Sedimentology & Stratigraphy exam 2

2 main processes acting on alluvial fans

Sediment gravity flows (Debris flow, Mud flow, Landslides)

Stream flow (Stream channel deposit, Sheetflood deposit)

Debris flow deposits

Deposits are poorly sorted, lack sedimentary structures except reverse graded bedding, large boulders, low porosity and permeability due to muddy matrix between clasts.

Mudflow deposits

Similar to debris flows. Consist mostly of sand size and smaller particles

Landslides

Rockfalls, slumps, slides and snow avalanches; occur near the top of the fan

Stream channel deposit

Long narrow bodies of poorly sorted, coarse-grained sediments

Sheetflood deposit

This layers of well-sorted sediment (larger particles deposited near the top), cross-bedded, laminated or structureless.

How do sediments change from proximal to distal end of an alluvial fan?

Proximal: steepest slope and coarsest sediments; poorly developed sedimentary structures; clast supported conglomerates within stream channels, gravity flow conglomerates are matrix-supported

Midfan: Intermediate-sized sediments. Mixed debris and stream flow deposits. Coarse-grained conglomerates. Sheet-like deposits of sand and gravel in between channels.

Distal: Sand and silt deposited sheets. Thing conglomerates may be present.

Conditions for braided streams formation

Sporadic discharge

Sparse vegetation

Abundant sediments

Types of bars in braided streams and how they form

Longitudinal bars: forms when streams initially slows down and sediments is dropped.

Lateral bars: Develop in areas with low energy along edges of stream channel

Sediment left behind by braided streams

Gravel, sands, little mud. Deposits fine upward.

What is helical flow?

The back and forth shifting of current.

Helical flow effect on point bars.

Causes point bars to build out into the stream and migrate downstream

Deposits of a channel

Coarse gravel, consolidated mud, waterlogged plant debris.

Deposits of a point bar

Gravel, sand, silt, and mud

Coarser sediments deposited on lower point bar, finer sediments carried up point bar

Deposits of a levee

Ripples and bedded sands, laminated mud

Deposits of a floodplain

Fine-grained sediments, plant debris, deposits resemble bouma sequence.

Deposits of an oxbow lake

Fine silt and mud, plant debris and shells of ostracodes and freshwater mollusks.

Where are deserts found?

Around 30 degrees latitude

Downwind of large mountain ranges

Interiors of large continents

Next to cold ocean currents

How does wind transport sediments?

Traction, saltation, and suspension

Advancing dune field

Lowest: Sand sheet (well sorted, frosted grains, low angle cross beds) (advancing dune fields)

Middle: Dune (Big cross beds shifting directions, frosted grains) (oasis)

Top: Interdune (Evaporite: gypsum, borax, trona, pseudomorphs. Silt: footprints, snails, mollusks) (oasis and evaporation of playa lake)

Factors that lead to dune sediment preservation

High water tables and systems stabilized by vegetation.

Compare and contrast lake and marine environments

Oceans and lakes share the same processes

lakes are low in energy, sedimentation rates are higher, tides are mostly negligible, lakes are very sensitive to climate, water chemistry is affected by climate

How do varves form?

Seasonal freezing of lake surface

Summer: light-colored coarse silts settle out into water

Winter: surface freezes and fine clays and organic matter settle out

Open lakes

Have an outflow of water and a stable shoreline

Siliciclastic sediment dominated (gravel and sand deposited along edges and sand carried to deeper parts)

Coal is commonly found due to plant growth in shallower parts

Closed lake

No major outflow and fluctuating shorelines

Perennial: carbonates muds, silts, sands, stromatolites

Ephemeral: Salt deposits, carbonate deposits such as travertine or tufa

3 main variables that affect the shape of a delta

River (sediment) input

Wave energy

Tidal energy

3 main delta types and how they form

Fluvial dominated: sediment input

Wave dominated: wave energy

Tide dominated: tidal energy

Deltaic environments with sediment and structures (lowest to highest): Prograding Delta

Prodelta: sandstone (cross-beds) and mudstone (laminated and bioturbation)

Delta front: sandstone (slump blocks)

Delta top: sandstone (bioturbation)

Lower delta plain: Sandstone (channelized, cross-beds,) and mudstone (bioturbation)

Upper delta plain: sandstone (cross-beds, channelized) mudstone (bioturbated, root casts)

What are cyclothems?

Cycle where delta lobe will switch back to where it was and deposit fresh sediment over the older delta deposits that sunk.

What are the hypotheses put forth on the formation of barrier islands? Which are likely or unlikely?

Upwards building of offshore bars (likely)

Former spits that have been breached (plausible)

Submergence of mainland ridges during sea level rise (plausible)

Ancient dunes that have been mostly covered during the last sea level rise (unlikely)

Lateral shifting of sand during sea level rise (plausible)

What leads to development of longshore currents?

Waves transport sediment at an angle and then travel out in a straight path.

What leads to the development of rip currents?

Water returning from the beach is forced between sand bars.

Beach environments with sediment and structure

(lowest) Lower shore face: silt (bioturbation, laminated) and sandstone (HCS)

Upper shore face: sandstone (shell fragments, cross-folds, cross-beds, ripples, skolithos)

Foreshore/beach face: sandstone (shell debris, biaxial direct cross-beds, maybe burrows)

(highest) Backshore: Sandstone (big cross-beds, burrows, root casts)

Storm deposits and what they look like

Commonly comprised of a thin basal lag deposit made of shells or rip-up clasts and they are overlain by sands. The sequence is capped by sands with HCS. The top is often bioturbated.

Lagoon

Comprised of interbedded mud, sand and peat deposits. Maybe contain brackish water fossils (oysters)

Tidal Channel

Erosional base or lag deposit of coarse sands or gravel at the base.

Tidal delta

Fine-medium grained cross bedded sands in the lower areas. Flaser bedded fine sands and mud in the middle. Layered muds in the upper part.

3 modes of water circulation within estuaries

Salt-wedge (stratified): present on microtidal coasts. Dominated by river flow.

Partially mixed: Mesotidal coasts so flow doesn’t dominate circulation

Fully mixed: Large tidal range, coupled with strong currents completely breaks down the stratification between the 2 bodies of water

Differences between tidal dominated and wave dominated estuaries

Tidal dominated: Tidal energy exceeds wave energy at mouth. (mud is deposited by flocculation of clays)

Wave dominated: Wave energy exceeds tidal energy at mouth. (mud characterized by planar layers of fine sediment and plant debris)

Lagoon deposits

Deposition is dominated by mud and silt with occasional sand washed into lagoon by storm.

Sands are often cross bedded with sets either dipping landward or seaward. Muds are typically bioturbated

Subtidal deposition

Sands with dunes and ripples, cross bedding and plane bedding

Intertidal deposition

Cross-bedded sands. Sand and mud deposited in between tidal channels.

Supratidal deposition

Mostly mud with mud cracks as a common feature

How to recognize ancient tidal flats?

Bimodal paleocurrent indicators

Tidal rhythmites

Occurrence of channel within muddy, bioturbated sediments

Flaser bedding