Gerald M. Richmond

Status of correlation of Quaternary stratigraphic units in the western conterminous United States

Abstract Deposits of Quaternary age from the Rocky Mountains to the Pacific Coast in the western conterminous United States represent a great variety of environments. The deposits include those of continental and alpine glaciers, glacial meltwater streams, nonglacial streams, pluvial lakes, marine environments, eolian environments, and masswasting environments. On two charts we have attempted to correlate representative sequences of deposits of many of these environments, based on published sources and recent unpublished investigations. Evidence for correlation is based mainly on stratigraphic sequence, soil characteristics, the amount of subsequent erosion and interlayered volcanic ash beds identifiable as to source. Chronologic control is based on numerous radiocarbon dates, U-series dates on marine fossils, and K-Ar dates on volcanic rocks. The Bishop volcanic ash bed and one of the Pearlette-like volcanic ash beds appear to represent significant regional key horizons, respectively about 700,000 and 600,000 years old. Rock magnetism is shown to suggest the paleomagnetic polarity at the time of rock deposition. Assigned land-mammal ages of included fossils help to put limits on the age of some units.

Comparison of the quaternary stratigraphy of the Alps and Rocky Mountains

Comparison of the glacial and periglacial deposits and soils of the Alps and Rocky Mountains suggests the following correlations. The Donau, Gu˝nz, and Mindel are correlated with the Washakie Point (Nebraskan), Cedar Ridge (Kansan), and Sacagawea Ridge (Illinoian). These glaciations are separated and followed by interglacials represented by thick deeply weathered soils, the last being the Mindel/Riss, or Sacagawea Ridge/Bull Lake (Sangamon). The Riss I (Paar) and Riss II (type Riss) Glaciations are correlated with the early and late advances of the Bull Lake Glaciation (early Altonian), and are believed to be different glaciations separated and followed by short interglacials represented by soils of intermediate development at these latitudes. The Alt-Wurm period of restricted ice in the Alps is represented only by local unnamed deposits in the Rocky Mountains. In the Alps, at least three interstadial soils occur between the Riss/Wurm soil and the oldest Main Wurm end moraine. These coincide in time respectively with the beginning and end of the Port Talbot II interstadial and with Plum Point interstadial. The three major end moraines of the Main Wurm are correlated with three or locally more end moraines of the early and middle stades of the Pinedale (Woodfordian). Moraines of the late stade of the Pinedale are equivalent to the late glacial and early postglacial moraines of post-Wurm but pre-Atlantic age in the Alps. K-Ar dating and tephrochronology in the Rockies suggest that the Washakie Point ended about 1.2 m.y. B.P., the Cedar Ridge about 700,000 B.P., the Sacagawea Ridge about 180,000 B.P., the last great interglacial about 130,000 B.P., and the interglacial separating early and late Bull Lake about 80,000 B.P. Radiocarbon dates from the Alps suggest that the Riss/Wurm was about 70,000–60,000 B.P., and that Alt-Wurm interstadials occurred at >50,000, 45,000–40,000, 34,000–32,000, and 28,000 B.P. Radiocarbon dates from rock shelter deposits in France indicate minor interstadials in the Main Wurm at 21,000–20,000 B.P. and about 17,000 B.P. Both the Main Wurm and Pinedale ended about 11,800 B.P.

Appraisal of the future climate of the Holocene in the Rocky Mountains

Abstract Consideration of the history of Holocene climate in the Rocky Mountains indicates that the over-all trend during the past 2500 yr has been toward increasing warmth, interrupted by cooler times of minor advances of cirque glaciers. Comparison of Holocene climatic history with the record of past interglacials in the region suggests that the present interglacial is not complete and that the climate may become first warmer and subsequently wetter before it is completed. Correlation of the timing of the regional glacial-interglacial record for the past 140,000 yr with the record of major sea level changes and with the calculated changes in the earths insolation suggest that the present interglacial may be completed within a few millenia and that it may be followed by a significant cooling of the climate.

Geology, palynology, and climatic significance of two pre-Pinedale Lake sediment sequences in and near Yellowstone National Park

Pollen analysis of a section of lake sediments at Grassy Lake Reservoir indicates a vegetational sequence changing from tundra, to spruce-fir-pine forest, to pine forest, to tundra at the top. Pollen analysis of a section of lake sediments on Beaverdam Creek indicates a tundra vegetation at the base, followed by a brief episode of spruce-fir forest and a return to a tundra vegetation at the top. The analyses of both sections suggest a cold to cool to cold climatic sequence, interpreted as interstadial in character. However, differences suggest that they represent separate interstadials. Pinedale Till disconformably overlies the lake deposits at Grassy Lake Reservoir. The upper sediments contain wood 14C dated at >42,000 yr; the lowermost interfinger with till shown to be more than about 70,000 yr old. The deposits at Beaverdam Creek grade upward into proglacial Pinedale deposits, contain an ash that is probably about 70,000 yr old near their base, and rest comformably on gravel that grades down into lake sediments containing wood debris suggestive of an older climatic amelioration. We conclude that the warmest part of the interstadial at Grassy Lake Reservoir is probably more than 70,000 yr old, and that the warmest part of the interstadial analyzed at Beaverdam Creek is slightly younger than 70,000 yr old.