INTRODUCTION

It is the purpose of the writers to give a brief discussion of the structural features of a part of southwestern Indiana as indicated by data on the West Franklin formation. This double-bedded limestone formation has been referred to, in Indiana, by different writers as Productal limestone, Double limestone, Argillaceous or Bituminous limestone, West Franklin limestone, and Somerville limestone. The name West Franklin was the first geographic name proposed.(FOOTNOTE 3) Hence in compliance with the generally accepted rules of priority, the writers use that name rather than Somerville, which was proposed at a later date by the geologists of the United States Geological Survey (FOOTNOTE 4) for identically the same formation.

FOOTNOTE 3. J. Collett, 13th Indiana Report (1884), pp. 61-62.

FOOTNOTE 4. M. L. Fuller, "Ditney Folio, Indiana," U. S. Geol. Survey Geol. Atlas, Folio 84 (1902), p. 2.

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In a detailed paper on the West Franklin formation, now in preparation, a complete discussion of the question of nomenclature will be included.

Character of the formation:
The West Franklin formation consists of a lower, hard, blue to gray, brecciated limestone member whose average thickness is about 5 feet; a middle shale member which is variable in thickness (maximum, 25 feet) as well as in lithologic character; and an upper, dense, gray crystalline limestone member whose average thickness is about 3 feet. The upper member is only very rarely brecciated. Both limestone beds are somewhat fossiliferous, but with one exception no fossils have been found in the shale. A thin coal bed with accompanying black thinly-laminated shale marks the shale member in many localities.

In many places only one of the limestone members is present, the other being absent either because of lack of deposition, as would be true if the lower were absent and the upper present, or because of removal by erosion subsequent to deposition, as would be true if the lower were present and the upper, and perhaps also the shale member, absent. Only in one place was the upper member found resting directly on the lower with the middle or shale member entirely absent. The shale is ordinarily present where the upper member is present and the lower either absent or represented by a calcareous nodular shale, but in many places it is missing where the upper member is absent and the lower present. In the last condition it may be difficult if not impossible to distinguish between the shale me ber of the West Franklin formation and the shale formation which separates the West Franklin from the overlying Merom sandstone. A period of erosion preceded the deposition of the Merom sandstone, so that in many places much, if not all, of the underlying shale formation is removed, and where erosion was extreme, part or all of the West Franklin formation is gone, and the Merom rests disconformably upon the shale-sandstone sequence immediately underlying the West Franklin.

Obviously, therefore, much discretion had to be used in interpreting the available structural data. In structural determinations the top of the lower member of the West Franklin formation was used as a datum. In its absence either the residual clay resulting from its decomposition, or the interval between it and the upper member, were used to reduce the elevations to one plane. In a very few places, where the entire formation is absent, the Merom sandstone was found resting on the shale series below the West Franklin and the contact between the shale and the overlying

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sandstone was used, with the necessary limitations, to approximate the elevation of the lower member.

Stratigraphic relations:
The West Franklin formation is approximately 900 feet above the base of the Pennsylvanian and about 350 feet below the highest Pennsylvanian formation in Indiana. It consistently occurs 300 feet above Coal V, which, with its superincumbent black shale and limestone, is the most prominent and widespread stratigraphic marker in the Coal Measures of the Eastern Interior coal field. This interval may decrease to 280 feet or increase to 315 feet or slightly more, because of the local thinning or thickening of the intervening strata. Such a variation is commonly found in the Pennsylvanian strata of southwestern Indiana; hence, it is to be expected.

The formation is a serviceable marker in wells and shows in most of the wells in the region down the dip from the outcrop. It is almost everywhere underlain by a series of thin sandstones and sandy shales. A shaly or laminated, or more rarely a thin-bedded sandstone almost everywhere occurs about 15 or more feet below the base of the formation. This common sandstone bed is rather resistant to weathering and erosion and is conspicuous because it ordinarily causes small waterfalls and cascades where the smaller streams cross it. Immediately overlying the West Franklin is a shale and sandstone formation which ranges in thickness from a few feet to 30 feet. In many places a thin coal is present in this formation. In most exposures, however, much of the formation is gone, having been erode before the deposition of the overlying Merom sandstone.

