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Orbital forcing and Mississippian sea level change: time series analysis of marine flooding events in the Visean Windsor Group of eastern Canada and implications for Gondwana glaciation
Orbital forcing is strongly expressed in spectra of time series delineating transgressive-regressive (TR) cycles in the late Visean Windsor Group in the Shubenacadie Basin of southern Nova Scotia. These spectra compare closely with those of synthetic, variously degraded time series constructed from summed sine waves representing orbital periods for the eccentricity, obliquity and precession-index as predicted for late Visean time. The introduction of significant new macrofaunal elements in the Windsor Group accompanied transgressions separated in time by 2.2 to 2.4 m.y., perhaps reflecting a longer-period orbital influence by the eccentricity or by modulation of the obliquity. Orbital forcing in the late Asbian substage was apparently dominated by the 100 k.y. eccentricity, whereas in the Brigantian, TR cycles responded mainly to forcing by the 400 k.y. eccentricity. High-magnitude sea-level change in late Visean successions elsewhere, and an orbital signature for Windsor Group TR cycles, suggest that orbitally forced waxing and waning of significant continental ice sheets is the most appropriate explanation for late Visean sea-level change. Ice sheets were fully developed by the end of the early Asbian substage. Sea-level high-stands, interpreted as the result of melting of continental ice sheets, represent a very small temporal percentage of the Windsor Group stratigraphic succession, and mark the late Visean as a cold period in Earth’s history. In this part of Gondwana, a late Asbian or even older start to Gondwana glaciation is indicated, contrasting with the Australian record which places the first significant glaciations of Gondwana in the early Namurian.
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