Rocks of Silurian Age
The term Hunton limestone was introduced in 1902 by Taff (1902) for the Devonian and Silurian limestones and dolomites in southeastern Oklahoma between the Sylvan shale (Maquoketa shale) and the Woodford chert (Chattanooga shale). The term Hunton limestone is a widely used name, especially in the subsurface of Oklahoma and Kansas, in places where it is impossible or immaterial to distinguish the various formations into which the Hunton can be divided. However, at least four limestone formations, two of which are of Devonian age and two of Silurian age, are now recognized in the Hunton rocks of Oklahoma.
At outcrops in southern Oklahoma, the upper or Devonian part of the Hunton consists of the Bois d'Arc limestone underlain by the Haragan shale. The lower or Silurian part consists of the Henryhouse shale underlain by the Chimneyhill limestone. These formations are separated from each other and from the overlying Woodford chert or Chattanooga shale and from the underlying Sylvan shale by unconformities of a magnitude adequate to account for the local absence of any of these formations. In some localities in Kansas and Oklahoma the Hunton is composed exclusively of Silurian rocks and in others exclusively of Devonian rocks. It is obvious, therefore, that the Hunton is a group of formations of variable continuity and character rather than a constant unit.
The most complete Silurian section in Missouri is in the southeastern part of the state where the following formations, listed in ascending order, are exposed: the Girardeau limestone, the Edgewood limestone, and the Brassfield limestone, of lower Silurian age, and the Bainbridge limestone of middle Silurian age. In southeastern Missouri, the equivalent of the Maquoketa shale in some localities is conformably overlain by Girardeau limestone. Some geologists consider the Girardeau limestone the final deposit of the Ordovician because no hiatus intervenes between it and the Maquoketa whereas there is a pronounced unconformity at its top.
The Chimneyhill limestone is the oldest Silurian formation in Oklahoma. It consists, in ascending order, of a basal white oolitic limestone member, a glauconitic limestone member, and a pink crinoidal limestone member (Reeds, 1911, pp. 258-260). According to Ulrich (1930, p. 73) the Chimneyhill corresponds, in ascending order, to the Noix limestone (of Edgewood age) of Missouri and to the Brassfield and St. Clair limestones of Arkansas. The Henryhouse shale which overlies the Chimneyhill in Oklahoma consists of earthy limestone and marly beds with intercalated shale. According to Ireland (1939, p. 190) both the Henryhouse shale and the Chimneyhill limestone are represented in the St. Clair of eastern Oklahoma. Ball (1939, p. 126) suggests the correlation of the Henryhouse shale of Oklahoma with the Bainbridge limestone of Missouri and Illinois.
Studies of Silurian outcrops in areas outside the Forest City basin reveal frequent interruptions of sedimentation. Most of the Silurian formations are separated from overlying and underlying strata by unconformities. Unconformities are also recognized or suspected between the several members of the Edgewood.
Ireland (1939), who studied the insoluble residues of Silurian and Devonian rocks of Oklahoma, reported that certain species of foraminifera are found exclusively in the Silurian. Ireland's work provides the criteria by which it is inferred that the Silurian rocks of northeastern Kansas were at one time continuous with those of Oklahoma and Missouri.
The Silurian rocks of northeastern Kansas consist of the Brassfield and Edgewood limestones which are represented in Oklahoma by the more inclusive term Chimneyhill. In the Coronado No. 1 Parks well, in sec. 16, T. 10 S., R. 8 E., Riley County, the Silurian rocks are 150 feet thick. Elsewhere in northeastern Kansas they are somewhat thinner, because the Silurian was cut off by erosion that preceded the deposition of the Devonian rocks. Thicker sections of the Silurian are known toward the north in southeastern Nebraska. Four lithologic zones have been differentiated in the Silurian of northeastern Kansas mainly by insoluble residues.
