cover 66% of the continental surfaces and probably most of the ocean floor (Blatt and Jones 1975).
The main reason for this wide areal extent is the chemical instability of igneous and metamorphic rocks under atmospheric conditions. Rocks and minerals are in equilibrium only under the set of physical and chemical conditions in which they form. Under different conditions,
they tend to move toward a new equilibrium state. Igneous and metamorphic rocks form at temperatures and
pressures much higher than those at Earth’s surface and
in environments containing less water, less oxygen, less
carbon dioxide, and very little organic matter. It is to be
expected that such rocks will be unstable and will undergo chemical and physical changes when brought to
the surface by tectonic, erosional, or isostatic forces.
These Changes in rocks constitute the process we call
weathering. On the basis of the Le Chatelier principle,
we would expect the products of this chemical change
to contain more water, more oxygen, more carbon dioxide, and more organic matter than the unweathered
rocks.
Sedimentary rocks consist almost entirely (>95%) of
three types: sandstones, mudrocks, and carbonate
rocks. Sand is defined as fragmental sediment 2.0 to
0.06 mm in diameter; mud, smaller than 0.06 mm. Carbonate rocks are composed largely of Cao03 (calcite or
aragonite) or CaMg (Co3)2 (dolomite) with other carbonates being rare. Despite the long-standing practice of
placing them in a separate category because\of their
chemical composition, the vast majority of carbonates
also originated as accumulations of mud-, sand-,’ or
gravel-sized fragments. As expected, there are transitional rocks that do not fit neatly in the three pigeon-holes. For example, coquina is a fragmental rock com-
posed of sand-sized (or fine gravel-sized) fragments of
fossil shells. Given its fragmental nature, it could be classified as a sandstone (or a. conglomerate) but because of
its chemical composition, it is customarily included in
the limestones. How should we classify a rock composed
of subequal amounts of clay minerals and microcrystalline carbonate material (marl)? There are no perfect
answers to such questions, only agreed-on compromises,
and sometimes not even those. Rocks termed detrital
or elastic are composed of sediment that has undergone
significant transport. The use of these terms is normally
restricted to silicate materials.
The most abundant sedimentary rocks are the mud-
rocks, which form 65% of all sedimentary rocks. A moment’s reflection about the mineralogy of igneous and
metamorphic rocks suggests why mudrocks dominate
the average stratigraphic section. Igneous and metamorphic rocks are composed of approximately 20% quartz
and 80% other silicate minerals; only the quartz is chemically stable under most surface conditions. The other
minerals (mostly feldspars) are unstable when exposed
at the surface and are altered to a variety of substances,
but mostly to clay minerals. Clay minerals are clay sized;
hence, mudrocks are the dominant sedimentary rock.
The quartz in crystalline rocks is very resistant to chemical attack and occurs chemically unchanged in both
mudrocks and sandstones. Many thousands of analyses
by X-ray, polarizing microscope and chemical techniques
have shown mudrocks and sandstones to have the aver-
age detrital mineral compositions listed in Table 11-1.
A weighted average shows that the detrital sediment in the
sedimentary column consists of 45%‘ clay minerals, 40%
quartz, 6% feldspar, 5% aggregated rock fragments,
and 4% others. About 85% is clay minerals or quartz, the
most stable minerals under most surface conditions.
Clearly, weathering has been a very effective process
through geologic time.
Types of. Sedimentary Rocks
The major types of sedimentary rocks are mudrocks
(65%), sandstones (20-25%), and carbonate rocks
(10-l5%), With all other rocks totaling less than 5%. How-
ever, these world averages vary Widely on both a re-
gional and a local scale, with some sedimentary basins
filled largely With detrital sediments in various propor-
tions and others dominated by nonclastics. For example,
the thick Cenozoic section of the northern part of the
Gulf of Mexico Basin contains about 90% mudrocks and
10% sandstones, With only minor amounts of nonclastic
rocks. The Michigan Basin, in contrast, contains only
18% mudrocks and 23% sandstones, but it has 47% car-
bonate rocks and 12% evaporate beds. Within a single
stratigraphic section, still greater extremes occur. In later
chapters we discuss each of the abundant types of sedi-
mentary rocks (and some of the less abundant ones) in
detail, but it is useful to introduce their general charac-
teristics here.
