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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 48 (1964)

Issue: 4. (April)

First Page: 528

Last Page: 528

Title: Penecontemporaneous Dolomite in Upper Silurian Cyclothems, South-Central Pennsylvania: ABSTRACT

Author(s): V. E. Gwinn, D. M. Bain

Article Type: Meeting abstract


Composition of carbonate rock units in carbonate mudstone cyclothems in the Tonoloway and Wills Creek formations (Swartz, 1955) appears to be positively correlated with types of primary structures, amount of organic material, state of oxidation of iron, relative abundance of fossils, and presence or absence of evaporite mineral pseudomorphs.

Dolomitic carbonates have a preferred association with the following: thin bedding and internal lamination, cut-and-fill structures, mud-cracks, and possible algal-mat lamination; higher proportions of siliciclastic detritus; and a low organic matter content as expressed by light color and usually an absence of fossils. Some dolomite beds have carbonate pseudomorphs after anhydrite and halite.

Limestone elements of the cycles, on the other hand, have these general characteristics: thicker bedding, general absence of thin laminae, and other structural evidence of shallow water or subaerial exposure seen in the dolomites; less siliciclastic detritus; more frequently fossiliferous, with more varied fauna; darker color with higher organic content; and iron generally present in the form of pyrite.

The excellent correlation of four features, primary structures, organic content, oxidation-reduction phenomena, and the amount of non-carbonate detritus, with carbonate composition appears to point to the operation of two causal factors in the repeated depositional-compositional cycle from calcareous to dolomitic carbonate: (1) shoaling of the water; and (2) a concomitant increase of salinity. Following this tentative conclusion, it appears that the dolomite are penecontemporaneous in development. The necessary Mg++ may have been derived from the relatively more saline waters present at the end of most of the cycles, with the dolomite forming while the carbonate debris was bathed in the basin waters during or immediately after deposition during earliest diagenesis. No "typi al" criteria of secondary dolomitization have been observed.

Additional study will be directed to analysis of boron content of clays in the intercalated mudstones, perhaps to carbon isotope fractionation in the cycles, and to the origin of a bed-thickness cycle superimposed on the compositional cycles (Lacey, 1960), which is not obviously correlated with the compositional cycle.

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Copyright 1997 American Association of Petroleum Geologists