THE EFFECTS OF GLACIAL AND FLUVIAL TRANSPORT ON DETRITAL APATITE MINERALS

Spatial patterns of erosion are increasingly studied with detrital minerals. Apatite minerals are dated with (U-Th)/He thermochronology to trace sediments back to their source rocks. These studies require apatite minerals that are large enough to analyze and ideally have good crystal form. An analysis of apatite grain shapes in sediment deposits can be useful to determine the accuracy of ages that can be obtained. This research investigates the quality of apatite grains in stream and moraine deposits in two canyons in the Teton Mountains. Stream samples from active channels near the mouth of each canyon and moraine samples close to the shores of Bradley and Jenny Lakes were collected and processed at Illinois State University. Samples were washed, sieved, and run through heavy liquids and the Frantz magnetic separator to isolate apatite grains. Next, apatite grains were examined with a petrographic microscope and classified based upon their relief and shape. Relief classifications include euhedral, subhedral, and anhedral. Shape classifications include P-P (prismatic-prismatic), N-P (nonprismatic-prismatic), N-N (nonprismatic-nonprismatic), S-P (sliced-prismatic) and AN (anhedral). Two preliminary samples from Cascade Canyon were analyzed, and include TMV-3 (moraine sample) and TTS-4 (stream sample). Sample TTS-4 was located within Cascade Canyon and sample TMV-3 was located on the Cascade Canyon moraine around Jenny Lake. Ten grains were analyzed from each sample. Sample TTS-4 contained a majority of subhedral, S-P grains while sample TMV-3 contained a majority of subhedral, N-P and AN grains. Sample TMV-3 contained fewer euhedral and P-P grains than sample TTS-4, which could be a result of abrasion and reworking within or under ice compared to the less intensive transportation by fluvial processes that sample TTS-4 underwent. Previous work with sample TMV-3 yielded only 3 useful apatite ages, which is in line with the frequency of broken and poorly shaped apatite grains. Further study of at least 50 grains from samples retrieved from both Cascade Canyon and Garnet Canyon sediment deposits will be analyzed to further assess how much glacial and fluvial processes break apart apatite grains, and how the frequency of broken grains may influence our ability to use ages to understand spatial patterns of erosion.