اربد

منتدى معلومات عامة
 
صفحة الاعلاناتالرئيسيةبحـثالتسجيلدخول
دخول
اسم العضو:
كلمة السر:
ادخلني بشكل آلي عند زيارتي مرة اخرى: 
:: لقد نسيت كلمة السر

pubacademy.ace.st--!>
pubacademy.ace.st--!>

شاطر | 
 

 Stratigraphy and Lithologic Correlation

استعرض الموضوع السابق استعرض الموضوع التالي اذهب الى الأسفل 
كاتب الموضوعرسالة
kingsam




مُساهمةموضوع: Stratigraphy and Lithologic Correlation    23/11/2010, 09:11

Stratigraphy and Lithologic Correlation
Pamela J. W. Gore
Department of Geology, Georgia Perimeter College
Clarkston, GA 30021
Copyright © 1982-2004 Pamela J. W. Gore

In this lab you will learn about sequences of sedimentary rocks and how they may be correlated, or traced between outcrops. Ideally, the rocks may be correlated directly by walking along the contacts between adjacent rock units, across the countryside. This is seldom the case, however, particularly where vegetation and soil cover make rock exposures poor (as in humid areas, such as the eastern United States). In other situations, geologists are interested in beds deep below the Earth's surface, and they must use drill hole and core data to correlate the rocks.
Geologists study rocks in outcrops(natural or man-made exposures such as road cuts or quarries), or in drill cores. When studying an outcrop of sedimentary rock, the most obvious feature is bedding(also called strata or layers). Although the rocks may be tilted or folded, the sediments were originally laid down in horizontal beds, which extended as continuous layers in all directions (such as a layer of mud on the sea floor), with the oldest layers on the bottom and the youngest layers on top (see Steno's Laws in Lab 7). A sequence of sedimentary rocks may be divided up into a number of lithostratigraphic unitsof various sizes.

LITHOSTRATIGRAPHIC UNITS
A lithostratigraphic unit is defined as a body of sedimentary, extrusive igneous, metasedimentary, or metavolcanic strata which is distinguished on the basis of lithologic characteristics and stratigraphic position (position in the rock sequence).
The smallest lithostratigraphic rock unit is the bed. A formation is a set of similar beds, and formations are the fundamental units of stratigraphy. By definition, formations are:


  1. Lithologically homogeneous (all beds are the same rock type or a distinctive set of interbedded rock types).
  2. Distinct and different from adjacent rock units above and below.
  3. Traceable from exposure to exposure, and of sufficient thickness to be mappable (formations are commonly hundreds of feet thick, but may be thinner or thicker).
  4. Formations must have names. Formations are usually named for some geographic locality where they are particularly well exposed. (This locality is referred to as the type section.) If the beds are dominated by a single rock type, this may appear in the name. (Also, to be valid, the name of a formation must be published in the geological literature.)
Examples of formation names are:
Chattanooga Shale
Rome Formation
Shady Dolomite
Tapeats Sandstone
Fort Payne Chert
Green River Formation
Dakota Sandstone
Red Mountain Formation
A set of similar or related formations is called a group.
Groups also have names: Knox Group
Conasauga Group
Chilhowee Group
Great Smoky Group
Pocono Group
Chesapeake Group

Subdivisions within formations are called members. Members also have names. A formation, however, does not have to contain members. Members may be designated to single out units of special interest or economic value, such as coal beds or volcanic ash layers.
Boundaries of lithostratigraphic units are placed where the lithology (or rock type) changes. They may be placed at a distinct contact, or may be set arbitrarily within a zone of gradation.
Virtually all lithostratigraphic units are "time transgressive" or diachronous (meaning that they, or their contacts, cut across time lines. For example, a particular unit of early Cambrian sandstone in southern California and Nevada may be traced continuously to the northeast, but in Colorado, it is late Cambrian, or roughly 35 million years younger.
The lithostratigraphic terms (bed, member, formation, and group) refer to sedimentary, volcanic, metasedimentary, and metavolcanic rocks only. Intrusive and highly deformed and metamorphosed rocks are called lithodemic units. The rocks in the Piedmont (which includes the Atlanta area) are primarily highly metamorphosed and intrusive igneous rocks, and should therefore be called lithodemic units. The fundamental lithodemic unit is the lithodeme (roughly equivalent to formation). The term "formation" should not be used for intrusive and metamorphic rock (according to the North American Stratigraphic Code of 1982). Unfortunately, a lot of mapping in the Piedmont predates this code, and many of the units on the available maps are referred to as formations. Ideally, the name of a lithodeme will be a location plus a lithologic term, such as:

