Georgia Geology Notes – 4 Provinces

Coastal Plain

Fall Line

Piedmont

Blue RidgeValley & Ridge*

*Some authors include part of the Appalachian Plateau with the Valley & Ridge Province, while others combine the Blue Ridge and the Piedmont.

Included in Appalachians

Map on left Adapted from Ga. Geologic Survey

state geologic map, 1977

Valley & Ridge is composed of Paleozoic Sedimentary rocks

Valley & Ridge*

Sandstones Shales Limestones Coal Conglomerate

Rocks deposited in a variety of sedimentary environments, shallow marine passive margins, shallow inland sea, delta and tidal flat deposits.

• Folding and faulting of Paleozoic sedimentary rocks – alternating anticlines & synclines – produced by compression during 3 periods of Appalachian uplift

Alleghenian Orogeny

Acadian Orogeny

Taconic Orogeny

Brief summary of Appalachian History.

Part I - Late Precambrian (700 to 600 m.y.) – Rifting of supercontinent Rodinia, opening of Iapetus Ocean.

Passive margins

Low-grade Metamorphosed sedimentary rocks are exposed in the western Blue Ridge Province.

These were deposited as rift-related sandstones, conglomerates, and shales.

The shoreline of the Iapetus Ocean might have looked like this – w/o the vegetation

Prior to Late Ordovician uplift – passive margin Iapetus Ocean sediments included sandstone, shale, and limestone.

Silurian limestone from Chattanooga area

Common Paleozoic fossils (photos from Univ. of Calif. Berkeley Museum of Paleontology website) in Ga. include:

Trilobites Crinoids

Brachiopods

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.

Part II – Rift zone became a subduction zone

Island arc collision with North America – Taconic Orogeny = uplift of “Taconic Highlands”.

Collision uplifted the “Taconic Highlands” to the East and down-warped a Foreland Basin to the west

NW SEClastic wedge (delta/tidal flat) Red Mt. Formation

Carbonates

Foreland Basin Foreland Basin

Each collision produced the same facies.

Igneous, metamorphics

The rising Taconic Highlands were eroded to produce river deltas and tidal flats, preserved 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..

Part III - Late Devonian (410 to 380 m.y.) – Acadian Orogeny – rejuvenation of highlands, new clastic wedges in NY State aka “Catskill Delta” contains fossil evidence of early vascular plants (below left).

Uplift in southern Appalachians not enough to produce new clastic wedges.

Part IV - Late Pennsylvanian (320 m.y.) – Alleghenian Orogeny. Stone Mt. and other granites emplaced in the Piedmont Province.

Highlands were again uplifted and eroded to produce clastic wedges (river deltas). The sandstones, shales, and coal beds associated with these clastic wedges can be seen on the Cumberland Plateau of GA, AL, and TN.

Plants and trees were well-developed enough to produce coal swamps in the river deltas.

Same outcrop – coal & shale were swamp deposits.

By the Pennsylvanian Period, plants were well-developed enough to produce large “coal swamps”

A close-up of typical fluvial sandstones

The Pottsville Sandstone is one of the clastic wedge units.

Blue Ridge Province

Piedmont Province

Combined Blue Ridge & Piedmont geologic map

Blue Ridge & Piedmont rocks are Late Proterozoic through Devonian (?), mostly metamorphosed rift-related sediments and volcanics, and some intrusions.

Blue Ridge & Piedmont Provinces –

• Highest topography in Georgia – 28 peaks > 4,000 ft. – Blue Ridge

• More igneous rocks in Piedmont – granites & diabase dikes.

• Piedmont topography is lower than Blue Ridge

• Generally, the metamorphic grade is lowest in the area adjacent to the Valley & Ridge Province (western Blue Ridge) and progresses higher to the southeast, across the Blue Ridge and Piedmont.

Low-grade to high-grade (left to right)

The rocks of the Blue Ridge & Piedmont also include Metamorphosed Igneous Rocks.

Ductile deformation in amphibolite, resulting from differential stress

The highest-grade igneous rocks are gene-rally in the greater Stone Mountain area.

Ductilly-deformed gneiss at Lake Lanier.

More examples of ductile deformation at Arabia Mt. – deformed garnet pods.

Partial melting and intense folding produce migmatites, seen on the Piedmont in the greater Stone Mt. area.

Chattahoochee River gravels over saprolite = Disconformity. Distance of travel produces well-rounded pebbles.

