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Environmental History of Grenola Limestone
Synthesis of the foregoing facts into an organized and continuous picture seems necessary. Southwestern Elk County during the time when the Grenola Limestone was laid down could be visualized from the following statement.
At first we may picture a low-level mud flat, composed of red clay and silt, crossed by slow-flowing rivers carrying fine sediment into a nearby marine basin. Then as saline waters flooded inland over the mud flat, iron-reducing bacteria and decaying organic debris changed the upper part of the red mud to green mud. As the water became progressively deeper, clams, small snails, Juresania, and ostracodes migrated slowly into the area, living on the bottom and feeding on minute organisms in the water. The water probably was brackish or perhaps freshened from time to time, keeping less tolerant animals away from these shallow waters. Deposition of new material formed a bottom of fine mud, composed of argillaceous material and disseminated calcium carbonate.
Breaking and sorting of shells was effected by waves and currents, and small shell fragments and debris were transported by waves, coming to rest in areas where such action was less intense or less frequent. In this slightly deeper and perhaps clearer water, small organic particles, gastropods, and ostracodes became nuclei for attachment of lime-secreting algae and small arenaceous foraminifers.
Gradually, as the water deepened, clams and snails moved into the shallower water and were replaced by delicate fronds and stalks of bryozoans and by brachiopods that attached themselves to the bottom with their muscular pedicle or by stout spines. Spiny echinoids moved or burrowed among the stems of crinoids. Ostracodes, which thrived in more argillaceous sediments, were supplanted by small arenaceous and calcareous foraminifers.
From time to time the water became slightly deeper, and many robust fusulinids populated the bottom, living either on top of or in soft carbonate ooze, which later lithified to limestone. Seemingly the foraminifers had little competition for this ecologic niche, as their tests are extremely abundant. Brachiopods and bryozoans either lived on fusulinid-populated bottoms or their tests were moved to slightly shallower molluscoid bottoms by gentle currents, because today both kinds of fossil are found in the same rock.
In this deeper water, colloidal and dissolved silica carried in by rivers may have gelatinized or precipitated and settled to the bottom, where agitation resulted in coalescence into nodules, which gradually hardened, incorporating organic particles. Finally, a slight shallowing took place, accompanied by a re-entrance of clams, gastropods, and algae, which was followed by more rapid shoaling and covering of the deeper-water carbonate sediments by argillaceous material. This was the beginning of deposition of the Eskridge Shale formation.
Conclusions
1. The Grenola in the type area is a well-defined traceable lithologic unit.
2. Black shale and plant fragments such as those found in the Neva, Burr, and Salem Point members farther north are not present in the type area.
3. The Sallyards, Legion, Burr, and Salem Point members are characterized by a molluscan fauna, accompanied in the Legion by a microfauna containing numerous specimens of Cavellina, Hollinella, and Knoxina.
4. Charophytes found below the Sallyards Limestone and in the Salem Point Shale probably indicate nearsbore conditions intermediate between the environments of deposition for green shale and molluscan limestone. Charophytes may be a microrepresentative of the Lingula phase.
5. Osagia found in the rocks is an intimate intergrowth of ?algae and Ammovertella.
6. Limestones of the Grenola Limestone may be divided on the basis of their organic textures into types as follows:
- Molluscan limestones
- Osagite limestones
- Pectinoid limestones
- Molluscoid limestones
- Brachiopod limestones
- Bryozoan limestones
- Echinoderm limestones
- Fusulinid limestones
7. Osagite beds at the top of the Burr and Sallyards members probably were formed: (a) in water approximately 60 feet deep, (b) under generally quiet conditions with slow accumulation of shell fragments, and (c) under conditions favorable for the growth of algae. The shell particles probably were derived from a shallow water zone characterized by roiling and approximating the plunge point in depth.
8. The Neva member contains a molluscoid and fusulinid fauna, and an equivalent microassemblage of Climacammina, Glyphostomella, and Bairdia.
9. Thickness of fusulinid- and chert-bearing limestones increases southward, indicating that deeper water prevailed to the south for a longer period of time than in the north.
10. The chert nodules in the Neva member seem to be primary.
11. The members of the Grenola Limestone may be classified in terms of Elias' cyclic phases as follows:
- Sallyards member, molluscan (4) phase, pectinoid and osagite limestone.
- Legion member, molluscan (4) phase, CaveIlina-Hollinella-Knoxina shale, and thin pectinoid limestones.
- Burr member,, molluscan (4) phase, pectinoid and osagite limestone.
- Salem Point member, Lingula (3) and molluscan (4) phases; charophytes, Cavellina, and pectinoid limestones.
- Neva member, brachiopod (6) and fusulinid (7) phases; molluscoid and fusulinid limestones; Climacammina-Glyphostomella-Bairdia shales.
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Kansas Geological Survey, Geology
Placed on web June 5, 2007; originally published June 1958.
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