Summary

Despite the common lack of absolute age constraints, ancient eolian dune seas and erg margins can record paleoclimatic proxies in the sedimentary record. Nested cyclities of eolian grainflow and wind-ripple laminae preserve random “snapshots” in time hinging from annual cycles to decadal climatic variations, influenced by periodic and quasi-periodic oscillators. Based on the established interpretation of annual cycles, nearly three hundred of these high-frequency seasonal/annual cycles in the Jurassic Navajo Sandstone of Zion National Park of southwestern Utah were measured in a continuous thickness series of transverse dune foresets. Harmonic analysis reveals prominent periodicities of approximately 30 and 60 years and several other decadal periodicities. These long bedform cycles are interpreted as climatic oscillations/fluctuations of flew related to decadal periodicities that may be driven by solar variability and/or monsoons.

Recognition of the levels and frequency of these well-preserved ancient eolian cycles can help refine tools to use in interpreting proxy paleoclimate signals. Although the distinction of autogenic vs. allogenic control on cyclic eolian stratification is controversial, there is currently a lack of other modern and ancient studies that can fully document similar rhythmic alternations from autogenic dune mechanics with the exception of superimposed dunes. Furthermore, bedform fluctuations produced by computer simulations suggest some external allogenic control such as climate that would in turn affect migration speed and asymmetry, dune position, and/or shifting wind patterns.

The understanding of past climatic regimes is contingent upon understanding the boundary conditions, such as the paleoclimatic extremes that have existed during earth history. Although additional data is required in order to test the significance of these preliminary results, the types of cycles encoded in the Jurassic eolian deposits may show stronger signals during the megamonsoonal Pangean paleoclimates and may later help identify more subtle climatic signals in modern or Holocene analogs (lacking the Pangean configuration and the megamonsoons).

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas