-1-

Prepared by:

Dr. Emad Samir Sallam

Lecturer of Stratigraphy,

Geology Dept., Faculty of Science,

Benha University.

-2-

I) Introduction

Sedimentary rocks are formed by materials, mostly derived

from the weathering and erosion of the pre-existing rocks (Igneous,

metamorphic or even sedimentary rocks), which are transported and

then deposited as sediments on the earth's surface. These sediments

are then compacted and converted to sedimentary rocks by the

process of lithification (rock formation). Diagenesis may be broadly

defined as the processes of physical and chemical change which take

place within a sediment after its deposition and before the onset of

either metamorphism or weathering (Fig. 1).

Fig. 1. Rock Cycle

-3-

An important part of applied stratigraphy is the description,

classification and interpretation of sedimentary rocks – a study known

as sedimentation or sedimentology. A distinction is made between

processes of sedimentation (weathering, transportation, deposition, and

lithification) and products of sedimentation – sedimentary rocks. A

sediment is a deposit of a solid material of earth's surface from any

medium (air, water, ice) under normal conditions of the surface. A

sedimentary rock is the consolidated or lithified equivalent of a

sediment.

A description of any sedimentary rock delineates its texture,

structure, and composition. Texture refers to the characteristics of the

sedimentary particles and the grain-to-grain relations among them.

Larger features of a deposit, such as bedding, ripple marks, and

concretions, are sedimentary structures. Sediment composition refers

to the mineralogical or chemical make-up of sediment.

Most sediments are of two main components, a detrital fraction

(pebbles, sand, mud), brought to the site of deposition from some

source area, or a chemical fraction (calcite, gypsum, and others)

formed at/or very near the site of accumulation. The components

may be mixed in any proportions, yielding sediments ranging from

pure types (quartzose sandstone) to intermediate types (such as shaly

limestone).

There are two main classes of sedimentary rocks, which differ in

their mode of origin, clastic rocks and nonclastic rocks.

The clastic rocks are represented by rocks such as conglomerates,

sandstones and shales. They are composed of rock fragments and

-4-

detrital grains, produced by the physical and chemical weathering of

older rocks. These particles are transported from their place of origin

by the flow of air, water and ice across the earth's surface,

accumulating as sedimentary deposits once the flow of transporting

medium so declines that the particles cannot be moved any further.

Deposition occurs progressively since the larger and heavier particles

require more energy to be transported in comparison with the smaller

and lighter particles. This results in a sorting of the particles to form

deposits of decreasing grain size away from their source.

The nonclastic or marine rocks (chemical and organic rocks)

comprise such rocks as limestones, dolomites, cherts, evaporites and

coal. These rocks are mainly composed of chemically or biologically

formed material produced in solution by chemical weathering, which

accumulates in sea water as the result of evaporation. This material in

solution may be abstracted by direct precipitation to form chemical

sediment (e.g. evaporites; gypsum, anhydrite and rock salt), or it may

be extracted by organisms to form shells and skeletons, which then

accumulate as organic sediment (fossiliferous limestone, chert). Coal

and oil shale considered as organic sediments since they derived from

the remains of tropical plants subjected to pressure and carbonization

processes.

Detrital rocks commonly have fragmental texture, whereas

chemical rocks have crystalline texture. Fragmental texture is

characterized by broken, abraded, or irregular particles in surface

contact, crystalline texture by interlocking particles, many having

crystal faces or boundaries.

-5-

The detailed texture of a sedimentary rock is largely determined

by the size, shape of the particles and by their arrangement in the

aggregate. In mechanically transported sediments (clastics, detrital

rocks), five properties of the particles influence the final texture of the

deposit (fig. 2):

1. Grain size,

2. Shape (roundness, shericity),

3. Orientation,

4. Mineralogical composition,

5. Packing.

Fig. 2. Bedding as the product of different combinations of grain composition, size, shape,

orientation and packing (modified after Pettijohn, Potter & Siever 1972, and Griffiths 1961).

-6-

II) Stratigraphic Concepts

Stratigraphy can be defined as the branch of geologic science

concerned with the description, organization and classification of the

stratified sedimentary rocks as a basis for interpreting the geological

history of the earth's crust. The study of the stratigraphy starts with

establishing the chronological order in which a sequence of

sedimentary rocks was originally deposited at the earth's surface. This

is known as the stratigraphic order of the sedimentary rocks. What is

known as a stratigraphic sequence can be established by placing the

sedimentary rocks in their chronological order.

All bedded rocks can be treated stratigraphically, i.e. to

establish age relations between beds. However, the term

“stratigraphy” is used primarily in the context of sedimentary

research. Stratigraphy involves the study, subdivision and

documentation of sedimentary successions, and on this basis to

interpret the geological history they represent. In order to reconstruct

an environment or special events in the geological past, regionally or

even globally, it is necessary to correlate sedimentary horizons from

different areas. It is important to establish which sedimentary units

were deposited at the same time or by the same or similar

sedimentological or biological processes, even if they are not quite

contemporary. Correlation is generally a question of what is possible

to achieve with the available amount and quality of data.

We usually have no possibility of determining definitely which

beds were deposited at the same time, but attempt to use all the

information available in the rocks. This information falls into five

main categories: (1) Rock composition and structures resulting from

-7-

sedimentological processes. (2) Fossil content, which is a result of

biological, environmental and ecological evolution throughout

geological history. (3) Content of radioactive fission products in minerals

or rocks which may be used for age dating. (4) Magnetic properties of

rock strata. (5) Geochemical features of sediments. These correlation

methods are so different that it has been found useful to work with

three forms of stratigraphy which can be used in parallel:

1. Lithostratigraphy: Classification of sedimentary rock types on the

basis of their composition, appearance and sedimentary structures.

2. Biostratigraphy: Classification of sedimentary rocks according to

their fossil content.

3. Chronostratigraphy: Classification of rocks on the basis of geological

time.

Sedimentary rocks are the basic materials of the science of

stratigraphy. Natural outcrops, excavations, quarries, mines, and well

bores serve to make these materials abundantly available for study.

Sedimentary rocks generally occur in the form of layers which were

deposited on top of one another to form a stratified sequence as

shown in figure (3). Each layer (greater than 1 cm thick) is known as

bed or stratum (pl: strata), whereas layer less than 1 cm is termed

lamina. The layered nature of sedimentary rocks which results from

such a mode of formation is termed bedding or stratification.

-8-

Fig. 3. Diagram showing the bedding or stratification of sedimentary rocks with beds of

different lithology (conglomerate, sandstone, sandy shale and limestone) separated from one

another by bedding planes.

Sedimentary rocks usually differ from one another in lithology,

which is a general term referring to the overall character of a rock, as

seen particularly in the field. The lithological differences reflected in

the beds of the sedimentary rocks are usually defined by differences in

mineral composition, grain size, texture, color, hardness and so on,

which allow the individual beds of sedimentary rock to be recognized

(fig. 4).

Fig. 4. Showing the bedding or stratification of sedimentary rocks with beds of different

lithology.

-9-

The surfaces separating the individual beds of sedimentary rock

from one another are known as bedding planes. They are commonly

defined by a more-or-less abrupt change in lithological character

which occurs in passing from one bed to the next. However, they may

simply be defined by planes of physical discontinuity between beds of

otherwise similar lithology, along which the rock will split.