The Merom sandstone in its exposures is characteristically a yellow, very friable, coarse-grained, highly cross-bedded, mica-bearing sandstone, 20 to 60 feet in thickness. In a few places it has been found to be 80 feet or more in thickness, but not in continuous exposure. Its disconformable base is made conspicuous by a thin conglomerate cemented by iron, some of which is in concretionary form. In some localities, however, these conspicuous characteristics of the Merom are lacking and the formation is composed of gray sandstone beds with much sandy shale, and it is difficult to distinguish it from other sandy formations commonly present in the Pennsylvanian of southwestern Indiana. It has been found resting on the shale and sandstone formation overlying the West Franklin formation, r sting disconformably on the upper, middle, or lower members of the West Franklin, and in a very few places even resting on the series of shales and sandstones below the West Franklin.

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This sandstone is ordinarily conspicuous in well logs and is of considerable assistance in identifying the West Franklin, because it is so consistently found either immediately above that formation or separated from it by only a few feet of shale, or shale and sandstone.

The following records show the character of the strata above the West Franklin formation and also between that formation and Coal V.

Table I. RECORD OF THE HAGEMANN WELL

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The following section was measured in the river bluff from West Franklin eastward to Priest's Bluff--a distance of slightly more than a mile.

The former locality is the type section of the West Franklin formation.

Table II. WELL NO. 3 (R. HOWE FARM)

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A careful study of this section shows marked lithologic and faunal changes in the strata both laterally and vertically. Especially conspicuous are the disconformity at the base of the Merom sandstone and the thinning of the members of the West Franklin eastward.

Outcrop area:
The West Franklin formation crops out in a sinuous line extending from the village of West Franklin on Ohio River southwest of Evansville in a northerly direction almost to Terre Haute (Figs. 1 and 2).

Starting at West Franklin the line of outcrop extends diagonally across Vanderburg County, thence northward through Gibson County, with isolated outliers on the east in the extreme eastern part of the county and in the northwestern corner of Warrick County, thence northward through central Knox County, thence slightly to the northwest through western Sullivan and southwestern Vigo counties, reappearing in Illinois in central-eastern Clark County. The line of outcrop also crosses from Indiana into Illinois in northeastern Knox County where it swings across the Wabash in a gentle curve just south of Palestine, Illinois.

South of Princeton outcrops are numerous and the outcrop can be traced with reasonable accuracy, but north of that city, due to the heavy

Table III. SECTION BETWEEN WEST FRANKLIN AND PRIEST'S BLUFF

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mantle of glacial drift on the one hand and the broad valley of the Wabash River on the other, outcrops are few and ordinarily are a considerable distance apart.

Age and correlation:
The West Franklin formation is of post-Allegheny age. Southwestward from the type section at West Franklin, Indiana, it is found exposed in the banks of Ohio River below Uniontown, Kentucky, at the Middle and Lower Highlands Reefs. It extends farther

Fig. 1. Outline map of the Eastern Interior coal basin showing the outcrop line of the West Franklin limestone in southwestern Indiana.

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southwest, rising more than 100 feet to Grundy Knob on the south.(FOOTNOTE 1)

Farther north, the West Franklin formation, which is well developed in the high river bluff at Merom, Indiana, is exposed a short distance southwest of Merom in the hills southeast of Palestine, Illinois (Fig. 2). From the outcrops in southwestern Vigo County, Indiana, it is only a short distance northwest to Ashmore and Big creeks, in central-eastern Clark County, Illinois, where the West Franklin is well developed and is overlain by a great thickness of typical Merom sandstone.

The similarity of the New Haven limestone of Illinois, and the sequence beneath, to the West Franklin and subjacent formations of Indiana, might suggest that both are of the same age, but it is more probable that the New Haven is a higher formation. This correlation, which must be settled by careful field work, was made long ago by Cox (FOOTNOTE 2) and later by Udden.(FOOTNOTE 3)

REGIONAL STRUCTURE

The strike, like the outcrop, approximately parallels the eastern boundary of the Eastern Interior coal basin (Figs. 1 and 2).

