Boggs S. Principles of Sedimentology and Stratigraphy
Подождите немного. Документ загружается.
U. S. S. R.
Turkey
16.5 Sedimenta Basin Fill
567
Figure 16.1 5
Aulacogen north of the Black and Caspian Seas
on the Russian platform. [After Burke, K., 1977,
Aulacogens and continental breakup: Annual Re
view of Earth and Planetary Sciences, v. 5. Re
produced by permission of Annual Reviews, Inc.]
the Reelfoot Rift of late Paleozoic age in which the Mississippi River flows, the
Amazon Rift in which the Amazon River flows, the Benue Tr ough of Cretaceous
age in which the Niger River is located, the aulacogen north of the Black and
Caspian Seas on the Russian platform (Fig. 16.15), and the Anadarko Basin in Ok
lahoma (Fig. 16.6). lmpactogens are structures similar to aulacogens in that they
formed at high angles to orogenic belts; however, they do not have a preorogenic
history.
Intracontinental wrench basins are hybrid basins that formed within conti
nental crust owing to distant collisional processes (e.g., the Quaidam Basin of
China). Successor basins are basins that formed in intermountain settings follow
ing cessation of local orogenic activity (e.g., the southern Basin and Range, Ari
zona). See Ingersoll and Busby (1995) and Sengor (1995) for details.
16.5 SEDIMENTARY BASIN FILL
The preceding discussion focuses on the structural characteristics of sedimentary
basins and the tectonic processes that create these basins. The particular concern
of basin analysis is, however, the sediments that fill the basins. This concern en
compasses the processes that produce the filling, the characteristics of the result
ing sediments and sedimentary rocks, and the genetic and economic significance
of these rocks. The fundamental processes that generate sediments (weathering/
erosion) and bring about their transport and deposition; the physical, chemical,
and biological properties of these rocks; the depositional environments in
which they form; and their stratigraphic significance are discussed in preceding
chapters of this book. The factors that control or affect these depositional
processes and sediment characteristics, discussed also in appropriate parts of
the book, include
568
Chapter 16 I Basin Analysis, Te ctonics, and Sedimentation
1. The lithology of the parent rocks (e.g., granite, metamorphic rocks) present in
the sediment source area, which controls the composition of sediment derived
from these rocks
2. The relief, slope, and climate of the source area, which control the rate of sed
iment denudation and thus the rate at which sediment is delivered to deposi
tional basins
3. The rate of basin subsidence together with rates of sea-level rise or fall
4. The size and shape of the basins
The processes that may cause basin subsidence are discussed briefly in Section 16.2
(see Figure 16.2). The rate of basin subsidence coupled with sea level fluctuations
controls the available space in which sediments can accumulate (accommodation;
see Fig. 13.15) at any given time, as well as affecting sediment transport and depo
sition. Thus, owing to continued subsidence, thousands of kilometers of sedi
ments may accumulate even in shallow-water basins.
The purpose of basin analysis is to interpret basin fills to better understand
sediment provenance (source), paleogeography, and depositional environments in
order to unravel geologic history and to evaluate the economic potential of basin
sediments. Basin analysis incorporates the interpretive basis of sedimentology
(sedimentary processes); stratigraphy (spatial and temporal relations of sedimentary
rock bodies); facies and depositional systems (organized response of sedimentary
products and processes into sequences and rock bodies of a contemporaneous or
time-transgressive nature); paleooceanography; paleogeography, and paleoclima
tology; sea-level analysis; and petrographic mineralogy as a means of interpreting
sediment source (Klein, 1987; 1991). Further, biostratigraphy provides a means of
establishing a temporal framework for correlating time-equivalent facies and sys
tems and to constrain timing of specific events, and radiochronology allows, in ad
dition, the dating of specific sedimentological events and stratigraphic boundaries.
Recent research in sedimentary geology and basin analysis has focused particular
ly on analysis of sedimentary facies, cyclic subsidence events, changes in sea level,
ocean circulation pattes, paleoclimates, and life history.