Stone Mountain Granite
Panola Granite
Lithonia Gneiss
Norcross Gneiss
Chattahoochee Palisades Quartzite
In this lab, we are primarily concerned with sequences of sedimentary rocks and occasional lava flows, so we will be using lithostratigraphic terminology.


STRATIGRAPHIC SECTIONS

Geologists study sequences of sedimentary rocks on a bed-by-bed basis. They measure the thickness of each bed, record the physical, chemical, and biological characteristics of the rock, and note the nature of the contacts (or bedding planes) between beds. Using these data, the geologist draws up a stratigraphic section for a particular sequence of rock. A stratigraphic section is a graphical or pictorial representation of the sequence of rock units. Standard symbols (called lithologic symbols) are used to refer to each rock type:

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 681x374 وحجمها 83 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


DRAWING A STRATIGRAPHIC SECTION

To draw a stratigraphic section, you must have data from a sequence of rocks. You will need to have data on the thickness of each bed, and all of the physical, chemical, and biological characteristics of that bed, as well as the character of its contacts. Before you start, you need to examine your data to determine the total thickness of the section you plan to draw. Then determine a proper scale so that the entire section will fit on your paper (such as, 1" = 100'). Draw a vertical column in which you will plot your data, and then mark off the thickness of each bed or formation using the scale you established. Draw in the contacts between units; if the contacts are erosional, you should use a wavy line. Once you have drawn in contacts, draw in the lithologic symbols for each unit. Information on fossils and sedimentary structures, etc. may be placed within the unit, or beside it using a special symbol or small sketch. Color may be illustrated with a special symbol, or by coloring your section. There are standard symbols which have been established by oil companies and core logging companies. You may use theirs (see a reference book), or create your own. Your instructor may give you further instruction on this.
Once you have drawn several stratigraphic sections for an area, you may begin to correlate them.

LITHOLOGIC CORRELATION
Geologists can draw stratigraphic sections for several outcrops (or cores) in an area, and then trace beds from one section to another. This is called lithologic correlation . Basically, correlation demonstrates the equivalency of rock units across an area. The sections being correlated are commonly miles apart. Basically, a correlation is a hypothesis that units in two widely separated sequences are equivalent. Clearly, the more unique characteristics that two sections share, the greater the probability that the correlation is correct.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة] Illustration of lithologic correlation
Correlation may be performed in several ways. Distinctive beds (calledkey beds or marker beds), distinctive sequences of beds, bed thicknesses, and unconformities may be traced between sections.
Key beds or marker beds tend to have some unusual, distinguishing feature which allows them to be readily identified, such as a bed of volcanic ash in a sedimentary sequence, or a bed of conglomerate in a sandstone sequence, or a bed of fossil shells or bones, or a bed of limestone in a shale sequence. Key beds or marker beds should also be laterally extensive, to aid in correlation over a large area.
Distinctive sequences of beds are also useful in correlation. For example, the sequence "limestone - dolostone - limestone" may be found within a thick unit of shales and siltstones, and correlated between sections.
In some cases, beds can be correlated between sections based on their thicknesses. One of the best examples of this is the correlation of laminations in cores from the evaporites of the Castile Formation in the Permian of western Texas and New Mexico. Cores were drilled about 9 miles apart, and the thickness of the laminations matches almost exactly.
Sometimes, one or more rock units are missing from the middle of a sequence. Close examination of the outcrop shows a sharp or irregular contact where the missing rocks should be. This contact is called an unconformity. Unconformities are surfaces which represents a gap in the geologic record, because of either erosion or nondeposition. Unconformities can be traced between stratigraphic sequences miles apart. Although unconformities may truncate rocks of many different ages, the sediments directly overlying the unconformity are roughly the same age.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة] Illustration of an unconformity causing beds to be missing from a sequence





