Rock-boring organisms as markers of stratigraphic breaks

Borings made by various kinds of organisms are characteristic of many disconformities from the Ordovician to the Recent. Borings are especially abundant on Jurassic, Cretaceous, and Tertiary discontinuity surfaces in shallow-shelf carbonate sequences. The organisms that made most of these borings are mollusks, sponges, various kinds of worms, barnacles, and algae. Of these groups the bivalve mollusks are the most common and most highly adapted borers.The recognition of the rock borings and their distinction from burrows made in unlithified sediment commonly is necessary for the identification of otherwise obscure disconformities. Rock borings in carbonate sequences imply stratigraphic breaks with histories of (1) emergence, (2) lithification, and (3) resubmergence; whereas, in the same sequences burrows do not necessarily imply any sort of stratigraphic break. Shapes of burrows and borings and relations with sediments and structures are reviewed as the essential criteria for recognizing borings and bored surfaces and for distinguishing them from burrows and burrowed surfaces.

Cretaceous and younger rock sequences of Texas and Mexico include many disconformities characterized by borings. The magnitude of the stratigraphic breaks which these bored surfaces represent ranges from local intraformational interruptions to major intersystemic unconformities. Examples of these surfaces are compared in terms of (1) surface morphology, (2) encrusting faunas, (3) shape and variation of borings, (4) corrosion features, (5) areal extent, and (6) lateral correlatives