Sed/Strat Exam 2

What are facies? What three elements are required for a facies? 

Physical, chemical, and biological characteristics to determine the deformational environment. Composted of glacial, alluvial, sandstone, and ichno.THE PROCESS 

Lithology (grain size, composition, color, texture), sedimentary structure (bedding, lamination, ripples), Clasts/fossils (ichnofossils, burrows, trace fossils, concretions) 

Together, these describe depositional processes and environment 

What do lithofacies tell you about? What do their associations tell you? 

Inform you of the process, a hierarchy of surfaces: bounding on separate surfaces are associations. E.g. fluvial channel + levee + floodplain = river system 

Components of lithofacies code: 

Commonly, three parts are used to make a facies code (after Miall, 1978): 

1. Grain size / lithology letter 

1. G = gravel, S = sand, F = fine-grained (silt/mud), M = mud 

2. Support or structure subscript 

1. m = matrix-supported, c = clast-supported, h = horizontal bedding, t = trough cross-bedding, p = planar cross-bedding, l = laminated 

3. Additional modifier(s) (optional) 

1. d = deformation, r = ripple lamination, f = massive (featureless), etc. 

Examples of common lithofacies codes: 

Gmm=massive, matrix-supported gravel -> glacial 

Gcm= clast supported massive gravel -> channel lag 

Sm = massive sand (rapid deposition) -> channel fill or turbidity flow 

Sh = horizontally bedded sand -> upper flow regime 

St = trough cross-bedded sand -> dune migration or fluvial channel 

Fl= laminated fine silt/mud -> floodplain or distal marine 

P = paleosol (soil horizon) -> subaerial exposure, climatic changes 

5 key elements to include in a stratigraphic column 

1. Verical scale 

2. Lithology 

3. Sedimentary structure 

4. Facies codes and interpretations 

5. Clasts, structures, fossils 

What are the elements of a good field sketch 

1. Scale 

2. Orientation 

3. Labels for lithology, structures, and contancts 

4. Bedding, strike/dip 

5. Annotations 

6. Title, location, and date 

What is the geometry of an alluvial fan? 

Cone, concave up cross section, thickest at the apex, thinning basinward 

Processes and facies across an alluvial fan 

Why: flow moves outward, loss of energy. There is coarser, poorly sorted debris at the apex 

Flow type near the apex 

• Near the apex: Flow is laminar or transitional because of: 

• High sediment concentration 

• High viscosity of debris flows 

• Rapid deposition and internal shear, not turbulence 

• Resulting deposits: 

• Massive, matrix-supported diamict (Gmm) 

• Poor sorting 

• Angular clasts 

• No internal stratification 

What would a stratigraphic column of a prograding alluvial fan look like? 

Coarsens upward: laminated clays and silts to cross bedded sands and gravels to massive debris flow gravels 

What are the properties used to classify a river system? 

1. Deviation from a straight path,  

2. Channels, 

3. Subdivisions based on bedforms 

3 types: Braided, meandering, and anastomosing 

What is a lateral accretion surface and how does it form? 

Low angle curved surface with point bar deposits of a meandering river. As water erodes the outer bank (cut bank) and deposits on the inner bank (point bar), sediment builds obliquely to the main bedding direction. 

Define thalweg and describe its relationship with sinuosity 

Thalweg: The deepest part of the river channel, which follows the path of maximum flow velocity.  

Relationship to sinuosity: In straight channels, thalweg runs near the center, in meandering rivers, thalweg shifts side-to-side, which drives channel migration and point-bar formation.  

REVIEW STRATIGRAPHIC COLUMNS OF RIVER TYPES 

Difference between a delta and estuary development: 

Rise: Estuary –> sand rich, typical of transgressive coastline RETROGRADE 

Fall: Delta -> river can advance toward the ocean 

What are the three delta types and their controlling processes? 

1. River dominated: fluvial outflow > waves and tides = birds foot shape, Mississippi 

2. Wave dominated: wave reworking > fluvial output = smooth, acruate coastline, Nile 

3. Tide dominated: Tidal currents > waves or river = mouth bars, Ganges 

What is a Gilbert Delta? 

The material density is greater than the body of water it is entering. Turbidity currents stack. The often form in lakes where river water is of the same density.  

Be able to draw a stratigraphic column for a prograding delta (river dominated) 

How do lakes form?  

Tectonics, glacial, volcanic, fluvial, deflation 

What is the thermocline, and why does it form in lakes? 

A transition layer in a thermally stratified body of water that separates zones of widely different temperature. It is a layer of rapid temperature change in warm surface water, to cold, dense bottom water. It controls oxygen levels, nutrient cycling, and sediment deposition. Creates alternations of organic rich deposits formed in winter months.Leads to lagerstatten 

What is the deepest and oldest freshwater lake on Earth, and how did it form? 