There are four basic types of unconformities:

  1. Angular unconformities
  2. Nonconformities
  3. Disconformities
  4. Paraconformities
1. ANGULAR UNCONFORMITIES

Angular unconformities are characterized by an erosional surface which truncates folded or dipping (tilted) strata. Overlying strata are deposited basically parallel with the erosion surface.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Angular unconformities
2. NONCONFORMITIES

Nonconformities are characterized by an erosional surface which truncates igneous or metamorphic rocks.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Nonconformities
3. DISCONFORMITIES


Disconformities are characterized by an irregular erosional surface which truncates flat-lying sedimentary rocks.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Disconformities
4. PARACONFORMITIES
Paraconformities are characterized by a surface of nondeposition separating two parallel units of sedimentary rock, which is virtually indistinguishable from a sharp conformable contact; there is no obvious evidence of erosion. An examination of the fossils shows that there is a considerable time gap represented by the surface.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]Paraconformity


SEDIMENTARY FACIES

A facies is a unit of sedimentary rock deposited in a particular sedimentary environment. A facies has distinctive physical, chemical, and biological characteristics which serve as clues that help the geologist to interpret the environment in which the rock was deposited. (Examples of sedimentary environments include beaches, rivers, lakes, deserts, alluvial fans, deltas, reefs, lagoons, tidal flats, etc.) You might refer to a red sandstone facies, or a mudcracked limestone facies, or a fossiliferous black shale facies.
LATERAL FACIES CHANGES
Beds may change laterally in thickness or in rock type, as a result of differences in the sedimentation rate, or environment of deposition. In these cases, a bed of rock may be in the same position in the sequence, but it is somewhat different in thickness or rock type. For example, a lateral change in rock type is caused by a lateral change in depositional environment; you could envision the deposits of a river passing laterally into the deposits of a floodplain, or possibly a delta. Or, you could envision beach sands passing laterally into deeper water silts, muds, and clays.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
llustration of lateral changes in bed thickness In some cases, a bed thins progressively in one direction until it pinches out. A pinchout may or may not be accompanied by the increase in thickness of an adjacent unit. In some case, the entire sedimentary section thins in a certain direction.


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Illustration of pinchout of a limestone bed and a sandstone bed, with three sections drawn to show the different facies that would be present in each.


WALTHER'S LAW AND VERTICAL FACIES CHANGES
The sedimentary sequence seen in outcrops is the result of different types of sediment being deposited in different sedimentary environments over time, producing a vertical sequence of different facies.
Lateral changes in facies are relatively easy to understand. Vertical facies changes may initially be somewhat puzzling. How does one layer of sedimentary rock come to overlie another? The vertical relationships between facies are explained by changes in sea level, or changes in subsidence and sedimentation rates.
As laterally-adjacent sedimentary environments shift back and forth through time, as a result of sea level change, facies boundaries also shift back and forth. Given enough time, facies which were once laterally adjacent will shift so that the deposits of one environment come to overlie those of an adjacent environment. In fact, this is how many (if not most) vertical sequences of sedimentary rocks were formed. This concept was first stated by Johannes Walther in 1894, and is called Walther's Law. Basically, in a conformable sedimentary sequence (i.e., one without unconformities), sedimentary units which lie in vertical succession represent the deposits of laterally adjacent sedimentary environments migrating over one another through time.
At any one time, sediment of different types is being deposited in different places. Sand is deposited on the beach, silt is deposited offshore, clay is deposited in deeper water, and carbonate sediment is deposited far from shore (or where there is little or no input of terrigenous sediment). Sedimentary environments (and facies) move as sea level changes, or as a basin fills with sediment.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة] Distribution of sedimentary facies

