea water, from deposition from lakes, by the decomposition of pyrite and resulting action on fragments of limestone, or by the action of sulphur springs on unweathered limestone.
The work of the last field season would seem to prove that both the salt and gypsum deposits originated from deposition, through the evaporation of sea water in a partly or entirely inclosed basin, and that they are of Lower Carboniferous age, being immediately overlain by the massive beds of the Greenbrier limestone and underlain by Lower Carboniferous sandstones.
Observed Relations.
The most striking fact in connection with the gypsum and salt deposits of this district is that they have been found in quantity only in the shales of the Maccrady formation along the Saltville fault. These shales also outcrop along the North Fork of the Holston southwest of Saltville, on the west side of the syncline, but so far as known neither gypsum nor salt has been observed in this area of the formation. Stevenson reported gypsum on both sides of the fault on the Miller and Buchanan tracts northeast of Saltville, but these observations seem to be in error in that the fault was not accurately mapped, which is not strange, for the altered Carboniferous limestone very closely resembles the Cambrian dolomite, and some of the red shales of the Cambrian closely resemble those of the Carboniferous.
aBull. U. S. Geol. Survey No. 213, 1903, p. 40G.
Origin Of The Deposits. 67
An effort has been made to obtain a carefully measured section of the Maccrady formation to determine the position of the gypsum and salt- bearing beds, but with scant success. In the broad flats where the gypsum occurs there are generally no outcrops except red clay and gypsum, and consequently there is little hope of solving the relation southwest of Saltville. Not even the base of the Maccrady, which is the most definite key horizon, is exposed there.
Northeast of Saltville there are a few good exposures, but generally where the gypsum occurs the inclosing rocks are soft clays and are hidden. The river cliff southwest of Maccrady is the best exposed section of these beds in the area, and the following details were measured there:
Partial section of Maccrady formation west of Maccrady, Va.
Thickness
Feet.
Dark crumbly fossiliferous shale and earthy gray limestones 50±
Alternate thick earthy limestone, calcareous shale, and thin crystalline
fossiliferous limestones 60
Massive-bedded bluish tough calcareous and argillaceous sandstone with
fossiliferous calcareous layers 25
Gray sandstone, weathering brown 5
Shaly earthy contorted sandy limestone 31
Hard thick-bedded bluish calcareous sandstone 20
Softer shaly earthy sandstone 30
Thick bed of earthy sandstone 6
Hard impure limestone, with chert nodules 8
Thick soft earthy sandstone 10
Shaly earthy limestone GO
Thick-bedded to shaly earthy sandstone 45
Covered, probably some red shale, shaly earthy limestone, and soft earthy
sandstone 225 ±
Red shaly sandstone and shale, mottled yellow 10
Red shale in part, rest covered
Red shaly sandstone, mottled yellow
Shaly gray sandstone, with phosphatic fish plates
Sandy shale, in part covered 20
Soft shale, light buff to dark drab; light-gray fire clay with rootlets,
leaves, and twigs at base 20
Black coaly fissile shale 1
Slabby blue even-grained irregular-bedded sandstone, weathering buff (top
of Price sandstone).
68 Salt And Gypsum Deposits Of Southwestekn Vibginia.
The next best partial section is just east of Watson Gap, 2 miles southwest of Broad Ford, which is as follows:
Partial section of Maccrady formation east of Watson Gap, Va.
Thickness
Feet.
Thin-bedded earthy limestone, with some hard dense beds 30
Purple fissile shale, with some earthy limestones 14
Fissile red shale 10
Micaceous red sandstone, mottled yellow
Fissile and crumbly red shale, mottled yellow 37
Hard yellow and red agglomeratic shale 1
Crumbly red sandstone and some yellow shale 10
Harder red sandstone, in part shaly 4
Red argillite and shale, with drab sandy concretionary masses
Greenish fire clay, with rootlets, red at surface 2
Crumbly and fissile red and yellow shale 30
Soft greenish micaceous sandstone, purplish at top 20
Soft yellow shale 4
Black fissile coaly shale 3
Thin sandstone and fire clay, with rootlets 1
Greenish fissile shale 10
Thin irregular-bedded sandstone 8
Sandy light-buff fire clay, with rootlets 3
Covered, probably thin sandstone and shale 10
Massive-bedded greenish-gray calcareous sandstone (top of Price sandstone).
Just east of Broad Ford is another fair exposure that shows the relations of the gypsiferous shales to the rest of the formation:
Partial section of Maccrady formation east of Broad Ford, Va.
Thickness
Feet. Soft red and green shale and
clay, with some soft thick brown sandstone and earthy limestone .
Red and green shale . . Red rippled sandstone.
