BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

VOL. 36. PP. 615-622 DECEMBER 30. 1925

MINERALIZATION OP THE PLATTEVILLE-DECORAH CON­ TACT ZONE IN THE TWIN CITY REGION1

BY CLINTON" R. STAUFFER

(Presented in abstract before the Society December 81, 1921^)

CONTENTS Page Introduction...... 615 Characteristics of horizons affected...... 616 Generalized section...... 616 The Platteville lim estone...... 616 The Platteville-Decorah contact...... 617 The basal Decorah sh ale...... 618 M ineralization...... 619 Crusted surface of the P latteville...... 619 Solution...... 620 Dolom itization...... 620 Pyrite deposition...... 621 Calcite deposition...... 621 Summary...... 622

I ntroduction

The middle of the Twin City (-Saint Paul) region forms an isolated patch of outcrops. It is an outlier from the great area to the south and east which is covered by these formations and to which it was definitely united until very recent (probably late Ter­ tiary) time. The characteristics of these formations, as observed in this outlier, are much the same as in the more extensive area. However, in both regions these formations differ somewhat in composition from place to place, and with this dissimilarity there is a corresponding variation in the chemical changes that have taken place since the deposition of the sediments. Recently rather extensive excavations and tunnelings have been made in the Minneapolis and Saint Paul area, which show especially the basal , all of the Platteville , and the upper

1 Manuscript received by the Secretary of the Society December 3 1 , 1 9 2 4 .

XLI—B u l l . G e o l . S o c . Am., V o l . 36, 1924 (615)

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part of the St. Peter sandstone. Since these cuts are in the fresh rock, quite removed from the outcrop and protected either by argillaceous beds of the Decorah shale or the compact boulder clays of the Kansan drift, they have afforded an unusual opportunity to study the unaltered Platte- ville-Decorah contact and the beds just below it. This constitutes a zone in which rather extensive mineralization has taken place and in which the order of these changes can be followed with a fair degree of certainty.

C haracteristics o f H o r iz o n s a f f e c t e d

GENERALIZED SECTION The following is a generalized section showing the relationship of the beds under discussion and is approximately that exposed during the con­ struction of the new Ford automobile factory in Saint Paul: Thickness

Decorah shale: Feet Inches ■9. Shale, argillaceous, soft, blue, alternating with occasional layers and lenses of blue limestone...... 30± 8. Limestone, gray to bluish...... 2 0 7. Shale, soft, gray to bluish w h ite...... 0 3 6. Limestone, dolomitic, hard, coarse, rough, brown in color.. 1 6 Platteville limestone: 5. Limestone, dolomitic, gray to bluish...... 8 0 4. Limestone, dolomitic, slialy, blue in color...... 6 0 3. Limestone, compact, hard, blue in color and with irregular slialy partings...... 12 0 2. Shale, bluish, somewhat calcareous and often sandy...... 3 0 St. Peter sandstone: 1. Sandstone, white, poorly cemented...... 70±

THE PLATTEVILLE LIMESTONE Exclusive of the shaly base (bed 2), usually regarded as transitional from the St. Peter sandstone, but probably a part of the succeeding formation, the Platteville is made up of three rather distinct divisions. These have no sure relation to the faunal content, but are dependent on the physical character of the sediments composing them and on their present chemical composition. The lowest of these divisions (bed 3) consists of about 12 feet of com­ pact, hard, blue limestone with numerous thin streaks of blue shale. The limestone layers are irregular and more or less nodular, yet continuous, and the thin shaly partings are an intimate part of the limestone; so that the whole separates into more or less massive beds at still more

