ROLE OF MUCUS SECRETION AND PYRITIZATION IN PRESERVING A RECORD OF BIOGENIC ACTIVITY IN A RESTRICTED ESTUARINE CHANNEL DEPOSIT

The Ingersoll shale, a carbonaceous estuarine clay lens within the Upper Cretaceous Eutaw Formation of eastern Alabama, contains abundant narrow, discontinuous, straight to meandering, rarely branched, virtually flat and mostly horizontal ribbons of essentially pure, mainly framboidal pyrite. These traces differ from previously described pyritized burrows in one or more respects (size, orientation and composition of burrow fills). Most important, Ingersoll structures differ from other horizontal to subhorizontal pyritized burrows with regard to their vertical dimensions and composition. The two-dimensional, flat aspect of Ingersoll structures and lack of admixed clastic detritus therein suggest that substrates for pyritization originated as thin organic sheaths, rather than compacted three-dimensional features. Hence, host substrates for mineralization are interpreted to have been mucus strands secreted by epibenthic or shallow endobenthic organisms, perhaps grazing gastropods and errant worms. Although the affinity of their progenitors cannot be inferred with high confidence, these structures indicate that: (1) bottom waters were sufficiently oxygenated to support at least small benthic organisms; (2) sulfidic conditions prevailed at very shallow substrate depths, at least in the microenvironments provided by mucus films; and (3) even sparse biogenic activity can have a significant impact on substrate biochemistry. Notably, without secretion of mucus and subsequent selective sulfide mineralization of parts thereof, these structures likely would not have passed through normal taphonomic filters; i.e., evidence for biogenic activity and environmental conditions would not have been preserved. The Ingersoll structures demonstrate that early diagenetic mineralization of biogenic structures, fueled by mucus or other labile organic substances, may be just as important as it is in preserving soft body tissues that characterize many lagerstätten in the body fossil realm.