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Americus Limestone Member of Kansas

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Appendix F: Description of the lithology of the Hamlin Shale and Americus Limestone members (Wolfcampian) in Kansas

In this appendix I give a generalized summary of the lithologies of the Hamlin Shale Member of the Janesville Shale (Admire Group) and the Americus Limestone Member of the Foraker Limestone (Council Grove Group). The generalized stratigraphic column (fig. 1) shows the vertical relationships of these units to associated units; fig. 3 shows the stratigraphic position of interpreted stages of inundation by the epeiric sea. Appendix G provides detailed lithologic descriptions at individual localities.

Hamlin Shale Member

In general, less than 2 m (7 ft) of the upper part of the Hamlin Shale Member is exposed; however, outcrops displaying 4 m (13 ft) or more of Hamlin shale occur at localities 15 and 27, in Lyon and Greenwood counties, respectively. The Hamlin shale, although highly variable from locality to locality, is characterized by shales and terrigenous mudstones, thin, laterally discontinuous limestones, boxwork structures in the upper part, and a lime-sand mudstone to grainstone capped by a thin shale or terrigenous mudstone at the top.

Shale and terrigenous mudstones are the dominant rock types of the Hamlin Shale Member. The shales and terrigenous mudstones, typically tan, gray, white, brown, or orange, are either fissile or blocky and are commonly calcareous. A purple to red noncalcareous shale with dolomitic nodules occurs low in the section at locality 15 in Lyon County. Blocky terrigenous mudstones tend to be more numerous toward the top.

A white to tan lime mudstone or indurated claystone with pervasive boxwork structures occurs near the top of the Hamlin Shale Member at many localities. The boxwork structures are horizontal and vertical fractures filled with calcite cement that tend to weather in relief. These rocks are typically void of fossils and other detritus; however, disseminated quartz-silt grains are abundant within the boxwork mudstones at localities 25 and 29.

Thin, laterally discontinuous limestone beds are common in the upper Hamlin shale. A stromatolitic unit, 0.1 m (0.3 ft) thick, occurs 0.13 m (0.43 ft) below the top of the Hamlin shale at locality 1 in Wabaunsee County. The domal, creamcolored stromatolites are peloid-alga boundstones, and they display pervasive fenestrae filled with pink gypsum. A layer of brecciated stromatolites covers the in situ stromatolites.

Laterally discontinuous beds of lime mudstone and wackestone, typically less than 0.1 m (0.3 ft) thick and generally containing peloids, ostracodes, or quartz silt, occur within 2 m (7 ft) of the top of the Hamlin at localities 2, 5, 6, 7, 9, 20, 27, 28, 29, and 31. These beds tend to be more numerous in the south, where they commonly display fenestral porosity, extensive in situ brecciation, and craze and joint fractures. The upper surface of a lime mudstone near the top of the Hamlin at locality 20 in Greenwood County displays a polygonal crack pattern. These thin limestones are either nonfossiliferous or contain a low-diversity fossil assemblage consisting of abundant ostracodes or, in some instances, rare gastropods. However, a wackestone [0.09 m (0.3 ft) thick] containing abundant Calcivertella foraminifers, bivalve fragments, ostracodes, and gastropods occurs just below the top of the Hamlin at locality 35 (the southernmost outcrop visited) in Cowley County. This wackestone, with its relatively high-diversity assemblage, is atypical of the Hamlin Shale Member.

The top of the Hamlin shale is typically marked by a distinctive orange rock, generally a lime-sand packstone or wackestone. This unit varies from an orange shale or terrigenous mudstone with a few sand-size lime grains to a conglomeratic lime-sand grainstone with mudstone lithoclasts of medium sand to pebble size. It varies in thickness from almost 0 to 0.3 m (1.0 ft) and is evidently not present at localities 29, 31, and 32. Crossbedding is generally observed where the unit is thick. Between-particle pore spaces are generally filled with orange terrigenous mudstone or by PE34 calcite cement. Ostracodes and well-rounded bivalve fragments are abundant to common, and Gyrogonites is typically present. Coal stringers, plant fragments, and petrified logs have been noted within this lime-sand rock at some northern localities (Bernasek, 1967; Fisher, 1980). The lower contact of this bed is abrupt and irregular, often with relief of 9 cm (4 in.).