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prominent partings. This is the building stone of the Twin City region and was formerly used on a rather large scald for such purposes. It con­ tains some fossils, which at certain places are quite abundant. Essen­ tially all of them preserve the shell, occasionally with even the mother- of-pearl luster. This portion of the formation has been changed but little, if at all, from its original composition as a calcareous mud on the bottom of the Ordovician sea, Numerous chemical analyses2 show it to run about 80 per cent CaC03, with only about 5 per cent MgC03. Most of the remainder is clay. The middle portion (bed 4) of the Platteville consists of about six feet of rather massive argillaceous dolomitic blue limestone which is often in part a true shale. It is quite compact and rarely fossiliferous, except in occasional pockets or patches where fossils may be common. Often these fossils are more or less broken up, as if they had been drifted together, but they are now preserved chiefly as molds. Analyses show this rock to contain about 26 per cent MgC03 and 46 per cent CaC03; the remainder is argillaceous material. The rock thus approaches a real dolomite, al­ though rather high in impurities. The upper 8 feet (bed 5) of the formation is a gray to bluish gray dolomitic limestone with little or no shale. These beds are often highly fossiliferous, and in places such remains are matted together like modem shells in the coquina of Florida. Except for the trilobites and lingulas, which still retain their original shells, these fossils are preserved as molds still more or less open and not deformed. Before the mineralizing changes, therefore, they were not only preserved as the original shell, but must have been almost undisturbed in the place where they fell when the animals died. Undoubtedly, therefore, when deposited this part of the formation was highly calcareous. At present, however, it is the most dolomitic part of the Platteville, having about 30 per cent MgC03 and only about 46 per cent CaC03. This is approaching the composition of the dolomitic of the Oneota (Lower Magnesian Series) and is even lower than the Oneota in its percentage of CaC03.

THE PLATTEVILLE-DECORAH CONTACT • Although the Platteville-Decorah dividing line throughout the State and in adjacent territory, as based on the fauna, is somewhat unsatis­ factory, the most convenient separating surfacc here, in the Twin City region, makes an exceedingly sharp line of contact.3 The top of the

2 The analyses for this paper were made by It. J. Leonard. 5 F. W. Sardeson : Am. Geol., vol. 22, 1808, p. 318.

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Platteville is slightly uneven and often somewhat pitted. Over this, and gradually disappearing downward through several inches or more, is a black stain caused by finely divided pyrite with a trace of manganese. At the top this stain is so concentrated that it forms a crustlike covering for the rock. Where this covering is well preserved the overlying Decorah separates readily from the Platteville; but where it was apparently broken through, when the later sediments were laid down, the Decorah is tightly welded to the Platteville, but still shows the dark band at the contact. Loose pieces of the Platteville incorporated in the basal bed of the Decorah are not abundant, but those found are also coated with the black stain, just as if they had lain upon the Platteville surface during the time it was receiving the coating. In a few instances the pyrite crust has been broken up and slightly mingled with the oncoming brown sediments of the basal Decorah. In such cases there may he formed a secondary, but less pronounced, band of blackened rock a few inches above the main one, but it is never so important, nor is there an upward permeation-of this stain corresponding to the downward penetration above described. As a break in sedimentation, this contact is not wholly unlike others that have occurred throughout the Ordovician or even far back into the of Minnesota. Some of these occur within formations and may have irregular surfaces associated with pebbles, but they have not developed the crusted surface and the accompanying pyrite stain to any­ thing like the perfection which it has attained at the top of the Platte- ville, nor is there the same evidence of the lapse of time, except at the well established contacts of formations. It seems safe to conclude, there­ fore, that the top of the Platteville, as here described, not only represents a break in sedimentation, but probably a real disconformity.