The thickness of individual beds generally varies from several

centimeters to a few meters, while their lateral extent may be

measured in kilometers. Accordingly, beds of sedimentary rock

typically form thin but very widespread layers, which eventually

disappear as their thickness decreases to zero. Such beds may retain a

lithological identity, or they may show a gradual change in

lithological character, as they are traced throughout their outcrop.

Some deposits of sedimentary rock show rapid changes in thickness

so that they do not have such a parallel-sided form. For example,

elongate bodies of sandstone and conglomerate are laid down in river

channels and along shore lines, wedge-shaped bodies of breccia and

conglomerate are banked against buried hills, volcanoes and coral

reef. However, such examples are exceptions to the general rule that

most beds of sedimentary rocks occur effectively in a tabular form as

parallel-sided layers which extend laterally for a considerable distance

before they eventually disappear as their thickness decreases to zero.

A bedding plane generally represents the original surface on

which the overlying bed of sedimentary rock was deposited. Such a

surface is simply known as a surface of deposition. Since sedimentary

rocks are deposited on the earth's surface, such a bedding plane

corresponds to this surface just before the overlying rocks were laid

-10-

down. Commonly, the earth's surface is very close to the horizontal

wherever sedimentary rocks are accumulating as the result of

deposition at the present day. For example, the alluvial plains and

deltas formed by the lower reaches of large rivers, and the shallow

seas into which they flow beyond the land, are typically flat lying

areas where considerable thickness of sedimentary rocks have just

been deposited. However, since a bedding plane is considered to

represent such a surface sedimentary deposition, it can be argued by

analogy with the present day that this surface was virtually horizontal

when the overlying bed of sedimentary rock was deposited.

This implies that sedimentary rocks were originally deposited as

nearly if not quite horizontal beds. Accordingly, sedimentary rocks

are generally deposited with what is known as a low initial dip, unless

they were deposited on a sloping surface. Such exceptions are only

likely where sedimentary rocks are deposited on the flanks of deltas,

coral reefs, volcanoes and hill slopes. The beds rarely have an initial

dip greater than 30º from the horizontal, even under these

circumstances. They are usually found to flatten out as they are traced

away from such prominences, until they eventually become

horizontal or nearly so.

-11-

III) The Stratigraphic Principles

First Principle: the Principle of Superposition: that "Sedimentary

layers are deposited in a time sequence, with the oldest on the bottom and the

youngest on the top". Logically a younger layer cannot slip beneath a

layer previously deposited. The principle arises simply because

sedimentary rocks are laid down as flat-lying beds on top of one

another, so that they become progressively younger in an upward

direction. In other words, the oldest rocks occur at the base of a

startigraphic sequence, while the youngest beds are found at the top.

Sedimentary rocks are commonly if not invariably found to be

affected by earth movements in such a way that the bedding is no

longer horizontal. For example, they may become tilted so that the

bedding is inclined in a particular direction. This is known as the

direction of dip, and the bedding is said to dip at a particular angle

from the horizontal in this direction, which corresponds to the dip of

the sedimentary rocks. The horizontal direction of right angles to the

dip of the bedding is known as the strike. How the dip and strike of a

bedding plane can be measured in the field will be described later.

Accordingly, earth movements can affect sedimentary rocks in

such a way that the bedding becomes increasingly inclined away from

the horizontal until it reaches a vertical position, beyond which it may

become overturned. The beds are then said to be inverted, even

although the bedding may still be inclined away from the horizontal.

The Principal of Superposition does not apply to sedimentary rocks

which have been affected by subsequent earth movements that they

are now inverted. It is often difficult to determine the stratigraphic

order of sedimentary rocks in such regions. However, it may be

-12-

possible to trace these rocks into areas of less complexity, where the

stratigraphic order can be determined according to the Principal of

Superposition.

Second Principle: the Principle of Original Horizontality of Beds:

that "most beds were once deposited as essentially horizontal beds".

Observation of modern marine and non-marine sediments in a wide

variety of environments supports this generalization (although cross-

bedding is inclined, the overall orientation of cross-bedded units is

horizontal).

Third Principle: the Principle of Uniformitarianism: that "the

present is the key to the past". The past history of our globe must be

explained by what can be seen to be happening now."

Fourth Principle: the Principle of Cross-Cutting Relationships: that

"a feature cuts across a bed must be younger than it". In geology, when an

igneous intrusion cuts across a formation of sedimentary rock, it can

be determined that the igneous intrusion is younger than the

sedimentary rock. There are a number of different types of intrusions,

including stocks, laccoliths, batholiths, sills and dikes. Faults are

younger than the rocks they cut; accordingly, if a fault is found that

penetrates some formations but not those on top of it, then the

formations that were cut are older than the fault, and the ones that are

not cut must be younger than the fault.

Fifth Principle: the Principle of Faunal and Floral Succession: that

"strata may be dated and correlated by the sequence and uniqueness of their

fossil content".

-13-

Sixth Principle: the Principle of Inclusions: that "fragments of an

older bed can be included in a younger bed, but not vice versa". For example,

in sedimentary rocks, it is common for gravel from an older formation

to be ripped up and included in a newer layer. A similar situation with

igneous rocks occurs when xenoliths are found. These foreign bodies

are picked up as magma or lava flows, and are incorporated, later to

cool in the matrix. As a result, xenoliths are older than the rock which

contains them.

هةادئ ػمن الطةلاج

(: Law of Superposition )هةدأ حؼاكب الطةلاج : الهةدأ األول

Nicholas Steno (1686 -1638 .)اكخرح ذا اهيتدأ ي كتل اهؾبهى االعبه شخ ف أ خخبتؼ ي اهضخر اهيخعتلج اهخ هى خخؾرط اه ؽيوج خش تبهخفوق : ص ؽو ا

. أ اهع فأ نل عتلج كد خنح تؾد اهعتلج اهخ خشفوب كتل اهعتلج اهخ خؾوب

فديب خخؾبكة اهضخر تشنل أفل فأ اهضخر ا . هذا اهيتدأ يدهال زيب اظباهعتلبح اهخ ف االشفل األكدى اهخ ف األؽو االددد ألب خنح ف فخرث زيج

(. 1)الدلج، نيب يظخ ف اهشنل

. وضح الحؼاكب ف حرشب الطةلاج زهىا هو الهو الى الشار: (1)الشكل

ايب ه خؾرظح اهعتلبح اه ؽيوبح اهخشج نبهخفوق ا اهع اهشدد اهخ خؤد تبهخبه اه كوة اهعتلبح ا ادفبؽب تبهخبه خش اهيكؼ االضو هب، فب خشخخدى عرق

اخر هخددد اهخؾبكة االه اهضدخ هذ اهعتلبح كتل خأذر ؽيوبح اهخش ي ذ اهعرق

-14-

اشخخداى اهيخدجراح هخددد اؽيبر اهعتلبح اشخخداى اهخرانة اهرشتج نبهخشللبح . اهعج ؽاليبح اهى اهخ ين ي خالهب خددد نل ي اهشعخ اهؾو اهشفو هوعتلبح

Law of Original Horizontality of )هةدأ األفلث األصمث لمطةلاج : الهةدأ الداىى

Beds:)

Nicholas Steno( 1686 -1638 )اكخرح ذا اهيتدأ ي كتل اهؾبهى االعبه شخ اهذ الدغ ا خرشة جزئبح اهضخر اهرشتج ددد تفؾل خبذر اهجبذتج األرظج . اظب

تذهم خن عتلج افلج يازج . ؽوب تبهخبه شلعب ف كبػ اهتدر ا ا دط رشتف دبهج جد ضخر رشتج يدج فذا . (2)هشعخ األرط، نيب يظخ ف اهشنل . تؾد خرشتب خضوتب (نبهع ا اهخفوق )دل ؽو اب خؾرظح هدرنج ارظج

حهل الطةلاج لمحرشب (B. )حرشب الرواشب ةشكل أفل (A. )وضح كاىوو الحؼاكب األفل: (2)الشكل

افلا ححى لو كاىج االرضث غر هشحوث حهاها ( Montgomery, 1997.)

