Physical Geology Introduction


Source, Slope and Sink

SOURCE Area : the composition of the source 
area exerts a great influence on the nature of 
the sedimentary rock(s) that can form -- 


	granite could alter to form clays (from the 
	feldspars) leaving quartz

	basalt, would alter rapidly to form clays 	 
	plus various iron-rich compounds

SLOPE refers to those processes which move 
solid material at the surface of the Earth




In addition to carrying solid particles, water 
can carry ions in solution.  Clays do not have 
the same composition as the materials from 
which they were produced.  Some ions are 
carried away in solution.

The SINK refers to a place of accumulation 
of previously transported material

	Granites in central Texas are weathered 
	chemically and physically.  The quartz is 
	transported by the river systems and some 
	eventually end up being deposited along 
	the Texas beaches.

	Will characteristics of the beach 
	environment be imposed on these grains?  
	Can a geologist infer the SINK from the 
	properties of a sedimentary rock?

Diagenesis and Lithification

Loose grains are called sediment

Sediment can be converted to sedimentary 
rock if the particles are either cemented 
together or compacted during burial.

As sediment is buried, changes in 
composition and texture may take place and 
sedimentary processes merge with 

Classification of Sedimentary Rocks

Clastic - particles carried from the source to 
the sink by slope processes .... grains do not 

Chemical - ions in solution are ³forced² to 
react to precipitate solid material.  If you leave 
a glass of water outside it will evaporate 
leaving a white film of calcium 
interpenetrating texture

Clastic Sedimentary Rocks
n About 75% of the Earth's continental crust is 
covered by sedimentary rocks

Sedimentary rocks form a thin covering 

About 85% of the sedimentary rocks consist 
of grains that were transported as solids

Grains between 2mm and 1/16mm in 
diameter are called SAND sized.  

Coarse Clastics

Conglomerates & Breccias > 2mm

Sandstones consist of framework grains plus 
matrix (finer in size)

Quartz Arenite - framework grains are Quartz

Arkose - framework grains are Feldspar

Litharentite - framework grains are rock 

Grains between 1/16mm and 1/256mm are 
called SILT

Grains less than 1/256 mm are called CLAY

SAND, SILT and CLAY are SIZE terms.  A 
sandstone is a sedimentary rock made up of 
SAND sized grains.

SANDSTONES account for about 20% of the 
Earthıs sedimentary rocks

CLAY and SILT sized grains are dominated 
by clay minerals

All clay minerals have a good cleavage.
If the cleavage planes are lined up parallel to 
each other the rock is said to be FISSILE - A 
SHALE is a fine grained clastic rock with 

If the cleavage planes are not oriented in 
parallel, the rock is said to be non-fissile.  A 
MUDSTONE is a fine grained clastic rock 
without fissility.

SHALES and MUDSTONES are the most 
abundant sedimentary rocks - ~60%.

Chemical Sedimentary Rocks

~15% of the sedimentary rocks are chemical - 
original transport was as ions in solution.

Carbonates - limestones and dolomites - 
calcium carbonate precipitated or extracted 
from solution(s)...shells....animal dies and shell 
transported and eventually deposited.

Evaporite Sequences - Carbonates, Sulfates 
and Salts

Mineralogical Maturity

Given time, minerals, with the singular 
exception of quartz, will react to form clays.

The more quartz in a coarse sedimentary 
rock the higher the mineralogic maturity.

A feldspar-rich sandstone (Arkose) is 
immature - not enough time for the feldspar 
to form clays

Shales and mudstones are mineralogically 
mature - clay minerals

Textural Maturity

Texturally Mature Coarse Clastics are WELL 
SORTED (particles are all about the same 

The quartz grains in these rocks tend to be 

The more persistent the energy level, the 
greater the textural maturity

Other Properties of Sedimentary Rocks

Porosity - the percentage of open space - 
remember the generalizations about packing 

The poorer the sorting, the lower the porosity

Permeability - a measure of the ³inter  
connectnedness of the pores - permeable 
units will transmit fluids
Sedimentary Structures
n Parallel Beds 

Cross Beds

Graded Beds

Ripple Marks


In broad terms environments of deposition 
can be viewed as Continental, Transitional 
(beaches and shorelines) and Marine

In Continental settings the supply of sediment 
may be high but the ³preservation potential² 
is usually low --too many things can happen 
before the sediment is lithified.

Transitional - supply of sediment may be 
high (at the mouth of a river) and 
preservation potential is usually high.

Marine - preservation potential is high but 
the supply of sediment may be low.

Much of the sedimentary section on Earth 
accumulated in the Transition Zone - deltas, 
barrier islands, beaches, etc.

Geologists study modern sediments to try and 
see what imprint the environment of 
deposition leaves on the sediment and 
resulting sedimentary rock.

Evaporite Basins



Evaporate 1 liter of marine water

		50% gone : carbonate : 2.8 g/cc

		80% gone : sulfate : 2.4 g/cc

		90% gone : salt : 2.1 g/cc


Sedimentary Facies

Focus on grain size - related to proximity to

source area

Sea Level Rises : encroachment of marine 
environment onto continental area (or, land 

Sea Level Falls : encroachment of continental 
environment onto marine area (or land rises)