Four seasons and spring tides relating to lunar phases; this is as far as most people experience cosmic cycles in their lifetime. However, our experience of natural cyclicity is limited to our short lifespans. In reality, natural or geological cycles operate slowly and over many tens or hundreds of thousands of years; they’re happening right now!
The study of ‘Sequence Stratigraphy’ works to tie geology with this cyclicity. It can be applied to the rock record at many different scales and, like most sciences, the study of ‘Sequence Stratigraphy’ is littered with technical jargon. This guide aims to bust through the jargon and provide a broad introduction to ‘Sequence Stratigraphy’, explaining some of the key terms and concepts that underpin this science.
What is Sequence Stratigraphy?
The present-day concept of ‘Sequence Stratigraphy’ was first introduced in 1977 by Vail et al.,. Initially it focussed on understanding seismic reflector geometries and was based on work undertaken at Exxon production Research Company in the 1960’s. Prior to this research, most unconformities (breaks in geological time that can be observed within the rock record) were thought to be tectonic in nature, with the role of sea-level and its effect on the sedimentary rock record less-well understood. With the introduction of Sequence Stratigraphy, this changed and sea-level variation was shown to provide a strong control on the sedimentary rock record, particularly at the coastline.
Sequence stratigraphy: A predictive tool
Today, oil and gas companies use sequence stratigraphy as a tool to predict where ancient sedimentary rocks (which form the majority of oil and gas reservoirs) are likely to have accumulated or where they may have been removed. To achieve this, sequence stratigraphy analyses the cyclicity of sea-level changes:
Cyclicity can be observed across large geological time spans, with cycles of 20.000, 40.000 and 110.000 years, depending on cosmic parameters. In combination with tectonics, this cyclicity helps to alternate between high and low sea level stands.
With a low sea level, more igneous (volcanic and plutonic) rocks on the continent are prone to erosion and weathering and subsequently form grains. These grains are transported, for example by rivers, to their depositional environment such as seas or oceans (a basin), where they settle and form a sediment. Over time the weight of the sediment ‘pushes’ (or loads) the material further down, making it more compact. This process can accumulate hundreds of meters of material in a basin, while the basin itself subsides (sinks).
Repetition in sea level cyclicity (i.e., high to low to high again) is called a sequence, with each high or low sea-level within a sequence called a system tract. In the rock record, systems tracts are best defined by analysing changes in the vertical order of ancient or palaeoenvironments. So, for example, if an ancient river system is capped by a shallow marine system, we know that sea-level had risen from low to high, flooding the land with sea water and pushing the river system further inland. Within each full sequence, there are 4 different system tracts.
A lowstand system tract (LST) is a scenario where sea level falls to, or is at its lowest level within a sequence. Sediments in lowstand system tracts sit above the maximum regressive surface – regression is best translated as sea level retreating – and the maximum regressive surface is typically exposed to the air and undergoes erosion, which creates an unconformity to the sequence below.
A transgressive system tract (TST) – this systems tract sits above the LST and represents initial sea level rise or trespassing of the sea over the land. Sediments that form at the top of a TST are said to be the most distal within that specific sequence. This means they are likely to have formed furthest from land and in the deepest water-depth compared to the underlying LST or overlying highstand. As sea level continues to rise throughout the TST system tract, the top of each TST is marked by a maximum flooding surface, representing the highest sea-level recorded within that specific sequence.
A highstand system tract (HST) is a scenario with highest sea level and sediments within a HST are deposited above the maximum flooding surface. Sediments within HST’s accumulate quicker than any remaining sea-level rise and so they form in a more proximal setting (i.e. they occur closer to the shoreline) compared to the underlying TST sediments.
A falling stage system tract (FSST) is deposited during falling sea level and sediments are forced to move towards the sea. FSST sediments are then overlain by LST sediments and the cycle/sequence starts again.
Originally, or traditionally, sequence boundaries (sb) are the frontiers, or surfaces, between each sequence. It is important to remember, however, that it will be easier to spot sequence boundaries in more proximal settings such as deltas and shorelines. Here, sequence boundaries are commonly exposed to air and erosion. In deeper settings, they form so-called non-conformities, where there is little change in environment above and below the surface/ sequence boundaries.
Further Reading about Sequence Stratigraphy
SEPM Stratigraphy Web have an excellent range of introductory Sequence Stratigraphy articles.