Petrography of the La Salle limestone (Pennsylvanian) La Salle County, Illinois

Includes bibliographical references.Includes illustrations and maps.Petrographic analysis of the typical eastern segment of the La Salle Limestone Member, a carbonate bank, has re­sulted in the recognition of six lithologically and paleon­tologically distinct microfacies: (1) replacement dolomite microfacies— a combination of dolomite and neomorphic calcite variably replacing the original allochems and matrix; (2) dolomitic biomicrosparite microfacies— a biomicrosparite in which burrows, algae, and stylolites were the sites of dolomitization; (3) sparite mlcrofacies— biopelsparite, biosparite, and Osagia intrasparite; (4) poorly washed blo- sparite microfacies— biosparite frequently distinguished by biomicrite laminae; (5) biomicrite microfacies— biomicrite containing Archaeolithophyllum and a varied fauna; (6) micrite microfacies— micrite and fossiliferous micrite having an abundance of phylloid green algae. The site and geometry of the carbonate bank were con­trolled by various factors— physiographic in the configura­tion of the postulated Pennsylvanian shoreline, delta, and possibly positive La Salle Anticline, and hydrographic in respect to water depth, tides, and currents on the shallow shelf of the epicontinental sea. In places favored by quiet conditions below wave base and at a depth providing suitable radiant energy, green algae established themselves upon a thin argillaceous poorly washed biosparite that mantled a black shale facies. They sheltered a fauna limited in numbers and variety. As the bank neared wave base, the green algae were replaced by an Archaeolltho- phyllum flora and a more abundant and diverse fauna. The algae produced most of the fine carbonate and were the agents which bound the soft sediment into bank form. Sub­sequently, several different types of sparites were depos­ited, reflecting the higher energy of a littoral regime. The restricted occurrence of dolomite suggests a final supralittoral environment for at least part of the time be­fore the La Salle Limestone was overwhelmed by the overly­ing red shale facies.M.S. (Master of Science