Depositional models are being increasingly used to better understand the
processes of basin filling and the effects of varying basin-filling parameters such
as sediment supply and sediment flux into basins (e.g., Jones and Frostick, 2002),
grain size, basin subsidence rates, and sea-level changes (e.g., Tetzlaff and Har
baugh, 1989; Angevine, Heller, and Paola, 1990; Cross, 1990; Slingerland, Har
baugh, and Furlong, 1994; Miall, 2000, Chapters 7, 9). Models may be either
geometric or dynamic. Geometric models begin by specifying the geometry of the
depositional system rather than calculating it as part of the model. Dynamic mod
els begin with consideration of the transport of sediment in the basin and use
some form of approximation to the basic laws that govern sediment transport and
deposition.
16.6 TECHNIQUES OF BASIN ANALYSIS
Analyzing the characteristics of sediments and sedimentary rocks that fill basins,
and interpreting these characteristics in terms of sediment and basin history, de
mands a variety of sedimentological and stratigraphic techniques. These tech
niques require the acquisition of data through outcrop studies and subsurface
methods that can include deep drilling, magnetic polarity studies, and geophysi
cal exploration. These data are then commonly displayed for study in the form of
maps and stratigraphic cross sections, possibly using computer-assisted tech
niques. In this section, we look briefly at the more common techniques of basin
analysis.
Measuring Stratigraphic Sections
16.6 Te chniques of Basin Analysis
569
To interpret Earth history through study of sedimentary rocks requires that we
have detailed, accurate information about the thicknesses and lithology of the
stratigraphic successions with which we deal. To obtain this information, appro
priate stratigraphic successions must be measured and described in outcrop
and/ or from subsurface drill cores and cuttings. We refer to this process of study
ing outcrops as "measuring stratigraphic sections"; however, the process also in
volves describing the lithology, bedding characteristics, and other pertinent
features of the rocks. Samples for mineralogic or paleontologic analysis may also
be collected and keyed to their proper position within the measured sections.
Thus, measuring and describing stratigraphic sections is commonly the starting
point for many geologic studies, and the measured-section data become an indis
pensable part of the studies. Such stratigraphic sections are referred to throughout
this book; see, for example, Figure 14.12.
A variety of teciques are available for measuring stratigraphic sections,
depending upon the nature of the stratigraphic succession and the purpose of the
study. The useful little book by Kottlowski (1965) describes these various meth
ods, and the equipment needed to carry out measurements, in considerable detail.
One of the most common methods involves use of a Jacob staff. A Jacob staff is a
lightweight metal or wood pole (rod) marked in graduations of feet or meters. It
is commonly cut to about eye-height and is used in conjunction with a Brunton
compass placed at or near the top of the staff. The technique is illustrated in
Figure 16.16. The clinometer of the Brunton compass is set at the dip angle of the
beds, allowing the staff to be inclined normal to the bedding to determine the true
stratigraphic thickness of the bed or beds. By stepping uphill and making a suc
cession of measurements, comparatively thick sections of strata can be measured.
After measuring several meters of section, the geologist commonly stops measur
ing for a time to describe the lithology and other pertinent features of the section
before resuming measurement. A lithologic column, together with appropriate de
scriptive notes, is recorded in a field notebook.
Preparing Stratigraphic Maps and Cross Sections
Stratigraphic Cross Sections
Once stratigraphic sections have been measured and described, they can be used
to prepare stratigraphic cross sections. Stratigraphic cross sections are used exten
sively for correlation purposes and structural interpretation, as well as for study
Figure 16.16
Schematic illustration of the jacob-staff technique for
measuring the thickness of stratigraphic units. By set
ting the clinometer of a Brunton compass attached
to the jacob staff to the dip angle of the beds, the
staff can be held normal to the bedding planes, yield
ing the true stratigraphic thickness (AB) of the mea
sured unit. [From Kottlowski, F. E., 1965, Measuring
stratigraphic sections: Holt, Rinehart, and Winston,
New York, Fig. 3.2, p. 63.]
570
Chapter 16 I Basin Analysis, Te ctonics, and Sedimentation
Fire 16.17
Schematic illustration of a
fence diagram showing in
tertonguing facies relation
ships in Permian strata
across the Paradox Basin,
Utah and Colorado. [From
Kunkle, R. P., 1958, Permi
an stratigraphy of the Para
dox Basin, in Sanborn, A.
