Sedimentary Environments, Grain Size, Structures, and Green River Basin Formations

Core concepts: erosion, transport, and deposition

• Erosion, transport, and deposition are the three fundamental processes in sedimentary geology:
  • Erosion: breaks down rocks and detaches sediment from its resting spot. It requires significant energy (often described as needing more “juice”) to move particles from their source.
  • Transportation: the movement of sediment from source toward sinks (e.g., by water, air, or ice). It carries sediment once it’s in motion.
  • Deposition: the settling out and accumulation of sediment when the transporting medium slows down or loses enough energy.
• Energy and grain size:
  • Grain size is a key indicator of the energy of the transporting medium. Higher energy tends to move larger grains; lower energy deposits finer grains.
  • If water velocity slows to below the threshold needed to move a given grain size, that sediment starts to deposit.
• Sedimentary-rock categories (clastic vs chemical):
  • Classic (clastic) rocks are made of physical fragments of rock.
  • Chemical sediments are made from dissolved ions that precipitate from solution.
  • Grain size in classic/clastic rocks helps infer the energy of the depositional environment.
• Applicability across media:
  • The grain-size approach is most often discussed for water-dominated environments but can be adapted for wind-driven settings; wind generally has less energy than water, so deposition of grain sizes can occur at different thresholds.

Sedimentary structures and what they reveal about flow

• Sedimentary structures describe how grains are arranged, not just their size. They help reconstruct flow direction and stability.
• Ripple marks:
  • Formed by flowing water or wind; persist after burial in the rock record.
  • Asymmetric ripples indicate one-directional flow (e.g., a unidirectional current in a river or persistent wind in deserts).
  • Symmetric ripples indicate bidirectional flow (e.g., waves in a shoreface).
• Dunes and dunes-like features:
  • Large-scale counterparts to ripples; shaped by persistent flow and provide clues about energy, depth, and duration of transport.
• Cross-bedding:
  • Occurs when sand is moved by currents and deposited on inclined surfaces; common in sandstone and conglomerate.
  • Indicates paleocurrents and channel migration or dune migration.
• Sedimentary ripples in different settings:
  • Asymmetric ripples form in unidirectional flow (river channels, wind-blown dunes).
  • Symmetric ripples form with bidirectional flow, such as in shallow marine environments affected by waves.
• Sedimentary structures in lakes and rivers: bars, ripple trains, and cross-beds can occur in channels and on floodplains.
• Mudcracks:
  • Form as mud dries and contracts, creating cracks; indicate subaerial exposure and intermittent drying.
  • Particularly common in floodplains or playalike lake surfaces that dry, rather than in continuously submerged marine settings.
• Bioturbation:
  • Disturbance of sediments by living organisms (burrowing animals, footprints, etc.).
  • Term: bioturbation (bio- meaning life, turbation meaning disturbance).
• Lake-specific sedimentary structures:
  • Lakes develop distinctive features such as bar deposits and evaporite sequences, which reflect hydrology, evaporation, and water balance.
• Bar formations in lakes:
  • Lakes can form sedimentary bars that influence local stratigraphy and preserve particular structures.
• Lagerstätten (exceptional fossil sites):
  • Lagerstätten are fossil-rich sites with exceptional preservation of organisms, sometimes including soft tissues; Laney Member is famous for pristine fossils.

Sedimentary rock types and what they tell us about environments

• Classic (clastic) sedimentary rocks:
  • Composed of fragments of other rocks (sandstones, conglomerates, mudstones).
  • Grain size and sorting provide energy and transport mode clues (e.g., river vs wind vs beach).
• Chemical sediments:
  • Precipitated from dissolved ions (e.g., evaporites like gypsum, halite; chemical precipitation in lakes).
  • High evaporation rates with standing water produce evaporite layers.

The Green River Basin formations and their lithology

• Three major formations in the study area (South Wyoming, parts of Utah and Colorado):

  • Green River Formation (one of the three major formations): named after the modern Green River, not necessarily an indicator of its past environment.
  • Wasatch Formation: often contains coarser clastic deposits (sandstone, conglomerate) and red/purple mudstones; abundant fossils; features such as cross-beds.
  • Washakie Formation: diverse sedimentology with red/purple/gray mudstones and variable sandstones; rich in fossils including primates and large mammals; cross-bedding present.

