General characteristics--The limestones of the Grenola exhibit the following general characteristics:
- The bedding planes between strata are all somewhat wavy. No very smooth bedding surface was observed.
- Colors range from dark blue gray to light yellow brown.
- Matrices are composed of clay and fine silt-size argillaceous material, mixed with clear or cloudy granular or subcrystalline calcite. The matrix is a consolidated calcium carbonate mud.
- With very few exceptions, all limestones contain discrete organic particles. These are the most distinctive features of the rock and will be described separately.
Organic components--Several characteristic associations and arrangements of whole fossils and fragmentary fossils were observed, which, taken together, make up what may be termed the organic texture of the limestone. Using particle length as a criterion, most limestones studied can be divided into two groups: those containing fragments predominantly 1 to 3 mm in length, and those containing longer particles.
Ammovertella and ?algae form common shell coatings in many limestones containing abundant small fossil fragments, which generally make up 70 to 90 percent of etched limestone surfaces. Rocks containing abundant, incrusted organic debris 1 to 3 mm long have an osagite organic texture.
In rocks containing longer fragmentary shells (Pl. 5B) such particles invariably make up less than 25 percent of etched limestone surfaces. Pelecypod valves are found on the surfaces of these limestones; therefore, contained shell fragments are probably those of mollusks, and the limestones are referred to as molluscan limestones. In a few molluscan limestones, algal coatings are found on one or both sides of long shell particles. If both sides of a particle are coated, it is probable that both surfaces were, at one time or another, exposed to light rays. If the incrusting material is the same in width on all sides, then all surfaces must have been exposed to light for approximately the same length of time.
In limestones containing long, fragmentary organic particles, three kinds of orientation with respect to the bedding were observed: (1) broken shells parallel to the bedding, all arranged with the convex side up; (2) organic particles lying at all angles to the bedding; and (3) fragments parallel to the bedding with either convex or concave side up.
Associated with large and small broken shells are minute high- and low-spired gastropods, ostracodes, sparse crinoid stems, and small spines. Gastropods and ostracodes are most numerous in limestones that contain organic debris less than 3 mm in length. The associated small spines are identified as brachiopod rather than echinoid spines, for cross sections of them show a concentric laminar structure, and not the minute honeycomb structure characteristic of echinoderms.
An important associate of shell fragments is the supposed algal incrustations. These have been given the generic name Osagia (Twenhofel, 1929). Because designation is based solely on shape, Osagia is a form genus. Working exclusively with thin sections, Johnson (1946) found that Osagia in the Grenola rocks consists of two groups of filaments or tubes, about 0.0018 and 0.4 mm in width, which he identified as Girvanella, an alga, and Nubecularia, a calcareous foraminifer, respectively. In thin sections prepared for the present study, no fine algal filaments were seen, but coarse tubes parallel to the surface of the shell nucleus were observed. The larger tubes could be seen standing out in relief on etched surfaces of the limestone, and coarse filaments freed from the limestone were found in insoluble residues from Osagia-bearing rocks. The freed tubes have been identified as Ammovertella, a common arenaceous Pennsylvanian-Permian foraminifer.
In regard to Osagia, Elias (1946) says "These fossil lumps and incrustations do not correspond to actual somata of living algae, but are merely accumulations of successive calcareous precipitations, presumably by the countless film-like aggregates of microscopic simple colonial algae, and are mixed with calcium carbonate precipitated by organisms other than algae, or (and) precipitated inorganically." This may well explain why the small algal filaments described by Johnson were not observed.
Associations of brachiopods, bryozoans, and fusulinids in limestone constitute another type of organic texture. Brachiopod shell fragments and whole brachiopods occur in the limestones, usually in association with ramose or fenestrate bryozoans, or both. Most fragmentary brachiopods and bryozoans lie flat, parallel to the bedding of the rock. Associated with these two dominant groups are calcareous foraminifers, crinoid stems, echinoid spines, and sparse gastropods and ostracodes.
In general, organic debris in brachiopod-bryozoan limestones occupies less than 50 percent of etched surfaces, which percentage is less than for beds having osagite organic texture. The brachiopod-bryozoan limestones contain a very small amount of argillaceous material, for commonly the only insoluble residue from several grams of limestone consists of a few small Ammovertella tests. In contrast with molluscan limestones, such tests and algal material are rare on the etched surfaces of brachiopod-bryozoan, or more conveniently, molluscoid, limestones.