(ed.), Guidebook to the ge
ology of the Paradox Basin,
Intermountain Association
of Petroleum Geologists,
Ninth Annual Field Confer
ence, Fig 1, p. 165.]
of the details of facies changes that may have environmental or economic signifi
cance. Cross sections may be drawn to illustrate local features of a basin, often in
conjunction with preparation of lithofacies maps (described below), or they may
depict major stratigraphic successions across an entire basin. In addition to mea
sud outcrop sections, the formation needed to prepare stragraphic cross sec
tions may be obtained from subsurface lithologic logs (which are prepared by
study of drill cores and cuttings) and/or mechanical well logs (petrophysical
logs). Most stratigraphic cross sections depict in two dimensions the lithologic
and/ or structural characteristics of a particular stratigraphic unit, or units, across
a
given geographic region. Several examples of such cross sections are given in
preceding chapters of this book. See, for example, Figure 12.15. Stratigraphic in
formation may be presented also as a fence diagram. These diagrams attempt to
give a three-dimensional view of the stratigraphy of an area or region (Fig. 16.17).
Thus, they have the advantage of giving the reader a better regional perspecve
on the stratigraphic relationships. the other hand, they are more difficult to
EXPLANATION
W NE
BE • a • ndstone, & $ll
SCALE
� CAONAT-&one & dolomite
� ARKE GRANE WH
IGNES and METAMOIC C
LOCAT•ON MAP
- p " f f"
� EVORITES - andrite & sum
16.6 Te chniques of Basin Analysis
571
construct than conventional two-dimensional diagrams, and parts of the section
are hidden by the fences in front.
Structure-Contour Maps
It is often desirable in basin studies to determine the regional structural attitude of
the rocks as well as the presence of local structural features such as anticlines and
faults. Structure-contour maps are prepared for this purpose. These maps provide
information about a basin's shape and orientation and the basin-fill geometry.
Structure-contour maps are prepared by drawing lines on a map through points of
equal elevation above or below some datum, commonly mean sea leveL Eleva
tions are typically determined on the top of a particular formation or key bed at a
number of control points. Elevation data may be obtained through outcrop study
and/ or subsurface interpretation of mechanical or lithologic well logs. After con
trol points are plotted on a base map, a suitable contour interval is selected, and
structure contour lines are drawn by hand or by use of computers. Figure 16.18
shows an example of a strucre-contour map.
Structure contours may also be prepared on the top of prominent subsurface
reflectors from seismic data (Chapter 13). Depth to a particular reflector may be
plotted initially as two-way travel time. Thus, the initial map shows contour lines
of equal travel time. If the seismic-wave velocity can be determined from well in
formation, the travel times can be converted to actual deps, allowing maps to be
redrawn in terms of actual elevations on the reecting horizon.
Structure-contour maps can reveal e locations of subbasins or depositional
centers within a major basin as well as axes of uplift (anticlines, domes). Structur
al features may be related also to syndepositional topography. Thus, analysis of
these maps can provide clues to local paleogeography and facies pattes. Struc
tural maps are useful also in economic assessment (e.g., petroleum exploration) of
basins.
Isopach Maps
Isopachs are contour les of equal thickness. An isopach map is a map that shows
by means of contour lines the thickness of a given formation or rock unit. A map
that
shows the areal variation in a specific rock type (e.g., sandstone) is an isoth
map. The thickness of sediment in a basin is determined by the rate of supply of
sediment and the accommodation space in the basin, which in turn is a function of
N
t
5 km
Figure 16.18
Schematic illustration of a structure-contour
map drawn on the top of a formation. The
contour interval is 20 m. The negative con
tour values indicate that this formation is lo
cated below sea level and is thus a buried
(subsurface) formation. Note the presence
of a syncline, dome, anticline, and fault.
572 Chapter 16 1 Basin Analysis, Te ctonics, and Sedimentation
Figure 16.19
basin geometry and rate of basin subsidence. Abnormally thick parts of a stra
graphic unit suggest the presence of major depositional centers in a basin (basin
lows), whereas abnormally thin parts of the unit suggest predepositional highs or
possibly areas of postdepositional erosion. Isopach maps thus provide informa
tion about the geometry of the basin immediately prior to and during sedimenta
tion. Furthermore, analysis of a succession of isopach maps in a basin can provide
information about changes in the structure of a basin through time.