Mudrocks
Mud particles are defined as grains less than 62 μm in
size (silt size is 62-4 μm; clay size is <4 μm). Because
Of this, clay-sized particles or small aggregates of them
(floccules) are easily kept in suspension by even the
Qweakest of currents and can settle and accumulate only
7m relatively still waters. Many such quiet-water environments exist in both nonmarine and marine settings, for
example, floodplains, deltas, and lakes on the continents and lagoons and areas below wave base in the
marine environment. Organisms can greatly accelerate
the settling of mud particles by ingesting them as individual particles and releasing them as coarser aggregates. The thickest accumulations of mudrocks occur
adjacent to ancient continental margins, with mudrock
thicknesses ranging up to at least 2000m in the central
Appalachians of Pennsylvania and in the Ouachita belt
in Arkansas.
Sandstones
Sand accumulates in areas characterized by relatively
high kinetic energies, that is, environments of rapidly
moving fluids. Examples of these environments include
desert dunes, beaches, marine sandbars, river channels,
and alluvial and submarine fans. However, some sands
deposited on the shallow seafloor are subsequently carried down to great depth in the sediment-water mixtures
called turbidity. currents. As a result, coarse-grained
sediments can occur in quiet water.
Typical sites of sand accumulation are elongate, such
as beaches and rivers, but in the geologic record the
sands deposited in these environments are commonly
sheetlike in character. This difference results from the
displacement of the depositional site through time; for
example, a beach migrates inland during a marine transgression, the migration resulting in a slight increase in the
thickness of the sand body but an extreme increase in its
width. It is possible, however, for a sand-dominated
beach-dune complex to exist at the same geographic locality for a long period of time. This stationary location
could occur in the tectonic setting of a. slowly subsiding
basin, resulting in pure sand deposits that are hundreds
of meters thick but relatively narrow in areal extent. The
Cambro-Ordovician quartz sands of the western United
States may be an example of this.
As with mudrocks, the environment of deposition
can commonly be related to mineral composition. Sands
deposited in loci of highest kinetic energy, such as
beaches and desert dunes, tend to be more quartz-rich
than the sandbars of sluggish rivers (Sedimentation seminar 1988). This enrichment in quartz occurs because of
the relative ease of weathering, breakage, and elimination of cleavable minerals, such as feldspar, 01 of foliated
fragments, such as shale or schist. However, it is not a
good idea to base an environmental interpretation of detrital mineral composition. Some fluvial sands contain
more than 90% quartz, and many modern beaches contain high percentages of feldspar and rock particles
carbonate Rocks
Modern carbonate sediments are rich in the hard parts
of marine organisms, and there is every reason to believe
this has been true of carbonates throughout Phanerozoic
time. However, there are some thick and extensive accumulations of carbonates in Precambrian successions that
lack shelled organisms, indicating that microbes are good
at fixing carbonate, too. Because of the great chemical
reactivity of calcium carbonate, most carbonate particles recrystallize sometime after deposition; thus, their
organic (biochemical) origin is not always evident. This
is particularly true of the microcrystalline particles that
form the bulk of ancient Phanerozoic limestones. Some
of these crystals may be inorganic chemical precipitates
rather than biochemical fragments.
Because a large proportion of carbonate particles is
organic in origin, the abundance of limestones is tied to
the occurrence of phytoplankton, which is at the base
of the food chain; the phytoplankton, in turn, is tied to
the depth of penetration of light into seawater. If there is
no light, there can be no photosynthesis and no phytoplankton. The depth of penetration of light in seawater
is shown in Table 1 12, and it is apparent that most light
is absorbed in very shallow depths. Because of this, most
organisms live within 10 m of the sea surface (photic
zone). However, the carbonate material generated in
these shallow waters need not be deposited there. Plank-
tonic, carbonate-shelled organisms such as Globigerina,
are abundant in the open ocean and can settle to the
deeper ocean floor. Few of these deep-sea carbonates
appear in the stratigraphic record because the shells
dissolve in the cold water and greater hydrostatic pressures of the deep ocean. As a result, most carbonate
rocks we see in the stratigraphic record are of very
shallow marine origin and originated in relatively warm
water. Carbonates deposited in cool neritic waters are
known but are difficult to recognize in the geologic
record. The best criteria are interbedding With glacio-marine rocks and a location inappropriate paleolatitudes.
