SEDIMENTOLOGIC, GEOMORPHOLOGIC, AND GEOCHRONOLOGIC RECORD OF LATE PLEISTOCENE AND HOLOCENE LAKE LEVEL CHANGES ALONG THE EASTERN SHORE OF YELLOWSTONE LAKE

This study presents new preliminary sedimentologic, geomorphologic and geochronologic results from multiple Quaternary study sites located along the eastern margin of Yellowstone Lake. Results suggest that, relative to the modern ground surface, the surface elevation of Yellowstone Lake was significantly higher than previously suggested during the past ~11ka.

A broad morphologic bench south of Columbine Creek near the southeastern end of the lake is interpreted to be the remnant delta plain/flood plain of a Gilbert-style fan-delta that formed along the margin of ancestral Yellowstone Lake. Exposed upper termination points of delta foresets documented in a measured stratigraphic section within the incised canyon cut by Alluvium Creek suggest that the lake surface elevation was ~22 m above the present day level during the time of latest delta formation. A radiocarbon date of 10,390 +/-150 cal yr BP associated with proximal fan-delta deposits indicate that the delta underwent construction during this time, around the Pleistocene-Holocene transition.

To the north of the Columbine/Alluvium Creek fan-delta, a series of well-developed constructional paleo-beach ridges reflect stable paleo-shoreline positions up to 10m higher than present day lake level. The excellent preservation of these beach ridges, close to the ancient front of the Gilbert delta describe above, suggests that construction of the beach ridges occurred after the delta formation and within the last ~10ka during subsequent lower stages of ancient Yellowstone Lake.

Although we did not recover any dateable material in the beach ridge succession, a sample of charcoal recovered from nearby fluvial floodplain deposits exposed in an abandoned channel ~50m from modern day Columbine Creek and situated immediately south of the beach ridge succession yielded a radiocarbon date of ~ 2795+/-55 cal yr BP. The location of the exposure suggests that the level of Yellowstone Lake was ~ 2m above the present day level at this time, coincident with the youngest of the shoreline beach ridges.

Further north, along Clear Creek, a radiocarbon date of 4100 +/- 130 cal yr BP associated with laminated floodplain/lacustrine deposits located ~ 4m above the present day lake level suggests that the lake surface was also elevated at that time relative to present.