The Glacial/Deglacial History of Sedimentation in Bear Lake (Utah and Idaho) Joe Rosenbaum, Walt Dean, and Richard Reynolds

Interpretations based largely on changes in provenance of detrital material and the transition from siliciclastic to carbonate sedimentation Catchment –Relation of Bear River to Bear Lake –Properties of fluvial materials Mineralogy and elemental chemistry Magnetic properties Lake sediment cores –Variations in provenance with age –Glacial history Comparisons to other records –Rocky Mountains –Lake Bonneville

Bear Lake and the Bear River Headwaters of the Bear River in the northwestern Uinta Mountains Bear River is the largest river in the Great Basin

Bear Lake & Bear River The Bear River did not enter the lake during much of the Holocene, but did during the last glacial period.

Geologic Map

Magnetic Properties of Fluvial Sediments “Magnetite” content –low in Uinta Mtns. and lower reaches of the Bear River – high in local Bear Lake catchment –about the same on east and west sides of lake “Magnetite” in the fluvial sediments is largely from dust –variety of magnetite and titanomagnetite grains that are derived from igneous rocks that do not exist in the catchment –fine-silt sized grains (<10  m)

Magnetic Properties of fluvial sediments “Hematite” content –higher in Uinta Mtns. than in other source areas. –about the same in local Bear Lake catchment as along lower reaches of the Bear River Note dilution of material from the headwaters of the Bear River downstream so that under present conditions Uinta Mountain material is a minor component of the river sediments.

Properties of 3 catchment areas Local Bear Lake catchment –High dolomite content (Mg/Ca) & magnetic susceptibility (MS) –Low HIRM Headwaters (Uinta Mtns.) –High HIRM and Al/Ti –Low MS “Lower” Bear River –Low MS, HIRM, & dolomite content

Cores Composite record from two 4-m-long cores –BL96-2: Largely carbonate-rich sediments overlying uppermost siliciclastic sediment –BL96-3: Very thin, incomplete Holocene record overlying more than 3.5 m of siliciclastic sediment GLAD800 –120 meters –Last 240,000 years

Composite Section and Chronology The composite record provides a nearly continuous record for most of the Holocene back to about 32 cal. kyr Siliciclastic sedimentation 32 – 17 cal. kyr Endogenic calcite or aragonite 14.5 cal. kyr - present

Ages 14 C ages –AMS ages on pollen concentrates (pollen + other refractory organic material) –Pollen is plentiful and well preserved in carbonate sediments –Pollen is sparse and in poor condition in the siliciclastic sediments –Ages are in good stratigraphic order and there is no apparent discontinuity

Properties vs age

MS vs. HIRM for Zone I Strong negative relation between content of material from Uinta Mountains and from the local Bear Lake catchment

Detrital material from the Uinta Mountains is glacial flour: High content of material from the Uinta Mountains requires enhanced erosion. There was extensive Pinedale-equivalent glaciation in the Uinta Mountains. Sediment with high content of Uinta Mountain material is finer-grained.

Properties vs age 2

Magnetic properties from BL96-3 and GLAD800 Unequivocal correlation based on magnetic susceptibility Base of BL96-3 just captures the base of the zone with high values of HIRM

Comparison to other records of glacial extent Upper Klamath Lake –Millennial-scale variations –Peak at about 19 to 18 cal. kyr Rocky Mts. –Peak at about the same time –Major deglaciation somewhat later Southern Uintas (Laabs) and Wasatch (Lips) –Cosmogenic ages indicate significantly later deglaciation

Comparison to Lake Bonneville Onset of glaciation at same time as initial growth of Bonneville Increase to max. glaciation corresponds to rise following SO Deglaciation occurs while Bonneville continues to rise River leaves Bear Lake (Zone IV) during fall from Provo shoreline Salinity increases (Zone V/VI boundary) during fall from Gilbert (i.e., at end of YD)

Summary Bear Lake contains glacial flour from the headwaters of the Bear River in the Uinta Mountains. The record suggests: –onset of glaciation at 32 cal. kyr –maximum glaciation from cal. –deglaciation during later stages of growth of Bonneville and earlier than indicated by recent dating of moraines in the southern Uinta and Wasatch mountains The Bear River abandoned Bear Lake cal/kyr The salinity of Bear Lake increased at the end of the Younger Dryas