Spatial Limitation of Sequence Stratigraphic Units

A short note for beginners/students

One of fundamental problems in our stratigraphic understanding is that we consider a stratigraphic unit as layers that may continue over a part of basin and may also correlate within and across the basin. This leads us to a troublesome procedure of correlation to build a sequence stratigraphic framework.

This post is a brief review on the correlation concept within a sequence stratigraphic perspective. I would like to cover briefly one point here that a sequence stratigraphic unit is spatially a limited unit.

A stratigraphic unit is always formed within a specific space (or accommodation space; do not want to complicate with terms). Within such a space, it may be altered or modified by post-depositional sedimentary and structural processes. Despite the fact of its origin; the unit would remain within the same space and may be reduced in volume (if eroded). Such a space would always be geographically restricted suggesting that a unit would be spatially limited.

If a unit is spatially limited, how does it define?

Mostly a stratigraphic unit is defined by its physical properties and time. The physical properties can be measured on surface and also in the sub-surface. Such measurements can thereafter lead us to define a stratigraphic unit. A sequence stratigraphic unit on the other hand relies much on the subjective time correlation. The time is primarily the relative geologic time (RGT). This puts another uncertainty - a sequence stratigraphic unit is time-limited.

Therefore, I would not be too wrong to say that a sequence stratigraphic unit is spatio-temporarily limited.

With such uncertainty, it would be wise if you correlate strata within a basin based on subjective time lines (or time base sequence stratigraphic surfaces) with or without physical properties. The time can be a regional boundary (e.g. Top Cretaceous, Top Jurassic and so forth) if it is correlated from basin center to the basin edge.

References:

Qayyum, F., C. Octavian, and P. F. M. De Groot, 2015, Historical developments in wheeler diagrams and future directions: Basin Research, 27, 336–350, doi: 10.1111/bre.12077.

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