4.1: Stratification

Stratification is an intentionally vague term used to describe laying in a sedimentary rock without implying any specific scale or thickness. Layering can be obvious or subtle ... you need to look for some sort of difference in grain size, mineralogy, cements, color, texture, texture, etc.

Beds and laminae

If you want to start talking about the scale of layering, we can break it down into two broad groups: Layers that are less than 1 cm thick are called laminae and layers that are more than 1 cm thick are called beds. If you need more precise terminology, a widely used table of terms is provided below.

Figure \(\PageIndex{1}\): Descriptive terms for the thickness of beds and laminae (Tenzinsonam995 via Wikimedia Commons; CC BY-SA 4.0).

Internal changes in grain size

Normal grading

Within some strata, it is possible that there might be a systematic change in grain size. Normal grading is when a bed shows an upward decrease in grain size. This is usually caused by a decrease in flow velocity or particles falling out of suspension.

Reverse (inverse) grading

Reverse (inverse) grading is exactly the opposite … the largest grains are at the top of the bed and the smallest are at the bottom. Reverse grading typically occurs in response to shaking or winnowing in cohesionless sediment (grainflows) which causes the smallest grains to work their way downward through gaps between the larger grains.

Massive beds

And although relatively rare, internally massive beds that lack any sort of internal stratification can form when sediment is rapidly dumped or when previously stratified sediments are reworked via bioturbation.

Figure \(\PageIndex{2}\): Grain size distribution with beds. A) Normally graded bed that fines upward (James St. John via Wikimedia Commons; CC BY 2.0). B) Inverse grading that coarsens upward (John Waldron via University of Alberta; CC BY-NC-SA 4.0). C) Crudely stratified to internally massive conglomeratic sandstone (Michael Rygel via Wikimedia Commons; CC BY-SA 4.0).

Bed geometry and contacts

Once you’ve identified bedding, you might find it useful to describe the overall form of the bed by thinking about whether the beds are continuous or not, whether the contacts are planar or wavy, and whether the contacts are are parallel or truncate each other. Another aspect of bedding that you can describe is the nature of contacts between beds. Specifically, we find that phrases like sharp, gradational, interbedded are very useful especially when paired with a semi-quantitative estimate of scale (ex: gradational over 5 cm).

Figure \(\PageIndex{3}\): Types and geometries of bedding contacts after North American Commission on Stratigraphic Nomenclature, 2005, North American Stratigraphic Code, American Association of Petroleum Geologists Bulletin, v. 89, no. 11, p. 1547-1591 and Campbell, C.V., 1967, Lamina, laminaset, bed and bedset, Sedimentology, v. 8, p. 7-26. (Page Quinton via Wikimedia Commons; CC BY SA 4.0).

Figure \(\PageIndex{4}\): Numerous different types and scales of bedding contacts are present in this exposure of a redbed paleosol complex. (Michael C. Rygel via Wikimedia Commons; CC BY SA 4.0).