THE BASAL DECORAH SHALE There is a marked change in the character of sediment from the fine­ grained, rather dense, gray dolomitic rock of the Platteville to the hard, coarse, rough, brown dolomitic limestone of the basal Decorah (bed 6). Most of the limestone layers occurring at intervals in the Decorah shale are highly calcareous, and the contained fossils, except the Mollusca, are usually preserved with the original shells; but the basal layer, 18 inches thick, is nearly 14 per cent MgC03 and about 54 per cent CaC03, the latter substance being 8 per cent higher than in the upper layers of the Platteville. Its fossils, except the lingulas, are mere holes in the rock, which evidently represent poorly preserved molds. Just above the basal layer of the Decorah occurs an inch and a half

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to three inches of soft, pale bine to gray or white, tough, sticky clay shale,4 which is almost impervious to water. While this thin bed in itself may not have been sufficient to prevent the upward movement of the mineralizing solutions, it was reinforced by the thick clay shales above, so that the mineral changes at higher horizons in this locality are prac­ tically negligible and have no definite relation to those below. It is thus evident that the upper limit of the zone of marked mineralization is rather definitely drawn by the thin bed of shale near the base of the Decorah, and that the lower limit is equally determined by the argilla­ ceous shaly beds of the middle Platteville.

M ineralization 4 CRUSTED SURFACE OF THE PLATTEVILLE The formation of the blackened upper surface of the Platteville seems to constitute an event quite independent of the other mineral changes that have affected this zone. The finely divided condition of the pyrite, which causes the stain, together with the fact that it is chiefly confined to the contact and does not penetrate the overlying rock, although it does coat fragments of the Platteville incorporated in the Decorah, leads one to suspect that it antedates the latter in deposition, and that it may have been precipitated by bacterial action during a time when no other deposi­ tion was taking place. In the salty lagoons bordering the Black Sea, Andrussow5 found a finely divided iron sulphide being deposited partly in the mud and partly as a coating on other substances. This is being precipitated by different types of bacteria and forms a deposit suggestive of the top of the Platte­ ville. At least two of the bacteria, Vibrio hydrosulfurous and Bacterium, hydro&ulfureum, separated from the Black Sea mud,6 are sulphate re­ ducers and in the process liberate hydrogen sulphide in quantity. This will attack the iron present in the sea water, producing a sulphide, or the iron sulphides may be formed directly through the iron bacteria, thus giving rise to black m ud; but the interesting point here is that the very presence of hydrogen sulphide, and therefore of the bacteria themselves, would foul the sea bqttom and render it unfit for the abundant develop­ ment of the Brachiopoda and Mollusca which were so characteristic of the

4 F. W. Sardeson calls this bentonite. Pan-Am. Geol., vol. 42, 1924, pp. 47-48. 5 N. Andrussow : La Mer Noir. Cong. Geol. Internat., Guide des exc., 7° Sess., Exc. 29, 1897, p. 7. 6 W. Omelianski: Der Kreislauf des Schwefels, in Franz Lafar’s Handbuch der tech- niscUen Mykologie, Band 3, Aufiage 2. Jena, 1904-5, pp. 214-220.

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life of Platteville time. This would help to bring about the break in sedi­ mentation here observed and may have been its chief cause, at the same time that the sulphide crust and its accompanying stain were formed.

SOLUTION Prom the present condition of the fossils preserved in the upper part of the Platteville and the basal Decorah, it appears that the first mineral change taking place was that of solution or the removal of large quanti­ ties of calcium carbonate. This affected especially the more soluble shells, which occurred in abundance in the upper Platteville, but left un­ affected those of phosphatic composition, such as the trilobites. Ap­ parently the water responsible for this change in the Twin City region was unable to penetrate appreciably the lower part of the Platteville or the overlying Decorah, in both of which shaly beds occur, for in the lower beds all fossils retain the shell, while in the upper beds all but the Mol- lusca retain the shell. DOTjQMITIZA tion The next change was dolomitization, during which a little more than one-third of the calcium was replaced by magnesium. This caused a cer­ tain amount of shrinkage, which rendered the rock slightly porous, but the fossils were not destroyed. In the Oneota, or lowest Ordovician, where this process has gone only a little farther, fossils are exceedingly rare, except in the cherty layers of certain localities, where they are re­ placed by silica, and in which layers the number of fossils suggests their probable abundance in that formation as a whole when it was a limestone high in calcium carbonate. If the destruction of fossils is due to the process of dolomitization, it would seem that the introduction of only a little more magnesium might have been sufficient to remove most, if not all, traces of life from the upper part of the Platteville. The fact that they are still so well pre­ served is regarded as an indication that a certain amount of solution had preceded dolomitization, and that when the latter process took place there was no mineral matter in these cavities that could be affected by it. The rock thus became porons, but the molds retained their shape. These two processes are always somewhat related, as dolonfitization necessarily means the removal of calcium, but by a process of substitution of mag­ nesium for half of the calcium in 2CaCOs, resulting in the formation of CaMg(C03)2.7 In the case under consideration, however, the shells of

