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Paleoenvironmental Significance of Zoophycos: Implications of Carboniferous Coal Beds Disturbed by Trace Fossils

Abstract

Analyzing ichnofauna during core description is a standard tool for evaluation of paleoenvironmental conditions within petroleum systems. Interest in fine-grained rock has re-energized the debate regarding shallow water vs. deep water deposition of organic-rich mudstone. Ichnofacies assemblages are central to the debate because Zoophycos is usually considered indicative of deep-marine and oxygen-poor environments. In this study of Carboniferous cyclothems, core samples show a pattern of coal directly disturbed by low-diversity Zoophycos-dominated strata. This pattern recurs thirteen times over 100 feet of strata. The coal formed during lowstand, whereas the Zoophycos-bearing strata mark the onset of transgression during which shallow, brackish-water bays developed over the coal beds. A single occurrence of Zoophycos-disturbed coal might represent the onset of a regional-scale transgression; however, the repetition of this association indicates frequent marine inundation of peat bogs. Most likely, Zoophycos records infaunal pioneers in low-oxygen, low-energy environments with highly variable salinity. This interpretation is consistent with a body of paleontological literature that cites Zoophycos as an indicator of low-oxygen, brackish-water, and similarly stressed environmental settings. However, outside the paleontological literature, Zoophycos is commonly cited as confirmation of a deep-water marine setting. This work further challenges the belief that Zoophycos is a reliable indicator of deep-water environments. When Zoophycos is part of a diverse ichnofauna, it probably has little environmental or no bathymetric significance. When part of a low-diversity assemblage it probably records a stressed environment where the Zoophycos trace maker was better adapted to the environment than other organisms.