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HISTORICAL GEOLOGY
LECTURE 2. SEDIMENTARY ENVIRONMENTS AND
PALEOGEOGRAPHY
Sedimentary rocks, much more so than igneous or metamorphic, provide a
geologic history of a region.
ANCIENT ENVIRONMENTAL CONDITIONS
LITHOLOGIC AND BIOLOGIC CHARACTERISTICS OF
SEDIMENTARY ROCK
INTERPRETATION OF ENVIRONMENT OF DEPOSITION
Examples….?
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Environmental Conditions:
Two major components = tectonic
setting and environment of deposition.
TECTONIC SETTING - loosely
defined, refers to presence/absence of
tectonic activity.
Some regions, usually younger, are
tectonically active - these are
OROGENIC BELTS:
Volcanism and vertical crustal
movements result in mountains,
which are high energy environments
and undergo rapid erosion – they are
therefore sediment sources; adjacent
lowlands and oceans become sites of
sediment accumulation.
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Other regions, usually older, are tectonically stable and more subdued
geologically. These are CRATONS - the ancient, eroded interior "core" of
continents; usually consisting of deformed igneous and metamorphic rocks.
Shield areas are exposed areas of cratons; platforms are parts of the craton
covered by a relatively undeformed blanket of sedimentary rocks. These
areas are much less active tectonically.
The tectonic setting influences:
a) the type of sedimentary rock e.g. rugged mountains -> high energy ->
conglomerates and sandstones.
Subdued hills, plains -> lower energy -> shales.
b) thickness of rock i.e. high rate of sediment supply -> thick accumulation of
sediment.
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ENVIRONMENTS OF DEPOSITION:
Environmental conditions influence many aspects of sedimentary deposits
such as texture, fossils and primary sedimentary structures.
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TEXTURE (SIZE).
Particle size in clastic sedimentary rocks reflects the ENERGY of the
depositional environment. E.g. (above) Nearshore - waves crashing on beaches > fairly high energy -> coarse textured deposits (pebbles/sand); offshore ->
progressively lower energy environments -> progressively finer textured
deposits - medium sand - fine sand - silt/mud - clay - carbonates (beyond landderived sedimentation in shallow tropical oceans).
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Other environments are also reflected by texture e.g.
Fine-grained sandstone
from desert sand dunes.
Coarse-grained conglomerate from debris
flows, pebbly beach, mountain stream.
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PRIMARY SEDIMENTARY STRUCTURES:
These are structures within the deposit that form during deposition e.g.
mud cracks -> swamps, tidal marshes (places that dry out – not marine)
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Stratification or layering indicates deposition in water e.g. rivers,
glacial meltwater, lakes, oceans.
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ripple marks -> dunes, tidal flats, river beds
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cross bedding -> deltas, sand dunes, river deposits
Planar cross beds, Woodbine sandstone,
Dallas.
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Planar cross beds in modern and ancient sand dunes.
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EXAMPLES OF SEDIMENTARY ROCKS RELATED TO DEPOSITIONAL
ENVIRONMENTS.
1. SANDSTONES:
Sandstones vary in quartz content, grain rounding & matrix percentage.
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a) Quartz sandstone - predominantly quartz grains ("clean sandstone").
Long transportation (quartz survives long transportation because it is
relatively hard). Distant from mountainous regions, tectonically stable.
Often form at coastlines, in deserts, on higher energy coastal plains and
river floodplains (e.g. Padre Island). Quartz grains make up 90%+ of rock
and the grains are well rounded. Cross beds and ripples are common.
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Clean quartz sandstone.
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b) Arkose - terrestrial;
derived from granitic
highlands, contain > 25%
feldspar grains (implies fairly
short transportation, because
feldspar is relatively soft and
erodes over long distances).
Commonly pink-red color.
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c) Graywacke – mixture of sand, clay and rock fragments ("dirty
sandstone"). Indicates tectonic activity, rapid erosion/sediment
accumulation, short transportation. Often deposited as turbidites
(submarine landslide deposits). Matrix is usually 30%. Beds are often
graded (sorted by size - coarse at the base, finer at the top).
