Prepared by: Dr. Emad Samir Sallam Lecturer of , Dept., Faculty of Science, Benha University.

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I) Introduction

Sedimentary rocks are formed by materials, mostly derived from the and erosion of the pre-existing rocks (Igneous, metamorphic or even sedimentary rocks), which are transported and then deposited as 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 after its and before the onset of either metamorphism or weathering (Fig. 1).

Fig. 1. Rock Cycle

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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 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 . 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 ).

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 . They are composed of rock fragments and

- 3 - 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 , dolomites, cherts, evaporites and . 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 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.

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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).

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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

- 6 - sedimentological processes. (2) 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. : Classification of sedimentary rocks according to their fossil content. 3. : 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 or (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.

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

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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

- 9 - 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.

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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

- 11 - 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".

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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

الهةدأ األول: هةدأ حؼاكب الطةلاج ) (:

اكخرح ُذا اهيتدأ يً كتل اهؾبهى االٖعبهٕ شخٌّٖ Nicholas Steno )-1686 1638(. ٌّٖص ؽوٓ اً: فٕ أٔ خخبتؼ يً اهضخّر اهيخعتلج ّاهخٕ هى خخؾرط اهٓ ؽيوٖج خشَّٖ تبهخفوق أّ اهعٕ فأً نل عتلج كد خنٌّح تؾد اهعتلج اهخٕ خشفوِب ّكتل اهعتلج اهخٕ خؾوُّب.

ّهِذا اهيتدأ يدهّال زيٌٖب اٖظب. فدٌٖيب خخؾبكة اهضخّر تشنل أفلٕ فأً اهضخّر اّ اهعتلبح اهخٕ فٕ االشفل ُٕ األكدى ّاهخٕ فٕ األؽوٓ ُٕ االددد ألٌِب خنٌّح فٕ فخرث زيٌٖج الدلج، ّنيب يّظخ فٕ اهشنل )1(.

الشكل )1(: ٌوضح الحؼاكب فً حرشٌب الطةلاج زهىٌا هو الٌهٌو الى الٌشار.

ايب هّ خؾرظح اهعتلبح اهٓ ؽيوٖبح اهخشّٖج نبهخفوق اّ اهعٕ اهشدٖد ّاهخٕ خؤدٔ تبهخبهٕ اهٓ كوة اهعتلبح اّ اٌدفبؽِب ّتبهخبهٕ خشٍّ اهيّكؼ االضوٕ هِب، فبٌَ خشخخدى عرق اخرْ هخددٖد اهخؾبكة االّهٕ اهضدٖخ هِذٍ اهعتلبح كتل خأذٖر ؽيوٖبح اهخشٍّ ّيً ُذٍ اهعرق

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ُّ اشخخداى اهيخدجراح هخددٖد اؽيبر اهعتلبح ّ اشخخداى اهخرانٖة اهرشّتٖج نبهخشللبح اهعٌٖٖج ؽّاليبح اهٌٖى ّاهخٕ ٖينً يً خالهِب خددٖد نل يً اهشعخ اهؾؤّ ّاهشفوٕ هوعتلبح.

الهةدأ الداىى: هةدأ األفلٌث األصمٌث لمطةلاج ) Law of Original Horizontality of :)Beds

اكخرح ُذا اهيتدأ يً كتل اهؾبهى االٖعبهٕ شخٌّٖ Nicholas Steno )-1686 1638( اٖظب. اهذٔ الدغ اً خرشة جزٖئبح اهضخّر اهرشّتٖج ٖددد تفؾل خبذٖر اهجبذتٖج األرظٖج ؽوِٖب ّتبهخبهٕ شلّعِب فٕ كبػ اهتدر اّ أ دّط رشّتٕ. ّتذهم خنًّ عتلج افلٖج يّازٖج هشعخ األرط، ّنيب يّظخ فٕ اهشنل ) 2(. ّفٕ دبهج ّجّد ضخّر رشّتٖج يٌدٌٖج فِذا ٖدل ؽوٓ اٌِب خؾرظح هدرنج ارظٖج ) نبهعٕ اّ اهخفوق( تؾد خرشتِب ّخضوتِب.

