Taphonomy of Middle Pennsylvanian Mazon Creek area fossil localities, Northeast Illinois; significance of exceptional fossil preservation in syngenetic concretions

Mazon Creek area fossil localities (Pennsylvanian: Westphalian D) in northeast Illinois provide an extraordinary preservational "window" for the study of Late Paleozoic terrestrial, fresh-water, and estuarine marine organisms. Marine animals were killed by episodic pulses of turbid fresh water associated with flooding from distributaries, and terrestrial organisms were introduced into coastal waters from upstream sources. Rapid burial entombed remains, commonly before significant decomposition had occurred, and very early diagenesis ensured fossil preservation as molds and composite impressions within sideritic concretions. The taphonomy of these deltaic, fossil-bearing beds is complex. Episodic engulfment of organisms is indicated by animal escape activity and by the presence of upended plant fragments. Numerous, distinctive, cyclic repetitions of siltstone-claystone laminae, each recording a single tidal (flood-ebb) event, are observed in areas yielding marine organisms; these indicate that rapid but extremely regular deposition was also important. Concretion formation was probably influenced, but not necessarily triggered, by decay processes. Siderite precipitation reflects three key conditions: the availability of iron, rapid burial of organic material, and a low to nonexistent supply of sea-water sulfate to centers of interstitial microbial activity. A regional (seaward) decrease in the quality of fossil preservation is observed away from the coastal depocenter, and an interval of sparsely fossiliferous facies (a taphonomic discontinuity) occurs between the delta complex and shell-rich, nonnal marine deposits. The preservatz.onal significance of early diagenetic siderite is discussed; we argue that paleoenvironmental interpretations of analogous or similar nearshore deposits lacking fossiliferous concretions must be made cautiously. The potential value of Mazon Creek-type facies in the study of other Pennsylvanian nearshore deposits, including many superficially "barren" shales, is stressed. The association of the fossilbearing concretions with inferred estuarine-deltaic facies points to the need for future actualistic studies of comparable modern coastal environments.