COMPACT LAGERSTATTEN: EXCEPTIONALLY FOSSILIFEROUS DEPOSITS FORMED BY MULTIPLE TAPHONOMIC FACTORS IN ISOLATED, TRANSIENT DEPOCENTERS

Most well-known conservation lagerstätten accumulated over extended time periods in sizeable depocenters—e.g., meromictic lakes, stratified marine basins, anoxic or hypersaline lagoons, or large estuaries—and, hence, are relatively thick and/or laterally extensive. However, as demonstrated by recent studies of the Upper Cretaceous Ingersoll shale (Eutaw Formation, eastern Alabama), processes that lead to exceptional preservation may act synergistically in small and transient depocenters as well, forming lagerstätten of limited breadth and thickness; i.e., compact lagerstätten. The Ingersoll shale, a small (<90 cm thick, <30 m wide) clay lens that accumulated in a shallow, narrow estuarine tidal channel, has yielded a diverse paleoflora, amber with fossil inclusions, and the largest collection of feathers thus far recovered from Mesozoic strata of North America. High sedimentation rates (~1 m/yr) associated with tidal rhythmite deposition was the most important factor contributing to fossilization; rapid burial insulated organic remains from the destructive effects of bottom currents, scavenging, and bioturbation. Clay, the dominant burial medium, cast structural details of buried remains and limited the extent of leaching and fossil degradation in burial diagenetic and weathering regimes. Pore-water oxygen deficiency, which developed in response to high organic influx, favored preservation by limiting bioturbation and by mediating early diagenetic mineralization. Early pyritization was important locally in conserving three-dimensional aspects of plant reproductive bodies and feather parts. Microbes governed redox conditions and consequent pyrite mineralization but also may have played a more direct role in fossilization by forming protective barriers or seals against physical and chemical degradation. Amber accumulation and preservation resulted from proximity to a plant community that included common resin-producers, concentration after limited transport of resin clasts by tidal currents, and rapid burial beneath reducing sediments. Similar compact lagerstätten formed by comparable interplay of taphonomic factors may be common in other marginal marine and continental successions but, due to their small volume and unremarkable field expression, may be easily overlooked.