Pockmark (geology)
A pockmark is a seabed or lakebed depression, typically circular or elliptical in shape, formed by the expulsion of fluids (liquids or gases) from the sediment. Pockmarks range in size from a few meters to several hundred meters in diameter and from centimeters to tens of meters in depth.
The formation of pockmarks is a complex process driven by various geological and biological factors. The primary driving force is the build-up of pore pressure within the sediment. This pore pressure can be generated by:
- Decomposition of organic matter: Anaerobic decomposition of organic matter produces biogenic gases, such as methane and carbon dioxide, which increase pore pressure.
- Sediment compaction: The weight of overlying sediments compacts the underlying layers, squeezing out fluids and increasing pore pressure.
- Gas hydrate dissociation: Gas hydrates are ice-like substances that contain methane or other gases trapped within a crystal lattice. Changes in temperature or pressure can cause hydrates to dissociate, releasing large volumes of gas and increasing pore pressure.
- Faulting and fluid migration: Faults and fractures can act as conduits for the migration of fluids from deeper geological formations towards the seabed, contributing to pore pressure build-up.
- Thermo-genic gases: Gases generated at depth as a result of the breakdown of organic matter by geothermal heating.
Once the pore pressure exceeds the overburden pressure (the weight of the overlying sediment), the fluid escapes through the seabed, creating a pockmark. The expulsion of fluid can be episodic or continuous, depending on the rate of fluid supply and the permeability of the sediment.
Pockmarks are often associated with active or recently active fluid expulsion features, such as gas seeps and chemosynthetic communities. They can provide valuable insights into subsurface fluid flow processes and the potential for hydrocarbon exploration. The presence of pockmarks can also pose geotechnical hazards to offshore infrastructure, such as pipelines and platforms, due to the potential for seabed instability and subsidence. Pockmarks can also alter local benthic ecosystems.