To construct an isopach map, the thickness of a formation or other strati
graphic unit must be determined from outcrop measurements and/ or subsurface
well-log data at numerous control points. The thickness of the unit at each of these
conol points is entered on a base map, and the map is contoured in the same way
that a structure-contour map is prepared. Figure 16.19 is an example.
Pa leogeologic Ma
p
s
Paleogeologic maps are maps that display the areal geology either below or above
a given stratigraphic unit. Imagine, for example, that we could strip off the rocks
that make up a particular formation (and all the rocks above that formation) to re
veal the rocks beneath-the rocks on which the formation was deposited. We
could then construct a geologic map on top of these underlying formations. Such
a map has been referred to as a subcrop map (Krumbein and Sloss, 1963). In a sim
ilar manner, the rocks above a formation or rock body may also be mapped. This
kind of map, looked at as though from below, is called a worm's eye view map or
supercrop map. Subcrop and worm's eye maps are commonly constructed at un
conformity boundaries; however, they can be constructed at the top and base of
any distinctive rock unit, whether or not an unconformity is present. The ma
purpose of such maps is to illustrate paleodrainage patterns, pattern of basin fill,
shifting shorelines, or gradual burial of a preexisting erosional topography (Miall,
2000, p. 250). To construct paleogeologic maps requires identification of the strati
graphic units that lie immediately below (or above) a given formation or other
satigraphic unit at numerous control points. Such data are gathered from out
crops or from subsurface well logs.
Lithofa cies Ma
p
s
Facies maps depict variation in lithologic or biologic characteristics of a strati
graphic unit. The most common kinds of facies maps, and the only kind discussed
here, are lithofacies maps, which depict some aspect of composition or texture.
Maps based on faunal characteristics are biofacies maps. Many kinds of lithofacies
Example of an isopach map of a hypothetical
formation drawn at a contour interval of
40 m. Note that the formation thickens to
more than 240 m in the basin low (deposi
tional center), thins over a basin high, and
thins to zero along the northwest and north
sides of the map.
16.6 Te chniques of Basin Analysis
573
maps are in use. Some are plotted as ratios of specific lithologic units (e.g., the
ratio of siliciclastic to nonsiliciclastic components) or as isopachs of such units
[e.g., sandstone isopach (or isolith) maps, limestone isopach (or isolith) maps].
Others examine the relative abundance and distribution of three end-member
components (e.g., sandstone, shale, limestone). Two kinds of lithofacies maps are
discussed here to illustrate the method: clastic-ratio maps and three-component
lithofacies maps. Additional examples are given in Krumbein and Sloss (1963) and
Miall (2000).
Clastic-ratio maps are maps that show contours of equal clastic ratio, which
is defined as the ratio of total cumulative thickness of siliciclastic deposits to the
thickness of nonsiliciclastic deposits, for example,
(conglomerate + sandstone + shale)
(l
imestone + dolomite + evaporite)
Values are computed for a number of control points, from outcrop or subsurface
data, and plotted on a map. The map is then contoured in the same manner as that
described for isopach maps. example is shown in Figure 16.20. Clastic-ratio
maps are usel for showing the relationship of lithologic units along the margin
of a basin in which both siliciclastic and nonsiliciclastic deposits accumulated.
Such maps provide limited information also about the location of the siliciclastic
sediment source.
Three-component (tangle) lithofacies maps show by means of patterns or
colors the relative abundance, within a formation or other stratigraphic unit, of
three principal lithofacies components. Figure 16.21 is an example of such a map
based on the relative thickness of sandstone, shale, and limestone. A teary dia
gram, called the standard diagram (inset in Fig. 16.21), is drawn using the three
lithofacies components as end members. The triangle is subdivided into fields,
each of which is indicated by a suitable patte or color. The thickness of each end
member component is measured at as many control points in outcrop or in the
subsurface as practicaL The relative values (normalized to 100 percent if neces
sary) at each control point are plotted on the teary diagram. The clastic ratio and
sand-shale ratio at each data point are calculated and used to draw contour lines
of clastic ratio (CR) and sand/shale ratio (SSR) on the map. These contour lines
allow the map to be divided into selected pattern areas corresponding to the pat
terns in the standard triangle. In this example, a progressive change in facies from
dominantly clastic material in the northwest part of the map to a section com
posed dominantly of limestone in the southe part is evident.
