QUATERNARY GEOLOGY OF THE SACRAMENTO AREA, CALIFORNIA: SIXTY YEARS OF INVESTIGATIONS AND STILL UNANSWERED QUESTIONS

The 2025 Sacramento Cordilleran Section meeting is taking place on late Quaternary sediments. The age and origin of these sediments were generally established about 60-yrs ago. The apparent near-featureless Sacramento-area plain is underlain by multiple, high-relief, gravel-filled channels of the lower American River, by buried paleosols, and by fluvial and distal-fan sediments all reflecting regional climatic change and local subsidence. The major sediments packets are formally named: the Arroyo Seco gravels (oldest), the Laguna-Mehrten transition zone, the Fair Oaks, the Riverbank and the Modesto formations, respectively. The origin and age of these sediments are deduced from limited natural outcrops, from a few quarry exposures, from an occasional roadcut, and from a myriad of water-well logs. The sediments are numerically dated by radiocarbon and uranium-series assay, and relatively by fossil assemblages, soil-stratigraphy and association with the marine oxygen-isotope chronology. Despite home to hundreds of government-employed geologists and consulting groundwater and engineering geologists, the Sacramento Quaternary still presents a plethora of unanswered questions: How do the Quaternary sediments and their associated buried and relict soils correlate with Sierra Nevada glaciations; where is all the expected loess; what has been the erosional impact of glacio-eustacy in the Sacramento-San Joaquin Delta; what is the influence of regional and possible local tectonism; and why have the American River channels "jumped" northward throughout the Quaternary. These are but a few practical questions that have waited at least 60 years for answers by new generations of Quaternary geologists.