Sediment color and weathered preglacial sources of Quaternary clays in southwestern Sweden

Abstract

The factors influencing clay color have been investigated using chemical, X-ray and microscopic methods. The olive gray colors are believed to be caused by organic matter and greenish iron silicates. Iron oxides mask the colors of clastic minerals when present as a precipitate on fine-grained particles. The iron oxides produce brownish hues. These precipitates are interpreted to have mostly developed prior to glacial erosion and transport in a weathering environment. Iron-oxides absorbed on clay surfaces were either removed or preserved due to the relative reducing or oxidizing conditions in the depositional and diagenetic environments. Low concentrations of reactive organic matter in the early glacio-marine sediments allowed the oxides to persist in bands of brownish clay (winter) which alternate with gray bands (summer). Distally and stratigraphically upwards, reduction of iron precipitates results in dominantly gray colored marine deposits. The prevalent occurrence of vermiculite, smectite, kaolinite and mixed-layer clay minerals and weathered feldspars and heavy minerals is also suggestive of a significant source of weathered material in addition to the erosion of fresh crystalline bedrock. The preservation of weathered material in the terrain despite repeated glaciations suggests a need to reconsider glacial erosion and sediment supply with respect to the Quaternary sediments in this area.

Key Words:

• Clay color
• iron-oxide precipitates
• clay minerals
• sediment source
• glaciomarine clays
• preglacial weathering
• Quaternary
• SW Sweden