Type section and history of name: The name Waldron Shale was used by N. M. Elrod (1883, p. 111) to replace the earlier designations Waldron beds and Waldron fossil bed that were used for the thinly interbedded clay, shale, and limestone overlying the quarry stone (presently known as the Laurel Member of the Salamonie Dolomite) near Waldron, Shelby County, Ind. The term Waldron there referred to the exposure on Conns Creek in the NE¼ sec. 6, T. 11 N., R. 8 E., which was one of the locations yielding the renowned fauna of more than 200 species described by James Hall of New York State and others. (See Cumings, 1922, p. 453-454.) Type Waldron exposures remain at this locality in the abandoned Standard Materials Corp. quarry.
Waldron rocks have long been known to extend along the southeastern Indiana outcrop area and into southern states. Much later, the term Waldron Formation was adopted by Pinsak and Shaver (1964, p. 29) for the dominantly dolomitic facies of this unit that had been traced by that time into
much of northern Indiana (Shaver and others, 1961, p. 13) except the far western counties and the northern two tiers of counties. Several modern reports further defined northern Indiana usage and even extended recognition into adjacent western Ohio (for example, Griest and Shaver, 1982, p. 377, 379-380, and Shaver and Sunderman, 1983, p 161-163). In this same period, Becker (1974, p. 18-20) defined the Waldron Shale occurrence in the more marginal part of the Illinois Basin in southwestern Indiana.
Use of the name Waldron Formation was discontinued in northern Indiana except in the far western counties, however, by Droste and Shaver (1982, p. 5, 11). They assigned the Waldron-equivalent rocks to the middle part of the then-new Pleasant Mills Formation. The principal reason for abandoning the term in northern Indiana was that the Waldron and Louisville (next above the Waldron) types of lithologies are interbedded in sequences as thick as 40 feet (12 m), thereby obviating easy recognition of the type boundary. This action has not been satisfactory, however, and the expression Waldron-equivalent rocks since has appeared in modern reports for northern Indiana. For this reason, therefore, the term Waldron is here reintroduced for use in northern Indiana as the Waldron Member of the Pleasant Mills Formation. This term is not everywhere useful in that part of the state, however, because of thick gradational Waldron-Louisville sequences in some places.
Description: As is apparent from the above discussion, the area of Waldron recognition in Indiana extends from the subsurface of southwestern Indiana (save for the parts or the whole of nine southwesternmost counties: Becker, 1974, fig. 9) eastward to the area of Silurian outcrop and northward to a line extending approximately from central LaPorte County to central Allen County, that is, to positions underlying the Fort Wayne Bank (Droste and Shaver, 1982, figs. 5-7). Characteristic Waldron lithology is hardly recognizable north of that line.
The Waldron is underlain, apparently conformably but through a rather thin interval of interbedded transitional lithologies, by the Laurel Member of the Salamonie Dolomite in southern Indiana, by the undivided Salamonie in some far western counties m northern Indiana, and by the Limberlost Dolomite Member in northern Indiana. The upper Waldron contact is also conformable, variably with the Louisville Limestone and the Louisville Member of the Pleasant Mills Formation. This contact, however, is involved in many places with a thick transitional zone as much as a few tens of feet thick.
The Waldron is typically a shale containing silt and fossiliferous limestone beds that are reeflike in many places. The terrigenous clastic content decreases westward and northward from the type area, however, so that in the southwestern subsurface, the unit becomes a dense argillaceous limestone; northwestward to its area of farthest recognition, it becomes a fairly pure dolomite bearing only shaly laminae or a faintly argillaceous appearance; and in areas in between the type area and northwestern Indiana, the Waldron consists generally of dark to mottled sublithographic to fine-grained limestone and dolomite exhibiting nodular carbonate structure and wraparound shale.
The Waldron averages about 5 feet (1.5 m) in thickness in its southern occurrence but is appreciably thinner or thicker in some places the Waldron equivalent is as thick as 20 to 30 feet (6 to 9 m) northward in some places and as thin as 1 foot (0.3 m) in the Ohio-Indiana border area southeast of Fort Wayne. Typical lithology is absent from some places by nondeposition as a terrigenous clastic unit, but in that circumstance it has generally been replaced by pure carbonate rocks in a reef facies. Such reef facies variably extend through the Waldron stratigraphic interval from subjacent into superjacent units or are confined within the Waldron interval in different places of observation.
Correlation: Hall (1882, p. 219-220) favored a middle Niagaran age for the Waldron comparable to part of the Rochester Shale of the northeastern states and was followed in that preference by Berry and Boucot (1970, p. 249-250) and Shaver (Shaver and others, 1970, p. 187). Physical tracing of Silurian units from the western New York standard westward across the Appalachian Basin (especially by Rickard, 1969, and Janssens, 1977) shows, however, that the Waldron must be assigned a Niagaran stratigraphic position well above that of the Rochester of western New York and also above that of the Rochester of Janssens (1977) of western Ohio. In western Ohio terms, this position is in the upper part of the Lockport Group where the upper boundary of this group has been extended upward to accommodate a westward facies change in the rocks lying above the Lockport of eastern locales in British terms its position is upper Wenlockian. (See, for example, Shaver and others, 1985.)
Conodonts have not yet been described from the Waldron in Indiana, although they have been from immediately overlying Louisville rocks (Rexroad, Noland, and Pollock, 1978), which are represented by the Kockelella variabilis Zone (late Wenlockian-early Ludlovian) .