Ichnology of American chalks: the Selma Group (Upper Cretaceous), western Alabama

Abstract

Chalk facies within the Selma Group, most of them impure, crop out only in a relatively small area of western Alabama and eastern Mississippi and grade laterally into calcareous terrigenous clastics. Body fossils and skeletal borings (described in a separate paper) are diverse and abundant in some units, especially the impure chalks. Major burrow forms, also distributed unequally among these units, include Chondrites sp., Cylindrichnus pustulosus n. sp., Planolites beverleyensis, Skolithos sp., Teichichnus rectus, Teichichnus zigzag n. sp., Thalassinoides paradoxicus, Thalassinoides suevicus, Trichichnus linearis, Zoophycos sp., and mineralized burrows. Minor forms include Palaeophycus sp., Rosselia spp., and mantled burrows.Characteristic trace fossil distributions especially include the striking dominance of Thalassinoides paradoxicus in chalk hardgrounds of the Arcola Member of the Mooreville Chalk and successive patterns of bioturbation in the lower unnamed member of the Demopolis Chalk. In purer chalks there, major burrow forms exhibit a chronologic sequence of ichnologic events, expressed by (1) cross-cutting relationships among burrows, (2) increased sharpness of younger burrow outlines, and (3) increased contrasts in color and composition of younger burrow fills. The sequence, interpreted to represent differences in depth of burrowing, consistency of the substrate, and type of biogenic activity, proceeds from vague burrow mottles and indistinct Thalassinoides sp., through increasingly distinct Thalassinoides suevicus, Zoophycos sp., and Teichichnus rectus, to sharply defined Teichichnus zigzag and Chondrites sp. Similar relationships, although less conspicuous, are discernible in the Mooreville and Prairie Bluff chalks.