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Petrology of Banzet lithologies
The petrographic characteristics of sandstone and shale units vary between core samples collected in northeastern Kansas and those of southeastern Kansas and northeastern Oklahoma. The outcrop samples, collected from western Missouri through southeastern Kansas and into northeastern Oklahoma, show some of these variations. In general, mudrocks become clayier southwestward from northeastern Kansas as kaolinite-rich silty shales give way to illite-rich clay shales, as determined by Aden (1982) using x-ray diffraction analyses. Likewise, sandstone bodies tend to be medium-grained lithic arenites in northeastern Kansas (Leavenworth County cores) and become more quartzose and finer grained to the south and west (fig. 44).
Figure 44--Ternary diagram showing composition of sandstone units in Banzet formation sampled from entire study area; see appendix B for data summary.
Clay mineralogy of mudrocks
By combining clay-mineral variation with paleontologic considerations, Aden (1982) recognized three distinct mudrock facies, which he interpreted to be related to depositional settings. Considering the clay components of these facies, it can be seen that the clay mineralogies vary as one moves vertically within measured surface sections and laterally along the outcrop belt and into the subsurface.
The "prodeltaic mud facies" of Aden (1982) is characterized by 1) high illite-crystallinity values, 2) mud percentages of 75% or greater, 3) kaolinite percentages less than 10%, and 4) mixed-layer clay percentages of less than 20%. These characteristics are best observed in the Neosho River Park section (fig. 45). In the units immediately below thin sandstone bodies, illite-crystallinity values drop and mixed-layer clay-mineral percentages increase upward, perhaps indicating short-lived increases in sedimentation rates (Aden, 1982).
Figure 45--Compositional variations in mudrock portions of Banzet formation at Neosho River Park section, Labette County, Kansas. Sample points and numbers are shown by arrows. Dots show relative abundances of eurytopic fossils (productid brachiopods plus ostracodes). I, illite; C, chlorite; K, kaolinite; and M, mixed-layer clays. Dashed line in right-hand column is placed at illite-crystallinity value = 1.2. See fig. 12 for exact location; from Aden, 1982.
Aden's (1982) "shoreline-coastal facies" include coal beds with underlying seatrocks, as well as relatively high percentages of siltstone and sandstone units. The clays in this facies are characterized by 1) illites with low crystallinity values, 2) kaolinite percentages ranging from 15% to 55%, and 3) mixed-layered clays accounting for 20-60% of clays. In addition, total clay content is generally less than 30% of rock samples. This facies is best exposed at the northeast Foster section in Bates County, Missouri (figs. 18 and 46).
Figure 46--Compositional variations in mudrock portions of Banzet formation at Northeast Foster section, Bates County, Missouri. Sample points are shown by arrows. Dots show relative abundances of eurytopic fossils (productid brachiopods plus ostracodes), and mf, other marine fossils found. I, illite; C, chlorite; K, kaolinite; and M, mixed-layer clays. Dashed line in right-hand column is placed at illite-crystallinity value = 1.2. See fig. 18 for exact location; from Aden, 1982.
A transition facies that Aden (1982) referred to as the "delta front facies" is found as thin units at several localities including the upper 2 m (6.6 ft) at the North Arma section in Crawford County, Kansas (figs. 16 and 47), and the lower 4 m (13.1 ft) of the type-Banzet section west of Walsh in Craig County, Oklahoma (figs. 9 and 48). Of the three mudrock facies recognized by Aden, the transitional facies is laterally more widespread than the others, occurring in many of the surface sections and in the cores he analyzed.
Figure 47--Compositional variations in mudrock portions of Banzet formation at North Arma section, Crawford County, Kansas. Sample points are shown by arrows. Dots show relative abundances of eurytopic fossils (productid brachiopods plus ostracodes). I, illite; C, chlorite; K, kaolinite; and M, mixed-layer clays. Dashed line in right-hand column is placed at illite-crystallinity value = 1.2. See fig. 16 for exact location; from Aden, 1982.
