Taconic Foreland Basin graptolites: age zonation, depth zonation, and use in ecostratigraphic correlation

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Abstract

Late Middle Ordovician graptoloids are stratigraphically zoned by depth of deposition as well as by age along an 83 km long downslope transect through a roughly four million year long continental-shelf and outer-trench slope sequence in the Mohawk Valley, New York. The distribution of graptoloid assemblages parallels the distribution of sediment types and benthic macroinvertebratc assemblages in showing the general, convergence-related marine transgression, secondary transgressive and regressive pulses, and topographic irregularities related to syndepositional block faulting. The relationship of ordination scores for graptoloid assemblages (which arc based on relative abundances of genera) to the samples' positions on downslope transects along bentonite beds, and to depth-related ordination scores for benthic macroin-vertebrate assemblages in the same samples throughout the sequence, quantitatively demonstrates zonation by depth of deposition. Rather like modern pelagic colonial tunicates, graptoloids were evidently zoned by depth in the water column, with Orthograptus spp. predominating in offshore waters above the oxygen minimum zone, Climacograptus spp. near the top of the minimum zone at 300 to 500 m depths, and Corynoides spp. well within the minimum zone. Destruction of sinking rhabdosomes by decomposers evidently accentuated zonation by depth of deposition, and may have contributed to the strong, broad oxygen minimum zone thought to have been characteristic of graptolitic shale basins. Stratigraphic sections' approximate relative sea level curves, calibrated in terms of graptoloid assemblage ordination score, give ecostratigraphic correlations of 2–3 × 105 year average accuracy when statistically cross-correlated between sections, and when cross-correlated with similar curves based on benthic macroinvertebratc assemblage ordination score. □Graptolithina, age zonation, depth zonation, paleoeco-logy, Middle Ordovician, ecostratigraphy, coenocorrelation.

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