Chalk and Related Deep-Marine Carbonates

Since the Jurassic, chalks and related carbonates have accumulated via multiple processes, under a broad range of water depths and in a wide array of marine environments. Trace-fossil suites in these deposits are highly variable, representing as many as six ichnofacies. Variability among Zoophycos Ichnofacies suites reflect variations in water depth, substrate consistency, and redox conditions in relatively quiet deep-sea, outer-shelf, and epeiric basins. Trace-fossil suites allied with the Cruziana, Glossifungites, and Trypanites ichnofacies, with notable exceptions, indicate deposition in shallower, periodically more energetic shelf-sea settings; suites of the latter two ichnofacies reflect depositional hiatuses and development of firmgrounds and hardgrounds formed by erosional exhumation or syndepositional cementation. Trace-fossil suites in redeposited carbonates are more enigmatic. While some calciturbidite suites are characteristic of the Nereites Ichnofacies, others may reflect the work of allochthonous tracemakers and may be more appropriately assigned to the Arenicolites Ichnofacies. As they reflect environmental and early diagenetic conditions, trace-fossil suites in chalks and related carbonates can help assess depositional facies, sea-level histories, paleoceanographic and paleoclimatic regimes, and petroleum potential.