Weather or change laterally into gypsum-bearing red plastic clays with secondary limestone layers 150
Gray shale with red sandstone
bed
Red shale and sandstone
Gray shale
Thin black fissile coaly shale 1
Earthy limestone and calcareous shale 12
Covered, probably in part soft earthy limestone 40+
Red sandstone and sandy shale (with unexposed gray sandstone, shale,
and carbonaceous seams to base of formation), estimated 130±
Origin Op The Deposits.
From the relations observed in the northeastern part of the area it may be stated that the gypsum does not occur in the lower red siliceous beds of the formation and probably not lower than 180 feet from the base; that thin-bedded argillaceous limestones which are characterized by a small spirifer resembling S. bifurcata generally occur near the top of this barren interval; that the gypsum seems to replace certain soft earthy sandstones, shales, and limestones in the overlying portion of the formation present in that part of the area.
Southwest of Saltville, where the surface exposures do not show the relations of the gypsum, the well records also do not aid much in their solution. From a glance at the records of the Mathieson Alkali Co.'s borings, kindly permitted by Mr. W. D. Mount, manager of the plant, no clue was gained as to the sequence of the gypsum and salt beds or of their relation to recognizable limestone, sandstone, or hard red sandy beds. The basal barren sandy beds were not observed, even in the deepest well. A generalized record of one of the typical wells of the Mathieson Co. illustrates the relative distribution of the gypsum and salt which prevails throughout most of the sections.
Generalized section of a well at Saltville, Va.
Thickness
Depth
Limestone and shale
Feet
Feet
Shale and gypsum .
Mostly shale with gypsum and some rock salt
Mostly limestone with shale, gypsum, and rock salt Mostly shale with gypsum and rock salt
Mostly rock salt with little shale
1,092
The record of a well on the Buena Vista Plaster Co.'s property at Plasterco, as given by T. L. Watson, is as follows :
Section of well at Plasterco, Va.
Thickness
Depth
Red clay
Feet
Feet
Clay and plaster
Impure plaster
Pure plaster
Slate and plaster
Xearly all plaster
Blue slate
Blue slate and plaster
Yellow soapstone
Pure plaster
Red rock with little salt
70 Salt And Gypsum Deposits Of Southwestern Virginia.
The distribution of gypsum throughout several hundred feet of strata in the wells at Saltville and Plasterco indicates that, even if the beds have a relatively steep dip, the gypsum has a wide vertical range in the south- western part of the area and may replace higher beds in the formation than occur at the surface in the northeast.
Conclusions.
It can not be determined positively from the well records whether the deposits are in thick continuous beds or, as has been found to be the condition in the mines at Plasterco, in detached segregated masses. The distinct interbedding, however, of the gypsum with limestone, shale, red clay, and rock salt in the Saltville wells precludes the idea that the deposits were formed in wash from the surrounding higher areas into a trough or lake, as suggested by Stevenson. The gypsum beds have nowhere been mined deep or far enough to determine how they change laterally into other sedimentary rocks. This must be inferred from such facts as can be gathered in the mines, on the surface, and in the well records.
The conclusion expressed by Eckel that the deposits are strictly sedi- mentary in origin, having been derived from the evaporation of confined bodies of water under salt-pan conditions, is believed by the writer to be only partly correct. The fact that the beds of almost solid gypsum 50 to 100 feet in thickness vary greatly, occurring at intervals along the belt of these rocks, with barren areas between, and, so far as known, not at all on the northwest side of the syncline away from the fault, does not harmonize with this view. That salt-pan conditions could be so local and still persist for so long a time as to form such thick beds of gypsum and that these conditions could be repeated over and over again in the same place while not occurring at all in intervening areas is highly improbable.
The facts that the gypsum is segregated in workable deposits in the Maccrady formation at intervals along a fault contact, with barren areas between, and that none occurs in the same formation, so far as known, where not adjacent to the fault, are more reasonably explained by assuming, first, that gypsum was originally deposited as disseminated grains and innumerable thin leaves with argillaceous and calcareous silt and earthy sand of the Maccrady formation in a partly inclosed arm of the sea, at times subjected to intense evaporation; second, that the gypsum was later concentrated in the same formation by ground waters, which, circulating along the fault, dissolved part of the disseminated calcium sulphate and redeposited it in adjacent gypsiferous beds, the gypsum being segregated
Origin Of The Deposits. 71
by chemical selection. The calcium carbonate in the calcareous silt was likewise dissolved by the meteoric waters and the gypsum has taken its place, possibly by direct replacement, the waters, being carbonated, dis- solving the calcium carbonate and depositing the calcium sulphate.
A sample of unaltered earthy limestone from the horizon of the gypsum- bearing clays of the Maccrady formation near Broad Ford was analyzed for F. A. Wilder, president of the Southern Gypsum Co., and was reported to contain 4 per cent of CaS04. Another sample from the limestone quarry across the river from the Mathieson Alkali works, analyzed in the chemical laboratory of the U. S. Geological Survey, showed 3.16 per cent of CaS04 present. This may represent the amount of disseminated gypsum present in the original calcareous silt.