• Within the Green River Formation there are units (members):

  • Wilkins Peak member: dominated by evaporites, especially trona (soda ash).
  • Trona is a major economic resource; approximately $90\%$ of US trona is mined here.
  • Laney (Lainey) member: characterized by oil shale and shale/siltstone alternations; famous for Lagerstätten and exceptional fossils (fish, birds, primates, turtles).
  • Oil shale indicates deeper, calmer, water with low oxygen in bottom waters; extended low-energy conditions that preserved organic-rich sediments.

• Kayaking through the sequence (for context):

  • Wilkins Peak: evaporites indicate high evaporation, shallow to intermittent water bodies; overall low-energy lacustrine setting with strong evaporite deposition.
  • Laney: deeper water, anoxic bottom waters; abundant preserved fossils; lagerstätten; deeper lake environment with continuous sedimentation and good preservation of organic matter.
  • Washakie: mix of sandstones and mudstones; varied fossils including primates; suggests river and floodplain with forests along banks; cross-bedding indicates channel migration and higher-energy fluvial processes.

• A sample lithology summary for quick reference:

  • Wasatch Formation: fairly coarse sandstone, conglomerate; red/purple/yellow mudstones; cross beds; fossiliferous (mammals, primates, rodents, reptiles, birds, insects).
  • Wilkins Peak (Green River Formation): very abundant evaporites (trona); shale and mudstone; minor sandstone; evidence of evaporation-dominated lacustrine conditions.
  • Laney Member: oil shale; shale/siltstone alternations; Lagerstätten; abundant fish and other fossils; deeper, oxygen-poor lake conditions.
  • Washakie Formation: mixed lithologies including cross-bedded sandstones and mudstones; abundant fossils (including primates and large mammals); forested channels and river-related environments.

• Notable environmental interpretations tied to these units:

  • Wilkins Peak represents a shallow, evaporative lake with significant salt deposition; evaporites are indicators of arid to semi-arid conditions with limited inflow and high evaporation rates.
  • Laney represents a deeper lake with low oxygen bottom waters, favorable for preserving delicate fossils (lagerstätten), and allowing organic-rich sedimentation to accumulate.
  • Washakie indicates a fluvial system with a river and floodplain, supporting forests and a diverse vertebrate fauna; cross-bedding suggests migrating channels and high-energy events along the floodplain.
  • Wasatch shows more traditional river-fluvial deposits with coarser clastic rocks and large counts of fossils, reflecting a dynamic landscape with forests and fauna.

• The sequence as a story (environmental change over time in the Green River Basin):

  • Start with a river channel and floodplain system with forests along the banks (Wasatch and related units).
  • Transition to a larger, wetter landscape with increased lake formation (Wilkins Peak evaporites indicate shallower, evaporative lakes).
  • Deep-water lake conditions develop (Laney) with anoxic bottom waters that preserve exceptional fossils.
  • Eventually, the basin evolves toward reduced hydrologic input and drying, but paleontological and sedimentary records preserve a diversified environment that includes continued river systems (Washakie) and forested floodplains.
  • The overall picture is one of long-term environmental change driven by fluctuations in climate, hydrology, and sediment supply.

• Key economic and scientific implications of the Green River Basin study:

  • Trona deposits from Wilkins Peak are economically significant; much of the US trona is mined here and used for glass making, baking soda, cosmetics, soaps, and more.
  • Laney Lagerstätten provide exceptional fossil records, including well-preserved fish, birds, and even primate remains, making this region scientifically invaluable for understanding Miocene ecosystems.
  • The Green River Basin contains a continuous geologic sequence that helps scientists interpret past climate, hydrology, and ecosystem changes across large timescales.

Interpreting rock records: formations, environments, and how to study for exams

• Formations and units:

  • Formations are large rock bodies with distinctive lithology and fossil content that represent particular environments and time intervals.
  • Within formations, rocks can be further subdivided into members (units) that share similar characteristics.
  • Reading the rock record involves tracing lithology, sedimentary structures, fossils, and sedimentary textures to reconstruct ancient environments and their changes over time.