At many localities a gray to orange shale, generally less than 1 cm (0.4 in.) thick, overlies the lime sand. This represents stage 2 in fig. 3. The remainder of the Hamlin shale represents stage 1.

Americus Limestone Member

The Americus Limestone Member generally consists of three limestones separated by shales. It ranges in thickness from 1.5 m (5 ft) at locality 1 in Wabaunsee County to 6 m (20 ft) at locality 23 in Greenwood County. The different subunits of the Americus change in thickness and, to some extent, in composition from locality to locality.

The lowermost bed of the Americus limestone, the focus of this study, is a white to tan limestone that ranges in thickness from 5 cm to 70 cm (2-28 in.) and pinches out north of Pottawatomie County. The upper surface and base of this limestone are relatively flat, except at localities 24, 25, and 27 in Greenwood County, where the base displays up to 0.36 m (1.2 ft) of relief. A stromatolite layer (fig. 3, stage 3) characterizes the base of this limestone at virtually all localities north of locality 28 in Elk County. These stromatolites are composed of peloid-alga boundstone. The peloids are irregularly shaped to oval and are generally coarse silt to fine sand sized. In thin section the peloids appear to float in PE32 calcite cement. Internal laminations are generally 1-6 mm (0.04-0.2 in.) thick. Typically, no grains other than peloids are incorporated in the boundstone layers. Exceptions are rare mollusk fragments, lime- sand grains, and palynomorphs.

Traces of algal filaments were noted in many samples. These traces occur in dense micritic laminae and appear in thin section as subtle horizontal threadlike features. The filaments, never longer than 0.5 mm (0.02 in.), have bright micritic hollow centers that are 0.02 mm (0.0008 in.) thick and dark micritic walls that are 0.01 mm (0.0004 in.) thick.

Rare pseudomorphs of evaporite crystals occur at locality 18 in Chase County. At localities 24 and 25 in Greenwood County, the stromatolite layer is brecciated and contains alveolar structures. The stromatolite layer ranges in thickness from 0.6 cm to 20 cm (0.2-8 in.) and displays a variety of morphologies, including thick horizontal-laminated to digitate layers, high domes, rip-up clasts, discontinuous patches with extensive fenestrae, and low-domed to flat layers. This stromatolite layer is typically encrusted with Spirorbis worm tubes, algae, and tubiform foraminifers [up to 19 cm (7.5 in.) thick at locality 27 in Greenwood County].

Two generations of encrustations are recognized. The first generation (fig. 3, stage 4) of encrustation includes Spirorbis worm tubes and incorporates tiny bivalves. The second generation of encrustation (fig. 3, part of stage 5) formed on the truncated upper surface of the first-generation encrustations. This second generation typically contains no Spirorbis worm tubes or bivalves. It consists of digitate growths of tubiform foraminifers and algae, incorporates bryozoans, and is typically penetrated by borings 3 mm (0.1 in.) in diameter.

Discrete plates [20 cm (8 in.) across and 5 cm (2 in.) thick] of Spirorbis-foraminifer-alga boundstone (fig. 3, included in stage 4) with fractured upper surfaces and pendant cements represent the lower limestone bed at its northern limits. The upper part of the lower limestone bed (fig. 3, stages 5 and 6; the lower contact of stage 6 is gradational and marked by the upward occurrence of typically normal-marine fossils) is laterally variable but is characterized by Calcivertella foraminifer- and ostracode-rich lime wackestones and packstones and peloid-intraclast wackestones to grainstones. Peloid mudstones occur at locality 5 in Lyon County and at localities 23 and 24 in Greenwood County. A lime-conglomerate-foraminifer-ostracode wackestone to packstone occurs at localities 29 and 32 in Elk County, and a foraminifer mudstone interlayered with foraminifer-brachiopod-gastropod wackestone with crinoid, bryozoan, and tiny bivalve fragments, sponges spicules, and ooid-coated grains occurs at locality 35 in Cowley County. Myalinid and pectinoid bivalves, productid brachiopods, crinoids, and a variety of foraminifers, including Globivalvulina, are abundant toward the top of the lower limestone at several locations.

