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01. Pendahuluan

MFS 3945

Salahuddin Husein

Jurusan Teknik Geologi Fakultas Teknik Universitas Gadjah Mada

2012

shddin © 2011 Silabus 1. 07/09/2012 : Pendahuluan 2. 14/09/2012 : Proses Sedimentasi 3. 21/09/2012 : Lingkungan Sedimentasi Darat 4. 28/09/2012 : Lingkungan Sedimentasi Transisi 5. 05/10/2012 : Lingkungan Sedimentasi Transisi 6. 12/10/2012 : Lingkungan Sedimentasi Laut Silisiklastik 7. 19/10/2012 : Lingkungan Sedimentasi Karbonat dan Evaporit

Ujian Tengah Semester

8. 09/11/2012 : Litostratigrafi 9. 16/11/2012 : Stratigrafi Seismik 10. 23/11/2012 : Stratigrafi Sekuen 11. 30/11/2012 : Stratigrafi Magnetik dan Biologi 12. 07/12/2012 : Kronostratigrafi 13. 14/12/2012 : Analisis Cekungan 14. 21/12/2012 : Analisis Cekungan

Ujian Akhir Semester

bad-news: tidak ada tanggal merah,

shddin © 2011 Buku Acuan

soft-copy available

shddin © 2011 Jargon

"Stratigraphy may be defined as the complete triumph of terminology over facts and common sense."

P.D. Krynine

Stratigrafi = bahasa latin : Stratum + graphia Ilmu yang mempelajari rock succession dan hubungannya dengan kejadian dan proses geologi dalam ruang dan waktu. Sangat penting untuk merekonstruksi lapisan tiap kejadian dari sejarah Bumi dan evolusi kehidupan di Bumi.

shddin © 2012 Definisi

Prinsip stratigrafi akan membantu untuk interpretasi ruang dan waktu geologi yang pernah terjadi.

shddin © 2012 Manfaat

shddin © 2012 Model dan Prediksi shddin © 2011 Fundamental Stratigraphy is one of the most demanding, fundamental, and interesting of geologic disciplines. It tries to reconstruct history with a few basic principles, requires both careful observation and wide-ranging imagination, and involves many individually fascinating subdisciplines. Yet, this is not how it appears to many people. Stratigraphy has a poor image because it is sometimes presented only as a way of classifying and organizing strata with "codes." Need to know:

(i) know how stratigraphy developed and how this constrains current approaches and dogmas; (ii) study all aspects of modern processes and environments as a basis for paleoenvironmental interpretation; (iii) understand the principles of correlation and dating as a basis for stratigraphic reconstructions; (iv) know some basics of geophysics, tectonics, climatology, and paleobiology in order to explain the development of sedimentary basins; and

shddin © 2011 Fundamental (v) make field observations in person, to understand the limitations of data and interpretations.

Explanations: First, it is essential to know the history of ideas in a subject, and how and why choices were made among different alternatives. Basic stratigraphic concepts were only slowly established after much discussion. It is important to have open-mindedness in the face of current dogma. Second, the conditions under which ancient stratified rocks formed can only be worked out by studying their modern counterparts and deriving principles from sedimentology and ecology that might help in this. Third, the relationships of environments in space and time require specific concepts of correlation and dating which are peculiar to stratigraphy. Fourth, it is impossible to interpret the stratigraphic development of an area, or sedimentary basin, without knowing some basic concepts in geophysics, tectonics, climatology, and biology. Fifth, personal observations on actual rocks are essential, otherwise people get completely out of touch with their material.

shddin © 2011 Mempelajari Stratigrafi Stratigraphy gives you techniques for working out earth history: it integrates diverse materials into a coherent view of how the earth and its life forms evolved. Though stratigraphy (literally writing about strata) is mostly about working out the history of sedimentary rocks, in order to do this you also need to know the effects of magmatism, metamorphism, tectonism, climatic change, and sea-level changes, and the effects of organic evolution. So, stratigraphy integrates data and concepts from many specialties, and in practice ends up as a much more comprehensive study than its name implies. Stratigraphy, perhaps most importantly, also helps you to understand how many economic materials formed and got distributed in the way they did and so will hopefully help you find more.

shddin © 2011 Mempelajari Stratigrafi For example, the Wembley Field is one of many isolated oil and gas reservoirs in the Middle Triassic in Alberta, Canada. Finding out why the oil is there, and where other similar oil and gas fields are, requires you to proceed logically through the various phases of stratigraphy.

(Brookfield, 2004; after Willis & Moslow, 1994)

shddin © 2011 Mempelajari Stratigrafi First, what are the actual oil-bearing rocks and how were they deposited?

The local (borehole) sections mostly consist of porous sands alternating with clays arranged in coarsening-upwards cycles, deposited by waves and currents as marine barrier island deposits.


Section of Halfway Formation with barrier-bar interpretation, and porosity and permeability of the oil- and gas-bearing units.

shddin © 2011 Mempelajari Stratigrafi Second, how are these sediments arranged spatially and how old are they?

In the absence of outcrops, space correlations have to be worked out from borehole logs and seismic sections which show the arrangement and thickness of the strata and environments and that the productive oil and gas wells are in linear sand bodies of a particular type of marine barrier bar.


