Monomineralic Grains Research Articles

Exotic pebble in La Salle limestone (upper Pennsylvanian), La Salle, Illinois

ABSTRACT An earlier discovery (1928) of exotic pebbles in a limestone located above the La Salle Limestone (base of Bond Formation, McLeansboro Group, Pennsylvanian), and a new one in the upper part of the La Salle Limestone, both in the vicinity of La Salle, Illinois, are described. The new occurrence is interpreted as an open marine, shallow water crinoid-brachiopod calcarenite containing irregular bedding planes with small NE-SW trending channels, along which sand-sized grains and pebbles reaching 2.5 cm diameter are present. These materials may be subdivided as follows: 20% igneous rocks; 20% high grade metamorphic rocks; 40% low grade metamorphic rocks; 10% sedimentary rocks and 10% monomineralic grains. The distribution of the exotic components is essentially bimodal associating a gravel fraction to a clay fraction. The latter represents the appearance of kaolinite in an environment of mixed layer clay minerals and montmorillonite. The mineralogical composition of the exotic pebbles indicates a wide source area of Precambrian terrane. The nearest possible one is located in northeastern Wisconsin, about 250 miles northeast of the investigated outcrop. An earlier fluvial transportation of the exotic pebbles of such a magnitude must be assumed, probably in relation to a change in base-level during typical regressive conditions of a cyclothem. The final distribution in the carbonate basin is assumed to have been as a submarine mudflow which travelled along a narrow path for about 10 to 20 miles, possibly triggered by storms or mild earthquakes. The former interpretation of transportation of the exotic pebbles by ice-rafting, with its climatic implications, is rejected.