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How do you recognize sequence stratigraphic surfaces in the field?

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Sequence stratigraphic surfaces are key features that define the architecture and evolution of sedimentary basins. They represent changes in relative sea level, sediment supply, and basin subsidence that affect the deposition and erosion of sediments. Recognizing them in the field can help you to reconstruct the paleogeography, paleoclimate, and paleoecology of the basin, as well as its potential for hydrocarbon exploration. In this article, you will learn how to identify the main types of sequence stratigraphic surfaces and how to use them to interpret the depositional history of a basin.

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Ali Lakzaie

Geoscience | 3D Geomodelling | structural geology

1 Boundaries and systems

The first step to recognize sequence stratigraphic surfaces is to understand the concepts of boundaries and systems. Boundaries are surfaces that separate different depositional sequences, which are packages of genetically related sediments. Systems are subdivisions of sequences that reflect changes in the position of the shoreline relative to the basin. There are four main types of boundaries and systems: sequence boundary, lowstand systems tract, transgressive systems tract, and highstand systems tract. Each one has distinctive characteristics and implications for the basin evolution.

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mohamad motasim

HYDRO - GEOLOGIST / GEOTECHNICAL ENGINEER
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First, the geological reading of the site is to know the lithological components and geological structures at the site. Secondly, using geophysical methods to estimate the depth and thickness of layers, groundwater levels, and geological structures in the ground. Third, drilling geological drilling points to ensure the quality, depth and difference of geological layers.

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Bernardo C. Aznarán Rengifo

Geólogo Senior Consultor & Superintendente de geología y Desarrollo de Proyectos
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Empezaría, recolectando la informacion geologica del área, todos los datos que se tengan, quizas de organismos gubernamentales ( en Perú, el INGEMMET). Tener un buen conocimiento de estructuras sedimentarias y procesos de sedimentación. Ir al campo y tener una buena observación, paciencia para interpretar en el campo. Hacer bosquejos o gráficos, definir bien los rumbos , buzamientos de los estratos. Esto despues , ponerlo en un software geologico , para verlo en 3D y con todo esto tener una buena interpretación. Pero importante sera, tener una buena data de campo, la observación es principal.

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2 Sequence boundary

A sequence boundary is a surface that marks a major drop in relative sea level, which causes erosion and incision of the underlying sediments. It can be recognized by the presence of angular unconformities, onlap patterns, erosional truncations, and fluvial channels. A sequence boundary is also associated with a lowstand systems tract, which is the sedimentary response to the low sea level. It consists of coarse-grained deposits that prograde into the basin, such as deltas, fans, and wedges. A lowstand systems tract can be identified by the presence of offlapping patterns, prograding geometries, and basinward shifts in facies.

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Gerardo G.

Basin Researcher | Geoscientist
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Key points of sequence boundaries, 1) Lowstand System Tract: rapid sea-level fall, 2) Transgressive System Tract: rapid sea-level rise, and 3) Highstand System Tract: it comes after a maximum flooding and it is probably the easiest surface to identify in seismic. Btw, I am an structural geologist more than a sedimentologist.

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3 Transgressive surface

A transgressive surface is a surface that marks a rise in relative sea level, which causes flooding and retrogradation of the shoreline. It can be recognized by the presence of ravinement surfaces, downlap patterns, transgressive lags, and marine flooding surfaces. A transgressive surface is also associated with a transgressive systems tract, which is the sedimentary response to the rising sea level. It consists of fine-grained deposits that backstep into the basin, such as shales, mudstones, and limestones. A transgressive systems tract can be identified by the presence of backstepping patterns, retrograding geometries, and landward shifts in facies.

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Ali Lakzaie

Geoscience | 3D Geomodelling | structural geology
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Transgressive surface: A transgressive surface is a surface that marks a rise in relative sea level, which causes flooding and retrogradation of the shoreline. It can be recognized by the presence of ravinement surfaces, downlap patterns, transgressive lags, and marine flooding surfaces. A transgressive surface is also associated with a transgressive systems tract, which is the sedimentary response to the rising sea level. It consists of fine-grained deposits that backstep into the basin, such as shales, mudstones, and limestones.

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4 Maximum flooding surface

A maximum flooding surface is a surface that marks the peak of relative sea level, which causes maximum flooding and aggradation of the shoreline. It can be recognized by the presence of condensed sections, maximum flooding surfaces, maximum flooding lags, and maximum biotic diversity. A maximum flooding surface is also associated with a highstand systems tract, which is the sedimentary response to the high sea level. It consists of fine- to coarse-grained deposits that aggrade in the basin, such as shales, sandstones, and carbonates. A highstand systems tract can be identified by the presence of aggrading patterns, parallel geometries, and vertical shifts in facies.

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5 Correlation and interpretation

The final step to recognize sequence stratigraphic surfaces is to correlate them across different sections and scales, and to interpret their significance for the basin history. Correlation can be done by using stratigraphic logs, seismic profiles, well data, and outcrop observations. Correlation can help to establish the chronostratigraphic framework, the depositional environments, and the sedimentary facies of the basin. Interpretation can be done by using sequence stratigraphic models, paleogeographic maps, paleoclimatic indicators, and paleoecological data. Interpretation can help to reconstruct the relative sea level changes, the sediment supply variations, and the basin subsidence rates that controlled the sequence stratigraphy of the basin.

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6 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?

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BUYANI TIMBA (SACNASP Cand.Nat.Sci)
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In the field, recognizing sequence stratigraphic surfaces involves careful observation of sedimentary rock layers and their characteristics. Key indicators include: 1. **Stratigraphic Terminations:** Identify abrupt changes in sedimentation patterns, such as the termination of one type of sediment and the abrupt onset of another. 2. **Unconformities:** Look for surfaces that represent breaks in the sedimentary record, like erosional surfaces or non-depositional periods, suggesting sequence boundaries. 3. **Transgressive and Regressive Surfaces:** Recognize surfaces that mark shifts between transgressive (sea-level rising) and regressive (sea-level falling) phases, often characterized by changes in sediment type and fossil content.

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How do you recognize sequence stratigraphic surfaces in the field?

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1 Boundaries and systems

2 Sequence boundary

3 Transgressive surface

4 Maximum flooding surface

5 Correlation and interpretation

6 Here’s what else to consider

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