COVEL CONGLOMERATE FROM LASALLE COUNTY (IL): AN UPPER CARBONIFEROUS TEMPESTITE OR TRANSGRESSIVE LAG DEPOSIT?

The Covel Conglomerate Member of the Pennsylvanian (Upper Carboniferous) Carbondale Formation in Illinois is a calcareous conglomerate, a lithology that is an anomaly in the well-recognized cyclothemic successions. Lithic conglomerates have been described from the basal portion of numerous Pennsylvanian sandstones (quartz and lithic arenites) and many bioclastic and algal-dominated limestones occur throughout most of the Middle to Upper Pennsylvanian.

Preliminary study of the Covel, sampled from near Marseilles, indicates the rock is very poorly sorted, grain-supported, and conspicuously lacks both graded bedding and bioturbation. Clast shapes are dominantly rounded to well-rounded but angular to subangular shapes are present. The majority of the clasts are calcareous lithoclasts, but siliceous lithoclasts and bioclasts (dominated by brachiopods) are common. Phosphatic lithoclasts and elongate (up to 5 cm), compacted, shale clasts also occur. Some slab-shaped clasts suggest subtle imbrication. The Covel is not a blanket deposit but consists of closely spaced lenses that only occur in the northern part of the Illinois Basin. The thickness of the bed reaches10 cm locally and up to 25 cm basinwide.

The lithology, texture, and sorting of the clasts and the general absence of internal bedding suggest very rapid deposition in a high-energy environment. Possible environments include low-latitude storm, earthquake-induced tsunami, or impact-induced tsunami. An alternative interpretation is that the conglomerate is a transgressive lag deposited during the initial sedimentation of the St. David Cyclothem.