Milankovitch cycles in the Juniata Formation, Late Ordovician, Central Appalachian Basin, USA

摘要

The Juniata Formation is a thick succession of prevalently red, cyclically bedded arenites, wackes, and mudrocks found in the Upper Ordovician of the Central Appalachian Basin, USA. In outcrops close to the study area, the Juniata cycles predominantly have the characteristics of regressive tidal flat deposits. Long and continuous well logs of the subsurface Juniata provide an unparalleled opportunity to investigate Milankovitch controls on the cyclic deposition. In the Preston 119 well, northern West Virginia, a 2700-ft long gamma-ray well log provides a high-resolution proxy of terrigenous siliciclastic flux to the northern Central Appalachian Basin shoreline, from the early Maysvillian (Reedsville Shale) to the Ordovician/Silurian transition (Tuscarora Sandstone). The gamma-ray cycles provide strong evidence for sea level oscillations forced by Milankovitch cycles with a dominant obliquity component. The strong obliquity signal is reminiscent of the obliquity forcing of Oligoccne climate and sea level following the glaciation of Antarctica. The Late Ordovician world analogously experienced glaciation of Gondwana, which straddled the South Pole; this may have involved ice sheet dynamics that generated obliquity-paced sea level oscillations that affected Late Ordovician shorelines worldwide. This Milankovitch-forced glacio-eustatic record from eastern North America joins other suspected Milankovitch-forced successions reported from the Late Ordovician of northern Africa, northwestern Australia, Scandinavia, and northeastern and eastern-central North America.