Figure 16.20
Example of a clastic-ratio (clastic/nonelastic)
map. The progressive increase in the ratio
from southeast to northwest across the map
indicates a progressively increasing percent
age of siliciclastic components in the strati
graphic section toward the northwest. Thus,
the source of the sediments must have been
located somewhere to the northwest. The
small arrows show the probable direction of
sediment transport.
57 4
Chapter 16 I Basin Analysis, Te ctonics, and Sedimentatio
n
Te xas
SSR 1
Fire 16.21
Oklahoma
I
I
I
I
I
\
.
Limestone
I
I
-
-
CR 1/4
1 Louisiana
I
I
Triangle lithofacies map of the Creta
ceous Trinity Group, southern U.S.A.
Note that the stratigraphic section
changes from predominantly siliciclastic
in the northwest part of the map to
predominantly limestone in the south
ern
part. Not all lithofacies shown in
the lithofacies triangle (e.g., shale) are
actually present in the mapped area.
[Redrawn from Krumbein and Sloss,
1963, Fig. 12.1 1, p. 464, as modified
from Forgotson, 1960.]
0
o
"
/
/
/
/
-
-
---
--
-
-
CR 1/4
/
-
Clastic ratio (CR)
sandstone shale
limestone + dolomite + dolomite
Sand/shale ratio (SSR)
sand
shale
100
200
Kilometers
Sandstone 8
1/8 Shale
Sandstone/shale ratio (SSR)
The degree of "mixing" of three rock components in a stratigraphic section
can be calculated mathematically by applying an entropy-like function. The func
tion is set up so that equal parts of (for example) sandstone, shale, and limestone
have an entropy value of 100. As the proportion of one end member or another in
creases, the entropy value becomes smaller, approaching zero as the composition
approaches that of a single end member. An entrop
y
map can be prepared from
these data by using an overlay of the entropy function on the lithofacies triangle
(Krumbein and Sloss, 1963, p. 467; Forgotson, 1960).
If more than three lithofacies are present in a stratigraphic section, the addi
tional lithofacies must be omitted (and the remaining three lithofacies normalized
to 100 percent) or combined with other lithofacies to yield a total of three lithofa
cies. For example, if a conglomerate facies is present, it could be combined wi
the sandstone facies, or an evaporite facies might be combined with a limestone
facies. Three-component lithofacies maps provide a convenient means of visualiz
g the relative importance of each lithofacies throughout a geographic area. Like
clastic-ratio
maps, however, they provide only a very rough guide to depositional
environments and sediment-source locations.
Computer-Generated
Maps
All of the maps discussed above can be drawn by hand, and geologists have been
drawing them this way for many years; however, such maps are now constructed
16.6 Techniques of Basin Analysis
575
more and more by computer. Computer application is particularly prevalent in
the petroleum industry, where basin analysis is a commonplace procedure. Com-
puters are able to handle large quantities of data, such as stratigraphic and struc-
tural data obtained from well records, and they allow these data to be easily
manipulated for a variety of statistical and mapping purposes. Appropriate base
maps are stored in the computer and the locaons of outcrop sections and subsur-
face wells can be easily plotted on these maps. Lithologic, structural, and strati-
graphic data obtained from study of outcrop and subsurface sections are likewise
stored in the computer. Selected data (e.g., thickness of lithologic units, structural
elevations) can be retrieved as needed and added to the base maps, which can
then be contoured by the computer by using appropriate software and special
printers to draw the maps. Thus, any of the maps described above can be generat-
ed by computer, as well as oer kinds of maps such as trend-surface maps.
Tr end-surface analysis allows separation of map data into two components: re-
gional trends and local fluctuations. The regional trend is mathematically sub-
tracted by the computer, leaving residuals, which correspond to local variations.
For example, the regional structural trend might be extracted from a structure-
contour map to more clearly reveal the nature of local structural anomalies.
