Abstract

This chapter presents parasequences—the next larger and more aerially extensive three-dimensional stratal unit of the stratigraphic hierarchy found consistently across most depositional environments. The parasequence scale is the key scale at which we interpret depositional environments, apply Walther’s Law to recognize significant stratal discontinuities, correlate and construct maps, and make tie-ins with well-log signatures.

A parasequence is a distinctive succession of relatively conformable beds and bedsets bounded by surfaces of flooding, abandonment, or reactivation and their correlative surfaces. They can be recognized in a variety of depositional settings through specific and objective attributes of their stratal stacking and bounding surfaces. The definitive interpretation of any particular interval as a parasequence requires two components: (1) recognizing its character in vertical section and (2) establishing its lateral distribution over an area significantly broader than any single component bedset or geomorphic element of the depositional environment (i.e., many hundreds of square kilometers). Although it might be difficult to recognize parasequences in a particular interval or location, they still are extremely likely to exist even there because they appear to be a universal feature of sedimentation.

This chapter introduces the general concepts of parasequences, illustrates those concepts with marine-shelfal examples, and provides practical guidelines for recognizing and correlating parasequences. It then discusses common variations in vertical and lateral aspects, presents an application to building quantitative models of depositional environments, and introduces the expression of parasequences in other depositional settings. Recognition of parasequences is essential because they are the building blocks of the next larger scales in the stratigraphic hierarchy: parasequence sets (systems tracts) and depositional sequences, which are discussed in Bohacs et al. (2022b, Chapter 6 this Memoir).

The following excerpt from table 12 of Lazar et al. (2022a, Chapter 3 this Memoir, p. 72) places parasequences in their context in the sequence-stratigraphic interpretation workflow:

3. Group the lithotypes and stratal characteristics at the bed and bedset scale into facies packages based on recurring associations of attributes.

4. Relate the stacking patterns of facies packages to the depositional environment:

   1. Facies stack in distinctive manners and proportions according to their depositional environment.

   2. Calibrate well-log response of each facies association or depositional environment.

5. Use the stacking patterns and lateral relations of rock packages from the different depositional environments, and their bounding surfaces, to interpret the sequence stratigraphy.

   1. Divide the stratigraphic section into large-scale packages bounded by significant surfaces: key discontinuities and major flooding surfaces (Chapter 3, figure 5).

   2. Group bedsets of depositional environments into parasequences. Group parasequences into parasequence sets according to their stacking patterns (progradational, aggradational, retrogradational), location relative to preexisting shelf break, and relations to bounding surfaces. Note the relation of the parasequence sets to the significant surfaces recognized in step V(A), and refine your estimation of systems tracts.