Sed Midterm

relief

how dark the lines around the edges are

Cleavage

a preferred fracture orientation

Pleochroism

grain changes color when rotated under plane-polarized light

Birefringence

distinctive colors seen in anisotropic minerals under cross-polarized light

twinning

crystals in the same grain formed with opposite orientation of their crystal lattice

Quartz

Typically colorless in plane-polarized light, shades of gray in cross-polarized light, has low relief

Calcite in thin section

Two cleavage planes, high relief, pale green and pink birefringence

Easiest way to distinguish different types of feldspars

twinning

Besides the different types of individual mineral grains in a sandstone, other constituents may include:

lithic grains, matrix, cement

Laminar flow

low flow rates

Turbulent flow

high flow velocities

rolling

clasts move along the bottom of the fluid column and stay in contact with the bed

saltation

particles jump or bounce along the bed surface

Suspension

turbulence keeps the particles in the fluid column

bedload

particles being carried by rolling and saltation

suspended load

the sediment load being carried in the fluid column without interacting with the bed surface

Normal grading

Grain size decrease from the bottom to the top of a single bed

Reverse Grading

Increase in grain size upward within a single bed

Fining upward

pattern of overall smaller grain sized upward through a number of beds

coarsening upward

pattern of overall larger grain sizes upward through a number of beds

Bouma Sequence

associated with turbidity currents

Downstream

Ripples that form under unidirectional flow have asymmetric shapes and cross-lamination that dip

Current ripple produces

cross-lamination

straight-crested dune produces

planar cross-bedding

sinuous-crested dune produces

trough cross-bedding

plane bedding produces

planar lamination

interbedded

alternations of thin beds of different lithology

cross-stratification

any layering that is at an angle to the depositional horizon

primary current lineation

ridges on bedding planes parallel to flow direction

stoss

upstream side of a ripple

lee

downstream side of a ripple

wave base

the depth to which surface waves affect a water body is the

Wave ripples

symmetric

Tidal currents which move water onshore

flood tides

Ebb tides

Tidal currents which move water offshore

mud drapes

thin layer of mud on foresets of cross-stratified sand

reactivation surface

a minor erosion surface within cross-stratification reflecting current reversals

Tidal bundle

a cyclical variation of thickness in foreset laminae in cross-beds reflecting variations in flow strength

Herringbone cross stratification

bipolar cross-stratification produced by alternating directions of ripple migration

flaser bedding

isolated thin drapes of mud in a cross-laminated sand

Tidal bundle

Flaser bedding

cross bedding

wave ripple-lamination

wavy bedding

climbing ripples

hummocky cross-stratification

ripple cross-lamination

lenticular bedding

Trough Cross bedding

Tidal Rhythmite

upstream

flute casts are elongated ridges with a steep narrow end that is oriented

scour marks

erosion of the underlying bed by turbulence in the flow

tool marks

an object carried in the flow marking the underlying bed

gutter cast

linear ridges and furrows on the bed surface

grooves

sharply defined elongate marks created by a dragging object

prod marks

indents in the lines of a bedding planes from a bouncing object

Flame Structure

type of deformation structure where mud projects into an overlying sandy bed

Dewatering structures

soft-sediment deformation structures formed by fluidization processes

Load Casts

Form when higher density sand partially sinks into the underlying mud to form downward-facing, bulbous structures

Skolithos

trace fossil assemblage that is dominantly characterized by small vertical tubes in sands

Zoophycos

trace fossil assemblage is mostly characterized by radial, sweeping forms on bedding plane surfaces

Cruziana

This trace fossil assemblage is the most diverse and contains complex networks of burrows that can be horizontal, vertical, and branching

Nereties

This trace fossil assemblage consists of intricate, regular patterns seen only on bedding planes and are typically feeding traces.

The distribution of ichnofacies is controlled by several environmental factors. The factors are:

Hardness of substrate, sedimentation rate, oxygen levels in the water, salinity of the water, quality/quantity of nutrient supply

Tropical regions

Chemical weathering is enhanced because of the higher temperatures and abundance of water.

Subtropical regions

Limited availability of water and absence of soil and vegetation leads to rapid physical weathering.

Polar and cold mountain regions

Physical weathering is the dominant process here.

Temperate regions

Both physical and chemical weathering are subdued in these regions.

Erosion and transport under gravity through downslope movement

landslide, rock fall, slumping, soil creep

Denudation

The lowering of the land surface through the combination of weathering and erosion is called

limestone

rocks made of calcium carbonate

calcite

the most common carbonate mineral

dolomite

calcium magnesium carbonate

aragonite

denser and harder CaCO3 mineral

ooid

spherical CaCO3 grain

pisoids

spherical CaCO3 grain >2mm diameter

peloids

homogeneous grains - probably fecal pellets

intraclast

grain that is a recycled piece of CaCO3

aggregate

consists of several fragments

The most common evaporite minerals in sedimentary rocks are

gypsum and anhydrite

Halite

evaporite mineral that is distinguished by its salty taste.

Chert

A fine-grained siliceous sedimentary rock made up of interlocking microquartz grains is called

Sedimentary log

Unidirectional indicators

Which type of paleocurrent indicator gives the absolute direction of flow?

Fence diagram

A diagram that shows how different sedimentary logs can be correlated and linked across various distances is called a

Flow axis indicators

Which type of paleocurrent indicator gives the linear orientation of the flow but does not distinguish upcurrent from downcurrent?

Rose

Paleocurrent data are normally plotted on a ____ diagram, which is a circular histogram.

lithostratigraphic unit

defined by rock type and stratigraphic position

biostratigraphic unit

defined by its fossil content

chronostratigraphic unit

defined by its absolute age

magnetostratigraphic

defined by magnetic properties

allostratigraphic unit

defined by its position relative to significant stratigraphic surfaces

isochronous

a surface representing a single moment in geologic time

Left side of diagram

angular unconformity

Right side of diagram

disconformity

Cross cutting

where igneous rocks are younger than the strata

Which characteristic CANNOT be used to define a formation?

age

True or False: A bedding plane typically represent much more time than beds of strata themselves.

True

True or False: Interpretation of facies should be based on the processes, not the specific environment, that formed the beds under observation.

True