(:Law of Uniformitarianism )اإلىحظاهث أو هةدأ الوحرت الواحدت : الهةدأ الدالخ

-15-

راد ت (uniform) ذا اهيضعوخ يشخق ي اهنويج االنوزج يخغى ا يخيبذل نبفج اهلا : ص ؽو ا. اخغبى ا ذتبح اهؾايل اهعتؾج اخجب ؽو شعخ االرط

".اهفزبج اهنيبتج ف اهن نبح يب خزال اددث

ين خظخ ذا اهيتدأ ي خالل اخخبذ اهربح نيذبل دد خؾد اهربح اهؾبيل / يذبل فس اهؾبيل اظب خالل . اهعتؾ اهيشتة هخن اهنذتب اهريوج ف اهكح اهدبظر

هذا فب يشبدخب هونذتب اهريوج اهيدفغج ف ضخر كديج خشبؽدب ؽو . اهؾضر اهلديج. خددد عتؾج اهيعلج اخجب اهربح ف خوم اهفخرث

تذهم فلد ؽرف اهجهج يتدأ اهخرث اهاددث خؾرفب تشعب اضتخ ي اهتدبح ف The present is the key to the" اهدبظر يفخبح هويبظ ))أ : ؽوى االرط،

past ") .) خشبؽدب ( نبهربح اهيب اهزالهزل غرب) أ أ يالدغخب هوؾاهى اهعتؾجؽو فى اهؾدد ي اهغار اهعتؾج اهخ شبدب ؽو شعخ االرط اهخ ؾد زي

. ددذب اه يال اهش

: ( Cross Cutting Relation )ػالكث اللواطغ : الهةدأ الراةغ

ص يتدأ اهلاعؼ ؽو أ أ ددث ضخرج أ فبهق لعؼ ددث ضخرج أخر فأ اهددث اهضخرج أ اهفبهق أددد ي اهدداح اهضخرج اهيلعؽج، نيب يظخ تبهشنل

(3 .)

(.Brice et al., 1997)وضح ػالكث اللواطغ ف هجهوػحو هو الصخور الىارث، : (3)الشكل

Law of Faunal and Floral )كاىوو حؼاكب األحاء الحواىث والىةاحث : الهةدأ الخاهس

succession :)

-16-

ص ذا اهيتدأ ؽو أ اهيخدجراح اهتبخج اهداج خخؾبكة اهاددث تؾد األخر . تشنل ذبتح غبى يددد، أ أ اهيخدجراح األكدى خن ف األشفل األددد خلؼ ف األؽو ؾد ذا اهيتدأ ي أى اهيتبدئ ف اهجهجب اهخأرخج دد ؾد األشبس ف خددد اؽيبر

ؽويب أ اهخؾبكة ف األدبء خجج هؾيوج . اهدداح اهضخرج خددد اهؾضر اهجهجج(. 4)اهخعر اهؾظ اشبس خلشى خأرخ األرط، نيب ف اهشنل

حخ الحظ ةدء . وضح حؼاكب االحاء خالل ػهود طةاك هحد هو الةركاهةري الى الكاهةري: (4)الشكل هموو شىث، ف حو حححفظ صخور الةركاهةري 545الؼصر الكاهةري ةظهور اصداف الحراموةاج كةل

(.Murck et al., 1999). هموو شىث500ةطواةغ الدكىوشوىا كةل حوال

(: Law of Inclusions)هةدأ األححواء : الهةدأ الشادس

ص ذا اهيتدأ ؽو ا اهعتلبح اهضخرج اهخ خدخ ؽو كعؼ ضخرج ي عتلج (. 5)ذبج يخخوفج ؽب فأ اهعتلبح األه األددد ي اهذبج، نيب يظخ ف اهشنل

ين خظخ ذهم تب جد كعؼ هعتلبح ضخرج ف عتلج أخر التد اب خنح أال ذى . خؾرظح هيوبح اهخؾرج ي ذى خرشتح ف اهعتلج اهذبج األددد

(. Brice et al., 1997)وضح ػالكث األححواء ف هجاهغ هخحمفث هو الصخور، : (5)الشكل

-17-

IV) Modern Stratigraphic Classification

Geologists can do no more than recognize, define, and name

the natural groupings of strata that are inherent in the local

stratigraphic section. In each case recognition and definition are based

on direct observations of the tangible features and characteristics of

the strata and are independent of interpretations regarding the

significance of these observations. It is convenient to refer to these

natural differentiates of the stratigraphic column as observable units.

The stratigraphic units may be differentiated into two main

classes: 1. Observable stratigraphic units, and 2. Inferential

stratigraphic units (table 1).

1- Observable Stratigraphic Units:

The multiplicity of tools and techniques available to present-day

stratigraphers for making observations on the characteristics of

sedimentary rocks exposed in outcrop or penetrated in bore holes

makes possible the recognition of a wide variety of tangible groupings

of strata based on these observations.

Two classes of observable units; a) Lithostratigraphic units

based on lithologic characteristics, and b) Biostratigraphic units

differentiated by paleontologic characteristics.

a) Lithostratigraphic units: Mappable assemblages of strata

distinguished and identified by objective physical criteria observable

in the field and in the subsurface studies (Groups, formations,

members, etc).

-18-

An outline of the classification of stratigraphic units is presented

in the following table.

-19-

- Formation:

The formation is the fundamental lithostratigraphic unit defined

in a bedded succession. An important property of a formation is that

it should be easily recognized in the field or a borehole due to its

composition (lithology). The formation is thus a mappable unit,

which can be visualized on an ordinary geological map (e.g. 1:50,000)

or recognized in the description of a bedded succession. It is

distinguished by: (1) definite lithologic composition or a distinctive,

interbedded or intergraded succession of lithologic types; (2)

observable lithologic separation from adjacent units above and below;

and (3) traceability from exposure to exposure (or from well to well in

the subsurface). There is in principle no limit to the thickness of a

formation, but it usually varies from a few tens to several hundred

meters. A 50–300 m thick sandstone, over- and underlain by quite

different rocks such as shale or limestone, would constitute a natural

formation. However, a formation will seldom be homogeneous, and

for more detailed mapping it will be useful to divide the formation

into smaller units.

Most formations occur as sheet-like bodies with a horizontal

extent which is very much greater than their vertical thickness. It is

commonly found that formations vary in thickness from a few meters

to several thousand meters, while it may be possible to trace

individual formations for several hundred kilometers. The boundaries

between stratigraphic formations usually appear to be parallel to the

bedding of the underlying rocks. Such a boundary is said to be

conformable wherever this is the case. This means that stratigraphic

sequences tend to accumulate as vertical successions of widespread

-20-

but flat-lying formations, which are conformable with one another

throughout their outcrop.