Figure 48--Compositional variations in mudrock portions of Banzet formation at type Banzet section, Craig County, Oklahoma. Sample points are shown by arrows. Dots show relative abundances of eurytopic fossils (productid brachiopods plus ostracodes). I, illite; C, chlorite; K, kaolinite; and M, mixed-layer clays. Dashed line in right-hand column is placed at illite-crystallinity value = 1.2. See fig. 9 for exact location; from Aden, 1982.
Sandstone petrography
The petrographic characteristics of sandstone units along the outcrop belt from western Missouri to northeastern Oklahoma seem to be similar. The textures vary within a narrow range from very fine grained to fine-grained sandstone. Mineralogically these units are quartzose with varying amounts of feldspar, chert, and mica. However, as mentioned and illustrated earlier (fig. 44), analyses of samples from subsurface cores show marked textural and mineralogical differences between northeast Kansas (Leavenworth County) and the southwestern portion of the study area in Coffee County, Kansas. With the exception of some of the subsurface specimens from Anderson County, Kansas, surface samples are more quartzose than those collected from subsurface cores. Diagenetic alterations are similar in an areas but vary in relative timing and extent between sandstone units.
Leavenworth County sandstones
Detrital mineralogy--The sandstones analyzed from the Leavenworth County cores are predominantly lithic arkoses (fig. 49), which range in grain size up to medium-grained sandstone (Nelson, 1985). Potassium feldspar ranges from 15% to 20% of the bulk sandstone, and albitic plagioclase constitutes 2-3%. Siltstone, shale, schistose metamorphic, and chert fragments account for 6-16% of bulk sandstone. Shale and sandstone fragments occur as rip-up clasts near the bases of upward-fining sandstone units, while the other rock fragments occur as sand-sized grains. Minor amounts of muscovite, biotite, and chlorite coarse-silt- to coarse-sand-sized grains are found throughout the sandstone units in this part of the study area. However, these micas are found in higher concentrations towards the top of fining-upwards sequences (Nelson, 1985). In addition to detrital minerals, seams of coalified plant debris and blebs of petroleum were common in the sandstones sampled from the Leavenworth County cores.
Figure 49--Ternary diagram showing composition of sandstone units in Banzet formation sampled in Leavenworth County, Kansas; see appendix B for data summary.
Diagenetic alterations--The general sequence of diagenetic alterations reported by Nelson (1985) is as follows: 1) pyrite formation as a product from reduction of organic matter, 2) crystallization of chlorite coatings on detrital grain surfaces, 3) formation of kaolinite pore fillings, 4) formation of quartz overgrowths on detrital quartz grains, 5) calcite cementation and partial replacement of authigenic quartz and detrital quartz and feldspars, 6) precipitation of iron carbonate, and 7) nearly continuous formation of illite and smectite clays (table 1). Many of these alterations took place prior to extensive sediment compaction. Nelson (1985) reported a separate late phase of potassium feldspar dissolution. However, feldspar dissolution more likely took place in conjunction with earlier carbonate replacement, followed by a phase of carbonate dissolution due to exposure to acidic fluids. This phase of dissolution was probably responsible for the creation of secondary porosity, which enhances the reservoir properties of the sandstone units throughout the study area.
Table 1--Diagenetic Sequences
| Lardner, 1984 | Nelson, 1985 | Reinholtz, 1982 |
|---|---|---|
| pyrite formation | pyrite formation | sediment compaction |
| clay coatings | chlorite coatings | pyrite formation |
| silica overgrowths | kaolinite fillings | siderite precipitation |
| illitization of smectite | quartz overgrowths | clay coatings |
| period of compaction | calcite precipitation and replacement |
quartz overgrowths |
| dissolution of feldspars | siderite precipitation and replacement |
iron-calcite precipitation and replacement |
| kaolinite pore fillings | illite formation | precipitation of gypsum |
| iron-calcite precipitation and replacement |
smectite formation | kaolinite sericite |
| seritization of feldspars | sediment compaction | oxidation of pyrite |
| hydrocarbon migration | feldspar (k) dissolution | |
| hydrocarbon migration |
Bush City and Centerville sandstones
Detrital mineralogy--The 39 sandstone specimens from the Bush City and Centerville trends of Anderson County, Kansas, form the most quartzose assemblage in the study area. According to Reinholtz (1982), the Bush City sandstones average 94% total quartz (monocrystalline [92%], and polycrystalline quartz [2%]), and the Centerville sandstones average 89% total quartz (85% monocrystalline, 4% polycrystalline). This, coupled with small amounts of feldspars and rock fragments (appendix B), puts these rocks into the quartz arenite and subarkose compositional fields (fig. 50).