Exfoliation slabs are characteristic of granites and gneisses. Produced by unloading…

Eventually this process produces the rounded shape of Stone Mt. and Mt. Arabia

Higher topography of Blue Ridge province can facilitate the growth of clouds due to the interaction of rising masses of moist air and cooler temps at higher altitudes.

The moun-tains of Rabun Co. get 76 in of rain/year. Augusta area gets 40 inches/year.

Orographic Effect

Etowah River tunnel through ridge for purpose of river diversion for 19th century gold mining - Blue Ridge Province. Rocks are tilted metamorphic rocks.

In the Blue Ridge Province (and isolated Piedmont sites) high gradients produce “V”-shaped, youthful creek valleys.

Piedmont streams are termed “mature” and have a more moderated gradient.

The youngest igneous rocks in Georgia are the diabase (basalt) dikes on the Piedmont, related to Pangea rifting during the Triassic and Jurassic Periods

Triassic/Jurassic diabase (basalt) dike – Vulcan quarry, Beaver Ruin Rd./I-85 one of dozens on the Piedmont – related to Pangea rifting

Passive margin

Coastal Plain Province

With the Triassic and Jurassic rifting of Pangea, a passive

margin (continental shelf) developed on the Gulf and

Atlantic Coasts

Geologic Time Scale

As the continent eroded, the land rose (remember Isostacy?), exposing the Late Cretaceous through Miocene Continental Shelf sediments. The remainder of the Coastal Plain is alluvial and fluvial, except for the coastal barrier islands.

The rising of the SE portion of North America produced an “Emergent Shoreline”

Fall Line – separates crystalline rocks of Piedmont from softer sediments of Coastal Plain.

Fall Line Cities – Columbus, Macon, Augusta

Marked by waterfalls and rapids.

Coastal Plain

PiedmontFall Line

Coastal Plain Province –Late Cretaceous to Holocene (Recent) deposits at shoreline. Age – 100 m.y. - >10,000 yr.Layered sediments, sands, clays, limestones.Late Cretaceous sediments – mostly deltaic, including Providence Sand.Paleocene – Oligocene sediments – mostly marine.Miocene – Holocene – mostly continental deltaic, fluvial and alluvial sediments, except near coast.Aquifers – uniform sediments, a few karst wells.

Aerial view of a Meandering River System on the Gulf Coastal Plain

Meandering streams are more common on the Coastal Plain, but they do sometimes occur on the Piedmont.

Eocene Epoch – warm temperatures, high sea level, 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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Residuum of Paleocene Clayton Fm. over deltaic Cretaceous Providence Sand

Excess sand from Providence Canyons erosion produces braided streams in canyon bottom, i.e., too much sand.

Sinkholes can form wherever limestone or marble is at the surface – Coastal Plain, Valley & Ridge, or Blue Ridge – Murphy Marble Belt.

Resources

Valley & Ridge Province

Oil & Natural gas (other states), Coal, Limestone, Barite.

Blue Ridge Province

Marble, minerals in pegmatites, Gold, Sulfide minerals (including Copper), Talc, Corundum.

Piedmont Province

Granite, minor Gold, Copper, Feldspar, minerals in pegmatites, minor Marble.

Coastal Plain

Limestone, Kaolin, Bauxite, Sand.

Georgia Geology Notes - Review

Valley and Ridge Province

Hard, compacted Paleozoic sedimentary rocks, folded and faulted during uplift of Appalachian Mts.

Blue Ridge Province and Piedmont Province

Hard, Precambrian to Paleozoic metamorphic rocks, folded, faulted, and metamorphosed during uplift of Appalachian Mts., with some igneous

Coastal Plain Province

Soft, uncompacted Late Cretaceous through Cenozoic sediments, layers inclined gently toward Atlantic Ocean and Gulf of Mexico (SW Ga.)

Georgia Geology Notes - Groundwater Resources

Valley & Ridge Province –

Fracture zones, karst features (caverns, enlarged fractures)

Blue Ridge Province –

Fracture zones, karst features (in Marble)

Piedmont Province –

Fracture zones

Coastal Plain Province –

More uniform aquifers (softer sand, limestone, karst features – Regional Floridan aquifer – Georgia and Florida.

Georgia Geology Notes - Geology and Ecology

• Geologic structures affect topography and stream channel distribution

• Rock types influence soil type • Soil type influences nature of rainfall infiltration • Soil type and topography influence vegetation

types • Hardness of rock influences type of porosity

and permeability and aquifer types • Topography influences climate and

microclimate

• Rock type influences chemistry and pathways of ground water