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Published reports and papers in press on the petrography and geochemistry of the Riley County kimberlites include Eastwood and Brookins , Rosa and Brookins , Brookins a, b, a, b, c, a, b, c, d, , and in press , Brookins and McDermott , Dyer and Brookins , and McDermott and Brookins in press.
Three other recent reports have mentioned the Riley County kimberlites. Wheeler has referred to them as klippen of an ancient overthrust, but this theory has been severely criticized by Franks O'Connor mentioned very briefly the Riley County kimberlites but included little of the more recently accumulated information. Acknowledgment is due H. Several faculty and student personnel of Kansas State University have been most helpful: I wish to thank P. Twiss, R. Vian, A. Sperry, R. Eastwood, the late F.
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Rosa, P. Dowling, R. Dyer, and R. O'Neil, of the U. Geological Survey, made the carbon and oxygen isotopic determinations. Special thanks are due E. Angino, of the State Geological Survey of Kansas, who encouraged the writer to compile the information for this report. According to Merriam there may have been a cover of only a few hundred feet of Cretaceous rocks overlying the lower Permian host rocks into which the kimberlites were intruded, and it is possible that the kimberlites may have come close to or even reached the surface.
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Kimberlite-wall-rock contacts have been exposed at the Randolph kimberlites by trenching at Randolph No. Geological Survey; at Randolph No. McDermott , and the contact at the northwestern edge of the Stockdale kimberlite has been infrequently exposed after very dry summers. The contacts of the Bala and Leonardville kimberlites are not exposed, and the Winkler kimberlite does not crop out. Some characteristics of the individual kimberlites are noted below. The Bala kimberlite Dreyer, forms a distinct topographic high in the form of a rounded knoll approximately 15 feet in height and feet in diameter.
The uppermost country rock is the Cresswell Limestone Member of the Winfield Limestone, and it is folded 0. However, the Stovall Limestone Member, a thin unit 1 to 3 feet thick overlain and underlain by much thicker shales, is subject to faulting in areas removed from the kimberlite. Core to a depth of feet from the Bala kimberlite is stored at Kansas State University.
The Stockdale kimberlite Rosa and Brookins, crops out in an irregular fashion in a stream bed. This is the only one of the five kimberlites which crops out to form a topographic low. The kimberlite-Fort Riley Limestone Member contact is thought to be a fault contact, as an opening partially filled with debris to a maximum width of 1 inch separates the two rocks. The dip of the limestone is nearly horizontal. Many samples of subsurface kimberlite and xenoliths were recovered from a 2. The stone was to have been used for ornamental purposes, but due to the high content of calcite this was not feasible.
The Leonardville kimberlite crops out in two very small mounds on the west side of a gently sloping hill.
The unexposed country rock is the Winfield Limestone. Magnetometer surveys indicate the Leonardville kimberlite to be the largest of the five exposed, but its outcrop area is the smallest, and it is very poorly exposed.
The Randolph No. Within a few feet of the contact, the dips in the limestone are nearly horizontal. No pyrometamorphic contact effects are noted, but the upwarping of the limestone indicates that the kimberlite was intruded into the country rock. The country rock has been exposed by trenching, and part of the geometry of the kimberlite has been outlined by drilling McDermott and Brookins, in press.
A mushroom-like cap suggests that the kimberlite may have come very close to the surface, a view proposed by Merriam In addition, weathered Fort Riley Limestone was recovered beneath the cap from one of the drill holes. The Winkler kimberlite does not crop out but forms the most striking topographic feature in northern Riley County--Winkler Crater, a foot-diameter topographic low which appears as an almost perfect circle on an aerial photograph but possesses crater-like morphology only in its west-southwestern portions Brookins, The entire northern half is dissected by small southward-trending streams that join a more pronounced eastward-trending stream, which divides the crater into two nearly equal parts.
The entire eastern "rim" has been eroded away, and the maximum relief 50 feet within the feature is found at that point. Drilling over magnetic anomaly has revealed the presence of a kimberlite-poor brecciated and partially recemented pipelike body. The highest rimrock of the feature is composed of the cherty, resistant Florence Limestone Member of the Barneston Limestone. Two other cherty, resistant limestones, the Schrover and Threemile Limestone members of the Wreford Limestone, were not encountered in the drilling, and the feature is therefore presumed to be due at least in part to solution promoted by the arching associated with the kimberlitic emplacement.
The exact structure of the circular feature and the kimberlite geometry remains to be investigated in detail. The geometry of the Riley County kimberlites and their relationship to the regional structure has been determined by magnetometer surveys reported by Dreyer , Cook , Dowell , and Brookins The trends of the magnetic fields surrounding each kimberlite are oriented more or less normal to the Abilene anticline Fig. The magnetic anomaly over the Winkler kimberlite differs from that of the other kimberlites, due, in part, to the f act that the main body is not exposed at the surface.
