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HISTORICAL GEOLOGY
LECTURE 8. EARLY PALEOZOIC GEOLOGY I.
Introduction
The Paleozoic is long – 7
periods covering almost
300 million years.
Therefore, we divide it
into an early and late
period. Cambrian +
Ordovician + Silurian =
early paleozoic (126 m.y)
Devonian +
Mississippian +
Pennsylvanian +
Permian = late paleozoic
(165 m.y).
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Both these "periods" have a similar geologic history
characterized mainly by large scale transgressions, orogenic
events and regressions.
Each period can be examined in terms of major factors that
shaped the geologic record - these factors are:
1. Paleogeography
2. Transgressions and Regressions
3. Orogenies
Why are these important?......
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1. Paleogeography:
Location of land and sea areas.
Land - often erosion, non deposition; exception =
arkosic sandstone + volcanics.
Sea (including epeiric seas) - deposition of clastics and
carbonates.
Paleolatitude - refers to the location of the North American
craton in relationship to the Equator and Poles.
Paleoclimate
Rock types
e.g. warm/hot climates -> carbonates, coral reefs, evaporites.
cold climates -> glacial deposits.
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2. Transgressions and Regressions
Occurred mainly because of tectonic plate movements.
TRANSGRESSION -> shallow ocean in PLATFORM areas
("Epeiric seas" - sea over a continent), which underwent
DEPOSITION and formation of sedimentary rocks.
REGRESSION -> land exposed - usually underwent
EROSION (with some exceptions, including arkosic
sandstones) and removal of rocks.
Because marine conditions often meant deposition and
terrestrial conditions often meant erosion, a major
transgression followed by a regression created a sequence of
strata capped by an UNCONFORMITY.
(on board)
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The green zone represents a cratonic
sequence (rocks laid down during a
transgression and regression). The
lower boundary of the green zone is a
transgression; the upper boundary is
a regression.
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The sequence is thicker and more complete near the
craton edges since these areas are the first to be covered by the
transgressing sea and the last to be uncovered by the regressing
sea. The Paleozoic contains a number of these major
depositional sequences - the SAUK in Cambrian time and the
TIPPECANOE in Ordovician/Silurian time.
Orogenies
Mostly resulting from plate convergence and continentcontinent collisions. Mountain chains were constructed along
craton margins, consisting of uplifted, folded, metamorphosed
marine and terrestrial deposits - often accompanied by intrusive
and extrusive igneous rocks (i.e. distinctive suites of rocks).
These mountain chains were subject to erosion and
became sediment sources for later deposits - shales +
greywackes in deep marine basins; quartz sands and shales in
shallow platform seas; fluvial sandstones and conglomerates in
terrestrial areas (i.e. more rocks).
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1. Paleogeography
750 million years
ago
Late Cambrian (514 million years ago):
Rodinia had broken up. The major land
masses (Laurentia, Baltica,
Kazakhstania, Siberia, China and
Gondwana) were moving apart.
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The North American
craton lay along the
Equator during
Cambrian time (Mexico Arctic Canada).
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CAMBRIAN
1. Paleogeography
The North American craton lay along the Equator
during Cambrian time (Mexico - Arctic Canada). The climate
was warm/hot. The craton land area was towards the
“northeast” (today) and eroding. Shallow platform seas existed
around the margins of the craton, especially in the south. The
deposits forming in these seas were gradational from sandshale-carbonate, depending on depth of water/distance from
shore.
The break-up of Rodinia had crated linear marine
basins (seaways) around the edge of the craton:
Appalachian seaway to the S.E./E.
Cordilleran seaway to the W.
Franklin seaway to the N.
These were sites for SHALE deposition.
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2. Transgressions/Regressions
The Cambrian was a time of TRANSGRESSION - the
SAUK TRANSGRESSION.
As the coastline moved inland, a typical transgressive
sequence was deposited:
COASTAL SANDS
SHALLOW MARINE SHALES
DEEPER MARINE LIMESTONES
Old at bottom
Young on top
A good example is provided by the Cambrian deposits
exposed in the Grand Canyon:
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Transgressive
sequence:
TEPEATS
SANDSTONE
(coastal sand)
BRIGHT ANGEL
SHALE (shallow
marine shale)
MAUV
LIMESTONE
(deeper marine
limestone)
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O: lower Cambrian. G: middle Cambrian. The sequence records the
transgression and the migration of the coastline from west to east.
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Due to the slow progress of the transgression, the same
sequence did not develop everywhere at the same time.
E.g. Bright Angel Shale is lower Cambrian in California;
middle Cambrian in the Grand Canyon. This is an
example of a “Diachronous Rock Unit” (a layer of rock of
non-uniform age).
Above: one layer of sandstone; younger to the left, older to the right.
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Sedimentary
environments near the
peak of the Sauk
transgression (late
Cambrian). This map
shows inland areas where
the late Cambrian
coastlines would have
been located. There are
coastal sandstones,
shallow marine shales and
deeper marine
carbonates. Orogenic
activity is recorded by
volcanic rocks.
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The end of the
Sauk
transgression
(Sauk
regression) is
marked by a
widespread
erosional
unconformity,
which in turn is
overlain by the
Tippecanoe
cratonic
sequence. The
erosion is
prominent
above an uplift
that formed in
the craton.
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Broad warping of the
craton formed these
basins and domes
during and after
deposition of these
rocks. Other uplifts
are related to
orogenic activity.
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Tepeats sandstone in the Grand
Canyon records lower Cambrian
Suak transgressive sands. St. Peter
sandstone records middle
Ordovician, Tippecanoe transgressive
sands.
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Are there any lower
Paleozoic rocks around
DFW?
Not exactly, they are west of
Denton and they are upper
Paleozoic (Pennsylvanian
and Permian). They are in
the Fort Worth Basin.
Lower Paleozoic rocks are
underneath them and are
exposed around the Llano
Uplift in the Hill Country.
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Section through the western part of the Fort Worth Basin
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3. Orogenies
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3. Orogenies
The Cambrian was mostly a stable period for North
America. The fragments of Rodinia were moving APART, so the
edges of the craton were mainly PASSIVE MARGINS -> no
convergence; no subduction; no orogenies.
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Seaways formed at
the craton edges;
these were sites for
coastal sands and
shales, shallow
water carbonates
and deeper marine
shales offshore.
E.g. a thick shallow
water carbonate
layer formed along
the Appalachian
Seaway from
Newfoundland to
Alabama in
Cambrian time.
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