Post-glacial evolution of a laterally confined lacustrine delta inferred from a multi-proxy study of a floodplain lake 

The pace and pattern of Holocene delta progradation is typically linked to sea-level and climate fluctuations, yet few studies have examined autogenic versus allogenic drivers of lacustrine deltas where lake level has remained largely stable. Coeur d’Alene Lake, impounded by outburst flood deposits from glacial Lake Missoula (ending ca. 15,000 yr BP), has a bedrock-controlled outlet. Since lake formation, two 45-km-long drowned tributary valleys have been infilled with sediment. Rapid progradation of the sediment feed point resulted in an elongated delta and a series of backswamp and lateral lakes. A 15.3-m sediment core was collected and radiocarbon dated from one such backswamp lake 22 km up-valley from the river mouth. Cluster analysis of grain size, elemental composition, and color identified four principal facies: prodelta, delta front, embayment, and backswamp lake. The prodelta phase (15,100–11,500 yr BP) with microlaminated sediments is consistent with isolation from main-lake seiche activity and prolonged seasonal ice cover during the Younger Dryas. Sedimentation rates increased through this period, culminating in delta-front facies (11,500–11,300 yr BP) with steeply inclined sand beds. The embayment phase (11,300–6800 yr BP) displays steady sedimentation and includes a couplet of airfall Mazama tephra, overlain by 133 cm of secondary tephra. This tephra input from the 3090 km² watershed likely contributed to delta advance, accelerated floodplain aggradation, and final isolation of the lake basin. The backswamp lake phase (6800 yr BP to present) is characterised by decreased fluvial silt input, increased organic content, and rare flood deposits. Elemental ratio trends are largely attributable to variations in grain size and organic matter, consistent with shifts in deltaic facies. This record is consistent with a narrow and elongated delta stranding multiple backswamp lakes through the Holocene, and supports the role of allogenic drivers, such as Early Holocene warming and the Mazama tephra, producing episodes of rapid progradation.