Since the formation is the basic unit in the mapping

sedimentary rocks, it must have a lithological character which allows

it to be distinguished from the adjacent formations. This character can

be defined in various ways. Some formations may contain beds of

only a single lithology, while others may be formed by alternating

beds of more than one lithology. A formation may be characterized

by a particular lithology, even if more than one lithology is present, or

by a particular association of different lithologies, which are often

arranged in the form of cyclic sequences.

Individual formations are designated by geographic names

derived from a locality or area of typical development of the unit e.g.

Minia Formaion, Arif El Naqa Formation, El Halal Formation and

so on.

Formations are generally giving a geographical name followed

by a term descriptive of the dominant lithology (Sudr Chalk

Formation, Abdallah Limestone Formation). However, if a formation

is composed of more than one lithology, it is simply called a

formation without any term descriptive of the lithology (Mokkatum

Formation, Minia Formation).

It has been common practice not to capitalize rock unit terms

where used in combination with geographic names (Esna shale, Sudr

chalk). The Stratigraphic Commission (1961) now recommends

capitalization of the initial letter of the term to conform with

-21-

treatment of other geographically identified features e.g. Kafr El

Sheikh Formation.

- Member:

The formation may be distinguished into smaller groupings of

strata, members, to provide greater detail in mapping or to single out

subsidiary units of special interest. For example, the Nubia Formation

in south Western Desert and Nile Valley is subdivided into the Taref

Sandstone Member and Qussier clastic Member (composed mainly of

shales alternated with sandstone and siltstone beds).

-22-

The members meet the same requirements as formations in

terms of mappability, litologic distinction, and designated type

sections. The only difference is that members are subdivisions of

formations and may not have more than local significance as

cartographic units.

Formations need not to differentiate into members. Specific

members may be recognized an utilized in only part of the area of

distribution of a formation, although no subdivisions at all, or other

members with different names and different limits may be applied

elsewhere. For example, the Jordan Sandstone is undivided

formation, but further north, along the Mississippi, differences in

grain size permit recognition of the Norwalk Member at the base of

the formation and the Van Oser Member above.

-23-

- Bed:

Beds of economic significance, such as coal beds, quarry layers,

oil or water sands, or ore horizons; and other distinct beds useful in

mapping, such as bentonites or fossiliferous horizons (for example,

"Trilobite beds") often receive names which are locally applied. The

application of "key beds" and "marker horizons" to problems of

stratigraphic classification and correlation is discussed in detail later.

- Group:

Two or more successive formations, related by lithology or by

position with reference to unconformities, may be assembled as a

group.

- Sequence:

Certain broad areas are united by a common tectonic and

depositional history, such as much of the continental interior of North

America. In such areas, the lithostratigraphy is marked by significant

interregional unconformities, which can be traced in outcrop and in

the subsurface over vast distances. The unconformities subdivide the

stratigraphic record of major continental areas into groupings of strata

that ma include groups and supergroups in vertical succession and

which are recognized over lager areas than any of their component

formal rock stratigraphic units. These unconformity-bounded masses

of strata of greater than group or supergroup rank have been called

sequences (Sloss, Krumbein, and Dapples, 1949; modified by Sloss,

1963).

As mentioned above, while formations may be divided into

members and beds, they may also be combined to form groups and

-24-

supergroups, defined on the basis of the lithological characteristics

which they share in common with one another. There is, therefore, a

hierarchy of startigraphic rock units arranged in order of decreasing

size:

Supergroup

Group

Subgroup

Formation

Member

Bed

In sedimentary basins which are partly or entirely buried in the

subsurface, for example the North Sea basin, formations and groups

are defined on the basis of records from wells, such as well logs. In

offshore areas where it is difficult to find enough geographical names

to give the stratigraphic units, other names are then also used,

including historical names, names of animals etc.

The rock units forming this hierarchy are known as rock-

stratigraphic units or (lithostartigraphic units) in order to distinguish

them from time-stratigraphic units.

-25-

b) Biostratigraphic units:

As the strata can be differentiated lithologically into rock units,

they can also be subdivided into other observable stratigraphic units

on the basis of their fossil content rather than on their lithologic

characteristics. Stratal units defined paleontologically are called

biostratigraphic units, and have boundaries which are drawn

between successive units at stratigraphic positions marked by changes

in the fossil content.

Biostratigraphy is based on the fossils in sedimentary rocks. Its

tasks are to group strata into units based on fossil content, and apply

these to correlate sedimentary successions. Biostratigraphy has made

it possible to correlate sedimentary rocks on a global basis, and forms

the foundation for global stratigraphic classification of sedimentary

successions in combination with radiometric and other age dating

methods. The biostratigraphic application of fossils is based on the

fact that during the course of geological time, biological evolution

took place whereby some species died out and new ones appeared.

Any particular species will therefore be represented only in sediments

deposited within a limited time span, and can be used to recognize

this time span or part of it.

Sedimentary rocks often contain fossils as the organic remains

of plants and animals, which were incorporated into the sediment at

the time of its deposition. It was early recognized that the fossils

preserved in a sedimentary rock are diagnostic of its geological age,

unless they have been derived from the erosion of older rocks. This is

simply a consequence of organic evolution whereby certain species

change into new and different forms, combined with organic

-26-

extinction whereby other species die out completely, with the passing

of geological time. Thus, each species is only present as a living

organism during a particular time span, before which it had not

evolved from a pre-existing form and after which it had either become

extinct or evolved into a new form. This means that sedimentary

rocks can be dated as belonging to a particular interval of geological

time, according to the assemblage of fossil species which they contain.

This principal allows stratigraphic correlation since it implies that

sedimentary rocks similar assemblages of fossil species are the same

age.

The biostratigraphic units are classified into 1) Assemblage

zone (subzone, zonule); 2) Range zone; and 3) Concurrent-rang

zone.

2- Inferential Stratigraphic Units:

The self-evident, observable lithostratigraphic and

biostratigraphic units are sufficient for depicting the descriptive

stratigraphy of any given area. Such units do not, however, lend

themselves to interpretation of the local startigraphic column in terms

of earth history. The latter purpose requires that stratigraphic units be

related to geologic time, and that this relationship be expressed

through some acceptable system of classification and terminology.

Such a system exists in the hierarchy of time units and time-

stratigraphic units, but these units are not self-evident nor directly

observable; rather they are inferential and are the result of complex

intellectual and philosophical processes.

-27-

The inferential units can be divided into: A)

Chronostratigraphic divisions which are subdivided into

Geochronological units (or Geological time units) and

Chronostratigraphic units (or time-stratigraphic units), and B)

Ecostratigraphic units.

A) Chronostratigraphic Divisions

The sedimentary rocks deposited during a particular interval of

geological time form a series of stratigraphic units which correspond

to the eras, periods, epochs and ages of the stratigraphic time-scale as

follows:

Eon Eonothem

Era Erathem

Period System

Epoch Series

Age Stage

The terms on the left constitute a hierarchy of geological time

units (or geochronological units) which is used to divide up geological

time, while the terms on the right represent the equivalent hierarchy

of time-stratigraphic units or (chronostratigraphic units), which is

used to divide up the stratigraphic record according to geological age.