A sea level rise is called a transgression. A transgression will produce a vertical sequence of facies representing progressively deeper water environments (a deepening-upward sequence). As a result, a transgressive sequence will have finer-grained facies overlying coarser-grained facies (fining-upward from sand at the bottom, and then to silt, and then to shale). Transgressions can be caused by melting of polar ice caps, displacement of ocean water by undersea volcanism, or by localized sinking or subsidence of the land in coastal areas.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة] Transgressive Sequence

A sea level drop is called a regression. A regression will produce a sequence of facies representing progressively shallower water environments (shallowing-upward sequence). As a result, a regressive sequence will have coarser-grained facies overlying finer-grained facies (coarsening-upward). Regression can be caused by a buildup of ice in the polar ice caps, or localized uplift of the land in coastal areas.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة] Regressive Sequence


We can easily see how transgressive and regressive sequences form. First, start with this basic situation:


Illustration of the formation of REGRESSIVE (A - D) and TRANSGRESSIVE (E - G) sequences.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 690x245 وحجمها 4 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Assume that sea level drops and the beach moves seaward.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 679x203 وحجمها 4 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Repeat for another sea level drop.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 682x214 وحجمها 5 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Now notice how the facies have migrated to keep their proper position relative to sea level.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 696x209 وحجمها 5 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Now begin again with the same basic situation.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 690x191 وحجمها 3 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Assume that sea level rises and the beach moves landward.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 694x199 وحجمها 4 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Repeat for another sea level rise.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 690x269 وحجمها 6 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Now notice again how the facies have migrated to keep their proper position relative to sea level.


The figure below illustrates a transgression followed by a regression, or a transgressive-regressive sequence. The part of the record deposited during the transgression is marked by an arrow labelled "T", and the part deposited during the regression is marked by an arrow labelled "R". Four facies are shown: a sandstone facies, a siltstone facies, a shale facies, and a limestone facies. Note that the facies pattern produces a broad V shape in vertical section. (Also note that the arrows for transgression and regression both point up, indicating change through time.)
Three "time lines" are shown. (In the geologic record, a "time line" could be represented by a thin volcanic ash bed, representing one particular eruption event.) Note that the lithologic units cut across the time lines; the facies are time-transgressive or diachronous. Note that the line marked "Time 2" bisects the V shape of the transgressive-regressive sequence. The point of maximum transgression (or regression) in a transgressive-regressive sequence is always a time line, marking the time of maximum transgression (or regression).

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 666x234 وحجمها 7 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Transgressive-regressive sequence The figure below is a diagrammatic representation of the V-shaped pattern produced by migrating facies during a transgression followed by a regression. Three stratigraphic sections are superimposed on the pattern to illustrate how the facies would appear in vertical section in three different locations. Note that the facies that are present in each are different, due to pinchout. Draw a dashed line down the center of the V connecting the maximum landward extent of each facies. This will be a time line, marking the time of maximum transgression.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]هذه الصورة تم تصغيرها تلقائيا . إضغط على هذا الشريط هنا لعرض الصورة بكامل حجمها . أبعاد الصورة الأصلية 688x511 وحجمها 12 كيلو بايت .
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Transgressive-regressive sequence with three superimposed stratigraphic sections.
الرجوع الى أعلى الصفحة اذهب الى الأسفل
theredrose




مُساهمةموضوع: رد: Stratigraphy and Lithologic Correlation    23/11/2010, 16:59

ابداع حقيقي
الرجوع الى أعلى الصفحة اذهب الى الأسفل
 
Stratigraphy and Lithologic Correlation
استعرض الموضوع السابق استعرض الموضوع التالي الرجوع الى أعلى الصفحة 
صفحة 1 من اصل 1

صلاحيات هذا المنتدى:لاتستطيع الرد على المواضيع في هذا المنتدى
اربد :: المنتدى العلمي :: فلك و جيولوجيا-
انتقل الى: