Part-4: Clastic Depositional Environment -Empower Your Geoscience Journey: Top Free Public Resources for Geological Learning

Depositional environment is a very interesting topic which involves geological analytical thinking to solve a problem. In this part i am sharing very key fundamental details & its resources which are critical for clastic depositional Environments.

A depositional environment simply refers to the specific type of place where sediments accumulate and become sedimentary rocks/formation. These environments vary greatly and are influenced by factors such as climate, tectonic activity, and the type of material being deposited. They play a crucial role in determining the characteristics of the resulting sedimentary rocks.

1. Significance of Depositional Environments

• Interpretation of Past Environments: By studying sedimentary rocks and their characteristics, geologists can infer the conditions of ancient depositional environments.
• Resource Exploration: Understanding depositional environments helps in locating natural resources such as coal, oil, natural gas, and certain types of minerals.
• Environmental Reconstruction: Provides insights into past climate changes, sea level fluctuations, and tectonic movements.
• Petroleum system elements : Depositional environments are crucial for hydrocarbon exploration for several reasons, primarily because the formation, migration, and accumulation of hydrocarbons (oil and natural gas) are directly influenced by the characteristics of these environments, This knowledge is used to build geological models and guide exploration drilling.
• Reservoir Quality: Depositional environments influence the texture, grain size, sorting, and mineral composition of reservoir rocks, affecting their quality. Understanding these properties helps in evaluating potential reservoirs.
• Reducing Exploration Risk: Detailed knowledge of depositional environments reduces the risks associated with exploration

There are several resources availble on Clastic Depositional environment, here i am highlighting few very concise references which are available free in public. However several case studies and published books can be referred for more clarity on this subject.

Brief Descriptions of sedimentary Environments can be obtained from the below resources,

• https://australian.museum/learn/minerals/shaping-earth/sedimentary-environments/
• https://en.wikipedia.org/wiki/Depositional_environment#
• https://web.archive.org/web/20090228063253/https://facstaff.gpc.edu/~pgore/geology/historical_lab/sedenvirons.htm

2. Categories of Depositional Environments

Depositional environments are divided into three main categories: terrestrial (on land), transitional (at the margins between land and ocean), and marine. Below table lists the depositional environments in these settings organized according to whether they are in clastic- or carbonate-dominated systems.

3. Factors Influencing Depositional Environments

Several Factors which influence clastic depositional systems asre shown in the below diagram,

4. 3D Block diagrams of Depositional Environments

Several 3D Block diagrams represening differnt sedimentary environments are provided as follows for better understanding can be found below

5. Key Characteristics of Depositional Environments

The table below includes specific environments where various types of sediments are deposited and common rocks, structures, and fossils that aid in deducing the depositional environment from examining a sedimentary rock outcrop.

Below Simplified tables shows what to look for in the rock record in specific Depositional Environment.

From different sources Similar tables provides a summary of the processes and sediment types, associated rocks that pertain to the various depositional environments .

6. Analog Examples of Depositional Environments

Below table provides several depositional environments and their analogue examples

Find below an Excellent read on Shallow-marine Sandstone Reservoirs, Depositional Environments, Stratigraphic Characteristics and Facies Model: A Review

https://scialert.net/fulltext/?doi=jas.2017.212.237

7. Role of Depositional Environment in Hydrocarbon Exploration

Depositional environments favouring the accumulation of petroleum source rocks are illustrated in above figure in the context of a section through a tectonic plate. The amount of kerogen present in sediments is a balance between bioproductivity, survival, and dilution by inorganic grains. we can consider some extremes as below:

