Table 1: Sedimentary Features Associated with Erosion Surfaces

 

Feature Ease of Recognition
Silt Lags in outcrop, careful examination reveals thicker silt beds (>5mm)

in hand specimen and on cut surfaces, beds of 1-2 mm thickness readily seen

Sand Lags in outcrop, careful examination typically reveals coarser beds, 5-20 mm thick

in hand specimen and on cut surfaces, beds of 1-2 mm thickness readily seen

Bone Beds typically a variant of sand silt lags, with comparable ease of recognition

larger bone fragments readily recognizable with hand lens on fresh surfaces

Pyritic Lags in outcrop, thicker pyritic lags, 10 or more mm thick, may be recognized via rusty stains or crusts of secondary white and yellow hydrous ferric sulfates

in handspecimen, layers as thin as 1 mm are readily seen on fresh surfaces

Conodont Lags may form a variant of sand/silt lags, with comparable ease of recognition

mostly forms almost pure layers of conodont material, 1-3 mm thick, that are best observed in handspecimen (bedding plane examination with hand lens)

Lingula Lags in outcrop and hand specimen, easily recognized only when shale is split along bedding planes

in thin section, shell cross-sections easily identified, but may be missed when Lingula shells are sparse on surface

Low-Angle Truncations typically not recognizable in outcrop

best recognized on cut/polished surfaces and in oversize thin sections

Soft Sediment Deformation deformation on the scale of tens of cm’s (e.g. with associated ball and pilow structures) can be recognized in outcrop

small scale deformation (mm’s to cm’s) best recognized on cut surfaces and in thin section

Sharp-Based

Shale Beds

recognition easiest on cut and polished surfaces and in thin section

very difficult and tentative in outcrop

 

 

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© Jürgen Schieber, UTA Department of Geology
Last updated: January 27, 2000.