Chapter 8: Continental (Terrestrial) Environments

Chapter 8: Continental (Terrestrial) Environments

Primary continental depositional settings:

Fluvial:

Alluvial fan

Braided stream

Meandering stream

Desert

Lacustrine

Glacial

Deltaic (Chapter 9) –marginal marine environments

Alluvial fan: deposits with gross shapes approximating a segment of a cone and exhibiting convex-up cross-sectional profile. Alluvial fans are typically poorly sorted and include abundant gravel-size detritus. Modern alluvial fans are particularly common in areas of high relief, generally at the base of a mountain range, where an abundant supply of sediment is available and vegetation is sparse.

Debris flow deposit: characteristically poorly sorted and lacking in sedimentary structures except possible reverse bedding in their basal parts. They may contain blocks of various sizes, including large boulders, and they are typically impermeable and nonporous owing to their high content of muddy matrix.

Mudflows are similar to debris flows, but contain mainly sand-size and finer sediments.

Landslides often provide source material for debris flows.

Stream flow deposit: often referred to as braided deltas, normally contain 20% sediment. Flows containing 20% to 40% sediment are considered hyperconcentrated.

Sheetflood: broad expanse of unconfined, sediment-laden runoff water moving downslope, commonly produced by catastrophic discharge.

Incised-channel flow: channels 1-4m high, incised into the upper fan, facilitating downslope movement of sediment flows & sheetflows.

River systems

Channel form:

Sinuosity: deviation of a channel from a straight path

Number of channels (single- meandering or multiple- braided and/or anastomosing)

Braided river: has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars. Braided river channels are common wherever a drastic reduction in stream gradient causes the rapid deposition of the stream's sediment load. Braiding occurs at lower slopes and/or discharge as bed material size decreases.

Anastomosed channel: a channel that both branches out (similar to a braided river) and subsequently reconnects (forming a single, meandering channel)

Avulsion describes the process where a stream abandons its old channel in favor of a new channel.

Sinuosity of channels increases with their width/depth for low-powered, single-channel streams, but decreases with width/depth for multiple-channel rivers. Sinuosity also increases for single-channel rivers, with a given discharge and slope, when bed material size decreases.

This discharge was initiated to help redistribute sediment downstream

How might the features you see in this picture compare to what you would observe in a low-gradient river?

Rocky banks and channel

As you continue down stream a channel becomes less rock, changing to gravel, then sand, and eventually fine-grained silt and mud = channel roughness.

Which would you expect to flow faster, the high gradient part of a stream or the low gradient part?

Why?

All streams erode particles from channel beds and banks and carry them downstream.

a. About 5 months after the eruption of Mt. St. Helens this channel of the upper Muddy river had formed in the soft erupted materials.

b. Note the increase in width and depth of the channel 12 months later.

As a stream slows down it drops some of its load. Heavier particles drop out first.

When a stream has a heavy load, sediment is often deposited in the stream channel itself to form bars or islands.

These deposits split the channel into smaller channels forming a braided channel.

As a meander becomes larger the channel migrates across the flood plain in the direction of erosion.

Oxbow Lake Development

Meandering River Systems

Mississippi River

Meandering channel deposition and stratigraphy

Braided channel deposition and stratigraphy

Braided River and Meandering River Deposits

Fig. 8.10

Eolian Desert Systems

Sand sea (erg): desert environment where eolian sands cover more than roughly 125 km2.

Dune field: desert environment where eolian sands cover less than roughly 125 km2.

Draas: Gigantic bedforms that may have wavelengths measured in kilometers and heights up to 400m.

Loess: wind-blown silt and dust that often accumulate far from their original source.

Wind is effective in transporting medium and finer-sized sand (less than 2mm in diameter)Through suspension and saltation. Grains larger than 2mm may undergo rolling and surface creep under high-velocity winds.

Deflation (Desert) pavement: lag deposits consisting of gravel-size particles that are too large to be transported by wind.

Desert subenvironments: Dune, interdune and sand sheet.

Fig. 8.15

Dune Forms

Interdune: a subenvironment bounded by dunes or other eolian deposits such as sand sheets.

Sand sheet: flat to gently undulating bodies of sand that commonly surround dune fields.

Eolian Systems

Dry systems: those in which the water table and its capillary fringes lie at depth below the depositional surface. Therefore, water has no stabilizing effect on the surface or near-surface sediment.

Wet systems: those in which the water table or its capillary fringe is at or near the depositional surface. Therefore, deposition, bypass and erosion along the substrate are controlled by moisture content and aerodynamics.

Stabilized systems: those in which factors such as vegetation, surface cementation, or mud drapes play a significant stabilizing role.

Accumulation space: the vertical space in which sediment accumulates.

Preservation space: the space below the baseline of erosion where sediment is preserved.

Navajo Sandstone

Lacustrine systems

Origin and size of lakes

Lakes can be created by:

Tectonics

Glaciers

Volcanic activity

Wind deflation

Fluvial activity

Modern lake sizes range from a few tens of square meters to the largest (Caspian sea) which is 436,000km2.

Open lakes: those that have an outflow of water and relatively stable shoreline and in which inflow and precipitation are approximately in balance by outflow and evaporation.

Closed lakes: those that do not have a major outflow and have fluctuating shorelines; inflow is commonly exceeded by evaporation and infiltration

Lake sedimentation

Varve: deposits formed in cold-climate lakes where thicker, light-colored, coarse-grained sediments accumulate during the summer months and finer, organic-rich dark sediments accumulate during the winter months, when the surface is covered with ice.

Open lake deposits

Closed lake deposits

Glacial Systems

Glacial environment zones

Basal or subglacial zone: influenced by contact with the glacial bed.

Supraglacial zone: in contact with the upper surface of the glacier.

Ice-contact zone: around the margin of the glacier.

Englacial zone: within the glacial interior.

Proglacial environment: depositional environments around the margins of the glacier that are influenced by the melting ice, but not in direct contact with the ice.

Periglacial environment: the area extending beyond and overlapping the proglacial environment.

Valley glaciers: relatively small ice masses confined within valley walls of mountains.

Piedmont glaciers: large masses or sheets of ice formed at the base of a mountain front where mountain glaciers have debouched from several valleys and coalesced.

Ice sheets: (continental glaciers) huge sheets of ice that spread over large continental area or plateaus.

Glacial moraines:

•Terminal moraine (end moraine): unsorted sediment that has accumulated at the front of the glacier.

•Lateral moraines (marginal moraines): accumulated concentrations of debris carried along the edges of the glacier where ice is in contact with the valley wall.

•Medial moraine: where the lateral moraines from two glaciers join.

•Ground moraine: an evenly distributed sheet of debris at the base of the glacier.

•Recessional moraine: a succession of end moraines deposited when the glacier retreats in pulses.

**See Figure 8.29 of text.

Alpine glacial deposits

Glacial marine sedimentation

Glacial Facies

Till: sediment deposited directly from glaciers to land.

Waterlaid till: glacial sediment carried in meltwaters and deposited in lakes or the ocean

Diamict: poorly-sorted, unconsolidated glacial deposits that cannot be proven to come from glacial sources.

Diamicton: consolidated diamict.

Kames: small mound-shaped accumulations of sand or gravel that form in pockets or crevasses in the ice.

Eskers: narrow, sinuous ridges of sediment oriented parallel to the direction of glacial advance. Eskers are the deposits of meltwater streams that probably flowed through tunnels within the glacier.

Facies of proglacial and periglacial environments

Glaciofluvial Glaciolucustrine

Marine glacial facies

Glacial deposits seen in vertical profile (Fig. 8.35)