Glacimarine Mudrock Provenance: Pb-isotopic and Elemental Analyses of Different Grain Size Fractions in the Yakataga Formation, Southern Alaska

Dylan P. Loss¹, John M. Jaeger¹, Ken D. Ridgway², George D. Kamenov¹, and Tania Villasenor^1
1University of Florida Department of Geological Sciences1
²Purdue University Department of Earth, Atmospheric, and Planetary Sciences

Fine-grain strata are abundant in glacial environments where diamicts are created in both terrestrial and marine settings. Evaluating the utility of diamicts as records of glacial dynamics depends in part on analyzing particle composition. We examine provenance of mudrocks of the glacimarine Neogene Yakataga Formation of southern Alaska to test the hypothesis that there has been a transition to more localized sediment delivery from the rapidly uplifting and exhuming St. Elias Mountains. The Yakataga is primarily composed of fine-grained diamictite, so previous detrital zircon analyses may provide an incomplete provenance. We analyzed the bulk sediment and silt fraction (15-63 μm) of mudrock Yakataga samples for elemental geochemistry and Pb isotopes. XRD analyses of the silt fraction reveals three dominant mineral phases present in most of the silt samples: quartz, albite, and chlorite. Pb isotopic silt analyses reveal mixed sources, but Pb overprinting on silt complicates its use. Pb isotopic values of residual silicate silt samples generally correspond with the isotopic values of bulk samples, suggesting that Pb overprint is size-selective. Elemental analyses are compared with published results from possible source rocks. Yakataga sections that unconformably overlie Eocene-Cretaceous strata have higher proportions of mafic source components, whereas sections conformable with Oligocene strata have higher proportions of felsic-derived material. Provenance of mudrocks of the Yakataga is consistent with detrital zircon results, but mudrock analyses provide stratigraphic context to provenance showing variation between sections. The difference between silt and bulk provenance signatures highlights the importance of utilizing consistent size fractions in mudrock analyses.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013