Geology of Estuaries and Estuarine Deposits

Lecture 9 - Estuaries and Estuarine Deposits

Overview

  • Focus on the nature and characteristics of estuaries, the differences between estuaries and deltas, and the role of sea level in shaping estuarine environments.

  • This lecture references the work of Reynolds (2017) and Boyd et al. (1992).

1. Introduction to Estuaries

  • Defining Characteristics of Estuaries

    • An estuary is a coastal waterbody where freshwater from rivers meets and mixes with saltwater from the ocean.

    • Estuaries typically form within pre-existing valleys or channels that are flooded during coastal transgression.

    • Requires:

    • A rise in sea level

    • Presence of a confining channel or valley (thus characterized as ephemeral features)

    • Eventually, estuaries fill with sediment over time.

2. Differences Between Estuaries and Deltas

  • Deltas versus Estuaries

    • Deltas protrude from coasts due to excess sediment supply relative to accommodation space.

    • Estuaries are typically confined to drowned river valleys where sediment is not the dominant aspect, unlike deltas which have a strong connection to sediment supply from rivers.

    • Two End-Member Types of Estuaries:

    • Wave-dominated estuaries

    • Tide-dominated estuaries

    • Coastal regions with limited sediment do not develop deltas but instead feature:

    • Coastal lagoons

    • Tidal flats

    • In estuaries, due to rising sea levels, the accommodation space generally exceeds sediment supply leading to gradual filling of the valley.

3. Estuarine Formation and Sedimentation

  • Formation Process:

    1. Existing fluvial system is buried due to shoreline transgression.

    2. Fine-grained sediments of the central lagoon are formed, often protected by wave-dominated barrier bars.

    3. River sand can eventually fill the area as it builds outward from the estuary mouth into the sea.

  • Temporary Nature of Estuaries:

    • They exist mainly during transgression when sediment fills the available accommodation space caused by sea-level rise.

4. Sea Level Change and Estuaries

  • Impact of Sea Level on Estuary Developments

    • Many estuaries are formed in flooded shelf valleys created during lowstands of sea level.

    • Valued for their high preservation potential.

    • As sea level rises, sediment supply rates decline, resulting in marine waters flooding these valleys to create estuaries.

    • Preservation Potential:

    • Estuaries situated in incised valleys are more likely to be preserved than other coastal features.

5. Preservation and Erosion in Estuaries

  • Ravinement (Scouring):

    • This is an erosion process caused by waves and tides during transgression, where up to 15 meters of stratigraphy can be removed.

    • Estuarine deposits typically provide a good record of transgressions despite the erosive effects of ravinement.

    • Transgressive Erosion:

    • Occurs when the substrate is eroded creating flat surfaces as a result of continuous marine transgression.

6. Modern Estuary Examples

  • Noteworthy modern estuaries include:

    • Mobile Bay

    • New Orleans

    • Galveston

    • Destin

    • Pensacola, among others in the Gulf of Mexico.

7. Estuaries and Incised Valley Fills

  • Incised Valleys:

    • Modelled under conditions of sea level fall (Greene et al., 2007).

    • Estuaries can contain bayhead deltas at their landward end which are formed by backfilling as the sea floods the valleys again.

    • For example, Mississippi Outer Shelf shows similarities with the existing systems due to backstepping and transgression event patterns.

8. Energy Distribution in Estuaries

  • Three Parts of an Estuary (after Dalrymple et al., 1992):

    • Outer Zone: Marine-dominated, with headward net bedload transport.

    • Central Zone: Low-energy environment facilitating net bedload convergence.

    • Inner Zone: River-dominated, influenced by marine conditions, net transport is seaward.

  • The degree of these zones depends on sediment availability and estuarine evolution stages.

9. Estuarine Facies

  • Important facies within estuaries include:

    1. Paired Mud Drapes: Show tidal influence due to slack-water deposition.

    2. Restricted Brackish Water Fauna: Indicative of specific ecological conditions.

    3. Inclined Heterolithic Stratification (IHS): Characteristic of sediment dynamics shaped by tides and river influence.

    4. Tidal Fluvial Deposits: Show evidence of mud-dominated environments with limited biodiversity in certain areas.

10. Definition of Terms and Processes

  • Inclined Heterolithic Stratification (IHS):

    • Interbedding of sand and mud deposits indicating original depositional dips, typical for tidal rivers and estuaries, formed by lateral growth of large bedforms.

11. Tidal Sedimentary Structures

  • Mud Drapes:

    • Form in tidal conditions due to the settling of mud during slack water periods, often in conjunction with wider sediment structures.

12. Sea Level Rise Implications

  • When sea level rises, it may influence the stratigraphy of estuaries and coastal environments, leading to a range of sediment deposit types reflective of changing conditions over time.

13. Conclusion

  • Understanding estuaries is crucial as they play a significant role in coastal dynamics, serve as indicators of sea-level changes, and contribute to sedimentary records critical to geological and environmental studies.

  • The stratigraphy of estuaries, marked by erosion and sedimentation histories, can provide insights into past climate and sea-level variations.