Salina Group
  

Niagaran and Cayugan Series,

Silurian System


Type area and use of name: The term Salina was first used by J. D. Dana in 1863 in the combination Salina Period, during which time the Guelph limestones and marls and limestones and salt of the so-called Saliferous Epoch were said to be deposited in central New York. The Salina once included rocks assigned to the Onondaga Salt (Saliferous) Group (Dana, 1880, p. 232-233), but after long evolution of the term in New York, the name Salina became restricted to Silurian rocks and also synonymous with most of the Cayugan Series, that is, with the Upper Silurian Series of other common North American terminology using a tripartite division of the Silurian System. (See Wilmarth, 1938, p. 1551, 1890-1892, and Rickard, 1975, pl. 2.)

The name Salina has been used traditionally in both the Appalachian and Michigan Basins (Rickard, 1969; Mesolella, 1978; Landes, 1945; Ells, 1962, and Janssens, 1977), variably as a formation or a group. In the latter basin the Salina Group reaches thickness greater than 2,500 feet (762 m) and consists dominantly of alternating carbonate rocks and salts. Southward the Salina lacks salts, becomes thinner (both depositionally and erosionally), and extends into a roughly wedge-shaped unit ranging in thickness from 500 feet (152 m) (northeastern Indiana) to as little as 50 feet (15 m) (central Indiana).

It was to part of this wedge that Pinsak and Shaver (1964, p. 47) first applied the name Salina Formation in Indiana, namely, to that part that lies buried north of the barrierlike Silurian feature in northern Indiana called the Fort Wayne Bank and to that outcropping part that had long been known as the Kokomo and Kenneth Limestones. After convincing evidence of a nearly complete facies relationship between typical Salina rocks of Michigan and reef-bearing rocks southward had been produced, Droste and Shaver (1982) extended the term Salina, as a group, to the whole of the wedge described above. (See precise limits defined in that source fig. 5.) The named subsidiary units of the group became the Pleasant Mills Formation (below, and including the Limberlost Dolomite, Waldron, and Louisville Members) and the Wabash Formation (above, and including the Mississinewa Shale Member and the Liston Creek, Kokomo, and Kenneth Limestone Members). The Salina Group, therefore, includes all the classically studied reef-bearing section in northern Indiana as well as the more basin-type Salina facies.

Description: The Salina Group, thus defined, includes a great variety of dominantly carbonate rocks, ranging from fine-grained shaly rocks to pure carbonate-mud rocks and to coarse-grained vuggy bioclastic and other-wise highly fossiliferous rocks including reef-framework rocks; also, from open-marine deeper water carbonate rocks to very shallow marine, ecologically restricted rocks. Bedded evaporites are unknown, however.

Environments of deposition tended to change cyclically (Droste and Shaver, 1977; Droste, Rexroad, and Shaver, 1980; Shaver and Sunderman, 1983); these changes are interpretable from the classically studied rock units and their boundaries. The several articles on these units (see names above) record the great lithologic complexities of the Salina Group.

The geographic boundaries of the group consist of an eroded edge in central eastern Indiana and of a defined vertical cutoff along the distal limits of the lowermost Salina unit (Limberlost). The cutoff extends northwestward from Shelby County and, farther on, northward to the northwesternmost counties. Thickness along this cutoff ranges from about 50 feet (15 m) (Shelby County) to 400 feet (122 m) (Newton and Lake Counties).

Much of this thickness change is due to pre-Middle Devonian erosion, as the top of the Salina Group is everywhere coincident with the Silurian-Devonian unconformity, which increases in magnitude (and extent of Silurian truncation) southeastward. The bottom of the Salina Group is everywhere conformable, with possible rare minor exception, with the underlying Salamonie Dolomite. A part of this relationship is very likely time transgressive. (See also Okla, 1976, for a thorough treatment of Salina rocks in Indiana.)

Correlation: Many isolated to systematic studies in Indiana have recorded, during the modern period of study of the Salina rocks defined here, dozens of occurrences of key index fossils whose reliabilities are firmly established in a sequential stratigraphy that takes account of the ecologic factors. In addition, stratigraphic sense has been made of the hundreds of once hopelessly stratigraphically/ecologically confused species that were the objects of classic study (Shaver, 1974b).

Each of the main groups S pentamerid brachiopods, conodonts, and ostracods S as well as the rarer but significant fossils S graptolites, acritarchs, and the mollusk Megalomus canadensis S now has a consistent stratigraphic meaning. Collectively they show that the Salina Group in Indiana ranges in age from late Wenlockian (middle or late Niagaran) into Pridolian (late Cayugan). Significantly, some of the same indicators are common between the classically studied reef-bearing section and the rocks classically correlated with the Salina of other states, that is, the Kokomo and Kenneth rocks. The commonness of lower Salina rocks with upper Niagaran rocks has also been shown in the Appalachian Basin states (Rickard, 1975; Mesolella, 1978; Janssens, 1977; and Patchen and Smosna, 1976).

The youngest Salina rocks of Indiana, probably eroded at the top, are older than the youngest Salina rocks of the Michigan and Appalachian Basins. Southwestward the Salina has age equivalence with the middle and upper parts of the Bainbridge Group of the Illinois Basin, but there, too, the youngest Bainbridge rocks are probably younger than the youngest Salina of Indiana. (See also Shaver and others, 1985, and the several articles in this compendium on the units making up the Salina Group.)






 
 
 
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