South Georgia rift

South Georgia Rift
Stratigraphic range: 215-175 Ma
Map of the South Georgia Rift
Type	Rift Valley
Unit of	Rift Valley associated with the break up of Pangea
Underlies	Thick coastal plain sediment
Area	~100,000 km2
Thickness	up to 3,500 metres (11,480 ft)
Lithology
Primary	(Surface) Limestone, Sand, clay, sandstone
Other	(subsurface) basalt, diabase
Location
Location	South Georgia, Southeast Alabama, South Carolina Lowcountry
Region	Southeastern United States
Country	United States
Extent	100,000 square miles, 160,000 square kilometers
Map of the southeastern US. The south georgia rift runs from southeastern Alabama to near Charleston, South Carolina

The South Georgia Rift (SGR) or the South Georgia Rift Basin is a large rift valley in the southeastern United States. It is the largest buried rift valley in the Eastern North American Margin (ENAM). ^[1] The rift runs from the western Florida panhandle and southern Alabama, through central Georgia, to southern South Carolina. ^[2]

Geology

The South Georgia rift is associated with the Central Atlantic Magmatic Province (a very large flood basalt eruption associated with the break up of Pangea) and is related to the Newark Supergroup rift valleys exposed at the surface to the north. ^[3] Like the exposed Newark-type basins farther north, the South Georgia Rift (including its eastern/central segment that is sometimes informally grouped as the "East Georgia" part of the rift system) formed as a Late Triassic–Early Jurassic continental rift during the initial stages of Atlantic opening, and is filled with non-marine "red bed" sandstones, siltstones, mudstones and conglomerates together with mafic igneous rocks (basalt flows and diabase sills). ^[1] Seismic and well data show that the basin is composed of multiple asymmetric half-grabens bounded by large normal faults, with syn-rift strata thickening toward the primary border faults, similar to other Eastern North American rift basins. ^{[4] [5] [6]}

Structurally, the South Georgia Rift is unusual among Eastern North American rift basins because it overlies the deep crustal suture between Laurentian crust and the Gondwanan Suwannee terrane. ^[7] Geophysical studies indicate that rifting here reactivated older Alleghanian-age structures along the Brunswick magnetic anomaly and related sutures, so that the basin floor steps down across inherited zones of weakness rather than forming a simple single graben. ^[5] This tectonic inheritance likely influenced both the location and geometry of the rift, including the development of sub-basins in central and eastern Georgia.

The syn-rift timeline is generally interpreted as Late Triassic at the base and early Jurassic at the top, with local amounts of Central Atlantic Magmatic Province basalts and intrusions emplaced around 201 Ma. ^[8] Petrophysical analysis of core and logs from wells such as Rizer #1 and Norris Lightsey #1 shows fluvial and alluvial-fan sandstones, siltstones and mudstones with variable porosity, interbedded with finer lacustrine intervals and cut by mafic sills. ^{[1] [9]} In the eastern (South Carolina/central–east Georgia) part of the rift, some of these sediments are overlain by one or more tholeiitic basalt flows; farther west in Georgia and Alabama, wells encounter only red-bed strata without preserved basalts, implying lateral variations in CAMP volcanic cover across the basin. ^{[8] [10]}

Sedimentation

Seismic studies show that the South Georgia Rift is composed of multiple syn-rift basins (half-grabens) separated by intervening structural highs. ^[4] These sub-basins were filled during Late Triassic rifting with a thick sequence of continental sediments. The basin fill consists largely of oxidized red bed sediments – interbedded sandstones, siltstones, mudstones, and conglomerates deposited by fluvial (river) systems and alluvial fans in an arid environment. ^{[1] [11]} Analysis of the rift's porosity profiles suggests the presence of distinct depositional environments, including zones associated with ancient lakes (lacustrine deposits) as well as fluvial channel and alluvial-fan deposits. ^[1] These observations indicate the rift valley featured intermittent lakes and rivers within an overall alluvial-plain setting, similar to other Triassic rift basins.

In the latest Triassic to earliest Jurassic, volcanic activity related to the Central Atlantic Magmatic Province accompanied the rifting. Basaltic lava flows and diabase intrusions are found within the upper part of the South Georgia Rift stratigraphy. ^[8] A prominent seismic reflector known as the "J horizon" has been identified as a Jurassic-age basalt layer extending across the region at depths of roughly 1–1.5 km near Charleston. ^[8] This indicates that a laterally extensive basalt flow solidified on top of the rift sediments. Diabase sills (sheet-like intrusions) are also present within the basin. The basalts and diabases together form a resistant igneous cap above the softer sedimentary red beds. Notably, the occurrence of basalt flows in the South Georgia Rift is regionally restricted – wells in the eastern part of the rift have encountered these volcanic units, whereas towards the west the basalts appear to thin out or be absent. ^{[8] [10]}