Oolitic zone--The lowest zone of the Silurian rocks recognized in northeastern Kansas consists of coarsely sucrose to crystalline dolomite characterized by the presence of dolomitized oolites. The oolites, like the matrix, are composed of coarsely sucrose dolomite and their surfaces are rough with minute crystals of dolomite. The concentric banding usual in oolites has been destroyed by recrystallization. The insoluble residues from one well revealed a few thin white cherty oolitic crusts but the residues from this zone are essentially noncherty. In many of the samples the oolites impinge upon each other and when drilled do not break free and are only partly and imperfectly rounded. Some of them resemble grains worn to roundish surfaces in drilling. In some samples from wells drilled with rotary tools the oolitic character is recognized by the shape of the interstitial cavities. The insoluble residues of the oolitic zone are less than 2 percent by volume and consist mainly of clear hackly quartz and drusy quartz. The oolitic zone is 15 to 30 feet thick and includes a band of nonoolitic dolomite at the base 5 to 15 feet thick.
White chert zone--The white chert zone which overlies the oolite member consists of similar coarsely sucrose white porous dolomite characterized by residues of opaque white chert. The amount of chert is ordinarily only 1 to 5 percent by volume, but in Riley County and in some other localities some samples from this zone yield 12 to 20 percent insoluble residues. In addition to the white chert, the insoluble residues include some gray chert which is slightly dolocastic. Quartz is absent or subordinate in the lower part of the zone, but where the zone is thick some quartz is present in the residues from the upper part but in amounts Iss than that of the white chert.
In the Empire No. 1 Schwalm well, in sec. 19, T. 12 S., R. 11 E., Wabaunsee County, traces of anhydrite were found in the residues of nearly every sample from this zone. The largest amount, only 1 percent, was found at the top of the zone, which is thicker in this well than in any other well examined. It is possible that at the top of this thick section the anhydrite was either locally deposited or locally preserved from erosion. However, in two wells in Shawnee County traces of anhydrite were detected at the base of the white chert zone or at the top of the oolite zone. Traces of crystalline anhydrite are common in the lower 150 feet of the Silurian in the Carter stratigraphic test well No. 4 in sec. 24, T. 4 S., R. 16 E., Brown County. It is not certain that anhydrite occurs in this zone exclusively in these counties, but it was not detected elsewhere. Where the anhydrite is associated with harder rocks, most of the anhydrite is reduced to mud in drilling and is lost when the samples are washed. Some anhydrite is also lost by solution when the samples are treated with acid. Failure to discover anhydrite in the samples, therefore, does not necessarily imply its absence. The discovery of anhydrite in the Silurian of Oklahoma by Ireland (1936, p. 1094) suggests that anhydrite may have been widely distributed.
Foraminiferal zone--Overlying the white chert zone, is a zone of coarsely surcose white dolomite which is indistinguishable from the underlying dolomites except by the insoluble residues. The insoluble residues from this zone are characterized by small numbers of arenaceous foraminifera accompanied by small amounts of several forms of clear secondary quartz (hackly, drusy, semiopaque, etc.) and by coarse angular silt or very fine sand. The residues are less than 1 percent of the volume in most samples, but they amount to 5 to 6 percent in some samples. The basal bed of the foraminiferal zone is characterized by traces of glauconite. The small and silty residues are to some extent characteristic of this zone. In several wells the presence of the silt had led to more careful preparation and re-examination of insoluble residues and the ultimate discovery of foraminifera.
Foraminifera from this zone were first observed in small quantity in the Empire No. 1 Schwalm well, in sec. 19, T. 12 S., R. 11 E., Wabaunsee County, but none could be found in other nearby wells although residues similar in other characteristics had been observed. When duplicate residues were prepared and examined it was discovered that in most wells the light hollow discoidal tests had been decanted in washing the residues because sufficient time had not been allowed to permit them to settle. Although nowhere present in large numbers, patient examination of the residues of samples from this zone has in almost every case revealed one or more foraminifera.
The foraminiferal remains consist in part of flat, closely coiled, discoidal tests having the aperture at the end of a tube extending sharply at right angles to the coil. These foraminifera resemble Lituotuba and Ammodiscus described by Moreman (1930) and Ireland (1939) from the Silurian of Oklahoma. Henbest expressed the opinion after casual observation of some of the specimens that some of them are new and undescribed species. Similar foraminifera have not been reported from the Devonian of Oklahoma. The foraminifera observed in Kansas have all been found in samples below the sandy zone that characterizes the base of the Devonian. It is concluded, therefore, that the rocks from the top of the Maquoketa up to and at least including the foraminiferal zone are of Silurian age.
