Micromorphological criteria for discriminating subglacial and glacimarine sediments: evidence from a contemporary tidewater glacier, Spitsbergen

Sedimentological and micromorphological evidence is presented from four sites within St. Jonsfjorden, western Spitsbergen. Sedimentological and geomorphological evidence is used to distinguish sites comprising undisturbed glacimarine sediments from glacially over-ridden glacimarine sediments and subglacial diamicts. Impregnated thin sections are used to define associations of micromorphological characteristics that allow diamicts formed under these different conditions to be discriminated. In situ glacimarine deposits display winnowed textures, dropstone structures and distinct vertical orientations of sand grains, whilst subglacial conditions result in structures indicative of deformation under moderate to high stress regimes. These criteria may help to determine the origin of ancient and Quaternary diamicts of questionable subglacial or glacimarine origin.