LINDGREN’S TERTIARY RIVERS, THE “AURIFEROUS GRAVEL” PALEOVALLEYS OF THE NORTHERN SIERRA NEVADA: WHAT IS AND IS NOT KNOWN ABOUT STRATIGRAPHY, IGNIMBRITE DEPOSITS, MORPHOLOGY, CHANNEL AND DRAINAGE AREA EVOLUTION (Invited Presentation)

Lindgren’s monumental 1911 The Tertiary Gravels of the Sierra Nevada of California remains highly informative and relevant. Recent work has added to the stratigraphy, channel morphology, and evolution of the paleovalleys, but major questions remain, especially about time of origin, paleoelevation, and location of headwaters through time. At least 15 ignimbrites that erupted from calderas in central Nevada were channelized in the paleovalleys, commonly reaching the Great Valley, and precisely date upper strata. Twelve 31.7-24.9 Ma ignimbrites crop out in the I80 corridor, and 10 (partly overlapping) 28.9-22.95 Ma ignimbrites are at ~38°N, near their southernmost extent. All correlate with widespread Nevada ignimbrites; all except one are tied to Nevada source calderas. Southward younging reflects southward younging of Nevada sources.

The timing of pre-volcanic strata, Lindgren’s auriferous gravels – variably deep/lower and bench/upper – is uncertain. Detrital dating shows that some upper parts are ≤42 Ma, including the famed Chalk Bluff paleoflora. Oldest parts are ≤85 Ma, probably mostly much younger.

Although Lindgren used the names “Tertiary Yuba, American, etc rivers”, the modern canyons are unrelated, morphologically different, and follow different courses than the paleovalleys. The paleovalleys are much wider than deep, typically 6-8 x ≤1 km, probably reflecting warm, wet early Cenozoic weathering, vs the narrow, v-shaped modern canyons. The Tertiary Yuba River cuts across the modern North and Middle Forks American River, then turns abruptly north near Foresthill for more than 40 km through Iowa Hill and Chalk Bluff to North Columbia where it turns southwest north of the modern South Yuba River. The Tertiary American River partly parallels but is south of the modern South Fork American River.

Published isotopic data indicate paleoelevations similar to modern day, implying no late Cenozoic uplift. Apparent tilting of ~10 Ma lavas requires significant uplift of the high Sierra Nevada. Resolving these disparate alternatives is difficult. Ignimbrite flow demonstrates that paleovalley headwaters were in central Nevada at least by 31.5 Ma. Similar paleovalley morphology in Nevada suggests they also developed during the warm, wet early Cenozoic. Detrital zircons demonstrate that the divide was at least as far east as western Nevada throughout the Cenozoic.