Turbidite and Lake Sedimentation Processes Explained

Erosional Base and Graded Sand Units

Erosional base is in the head, and deposition is in the body.

Normal Graded Sand Unit (Ta)

Rapid deposition from suspension.

Planar Stratified Sand Unit (Tb)

Deposition on an upper stage plane bed, indicating flow deceleration, reduction of deposition rate, and establishment of a well-defined bed over which there was both bed load and suspended load movement.

Cross Laminated Ripple Unit (Tc)

Further decrease in flow velocity with an appreciable deposition rate.

The Overlying Planar Laminated Mud Unit (Td)

Deposition from the tail.

The Massive Mud Unit (Te)

Fallout from the turbulent buoyant plume.

Lakes: Palaeoclimate Indicators

Two key features of lakes:

1. They are among our best indicators of continental palaeoclimate.
2. Abrupt variation of sedimentary facies in vertical sequences results from shifting shorelines and biochemical fluctuations in lake waters.

Thermally Stratified Lakes

• Epilimnion: Upper, warm, oxygenated, and circulating layer.
• Hypolimnion: Lower, cooler, and relatively undisturbed region. It may be anoxic, which favors the preservation of organic matter on the lake floor.
• Thermocline: Zone where temperature decreases rapidly with depth.

Lakes which circulate completely to the bottom at the time of winter cooling are termed holomictic. Lakes undergoing only a partial circulation, leaving a permanently stagnant bottom layer, are termed meromictic.

Clastic Sedimentation

Sediment inputs are determined by:

1. Nature and size of the surrounding drainage basins.
2. Seasonal changes in fluvial discharges.

Deltas

The sedimentological characteristics of deltas in lakes are determined by:

• Water Salinity: Salinity contrast between inflow and standing body of water are minimal. This influences the way in which the two waters mix.
• Tidal effects are negligible.
• Wave modification tends to be modest.

Varve Formation

A varve is an annual layer of sediment or sedimentary rock.

The Origin of Varves

Varves occur in calm currents, originating from seasonal temperature changes. The white varves are associated with diatoms, and the beige ones with terrigenous material.

Delta and Estuary Systems

Delta

Sediment derived from the river that feeds it. Characterized by regressive depositional systems.

Estuaries

Sediment derived from the marine and the fluvial realm. Characterized by transgressive depositional systems.

• Hypopycnal: Density of outflows of the river is less dense than the basin.
• Homopycnal: Outflow has negligible density contrasts.
• Hyperpycnal: Outflow is denser than basin water.

Delta Environments

Environments include Prodelta, Delta front, and Delta plain.

Delta Plain

Mostly subaerial part of the delta complex (dominated by fluvial processes). Contains distributary channels.

Delta Front

Shallow subtidal part of the delta above wave base (mix of fluvial and wave/tide processes).

Prodelta

Deeper subtidal part of the delta below wave base.

Deltaic Sequences

Features include coarsening- and thickening-upward sequences. The distributary trunk channels erosively overlay the mound bar deposits. Relevant surfaces exist for sequence stratigraphy.