Georgia Geology Notes Part II

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Georgia Geology Notes Part II
Late Precambrian (600 m.y.) rifting of
super-continent Rodinia. Existing crust
(Grenville-age, 1 b.y.). Preserved in Pine Mt.
area and between Canton Cartersville.
Graben basins developed
Laurentia
Widening rift zone grabens infilled with basaltic
volcanics sediments from weathering of exposed
portions of Grenville crust. Mecham River
Formation western Va. metamorphosed graben
fill.
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Continued rift-opening of the Iapetus Ocean
formed a continental shelf (passive margin) along
the east coast of N. America. Sinking of
continental margin rising of Cambrian seas
deposited transgressional sequence. Passive
margin deposition continued into Ordovician Period
Iapetus Ocean 600 mi. max. width
Meanwhile, in the center of the Iapetus Ocean,
the rift zone activity ceased and the plate
margin west of the rift began to sink into the
mantle.
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Common Paleozoic fossils (photos from Univ. of
Calif. Berkeley Museum of Paleontology website)
in Ga. include
Brachiopods
Trilobites Crinoids
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An east-dipping subduction zone formed along with
an island arc system, resulting in the gradual
closure of the Iapetus Ocean. Carbonate
(limestone) deposition continued along the
passive margin until the collision of the island
arc and the continent Taconic Orogeny Late
Ordovician Period
Passive margin
Rifted continental margin
Oceanic crust
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Collision down-warped passive margin and caused
the deformation and metamorphism of margin and
rift-related sediments and the uplift of Taconic
Highlands to the east
NW SE
Clastic wedge (delta)
Carbonates
Starved basin
Foreland Basin
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The change from Late Ordovician passive margin
carbonates to clastic wedge sedimentation is
recorded in the Taylor Ridge roadcut, Ringgold,
Ga..
Photos by P. Bouker I-75 at Ringgold
Ripple-marked mud-cracked siltstone from
tidal-flat environment of clastic wedge is
preserved in the Late Ordovician Sequatchie Fm.
and the Silurian Red Mountain Fm..
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• Other tectonic events included Late Devonian
  Acadian Orogeny (mostly NE N. America) and the
  Pennsylvanian Alleghenian Orogeny.
• Each orogeny rejuvenated the Taconic Highlands
  and triggered the growth of new clastic wedges to
  the NW.
• Devonian clastic wedge, aka Catskill Delta
  contains fossil evidence of early vascular plants
  (below left).

Archaeopteris is one of these first trees.
By the Pennsylvanian Period, plants were
well-developed enough to produce large coal
swamps
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Between and during orogenic events, marine
sediments occasionally were deposited in the
Valley Ridge Province. The last deposition of
Paleozoic limestones in NW Georgia occurred
during the Mississippian Period. Fossils include
blastoids and horn corals.
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• During the 3 Paleozoic episodes of Orogeny
  the rocks now preserved in the Blue Ridge and
  Piedmont Provinces were undergoing metamorphism
  and deformation and some intrusive igneous rocks
  were added, e.g. Stone Mt. granite, Elberton
  Granite, etc..
• Some common minerals in these igneous and
  metamorphic rocks include Staurolite, Muscovite,
  Garnets, Beryl, Kyanite, Feldspar, Corundum,
  Gold, Platinum, Diamonds and others.

Many of these minerals are preserved in heavy
mineral sands (black sands) in local streams.
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• The Triassic/Jurassic initiation of continental
  rifting produced a swelling cracking of the
  continent, causing the intrusion of diabase dikes
  and the probable eruption of flood basalts at the
  surface.

To the east, graben basins were opening. As the
Atlantic Ocean began to open, the continental
margin cooled and subsided, causing flooding of
the Piedmont and establish-ment of a new
continental shelf. The graben basins in Georgia
were submerged and covered by Jurassic (?) and
Cretaceous sediments of the Coastal Plain.
During this time, an apron of river delta
sediments were being deposited along the Atlantic
shoreline.
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Passive margin
Early Mesozoic Era
Oceanic crust
Rifted continental margin
The Cretaceous-age Providence Formation deltaic
sands have down-dip marine facies equivalents.
Deltaic sediments are present in the subsurface
of Burke County, Ga., also.
A post-Cretaceous transgression deposited the
overlying Clayton Formation (limestone) during
Paleocene time.
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The shoreline and nearshore clastic sediments
were derived from the weathering and erosion of
the exposed Piedmont, Blue Ridge, and Valley
Ridge rocks.
The Middle Eocene Perry Sand was deposited in a
shoreline environment. Downdip (seaward), the
Blue Bluff Member of the Lisbon Formation
consists of calcareous clays and further is the
Avon Park Limestone (in Florida). Kaolins (in
central Georgia) were derived from the chemical
weathering (hydrolysis) of feldspar-rich granites
on the Piedmont.
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Periodic transgressions and regressions
(and periods of erosion) resulted in the present
Coastal Plain geology. From the Eocene Epoch
through the Late Oligocene Epoch, the present
Georgia Coastal Plain was likely covered by the
Atlantic Ocean. During the Late Eocene and the
Late Oligocene, during particular periods of high
sea level, the possible seashore was in the
Atlanta area. From the Miocene Epoch on, the
Coastal Plain gradually became more exposed due
to gentle uplift of the continent.
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During the warm climate of the Eocene Epoch and
the high sea level, there was widespread
deposition of shallow-water limestones and much
fossil biodiversity. This particular sand dollar
(echinoid) is from SE of Perry, Ga..
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Erosion removed most of the Late Oligocene
limestones from the upper portions of the Coastal
Plain, but residuum and silicified fossils leave
the evidence of the higher sea level. As the
continent underwent weathering and erosion,
gentle uplift occurred, progressively exposing
the Coastal Plain. Miocene and later sediments
consist of fluvial and alluvial sediments
(related to deltas and rivers.) Chemical
weathering (dissolution) of limestones in the
Dougherty Plain (Albany area) and other areas)
Burke, Screven, Bulloch Counties, resulted in the
development of local karst conditions. More may
be added
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