Sedimentary Environments

 Facies

A body of rock with specific characteristics. Defined on the basis of colour, bedding, composition, texture, fossils and sedimentary structures.

   Facies Sequences

A sequence of facies as the pass from one to another. The sequence can have an abrupt or erosive boundary or a hiatus. In this case there is a coarsening upward sequence or a fining upward sequence. This is important because grain size is an indication of the hydraulic power with which the grain is deposited. An upward coarsening of the particle indicates increased flow an upward fining would indicate low flow.

              Continental Environments

   Alluvial Fans

Localized and their shape approximates a cone. They develop in regions of high relief where there is an abundant supply of sediment.  They develop in humid environments but are best known in and regions where erosional process are NOT as active.

The size of the basin receiving the sediment determines the size of the fan. Most fans have an upward concave profile.

Lithology – matrix and clast supported conglomerates texturally and compositionally immature argillaceous sandstones. Often red beds.

Structures – generally massive some ross-bedding and imbrication in the conglomerates.

   Braided Stream Environments

Areas where river flow diverges and merges. The pattern is as a result of bars that develop and split the river flow.

Longitudinal bars – channel shaped

Curved bars – usually extension and modification to the flanks of longitudinal bars.

Transverse bars – common in sandy low sinuosity streams.

Crevase splay deposits are thin sandstones within floodplain claystones and siltstones deposited by meandering rivers that have breached its banks.

Meandering rivers tend to stay in narrow meander belts building facies of point bar and flood plain. This produces a ribbon like geometry of sand stone bodies. The belts can change course by a process known as avulsion.

Lithology – coarse sand and conglomerates often red with NO organic material. Sandstones Arkosis and/or lithc. Thin intraformational conglomerates are common.

Structures – tabular and trough cross-stratification caused by migration of bars sand waves and dunes. Imbrication is common. Minor cross lamination caused by ripple migration in a abandoned channel or in near full channel.

   Meandering River Deposits

More developed distribution of channel processes and greater distinction between channel and over bank deposits than braided streams. They occupy only a small portion of the flood plains at a time. Point bars have approximately horizontal surface at about the same level as the flood plains. Scroll bars are ridges of sand that develop some distance down the point bar surface and are elongated approximately parallel to the contours of the surface.

A fining upward sequence and are cross bedded with upward reduction of set size.

Lithology – medium to fine sandstones claystones and siltstones with a roughly 505/50 ratio of sandstones to mudrocks. Contains conglomerates and generally arkosic, caliche may be present.

Structures – channel surfaces, flood plain deposits may have thin crevasse.

              Eolian Facies

Sediments that are reworked and laid down through the action of wind currents. Ancient eolian sediments are composed primarily of sand. Usually results in the deposition of sand that is lean and relatively homogenous in size.

Lithology – fine to coarse sandstones well sorted well rounded. Generally quartz areite. Sharp differences in grain size between laminae. May contain minor claystone deposits.

Structures – dominated by large scale cross-bedding. May show mud flakes or adhesion ripples. Often have intraformational unconformities.

              Barrier Islands & Near Shore Environments

Usually long narrow sand bodies that occur within deltas along deltas and in oceanic an lacustrine environments with no connection to deltas. More likely to form when there is a steady supply of sand to the coast the depositional basin has a limited tidal range and the coastal environmental is stable with a low gradient. They are formed by wave processes.

Backshore – above mean high tide with low angle land and diping laminae maybe eolian dune at top.

Foreshore – between high and low tide swash zone. Low angle seaward dipping laminae. Heavy mineral concentrations.

Shoreface – between low water mark and fair weather wave base. Structurally varied with low angle seaward dipping lamination dune crossbedding. Abundant bio-turbidation.

Upper Offshore – below fair weather base but above storm weather base. Interbedded thin sandstones and claystones may be present. Maybe intense bio-turbidation.

              Deltaic Environments

Distinct extensions from the shoreline where rivers enter the marine or freshwater depositional basins and supply sediments faster than the basin can redistribute and process them. The general morphology is dependent on the type of sediment, rate of deposition, hydraulic gradient, the energy and the flow of the basins currents.

   Delta Models

  1. Highly constructive deltas formed by fluvial processes
    1. Lobate
    2. Birdsfoot
  2. Highly destructive deltas dominated by basinal processes
    1. Wave dominated
    2. Tide dominated

   Fluvial Dominated Deltas

Unidirectional flow patterns. Highly sinuous patterns are common however braided patterns can be developed. They resemble alluvial channel deposits with erosive basal sequences. Overall fining upward.

They tend to be defined by flood generated events:

  1. Overbank flooding
  2. Crevasse splays. Sediments deposited over a small area.

   Tide Dominated Deltas

Prominent in medium to high tidal ranges. During high sea level marine waters can be trapped in the delta plain. As the tide drops the currents will predominate as the waves escape.

   Wave Dominated Deltas

Distribute the sediment o the delta front. This results in a regular beach shoreline, with a minor protrusion at the month and a steep delta front.

       Growth Faults in Alluvially Dominated Deltas

Results from uneven compaction and found in deltas with high shale to sand ratio. The over pressured clays are abnormally high in porosity and relatively plastic when compared to adjacent sediments.

              Submarine Fans and Turbidites

Turbidity currents deposits deposit sediments these currents are rolling turbulent masses of flow that have dispersed sediments in them. They remain in suspension because of the hydraulic energy in the flow. They are usually deposited on the upper surface of continental slopes, eventually the deposited material will lose its cohesive strength and the surrounding waters will churn the sediments producing the turbide flow. Heavier sediments settle first, followed by smaller grains and then clays.

Distal Fan Turbidites – very thin with no scour marks.

Proximal Turbidites – can have abundant thick lenticular channel fill sand inter-bedded with more classic turbidites.

Lithology – turbidites are transported in chaotic flow regimes

   Exploration Considerations

Turbidite deposits are of recent interest in exploration because they are rich in hydrocarbons. Oil migration will be toward the fan apex. An anticline may develop seaward and could entrap large oil reservoirs.

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