Print this pageCyclostratigraphy of a Lower Triassic Outer Shelf Sequence: Nested Cycles with Frequencies ranging from Decadal to Millenial, and Possibly Milankovitch
WOODS, ADAM D., and DAVID J. BOTTJER
A portion of the Lower Triassic Union Wash Formation exhibits cyclicity which appears to range from the decadal-scale to possibly the Milankovitch-scale. The Union Wash Formation is a micritic limestone which was deposited along the shelf of western North America from the early Smithian to the late Spathian with the cyclic portion representing approximately 1 million years of Union Wash Formation deposition. Much of the cyclic sequence exhibits bedding-scale (i.e. cm-scale) bundling of laminations into couplets of alternating light-dark beds, and shifts in bed couplet thickness occur at what appear to be millenial-scale frequencies. The laminations themselves appear to represent decadal-scale depositional cycles based on average lamination thickness, and the length of time represented by the entire cyclic sequence. Centimeter-scale bundling is not apparent in the entire sequence, however, suggesting that longer-term variations were present in the system. The entire system appears to have been driven by climatic influences which were manifested as changes in upwelling intensity along this portion of the western coast of North America during this time. Shifts in upwelling intensity are suggested by changes in organic matter content and carbon isotopic values. Dark laminae on average have greater organic matter contents than the light laminae; in addition dark laminae in general have more positive delta{13}C carbonate carbon values than the light laminae. This data, coupled with a tendency for the light laminae to have a higher quartz contents suggests shifts in wind directions caused changes in upwelling intensity with lighter laminae representing times of offshore winds and sluggish coastal upwelling and dark laminae representing times of southerly winds and enhanced coastal upwelling. Shifts in wind direction may be related to the presence of the Pangean "megamonsoon" which was strongest during the Early Triassic, and which may have effected climates far from the locus of the monsoon over the Tethyan Sea.