L6 - Deserts and Lakes

Understanding Past Depositional Environments

  • Recent lectures focus on piecing together geological history based on sediment analysis, which involves studying layers of sediment deposits to infer the conditions that led to their formation. This analysis helps reconstruct ancient environments, telling a story of Earth's climatic and tectonic past.

  • Key locations for observation, like Westbury on Thames, provide insights into past paleo-environments due to their well-preserved sedimentary records. Such sites often exhibit diverse facies that reveal a wealth of information about historical climate changes, sediment transport mechanisms, and ecological conditions of past eras.

Focus Areas: Desertic and Lacustrine Environments

  • Today’s lecture concentrates on different types of deserts followed by discussions on lakes. Deserts, defined primarily by their low precipitation levels, can offer a glimpse into Earth's arid historical conditions, while lakes, even in desert regions, play a vital role in understanding past water bodies and climate fluctuations.

  • Explore the relationship between modern desert types and historic continental configurations, emphasizing how tectonic movements, atmospheric conditions, and geographical features have shaped current landscapes and influenced sedimentation patterns.

Fluid Dynamics in Sediment Transportation

  • Fluids, such as water and air, are essential in transporting grains across various landscapes. Their dynamic properties affect how sediments are moved, sorted, and deposited, playing a key role in shaping geological features.

  • The structural features of landscapes, including dunes, river channels, and lake beds, reveal much about the transportation process and the historical context in which these features were formed, informing predictions about future geological developments.

Facies Analysis

  • Definition: A key concept in sedimentary geology defining rock bodies with specific characteristics that reflect their conditions of formation. Understanding facies is crucial for interpreting the geological history of an area.

  • Importance of integrating multiple strands of evidence:

  • Sediment Composition: Examining the type of grains helps determine geological processes, such as erosion, transportation, and deposition rates.

  • Sedimentary Structures: Identifying features like bedding, cross-bedding, and ripples indicates past flow patterns and energy conditions in the depositional environment.

  • Fossils: Both body fossils (remains of organisms) and trace fossils (evidence of organism activity such as tracks or burrows) are crucial for interpreting past life and environmental conditions.

  • Contextual Information: Assessing surrounding rock types and features provides a broader understanding of the geological history and paleoecology of an area, allowing for more accurate reconstructions of past environments.

Researching Desert Environments

  • Deserts are defined by precipitation levels of less than 250 mm/year, although they can exhibit a wide range of temperatures and ecological systems. They are characterized by extreme conditions that affect both the biological and physical processes occurring within them.

  • Can include both hot and cold deserts, with Antarctica being a prime example of a cold desert, highlighting the diversity of desert environments beyond typically warm and sandy locales.

  • Sediment exposure due to vegetation absence is a hallmark of desert landscapes, enabling significant wind erosion and aeolian transport, which shape the topography and sedimentary structures found in these regions.

Types of Deserts

  • Subtropical Deserts: Created by dry air descending in subtropic regions, leading to formations like the Sahara, where high temperatures and minimal rainfall dominate the environment.

  • Rain Shadow Deserts: These mountains create rain shadow effects, causing deserts to form on the leeward side; notable examples include regions like Patagonia and Tibet.

  • Coastal Deserts: These deserts develop due to cooler air from oceans that inhibits moisture retention, such as the Namibian desert, which exhibits unique ecosystems due to its marine proximity. Onshore winds wick away moisture

  • Interior Deserts: Formed in the interior of large landmasses where moisture fails to penetrate, resulting in stark and arid landscapes, exemplified by the Gobi desert.

  • Polar Deserts: These regions maintain low precipitation levels, significantly influenced by wind patterns; Antarctica stands out as the largest desert, illustrating the varied nature of desert classifications.

Wind Impact on Sediments

  • Grain Transport Mechanisms: Saltation, where grains bounce along the surface, and suspension, which involves lighter grains being lifted into the air, are affected by wind speeds, which are critical for transferring materials across desert landscapes.

  • Implications for sediment structure: Large dunes form in desert environments, resulting in distinct sedimentary deposits in the geological record that elucidate the processes at play in these arid systems.

  • Characteristic deposits include well-sorted quartz sand, indicative of aeolian processes, which can serve as a record of prevailing wind directions and strengths throughout geological history.

Geological Record and Fossils

  • Fossils within sedimentary records provide significant evidence of past environments and climate conditions, helping to paint a picture of Earth’s history.

  • Desert signatures, often characterized by rounded grains, frosted appearances, and varied sediment structures, can be diagnostic tools in understanding sedimentary processes and past climatic conditions.

Lakes

Balance of evaporation vs inflow is important in determining lake levels and chemistry, like for determining salinity

  • Hydrologically closed

    • Supply of dissolved ions is high and evaporation is also high, evaporation makes saline waters

  • Hydrologically open

    • Supply balances outflow, so lake level remains constant and fresh

Hydrology of freshwater lakes

Below 10-20 m, the lake is unaffected by wind driven waves and thermal stratification is important. Warm water lies above cold water, so stratification is stable, but this bottom water is unmixed due to lack of circulation therefore the bottom water is anoxic.

Fine, low density sediment from rivers ‘overflow’ into the warm, oxic (epilimnion) section of the lake and rains down on top of the ‘underflow’ in the cold, anoxic section (hypolimnion) which is high density sediments deposited as turbidite

Types of Lake Sediment

  • Siliciclastic sediment - Detrital grains supplies by rivers and streams

  • Biochemical sedimentation (when terrigenous input is low) - carbonates produced biologically and through direct chemical precipitation, evaporite minerals

  • Organic production - freshwater photosynthetic organisms (diatoms and coccolithofores)

Mixtures in Lake sediments

  • Clast dominated system

    • Similar to marine environment, beaches, deltas, deepwater turbidites and suspended sediment settling

  • Clast starved freshwater system

    • Carbonate production either organically through bacteria or inorganically precipitated, or from breakdown of calcareous algae or shells

Within deeper parts of the lake, laminations are preserved because of a lack of benthic life due to the anoxic nature of the hypolimnion, meaning no bioturbation. There are also no currents or waves

Laminations as a result of seasonal processes (dark and light bands) are called varves and occur as a result of organic matter deposition

Lake Formations in Desert Areas

  • Lakes more commonly form in desert environments than in humid regions, often resulting in unique geological formations and stratigraphy that reflect variable hydrological conditions, such as evaporation rates and seasonal changes.

  • They can stratify into layers based on seasonal and chemical variations, which are essential for reconstructing past climates and understanding ecological dynamics.

  • Anoxic layers in sediment profiles could suggest prolonged periods of stability without disturbance, thereby preserving fine sediment layers that can be studied for changes over time.

Sediment Characteristics of Lakes

  • Lakes generally have diverse chemical compositions influenced by inflows of freshwater, evaporation impacts, and biological activity within the lake, making them key sites for sedimentological studies.

  • Distinguishing factors, such as the presence of biogenic sediments or evaporite formations, can reveal vital information about the environmental conditions prevalent at the time of deposition.

Ephemeral Lakes and Sediment Dynamics

  • Ephemeral or playa lakes: These lakes depend on seasonal water changes and can have distinct sedimentary records reflecting desiccation processes, mineral precipitates, and varying climatic conditions over time.