COMPARATIVE SEDIMENTOLOGY AS A TEMPORAL MEASURING STICK: THE MIDDLE TRIASSIC LATEMAR CYCLES, THE DOLOMITES, N. ITALY

Recent work attempting to measure the duration of the well known Latemar cycles has relied on a composite dataset of ash-fall zircon dates and magnetostratigraphy to conclude that Latemar cycles and bundled megacycles formed rapidly enough as to exceed even a precessional astronomical forcing mechanism (1-2 kyr per cycle, 5-10 kyr per megacycle). However, use of comparative sedimentology forces a reconsideration of these conclusions, particularly when rates of formation of like facies from the modern are cross-applied to the geologic past. Facies-to-facies comparative sedimentology suggests that the average Latemar depositional cycle required at least 5-6 kyr for sediment to be deposited. This estimate does not count additional time for attainment of lag depth or prolonged subareal exposure and tepee formation, which may add tens of kyr of estimated time in some examples. In addition, a review of geological literature focused on the dating of modern carbonate depositional cycles concludes that modern carbonate allocycles from around the globe have an age of 5-7 kyr, and that many of these have not yet aggraded to sea level. The use of comparative sedimentology as a temporal measuring stick provides a contrast to the radiometric dates used to construct a purely sub-Milankovitch cyclostratigraphic framework for the Latemar cycles and suggests that carbonate allocycles form at multi-millennial rates.