Lake Baikal in Russia formed 30 million years ago. It is 1,600 m deep, as a result of continental rifting. Holds 20% of Earth’s unfrozen freshwater. 

Describe the relationships between precipitation, evaporation, and accommodation space and the three different types of lakes based on these relationships 

P>E = overfilled -> fluvial  

P=E= balanced -> fluvial-lacustrine fluctuations 

P<E = underfilled -> evaporative 

How do varves form and what might they inform about climate 

• Varves = annual couplets of light (summer) and dark (winter) sediment layers in lake deposits. 

• Summer layer: coarser, lighter silt and sand — from runoff and higher energy. 

• Winter layer: fine, darker clay and organic matter — from calm conditions and low energy. 

• Significance: 

• Represent annual deposition → used to determine seasonality and sedimentation rates. 

• Variations in varve thickness indicate climate changes (e.g., warmer years = thicker summer layers). 

When was the Eocene Epoch? Why where there so many lakes in the Rockies in this time? What do the Laggerstatten of the Green River Formation tell you about the pale-lake environment? 

56-33 MYA. Laramide orogeny created accommodation space for lakes. The subduction plate rolled back. The warm humid climate created overfill. Mixed aquatic and terrestrial input to freshwater basins.  

What are three lake types and 3 specific facies found with each 

1. Balanced: Fluvial –lacustrine 

1. Troughs, bars, root casts, lamination, mudstone/limestones 

2. Overfilled: Fluctuating/Profundal 

1. Distinct shoaling cycles, mudstone, siltstone, sandstone, fine lamination, mudcracks 

3. Underfilled: Evaporational 

1. Wet-dry cycles, evaporites,mudstones, eolinite, mudcracks 

Define bioherm, biostrome, and carbonate mound 

Bioherm: lens or moundlike body of organic reef 

Biostrome: bedded, tabular body of carbonate rock 

Mound: non-framework, microbial or micritic 

Primary Reef builders 

Four properties that make a good reef-builder 

1. Longer lifespan and larger size 

2. Increased regenerative ability 

3. Indeterminate growth 

4. Adaptability to different environments 

Reef Sub Environment and Dunham Facies Classification: 

Typical cross section of a reef:  

From ocean to lagoon:  

Fore-reef->crest/core->flat->back reef 

Fore: rudstone, floatstone 

Reef core: boundstone / bafflestone 

Reef flat: bindstone, algae 

Lagoon Packstone-wackestone (fine-grained carbonates) 

🌊 1. Broad Classifications of Coastlines 

➡ In short: 

• Erosional coasts = losing land (high wave energy, limited sediment supply). 

• Depositional coasts = gaining land (high sediment input, strong tidal or wave reworking). 

🌊 2. Four Main Types of Flow in Marginal Marine Environments 

🪸 3. Cross Section of a Marginal Marine Environment 

Here’s what a schematic cross-section (from land to sea) would include: 

Land  →  Delta / Estuary  →  Tidal Flat  →  Lagoon / Shelf 
|           |                   |                 | 
|           |                   |                 | 
Fluvial → Mixed → Tidal → Wave-dominated 
 

🌊 4. Rock Record and Sea-Level Change (Walther’s Law) 

➡ Walther’s Law: Facies that form next to each other laterally will succeed one another vertically if sedimentation is continuous. 

🌊 5. Typical Tidal Sedimentary Features 

🧭 Summary of Key Relationships 

• Erosional vs Depositional coastlines → energy & sediment balance 

• Four flows → define dominant sedimentary structures 

• Cross section → lateral transitions follow Walther’s Law 

• Sea-level rise/fall → creates distinctive vertical facies patterns 

• Tidal features → diagnostic of alternating bidirectional flow 

Sedimentary deposits produced in glacial systems: 

2. Primary sedimentary processes controlling deposits 

3. Sorting and depositional environment 

• Poorly sorted sediments (e.g., till, diamictite) form from direct ice deposition, where all grain sizes are mixed together with little transport sorting. 

• Well-sorted sediments (e.g., sand, laminated silt/clay) form from meltwater or marine suspension settling, where hydraulic processes separate grains by size. 

Interpretation: 

• Poor sorting → direct ice deposition (subglacial or grounding zone) 

• Good sorting → meltwater- or current-dominated environments (outwash, distal glaciomarine) 

4. Sedimentary evidence for identifying a glacial till (diamict) 

Key diagnostic features: 

• Matrix-supported texture (clasts “floating” in muddy matrix) 

• Angular to subrounded, faceted, and striated clasts 

• Lack of bedding or grading (massive structure) 

• Compaction and shear deformation 

• Association with glacial erosional surfaces or striated pavements 

5. Terrigenous sediment trend (subglacial → open marine) 

• Subglacial: Dominated by coarse, terrigenous material (till, gravel, sand). 

• Grounding zone: Still largely terrigenous, but with more fine-grained muds. 

• Sub–ice shelf: Mix of terrigenous mud + biogenic fine sediment. 