1. Low productivity—Aeolian (wind-blown) desert sandstones where land plant growth is restricted due to the extremely arid environment.
2. High productivity—Areas of ocean upwelling where the enhanced supply of nutrients fuels explosive growth of phyto- and zooplankton.
3. Poor survival—Chalk composed almost exclusively of coccolith skeletal debris (high bioproductivity) but where all the coccolith body tissues are destroyed by bacterial activity under strongly oxic open marine conditions.
4. Excellent survival—Early rifting phases of oceans where both optimum sedimentation rates and anoxic conditions of the lakes and enclosed seaways promote high rates of organic matter preservation.
5. Strong dilution—The prolific sediment supply of major deltas produces organic lean delta-front and pro-delta sediments despite high bioproductivity.
6. Minimal dilution—Coals where the lignocellulosic and other tissues of high-productivity land plants are well preserved in a delta-top environment starved of (or bypassed by) mineral grains.

Thus to summarize the optimum oil-prone source rocks are deposited where anoxia develops in an aqueous environment enjoying high rates of sedimentation. The combination of high sedimentation rates and oxic environments favours the accumulation of gas-prone source rocks such as coals.

8. Types of data used to determine the Depositional Environment

One of the initial steps in depositional environment analysis is facies analysis. This involves the description and interpretation of conventional core samples from a clastic reservoir. Core description allows for the subdivision of these cores into lithofacies, which are defined by characteristics such as lithology, grain size, physical and biogenic sedimentary structures, and stratification, all of which relate directly to the depositional processes that created them. Lithofacies and lithofacies associations (groups of related lithofacies) serve as the fundamental units for interpreting depositional environments, Example provided as below,

Below is a simplistic 3D depositional environment model with basic log response for reference,

9. Quick Exercises to test your knowledge

Below are few key resources with examples and exercises to test your knowledge and improve understanding.

• https://open.maricopa.edu/geologylab/chapter/putting-it-all-together-part-two-depositional-environments/
• https://web.archive.org/web/20081202011925/https://facstaff.gpc.edu/~pgore/geology/historical_lab/sedenvirons_exercises.html
• https://web.archive.org/web/20071023190137/https://gpc.edu/~pgore/geology/historical_lab/sedenvexer.html
• https://pressbooks.bccampus.ca/earthhistorylab/chapter/exercise-2-2-identifying-depositional-environments/
• https://pressbooks.bccampus.ca/earthhistorylab/chapter/exercise-2-3-introducing-sedimentary-facies/
• https://pressbooks.bccampus.ca/earthhistorylab/part/lab-2-sedimentary-environments-and-facies/
• Laboratory Manual for Introductory Geology (Page-241 onwards)

References

• “Sedimentary Rocks And Their Environments” | Temple University Press and North Broad Press (manifoldapp.org)
• https://geo.libretexts.org/Bookshelves/Geology/Book%3A_An_Introduction_to_Geology_(Johnson_Affolter_Inkenbrandt_and_Mosher)/05%3A_Weathering_Erosion_and_Sedimentary_Rocks/5.05%3A_Depositional_Environments
• https://www.geological-digressions.com/depositional-systems-and-systems-tracts/
• https://www.sciencedirect.com/topics/earth-and-planetary-sciences/depositional-environment
• https://www.researchgate.net/figure/A-schematic-model-display-the-different-depositional-environment-and-the-the-interpreted_fig8_362078147
• https://opentextbc.ca/physicalgeologyh5p/chapter/depositional-environments-and-sedimentary-basins/
• https://booksite.elsevier.com/9780444528186/htm/reschar_chapter_01/figure_1_18.htm
• https://web.archive.org/web/20090228095248/https://facstaff.gpc.edu/~pgore/geology/historical_lab/environmentchart.htm
• https://pressbooks.bccampus.ca/earthhistorylab/chapter/depositional-environments/
• https://wiki.aapg.org/Lithofacies_and_environmental_analysis_of_clastic_depositional_systems
• Basics--Table of Depositional Environments (wvc.edu)
• Chapter 2: Earth Materials – The Story of Earth: An Observational Guide (pressbooks.pub)
• File:Compositional and Textural Maturity.jpg - Wikimedia Commons

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