The geology of the ACE Basin has a complex, long-lived history and covers a time period of 250 million years. This history, however, cannot be separated from the events that developed the preserved stratigraphy and geomorphology of the entire Coastal Plain of South Carolina. These events, like much of the geology of the earth, are related to the assembly and subsequent breakup of the supercontinent Pangea (see inset "a") . The breakup of Pangea at the beginning of the Mesozoic, approximately 250 million years ago (Ma), led to the opening of the Atlantic Ocean and to the development of the continental margins of what is now Europe, North America, Africa, and South America. Refer to the geologic time scale for the time period of the Mesozoic and other periods described in this section.
The breakup of Pangea also influenced climatic change. As the landmasses separated, different ones drifted into polar regions or collided again. Changes in oceanic and atmospheric circulation around polar landmasses led to glaciation (Kennett 1977). Collision of landmasses and the resulting younger orogenic uplift (mountain building) also influenced both oceanic circulation and regional weather conditions that subsequently triggered glaciation (Miall 1997). Different periods of climate change drove the physical processes that created much of the geomorphology of the Coastal Plain described in other sections of this product.
When viewed from the perspective of geologic time, physical processes within the earth initiate much of this change as oceanic and climatic feedback processes modify an area's geology. This review briefly discusses these changes and how they influenced the geology of the Coastal Plain of South Carolina over the last 250 million years. Particular emphasis is placed on the events and processes that define the geology of the ACE Basin.
The Piedmont basement of the Coastal Plain has not been well studied. Different writers have discussed the basement in general terms and identified the structural geology from different data sets. Both Heron and Johnson (1966) and Siple (1969) suggested that a structural feature exists in coastal Beaufort and Colleton Counties. In their atlas of the geology of the Coastal Plain, Colquhoun et al. (1983) showed a fault trending east-west from the mouth of St. Helena Sound to the Savannah River. These writers referred to the fault as the Garner-Edisto fault and implied that the fault extends into the basement. Such basement features are the foundation on which ecosystems develop; and in many situations, it is basement features that stimulate physical change, that is, subsidence, uplift, or earthquakes.
Colquhoun et al. (1983) mapped the Garner-Edisto fault only to the mouth of the St. Helena Sound. Eastward projections of the Garner-Edisto fault suggest that the fault becomes part of the offshore Helena Banks fault zone. The Helena Banks fault zone was identified with seismic-refection profiles during a study of young faults that may be earthquake sources in the Charleston area (Behrendt and Yuan 1987). Behrendt and Yuan (1987) proposed that these faults resulted from the movement of an older zone of basement weakness.
Westward projection of the Garner-Edisto fault connects with mapped structures in southern Georgia. Chowns and Williams (1983) had previously mapped a zone of basement weakness to the same locality where the Garner-Edisto fault in the Coastal Plain sediments crosses the Savannah River, and they proposed that this zone of basement weakness may represent a suture between the Piedmont rocks of North America to the north and rocks with similar to those in Africa to the south. If these interpretations are correct, this zone of basement weakness was created when Africa collided with North America 265 million years ago. The existence of a suture implies that a structurally complex zone is present in the basement and that rocks with both African and North American affinities underlie the ACE Basin.
Subsequent breakup of Pangea began to exploit such zones of basement weakness in eastern North America in the early Mesozoic (~250 Ma). Chowns and Williams (1983) had suggested that the location of the South Georgia rift that opened in the late Triassic to early Jurassic (Daniels and et al. 1983) was influenced by a zone of basement weakness. Swanson (1986) subsequently pointed out that Mesozoic basin development was accentuated by the reactivation of zones of basement weakness, particularly in the southern Appalachians.
Line drawings by Heck (1989) of Consortium of Continental Reflection Profiling (COCORP) seismic reflection profiles collected sub-parallel to the Savannah River in eastern Georgia show the South Georgia rift to consist of a series of fault-bounded valleys. These relations also imply that the axis of the South Georgia rift underlies the ACE Basin. The South Georgia rift may be related to the opening of the Gulf of Mexico as the North American and South American plates began to separate during the Triassic (Daniels et al. 1983). The rift may also connect spreading centers in the Gulf of Mexico and Atlantic Ocean (Chowns and Williams 1983). A jump in the spreading center in the middle Jurassic (~175 Ma) led to the opening of the Atlantic Ocean (Thomas et al. 1989).