• Open marine: Mostly fine-grained hemipelagic or biogenic sediment with low terrigenous input. 

👉 Trend: Coarse → Fine; Terrigenous → Biogenic with distance from ice. 

6. Stacking of tillite–diatomite couplets and climate interpretation 

• Tillite (glacial diamictite): Represents glacial advance and cold conditions (ice at sea level, terrigenous input). 

• Diatomite (biogenic sediment): Represents glacial retreat / interglacial conditions — open water, high productivity. 

• Stacking of these couplets: 

• Alternating tillite–diatomite sequences = glacial–interglacial cycles 

• The thickness and frequency of couplets can indicate duration and intensity of climate fluctuations (e.g., Milankovitch-scale cycles) 

7. Stratigraphic columns 

(Described; you can sketch from these descriptions) 

a) Glacial Retreat (ice margin moving landward) 

• Bottom → Top: 

• Basal lodgement till / diamictite (massive, striated clasts) 

• Overlain by stratified sand and gravel (meltwater outwash) 

• Grading upward into laminated silts and clays (varves) 

• Capped by diatomaceous muds (open marine, ice-free) 
  → Shows decreasing grain size, increasing biogenic content = warming & retreat. 

b) Glacial Advance (ice margin moving seaward) 

• Bottom → Top: 

• Open marine diatomite / mud (biogenic, laminated) 

• Dropstone-bearing glaciomarine mud 

• Deformation structures (shear from advancing ice) 

• Lodgement till / diamictite (massive, poorly sorted, striated clasts) 
  → Coarsening upward and loss of biogenic material = cooling & ice advance. 

4 components of a continental margin 

2. Sedimentary Structures Indicative of Shelf Processes 

🪸 3. Structural Features Forming Topographic Highs Along Shelf Edge 

1. Reef or carbonate buildups – biologically constructed highs (bioherms, carbonate platforms). 

2. Submarine ridges / sand ridges – formed by tidal or storm currents along shelf edge. 

3. Salt diapirs or structural uplifts – due to halokinesis or tectonic compression/uplift. 

Other examples: shelf-edge deltas or contourite drifts (sediment mounds from deep ocean currents). 

🌊 4. What is a Geostrophic Current? 

A geostrophic current is a large-scale ocean current in which the Coriolis force (from Earth’s rotation) balances the horizontal pressure gradient. 

• Flow runs parallel to lines of constant pressure (isobars) rather than directly downslope. 

• Common along continental slopes and shelves where currents (e.g., the Gulf Stream) move sediments as contourites. 

Formula (qualitatively): 
Pressure gradient force ↔ Coriolis force ⇒ stable, steady flow. 

🌪 5. The “Perfect Storm” – Three Processes Raising Sea Level 

A “Perfect Storm” that devastates coastlines combines three additive processes that raise local sea level: 

➡ Combined, these produce catastrophic coastal flooding — especially on gently sloping shelves or during high tide. 

🌊 6. Hummocky Cross-Stratification (HCS) 

Definition: 
A sedimentary structure consisting of gently curved, concave-up and convex-up laminae formed by storm wave oscillations and combined flow (oscillatory + unidirectional). 

Formation Process: 

1. A storm generates oscillatory waves that reach below fair-weather wave base. 

2. Combined flows (oscillatory + weak unidirectional return flow) create alternating laminae of sand and silt. 

3. After storm wanes, finer sediments settle out, forming low-angle cross-lamination. 

Depth / Setting: 

• Found between fair-weather wave base (~5–10 m) and storm wave base (~200 m) — i.e., middle to outer shelf. 

• Indicates storm-dominated shelf environment (tempestite deposits). 

🧭 Summary of Transport Mechanisms 

1. Gravity flows (turbidity currents) → rapid downslope movement of shelf sediments. 

2. Thermohaline / geostrophic currents → long-distance transport parallel to slope (contourites). 

3. Biogenic rain → slow, vertical settling of skeletal remains. 

4. Aeolian dust input → fine-grained clays settling slowly through the water column. 

🪨 2. Turbidites vs. Contourites 

🌊 3. Morphology of Turbidite Fans & Source Material 

Submarine fans form when turbidity currents exit submarine canyons and spread out over the continental rise or abyssal plain. 
Their geometry depends strongly on grain size and sediment supply: 

➡ Why this matters for petroleum systems: 
Submarine fan turbidites often form reservoir rocks, capped by mud-rich levees or pelagic drapes that act as seals. The fan geometry controls reservoir continuity and connectivity in the subsurface. 

🧬 4. Biogenic Sediments (Oozes) 

Example: 

• Coccoliths → form chalk (fine white limestone) 

• Diatoms → form diatomite, later silicified into chert 

🧱 5. Red Clay Deposits 

➡ Form in areas far from land and below CCD (no carbonate preservation). 

🧭 6. Summary Table