ABSTRACT

The Meagher Formation conformably overlies the Wolsey Shale and can be divided into three distinct lithologic units. The lowest unit is a mixture of micrite and microsparite and locally includes significant amounts of argillaceous material, fossil debris, and calcispheres. The unit contains blue and gold mottling and irregular patches of dolomite. The middle unit is an oolitic, pelletoidal, intraclastic sparite. The allochems commonly form crossbedded sediments and exhibit incipient dolomitization. Recrystallization and dolomitization are evident in this unit but are not volumetrically abundant. The uppermost unit is characterized by thin blue and gold mottled beds of biosparite and locally contain pellets and algal pisolites.

The deposition of the Meagher Formation took place on a stable shelf dotted by marine banks and shoals. Oolites formed in the more shallow, highly agitated waters in and around the banks, by a combination of accretion and/or precipitation, and abrasion processes. Fluctuations in sea level subjected these semiconsolidated sediments to sporadic wave attack, as attested to by intraclasts made up of all previously deposited carbonate sediments.

Diagenetic alteration ranges from very slight to almost complete obliteration of original structure and texture in some parts of the Meagher Formation, as a result of dolomitization, recrystallization, and the formation of stylolites.

Mottling in the Meagher Formation is classified into three types: depositionally controlled, organically controlled, and diagenetically controlled. Mottling, with the exception of the depositionally controlled type, is associated with dolomitization, recrystallization and iron-staining.

Dolomitization related to mottling took place during the period of lithification when sea water was still incorporated in the semisolid sediment. Incipient dolomitization of allochems has been a product of diagenetic differentiation and has resulted from the unmixing of a solid solution of magnesium and calcium carbonate.