• Example environments and their typical rocks/structures (as discussed in class):

  • River channel (fluvial): coarse sandstone and conglomerate; cross-bedding; large grain sizes; higher energy; possible fossils of terrestrial animals.
  • Floodplain: mudstone, siltstone; red soils indicating ancient soil development (paleosols); occasional cross-beds; evidence of seasonally variable sedimentation.
  • Desert/wind-dominated environment: cross-bedded sandstones; dunes; relatively coarse grains; evaporite associations in very arid cases.
  • Shallow lake: fine mudstone and shale; possible evaporites in arid periods; occasional cross-bedding; muscles for aquatic life preserved in lagerstätten (Laney as a prime example).
  • Deep lake with low oxygen: oil shale and fine silts; excellent fossil preservation due to rapid burial and anoxic bottom waters (Laney).
  • Coastal or shoreline zones: waves and tides produce symmetric ripple patterns and sometimes cross-bedded sands, depending on energy and environment.

• How this relates to exam questions (as hinted in the lecture):

  • You may be asked to identify an environment from a description that includes a few rock types, structures, and fossil content.
  • You may be asked to explain a sequence that represents sea-level or hydrologic changes, using the rock record as evidence.
  • You may need to describe environments with key features: rock type, sedimentary structures, common fossils, and their implications (e.g., forests along a floodplain vs. a desert lake).
  • Expect questions that require short answers (16 questions, multiple choice or one-word responses) and longer, more explanatory responses (6 questions, e.g., paragraph or diagram-based).

• Practical study tips mentioned in class:

  • Review module four materials on depositional environments (lake, river, seafloor, coastline, desert, floodplain, coral reefs).
  • Practice recognizing environments by combining lithology, sedimentary structures, and fossil content.
  • Use the provided prompts to describe an environment in terms of rock type, structure, and fossils.

Quick glossary of key terms from the lecture

• Erosion: wearing away of material from a surface by natural forces.
• Transport: movement of sediment by wind, water, or ice.
• Deposition: accumulation of sediment in a new location.
• Grain size: the diameter of sediment grains; a primary control on depositional energy.
• Clastic (classic) sedimentary rocks: rocks composed of fragments of other rocks.
• Chemical sediment: rocks formed from precipitated minerals dissolved in water.
• Sedimentary structures: features formed during sediment deposition (e.g., ripple marks, cross-bedding, mud cracks, bioturbation).
• Ripple marks: small-scale ridges formed by flowing fluid; asymmetric ripples indicate unidirectional flow, symmetric ripples indicate bidirectional flow.
• Cross-bedding: inclined depositional surfaces formed by migrating dunes or channels; reveals paleocurrents.
• Mud cracks: cracks formed by drying and contraction of mud; indicate subaerial exposure and intermittent drying.
• Bioturbation: disturbance of sediment by living organisms (burrows, footprints).
• Lagerstätten: fossil sites with extraordinarily well-preserved fossils.
• Evaporite: sedimentary rock formed by evaporation of water, leaving behind salts or minerals (e.g., gypsum, halite, trona).
• Trona: a commercially important evaporite mineral used in glass manufacture, baking soda, cosmetics, soaps, etc.; major deposits in Wilkins Peak (Green River Formation).
• Lagerstätten in Laney: exceptional preservation of fish, birds, and other fossils, often with soft-tissue impressions.
• Paleosol: fossil soil preserved in sedimentary rock, indicative of past terrestrial vegetation and soil formation.
• Bar (sedimentary structure): a sandbar feature in fluvial or lacustrine settings that can affect local stratigraphy.
• Formations: large stratigraphic units with recognizable lithology and fossil content; can be subdivided into members.
• members: subdivisions within a formation that share particular characteristics.

Exam-ready recap: core patterns to recognize

• High-energy environments (river channels, dominant currents): coarse sand and gravel, abundant cross-beds, large grains.
• Low-energy lacustrine environments: fine muds and silts, potential oil shales, very slow sedimentation, possibility of anoxic conditions.
• Evaporative environments: evaporites (e.g., trona), high salt precipitation, often in arid basins with limited inflow.
• Paleoclimates inferred from soils and fossils: colorful paleosols and fossil primates suggest forested regions; fossil fish and birds indicate aquatic ecosystems.
• The broader story in the Green River Basin shows dynamic interplay among rivers, floodplains, lakes (shallow to deep), and deserts, driven by climate and hydrologic changes over time.