Sand-sized fragments of carbonate-shelled organisms
are not difficult to recognize and identify in thin sections
of limestones, at least to the level of phylum and class
(see Chapter 15). Sometimes even the genus and species
can be specified, and rather detailed reconstructions of
depositional environments can be made on the basis of
these and other data. For example, analyses of the relative amounts of the various isotopes of oxygen that are
present in the calcite of crystallized shell material
can be used to determine the temperature of the water
in which the organism lived. Because organisms are very
sensitive to their environment, limestones can be gold
mines of information about the shallow marine waters of
the geologic past.
Depositional Basins and Plate Tectonics
1. The light sialic material that forms a large proportion of continental masses is limited in volume, with 5
the result that continental areas constitute only
about 30% of Earth’s surface at present. The marine
areas are sediment traps that cover 70% of Earth ’s surface. These percentages have varied significantly
through geologic time, but marine areas have always
dominated.
2. Because of the movement and subduction of oceanic
lithosphere at the edges of some continental blocks
(Figure 11-2), topographically depressed areas
(trenches) with adjacent easily eroded highland areas
exist at active continental margins.
3. There has been a pronounced tendency through time
for broad areas of the continental blocks to be in-
vaded by shallow marine water (epicontinental
seas). The resulting shallow-water marine sediments
are laterally extensive on the craton, although typi-
cally they are thin. Figure 11-3 shows the number and
extent of the major global transgressions and regres-
sions of these seas, as determined from seismic
records; but many more minor transgressive-regressive
sequences are known to be present. The tectonic ex-
planation for at least the major cycles is the chang-
ing volume of the oceanic rift system. An increase in
volume of the mid-ocean ridges results in a decrease
involume of the ocean basins and subsequent flood-
ing or” the low-lying parts of continents. It is clear that
there have been two first-order transgressions, the
first extending from Early Cambrian to Late Cam-V
bn'an (or Ordovician) time and the second from Early
Jurassic to Late Cretaceous time. Extensive conti-
nental flooding also was prominent from Cambrian
through Mississippian time. The causes of higher~
order cycles might be the waxing and waning of ice
sheets or regional tectonic movements. Phanerozoic
continental glaciations are known to have decurred
in every geologic period except Cambrian, Triassic,
Jurassic, and Cretaceous.
4. Continental deposits are, by definition, formed above
sea leveland hence are subject to removal should the
rate of accumulation fall behind the erosion rate; It
is no accident that stratigraphic sections on the con-
tinental blocks contain many more large unconformi-
ties than deeper marine or oceanic sections.
Divergent Margins
Rifted intracontinental margins are important sites of
sediment deposition, and the United States contains sev-
eral that have been extensively studied, including the
Basin and Range rift syStem, the Rio Grande Rift, and the
Triassic rift system that preceded the formation of
the Atlantic Ocean (Figures 11-7 and 11-8). (See pages
225-229 for Figures 11-7-11-14.) The rifting process re-
sults in half-grabens, oriented perpendicular to the ma-
jor stress component. In the case of the eastern United
States, the half-grabens are paiallel to the sides of the
ocean basin, and an equiv alent set occurs in Nmthvvest
Africa (Seng or 1995). A few grabens can be oriented at a
large angle to the coastline, in a 1eentrant.
Early in the rifting process, richly feldspathic non-
marine sediments may be deposited from rising granitic
highlands. As continued crustal separation induces sub-
sidence along the zone of incipient continental rupture,
the floors of the main n'ft valleys become partially or in-
termittently flooded to form proto-oceanic gulfs. Re-
stricted conditions in these basins may promote the dep-
osit-ion of evaporites in suitable climates. For example,
layers of evaporites Z5 to 7.5 km thick are present in the
subsurface beneath parts of the Red Sea. Extensive evap-
orites several thousand meters thick are known also
from coastal basins on both sides of the Atlantic Ocean,
Where they apparently represent dismembered portions
of the same elongate trend of evaporite basins. The Juras-
sic Louann Salt at the base of the sediment pile in the
Gulf of Mexico is such a deposit.
