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Depositional Environment of the Wreford Megacyclothem

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Summary

The Wolfcampian Wreford megacyclothem comprises the Wreford limestone, part of the underlying Speiser shale, and part of the overlying Wymore shale member of the Matfield formation. The Speiser shale consists of three widely recognizable units including, in upward order, (1) basal red shale and green ostracode-bearing shale, (2) a molluscan-limestone unit containing Aviculopecten and Septimyalina, and (3) a calcareous shale best characterized by Derbyia, Dictyoclostus, Composita, and Chonetes. Sandstone is locally intercalated in the red shale; grayish-yellow mudstone commonly lies below the molluscan limestone.

The Threemile member of the Wreford limestone comprises, in ascending order, (1) a persistent chert-bearing member containing a brachiopod-bryozoan fauna, (2) a likewise persistent shaly phase containing a mixed fauna, (3) chert-bearing limestone, (4) chalky, less cherty limestone characterized by bryozoans and some corals (unit absent in southernmost Kansas), and (5) cherty limestone (northern and central Kansas) or algal-molluscan limestone (southern Kansas).

The middle member of the Wreford limestone, the Havensville shale, generally includes, in northern Kansas, the following beds in upward order: (1) calcareous shale, (2) molluscan limestone, (3) mudstone, (4) molluscan limestone, and (5) calcareous shale; one or more of these units is commonly poorly developed. Each unit contains a fauna resembling that in equivalent phases of the Speiser shale. Green shale is seen locally in the middle Havensville. In southern Kansas an algal-molluscan limestone that lies in the middle of the member is underlain by mudstone and shale, and overlain by calcareous shale.

The Schroyer limestone, uppermost member of the formation, comprises a basal cherty limestone, a middle calcareous shale, and an upper algal limestone. The calcareous shale contains a thin cherty limestone in northern Kansas. The faunas in these units are similar to those in lithologically equivalent units lower in the section.

The Wymore shale includes, in upward order, green shale, red shale, green shale, and calcareous mudstone or very silty limestone.

Recurrence of lithologic types, each with characteristic fossils (or lack of them), suggests cyclic sedimentation. Two nearly complete cycles and parts of two others are represented in the sections studied. The complete cycles, each divided into transgressive and regressive hemicycles, are the Threemile cyclothem and the Shroyer cyclothem; these combined constitute the Wreford megacyclothem.

Strata in the transgressive hemicycle of the Threemile cyclothem include sandstone and red shale of continental origin successively overlain by marine green shale, mudstone, molluscan limestone, calcareous shale, and chert-bearing limestone. The organic and inorganic constituents of the various phases indicate marine deposition progressively farther from shore as one proceeds from green shale to cherty limestone. Chalky limestone reefs, special developments in the chert-bearing limestones, were formed during maximum transgression in central Kansas. Intermingling of molluscan and calcareous-shale phases is common in southernmost Kansas.

The Threemile regressive hemicycle includes cherty limestone, calcareous shale, molluscan limestone, mudstone, and, rarely, green shale. South of the reef limestone exposures, the upper Threemile limestone contains an algal-molluscan limestone unit the depositional environment of which undoubtedly was tempered by the presence of the reefs; the regressive hemicycle in this area terminates in mudstone of the basal Havensville shale. The ascending order of phases indicates progressively shoreward deposition.

In northern Kansas the Schroyer transgressive hemicycle includes green shale, mudstone, molluscan (locally algal) limestone, calcareous shale, and cherty limestone, in upward order. In southern Kansas the transgressive hemicycle includes mudstone, algal-molluscan limestone, calcareous shale, and cherty limestone.

The Schroyer regressive hemicycle is uniformly developed over much of Kansas. Included are cherty limestone, calcareous shale, algal limestone, green shale, and red shale. In northern Kansas, minor reversal during regression caused deposition of a widespread cherty limestone bed in the calcareous shale.

Subsurface sections far west of the outcrop contain red shale, green shale, and cherty limestone in the same relative stratigraphic positions as at the surface. The environments favoring deposition of these phases thus had wide geographic distribution in both north-south and east-west directions, although presumably a single environment did not prevail throughout the area at any given time.

The stratigraphic section above the Wreford limestone as high as the Winfield limestone shows repetition of lithologies and faunas that can be demonstrated to represent parts of five cyclothems. Of these the Barneston cyclothem is most nearly complete.