These time-stratigraphic units are named after the particular intervals

of geological time when they were deposited. Thus, the rocks of the

Tertiary System were deposited during the Tertiary period. They are

formed by the Pliocene, Miocene, Oligocene, Eocene and Paleocene

Series, which were deposited during the corresponding epochs of the

-28-

Tertiary period (see Table 2). Since epochs and series often have no

specific names, individual periods are often divided into early, middle

and late epochs, while the corresponding parts of the equivalent

system are termed lower, middle and upper series of that system. For

example, the rocks of Lower Carboniferous Series were deposited

during the Early Carboniferous Epoch.

The rocks of a stratigraphic series can be further divided into

stages, characterized by fossil assemblages which are considered to be

diagnostic of their geological age. However, it is the longer intervals

of geological time, and the corresponding parts of the stratigraphic

record, which mostly concern us in the analysis of geological maps in

terms of geological history.

Geological time units (geochronological units), segments of

continuous geologic time (Periods, Epochs, and Ages). Time-

stratigraphic units (chronostratigraphic units), assemblages of strata

representing deposition during distinct time units (Systems, series,

and stages).

Periods and Systems. As shown by the history of stratigraphic

classification, the classical systems were the first widely applied time-

stratigraphic units. The systems, and the periods which represent their

times of deposition, remain the fundamental units of geochronology.

Epochs and Series. Systems are subdivided into smaller time-

stratigraphic units called series. Each series represents a portion, or

epoch, of geologic time within a period.

-29-

Ages and Stages. Epochs of geologic time are divisible into ages

which are represented by time-startigraphic units below series rank,

termed stages.

Eras. The grouping of geologic periods in larger time units,

eras, is widely accepted and used in the discussion of earth history,

although no time-stratigraphic term is applied to the strata deposited

during an era.

Eons. In writing and speaking of geologic history, it is

frequently necessary to differentiate between Precambrian and later

time. "Precambrian rocks" or "pre-Paleozoic time" effectively dispose

of part of the problem. Since Precambrian eras are seldom applied,

geologists cannot utilized "post-Proterozoic" for reference to later

rocks and events, and most writers shrink from using the logical but

awkward term, "post-Precambrian". Many ways around this dilemma

have been suggested, but none has been widely adopted. Perhaps the

use of "Cryptozoic Eon" for Precambrian time and "Phanerozoic

Eon" for later time, as proposed by Chadwick (1930) has the greatest

possibility of successful application, but these terms are seldom

encountered in geologic literature.

The Cryptozoic Eon is subdivided into Archeozoic and

Proterozoic eras, while the Phanerozoic Eon is subdivided into

Paleozoic, Mesozoic, and Cenozoic eras.

-30-

The Stratigraphic Time-scale

The evolution and extinction of organisms gives a definite and

recognizable order to the succession of fossil assemblages which are

preserved in sedimentary rocks. This means that the fossil record can

be used to erect a stratigraphic time-scale which allows geological

time to be divided into distinct units. Arranged in order of decreasing

length, these units are known as Eras, Periods, Epochs and Ages.

These terms refer to particular intervals of geological time as defined

by fossil record. Thus, geological history over approximately the 560

million years is divided into the Paleozoic, Mesozoic and Cenozoic

Eras. Each of these eras is divided into a number of periods, as shown

-31-

in table 2. Each period is formed by a number of epochs, and this

division of geological time can be continued by dividing each epoch

into a number of different ages. However, it is rarely possible to

recognize such a short interval of geological time on a world-wide

basis.

B) Ecostratigraphic Units. A stratigraphic column may be

subdivided into other inferential units by consideration of the

environmental significance of the rocks and their contained fossils.

The environment of deposition, like the time of deposition is not a

directly observable attribute of sedimentary rocks. However, ecologic

analysis and interpretation of mineralogy, depositional structures and

textures, and fossil fauns make possible the recognition of vertically

successive ecostratigraphic units or ecozones.

TABL E 2:

-32-

V) Stratigraphic Procedures

Introduction:

At this point it is necessary to become familiar with a number of

operations and procedures used in gathering and analysis of

stratigraphic data and materials. This part intended to produce a

variety of procedures constantly utilized in academic and applied

stratigraphy.

OUTCROP PROCEDURES:

1- Measured Sections

Accurately measured and properly described stratigraphic

sections form the basis for the most studies of strata in outcrop. From

measured sections are derived data used in correlations, as well as

information on thickness and lithologic variations, positions of

faunas, and stratigraphic relations of various rock units. The approach

toward completion of stratigraphic work in any outcrop study is

largely a function of the number of sections measured, studied and

analyzed.

The methods and procedures of section measurement cannot be

summarized in a general fashion, since each area presents its own

problems. A measured section is useful only if it yields the data

required on lithology, paleontology, stratigraphic relations, and

thickness for the particular problem in question. In reconnaissance

stratigraphy, which involves the correlation of units over wide areas,

it is often more important to measure ten or twenty sections in only

-33-

moderate detail to establish general relationships than to spend a field

season in detailed work on one section.

2- Selection of Sections To Be Measured

The proper choice of locations for section measuring is an

important factor in determining the value of the results and the

efficiency in procuring such results. In some areas, the choice is

limited by the lack of good exposures, whereas in areas of numerous

exposures, choice is made on the basis of spacing between sections,

amount of the strtaigraphic column present, degree of exposure or

cover, structural simplicity, and accessibility.

Spacing is important, since, in the stratigraphic analysis of any

area, it is necessary to get as wide a coverage as possible within the

time available for field work. Stratigraphic studies usually involve

either the entire stratigraphic column of an area, or certain rock or

time-stratigraphic units. In the first case, the sections chosen should

permit the measurement and study of as much of the total column as

possible. Furthermore, they should begin and end with horizons that

may be correlated with adjacent sections in order that the column

may be pieced together.

Where individual units, or groups of units, are involved, the

selected sections should, if possible, expose the tops and bottoms of

the portion of the column concerned.

Often it is also necessary to sacrifice other factors in order to

choose an uncomplicated section. Gentle folding and obvious

transverse faults offer no special difficulty, but complex structures are

serious deterrents to accurate section measuring. The steep or

-34-

overturned limbs of folds often appear to offer ideal sections for

measurement, but such locations are subject to radical thinning,

elimination of incompetent units, undetectable repetition, or

elimination by strike faults. There have been published examples of

sections in which units appear with abnormal thicknesses, later

proven to be the result of isoclinal's folding, and of other cases of

sections measured and recorded upside down.

3- Description of Measured Sections

Adequate description is required in order to extract the

maximum amount of the stratigraphic information from any exposed

sequence of strata. The description of a measured section should

include observations on the thickness of units, their stratigraphic

relationships, lithology, stratification, internal structures, weathering

behavior, and paleontology.

In dealing with the thickness and other attributes of strata in a

measured section, reference is usually made to two kinds of "units".

First there are such formal rock units as formations and members,

recognizable in all or part of the area involved. Each formation or

member is further divisible at the locality of section measuring

according to lithology, bedding, exposure, weathering effects, or some

other feature, into convenient subunits which permit the recording of

differences in character within formations and members. Subunits are

usually individual beds or groups of beds differentiated from those

above and below by unity of color, texture, or gross appearance.