Convergent Margins
Along conve1gent margins where an oceanic plate is subducted unde1 a continental plate (see Figure 11- 5), several different types of basins are formed which differ not
only in location relative to the convergent margin, but in
the type of sediment fill as well. They are termed trench-
slope basins. forearc basins, intraarc basins, and retroarc
basins.
Trench- Slope Basins. The basaltic crust of an oceanic plate is covered by abyssal deposits such as carbonate or siliceous oozes and brown clay. Before being
subducted beneath the continental plate, these deposits
are covered by significant thicknesses of trench-slope
turbidites, a significant part of which becomes plastered
onto the side of the continent and later uplifted (Underwood and Moore 1995). Several subbasins may be formed
Within the trench-slope complex (Figure 11-9). The sediments in these basins are derived from both the trench
slope and from longitudinal transport along the trench axis, and the accumulated sediments hold the world record for the widest spread in grain size of any deposit.
Some of the slabs that fall into the trench from the slope
are as large a railroad car, and in areas of limited outcrop,it can be difficult to distinguish between an in-place sequence and one of these huge gravel fragments. These
trench deposits are called mélanges, chaotic mixtures
of very large fragments of older sedimentary and crystalline rocks in a pelitic matrix. The Mesozoic Franciscan Formation in northern California contains mélanges,
and similar deposits are forming today along the western
margin of North America.
Forearc Basins. Above and landward of the trench
lie forearc basins (Figure 11- 10) which overlie older, deformed orogenic belts or perhaps oceanic or transitional
crustal material (DiCkinson 1995). Forearc basins receive sediment'mainly from the extensive nearby arc
structures, where not only volcanic rocks but also plutonic and metamorphic rocks exposed by uplift and erosion serve as sources. Sources can also include local uplands along the trench-slope break or Within the arctrench gap itself. The sandstones are characteristically
rich in volcanic and plutonic rock fragments and feldspar
grains of variable composition. There may be little trans-
fer of sediment into the subduction zone from the fore-
arc basins; frequently, the forearc basins seem to Com-
pletely override the subduction zone. The Great Valley
sequence of late Mesozoic-early Cenozoic age in Califor-
nia is an example of a» forearc basin deposit. A modern
example is at the eastside of the Bay of Bengal on the
east side of the Sunda trench. Others lie along the west-
ern coast of South America on the east side of the Peru-
Chile trench and immediately south of the volcanic Aleut-
ian Islands.
By inference from the b‘athymetry of modern forearc
basins and from the sedimentologv of older sequences
assumed to have been deposited in similar settings, a
forearc basin can contain a variety of facies. Shelf and
deltaic or terrestrial sediments, as Well as turbidites,
occur in different examples. The local bathymetry is
controlled by the elevation of the trench-slope break, the
rate of sediment delivery to the forearc basin, and the
rate of basin subsidence. Different facies patterns occur
in various basins.
Intraarc Basins. Landward of the forearc basins lie
intraarc basins, located among the many volcanoes of
the island arc (Smith and Landis 1995). The sediments
within the basins are dominated by volcanic debris in-.
terbedded with flow rocks. A map of the many intraarc
basins associated With the Aleutian convergent margin is
shown in Figure 11-11. An ancient example is the late
Miocene High Cascade Graben in central Oregon, Which
formed Within the late Miocene arc and contains the
present arc. The fundamental characteristic of "sedimen-
tation in volcanic environments, compared to other clas-
tic settings, is the production of sediment independent
of weathering processes. The sediments accumulate
essentially contemporaneously With the parent volcanic
activity.
Nonvolcaniclastic sediments may be locally signifi-
cant in intraarc basins. These sediments contain abun-
dant feldspar and igneous and metamorphic material de-
rived from exposed plutons Within the arc and from high-
standing plutonometamorphic terranes located periph-
eral to the arc. There may also be pelagic carbonate or
sediment accumulated in deep intraarc basins between
Widely spaced volcanoes. Volcanic ash is commonly in-
terbedded with these sediments.
