EVIDENCE FOR LAKE IROQUOIS BREAKOUT FLOODS IN THE HUDSON VALLEY AT 13.2 CAL KA FROM COREHOLES AT SANDY HOOK, NJ

When ice retreated from Covey Hill on the north side of the Adirondack Mountains, Lake Iroquois in the Ontario basin lowered in two steps to the level of Lake Vermont in the Champlain Valley. These two drops released 700 km³ (Iroquois breakout) and 2500 km³ (Frontenac breakout), respectively, down the Hudson Valley (Rayburn and others, 2005, 2007). They are dated to between 13.4 and 13.0 ka by radiocarbon dates on wood and bone from pre- and post-flood lake sediments in the Champlain Valley (Rayburn and others, 2005, 2007). This timing is supported by a rapidly deposited 30-m thick section of estuarine silt at Sandy Hook, NJ, sampled in two coreholes and dated to 13.2 ka. Sandy Hook is a spit built northward within the past 5 ka over the Raritan paleovalley, a tributary to the Hudson shelf valley, which is 13 km east of the Hook. The Raritan, which did not itself conduct Hudson meltwater, acted as a backwater for the breakout floods. The 30-m section of estuarine silt in the deepest part of the paleovalley at the north end of the Hook overlies fluvial gravel at a depth of 84 m with Raritan basin provenance. Five dates on plant material and wood at depths between 81 and 55 m in this core had median ages between 13.15 and 13.35 ka. Three dates on plant material and wood from the same unit sampled in a second corehole 2 km to the south at depths between 59 and 50 m had median ages between 12.99 and 13.30 ka. All the dates overlap at 2 sigma calibration uncertainty and the medians average to 13.23 ka (2 sigma range of 12.88-13.48 ka). Above the flood silt there is a depositional hiatus, and overlying estuarine sediments date to 12.1 ka and younger above 50-55 m in depth. This hiatus corresponds to a local plateau in relative sea level at -50 m between 13.5-11.5 ka, when rebound in the area kept pace with eustatic rise. The flood deposit consists of gray micaceous silt and clayey silt with fine organic matter and wood and thin lenticular beds of cross-laminated fine sand. There is no medium or coarse sand or gravel. These properties are consistent with a backwater setting infilled with silt and clay washed from lake and estuarine deposits in the Hudson Valley. The floods predate the Younger Dryas but correspond to a lesser cooling at 13.3-13.2 ka recorded by δ¹⁸O depletions in Greenland ice cores (Greenland interstadial 1b of Rasmussen and others, 2014).