Silurian Thalassinoides in an offshore carbonate community, Wisconsin, USA

Abstract

Abstract Silicified boxwork of the ichnogenus Thalassinoides in the Silurian (Wenlockian) Racine Formation, Milwaukee, Wisconsin (USA) represents open-burrow systems that extended decimeters below the sediment surface in offshore carbonate muds of an intracratonic basin. The most completely-silicified burrows contain a core of organic-rich, spiculitic chert, a surrounding zone of chert, and an outer zone of mixed chert and dolomite. Burrows reach 3 cm in diameter and include larger “turning chambers”; they were probably produced by an arthropod. Bioturbation affected the epibenthic fauna by extensive vertical mixing of skeletal material and possibly by construction of small mounds at the sediment surface. The same skeletal fauna of siliceous sponge spicules, silicified crinozoan ossicles, and a diverse array of other groups occurs in both burrows and surrounding dolomite matrix. Brachiopods, including Dicoelosia and Skenidioides , indicate Silurian Benthic Assemblage 5, which represents an outer shelf setting. Trilobite sclerites are more common within Thalassioides burrows than in the surrounding matrix. Two enrolled calymenid individuals and one molted calymenid exoskeleton occur within Thalassinoides , suggesting that commensal trilobites occupied the burrow systems.