What are the main controls and feedbacks on parasequence development and architecture?

1 Relative sea level

The rise and fall of relative sea level (RSL) is the primary driver of parasequence formation. RSL is the result of the interaction between global sea level (eustasy), local subsidence or uplift (tectonics), and sediment loading or compaction (isostasy). When RSL rises, the shoreline migrates landward, creating a transgressive surface of erosion (TSE) that marks the base of a parasequence. When RSL falls, the shoreline moves seaward, generating a regressive surface of marine erosion (RSME) that defines the top of a parasequence. The TSE and RSME are important sequence stratigraphic surfaces that can be recognized in well logs, seismic data, and outcrops.

2 Sediment supply

The amount and type of sediment delivered to the basin influences the shape and thickness of parasequences. Sediment supply depends on factors such as climate, drainage area, relief, vegetation, and weathering. High sediment supply can cause parasequences to prograde seaward, forming wedge-shaped or tabular geometries. Low sediment supply can result in parasequences that retrograde landward, creating bell-shaped or sigmoidal geometries. The sediment supply also affects the grain size, sorting, and composition of the parasequence facies, which have implications for reservoir quality and diagenesis.

3 Accommodation space

The accommodation space is the volume available for sediment accumulation in the basin. It is controlled by the balance between RSL and sediment supply. When accommodation space increases, parasequences tend to stack vertically, forming aggradational or backstepping patterns. When accommodation space decreases, parasequences tend to stack laterally, forming progradational or downstepping patterns. The accommodation space also determines the degree of exposure and reworking of the parasequence surfaces, which can create unconformities, diastems, or hardgrounds.

4 Autocyclic processes

Autocyclic processes are internal feedbacks within the depositional system that can modify the parasequence architecture. They include factors such as sediment transport, deposition, erosion, compaction, and bioturbation. Autocyclic processes can create variability in the parasequence geometry, thickness, facies, and connectivity. For example, channel migration, delta switching, or storm events can create local incisions, avulsions, or lobes within a parasequence. Autocyclic processes can also generate heterogeneities and complexities in the reservoir properties and fluid flow of the parasequences.

5 Allogenic factors

Allogenic factors are external forces that can affect the parasequence development and architecture. They include factors such as climate change, tectonic activity, eustatic fluctuations, and basin configuration. Allogenic factors can create regional or global changes in the RSL, sediment supply, and accommodation space, resulting in shifts in the parasequence stacking patterns, facies associations, and sequence boundaries. For example, glacial-interglacial cycles, orogenic events, or basin inversion can create major changes in the parasequence stratigraphy.

6 Parasequence applications and case studies

Parasequences are useful tools for sequence stratigraphic analysis and correlation, as they can provide information on the depositional environment, relative sea level history, and basin evolution. Parasequences can also help to characterize the reservoir quality, connectivity, and heterogeneity of the sedimentary formations, as they reflect the variations in the grain size, sorting, composition, and diagenesis of the facies. Parasequences have been applied and studied in various settings and scales, such as carbonate platforms, siliciclastic shelves, fluvial-deltaic systems, and deep-water fans.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?