In addition to the facts mentioned above pointing to this conclusion, several other observations may be cited. The occurrence of large crystalline sheets of selenite in the granocrystalline mass and especially of small veinlets of satin spar in the otherwise barren inclosing clay, affords positive proof that solution and redeposition may have taken place to some extent. The massive gypsum has the appearance of bedding, due to the banding of gray impurities, but on close observation this is found to be not sedimentary banding parallel to the inclosing strata but concentric banding parallel to inclosed bodies of "black rock," fine particles of the argillaceous material producing the dark banding. These argillaceous masses may have resulted from less soluble clayey masses in an otherwise calcareous gypsiferous bed which was gradually encroached upon during the concentration of the gypsum and particles of it were left as banded impurities in the gypsum; similar drab argillaceous concretionary masses were observed in the red argillite 94 feet above the base of the Maccrady formation on the road east of Watson Gap. Or, on the other hand, the argillaceous impurities may have been segregated in the rounded masses by chemical repulsion during the concentration and purification of the gypsum. At least, both the banding of the gypsum and the rounded masses of argillaceous "black rock" appear to have resulted from the secondary segregation of the gypsum. The red plastic clay that generally incloses the gypsum is probably the fine argillaceous impurity of the earthy limestone left as a residuum, expelled by the crystalline segregation of the gypsum, and stained red by contained iron highly oxidized when set free during the process. Thin layers of fine- grained limestone in the gypsiferous clays were apparently redeposited from solution as another secondary mineral.
This theory as to the method of the concentration of gypsum is not new, for it has been proved beyond much doubt that the remarkable domes
72 Salt And Gypsum Deposits Of Southwestekn Virginia.
of salt and gypsum in Louisiana and Texas were formed by the deposition of these minerals along spring lines at the exposed intersection of fissures or faults/ having been dissolved and transported from some deeper-lying beds. Secondary limestone, apparently similar to the crackled layers in the clays of the Holston Valley area, also occur in the domes associated with the salt and gypsum. The fact that the Louisiana deposits were derived from lower beds suggests the possibility that the salt and gypsum in the Holston Valley area were also derived from beds at a lower horizon, that the solutions rose along the fault, and that these minerals were deposited at or near the surface in their present position. This explanation, however, is untenable, inasmuch as none of the older formations which outcrop to the west on the slopes of Clinch Mountain — not even the representative of the Salina, the great salt and gypsum bearing formation of New York — contain deposits from which these minerals could have been derived, and furthermore, as such strictly secondary deposits would be found only at or near the surface, whereas the Holston Valley deposits occur interbedded in the Maccrady formation to considerable depths.
If the theory of secondary concentration above suggested is the correct explanation of the origin of the gypsum in the Holston Valley area, it accounts for the absence of the mineral in quantity on the west side of the syncline away from the fault, the occurrence of natural outcrops of gypsum close to the fault, and the greater thickness of the deposits toward the southeast, as developed by borings in the Saltville, Plasterco, North Holston, and other tracts tested. In accordance with this theory it may be predicted that the gypsum will be found to extend under the overthrust Cambrian dolomite as far as the Maccrady formation is at the fault contact, and when the deposits near the surface are worked out deeper mining may be carried in this direction.
The beds of rock salt undoubtedly had the same origin as the gypsum and may be regarded as concentrations of somewhat saliferous beds, the associated calcium carbonate of the earthy limestone being dissolved out and its place taken by salt, segregated by solution and redisposition through chemical selection. Whether workable beds will be found asso- ciated with all the gypsum deposits can not at present be determined, but where salt has not been encountered in mining the gypsum there is still a prospect that it may be discovered at greater depth close to or under the overthrust dolomite. This is especially true southwest of Saltville,
aHarris, G. D., Rock salt: Bull. Louisiana Geol. Survey No. 7, 1907; Oil and gas in Louisiana: Bull. U. S. Geol. Survey No. 429, 1910.
Summary. 73
where the overriding Cambrian limestone conceals most of the Maccrady formation, as it is apparently turned under in a minor anticline next to the fault. Southwest of Plasterco both salt and gypsum may be expected along the fault some distance from its outcrop under the overthrust mass where the Maccrady formation is probably at the fault contact. This may be proved by either drilling through an unknown thickness of tough dolomite southeast of the fault or boring diagonally under it in the soft rocks at the fault contact.
Summary.
The gypsum and salt deposits of southwestern Virginia described in this report are believed by the writer to have been derived from calcareous- argillaceous sediments which originally contained disseminated gypsum and salt precipitated in a partly inclosed arm of the sea during the deposition of the Maccrady formation, these minerals having been con- centrated in the same formation by ground waters which circulated along the fault contact between the Carboniferous and Cambrian rocks, dissolved the calcium carbonate from the earthy limestones, and segregated the gypsum and salt in the gypsiferous and saline beds by chemical selection.
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