REVISITING MILANKOVITCH SIGNALS IN PLATFORM CARBONATES

Platform carbonate formations are often organized into extensive vertical stacks of meter-scale shallowing-upward cycles. This cyclicity has been attributed to autocyclic and/or allocyclic mechanisms that controlled deposition. Milankovitch-forced sea level oscillations are thought to be an important allocyclic mechanism, with periodicities finely tuned to carbonate productivity and subsidence rates that characterize platform environments. Tidal flat progradation is considered to be an important autocyclic mechanism, requiring external forcing from subsidence only. The relative importance of autocyclicity versus allocyclicity is not understood. Other climatic signals (not related to sea level change) recorded in carbonate platform sequences have yet to be discovered. Compounding these unknowns is the belief that non-deposition is the prevailing condition in platform environments, and therefore, sea level or other climatic signals are too disrupted to reconstruct adequately. Fortunately, all of these problems can be revisited using new modeling and analysis tools that have been developed over the past decade; some of these tools will be highlighted in this talk. Assessing Milankovitch signals in platform carbonates is important for understanding the forcing of the marine sediment record prior to the evolution of pelagic carbonate producers. Moreover, the astronomical parameters that drive Milankovitch cycles are expected to have been different in the deep past, when platform carbonates were the primary recorders of marine environments. The successful retrieval of Milankovitch signals from the pre-Jurassic platform carbonate record can provide key empirical data for validating astronomical models of the ancient Solar System.