Subdivisions in relatively homogeneous strata are sometimes

split on the basis of a horizon of chert nodules in a limestone, a

-35-

covered interval, a change in the resistance to weathering, or some

similar minor feature.

The staratigraphic relationships of all rock units, such as

members and formations, at the point of section measurement should

be thoroughly investigated and recorded as being transitional, sharp,

disconformable, or unconformable.

The lithology of each unit is described in terms of the dominant

rock type (Sandstone, shale, limestone, and so forth); the texture

(coarse, medium, or fine-grained; sorting; shape; and roundness); the

color (of fresh surface); and the mineralogy of the detrital particles

and cement, if any. Associated lithologies are separately recorded

with mention of the percentage of each type present and the manner

of its association with the dominate type. Figure – illustrates a typical

field description of a measured section.

Where fossiliferous units are encountered, adequate description

includes a list of the faunal elements present, and some measure of the

relative number of fossils representing each genus or species. Accurate

paleontologic description usually requires the collection of

representative fossils for later study, if they can be gathered in

identifiable form.

-36-

4- Lithologic Sampling

In addition to observations made in the field, modern

stratigraphic work requires a refinement of lithologic detail by

microscopic examination and laboratory treatment. For these

purposes, it is necessary to collect a sample from each subunit in the

measured section. Normally, the samples need not be large – less than

handful in most cases – but even small samples become a considerable

-37-

burden when thick sections involving tens or hundreds of units are

studied. However, the value of "bringing the stratigraphic section into

the laboratory" is more than worth the effort expanded.

It should be noted that the systematic sampling procedures

discussed above, as well as the parallel procedures for paleontologic

collecting discussed below, are for the purpose of preparing a more

complete description of the stratigraphic section through microscopic

and laboratory analysis.

5- Fossil Collecting

Fossils collections should be limited to representative specimens

of the genera and species present and the relative abundance of each

noted. Workers familiar with the biologic elements encountered will

be able to enter field identification in the notes without making

extensive collections, whereas others with less paleontologic

experience will have to collect more frequently for later identification.

Certain fossil types, such as bryozoa, corals, and lager foraminifera,

require laboratory techniques for identification.

If microfossils are sought, adequate samples for later

examination must be collected in addition to the smaller samples. In

any case, all fossil collections should be clearly labeled with the

number of the subunit from which they are derived, since such

collections lose most of their startigraphic value if their proper

position in the measured section cannot be assigned.

-38-

6- Measuring Horizontal Strata

Measurements in horizontal strata do not require horizontal

control; therefore, any means of accurate measurement of elevation

can be adapted to section measuring. A number of methods are used,

the choice depending on the degree of accuracy required and the

topography of the area under investigation.

One of the most applied methods utilizes the hand level, the unit

of measurement being the eye-level height of the worker. Fig.5.

illustrate the method. The hand level is portable and easy for one man

to use. Hand leveling is well adapted to measure on moderate slopes,

and is widely used in the flat-lying strata.

Where very steep slopes or benches and cliffs are encountered,

the Jacob's staff or "Jake-stick" method is applicable, as illustrated in

Fig. 6. This procedure employs a rod, usually five feet in length,

which may be either specially prepared and subdivided or cut from

local timber. Measurements are accomplished in a series of five-foot

steps, zigzagging up the slope to avoid obstructions and covered

areas.

-39-

6- Measuring Inclined Strata

Sections measured in non-horizontal strata require modified

methods, since the dip must be accounted for in calculation of

thickness. If measurement is done on steep slopes with good

exposures, the Jacob's staff can be used by holding it at right angles to

the bedding planes and by five-foot intervals. Kummel (1943) has

described a clinometer attachment for a Jacob's staff which permits

accurate five-foot readings in the absence of perpendicular joints.

The method most commonly applied in measuring inclined

strata is traversing with Brunton and tape, illustrated by figure 3-5 . It

is often possible to stretch the tape across the subunits at right angles

to the strike to obtain the slope distance of outcrop. Then, from slope

angle and dip, the stratigraphic thickness of the subunit can be readily

computed, trigonometrically or graphically.

-40-

7- Laboratory Study of Outcrop Samples

Many details of lithology cannot be sufficiently studied in the

field, some because of insufficient time during section measuring,

others because they are not readily observable with a hand lens. Field

descriptions of carbonate rocks and fine-grained clastics seldom do

justice to the wealth of valuable detail these sediments can yield to

microscopic study.

When sandstone grains are split into heavy and light fractions

by gravity separation in heavy liquids, many mineralogical details not

detectable in the gross sample are exposed. Such data are useful in

correlation and in studies of source and depositional environments of

sediment.

Many textural details of clastic rocks cannot be adequately seen

in a routine microscopic examination. The particle-size distribution of

sandstone, silts, and shales must be determined by mechanical

analysis usually by sieving and settling velocity techniques.

Differential thermal and X-ray analyses of shales can be interpreted in

-41-

terms of clay minerals present and thus implement detailed lithologic

description and analysis.

8- Presentation of Outcrop Data

a) Written Descriptions. A written description of every measured

section is presented which include all the data gathered in the field

and in the laboratory for each rock unit, as well as the results of

paleontologic investigation.

b) Geologic Cross Sections. Geologic cross sections conveniently

illustrate both stratigraphy and the relationship of the stratigraphy and

topography. Geologic cross sections may be used to illustrate the

position and occurrences of various stratigraphic units within a

mountain or sedimentary basin (fig. 3-6). They cannot, however,

show much stratigraphic or lithologic detail without great vertical

exaggeration of scale and consequent distortion of relief and structure.

c) Columnar Sections. Columnar sections are the most useful and

familiar graphic means of expressing the stratigraphic data of the

measured sections. Columnar sections show the sequence,

interrelations, and thickness of stratigraphic units and illustrate their

lithology by conventional symbols.

The selection of vertical scale makes possible the expression of

whatever degree of detail is available or desired. Each unit is shown

with the proper scale thickness in its proper position in the column.

Symbols expressing the chief lithologic attributes of each

subunit and certain accessory features are entered within the main

body of the column. The position and distribution of chert and other

-42-

significant details, the spacing of bedding planes, cross-lamination,

and unconformities can be indicated.

Color and texture are best shown by separate strips of symbols

along one or both sides of the column.

The range of paleontologic zones may be represented by

brackets or arrows adjacent to the column, each zone being identified

by its characteristic species or by a symbol referring to an annotated

faunal list.

A properly organized columnar section is capable of expressing

practically all the significant physical and biological data obtained

from measurement and analysis of a stratigraphic section.

-43-

-44-

d) Stratigraphic Cross Sections. Stratigraphic cross sections differ

from geological cross sections in that they do not attempt to illustrate

the topographic profile, and structure is either restored or

diagrammatically expressed. Moreover, vertical scale is greatly

exaggerated in order to show stratigraphic detail. Cross stratigraphic

sections are drawn by arranging series of columnar sections side by

side in proper geographic sequence. In indicating actual distance from

one location to another, a uniform spacing between columns may be

used, or irregular spacing may be used to show the actual separation.

Lithologic and faunal correlations are indicated by lines connecting

the proper horizons form column to column.

-45-

VI) Stratigraphic Relationships

The present chapter attempts to examine the fundamentals

governing the relationships among sedimentary bodies and seeks for a

usable language by which the relationships may be expressed.