Borehole data

Because the South Georgia Rift is buried under over 11,000 feet sediment deposits, much of what is known about its geology comes from boreholes and geophysical surveys. Oil and gas exploration wells in the 1970s first confirmed the existence of Triassic red beds beneath the Coastal Plain. For example, the Clubhouse Crossroads well in South Carolina encountered reddish sandstones and shales at depth, indicating a Mesozoic rift basin (later termed the Dunbarton Basin) in that area. ^[12] Since then, numerous boreholes have been dug in the rift basin, showing the thickness and makeup of its fill. Data shows that the South Georgia Rift's sedimentary sequence is several kilometres thick in places. ^{[13] [14]}

Drill holes have confirmed key aspects of the rift's stratigraphy. In southern South Carolina, the Dorchester 211 and Clubhouse Crossroads #3 wells penetrated basalt flows and diabase sills overlying Triassic sedimentary layers, directly verifying the CAMP volcanic section in the subsurface. ^[8] By contrast, wells in the Georgia portion of the rift (such as those in the Riddleville area) encountered thick red beds but no basalt, suggesting that the lava flows did not extend as far westward. ^[4] The overall thickness of the rift fill is substantial: the deepest test well, Norris Lightsey #1, was drilled in 1984 in Colleton County, South Carolina (about 70 km (43 mi) west of Charleston) and reached a total depth of about 4,000 m (13,123 ft). It penetrated more than 800 m (2,624 ft) of Triassic red-bed strata (sandstone and shale) before reaching pre-rift basement, making it the well with the largest known Triassic section in the basin. ^{[15] [16]}

In 2015, a dedicated stratigraphic test well (Rizer #1) was drilled in the southeastern part of the rift to obtain core samples and modern geophysical logs for carbon storage research. The Rizer #1 borehole confirmed the expected sequence of red beds and correlated well with seismic reflections and the nearby Norris Lightsey well, improving understanding of the rift's subsurface structure. ^{[17] [18]}

Relationship to the Newark Rift Basin

See also: Newark Supergroup

The South Georgia Rift formed as part of the same extensional event that produced the Newark Supergroup rift basins of eastern North America. Both the South Georgia Rift and the Newark Basin (along with related basins east of the Appalachian Mountains) developed during the Late Triassic–Early Jurassic breakup of the supercontinent Pangaea. Accordingly, the stratigraphy of the South Georgia Rift is broadly similar to that of the Newark-type basins to the north. ^{[1] [19]} In the Newark Basin (exposed in New Jersey and Pennsylvania), multiple lava flows of CAMP (for example, the Orange Mountain Basalt) are above Triassic sedimentary formation. By comparison, the South Georgia Rift – though buried – preserved at least one extensive basalt flow (the J horizon) above its Triassic deposits, as evidenced by drilling and geophysical data. ^[8] Because the South Georgia Rift is buried under the Coastal Plain, its sedimentology and palaeoenvironments have been a part of the better-studied Newark Supergroup outcrop basins. For example, the presence of lake-bed mudstone sequences in the South Georgia Rift has been theorized based on the Newark Basin's abundant lacustrine deposits, although well evidence in the SGR is sparse. ^[1]

Despite these similarities, the South Georgia Rift is also different from the Newark Basin in significant ways. The South Georgia Rift is much larger in areal extent and is entirely buried, whereas the Newark and other northern rift basins are comparatively smaller and exposed at the surface. The SGR is also situated along a different tectonic setting. It straddles the ancient suture between the North American continental crust and the Suwannee terrane, a fragment of African (Gondwanan) crust. ^[20] Geological studies suggest that the rift basin's southwestern portion lies atop the Suwannee terrane, whereas the Newark Supergroup basins to the north which is on Appalachian (Laurentian) basement. The South Georgia Rift formed on crust that had a different composition and structural history than the crust underlying the Newark Basin. This inherited difference may have influenced the South Georgia Rift's structure The rift may have exploited pre-existing weaknesses along the Alleghanian suture zone. ^[5] Additionally, the South Georgia Rift's subsidence and magmatism appear to have been somewhat different compared to northern basins, with peak rifting and volcanism occurring slightly later in the south. ^[4]

Extent of the rift

The South Georgia Rift is the largest known Mesozoic rift basin on the Eastern North American Margin. ^{[5] [21]} It extends in a southwest–northeast orientation for roughly 500 km, crossing several state boundaries under the Atlantic Coastal Plain. The rift's southwestern end lies in southern Alabama and the Florida panhandle, near the northern Gulf of Mexico. From there it runs northeast through southern Georgia (approximately along the Fall Line region) into South Carolina, reaching its northeastern terminus around the Charleston area. ^[21] The buried rift underlies a broad swath of the Southeast—estimates of its area range from about 87,000 km² up to around 160,000 km² (34,000–62,000 sq mi) when offshore portions are included. ^[5] The rift valley itself is entirely subsurface; there are no surface outcrops of Triassic rock in the region, as the basin is everywhere covered by later sediments.