Figure 50--Ternary diagram showing composition of sandstone units in Banzet formation sampled in Anderson County, Kansas; see appendix B for data summary.
The Bush City sandstones are very fine to fine-grained, while the Centerville is somewhat coarser grained and more arkosic (Reinholtz, 1982). However, the sandstone samples from the Bush City are from three lithofacies: 1) subequal amounts of interstratified, very fine grained sandstone and shale; 2) thin, ripple-laminated siltstone and very fine grained sandstone and silty shale; and 3) a mostly mudrock facies containing isolated lenses of siltstone and very fine grained sandstones. These samples only represent the finest grained sandstones found in the Bush City trend. On the other hand, the Centerville sandstone samples are from two lithofacies consisting of massive, fine-grained micaceous sandstone and fine-grained sandstone with lesser amounts of silt and clay shales. These samples represent a somewhat coarser portion of this trend. Thus, the differences between the two groups of samples may be due entirely to textural and environmental differences.
Diagenetic alterations--The diagenetic alterations reported by Reinholtz (1982) for the Bush City and Centerville sandstones are similar to those observed in Leavenworth County and elsewhere in the study area. However, Reinholtz suggested that sediments in Anderson County underwent some degree of compaction before any authigenic materials were found. In addition, he recognized two phases of carbonate precipitation, one of which preceded the formation of authigenic chlorite coatings on quartz grains (table 1).
As mentioned above in our discussion of the detrital components of the Bush City and Centerville sandstones, these specimens are more quartzose than sandstones studied in adjacent areas. This may be due to more pervasive early-phase carbonate cementation that may have been synchronous with, or preceded by, feldspar dissolution.
One observation made by Reinholtz (1982), neither I nor other workers in the area have confirmed. This is a late phase of gypsum cementation. The minor amounts of gypsum observed by Reinholtz may have formed as a by-product from the late oxidation of pyrite to iron oxide. This reaction releases sulfur in the form of sulfate ions which combine with calcium ions in formation waters to form gypsum.
Coffey County sandstones
Detrital mineralogy--Core samples available from wells in Coffey County, Kansas, contained two distinct sandstone-rich lithofacies (fig. 24) which were analyzed by Lardner (1984). The stratigraphically lowest of them consists of fine-grained, ripple-laminated, subarkoses and arkoses. The higher lithofacies consists of fine- to medium-grained, fossiliferous subarkoses and arkoses. The ripple-laminated sandstones are moderately well-sorted, angular to subangular, with detrital minerals consisting of 36-44% monocrystalline quartz, 9-16% potassium feldspars, 1-4% plagioclase, 1-2% rock fragments, and 4-15% micas (biotite plus muscovite; fig. 51A). The fossiliferous sandstones contain 20-45% quartz and similar amounts of the other constituents with the exception of detrital micas, which are much less abundant in this lithofacies (fig. 51B).
Figure 51--Ternary diagram showing composition of sandstone units in Banzet formation sampled in Coffey County, Kansas. A) lower sandstones (lithofacies "B" on fig. 22); B) upper sandstones (lithofacies "D" on fig. 22); C) total Coffey County samples; see appendix B for data summary.
