Summary

Rhythmically-bedded sequences offer a means of examining high-frequency climatic oscillations which otherwise would be impossible to study. One such sequence is a portion of the Union Wash Formation of east-central California, which contains approximately 500 m of mixed siliciclastics and carbonates as rhythmically-bedded siliceous and calcareous mudstones and micritic limestones. The sedimentary rocks of the Union Wash Formation were deposited on the shelf edge to slope of western North America during the latter half of the Early Triassic (Smithian-Spathian), with the cyclic portion representing approximately 2 million years of deposition (from ca. 245 - 243 Ma; Gradstein, et al. 1995). Several orders of cyclicity appear to be present in the Union Wash Formation. The dominant cyclicity is an outcrop-scale variation between siliciclastics (laminated mudstones) and carbonates (bedded micritic limestones), with the cyclicity apparently the result of rhythmic changes in detrital input. High detrital input led to deposition of the laminated mudstones while low detrital input led to deposition of the laminated and bedded micrite. In addition, higher-frequency shifts in detrital input appear to cause cyclicity in the laminated mudstones while shifts in eolian input (i.e., silt) during times of low detrital input appear to have caused the pattern of alternating beds of dark, nearly pure micrite beds and light, silty micrite beds. Outcrop-scale cyclicity is not present through the entire sequence, however; the lowermost ca. 150 m of the sequence only exhibits rhythmic laminations. Spectral analysis of time series at all three scales reveals a wide variety of periodicities present in the sequence ranging from Milankovitch-scale cycles to decadal-scale cycles. Analysis of the amount of detrital input via percent potassium data demonstrates that at the outcrop scale, the lower half of the rhythmically-bedded sequence is dominated by Milankovitch-band precessional cyclicity, while the upper half of the rhymically-bedded sequence is dominated by Milankovitch-band obliquity cyclicity. The bedding-scale rhythms, meanwhile, appear to contain evidence for depositional control by several centennial and millennial frequencies, as well as the 78-year Gleissberg solar cycle, which is caused by expansion and contraction of the solar radius. The mm-scale alternations contain evidence which suggests the presence of the sunspot and Hale solar cycles (at 11 and 22 years respectively), as well as by what might be an ENSO-type oscillation. It appears as though the rhythmic beds of the Union Wash Formation were driven by changes in the strength of the Pangean megamonsoon. During times of strong seasonality in the northern hemisphere (precessional cycle dominant) or less axial tilt (obliquity cycle dominant), the megamonsoon would have strengthened due to greater heating of the tropical regions. Climatic models (Kutzbach, 1989) and lithologic data (Parrish, 1993) indicate that during the Triassic the megamonsoon may have drawn water from the Panthalassic Ocean when the monsoon was at its greatest strength. When the monsoon was strongest, winds in the region of Union Wash Formation deposition would have been oriented parallel to shoreline, leading to enhanced upwelling and deposition of cm-scale alternations between the dark, nearly pure micrite beds (strong upwelling) and the light, silty micrite beds (slightly weaker upwelling with variable winds including an offshore component that led to greater eolian input). Less seasonality or less axial tilt would have led to less heating in the tropics, a weaker monsoon, and apparently wetter conditions which led to greater detrital input and deposition of the laminated mudstones. It also appears that solar variability led to the deposition of high-frequency cycles, perhaps as the result of subtle changes in rainfall amounts.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah