4.1: Stratification
Stratification is an intentionally vague term used to describe layering 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 table of more specific terms is provided below.
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 .
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).
Resources and Readings
- North American Commission on Stratigraphic Nomenclature, 2005, North American Stratigraphic Code, AAPG Bulletin, v. 89, no. 11, p. 1547-1591
- Campbell, C.V., 1967, Lamina, laminaset, bed and bedset, Sedimentology, v. 8, p. 7-26.