Igneous activity began and increased over the future continental margin of South Carolina with the opening of the Atlantic Ocean. In the Coastal Plain region, igneous activity was characterized by surface basalt flows, the emplacement of diabase dikes, and large mafic-ultramafic intrusions (Daniels et al. 1983). A blanket-like basalt flow, approximately 250 m thick, is buried in southern South Carolina, and this period of volcanic activity was probably related to the opening of the Atlantic. Diabase dikes are the most easily recognized igneous features on geophysical maps and are abundant underneath the Coastal Plain of South Carolina (Daniels et al. 1983). One of the best-defined mafic-ultramafic intrusions under the South Carolina Coastal Plain is directly under St. Helena Sound.
During the late Jurassic, sea-floor spreading and continental drift separated North America and Africa (see inset "b") (Miall 1997). Continental margins began to evolve as sea-floor spreading led to the opening of the Atlantic Ocean. Marginal subsidence followed and provided accommodation space for the deposition of sediments. Subsidence was enhanced by rapid sediment deposition of materials from the Appalachians (Dewey 1982), and both processes contributed to flexure and coastal onlap. These processes led to a rise in sea level and a submergence of the area that has come to be known as the Coastal Plain. Coastal onlap spread the sediment load over a wider area of the margin and developed a sediment geometry that has come to be known as "the steer's head" (Dewey 1982). The geometry consists of a thick sedimentary pile near the continental margin (the head) that tapers inland (the horns). The Cretaceous stratigraphy of the Carolinas clearly shows this steer's-head geometry (Watts et al. 1982).
Regionally, the preserved Cretaceous sediments are a succession of sandstone layers (siliciclastics) deposited in a range of fluvial, deltaic, and marine environments (Colquhoun et al. 1983; Owens and Gohn 1985; Sohl and Owens 1991). Regional cross sections led Colquhoun et al. (1983) to divide these siliciclastics into four depositional sequences (depositional activity during submerged periods) separated by bounding unconformities (breaks in geologic time and deposition). Following the development of each unconformity, renewed onlap progressively moved each younger Cretaceous depositional sequence further inland across the Coastal Plain (Sohl and Owens 1991). Progressive onlap and sequence thinning defined the steer's-head geometry as each depositional sequence was spread over a wider area of the margin.
Tertiary sediments unconformably overlie Upper Cretaceous strata (Colquhoun et al. 1983; Nystrom et al. 1991; Colquhoun and Muthig 1991). Preserved stratigraphic relations indicate that fluvial to open-shelf deposition continued into the Lower Paleocene (Colquhoun and Muthig 1991; Nystrom and others 1991); and, in overview, Lower Paleocene depositional patterns seem to differ little from those of the Cretaceous. Mapping of Paleocene facies (depositional environments) also shows that carbonate deposition was localized in the St. Helena Sound area (Colquhoun and Muthig 1991). Carbonates had been previously deposited near the top of the preserved Cretaceous section over different parts of the Coastal Plain.
Upper Eocene and Lower Oligocene depositional patterns mimic the lower and middle Eocene sedimentary patterns. Isopach maps of preserved strata show that the St. Helena Sound area continued to be a center of deposition, or depocenter (Colquhoun et al. 1983). Carbonate sedimentation again spread over the Lower Coastal Plain during this period as laterally equivalent siliciclastics were deposited in the Upper Coastal Plain (Nystrom et al. 1992). This depositional pattern changed, however, during the Lower Oligocene when glacioeustatic events were superimposed on the Coastal Plain.
Stratigraphic data indicates that the Garner-Edisto fault developed during the Paleocene to Eocene (Colquhoun et al. 1983). Fault movements began in the basement during the Upper Paleocene and were characterized by southward draping, or down warping, of depositional sequences (Colquhoun et al. 1983). Faulting perpetuated upward through the overlying sedimentary pile during the middle Eocene and produced approximately 125 meters of down-to-the-south displacement.
Reactivation of pre-existing zones of basement weakness should be expected and may influence the locations of faulting (Etheridge 1986). The orientation of the east-west striking Garner-Edisto fault during the middle Eocene regime of northeast-southwest-oriented crustal compressive stress (Janssen et al. 1995) suggests that a component of strike-slip (crustal movement) also developed and that reactivation was characterized by oblique movement. Oblique down-to-the-south displacement would have produced additional accommodation space for sediment deposition along the margin of the coast.
