Chapter 4

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21 févr. 2014 (il y a 3 années et 6 mois)

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Chapter 4

Sedimentary Structures

Bed:

Tabular or lenticular layers of sedimentary rock that have lithologic, textural,
or structural unity that clearly distinguishes them form strata above and below.

Bedding planes (Bounding planes):

The upper and lower surfaces of beds.

Sedimentation units:

thickness of sediments deposited under essentially constant
physical conditions.

Laminae:

layers less than 1 cm thick.

Subdivision
: an informal unit arising from distinctive associations of sedimentary
structures such as plane or ripple laminae.

Lens
: Lenticular shaped deposit of different composition, texture, cementation or
color than that of the surrounding material.

Band
: Similar to lens but a intermittent, linear deposit that differs from the
surrounding material.

Amalgamation surface
: a marked discontinuity between two beds of similar
composition.

Amalgamated bed
: a bed separated by an amalgamation surface.

Layer
: an informal reference to any bed or stratum of rock.

Terms:


Beds


Bedding planes


Sedimentation Units


Laminae


Subdivision


Lens


Band


Amalgamation surface


Amalgamated bed


Layer

Bedding Surfaces

Bedsets
(Groups of similar beds


Simple or Composite)

Graded bedding
: sedimentation
units characterized by distinct
gradations in grain size. These
can be normal (larger grains on
the bottom gradually changing to
smaller grain on the top, or the
opposite and less
-
common,
reverse grading.

Bedform development as a function of water flow velocity

Page 84

Pebble Orientation and Current Direction

Imbrication

Ripple formation in a flume…

Symmetric ripples (Oscillation ripples) are formed by
wave action.

Sediment Transport within an antidune

Tabular cross
-
bedding
: cross
-
bedded units that are broad in lateral
dimensions with respect to set thickness and that have essentially
planar bounding surfaces.

Trough cross
-
bedding
: cross
-
bedded units in which one or both
bounding surfaces are curved.

Flaser bedding
: ripple bedding in which thin streaks of
mud occur between sets of cross
-
laminated or ripple
laminated sandy or silty sediment. More sand than mud.

Lenticular Bedding
: a structure formed by interbedded mud
and ripple cross
-
laminated sand in which the ripples or sand
lenses are discontinuous and isolated in both a vertical and
horizontal direction. More mud than sand.

Irregular Stratification

Convolute bedding and lamination
: a structure formed by complex
or intricate crumpling of beds or laminations into irregular, generally
small
-
scale anticlines and synclines.

Flame Structures
: wavy flame
-
shaped tongues of mud that
project upward into an overlying layer, which is commonly
sandstone.

Ball and pillow structures
: present in the lower part of
sandstone beds and less common in limestone beds, that overlie
shales. They consist of hemispherical or kidney
-
shaped masses
that show internal laminations.

Synsedimentary Folds and Faults (slump structures)

D
é
collement structure

Convolute lamination

Dish and pillar structures

Scour and fill structures

Bedding
-
plane markings

Groove and Flute casts

Sole markings:

owing to erosion of a mud bottom followed by the
deposition of coarser material.

Index Fossils

Trace Fossils

Bedding
-
plane markings

Syneresis cracks, precipitation imprints, mudcracks, rill marks &
parting lineation.


Syneresis cracks

Precipitation imprints

Mudcracks

Rill marks

Parting lineation

Ichnofacies

are trace fossil assemblages that indicate a specific
environment.

The type of trace fossils that may be encountered within ichnofacies
include:

Repichnia
: crawling or walking traces (any trace made during
locomotion.

Fodichnia
: feeding structures (usually infaunal burrows made by
deposit feeders that systematically mine for food.)

Domichnia
: burrows used primarily for dwelling as opposed to
feeding.

Cubichnia
: behavioral traces including resting or nesting traces.

Pascichnia
: traces made by grazing herbavores, usually at the
sediment/water interface.

Schematic representation of the relationship of the characteristic
trace fossils to sedimentary faces and depth zones in the ocean.

Stromatolites
:
organically formed laminated structures
composed of fine silt or clay
-
sized sediment, or more rarely,
sand
-
size sediment.

Paleocurrents