A HEINRICH 4-AGED UNCONFORMITY IN THE SEDIMENTS OF PLUVIAL LAKE CHEWAUCAN, OREGON
A prominent unconformity in the basin-bottom sediments of Pluvial Lake Chewaucan sugest that this lake was dry sometime between 42 and 37 cal kyr BP, at the end of the Bond Cycle repetition that terminated with Heinrich Event 4. The unconformity is marked by a basal, fining upward sand layer which separates sediments of distinctly different lithologies. The sediments below the unconformity are massive and lighter colored with a mean grain size of 25 microns while the sediments above are finely layered, darker-colored, and finer grained (15 microns). There are also distinctive discontinuities in magnetic measurements across the unconformity. All three magnetic mineral concentration parameters (k, ARM, and IRM) change abruptly from low to high concentration upsection. Declination and inclination jump 40-50˚ westerly and 25˚ to steeper values across the boundary, respectively.
The attributes of the sediments below the unconformity are consistent with those of shallower sediments whereas those of the sediments above are consistent with sediments deposited in deeper water. The paleomagnetic secular variation (PSV) below the unconformity is correlative to the PSV entering the Laschamp magnetic excursion as recorded in sediments from the western North Atlantic Ocean, but the signal is cut off at ~42 cal kyr BP. The PSV signal picks up again above the unconformity at <38 cal kyr BP.
The missing interval centered at 39-40 cal kyr BP suggests that the Summer Lake subbasin of Pluvial Lake Chewaucan dried up at that time. The exact timing of the lowstand is dependent on the thickness of the sediments deposited after 42 cal kyr BP that were eroded and/or deflated off the dry lake surface after the lake had dried up. The average of three bulk organic radiocarbon dates below the unconformity (~42 cal kyr BP) and three above (~33 cal kyr BP) support this hypothesis.