Toward the south the structure contours change to an east-west trend, paralleling the Rough Creek fault. In the northern part of the area near Merom, the structure contours double back, indicating a syscline north of Vincennes and a possible terrace structure west of Sullivan. This terrace structure may in part account for the oil production in the several small fields northwest of Sullivan. Elsewhere throughout the area the general strike is north-south and the dip is in a general westerly direction at about 25 feet per mile.

FOOTNOTE 1. This interpretation is not in accord with that of Lee ("Geology of the Kentucky Part of the Shawneetown Quadrangle," Kentucky Geol. Survey (1916), p. 53) who states, "North of the Rough Creek fault the structural deformations are much more gentle. From Spring Grove, where the Herrin coal outcrops, the beds dip west of north at the rate of about 125 feet to the mile. Toward the east, outside the area, the dip of the beds seems to be east or northeasterly. The coals at Morganfield, 5 miles distant, are 225 feet lower than those near Spring Grove, and at Uniontown, 3 ½ miles northeast of the quadrangle, the coals are 100 feet lower than at Spring Grove but nearly 300 feet higher than at Grundy Knob. These facts suggest that a pitching anticline extends northeastward from the vicinity of Spring Grove toward Uniontown." He also states (page 39) that the limestone exposed on Grundy Knob is 590 feet above Coal No. 9 (Coal V of Indiana) and that it is probably higher than the limestone outcropping in the river bank below Uniontown. Finally, his structure map does not seem to be consistent with either of the preceding statements. The writers consider the limestone formations at West Franklin, in the river bank below Uniontown, and on Grundy Knob as being of the same age, and they present structural and stratigraphic data to support this statement in a detailed paper now in preparation.

FOOTNOTE 2. "Geology of Gallatin County," Geol. Survey of Illinois, Vol. 6 (1875), p. 212.

FOOTNOTE 3. "Notes on the Shoal Creek Limestone," Geol. Survey of Illinois Bull. 8 (1907), p. 118.

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Fig. 2. MAP SHOWING OUTCROP AND REGIONAL STRUCTURE OF THE WEST FRANKLIN LIMESTONE FORMATION IN SOUTHWESTERN INDIANA AND ADJACENT PORTIONS OF KENTUCKY AND ILLINOIS By Robert R. Shrock and Clyde R. Malott 1929

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DETAILED STRUCTURE

The general stratigraphic and structural setting for the West Franklin formation having been given, it remains to point out some of the irregularities of the structure contours and what these irregularities may indicate.

Since numerous producing structures in southwestern Indiana are known to be small both in size and closure, closures of 20 feet or more are worthy of consideration as possible oil and gas reservoirs.

The reader, in considering the following descriptions, should follow closely the detailed map (Fig. 3).

From West Franklin to Evansville the beds strike northeast-southwest and dip rather uniformly (about 25 feet to the mile) toward the northwest. The data upon which the contours for this area are based were obtained with aneroid barometers; hence, the lines may vary slightly from absolute accuracy. From Evansville northward all data were obtained with a planetable and telescopic alidade.

Near the northern parts of Sections 22 and 23, T. 6 S., R, 11 W., a shallow syncline with an east-west trend flanks the narrow sinuous anticline which extends from the northern part of Evansville northwest and north to the northern part of Sec. 2, T. 6 S., R. 11 W. Approximately paralleling this anticline on the north is a shallow syncline which separates the anticline from the broad terrace-like structure, with two small domes, in the vicinity of Highland and Zipp.

At McCutchanville a small nose is present, flanked on the east and west by shallow synclines in Sections 27 and 34, T. 6 S., R. 10 W., and Sections 20, 21, 28, and 29, T. 6 S., R. 11 W., respectively.

One and one-half miles southwest of Elliott, in Sec. 2, T. 5 S., R. 10 W., the north end of an apparent anticline is present. Owing to inadequate data, the complete outline of this structure could not be determined. It is flanked on the west by a rather sharp syncline with a general north-south trend.

In Sections 23, 24, 26, and 27 a broad anticline is present, flanked on both the west and south by synclines.

