Petrographic Analysis


Current description
Hand specimen description
The specimen is very thinly to thinly bedded, very pale grey (Munsell value 10YR 8/1) sedimentary rock. The layering is defined by contrasting porous, quartz-rich sandy bands weakly stained with secondary iron oxyhydroxides ("limonite") and low-porosity, quartz-poor muddy bands.
Microscopic description,
The sandy layers referred to above show the following features:
· framework : matrix : porosity = 40 : 55 : 5, but some layers with more matrix
· mineralogy of framework
quartz 60%
lithics 30%, most of phyllitic or mica-schist fragments, but also including quartz fine siltstone
feldspar trace
muscovite trace
zircon trace
matrix mineralogy indeterminate (probably mostly phyllosilicates)
· framework texture
grain size ~20-200 µm (medium silt-fine sand; average very fine sand)
grain-size distribution wide (i.e. sorting poor)
grain roundness angular-very angular
broadly uniform orientation of elongate grains parallel to stratification
porosity of sandy layers ~5%, but up to ~10%
grain contacts varied: point, straight, interpenetrant and sutured.
The muddy layers are as follows:
· framework : matrix = 30 : 70 (or lesser proportions of framework)
· mineralogy is probably mostly phyllosilicates, with the coarse fraction dominated by quartz and subordinate lithics
· grain-size variously dominated by coarse or medium silt, but ranges to fine sand
· internal sedimentary structures
very thin laminae of fine-very fine sand, discontinuous on scale of thin section
rare, enigmatic stringers of quartz fine-very fine sand, vertically oriented, sinuous in form and one or two grains wide within muddy host.
Classification
A. Textural classification
The textural classification is that of Folk (1980).
· The sandy layers are a sandy mudstone (transitional to muddy sandstone).
· The muddy layers are sandy mudstones and mudstones.
B. Dott classification
The Dott classification of 1963 uses both grain size and composition.
· The sandy layers are a lithic greywacke.
· The muddy layers are mudstones.
 
Revised description

(This assumes that the samples I examined are representative of the unit as a whole).

The Eidsvold Siltstone is a pale grey, finely bedded sedimentary mudstone with a well developed parting parallel to bedding.   The layering is due to a variation in grain size, where sand-bearing layers alternate with muddy layers.   The sand fraction consists mostly of quartz and rock fragments and is rarely coarser than fine sand grade.