Control on frequency hierarchy in sequences of meter-scale carbonate cycles

Proponents of allocyclic (eustatic) control of meter-scale carbonate cycle formation commonly point to the presence of an hierarchy of cycle thickness as compelling evidence for a Milankovitch origin of high-frequency sea-level change during cycle deposition. When assuming constant subsidence and sedimentation rate, thickness is directly related to duration of eustatic sea-level change (the Fischer plot assumption), and ratios of individual cycle period to megacycle period are then found to be similar to ratios of Milankovitch orbital variations, typically four or five to one. However, a variety of computational models of carbonate cycle formation indicate that a single period of eustatic sea-level change will give rise to multiple shallowing upward sequences when depth-dependent parameters of sediment generation are also included. When employing constant sedimentation and subsidence rates, as well as a depth-controlled flooding lag time for the initiation of sediment accumulation, patterns of cycle hierarchy are seen to vary both with amplitude of sea-level change and with subsidence rate. Low sea-level amplitudes and intermediate subsidence rates give rise to the most complex stacking patterns. Because patterns of cycle hierarchy are very sensitive to inferred rates of subsidence, the number of cycles produced during any single sea-level rise will depend upon lateralmore » position of a sequence along any shelf undergoing differential subsidence. Hence, the presence of cycle-megacycle thickness/period ratios either within or outside the Milankovitch range makes no comment on the role of multiple forcing functions. As such, the presence of a frequency hierarchy in cyclic carbonate sequences can not be used to argue for an orbitally forced origin of upward-shallowing carbonate cycles.« less