The original shapes of quartz silt grains: A test of the validity of the use of quartz grain shape analysis to determine the sources of terrigenous silt in marine sedimentary deposits

Abstract

The results of a study of the original shapes of quartz silt grains that are released from plutonic, metamorphic, and sedimentary rocks of varied age, origin and lithology are reported. The purpose of the study is to demonstrate the validity of quartz grain shape analysis as a technique for determining the sources of terrigenous silt within marine sedimentary deposits by showing that different sources of terrigenous silt release quartz silt grains with different shapes. There are four major types of sources that release quartz silt grains with different and distinctive shapes. Crystalline rocks release a mixture of crystalline grains (with subrounded and subangular outlines, crystalline nodes, and grain embayments) and fractured grains (with angular outlines, microfractures, deformation sheets, and fracture faces). Quartz-cemented sedimentary rocks release overgrown grains (with subangular to angular outlines and quartz overgrowths) and lesser numbers of rounded grains (with rounded and subrounded outlines and abraded grain surfaces). Glacial deposits release fractured grains (with angular outlines, microfractures, deformation sheets, fracture faces, micro-blocks, and comminution debris). Lastly, quartzose multicyclic sediments release abundant rounded grains (with rounded or subrounded outlines and abraded and weathered surfaces). Therefore, the results indicate that quartz grain shape analysis is generally a valid technique for determining the sources of silt. However, there are exceptions to this generalization, for there are some sediment sources (e.g., first-cycle coastal plain sediments) that inherit and release grain shape types which are released from other sediment sources (e.g., crystalline rocks). Thus, the application of quartz grain shape analysis to the study of the sources of terrigenous silt for a given study area requires some knowledge about the major sediment sources which are available to that study area, for this knowledge will define the limitations of this sourcing technique.