The syncline shown in Sec. 35, T. 3 S., R. 10 W., is somewhat of a puzzle. Certain features about the outcrops in its vicinity indicate the possibility of a small fault, but it seems best to consider the structure as a sharp synclinal flexure. This syncline is separated from a shallow depression on the west by a narrow anticlinal ridge beginning in Sec. 11, T. 4 S., R. 10 W., and continuing in a northerly direction to Sec. 26, T. 3 S., R. 10 W.

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Fig. 3. STRUCTURE MAP Based on exposures of the West Franklin limestone formation (upper Pennsylvanian) in Vanderburg and southern Gibson counties, Indiana By Robert R. Shrock and Clyde R. Malott 1929

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If more detailed data were available for the area between West Franklin and Evansville, it is very probable that the general uniform dip shown on the map would be complicated by small domes and basins similar to those on the north.

ORIGIN OF THE STRUCTURES

It is hardly necessary to state that the regional dip of the geological formations of southwestern Indiana is caused by their situation on the west flank of the Cincinnati arch. Little of the general dip of the formations may be regarded as initial. The larger structural and regional relationships indicate that the regional dip is dependent almost wholly on the diastrophic movements which resulted in the making of the Indiana-Illinois coal basin and the Cincinnati arch.

Locally, however, as indicated by the detailed map (Fig. 3), the general regional structure is complicated by small domes or anticlines and basins or synclines. The irregularity in shape and distribution of these structures, their relatively small size and low dips, and their variable alignment, together with the absence of faults, all indicate that some origin other than regional deformation must be appealed to in accounting for them.

On several different occasions, while doing structural work on the coals of southwestern Indiana, the senior writer (Mr. Malott) has found evidence supporting the theory that many of the small irregular domes and basins, or anticlinal and synclinal flexures, were formed as a result of differential compacting or subsidence of the muds which were deposited either on a sea bottom of some topographical diversity or over and about a sand lens which was deposited practically contemporaneously with the overlapping muds.

In either case differential settling and compacting of the muds would bring out in relief the buried hills or sand lenses as anticlines or terraces, which would become less pronounced upward through the overlying formation.(FOOTNOTE 1)

FOOTNOTE 1. The idea of local domes and basins being formed by the differential compacting of muds over and about a mass which could be compacted but little was proposed long ago in geological literature and has been elaborated and used very frequently since its proposal. Reference will be made here only to a few of the more recent papers, for a detailed review of the literature on this subject is beyond the scope of this paper. See: E. Blackwelder, "The Origin of the Central Kansas Oil Domes," Bull. Amer. Assoc. Petrol. Geol., Vol. 4 (1920), pp. 89-94; L. P. Teas, "Differential Compacting the Cause of Certain Claiborne Dips," Bull. Amer. Assoc. Petrol. Geol., Vol. 7 (1923), pp. 370-78; H. D. Hedberg, "The Effect of Gravitational Compaction on the Structure of Sedimentary Rocks," Bull Amer. Assoc. Petrol. Geol., Vol. 10 (1926), pp. 1035-72; R. W. Brown, "Origin of the Folds of Osage County, Oklahoma," Bull. Amer. Assoc. Petrol. Geol., Vol. 12 (1928), pp. 501-13; and R. W. Brown, "Occurrence of Folds of the Osage Type," Bull. Amer. Assoc. Petrol. Geol., Vol. 12 (1928), pp. 675-77. In the last paper Brown states (p. 676) that folds of Osage type probably occur in Indiana in the Ditney quadrangle. See also C. M. Nevin and R. E. Sherrill, "Studies in Differential Compaction," Bull. Amer. Assoc. Petrol. Geol., Vol. 13 (1929), pp. 1-22.

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An example of a dome which probably owes its origin and contour to differential compacting of sediments about and over a buried sand lens has been mapped by Malott approximately 1½ miles northwest of Linton, Greene County, Indiana.

The contours drawn on Coal IV show a nearly round dome with a diameter of about a mile and a closure of more than 40 feet. A test well, drilled on the top of this dome to Coal III, showed a lessening of approximately 16 feet of the normal interval between Coal III and Coal IV. Subsequent drilling for oil showed a thick sandstone below Coal III with no folding below the Mansfield sandstone, the basal formation of the Pennsylvanian of Indiana.

