THE PROSPECT OF COMPACT ESTUARINE LAGERSTATTEN (CEL)

Fossil conservation lagerstätten are sedimentary deposits that contain unusually well-preserved fossil biotas, commonly including soft parts. Given their importance to paleontology and sedimentary geology, it is critical to understand the processes by which they form and to develop criteria that can be used to prospect for additional exceptionally preserved fossil biotas. Many conservation lagerstätten accumulated over extended time periods in sizeable depocenters—meromictic lakes, stratified marine basins, anoxic or hypersaline lagoons, or large estuaries—and, hence, are relatively thick and/or laterally extensive. However, conditions conducive to exceptional fossil preservation may develop in small and transient depocenters as well, forming relatively compact lagerstätten that, due to limited breadth and thickness, may go unnoticed. Transgressive estuarine sequences hold particularly high potential for discovery of new compact lagerstätten, here referred to as compact estuarine lagerstätten (CELs). As demonstrated by the Ingersoll shale, a small yet extraordinarily fossiliferous deposit in the Cretaceous (Santonian) Eutaw Formation of eastern Alabama, the various processes that lead to exceptional preservation may act synergistically in certain restricted estuarine environments. The Ingersoll shale accumulated in a small bayhead-delta channel wherein preservation of a diverse paleoflora, amber with fossil inclusions, and feathers was favored by rapid tidal deposition of clay-rich carbonaceous sediments, anoxic pore waters, early diagenetic pyrite mineralization, and perhaps microbial sealing and/or replacement. We predict that preservation processes have interacted similarly to form as-yet unrecognized compact lagerstätten in estuarine settings comparable to that of the Ingersoll shale. Exploration for additional CELs, guided by sequence stratigraphic principles, may yield other small but spectacular glimpses of life in the past.