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Hand specimen and thin section of graywacke.
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d) lithic sandstone - typical of deltaic deposits e.g. Mississippi delta. Matrix
< 15%. Transitional between quartz sandstones and graywackes.
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SHALES: Form in similar environments to sandstones, only deposited under
lower energy conditions (i.e. "quieter" locations) -> finer particles (clay, silt).
Shallow marine, marshes, lakes, lower energy coastal plains and floodplains.
Finely layered, often fissile. Common fossils.
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CARBONATES: Most common = limestone (calcium carbonate). Formed by
abundant marine organisms and the precipitation of calcium carbonate from
sea water. Warm, clear, shallow tropical oceans - particularly common in
platform areas.
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NAMING ROCKS:
Because sedimentary rocks are so useful in deciphering the past,
there must be a way of precisely referring to them to avoid confusion or
errors. The fundamental rock unit is the FORMATION - "a lithologically
distinct body of rock”. A formation normally has 2 names; the first its
geographic locality, the second its lithology eg. DENTON SANDSTONE.
However, a formation is not necessarily of a uniform lithology; for
example, a thick sandstone with interbedded shale layers could be a
formation: in these cases the locality name is simply followed by "formation"
i.e. DENTON FORMATION. The lithologically distinct subdivisions of the
formation are referred to as MEMBERS. Several adjacent formations may
also be combined into a GROUP. MEMBERS, FORMATIONS and
GROUPS all constitute ROCK UNITS.
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Time
unit
Rock
units
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The major inadequacy in this approach is that no regard is made to TIME
BOUNDARIES and, consequently, formations may be DIACHRONOUS:
This sandstone layer may be a lot older to the right than to the left.
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CHRONOSTRATIGRAPHIC UNITS (or Time-Rock Units).
To overcome this major shortcoming of rock units; the
chronostratigraphic unit was introduced; this refers to sedimentary rocks
deposited during the same time period. Unlike rock units, lithology can vary
greatly within chronostratigraphic units, and the upper and lower boundaries
consist of time-planes. The fundamental chronostratigraphic unit is the
SYSTEM, which corresponds to a PERIOD of geologic time.
The importance of the chronostratigraphic unit is that it enables
PALEOGEOGRAPHIC MAPS to be constructed (i.e. maps showing the
geography of a region at a given instant in the geologic past). A good example
would be to divide a chronostratigraphic unit into continental (terrestrial)
and marine deposits and thereby map the location of former shorelines.
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FACIES. The example shows how time-rock units can be used to map
paleogeography - but an important question is.."how is the distinction
between terrestrial and marine deposits made?" The answer obviously
involves PALEOENVIRONMENTAL INTERPRETATION and introduces
another concept in historical geology - THE FACIES CONCEPT.
A FACIES is a description of a sedimentary rock which is intended
to aid in the interpretation of paleoenvironments. Examples:
1. Trough cross-bedded, clean well-rounded, quartz sandstone.
2. Reef-coral limestone.
3. Organic-rich, shales, containing abundant clam fossils and bulrush
pollen.
So a paleogeographic map is also a FACIES MAP. A series of
successive facies maps -> shifting of facies through time -> changing
environmental conditions.
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SEA-LEVEL CHANGES:
Examples of major changes in environmental conditions are sealevel changes, which have occurred frequently in the geologic past. Global,
or EUSTATIC, sea-level changes have resulted in inland seas, referred to as
EPEIRIC SEAS, covering as much as 2/3 of the North American continent.
Much of the sedimentary rock record of the Paleozoic and Mesozoic Eras
were deposited during these periods of marine inundation. Characteristic
vertical facies sequences are created by transgressions and regressions.
Coastal facies
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In a marine regression (falling sea-level) nearshore facies migrate out over
offshore facies, resulting in a coarsening-up stratigraphic sequence.
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The opposite occurs in a marine transgression, resulting in a fining-up
sequence.
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