A review by Karen K. Shell ESS 433 Relative Dating Techniques Sierra Nevada, California R.M. Burke and Peter W. Birkeland A review by Karen K. Shell ESS 433

Overview Defining Glaciations using RD Techniques Field Case Studies Questions Environmental Factors Weathering Criteria Moraine Morphology

Environmental Factors Setting of Deposits Vegetation Lithology

Environmental Factors Settings of Deposits Valleys Elevation of valleys Constraining bedrock Location with respect to mountain range

Environmental Factors Vegetation Forests Brushes Smaller Trees Grasses Soil Development

Environmental Factors Lithology Importance of minimizing variables Limit to particular grain sizes and weathering resistant minerals Granodiorite or Quartz monzonite sampled Correlation between sample sites

Weathering Criteria Moraine Surface Boulders Fresh-to-weathered ratio Pitted-to-non pitted ratio Pit Depth Maximum height of resistant mafic inclusions Rind-to-no rind ratio

Weathering Criteria Moraine Surface Boulders Average rind thickness Hammer-blow weathering ratio Surface boulder frequency Granitic boulder-to-non granitic boulder ratio Split-to-non split ratio Oxidation ratio

Weathering Criteria Moraine Surface Boulders (continued) Fresh-to-weathered ratio Requires 50% of boulder to exhibit weathering Rough texture Pitted-to-non pitted ratio Has one or more concave depressions (pits) Pits are not required to be circular in shape Counts are made separate from boulder counts Caused by break down of grains

Weathering Criteria Moraine Surface Boulders (continued) Pit depth Depth is measured from lowest point of pit to current boulder surface Maximum height of resistant mafic inclusions Measured from top of inclusion to average position of the adjacent rock surface Rind-to-no-rind ratio Discolored parallel to outer surface

Weathering Criteria Moraine Surface Boulders (continued) Average rind thickness Measured after rind-to-no rind ratio Measured to nearest millimeter Hammer-blow weathering ratio Based on sound heard when striking boulder with hammer. Determines whether boulder is fresh, weathered or grusified Surface boulder frequency Measurement of boulder frequency along crest of moraine within an area abundant with boulders

Weathering Criteria Moraine Surface Features (continued) Granitic boulder-to-non granitic boulder ratio 50 samples taken on boulders greater than 50 cm in diameter Split-to-non split ration Breaks appear along planar cracks Not the result of spalling Oxidation Ratio Relative visual discoloration within and between 50 samples measured

Weathering Criteria Subsurface Features Grusified granitic boulders beneath surface Soil properties

Weathering Criteria Subsurface Features (continued) Grusified granitic boulders Diameters of 30 cm or greater are measured Grusification is the intense disintegration of grains throughout the clast Classification Fresh Grusified Grusified and un-oxidized Grusified and oxidized

Weathering Criteria Subsurface Features (continued) Soil Properties Measured in field and analyzed in laboratory Extracted from pits dug along moraine crests. Field Properties Horizonation Color Texture Consistence Structure pH Other diagnostic features (clays, carbonates, etc.) A soil horizon is a specific layer in the soil which parallels the land surface and possesses physical characteristics which differ from the layers above and beneath. B Horizons are commonly referred to as ‘subsoil’, and consist of mineral layers which may contain concentrations of clay or minerals such as iron or aluminium, or organic material. In addition, they are defined by having a distinctly different structure or consistence to the A horizon above and the horizons below. They may also have ‘stronger’ colours (ie higher chroma) than the A horizon

Weathering Criteria Subsurface Features (continued) Soil Properties (continued) Laboratory Properties Particle size distribution Percent loss during ignition Organic matter pH Dry color

Weathering Criteria Moraine Morphology Width of crest of moraine Angle of slopes of moraine Inner and outer slopes Possible Downfalls Precision and accuracy require a single person to take measurements

Defining a Glaciation Using RD Techniques How much variation occurs between deposits? How much variation occurs between a stade? Splitters vs. Lumpers Burke and Birkeland’s methodology A stade is a period of quiescence between glacial cycles that usually lasts less than 10, 000 years.

Defining a Glaciation Using RD Techniques (continued) Splitters Researchers that make the subdivision at the maximum interval of the sequence, namely at the stade level. Subtle variations are of greater importance Good for determining moraine positions and cross-cutting moraine relationships Lumpers Researchers that are more inclined to make distinctions between glaciations at larger scale Consistent recognition of weather features between research locations (valley to valley)

Defining a Glaciation Using RD Techniques (continued) Burke and Birkeland’s Methodology Methodology largely influenced by the focus of the research on moraines that are in close proximity (adjacent, local) Determined that no single value could be set for determining glaciations in all locales

Defining a Glaciation Using RD Techniques (continued) Burke and Birkeland’s Methodology Burke and Birkeland settled for a magnitude of 2 times or greater in variation to separate glaciations Allows for distinctions to be made between first-order glaciations rather than secondary fluctuations of the first-order glaciations

Field Case Studies Bloody Canyon

Field Case Studies Bloody Canyon Previous Studies Some studies divided morphological features into four glaciations: Mono Basin, Tahoe, Tioga and Tenaya. (McGee, 1885; Russell, 1887; others) Other proposed two specific glaciations prior to Burke and Birkeland: Tioga and Tahoe (Putnam, 1949; Kistler, 1966)

Field Case Studies Bloody Canyon (continued) Burke and Birkeland’s Assessment Tioga Lie above Tahoe deposits and through RD Techniques is similar in morphology, lithology, etc to the Tenaya deposits. Determined that Tioga and Tenaya are part of the same glacial event. Younger than Tahoe deposits as determined by degree of weathering and superposition

Field Case Studies Bloody Canyon (continued) Burke and Birkeland’s Assessment Tioga Evidence? Presence of ash deposits above Till Absence of clay build-up except near the top of the profile Granitic boulders are “fresh” throughout the soils

Field Case Studies Bloody Canyon (continued) Burke and Birkeland’s Assessment Tahoe Due to extent of moraines, several sites were studied Vegetation at each of the study sites is crucial to correlation of RD data Ideal scenario would be the absence of vegetation or if not available, two sites of similar vegetation

Field Case Studies Bloody Canyon (continued) Burke and Birkeland’s Assessment Tahoe Evidence? Little difference in subsurface age between Mono Basin and Tahoe deposits Minimal soil development Both have clay development in the B-horizon of the soils. Tahoe deposits exhibits clay in horizons above and below the B-horizon Clay development is the result of eolian influx and in situ weathering

Field Case Studies Bloody Canyon (continued) Burke and Birkeland’s Assessment Tahoe Evidence? (continued) Surface boulders are limited and when found a moderately to completely grusified. Since differentiation between the sub-surfaces cannot be distinguished from one another, the Mono Basin and Tahoe deposits cannot be proven to be the result of separate glaciations. 7. Only distinction is the moraine morphology (Mono Basin exhibits less relief than Tahoe Moraine).

Discussion Questions What are the positive/negative aspects of the RD Techniques? Does the RD technique have differing results when applied to different types of glaciers (i.e. warm-based vs. cold-based, etc)? Is the elimination of the Mono Basin and Tenaya glaciations valid? Is the justification of order of magnitude for determining/separating glaciations a valid way to go about distinguishing glaciations?