Computer-generated maps are not necessarily better or more accurate than hand-
drawn maps. Their principal advantage is in the ease and rapidity with which
they can be drawn. See, for example, Robinson (1982) and Jones, Hamilton, and
Joson (1986).
Pa leoent Analysis and Pa leoent Maps
Paleocurrent analysis is a tecique used to determine the ow diction of an
cient currents that transported sediment into and within a deposional basin,
which reflects the local or regional paleoslope (see Chapter 4, Section 4.6). By in
ference, paleocurrent analysis also reveals e direction in which the sediment
source area, or areas, lay. Further, it aids in understanding the geometry and trend
of lithologic units and in interpretation of depositional environments. Paleocur
nt analysis is accomplished by measuring the orientation of directional features
such as sedimentary structures (e.g., flute casts, ripple marks, cross-beds) or the
long-axis orientation of pebbles. Numerous orientation measurements must be
made within a given stratigraphic unit to obtain a statistically reliable paleocur
nt trend. Grain-size trends, lithologic characteristics, and sediment thickness
may also have directional significance when mapped, as previously discussed.
example of a paleocurrent map, constructed mainly on the basis of cross-bedding
orientation, is shown in Figure 16.22. Note that the average (statiscal) paleoflow
direction indicated by this map is from the northwest to southeast, suggesting that
the sediment source area lay somewhere to the northwest. For more detailed dis
cussion of the application of paleocurrent alysis to interpretation of sediment
dispersal patter and basin-filling mechanisms, see Potter and Pettijohn (1977,
Chapter S).
Siliciclastic Petrofacies (Provenance) Studies
The composition of siliciclastic sediments that ll sedimentary basins is deter
mined to a large extent by the lithology of the source rocks that fished sedi
ment to the basin, as well as by the climate and weathering conditions of the
source aa. Therefore, analysis of the particle composition of siliciclastic mineral
assemblages (and rock fragments) provides a method of working backward to un
derstand e nature of the source aa. We commonly refer to such study as prove
nance study, where provenance is considered to include the following: (1) the
lithology of the source rocks, (2) the tectonic setting of the source aa, and (3) e
576
Chapter 16 I Basin Analysis, Te ctonics, and Sedimentation
Figure 16.22
Example of the use of paleocurrent data
to locate source areas, Brandywine gravel
of Maryland. The contours show modal
grain size (in mm). [From Potter, P. E., and
F. ]. Pettijohn, 1977, Paleocurrents and
basin analysis, Fig. 8.9, p. 282. Reprinted
by permission of Springer-Verlag, Berlin.]
"
Cross-bedding
direction, moving
average
._ Modal size (mm)
0 5 10 15 mi
0
10
20 km
climate, relief, and slope of the source area. Provenance studies provide important
information about the paleoclimatology and paleogeography of the basin setting.
The lithology of the source rocks is interpreted on the basis of the kinds of
minerals and rock fragments present in siliciclastic sedimentary rocks, particular
ly in sandstones and conglomerates. For example, the presence of abundant alkali
feldspars suggests derivation from granitic source rocks; abundant volcanic rock
fragments suggest derivation from volcanic source rocks; and so on. The siliciclas
tic mineralogy also provides information about the tectonic setting (continental
block, magmatic arc, collision orogen); sediment derived from continental block
settings is likely to be enriched in quartz and alkali feldspars; sediment derived
from a magmatic arc is dominated by volcanic rock fragments and plagioclase
feldspars; and sediment stripped from highlands along orogenic collision belts is
characterized particularly by an abundance of sedimentary and metasedimentary
rocks fragments. Climate, relief, and slope of the source area are more difficult to
interpret from siliciclastic mineral assemblages, but some clues are provided by
quartz/feldspar ratios and by degree of weathering of feldspars. The essentials of
provenance analysis are discussed in greater detail in Chapter 5, Section 5.6. Zuffa
(1985); Morton, Todd, and Haughton (1991), and Johnsson and Basu (1993) pro
vide a broader and more detailed view of provenance analysis.
Geophysical Studies
Geophysical investigations, including both seismic and paleomagnetic studies of
various kinds, play an important role in basin analysis. Seismic techniques are