Conclusions

  1. The various phases of the Wreford megacyclothem are assignable to depositional environments and can be shown to lie in an orderly and logical stratigraphic sequence comprising two nearly complete cyclothems.
  2. Fine-grained channel sandstones in the Speiser shale of Cowley County were deposited along a stream that flowed across a broad, low alluvial plain.
  3. Red shales seem to be subaerial deposits laid down on a broad, low alluvial plain bordering the Early Permian sea. The red color is caused either by oxidation of the soil from which the sediment was derived, by subaerial oxidation after deposition, or both.
  4. Green shales are the product of sedimentation in the zone of marine deposition nearest shore. Reduction of iron and organic material permits dominance of green color, which is due to the presence of green clay minerals. The restricted fauna indicates a brackish-water environment.
  5. Grayish-yellow mudstone was commonly deposited next offshore from the area of green-shale accumulation. More abundant organic matter accounts for the difference in color. Dominance of ostracodes suggests a brackish-water environment.
  6. Molluscan limestone represents marine deposition in water of more nearly normal salinity in an offshore zone where some turbulence resulted from wave activity. The calcium carbonate is believed to be chiefly a product of shell disintegration and wave agitation.
  7. Calcareous shale represents deposition in a normal marine offshore area where wave agitation was not strong. Conditions favorable to many forms of life resulted in prolific faunal development.
  8. Chert-bearing limestone was deposited in relatively deeper, clear, offshore water of normal salinity. The calcium carbonate is primarily an accumulation of shell fragments, but it also contains algal and inorganic components.
  9. The chalky limestone is believed to comprise a system of reefs, which have an arcuate, seawardly convex pattern. The reefs seem to be more or less connected and are probably of the offshore-barrier type. Most of the calcium carbonate is organic in origin, algae and bryozoans having contributed heavily to its accumulation. Wave agitation of warm water probably caused chemical precipitation of some calcium carbonate, also.
  10. The reef in the Threemile limestone of central Kansas possibly had an important effect on sedimentation. The fore-reef sediments display the best cycles of sedimentary rocks; the back-reef facies includes algal-molluscan limestone in the upper Threemile member, which overlies southern equivalents of the chalky limestone, and similar beds in the Havensville shale in Cowley County.
  11. On the basis of geographic, stratigraphic, and lithologic relationships, the compact noncalcareous chert is thought to be a primary inorganic precipitate. The concentrically layered calcareous chert represents slowly deposited inorganic silica in which calcium carbonate was incorporated; the present aspect is due to diagenetic alteration of the original sediment. The chert is observed mainly in limestones containing brachiopod-bryozoan-echinoderm-coral faunas; hence optimum conditions for silica deposition occurred during the maximum stage of transgression and earliest stage of regression.
  12. Noncalcareous chert is progressively less abundant to the south in Kansas; calcareous chert is more abundant southward. Most chert of the latter type is attributed to deposition nearer shore where the environment was intermediate between that suited for rapid precipitation of silica and that in which silica was precipitated very slowly.
  13. Algal limestone was deposited in a special sort of environment and is a regressional equivalent of molluscan limestone. There is a constant association of algae and mollusks in these beds. The profusely algal limestones may have resulted from a hypersaline environment, and algal beds that contain large boring clams suggest a brackish environment of deposition.
  14. The upland area affecting Wreford sedimentation probably lay not far to the south or southeast. Plant fossils provide inconclusive evidence for high ground in the south and low-lying land areas adjacent to the sea in northern Kansas.
  15. Broad-leaved-conifer and seed-fern fossils indicate that a warm climate prevailed during Wreford deposition.
  16. The cycles of the Wreford megacyclothem contain the following lithologic types, arranged in stratigraphic order:
Threemile cyclothem Schroyer cyclothem
2. Green shale* 1. Red shale
3r. Mudstone 2r. Green shale
4r. Molluscan limestone 4r. Algal limestone
5r. Calcareous shale* 5r. Calcareous shale
6r. Cherty limestone 6. Cherty limestone
6. Chalky limestone* 5t. Calcareous shale
6t. Cherty limestone 4t. Molluscan limestone
5t. Calcareous shale 3t. Mudstone
4t. Molluscan limestone 2. Green shale*
3t. Mudstone *absent in southern Kansas
2t. Green shale †absent in northern Kansas
1t. Red shale  
0. Sandstone†  
  1. Individual phases, as well as complete cyclothems, are commonly traceable for as much as 200 miles in Kansas even though they may be very thin. This phenomenon suggests that some depositional environments extended almost unmodified over broad areas during the time in which the Wreford megacyclothem was deposited.

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Kansas Geological Survey, Geology
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