Geology: Stratigraphy, Weathering, Soils & Sediment Grain Size

Strata

layers characterized by unifying properties that distinguish from others

Steno's Laws

(superposition, original horizontality, original lateral continuity

Arduino

(Primary, Secondary, Tertiary, Quaternary)

Correlation

establishing equivalency

Lithostratigraphic Units

based on lithology such as the formation

Formation

a mappable unit, fundamental unit of lithostratigraphy

Lithostratigraphy

organization of strata based upon lithology

Chronostratigraphy

organization of strata based upon age relations into time-rock units

Geochronology

abstract time units (memorize these three words and their definitions)

Facies

lateral or vertical variations in sediments (lithofacies) or fossils (biofacies)

Walther's Law

Lateral variations in facies are observed as vertical variations

Multiple Working Hypotheses

A family of hypotheses that are testable

Physical (mechanical) weathering

disintegration without chemical change

Chemical (decomposition) weathering

chemical reactions change rocks and minerals into more stable compounds.

Thermal expansion

the rock surface expands more than the interior, creating stresses

Frost wedging

the volume of water increases 9% on freezing

Stress release

Release stress and the rocks expand

talus slope

pile of rock at base of slope

Goldich Stability Series

know the minerals that weather first; the more oxygens in the mineral that are shared...the more resistant the mineral is to decomposition

reverse bowens reaction series

Solution

Dissolution of soluble minerals in water or with the help of acid

Oxidation-reduction

converting iron in a mineral to rust

Hydrolysis

a reaction between H+ and OH- ions of water and silicate minerals; important in weathering feldspar and turning it into clay

Hydration (dehydration)

add or lose water to get new minerals

Ion exchange

the substitution of ions in solution for those held by mineral grains, effective in clays

Effect of climate on weathering

chemical weathering is faster in warm wet climates

pH

- acidity - Normal pH values for surface and groundwaters range from 4-9. Some minerals readily break down in acidic water. Silica, iron and aluminum do not.

Eh

redox - some minerals are more susceptible to chemical weathering in oxidizing environments

Ion mobility

encourages chemical weathering

Paleosols

ancient or buried soil

Humus

dark-colored, carbon-rich material made of decomposed organics

Soil profiles

sequence of layers; (parallel horizons) formed from leaching and accumulation processes

Soil horizons

distinct soil layers

O horizon

organic horizon. Consists of humus. Adds CO2 and organic acids to soil.

A horizon

humus from O horizon mixed with mineral matter. Zone of leaching (groundwater percolating through carries ions and clay further down)

B Horizon

mostly mineral matter with little humus. Zone of accumulation (ions and clays leached from above accumulate)

C Horizon

Partially Weathered parent material

Grain form

Relative lengths of the three major axes; affects the hydraulic properties of a clast

Sphericity

A measure of how closely the shape of an object resembles that of a perfect sphere; grains get more spherical with increased transport

Zingg diagram

uses two shape indices b/a and c/b to define 4 shape fields

Roundness

The degree of sharpness of the corners and edges of a grain; grains get rounder with increased transport

Surface texture

Surface of sand grains and pebbles can be frosted, scratched, with pits & ridges

Grain size - fundamental property of sediments

Udden Wentworth grade scale

boulder (>256 mm); cobble (64-256 mm); pebble (4-64 mm); granule (2-4 mm); sand (63 microns-2 mm); silt (4-63 microns); clay (

Sand sizes

Very coarse upper= 1.410-2.00mm

Very coarse lower= 1.00-1.410mm

Coarse upper= 0.710-1.000mm

coarse lower=.500-.710mm

medium upper=.350-.500

medium lower=.250-.350mm

fine upper=.177-.250

fine lower=..125-.177

very fine upper=.088-.125

very fine lower=.062-.088

Mafic minerals

magnesium and iron = dark black/green in color - hornblende, pyroxene

Opaque heavy minerals

magnetite, ilmenite

Non opaque minerals

Last to chemically weather - Z,R,Ts

ZRTs

Zircon tourmaline rutie

Phi size

for mathematical convenience (-log2d) - use the negative of the base 2 log

bigger phi means smaller grain

Measuring gravel

direct measurement with ruler or calipers

Stokes law

for analyzing mud grain size, particle settling velocity increases with the diameter of the particle