Intraarc basins are the least studied of all basin types
because very few volcanic arcs are well mapped and few
sedimentological studies have been made in what is gen-
erally considered volcanic terrane. There is very little'di-
rect knowledge of the nature of the sedimentary and vol-
canic fills. Ancient volcanic arc sequences are commonly
highly deformed and/or metamorphosed by
later tectonic dismemberment or plutonic
activity.
Backarc Basins. The sedimentary
record of backarc, or retroarc, basins in-
cludes fluvial, deltaic, and marine strata as
much as 5 km thick deposited in terrestrial
lowlands and epicontinental seas along elon-
gate, cratonic belts between continental mar-
gin arcs and cratons (Figure 11-12). Sediment
dispersal into and across retroarc basins is
from highlands on the side toward the mag-
matic arc and from the craton toward the
continental side (Klein 1985). The Sea of
J apan'is a modern example of an extensional
retroarc basin. Ancient examples of com-
pressional (thrust-faulted) retroarc basins
are the Upper Cretaceous basins of the West-
ern Interior and Rocky Mountain region of
North America
The'major sediment types in backarc
basins are pelagic fallout, airborne ash, frag-
ments of volcanic rock, and planktonic car-
bonate and siliceous sediment. The volcanic
arc is the major source of the volcaniclastic
debris carried into the backarc basin, and
the amount and distribution of sediment is
controlled by the location of volcanic cen-
ters during the history of the basin. Much of
the sediment that is not deposited from the
air is deposited from submarine gravity
flows. In ancient rock sequences it is often
difficult to distinguish backarc basins from
intraarc basins.
Foreland Basins. These basins form
along the flanks of the continent, on the land-
ward side of the marginal orogenic belts.
They are inland of the volcanically controlled
retroarc basins and postdate them. Such
postorogenic deposits are called molasse.
In the United States, a well-studied example
of Paleozoic age exists immediately to the
west of the Appalachians (Figure 11-13); a
Mesozoic example is the interior seaway to
the east of the Rocky Mountains (Jordan
1995). Foreland basins are thousands of kilometers long
and last for tens of millions of years.
Intracratonic Basins
Most intracratonic basins overlie fossil rifts that have
been reactivated. The best known in North America are
the Hudson Bay Basin, Williston Basin, Illinois Basin,
and Michigan Basin (Figure 11-14). The lattertvvo over-
lie arms of the Reelfoot Rift in Which the Mississippi
River flows. Sediment thicknesses in the basins are typ-
ically 1000 to 5000 m greater than those in the geographic
surroundings. North American nitracratonic basins are all
oval in plan view and saucer-shaped in three dimensions
f but are dissimilar in sediment content. The Michigan Basin
j is filled with 4500 m of Cambrian to Carboniferous
coastal and shallow shelf sediments, mostly carbonate
rocks and evaporites. Shale and sandstone are subordi-
nate. The sediment fill is similar in the Williston Basin,
but the Illinois Basin contains abundant clastic deltaic
sediments and oil-bearing carbonates.
Climate
The effects of climate on the amounts and types of sed-
imentary accumulations are not always easy to docu-
ment except in extreme cases. For example, bauxite and
ferr‘uginous laterite are clear indications of moist tropi-
cal conditions, and tillite indicates a glacial regime. But
regional patterns of temperature and precipitation have
always been present on Earth and have affected sedi-
mentary patterns. The fermation of the most extensive
marine limestones requires warm temperatures through-
out most or all of the year. What are the controls of re-
gional climatic patterns and how have they varied
through geologic time?
One important control of regional climatic variations
results from changes in insolation with latitude. These
changes occur because the Sun’s radiation strikes Earth’s
surface closer to the vertical at low latitudes than at
higher ones. The lower angle of incidence at higher lati-
tudes means that temperatures there will always be lower
than at the equator, although it is not clear that the dif-
ference is always large enough to promote ice formation
and glaciation. There are several modifying factors:
1. Variations in the amount of carbon dioxide in the at-
mosphere affect climate and sedimentation, with
more carbon dioxide causing higher temperatures
through a greenhouse effect.