الشكل )2(: ٌوضح كاىوو الحؼاكب األفلً. )A( حرشب الرواشب ةشكل أفلً. )B( حهٌل الطةلاج لمحرشب افلٌا ححى لو كاىج االرضٌث غٌر هشحوٌث حهاها .)Montgomery, 1997(

الهةدأ الدالخ: اإلىحظاهٌث أو هةدأ الوحٌرت الواحدت ) Law of Uniformitarianism(:

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ُذا اهيضعوخ يشخق يً اهنويج االٌنوٖزٖج يٌخغى اّ يخيبذل ) uniform( ّٖراد تَ اٌخغبى اّ ذتبح اهؾّايل اهعتؾٖٖج ٌّّاخجِب ؽوٓ شعخ االرط. ٌّٖص ؽوٓ اً: نبفج اهلّاًٌٖ اهفٖزٖبّٖج ّاهنٖيٖبّتج فٕ اهنًّ نبٌح ّيب خزال ّاددث".

يذبل/ ٖينً خّظٖخ ُذا اهيتدأ يً خالل اخخبذ اهرٖبح نيذبل دٖد خؾد اهرٖبح اهؾبيل اهعتؾٖٕ اهيشتة هخنًّ اهنذتبً اهريوٖج فٕ اهّكح اهدبظر. ُّٕ ٌفس اهؾبيل اٖظب خالل اهؾضّر اهلدٖيج. هذا فبً يشبُدخٌب هونذتبً اهريوٖج اهيدفّغج فٕ ضخّر كدٖيج خشبؽدٌب ؽوٓ خددٖد عتؾٖج اهيٌعلج ّاخجبٍ اهرٖبح فٕ خوم اهفخرث.

ّتذهم فلد ؽرف اهجّٖهّجًٖ يتدأ اهّخٖرث اهّاددث خؾرٖفب تشٖعب اضتخ يً اهتدِٖٖبح فٕ ؽوّى االرط، ُّّ : أً )) اهدبظر يفخبح هويبظٕ " The present is the key to the past" ((. أٔ أً يالدغخٌب هوؾّاهى اهعتؾٖٖج ) نبهرٖبح ّاهيٖبٍ ّاهزالهزل ّغٖرُب( خشبؽدٌب ؽوٓ فِى اهؾدٖد يً اهغّاُر اهعتؾٖٖج اهخٕ ٌشبُدُب ؽوٓ شعخ االرط ّاهخٕ ؾّٖد زيً ددّذِب اهٓ يالًٖٖ اهشًٌٖ.

الهةدأ الراةغ: ػالكث اللواطغ ( Cross Cutting Relation ):

ٌٖص يتدأ اهلّاعؽ ؼوٓ أً أٔ ّددث ضخرٖج أّ فبهق ٖلعؼ ّددث ضخرٖج أخرْ فأً اهّددث اهضخرٖج أّ اهفبهق ُّ أددد يً اهّدداح اهضخرٖج اهيلعؽّج، نيب يّظخ تبهشنل .)3(

الشكل )3(: ٌوضح ػالكث اللواطغ فً هجهوػحٌو هو الصخور الىارٌث، )Brice et al., 1997(.

الهةدأ الخاهس: كاىوو حؼاكب األحٌاء الحٌواىٌث والىةاحٌث ) Law of Faunal and Floral :)succession

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ٌٖص ُذا اهيتدأ ؽوٓ أً اهيخدجراح اهٌتبخٖج ّاهدّٖاٌٖج خخؾبكة اهّاددث تؾد األخرْ تشنل ذبتح ٌّغبى يددد، أٔ أً اهيخدجراح األكدى خنًّ فٕ األشفل ّاألددد خلؼ فٕ األؽوٓ. ّ ؾٖد ُذا اهيتدأ يً أُى اهيتبدئ فٕ اهجّٖهّجٖب اهخأرٖخٖج دٖد ؾٖد األشبس فٕ خددٖد اؽيبر اهّدداح اهضخرٖج ّخددٖد اهؾضّر اهجّٖهّجٖج. ؽويب أً اهخؾبكة فٕ األدٖبء ُّ ٌخٖجج هؾيوٖج اهخعّر اهؾظّٔ ُّّ اشبس خلشٖى خأرٖخ األرط، ّنيب فٕ اهشنل )4(.