The continued breakup of Pangea and subsequent continental drift in the early Oligocene initiated a major glacioeustatic event (Miall 1997). Glaciation followed the breakup of Australia and Antarctica as Antarctica drifted into polar regions with the opening of the Drake Passage and the Tasman seaway (Eyles 1993). The development of circumpolar currents essentially isolated Antarctic weather systems from the rest of the southern hemisphere (Kennett 1977), and this "icehouse" effect created a land-based ice cap. This sequence of feedback processes of oceanic currents and glaciation resulted in the major sea-level fall of the early Late Oligocene (Eyles 1993; Miall 1997).
Based on paleogeographic maps of Harris and Zullo (1991), sea-level fall related to the early Oligocene glaciation shifted the shoreline a minimum of 200 kilometers to the east-southeast across the Coastal Plain. At present, most Lower Oligocene units in the South Carolina Coastal Plain have been completely eroded (Harris and Zullo 1991). The erosion of these Lower Oligocene units may only be partially related to glacioeustatic exposure (i.e. sea-level fall). The location of preserved Lower Oligocene carbonate units in eastern Georgia and in the vicinity of St. Helena Sound (Huddlestun 1993) implies that uplift to the north-northeast tilted the region and protected the carbonates to the south-southeast from erosion.
Lower Oligocene erosion was extensive and cut channels in the underlying Eocene strata (Harris and Zullo 1991). These channels were filled by Upper Oligocene sediments prior to glacioeustatic sea-level fall at the beginning of the Miocene (Miller et al. 1985). The rate of sea-level fall decreased during this period, but margin subsidence exceeded the rate of fall and allowed onlap and deposition while sea-level was still low (Miller et al. 1985). Glacioeustatic sea-level fall occurred again in the middle Miocene when a large continental icemass became a permanent fixture of Antarctica (Eyles 1993). Erosion related to these events may explain why very few Miocene units are preserved in South Carolina.
In the middle Miocene, a flood of coarse fluvial siliciclastics spread southward from the Piedmont over the Coastal Plain. These rapidly deposited fluvial siliciclastics formed a broad apron of immature braided-stream and other riverine deposits and were the result of accelerated erosion caused by active uplift of the Atlantic margin. The appearance of these fluvial siliciclastics in the stratigraphic record represents an unparalleled event in the history of the margin (Nystrom et al. 1991). With the global changes that were occurring in the middle Miocene, a combination of tectonics and climate change probably produced the coarse fluvial siliciclastics (Nystrom et al. 1991) and the seaward tilting of the Coastal Plain (Watts et al. 1982).
Sea level progressively rose again in the early Pliocene and early late Pliocene, and subsequent high stands can be related to glacioeustatic change. During this time, Pliocene sea level rise out-paced the rate of subsidence on the margin. Submarine erosion of older sequences occurred as a result of a lack of accommodation space for the younger depositional sequences. Sea level rose to the approximate limits of the older middle Eocene carbonates as waves cut the Orangeburg scarp . Facies patterns show that carbonates were prominent in the four onlapping Pliocene sequences, and the most extensive sequence spread carbonates over a portion of the Coastal Plain below the Orangeburg scarp (Willoughby et al. 1999). These relations imply that during Pliocene onlap the Gulf Stream flowed over the Coastal Plain along the Atlantic margin from the Gulf of Mexico.
Glacioeustatic events began to intensify after the middle Miocene as climatic and oceanic conditions were modified by plate tectonic collision. In the middle Miocene, collision of India with Eurasia uplifted the Himalayan ranges and the Tibetan plateau (Miall 1997). Disruption of air masses above 16,000 feet led to regional cooling of the northern hemisphere (Helfert 1998, pers. commun.). At the beginning of the late Pliocene, the Isthmus of Panama collided with South America and closed the Central American seaway (Keigwin 1978). Separation of the Atlantic and Pacific Oceans modified both the directions and intensities of oceanic currents (Keigwin 1978; Denny et al. 1994). The sudden appearance of ice-rafted material in deep-sea North Atlantic sediments is interpreted as the first indication of northern hemispheric glaciation (Cronin 1981). The combination of climatic cooling and the closure of the Central American seaway is considered by some as the triggering mechanisms for northern hemispheric glaciation in the late Pliocene and Pleistocene.