2. Changes in the relative proportions of land and sea.
Water has a muchlower albedo (reflectivity) and
higher heat capacity than rocks and vegetation on
the land surface. As a. result, transgressions of the
sea cause both regional warming and a moderation
of extremes in temperature variation. The major fac-
tor controlling sea level is the formation of mid-ocean
ridges, and hypsometric studies indicate that at least
10% of the area of exposed continents is flooded for
each 100 m of sea-level rise. Grogenic movements on
the continents are a secondary control of tempera-
ture distribution.
3. The fragmentation of tectonic plates and resulting
drift of landmasses through tens of degrees of lati-
tude (Figure 11-15) can result 111 vast changes in sed-
iment distribution patterns. But even an apparently
small latitudinal difference can mean the difference
between tropical rain forest and and desert, as can
be seen today in northern Africa Continental drift
can also alter the path of oceanic currents and con-
sequently the temperatures on land areas adjacent to
the currents. For example, both central England and
southern Labrador lie at 53° north latitude, but only
England is easily habitable, a result usually attrib-
uted to the warming effect of the Gulf Stream-North
Atlantic Drift.
4. The location and orientation of mountain ranges on
the continents can greatly modify the overall latitudi-
nal control of climate. The extensive desert zone in
the western United States to the east of the Sierra
Nevada range provides a clear example of this effect.
5. Climatic variations may also result from changes in
magnetic activity in the outer layers of the Sun and
interactions between cosmic radiation and the solar
Wind.
Geologic data are almost always insufficient for pa-
leoclimatic reconstruction. The problem may be the ab-
sence of strata because of erosion, diagenetic alter-
ation, or dissolution of minerals and organic matter, or it
may be that many climate parameters leave no discern-
able mark in the rocks. The most useful information
comes from environmentally sensitive fossils, including
spores and pollen, which can yield information about
temperature, precipitation, and elevation or bathymetry.
In some stratigraphic sections there are lacustrine and
glacial deposits that permit detailed paleoclimatic infer-
ences. Textures and structures of sediments can be in-
formative as well, examples being eolian cross-bedding in
sandstones (see Chapter 13) and fenestral structures
in limestones (see Chapter 15).
Perhaps the greatest insufficiency in reconstructing
paleoclimates is the lack of adequate numerical dating.
The precision of radiometric dating of pre-Miocene rocks
is no better than 104 years, but the Pleistocene record
bears Witness to the magnitude of climate changes pos-
sible in only 103 years or less. For example, 15,000 years
ago, large parts of the now arid states of Utah and Nevada
were covered by lakes; and 5000 years ago major river
systems ran through what is now the Sahara Desert.
k Summary
Sedimentary rocks are composed almost entirely of
mudrocks (65%), sandstones (20-25%), and carbonate
rocks (10-15%). Clay minerals and quartz grains form
about 85% of the mineral grains in detrital rocks. The
thickness of sedimentary rocks on the continents ranges
from 0 m over extensive areas such as the Canadian
Shield and the Siberian Shield to more than 20,000 m in
the deepest parts of some basinal areas; the lower limit\
is set by the local geothermal gradient and the suscepti-
bility of the minerals to metamorphism. u
Mudrocks are composed of 60% clay minerals, which,
because of their small grain size, can accumulate only in
areas of low ldnetic energy. Sandstones dominate in ar-
eas of high kinetic energy such as beaches and desert
dunes. The occurrence of carbonate rocks is controlled
primarily by the depth or” penetration of light into the
sea; thus, most carbonate rocks accumulate within a few
tens of meters of the sea surface.
The location and size of depositional basins are con-
trolled by plate tectonics. Many distinct types of struc-
turally controlled accumulations of sediments can be
recognized, and some have distinctive petrologic charac-
teristics. The mineral composition of the sediments in
each type is determined by its location with respect to a
continental margin, the nature of the underlying crustal
material, the types of plate boundaries nearest the basin,
and climate.
The main control of climate is latitude and the latitudes of depositional sites are determined by plate movements. Other important climatic controls include the
amount of carbon dioxide in the atmosphere, changes in
the relative areas of continent and ocean surface, and
the orientation of mountain ranges on the continents.
Walang komento:
Mag-post ng isang Komento