Black, brown, and gray shale (fig. 3, stage 7), up to 2.5 m (8.2 ft) thick, overlies the lower limestone, except where the lower limestone coalesces with the middle limestone of the Americus at localities I and 4 in Wabaunsee County. This shale is generally fissile and calcareous and commonly displays orange and tan layers. Fisher (1980) reported sparse fragments of crinoids, brachiopods, and bivalves in this shale interval. An interbedded tan to orange sandstone and silty shale interval, 0.5 m (1.6 ft) thick, occurs in the middle of the shale at localities 20 and 21 in Greenwood County. The sandstone consists of very fine sand sized quartz grains, mica, few ostracode valves, and quartz cement in the pore spaces.

The middle limestone (fig. 3, stage 8) of the Americus Limestone Member is typically a gray crinoid wackestone, 0.3 m (1 ft) thick in the north to 0.76 m (2.5 ft) thick in the south. This resistant limestone, informally referred to by some researchers as the main ledge (Harbaugh and Demirmen, 1964), weathers into distinctive slabs. The upper surface is flat, whereas the base commonly displays relief of up to 0.23 m (0.75 ft). Typical fossils include crinoid debris, commonly with a diameter of 1 mm (0.04 in.); brachiopod fragments; fusulinid, Nodosia, and biserial foraminifers; bryozoans; bellerophontid gastropods; and rare trilobites. A packstone composed of abraded bellerophontid gastropods occurs at the base of the limestone at localities 12, 14, 16, and 17 in Lyon County. At locality 16 this gastropod packstone also contains abundant myalinid fragments, many granule-size, bored mudstone lithoclasts, and a few bryozoan fragments. Pink geodes of both quartz and calcite occur at locality 4 in Wabaunsee County. Chert containing abundant fusulinids is a common feature at most outcrops south of locality 25.

A gray to yellow calcareous shale or terrigenous mudstone (fig. 3, stage 7) typically separates the middle limestone from the upper limestone. This shale ranges in thickness from 0.07 m (0.2 ft) at locality 35 in Cowley County to 1.4 m (4.6 ft) at locality 12 in Lyon County. At several northern localities the upper part of this shale contains lenses of fusulinid packstone that may represent the poorly developed base of the upper limestone of the Americus.

The upper limestone (fig. 3, stage 8) of the Americus Limestone Member is generally less resistant and more laterally variable than the middle limestone. In the north the upper limestone varies in Wabaunsee County from a gray brachiopod-crinoid wackestone [0.11 m (0.36 ft) thick] with pink quartz geodes at locality 1 to a 0.52-m-thick (1.7-ft-thick) friable brachiopod wackestone with abundant whole brachiopods at locality 2. From locality 12 in Lyon County to locality 20 in Greenwood County, the upper bed is divided into an upper 0.25-m-thick (0.82-ft-thick) brachiopod mudstone; a 0.11-m-thick (0.36-ft-thick) silty shale; and a lower 0.1-m-thick (0.3-ft-thick) fusulinid packstone. At locality 23 in Greenwood County the upper bed is again two limestones: an upper 0.36-m-thick (1.2-ft-thick) cherty fusulinid wackestone and a lower 0.57-m-thick (1.9-ft-thick) cherty fusulinid-crinoid wackestone, separated by 0.2 m (0.7 ft) of calcareous shale with abundant fusulinids. South of locality 25, the upper limestone is a relatively resistant fusulinid wackestone [0.7 m (2 ft) thick] with a distinctive chert layer in the middle.

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Kansas Geological Survey, Geology
ISBN: 1-58806-107-8
Placed on web Sept. 8, 2011; originally published 1992.
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