Shenkel, Dept. The reason for the discrepancy between the various investigations is not clear, but presumably it is due to instrumental error. No negative anomaly was found on the north side of the Winkler kimberlite, although the gradient is extremely steep on that side Dowell, ; Brookins, The importance of these magnetometer studies is that a similar geometry, that of a steeply dipping pipelike body, is common to all the Riley County kimberlites Table 2.
Further, a similar mechanism of emplacement for all of the kimberlites is suggested even though local variations exist. According to Dawson a , the pipes of Riley County should be classified as intrusive kimberlite breccias, probably originating from diatremes as opposed to dikes.
Although the age of the kimberlites has been definitely placed as post-Chase Group early Permian from field relationships, most previous investigators have suggested a Cretaceous age for the emplacement.
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I do not agree that these rocks were emplaced under the same conditions as those in Woodson County, because there is abundant evidence for high-temperature contact metamorphism in the country rocks surrounding the Woodson County ultrabasic rocks, whereas such effects are absent in Riley County. Both of these sets are readily discernible in the lower Permian rocks and can be traced into the Dakota sandstone of late Cretaceous age in northwestern Riley County, but are not found in the outcrops of the kimberlitic rocks.
On this evidence I believe the rocks to be post-Dakota in age, a view in agreement with Jewett, Byrne, et al. The pipes are thought to be pre-Kansan, as glacial debris is found in Winkler Crater. A much younger age than late Cretaceous possibly can be inferred from the presence of the weathered Fort Riley Limestone recovered from beneath the cap rock at the Randolph No. The following description is directly applicable only to the five exposed kimberlites, and the texture of the Winkler kimberlite can only be inferred. The kimberlites have a light- to dark-green, brecciated and porphyritic appearance and are cut by numerous veins of calcite filling joints and other fractures Brookins, c.
Some color zoning is apparent Rosa and Brookins, , and flow banding is evident in a few places. The kimberlites can be divided into three groups based on their texture, number and nature of xenoliths, and phenocryst mineralogy. Ilmenite and magnetite phenocrysts are present in extremely small amounts. Pyrope and phlogopite are absent. The few xenoliths present are completely serpentinized. The only pronounced similarity between the Randolph kimberlite and the other kimberlites is its light-green color and abundance of calcite veins.
The Stockdale and Leonardville kimberlites are highly porphyritic and brecciated. They contain megascopic phenocrysts of dark-red pyrope, ilmenite, chloritized phlogopite, magnetite, and serpentinized olivine, as well as many cognate and accidental xenoliths. The xenoliths vary in size from barely megascopic to 40 cm in maximum dimension. Some are highly altered, others scarcely altered at all. The degree of roundness varies also. In general, the smaller and more highly altered xenoliths are more rounded than larger, less altered xenoliths.
Most of the xenoliths, including the nearly unaltered ones, contain some type of apparent reaction rim. The Winkler kimberlite contains abundant red pyrope, ilmenite, cholritized phlogopite, and serpentine pseudomorphs after olivine and is on this basis tentatively grouped with the Leonardville and Stockdale kimberlites pending further investigation. The Bala and Randolph No. Moore and Haynes have reported biotite that occurs in large books in the Bala kimberlite, but I have observed none.
The Bala kimberlite also contains large 0. Due to local accumulation of accidental xenoliths of the surrounding country rocks, the Bala and Randolph No. Cognate xenoliths are much less abundant than in the Leonardville and Stockdale kimberlites. Finally, the "chromite" from the Bala Moore and Haynes, is almost certainly ilmenite, as chromite is known from the kimberlites as a minor accessory mineral only Brookins, d.
The kimberlites have an approximate serpentine-carbonate mineral ratio of The kimberlites show variations in mineralogical composition and texture. Olivine 0. The phenocrysts usually contain a reaction rim of some sort, often fine-grained serpentine with a columnar structure, and chlorite with occasional secondary amphibole in part tremolite. Clino- and ortho-pyroxenes 0. It may also contain exsolved magnetite. Magnetite occurs in several generations. It occurs as primary phenocrysts and microphenocrysts 0.
In addition, it is a common secondary product of serpentinization of ferromagnesian minerals and is commonly found oriented parallel to cleavage or parting planes and in reaction rims. Another generation of magnetite of uncertain origin occurs in primary carbonate veins. Phenocrysts or xenocrysts?
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In these latter carbonates, serpentine replaces calcite, and later fractures contain calcite with a random optical orientation. Pyrope varies from 0. It possesses a subhedral to anhedral outline and is commonly rimmed by kelyphite, which may be partially or completely removed during emplacement. Fractures within the grains are commonly filled with calcite. Some grains contain minute inclusions of rutile; other grains contain naturally decorated dislocations in otherwise unaltered portions. Phlogopite 0. It exhibits undulatory extinction, and some grains are bent or broken while others possess "kink bands.