The base of the Devonian rocks in northern Missouri is marked by sandy limestone and calcareous sand. In following this sandy member from well to well in the well cuttings it was discovered that toward the west an increasing thickness of dolomite intervenes between the Maquoketa shale and the base of the Devonian. The presence of Silurian rocks had, therefore, been suspected before the discovery of Silurian foraminifera. The foraminiferal zone is 20 to 50 feet thick. The limits of the zone are based on the recognition of foraminifera in the residues, in some cases represented by a single test. The characteristic silty nature of the residue of some samples in which, however, foraminifera were not found suggests that they are a part of this zone and that locally the thickness of the zone may be greater than indicated by the observed fossils.
In the Arab No. 1 Ogle well in sec. 9, T. 1 N., R. 14 E., Nebraska, both the oolitic zone and the white chert zone are missing. In this well the foraminiferal zone rests unconformably upon the weathered and eroded surface of the Maquoketa shale, suggesting uplift and erosion of the Maquoketa at the beginning of Silurian time and overlap of Silurian rocks. This relation is probably the expression of the unconformity recognized in the outcrops at many places in the Mississippi valley at the base of the Brassfield. Similarly, in the McNab No. 1 Fritz well, in sec. 4, T. 12 S., R. 14 E., the white chert zone of the Silurian is missing and the foraminiferal zone immediately overlies the oolitic zone. An unconformity is recognized in the Mississippi valley outcrops also between the Brassfield and the underlying Edgewood limestone.
Drusy quartz zone--The drusy quartz zone, which overlies the foraminiferal zone (fig. 7), consists of coarsely surcose white dolomite similar to earlier Silurian dolomite. The insoluble residues constitute 2 to 4 percent of the samples and consist of small particles of angular, drusy, hackly, clear to semiopaque quartz. Some opaque white chert also occurs in the lower part.
A change in the character of the dolomite occurs in many wells at the contact of the basal Devonian rocks (identified by the sand content) and the underlying Silurian dolomite. The Silurian dolomite is a coarsely sucrose white dolomite with angular microscopic vugs lined with dolomite crystals. The Devonian dolomite is sucrose but generally dense and more or less impervious. Toward the west this change in lithologic character occurs at somewhat varying intervals up to 50 feet above the base of the drusy quartz zone. In the absence of a sandy zone at the base of the Devonian the contact is tentatively placed at this change in lithology. This, however, is not an entirely satisfactory expedient for in some wells there are alternating beds of dense and porous dolomite of unknown age.
Correlation--The Silurian of the Forest City basin is believed to be the correlative of the Chimneyhill limestone of Oklahoma. The basal dolomitized oolite zone corresponds to the Noix oolite member of the Edgewood limestone of Missouri. The correlation of the overlying zone which yields insoluble residues of white chert is uncertain, but it also may be a part of the Edgewood. The fairly regular thickness of the overlying foraminiferal zone and the considerable irregularity in thickness of the underlying white chert zone suggests unconformable relations between them (see fig 7). There is certainly an unconformity at the base of the foraminiferal zone in the Arab-Ogle well in Nebraska where the foraminiferal zone overlies an abnormally thin section of weathered Maquoketa shale and also in the Carter stratigraphic test well No. 4, in sec. 24, T. 4 S., R. 16 E., where also the foraminiferal zone rests on the Maquoketa (fig. 7). The presence of glauconite at the base of the foraminiferal zone suggests its correlation with the corresponding glauconitic limestone of the Chimneyhill of Oklahoma which has been correlated with the Brassfield by Ball (1939). In the Arab No. 1 Ogle well, the foraminiferal dolomite zone contains some pink and salmon-colored dolomite, a characteristic of the Brassfield, but these colors are not as conspicuous in this zone in Kansas as in the outcrops, although fragments of pink limestone have been observed in some well samples. Pink and reddish limestones have been noted by Ball (1939, p. 119) in the upper part of the Brassfield or its correlatives from Indiana to Oklahoma, and the occasional occurrence of this lithologic feature in Kansas is additional reason for regarding this zone as of Brassfield age. The correlation of the drusy quartz zone is uncertain; it may represent part of the upper Brassfield of Arkansas or the still younger St. Clair of that state.
Kansas Geological Survey, Forest City Basin
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Web version July 2005. Original publication date Dec. 1943.