Sand thickness map of Halfway Formation

shddin © 2011 Mempelajari Stratigrafi Second, how are these sediments arranged spatially and how old are they?

Further detailed studies showed that the oil and gas is concentrated specifically in the well-sorted and porous sandstones deposited in active tidal inlets and ebb-tidal deltas whose distribution could be plotted in sections and on thickness maps.

Dip section across Wembley Field showing tidal channels, ebb-tidal delta environments, and oil- and gas-producing intervals .

Distribution of tidal inlet sand units. (Brookfield, 2004; after Willis & Moslow, 1994)

shddin © 2011 Mempelajari Stratigrafi Why is the oil and gas concentrated in the Halfway Formation, when many other underlying units also show coarsening-upwards barrier-bar sections?

Small rises in sea level, which led to the formation of transgressive barriers from eroding shoreline sand deposits, alternated with periods of stable sea level, during which seaward migration of barriers led to burial of the transgressive barriers with fine-grained impermeable back-barrier and coastal plain sediments. The oil and gas reservoirs can be located by tracing the condensed clays at the base of each marine transgression up-dip into the transgressive barriers.

Model for the development of a transgressive barrier and (d) location of successive transgressive barriers


shddin © 2011 Mempelajari Stratigrafi

Coastal morphology (Kendall, 2005)


Transgressed barrier-islands (Kendall, 2005)

shddin © 2011 Mempelajari Stratigrafi Third, what is the history of the area?

Regional stratigraphic studies show that it formed part of a subsiding passive margin shelf in western Canada. Inherently unpredictable evolution of discontinuous barrier islands was controlled by the interactions of tides and sea-level changes, The ages come from marine fossils fitted into the standard geologic time-scale,

Thickness (in meters) of Middle Triassic sediments on western Canadian margin


shddin © 2011 Mempelajari Stratigrafi Phase 1: Basics 1. identify and classify minerals, rocks, and fossils accurately; 2. infer the processes that formed the minerals, rocks, and fossils from field

and laboratory studies of the effects of modern physical, chemical, and biological processes;

3. recognize the ancient depositional (and rarely non-depositional) environments, by comparing the variety, intensity, and periodicity of processes in modern environments with those inferred in ancient rocks successions;

4. map the obtained data on maps and sections of various types.

shddin © 2011 Mempelajari Stratigrafi Phase 2: Tracing environments in space and time 1. An overview of the area studied involves a preliminary survey of what work

has already been done, together with an analysis of the type and distribution of the rocks and the problems in studying them. The first can be done in a library and/ or by talking with previous workers. The second involves areal studies of the surface using remote sensing (e.g. satellite and aerial photographs), and by actual reconnaissance on foot or by some form of transport; and studies of the subsurface with geophysics (e.g. seismic profiles) and boreholes.

2. The description of local sections involves measuring STRATA, describing their attributes (including composition, texture, structure, and fossil content), and working out the processes that formed the sediments and the succession of depositional environments present in the sections. For this you need to know the basic principles of sedimentology and ecology.

shddin © 2011 Mempelajari Stratigrafi Phase 2: Tracing environments in space and time 3. The correlation of local sections in space and time involves the physical

tracing and mapping of rock units, their relative dating by means of fossils and other methods, and dating by means of radiometric dating or cross-correlation with standard time-scales. You need to know how environments and organisms vary and differ in a variety of sedimentary basins; how to plot these variations on maps and diagrams (and the advantages and limitations of different methods); how relative time units are constructed; and how radiometric and other methods of dating are done.

4. The reconstruction of sedimentary basin history involves synthesis, often on maps and cross-sections, of trends in rock type, petrology, facies, thickness, and so on.

shddin © 2011 Mempelajari Stratigrafi Phase 2: Tracing environments in space and time The three steps following the initial overview are summarized as stratinomic analysis (giving the depositional dynamics of the sediments), facies analysis (showing the lateral and vertical variability or facies dynamics of the sediments), and basin analysis (explaining how the basin evolved during sedimentation).

Dynamic stratigraphy

(Brookfield, 2004; after Aigner, 1985)

shddin © 2011 Mempelajari Stratigrafi Phase 3: Interpreting geologic history

Interpreting geologic history involves evaluating the effects of controlling processes such as tectonics, sea-level changes, climate, and biology (the effect of organisms) on sedimentary basin history. It requires the ability to synthesize large amounts of data from many fields. On the grand scale it involves the correlation of histories of individual basins and intervening areas to give a worldwide picture of the development of a planet, and requires wider consideration of the stratigraphic peculiarities and problems of different time periods on these planets.

Each of these phases depends on the competence of the preceding phase. Thus, poorly described local sections inevitably result in poor stratigraphy, poor environmental reconstructions, poor correlations, and lousy reconstructions.


(Kendall, 2005)


(Kendall, 2005)

sediment


(Kendall, 2005)

stratum or bed

strata


(Kendall, 2005)

succession or

stratigraphy or

strata


(Kendall, 2005)

students doing stratigraphy


(Kendall, 2005)

basin

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