Rawles (FOOTNOTE 1) concludes that

Most of the domes of this type are not perfectly developed, usually being terraces closed on but three sides. They are often connected by saddles with other domes. A line of these may be found on a line running southeast from Terre Haute, Indiana.

Although no examples have come to the writers' attention in which structures are due to differential compacting of sediments upon and about buried topographical eminences, such structures may very possibly occur in southwestern Indiana. It seems best, therefore, to consider the low domes and shallow basins shown by contours on the lower member of the West Franklin formation as owing their origin to differential compacting or settling of sediments, chiefly muds, over and around large sand lenses below the formation, rather than to any of the other processes previously mentioned, particularly regional deformation.

SUMMARY AND CONCLUSIONS

The West Franklin formation, consisting of a double-bedded limestone of post-Alleghenian age occurring about 300 feet above Coal V, is an important key horizon in the Pennsylvanian of southwestern Indiana. Both its line of outcrop and the strike of its structural contours approximately parallel the eastern boundary of the Eastern Interior coal basin. The formation continues into Illinois on the north, cropping out along Ashmore Creek in Clark County and in the hills southeast of Palestine in Crawford County; and into Kentucky on the south, cropping out in the bank of Ohio River below Uniontown and in Grundy Knob in Union County.

The regional dip of the formation is toward the west at 20-30 feet to the mile. In the southern part of the outcrop area, detailed structural

FOOTNOTE 1. Unpublished M. A. thesis at Indiana University, 1926. Quoted with the author's permission.

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maps show small low domes and shallow basins complicating the uniform regional dip. These minor structures, which are possible reservoirs for oil and gas, are thought to have been formed by differential settling and compacting of muds over and about bodies of sand or sand lenses.

DISCUSSION

GAIL F. MOULTON, Urbana, Illinois:
The paper by Robert R. Shrock and Clyde A. Malott contains valuable geologic information on southwestern Indiana, and it is an important contribution to the geology of that region. Their conclusion, however, regarding the origin of the minor structural features in the region should not be applied generally. The casual reader, not familiar with the region, would be led to infer that the authors intend a general application of the conclusion that unequal compacting of sediments of Pennsylvanian age has resulted in the development of local structural "highs" over the thicker sand bodies. Although the detailed structure map includes an area which extends only a moderate distance north of Ohio River, the authors cite the case of a minor structure nearly half way to the northern part of the st te in support of their conclusion. Thus a general application of the conclusion seems to be indicated. Some of the Indiana oil pools seem to provide contradictory evidence on this point.

The southwestern Indiana oil pools are unlike most of those in southeastern Illinois in that they are generally of smaller extent and rarely include areas of more than 500 acres. This condition is due both to the small size of the local domes on which the pools occur, and to the irregular character of the producing sand. The structures generally have a closure of 30 feet or less in the producing beds, and most of them have much less than this amount in the surface formations. Consequently, the type of minor structural irregularities shown on the detailed structure map included in the paper by Shrock and Malott is the type with which most of the oil production of the region is associated.

The following list of the producing oil pools of southwestern Indiana is incomplete, but includes all of those on which the writer has sufficient structural data to determine the relation between the structure of the near-surface formations and the oil-producing formations. Of these pools, all except the Tri-County field are indicated by the presence of favorable structure either in the West Franklin formation or in one of the lower coals in the Pennsylvanian system.

Table

In most of these pools the structure of the Pennsylvanian is similar to that of the Chester and deeper producing formations, but is not nearly as pronounced.

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Consequently, it seems to the writer that the only just conclusion to be drawn regarding the origin of the structures, is that, although many of the minor structures in southwestern Indiana are probably caused by unequal compaction of sediments of Pennsylvanian age, fairly complete data for a number of oil pools in the region show that some minor structures in the surface formations indicate the presence of more pronounced structures in the underlying formations. In prospecting for oil on these structures, failure may result from adverse sand conditions in the prospective producing formations; in such cases, the structure should not be condemned.

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