341 Sediments, Clastic (Posters)

Wednesday, 28 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

ROLE OF SORTING ON THE COMPOSITION OF SILICICLASTIC SEDIMENT: IMPLICATIONS FOR INTERPRETING PROVENANCE AFTER LIMITED TRANSPORT IN AN ARID CLIMATE

Paper number: 341-16
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

This study tested whether transport distances (< 500 m) have the capacity to shape the geochemistry of sediments across grain sizes derived from a single source due to sorting. In the Stepladder Mountains, Mojave Desert, CA, a < 1 km2 watershed allows for a controlled study to understand how modern sediments acquire their composition from a single granodioritic source. The study area naturally minimizes the effects of chemical weathering and source mixing. Sediments are naturally sorted into distinct grain-size populations, ranging from very fine sand to gravel within a single, alluvial channel. Sediment samples representative of each population were petrographically and geochemically analyzed in order to test the effectiveness of commonly used discrimination diagrams. Sediments became proportionally enriched in plagioclase and biotite and depleted in K-feldspar and quartz with decreasing grain size. Major elements were plotted in Al2O3–CaO*+Na2O–K2O and Al2O3–CaO*+Na2O+K2O–FeO+MgO compositional space and indicate that a negligible degree of chemical weathering was involved in sediment production. Trace element plots normalized to average granodiorite bedrock show strong enrichments in elements thought to be immobile during sedimentary processing (Cr, Co, and Sc) across nearly all sediment samples. Using any of these elements as ratios (Th/Co, La/Sc, Th/Sc, Cr/Th, Zr/Sc) in provenance discriminating plots reveal that sediment was formed source mixing, as sediments in these plots considerably deviate from bedrock composition due to the control grain size has on geochemistry. Sediment rare earth element (REE) contents also deviate from source composition and show an increase in total REE content, decrease in fractionation of light REEs and heavy REEs, and an increase in the magnitude of the negative Eu anomaly with decreasing grain size. Variations in sediment composition and thus geochemical ratios result from mineral sorting during transport, no matter how short. Thus, strong caution must be used when using discrimination diagrams to interpret sediment and sedimentary rock provenance.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-286768

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.