Internally, the South Georgia Rift is composed of a system of interconnected basins rather than a single monolithic graben. Several named sub-basins have been identified, particularly along the rift's margins. On the north-western side of the rift, two major sub-basins separated by a basement high have been recognised: the Riddleville Basin in east-central Georgia and the Dunbarton Basin in western South Carolina. ^{[21] [5]} These sub-basins were first delineated through aeromagnetic and gravity surveys combined with well data. The Riddleville Basin (in the vicinity of Riddleville and Statesboro, Georgia) contains at least about 2.2 km of Triassic–Jurassic strata, making it one of the thickest sections of the rift fill. ^[22] The Dunbarton Basin, farther northeast near the Savannah River site in South Carolina, has about 1 km of Triassic sedimentary fill according to geophysical interpretations. ^[21] Both sub-basins are separated from the main central trough of the SGR by a broad structural horst. In the central (more southerly) part of the rift, the basin fill is even thicker – a maximum thickness of approximately 3.5 km of sedimentary and igneous rock has been inferred near Statesboro, Georgia. ^[4] Toward the rift's edges, the Triassic fill thins out against the rift border faults and gradually onlaps onto the adjacent basement. The overall structural footprint of the South Georgia Rift (often called the Southeast Georgia Embayment in older literature when including the overlying strata) continues offshore: the rift likely extends beneath the Atlantic continental shelf off Georgia and adjacent Blake Plateau. ^[5]

Seismic activity

The South Georgia Rift lies within the stable intraplate region of North America, but its presence has caused seismicity in the southeastern United States. In general, the rift area has low levels of earthquake activity. One huge exception is the Charleston seismic zone at the northeastern end of the rift. This is one of the most notable seismic areas on the U.S. East Coast and east of the Mississippi River. The Charleston seismic zone, centered around Summerville, South Carolina, lies entirely within the mapped boundaries of the South Georgia Rift basin. ^[23] This zone is characterised by clusters of small earthquakes and occasional moderate tremors. The modern earthquakes align with the subsurface rift structure, suggesting that ancient faults associated with the rift are still capable of being reactivated under the current stress regime. ^[24] The overall seismicity of the South Georgia Rift region is not uniform; it is concentrated in this northeastern segment (coincident with the Dunbarton sub basin and related structures) while the rest of the rift zone (extending through Georgia and Alabama) is comparatively quiet.

1886 Charleston earthquake connection

See also: 1886 Charleston earthquake

Charleston, SC Earthquake - 1886

Charleston, SC Earthquake - 1886

On 31 August 1886, a powerful earthquake (magnitude estimated between 6.9 and 7.3) struck the area near Charleston, South Carolina at the northeastern segmant of the South Georgia Rift. Centered around the town of Summerville, just northwest of Charleston, the quake was the most damaging ever recorded on the U.S. East Coast. It caused extreme shaking (intensity X on the Mercalli intensity scale) at the epicenter region, levelling or severely damaging most buildings in Charleston. ^{[25] [26]} Between 60 and 100 lives were lost, and damage was reported as far away as Columbia, South Carolina, and Savannah, Georgia. The earthquake was felt over an enormous area. Reports came from as far north as Boston and as far west as the Mississippi River, du to efficient transmission of seismic waves through the rigid crust of eastern North America. The 1886 event remains one of the largest intraplate earthquakes in North American history. ^[27]

Geological evidence indicates that the 1886 Charleston earthquake resulted from sudden slip on an ancient fault associated with the South Georgia Rift. The earthquake occurred far from any plate boundary, so its researched for a long time. Many researchers now conclude that the earthquake likely originated on a reactivated normal fault from the Triassic rift system. ^{[28] [29]}

Recent investigations have shed new light on the cause of the earthquake. A re-examination of nineteenth-century surveys, together with modern numerical modelling, scientists identified a right-lateral offset of about 4.5 m; ±0.3 m (14.76 ft ± 1 ft) on a section of the South Carolina Railroad southeast of Summerville and uplift of roughly 1 m (0.62 ft) near the town. ^[30] These observations are consistent with coseismic slip on a west-dipping fault beneath Summerville, commonly referred to as the Summerville fault. The fault aligns with one of the ancient rift-basin fault zones mapped in the South Georgia Rift, strongly linking the 1886 earthquake to the rift's geologic structure. ^[31]

References

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