الشكل )4(: ٌوضح حؼاكب االحٌاء خالل ػهود طةاكً ٌهحد هو الةرٌكاهةري الى الكاهةري. حٌخ ٌالحظ ةدء الؼصر الكاهةري ةظهور اصداف الحراٌموةاٌج كةل 545 همٌوو شىث، فً حٌو حححفظ صخور الةرٌكاهةري ةطواةغ الدكىوشوىٌا كةل حوالً 500 همٌوو شىث. )Murck et al., 1999(.

الهةدأ الشادس: هةدأ األححواء )Law of Inclusions(:

ٌٖص ُذا اهيتدأ ؽوٓ اً اهعتلبح اهضخرٖج اهخٕ خدخّٔ ؽوٓ كعؼ ضخرٖج يً عتلج ذبٌٖج يخخوفج ؽٌِب فأً اهعتلبح األّهٓ ُٕ األددد يً اهذبٌٖج، ّنيب يّظخ فٕ اهشنل ) 5(. ّٖينً خّظٖخ ذهم تبً ّجّد كعؼ هعتلبح ضخرٖج فٕ عتلج أخرْ التد اٌِب خنٌّح أّال ذى خؾرظح هِيوٖبح اهخؾرٖج ّيً ذى خرشتح فٕ اهعتلج اهذبٌٖج األددد.

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

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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).

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An outline of the classification of stratigraphic units is presented in the following table.

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- 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

- 19 - 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 , 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

- 20 - 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).

- 21 -

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.

- 22 -

- 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.

- :

Two or more successive formations, related by lithology or by position with reference to , 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 -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

- 23 - 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.

- 24 - 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 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 - 25 - 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.

- 26 -

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

Era Erathem

Period System

Epoch Series

Age

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

- 27 -

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 .

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.

- 28 -

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.

- 29 -

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

- 30 - 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. TABL E 2:

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.

- 31 -

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

- 32 - 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

- 33 - 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

- 34 - 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.

- 35 -

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

- 36 - 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.

- 37 -

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.

- 38 -

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.

- 39 -

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

- 40 - 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

- 41 - 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.

- 42 -

- 43 -

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.

- 44 -

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

- 45 - 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

- 46 - 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

- 47 - 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.

- 48 -

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.

- 49 -

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

- 50 - 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, . The (Triassic), the slope forming unit in the central portion of the picture, has a very sharp contact (black line) with the overlying (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.

- 51 -

Quaternary sediments

Granite

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

- 52 - 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.

ػدن الحوافق : ؽتبرث ؽً شعخ يخؾرر ٌشتٖب ّاهذٔ ٖيذل يجيؽّخًٖ يً اهعتلبح ، اهشفوٓ يٌِب يعّٖج أّ يجؾدث ّاهؾوٖب أفلٖج .

أىواع ػدن الحوافق :

ؽ دى اهخّافق اهزأّ Angular Unconformity: ٖيذل شعخ ٖفضل تًٖ عتلبح يبئوج ّأخرْ يبئوج ّهنٌِيب يخخوفخًٖ فٕ اهيٖل.

عدم التوافق الزاوى

ؽ دى اهخّافق االٌلعبDisconformity ٕؽ: ٖنًّ ؽوٓ شنل شعخ يخؾرر تًٖ عتلبح يخّازٖج أّ يبئوج أّ أفلٖج .

عدم التوافق االنقطاعي

- 53 -

 ال حوافق Nonconformity : ٖخنًّ ُذا اهػٌّ يً ؽدى اهخّافق ؽٌديب ٖخى خرشٖة يجيؽّج يً اهعتلبح اهرشّتٖج فّق ضخّر ٌبرٖج أّ يخدّهج .

الال توافق

شةه حوافق Paraconformity ٖخنًّ ُذا اهػٌّ يً ؽدى اهخّافق ؽٌديب ٖخى خرشٖة يجيؽّخًٖ يً اهعتلبح اهرشّتٖج هِيب ٌفس اهيٖل ، ّٖفضوِيب شعخ خؾرٖج غٖر ّ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.

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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 - 55 - 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.

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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 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.

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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

- 58 - 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.

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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 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

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