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Abstract:

Carbonate sediments of tectonically quiescent continental interiors are nearly ideal for precisely tracing eustatic sea-level change during major transgressions. Over roughly 10 million years during the later Middle Ordovician (Rocklandian through the middle Denmarkian stages), sea levels measured in the American Midwest rose about 10 m relative to the continent. Because the sediment accumulation rate in the epeiric sea was proportional to water depth, the time trend of sea level can be reconstructed from cumulative sediment thickness and from measurements on water depth throughout a stratigraphic section. Sea level is reconstructed as a function of time for six sections in the midwestern United States, and the reconstructed time trends are compared for common eustatic components based on the section time correlations by geochemically fingerprinted volcanic-ash layers. Relative water depth is measured through gradient analysis of fossil assemblages by reciprocal averaging ordination. Sample ordination scores are calibrated as a measure of absolute depth by use of the offshore depth estimated from stratigraphic expressions of the shoreline edge effect in lithospheric flexure. Sea-level change during the Middle Ordovician transgression had at least two components: (1) a steady rise at a slowly varying rate around 1 m per million years (2) pulses no more than 0.1 to 1 million years long during which sea level fell roughly 1 m and then rose about the same amount. The long-term trend is attributable to steady decrease in the mean age of oceanic lithosphere. The pulse correlations from section to section and the remarkably small sea-level changes involved testify to the tectonic quiescence, spatial homogeneity, and essential tidelessness of the epeiric sea, and to the precision of its stratigraphic record. Pulses were probably related to climatic fluctuations, and their association with particularly frequent volcanic-ash deposition is suggestive that climatic effects of increased explosive volcanism may have been controlling factors.

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