Following the Pliocene high stand, progressive sea-level fall, or regression, from the Orangeburg scarp is marked by the eroded remnants of major transgressive-regressive cycles (Willoughby et al. 1999). Each younger offlap sequence is found seaward of the Orangeburg scarp and at a lower elevation over the Middle and Lower Coastal Plain (Colquhoun and Muthig 1991; Soller and Mills 1991). The landward extent of each younger transgressive-regressive cycle is commonly marked by a distinct scarp or a break in elevation (Colquhoun 1974; Colquhoun and Muthig 1991; Soller and Mills 1991). Such scarps are considered erosional, presumably wave-cut, but they may also be a shoreline, a barrier island shoreline, an estuary valley wall, or a riverbank (Colquhoun 1974). Since scarps may have compound origins, the toe of the scarp may mark more than one transgressive highstand (Soller and Mills 1991).
The offlap sequences are considered cyclic because each sequence consists of transgressive and regressive facies (Colquhoun 1974). The facies arrangement was determined by changes in sea level. As sea level increased, a deepening-upward sequence of marine sediments was deposited; and as sea level fell, a shallowing-upward sequence of marine, barrier, backbarrier (marsh), and fluvial sediments was deposited over the eroded remnants of the previous transgressive sequence (Colquhoun 1969; Colquhoun and Muthig 1991; Soller and Mills 1991). During protracted periods of standstill, deltaic deposition allowed the formation of secondary barrier islands (Colquhoun 1974). Fluvial systems that were part of the later, regressive sequence eroded their lateral equivalents of marine sediments of the shallowing-upward marine sequence (marsh, barrier island, and shelf sequences). In some areas, only eroded remnants of these facies are preserved on the depositional surface (Willoughby 1999).
Soller and Mills (1991) report that, in the area of St. Helena Sound, elevations are quite low and estuaries are extensive C they also suggest that some sort of tectonic mechanism is responsible for this broad, complex area. The geomorphic study of Pliocene and Pleistocene shorelines by Winkler and Howard (1977) shows that the shorelines are gently warped or folded, from the "high" shoreline elevations in the Cape Fear Arch region to the "low" shoreline elevations in southern South Carolina and eastern Georgia. Such folding simply reflects vertical movement of one area relative to another (Cronin 1981).
Map patterns suggest that a "low" area was localized in the ACE Basin area in the late Pleistocene (Colquhoun 1969; McCartan et al. 1984). Map patterns show that, in the Lower Coastal Plain, late Pleistocene scarps that are sub-parallel to the coastline northeast of Charleston step landward approximately 40 kilometers in the St. Helena Sound area and only become subparallel to the coastline again near the mouth of the Savannah River. Older Pliocene and Pleistocene scarps to the northwest do not show this landward indenting pattern, which suggests that active down warping in the late Pleistocene allowed onlap to occur in a regressive sequence.
The folding of the coastline in the vicinity of the ACE basin demonstrates the sensitivity of the Coastal Plain region to vertical movements. Vertical movements are also expressed in other ways. Subtle topographic highs and morphologic changes in rivers indicate a northeast-trending zone of tectonic uplift exists northwest of Charleston (Marple and Talwani 1993). Regional studies indicate that this zone of uplift is approximately 200 kilometer long and 15 kilometer wide. The zone of uplift appears to have deflected the Edisto River (Doar pers. comm.). The South Fork Edisto River flows southeast out of Saluda County; and past its confluence with the North Fork Edisto River, the Edisto River continues to flow in nearly a straight line southeast until it reaches the northwest margin of the zone of uplift. At this locality, the Edisto River was deflected abruptly to the south into the ACE Basin. If the zone of uplift were not present, the Edisto River would connect with the Ashley River and flow into Charleston Harbor (Soller and Mills 1991).
This review summarizes a series of stratigraphic, tectonic, climatic, and oceanic events that influenced the geology not only of the ACE Basin, but of the entire Coastal Plain of South Carolina. The message is that the earth is sensitive to physical change and that such changes control ecosystems. As the plate tectonic processes initiated physical change, oceanic and climatic feedback processes modified the preserved geology. The effects that these feedback processes had on the geology clearly indicate the sensitivity of the Coastal Plain to any type of change.
W. Clendenin, SCDNR Geological Survey
R. Willoughby, SCDNR Geological Survey
C. Niewendorp, SCDNR Geological Survey
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