Sorting

A measure of grain sizes present and the scatter of these sizes around the mean

Textural maturity

with transport, clays are removed; sand grains become better sorted and rounded

Major minerals (framework grains)

chemically mature sand is mostly quartz

Accessory minerals (>1-2%):

mafics, opaque heavy minerals, Z,R,Ts

Rock fragments:

igneous, sedimentary, metamorphic

Matrix

fine-grained material (silt and clay) filling spaces

deposited at the same time

Chemical cements

also fills spaces; glues rocks together

deposited after

Chemical maturity

with transport, all minerals but quartz are removed

Conglomerate

rounded gravel sized clasts

breccia

angular gravel sized clasts

oligomict

almost all one composition

polymict

clasts are of mixed lithologies

epiclastic

normal weathering of rocks

Volcaniclastic

from volcanic eruptions

cataclastic

in faults

Clast (or grain) supported conglomerate

Matrix-supported conglomerate

15-70% matrix

Arenites

sandstones, could have any minerals, less than 10% mud

Quartz arenite

>90% quartz

Arkose

abundant feldspar (less than 90% quartz and more feldspar than lithics)

Lithic arenite

abundant lithics (less than 90% quartz and more lithics than feldspar)

wacke

more than 10% clay and silt

Mudstones and shale

Mudstones and shale

Shale

has fissility

fissility

(the ability or tendency of a rock to split along flat planes of weakness)

The ability of a fluid to entrain, transport, and deposit sediment depends on

fluid density, viscosity, and flow velocity

Density

gm/cm3, affects the ability of force to impact particles within a fluid and on bed

viscosity

resistance to deformation; affects turbulence, sediment transport, and sediment settling

laminar flow

smooth and tranquil, streamlines do not cross; at low velocities, smooth beds, and viscous fluids

Turbulent flow

chaotic flow, streamlines do cross; at high velocities, rough beds, and low viscosity fluids

Reynolds Number

Reynolds Number = (VRh*ρf)/𝛍

Reynolds number = (fluid inertial forces)/(fluid viscous forces)

Re below 500 the flow is laminar

Re between 500-2000 is transitional

Re above 2000 is turbulent

Bedload

[saltation and traction (rolling and sliding)], suspended load, dissolved load

Entrainment

processes involved in picking up grains

Bernoulli effect

When incompressible fluids reach a constriction, they must speed up to maintain a constant volume flow rate. Points of higher fluid speed will have less pressure than points of slower fluid speed.)

Sediment Characteristics that affect entrainment

density of sediment, shape of grain; diameter of sediment grain

Hjulstrom's Curve

mean velocity at which particles of various sizes are eroded, transported and deposited

Bedforms

regular undulations that form on the bottoms of sandy channels

Fr (Froude number)

inertial forces/gravitational forces; Fr >1 = upper flow regime, Fr

Sediment Gravity Flows (mass wasting

sediment transported independently of fluid by gravity

Mud or debris flow

floating pebbles, some beds exhibit reverse grading

Grain flow

air or water trapped between the grains acts as a lubricant and does not propel the grains.

Fluidized flow

grains supported by pore water in between

Turbidity Currents

sediment is supported by upward turbulence of the fluid in the flow.

Primary Sedimentary Structures

formed by the current that deposited the sediments...indicates conditions of transport & deposition

Secondary Sedimentary Structures

formed shortly after deposition

Bedding

Tabular layers of sediment that have lithologies, or textural, or structural unity that distinguishes them from layers above and below

lamination

Bed sets (simple)

Groups of beds of differing composition and texture

Planar stratification

Parallel, planar (not necessarily currently horizontal) beds; deposition on a flat surface

Graded bed

a gradual change in grain size from coarse to fine (normally graded beddin g) or fine to coarse (inversely graded bedding) upsection