A 740,000-yr-long Mohawk Lake record, Mohawk Valley, northeastern California, USA

Abstract

ABSTRACT Mohawk Valley lies in northeastern California on the margin of the northernmost Sierra Nevada and was occupied by Mohawk Lake during much of the middle and late Pleistocene. Throughout that time, the Sierra Nevada ice cap repeatedly extended northward into Mohawk Lake, and ice-contact deltaic sediments were deposited along the valley margins and in the valley bottom. Nearly 200 m of lacustrine and deltaic sediments are now well exposed along streams draining the Sierra Nevada. Tephra beds deposited within the deltaic sediments allow correlation of stratigraphic sections around the valley margin and, together with geomorphic evidence of former lake levels, permit interpretation of a Mohawk Lake history as far back as 740 ka. Mohawk Valley changed from a through-flowing fluvial setting to an intermittent closed basin sometime before 740 ka. After this change occurred, relatively small lakes intermittently formed in Mohawk Valley until ca. 600 ka, when the lake dramatically deepened. Mohawk Lake fluctuated in size over the next ~400,000 yr and increased in size to its highest levels after ca. 200 ka, possibly due to drainage integration with the upstream Lake Beckwourth. After this time, Mohawk Lake spilled over its westward sill, incrementally eroding and lowering lake levels until Mohawk Lake was emptied by ca. 7 ka.