Diagenetic alterations--Lardner (1984) reported similar diagenetic alterations in both of the sandstone lithofacies that he studied (table 1). As in the other sandstone samples analyzed, the Coffey County sandstones contain early authigenic pyrite, clay coatings, and quartz overgrowths. The primary difference between these sandstones and others may be the distributions and timing of carbonate cements. Lardner (1984) observed evidence of appreciable compaction prior to carbonate cementation in both sandstone lithofacies, indicating that only a late phase of carbonate cementation affected these rocks. The pervasiveness of iron-rich calcite cement varies considerably between the ripple-laminated and the fossiliferous lithofacies. The former is characterized by minor carbonate cementation and, as a result, has up to 25% preserved porosity. The latter has only trace amounts of porosity with pervasive calcite cement filling what may have been pore spaces at one time.
Sandstones from the southeastern Kansas-northeastern Oklahoma outcrop belt
Detrital mineralogy--Denesen (1985) concentrated his petrographic analysis on the sandstones below the stratigraphic horizon of the Iron Post coal. These sandstones are subarkosic and are the most quartzose of the sandstones studied except those that were discussed earlier from Anderson County, Kansas (fig. 52). Two types of subarkoses are present. One is fine grained, moderately well sorted, ripple stratified, and lies with a gradational basal contact above silty shales. The other is medium grained, moderately sorted, micaceous, and crossbedded with sharp contacts with underlying mudrocks. Potassium feldspar constitutes from 6% to 13% of bulk sandstone samples, while albitic plagioclase constitutes less than 2% of the bulk sandstones.
Figure 52--Ternary diagram showing composition of sandstone units in Banzet formation sampled along outcrop belt in southeastern Kansas and northeastern Oklahoma; see appendix B for data summary.
Denesen (1985) made an interesting observation concerning the condition of feldspar grains. The majority of potassium-feldspar grains are partially altered, while the albitic-plagioclase grains are clean. This argues against the contention that plagioclase grains are less abundant due to their more extensive diagenetic alteration or weathering than the potassium feldspars. Apparently, the siliciclastic source contained a high ratio of potassium to plagioclase feldspar.
Diagenetic alterations--Although the detrital mineralogies of the subarkoses in the southern portion of the study area are similar, the bulk compositions vary. The two sandstone types that Denesen (1985) recognized have two distinct diagenetic styles. The moderately well-sorted, fine-grained sandstone consists of up to 5% clay matrix. Sandstone units of this type have authigenic kaolinite, chlorite (?), minor amounts of quartz overgrowths, and point-counted porosities up to 13%. Some of these units have pervasive calcite cement and show no pore space. The second type of subarkose, which has up to 5% muscovite, has similar diagenetic alterations except that it has significant amounts of pyrite and iron oxides and lacks carbonate cements. This sandstone type showed up to 17% point-counted porosity.
The fine-grained, rippled sandstone seems to be marine influenced, perhaps accounting for the pervasive calcite cementation of some of its units. The medium-grained, crossbedded sandstone with sharp basal contacts may have been more influenced by freshwaters that were not chemically suited for pervasive calcite precipitation.
Discussion
The sandstone specimens Reinholtz (1982), Lardner (1984), Denesen (1985), Nelson (1985) and I studied represent sands that were deposited in a variety of depositional environments at slightly different times. They have in common a tectonic setting along the margins of an epicontinental seaway and mineralogically similar siliciclastic source areas. When comparing the compositional data from the detailed study areas just discussed, we see a sequence from lithic arkoses in the northeast (Leavenworth County, Kansas) to subarkoses in the southwest (Coffey County, Kansas) and south and along the outcrop belt of southeastern Kansas and northeastern Oklahoma (fig. 53). The exception to this trend seems to be the Bush City and Centerville trends of Anderson County, Kansas, where sandstones are sometimes quartz arenites as well as subarkoses (fig. 50).
Figure 53--Ternary diagram showing composition of sandstone units in Banzet formation sampled from entire study area, excludrig Anderson County, Kansas; see appendix B for data summary.
Diagenetic styles also varied depending upon original sediment composition and the position of sandstone units within stratigraphic sequences. These factors, along with their impact upon the distribution of petroleum within the upper portion of the Cherokee Group, will be discussed after we consider the depositional environments that hosted sandstone deposition.
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Kansas Geological Survey, Geology
Placed on web Oct. 27, 2010; originally published 1989.
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