Abstract

A quantified petrographic analysis of a McKenzie core from Wayne County defined a suite of environments from shelf-edge to lagoonal to peritidal. Q-mode cluster analysis of 34 variables for 80 thin sections generated ten clusters which were then characterized on the basis of mean variable abundance. These characterized clusters represent deposition in nine different environments. Near the base of the McKenzie, rocks of a bioherm core arc characterized by crinoids and Cladopora . As the mound grew above wave base, stromatoporoids and Favosites in a micrite matrix capped the bioherm. A marine regression combined with upward growth of the mound established a littoral environment, and sandy dolomite formed within tidal channels. The transition to an oolite shoal indicates a slight rise of sea level. Oolitic dolomite represents the seaward slope and crest of the shoal whereas oolitic limestone (ooids and pellets) was deposited on the backslope. The overlying subtidal lagoonal sediments include pelletal and ostracode limestone. The remainder of the formation was laid down in a peritidal environment, indicating continued regression. Pelletal dolomite formed on a lower intertidal mud flat, and pyritiferous dolomite is of an intertidal/supratidal origin. R-mode cluster analysis grouped the 34 variables into five genetic associations: (1) transported minerals such as quartz, tourmaline, and zircon; (2) secondary minerals--dolomite and pyrite; (3) tidal-flat components--gypsum, anhydrite, oncolites, and intraclasts; (4) elements of low-energy, high-diversity environments--eleven fossil taxa plus micrite and clay minerals; and (5) elements of high-energy, low-diversity environments, like ooids, calcite cement, ostracodes, and gastropods.

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