Age: Pennsylvanian, Carboniferous.
The exceptional preservation of the fossils in the concretions, especially those that are soft bodied, and the presence of fossilised upright tree stems, perpendicular to the plane of bedding, suggests that the fossils may have been preserved by rapid engulfment during catastrophic events [1, 7]. Aquatic organisms from the more estuarine environments may have been killed by episodic inundations of turbid freshwater during periods of flooding . Escape burrows, produced by bivalves and other organisms, are apparent in several of the sediment layers, although some horizons are completely devoid of trails or disturbances, suggesting rapid deposition of large quantities of sediment to a great depth [1, 7]. The fossils show little or no disturbance, indicating that they were buried quickly, with little scavenging . The soft-bodied organisms may have been buried alive in such conditions, which inhibited anaerobic decay by decomposers .
The Mazon Creek fossils are preserved as moulds or composite impressions in the concretions . The plant fossils are generally preserved as moulds, their internal cavities infilled by calcite, kaolinite or sphalerite and often with a remaining element of coaly organic material [1, 7]. Shelly molluscan faunas, cephalopods and holothurian pharyngial rings are also preserved in the form of moulds . Jellyfish or medusae commonly form composite moulds, reflecting the compression and superposition of external details by loading of the animal on to the surrounding muds . Arthropods and some worms generally form impressions or thin surficial films, occasionally still lined with degraded organic cuticle or skeletous chitin, while gut contents may also be preserved [1, 7].
The concretions are composed of 80% carbonate, and their preservation indicates that they began to form during relatively early diagenesis [1, 3]. Nucleation of the concretions around organic matter may have commenced before the start of significant decay processes [1, 3]. The precipitation of siderite relates to three variables: the availability of iron; the rapid burial of organic material; and most importantly, the very low or nonexistent supply of seawater sulphate to the nuclei of microbial activity . Therefore, siderite precipitation was initiated following bacterial methanogenesis after the interstitial seawater sulphate had been exhausted by sulphur-reducing bacteria [1, 3].
In summary, the general abundance of sideritic concretions in the Francis Creek Shale may be elucidated by the supply of rapid sedimentation, an unstable supply of sulphate in the estuary, the entrapment of iron, and a large quantity of organic material .