Lithosome: The term lithosome is proposed for these masses of

essentially uniform lithologic character which has vertical and lateral

relationships with adjacent masses of different lithology.

1. Vertical Relationships among Lithosomes

The vertical relationships between successive sedimentary

bodies are the most easily visualized, since they can be studied at a

single point of observation, either in outcrop or in the subsurface,

without involvement with the problems of lateral changes in

character.

1.A. Conformable Relationships

Surfaces of contact between vertically successive lithosomes are

considered conformable if there is no significant evidence of

interruption of deposition between adjacent units. Conformable

contacts may be abrupt, gradational, or intercalated. In each case, the

change in lithologic character reflects a shift in the conditions of

deposition or in the materials brought to the site of deposition.

Abrupt Contacts. Abrupt, yet strictly conformable, contacts

between vertically successive lithosomes are relatively rare. Where

such contacts can be observed, they do not persist over large areas but

commonly pass laterally into unconformable relationships. Abrupt

-46-

conformable contacts resulting from primary causes are most

frequently encountered in areas of very slow deposition, in which

changes taking place over span of thousands of years are represented

by accumulations measured in fractions of inches.

Secondary post-depositional effects can result in sharply defined

lithosomes. Dolomitization, for example, may selectively effect

certain elements of a previously uniform limestone lithosome,

producing abrupt distinctions in color, grain size, and resistance to

weathering.

Gradational Contacts. Normally, the changes in material and

conditions that differentiate successive sedimentary bodies are gradual

throughout a variable thickness of stratigraphic section. The resulting

gradational contacts are of two types, mixed and continuous

contacts. Mixed gradation occurs where two distinct sediment types

grade from one to the other. Sandstone, for example, may grade

upward into shale by gradual admixture of clay. Starting with a clay

matrix in the voids between sand grains, the composition may change

to sand grains enclosed in clay laminae, then to "floating" sand grains

in a mass of clay, and finally to pure, sand-free, clayey shale.

Continuous gradation involves the progressive changes in a single

sedimentary parameter, without mixing of end members. Examples

are found in sand-to-shale gradations in which there is a progressive

reduction in grain size from sand to silt to clay.

Intercalated Contacts. The majority of intercalated lithosome

contacts are problems of formation and member definition rather than

questions of lithosome differentiation. When as, for example, in a

sand body that becomes interbedded with shale before giving way to

-47-

an overlying shale body. The interbeds are seen to be tongues

extending from the main bodies of shale and sandstone, each of which

exhibits abrupt or gradational contacts with lithosomal tongues above

and below.

1.B. Unconformable Relationships

Relationships between lithosomes separated by a surface of

non-deposition or erosion are unconformable, and the separating

surface is an Unconformity.

What is an Unconformity?

An unconformity is a buried erosion surface. It is a surface of

rock that was exposed on the Earth's surface and was then covered by

younger layers. Unconformities are important because they represent

missing intervals of the geologic record, like pages missing from a history

book.

An unconformity is a buried erosional or non-depositional

surface separating two rock masses or strata of different ages,

indicating that sediment deposition was not continuous. In general,

the older layer was exposed to erosion for an interval of time before

deposition of the younger, but the term is used to describe any break

in the sedimentary geologic record.

Unconformities are gaps in the geologic record that may

indicate episodes of crustal deformation, erosion, and sea level

variations. They are a feature of stratified rocks, and are therefore

usually found in sediments. They are surfaces between two rock

bodies that constitute a substantial break (hiatus) in the geologic

-48-

record (sometimes people say inaccurately that "time" is missing).

Unconformities represent times when deposition stopped, an interval

of erosion removed some of the previously deposited rock, and finally

deposition was resumed.

Commonly four types of unconformities are distinguished by

geologists:

Fig. Unconformities within the stratigraphic succession comprising

the wall rocks of the Grand Canyon, Arizona.

1- Angular Unconformity: An angular unconformity is an unconformity

where horizontally parallel strata of sedimentary rock are deposited on

tilted and eroded layers, producing an angular discordance with the

overlying horizontal layers. The whole sequence may later be

deformed and tilted by further orogenic activity.

-49-

Angular Unconformities are those where an older package of

sediments has been tilted, truncated by erosion, and than a younger

package of sediments was deposited on this erosion surface. The

sequence of events is summarized in the pictures at left. First:

subsidence and sediment deposition occurs; Second: rocks are uplifted

and tilted (deformation); Third: erosion removes the uplifted mountain

range; Fourth: subsidence occurs, the sea covers the land surface, and

new sediments deposition occurs on top the previous land surface.

Then the cycle may repeat. (fig. ).

Fig. Development of an angular unconformity. Angular unconformity. An

angular unconformity can form when rocks that were initially horizontal are

tilted, eroded and subsequently buried beneath younger layers of sediment.

For geologists, one of the most famous angular unconformity is

the Grand Unconformity in the Grand Canyon of Arizona. Here tilted

sedimentary rocks of Precambrian age (lower half of photo 1) are

overlain by younger sedimentary rocks of Phanerozoic age (Cambrian

and younger, upper half of photo). The two packages of strata are

clearly separated by an angular unconformity that is best seen just left

of the center of the photo.

-50-

2- Disconformity: a surface separating essentially parallel strata,

indicating the absence of beds represent certain ages due to a period of

erosion or no deposition.

Disconformities are also an erosion surface between two

packages of sediment, but the lower package of sediments was not

tilted prior to deposition of the upper sediment package.

Disconformities are marked by features of subaerial erosion. This type

of erosion can leave channels and paleosols in the rock record. The

sequence of events is as follows: First: subsidence and sediment

deposition; Second: uplift and erosion; Third: renewed subsidence and

deposition. Because the beds below and above the disconformity are

parallel, disconformities are more difficult to recognize in the

-51-

sedimentary record. In the diagram below, the disconformity is

indicated by an irregular black line between the 3rd and 4th rock unit

from the bottom.

This picture shows a disconformity in Capitol Reef National Park, Utah. The Chinle Formation

(Triassic), the slope forming unit in the central portion of the picture, has a very sharp contact

(black line) with the overlying Wingate Sandstone (uppermost Triassic, forms steep cliff). This

contact is considered a disconformity.

3- Nonconformity: a surface separating the igneous or metamorphic

rocks from the overlying sedimentary starta. They usually indicate that

a long period of erosion occurred prior to deposition of the sediments

(several km of erosion necessary). In the diagram at left, the

igneous/metamorphic rocks below the nonconformity are colored in

red.

-52-

A nonconformity in the Obigarm River gorge in Tajikistan. The red line

points out the irregular contact between Precambrian granitic rocks (black

color at the base) and the Quaternary sediments (grey color at the top).

4- Paraconformity: A paraconformity is a type of unconformity in

which strata are parallel; there is little apparent erosion and the

unconformity surface resembles a simple bedding plane. It is also

known as nondepositional unconformity or pseudoconformity

Barrell (1917) introduced the term diastem to refer to the slight

discontinuities in marine sediments that indicate minor interruptions

in deposition. It should be distinguish between major breaks in the

sedimentary record (unconformities) and minor interruptions of brief

duration (diastems). In order to make this distinction, diastems were

Granite

Quaternary

sediments

-53-

defined as breaks in the record involving a "bed or series of beds" but

not an entire formation. The implication that a formation has a

definite time value is no longer acceptable, but the differentiation

between major and minor discontinuities remains valid and useful.

