Cyclic Eolian Stratification on the Jurassic Navajo Sandstone, Zion National Park: Periodicities and Implications for Paleoclimate

Zion National Park and related areas of southern Utah contain exposures of the largest ancient wind-blown (eolian) dune system known in North America, preserved in the Jurassic Navajo Sandstone. Despite the common lack of absolute age constraints, ancient desert sand seas (ergs) and their erg margins can record valuable paleoclimatic proxies in the sedimentary record. The central erg portion of the Jurassic Navajo Sandstone of southwestern Utah contains nested cycles of eolian grainfall and wind-ripple laminae. These rhythmic alternations preserve random “snapshots” in time ranging from annual cycles to decadal climatic variations, influenced by periodic and quasi-periodic oscillators. Based on the established interpretation of annual cycles, nearly 300 of these high-frequency seasonal/annual cycles in the Navajo Sandstone of Zion National Park 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 flow related to decadal periodicities that may be driven by solar variability and/or seasonal precipitation (moisture changes). This study demonstrates the utility of spectral analysis as a quantitative tool for interpreting periodic paleoclimatic oscillators in eolian environments. Geology of Utah’s Parks and Monuments 2000 Utah Geological Association Publication 28 D.A. Sprinkel, T.C. Chidsey, Jr., and P.B. Anderson, editors Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112-1183 Department of Geology, Kansas State University, Manhattan, KS 66506 are well understood, it is generally either too difficult or too expensive to create the types of trenches that would be necessary to study long records of modern deposition. Thus, exposures of ancient eolian dunes can provide data that has not been studied in the modern record. Although decadal cycles may be present in modern dunes, eolian workers indicate that they haven’t really looked for such cycles (N. Lancaster, verbal communication, 1997; G. Kocurek, verbal communication, 1998). “Reverse uniformitarianism, ” the concept of the ancient as the key to the present or future, may prove fruitful for understanding the modern record. Recognition of these cycles may be useful in paleoclimate modeling and examining parameters (such as paleogeography) that could have accentuated climate shifts. Cycles encoded in the Jurassic eolian rocks may show prominent periodicities of Mesozoic paleoclimates that can later be used to identify more subtle climatic signals in modern or Holocene analogs. Furthermore, identification of the magnitude and frequency of these well-preserved ancient cycles also help refine tools (imageand harmonicanalysis methodologies) to use in interpreting proxy paleoclimate signals. GEOLOGIC SETTING AND STRATIGRAPHY The Jurassic Navajo Sandstone and its related equivalents form the largest erg deposit in North America (Peterson and Turner-Peterson, 1989). This spectacular erg deposit is generally flat lying and well exposed in the Colorado Plateau. The Navajo Sandstone is the uppermost formation of the Glen Canyon Group (figure 1) that has been regionally traced over the Colorado Plateau (Pipiringos and O’Sullivan, 1978; Peterson and Pipringos, 1979). The Navajo Sandstone is underlain by the Kayenta Formation of the Glen Canyon Group, and overlain unconformably (J-1 surface) by the San Rafael Group. Previous work on this erg (for example, Blakey and others 1988; Peterson 1988; Verlander 1995; Blakey and others 1996; Kocurek 1999) provides the framework for the detailed examination of cyclic cross-beds presented here. Jurassic paleogeography and paleoclimatology is summarized in Kocurek and Dott (1983), Blakey and others (1988), Chandler and others (1992), Blakey (1994), Parrish and Peterson (1988), and Peterson (1994). In this study, a locality was chosen along State Highway 9, 2.4 kilometers west of the east gate in Zion National Park (figure 1) which provides a thick series of cyclic cross-beds for evaluation. CYCLIC CROSS-BEDDING Expression of Cyclic Eolian Stratification Cyclic cross-beds are distinguished by alternations of eolian stratification types and fine internal structures originally delineated and defined by Hunter (1977, 1985). These lee-face stratification types and structures include: climbing-wind-ripple laminae, grainfall lamination, and grainflow/sandflow or avalanche tongues. In the lee face of dunes, wind ripples occur where tractional processes dominate. Both grainfall (from salting grains passively falling out of suspension) and grainflow (from grains that exceed the angle of repose and avalanche down the slipface) occur where gravitational processes dominate. M.A. Chan and A.W. Archer Cyclic Eolian Stratification on the Jurassic Navajo Sandstone, Zion National Park