Another dissimilarity between molluscan and molluscoid limestones is the nature of preservation of the shell fragments. The substance of mollusk shells usually is recrystallized into brown or white calcite crystals, whereas fine original internal structure of brachiopod and bryozoan fragments commonly is preserved. The organic particles show on etched surfaces as white, microcrystalline calcite, generally with laminar structure. This difference is explained by the dissimilarity in crystal structure of the shell material in mollusks and molluscoids.
Limestone groups--The gross lithologic features of the Grenola limestones in the type area are not diagnostic. In order to distinguish one limestone sample from another, stratigraphic sequence being ignored, it is necessary to rely on organic textures. On this basis, the groups of limestone indicated below are differentiated. Because only Grenola limestones were studied, the limestone groups suggested may be useful in the examination of other marine limestone formations, but an exhaustive stratigraphic study of organic textures of limestone would undoubtedly reveal characteristic groups of fossil debris not found in the Grenola rocks. In regard to classification of sedimentary rocks, Pettijohn (1949) says "The only test of significance is whether the characteristics are or are not basic to understanding of origin." The organic particles are the only characteristics of the Grenola limestones that are useful for cyclic interpretation and therefore the only ones that are "basic to understanding of origin."
These groups have been distinguished by laboratory examination of etched surfaces of the limestones, but probably they may be identified also in the field with the aid of a hand lens. The limestone groups are as follows:
1. Molluscan limestones. The organic content of these consists chiefly of complete or fragmentary pelecypod valves and small high- and low-spired gastropods. The molluscan limestones may be divided further into osagite limestones, which contain many small Osagia-incrusted pelecypod shell fragments and gastropods; and pectinoid limestones, which contain long pelecypod shell particles, generally free of algal incrustations.
a. Osagite limestones. These are composed of very abundant small (1 to 3 mm) molluscan shell particles, and common small gastropods and ostracodes, many of which are surrounded by a thin algal crust containing Ammovertella tests (Pl. 1B, 2B). Organic debris in osagite limestones ranges from 1 to 5 mm in length, the average being 2 mm. A maximum of 150 shell particles was counted on one square inch of etched limestone surface. Total organic debris occupies 70 to 90 percent of etched surfaces. The narrow range in size of all the small fossil particles (gastropods, ostracodes, and shell fragments) is the result of wave or current sorting.
b. Pectinoid limestones. These contain common long thin convex valves of pelecypods (Pl. 6A), which are predominantly, if not exclusively, Pecten-like forms and hence appropriately called pectinoid. It is impossible to identify the fragments definitely as belonging to genera of the Pectinacea. Associated with pelecypod particles are small gastropods and ostracodes, but algal incrustations, so abundant in osagite limestones, are generally lacking. Where nearly all fragments in a limestone are oriented with their convex side up, it is likely that such orientation was effected by current or wave action. Where particles exhibit random orientation, or several have their concave side up, it is probable that little or no current action influenced sedimentation.
2. Molluscoid limestones. These contain a predominance of particles derived from brachiopods and bryozoans.
a. Brachiopod limestones. Fossils in these consist mostly of entire valves or fragments of brachiopods. Included also are limestones that contain very few brachiopod fragments other than spines.
b. Bryozoan limestones. Ramose or fenestrate bryozoans, or both, predominate over other fossils in this kind of limestone, commonly associated with small multichambered calcareous foraminifers (Pl. 6B). The bryozoan remains may be broken particles or whole colonies. A rock containing both brachiopods and bryozoans may be classed simply as a molluscoid limestone.
3. Echinoderm limestones. Limestones that contain organic remains composed mostly of echinoderm particles are less common than type 2a or 2b, and only a few of them were observed in the Grenola. Echinoid spines and plate fragments associated with crinoid stems and plate fragments are common in these limestones. The minute honeycomb internal structure is preserved in most of the echinoderm remains.
4. Fusulinid limestones. Any limestone containing sparse to very abundant fusulinids is here classed as a fusulinid limestone, although molluscoid debris is present in all limestones containing fusulinids (Pl. 6C). The reason for this procedure is that fusulinids constitute the only indication of the culminating transgressive phase of the cyclothem, making this arbitrary division necessary. Robust "Triticites" is the common fusulinid, and may constitute 50 percent of an etched surface.
Plate 6--[Web versions enlarged to show more detail.] A. Small algal mass in pectinoid limestone, lower part of Sallyards member, locality 3. Composita and pectinoid shells attached to top and side of algal mass (x2.4) B. Bryozoan limestone, Neva member, locality 4 (x2.4). C. Fusulinid limestone, Neva member, locality 4 (x4).
Kansas Geological Survey, Geology
Placed on web June 5, 2007; originally published June 1958.
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