ؽتبرث ؽ شعخ يخؾرر شتب اهذ يذل يجيؽخ ي اهعتلبح ، اهشفو يب يعج :ػدن الحوافق .أ يجؾدث اهؾوب أفلج

: أىواع ػدن الحوافق

ؽدى اهخافق اهزاAngular Unconformity : يذل شعخ فضل ت عتلبح يبئوج أخر .يبئوج هنيب يخخوفخ ف اهيل

الزاوىعدم التوافق

ؽدى اهخافق االلعبؽDisconformity : ن ؽو شنل شعخ يخؾرر ت عتلبح يخازج أ .يبئوج أ أفلج

عدم التوافق االنقطاعي

-54-

ال حوافقNonconformity : خن ذا اهػ ي ؽدى اهخافق ؽديب خى خرشة يجيؽج ي .اهعتلبح اهرشتج فق ضخر برج أ يخدهج

الال توافق

خن ذا اهػ ي ؽدى اهخافق ؽديب خى خرشة يجيؽخ ي Paraconformityشةه حوافق . اهعتلبح اهرشتج هيب فس اهيل ، فضويب شعخ خؾرج غر اظخ

شبه توافق

Recognition of Unconformities

There are many criteria available for the recognition of the

unconformable surfaces, both in outcrop and in the subsurface. These

criteria falls into three classes: sedimentary, paleontologic, and

structural. The following treatment lists several of each.

-55-

Sedimentary Criteria. More than twenty sedimentary criteria

have been proposed. The more important ones include the presence of

a basal conglomerate, residual (weathered) chert, buried soil profiles,

and zones of glauconite, phosphatized pebbles, or magniferous zones.

The first three criteria are generally accepted as evidences of

subaerial disconformities. The latter three, represent submarine

disconformities or diastems, indicative of solution or nondeposition.

Paleontologic Criteria. More than a half dozen paleontologic

criteria are known, among which three are the most significant.

Abrupt changes in faunal assemblages, gaps in evolutionary

development, and the occurrence of bone and tooth conglomerates

represent generally accepted criteria.

Perhaps the most decisive criterion of an unconformity is an

abrupt phylogenetic change from one fossil assemblages to another in

vertical succession of the rocks. If the beds below a stratigraphic

surface contain Lower Silurian fossils and the beds above contain

Pennsylvanian forms, a subaerial unconformity of major proportions

is established.

Less marked faunal changes indicate unconformities of lesser

magnitude. A gap in the orderly evolution of a single organism

through a vertical series of beds is indicative of a hiatus.

Structural Criteria. Four structural criteria are recognized.

Discordance of dip above and below a contact is a distinctive criterion

of an angular unconformity. An undulatory surface of contact which

cuts across bedding planes of the underlying formation marks a

disconformity caused by emergence and erosion.

Truncation of dikes at a surface of contact, with no evidence of

thermal alteration of the overlying beds, marks subaerial

unconformities caused by erosion. Similarly, relative complexity of

-56-

faults above and below a surface of contact may indicate an erosional

disconformity. If the lower formation is more complexly faulted, with

abrupt cessation of fault planes at the contact, an unconformity is

established.

2- Lateral Relationships Among Lithosomes

All sedimentary bodies, large and small, widespread and local,

have laterally bounding peripheries. In many instances, the lateral

boundary is the result of erosion and the edge of the lithosome no

longer represents its original depositional limits. Or, a particular

lithosome may be bounded by lateral passage into beds of a different

lithology. Such lateral termination of a lithosome may involve pinch-

out, intertonguing, or lateral gradation.

Pinch-out

The term pinch-out is applied to the termination of a lithosome,

a sandstone for example, that thins progressively to extinction. Pinch-

out may be accompanied by an increase in the thickness of an

adjacent body (a shale, for instance) which may lie above, below, or

both above and below; or, pinch-out may involve thinning or

convergence of the stratigraphic section. In a typical pinch-out, the

lithologic character of the lithosome is maintained right to the feather-

edge of the body. Commonly, the angle formed by the converging

upper and lower surfaces of the lithosome is very small – one degree

or less. However, in some bodies such as reefs and channels,

significant thicknesses may be reduced to nothing in a few hundred

feet.

-57-

Pinch-outs are among the most eagerly sought after

sedimentary relationships, since many stratigraphic-trap oil and gas

fields have been discovered along the pinch-out zones of porous and

permeable lithosomes.

Sandstone pinch out in shales in the Jurassic Morrison Fm. near

Green River.

Intertonguing

Some bodies of sediment disappear and are lost in laterally

adjacent masses owing to splitting into many thin units, each of which

reaches an independent pinch-out termination. The resulting

intertonuing zone has many vertically successive intercalations of thin

representatives of two lithosomes. The numbers of tongues increases

with the distance from the main mass of either sedimentary body,

reaching a maximum as tongues split again and falling as individual

tongues pinch out.

-58-

This view looking down Golden Canyon shows red and yellow interbedded strata of the

Furnace Creek Formation on an unnamed hill (between Golden Canyon and Gower Gulch).

The inter-tonguing beds perhaps reflect changing shoreline depositional environments

(between terrestrial and lake deposits) within an ancestral intermountain basin in the mid-to-

late Pliocene (about 3 million years ago) before the formation of modern Death Valley (Hunt

and Mabey, 1966).

White Sands National Monument, New Mexico. Latex peel of vertical cross section of dune,

showing intertonguing cross-wind deposits and fade-outs from avalanching. 1959.

Lateral Gradation

In lateral gradation, there is no necessary thinning of rock units.

Instead, lithosomes are commonly terminated by gradual replacement

of their lithologic characteristics by those of another type. Lateral

gradation may be mixed or continuous, similar in nature to the

vertical gradational relationships described earlier, except that the

lateral rate of change is normally very low, and distances of hundreds

of feet, or even miles, must be traversed in order to observe the same

-59-

magnitude of change exhibited within a few beds in vertical

succession.

Mixed lateral gradation of sandstone to shale is typical of the

“shale-out” of petroleum geologists and is responsible for oil and gas

trapping where the zone of gradation is relatively narrow. Similar

relationships are common between carbonate rocks and shale and

between carbonates and sandstone. Continuous gradation is

exemplified by many sand-to-shale patterns and is also common

among carbonates, as in the gradual change of a coarse biogenic

limestone to a micritic lithesome.

-60-

VII) Principles of Correlation

Stratigraphic correlation is the demonstration of equivalency of

stratigraphic rock units. Recognition of the distinctions that separate

rock-stratigraphic, biostratigraphic, and time-stratigraphic units makes

it apparent that “equivalency” may be expressed in lithologic,

paleontologic, or chronologic terms. Therefore, most stratigraphers

apply the noun “correlation” the verb “correlate” and the adjective

“correlative” to lithostratigraphic equivalency as well as to time-

stratigraphic equivalency. In each situation, it is necessary to identify

the nature of the equivalency, the kind of correlation involved (such

as lithologic correlation, biostratigraphic correlation, and so on).

Correlation is an essential element of most stratigraphic investigations

and typically occupies a major portion of a stratigrapher’s time.

Without correlation, the facies relationships, and others discussed in

the preceding chapter, would be meaningless and the rational

treatment of the analytical aspects of stratigraphy would be

impossible.

A) Correlation of Lithostratigraphic Units