1 C. R. Van Hise : U. S. Geol. Surv. Mono. 47, 1904, p. 798. Also F. W. Clarke: U. S. Geol. Surv. Bull. 095, 1920, p. 557.

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the fossils have not been dolomitized, bnt entirely removed, which in itself seems to indicate that they were carried away in solution before the magnesium-bearing waters entered the rock.

PYRITE DEPOSITION After the shells had been removed by solution, iron-sulphide-bearing waters entered the same zone and a thin, continuous coating of pyrite was deposited as a lining over the inside of most of the molds, partially filling nearly all other openings. Some of the cracks and joints were thus con­ verted into miniature veins, while some parts of the rock, especially the basal layer of the Decorah, are highly impregnated with it. Pyrite and marcasite are common constituents of the Ordovician rocks of this region, but it is rarely that the order of their deposition can be so definitely fixed as in this case. As pointed out above, it is certain that part of the iron was introduced at an early date, but the greater part of that now occur­ ring in the upper Platteville and basal Decorah was brought in after solu­ tion and dolomitization. This is shown by its relation to the fossil molds. In many places the pyrite is finely crystalline, showing well developed cubes, many of which are twinned. Small quantities of sphalerite and even galena may occasionally be found, and their relation to the iron sul­ phide suggests that they may have been introduced with the pyrite.

CALCITE DEPOSITION After the period of pyrite deposition, solutions charged with calcium bicarbonate entered this zone, and the cavities that were earlier lined with pyrite received a coating of calcite which developed a variety of crystal forms. Many of the cavities were completely filled, while others re­ mained as irregular geodes. The smaller of these may be separated from the limestone as nodules of pyrite, which, when broken open, prove to be filled with masses of calcite crystals. In the matted masses of fossils, referred to as characteristic of parts of the upper beds belonging to the Platteville, the surfaces of the molds sparkle with the innumerable calcite crystal faces, and the identification of the species is thus often rendered difficult. Some calcite occupying pore spaces within the rock itself was apparently introduced at this time, but this is not quite so obvious. How­ ever, a complete reversal of the type of solutions passing through the rock had taken place. . Instead of the dissolving or substituting liquids, the waters were charged with calcium carbonate ready for deposition. The calcite crystals then formed are quite pure, running as high as 98.69 per cent CaC03. The order of mineral change taking place has thus con-

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formed to that of the mineralized region of Wisconsin8 and other similar, areas. The changes now taking place at the outcrop of this horizon are chiefly leaching of the carbonate anti disintegration of the pyrite. The miner­ alization here presented is thus soon obliterated, and there is left at such places no adequate record of the sequence of events that have taken place.

S u m m a r y

Exclusive of the pyrite crust at the top of the Platteville, it appears that the Platteville-Decorah contact zone has passed through four suc­ cessive stages of mineral changes. The first of these was solution affect­ ing chiefly the highly calcareous shells. Then followed dolomitization, during which magnesium was substituted for part of the remaining cal­ cium. Iron-bearing solutions next deposited pyrite in the pore spaces, cracks, and cavities. Later the pyrite coatings were covered by calcite, which either filled the cavities or converted them into geodes.

8 U. S. Grant: Wis. Geol. and Nat. Hist. Survey, Bull. 14, 1906, p. 74.

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