The name “Gaskin granite” was first applied to the granite in borehole W12509 by Winston (1992). The term was later broadened to Gaskin intrusive complex () to include a granodiorite in borehole W12497 and other granitoids in the area. A region of anomalously low gravity enclosing these boreholes (fig. 4) is interpreted as the extent of the Gaskin intrusive complex () in northwestern Florida.

Zircons from the granite in borehole W12509 have Neoproterozoic ages (656±38 Ma [mega-annum, million years before present]; n=6; Deasy and others, 2023). Euhedral muscovite flakes from the same sample (cover photograph; Deasy and others, 2026b, figs. 19 and 21) yielded an argon-argon (⁴⁰Ar/³⁹Ar) plateau age of 653.8±3.4 Ma (Deasy and McAleer, 2022). This is interpreted as very rapid cooling from crystallization at the minimum depths and temperatures necessary to stabilize magmatic muscovite (~12‒15 km and ~650 degrees Celsius [°C]). Zircons from drill cuttings of granodiorite from southern Alabama have been dated to 625±2 Ma (Heatherington and Mueller, 1994). Neoproterozoic ages are tentatively presumed for the other granite (W12309) and granodiorite (W12497) that represent this unit in Florida.

Although the rocks representing this unit all have metaluminous compositions (fig. 6), samples from each of the three boreholes are otherwise petrographically, geochemically, and mineralogically distinct (Deasy and others, 2024a, b). The Gaskin “type” granite of borehole W12509 contains magmatic muscovite and no mafic silicate minerals. Magnetite is common and commonly intergrown with apatite; the abundance of these two minerals increases significantly with depth. Abundances of rare earth elements (REEs) in this rock increase with depth across a ~30-ft sample interval (n=5), a trend which correlates with the abundances of apatite and magnetite (fig. 7; Deasy and others, 2026b, figs. 28‒32). This is interpreted as resulting from internal fractionation and differentiation. All samples of this rock have moderate europium (Eu) anomalies (Eu/Eu* = Eu / (Sm × Gd)^1/2 = 0.49‒0.60, n=5; fig. 7A) indicating fractionation and loss of plagioclase during its crystallization history. (The term “Eu/Eu*” is a standard notation for the europium anomaly, which is calculated by dividing measured Eu concentrations by the geometric mean of the measured concentrations of samarium [Sm] and gadolinium [Gd]. That denominator is abbreviated “Eu*.”) In contrast, the biotite-bearing granite from borehole W12309, which contains only trace amounts of magnetite and ilmenite, has low overall REE abundances and exhibits a weak Eu anomaly (Eu/Eu* = 0.86; fig. 7B). Samples of the augite- and hornblende-bearing granodiorite in borehole W12497 have consistently higher and highly consistent REE abundances across the ~164-ft sample interval (n=9; fig. 7B). This is interpreted as the product of magmatic homogeneity. These samples have weak Eu anomalies (Eu/Eu* = 0.74‒0.91), indicating minimal fractionation and loss of plagioclase. This rock contains enough magnetite to be discerned in whole-rock powder X-ray diffraction analyses (1‒2 weight percent; Deasy and others, 2024b). Taken together, this evidence suggests that each of these three boreholes penetrated discrete intrusions, each with its own petrogenetic history, rather than a single large pluton as previously hypothesized.

The Gaskin intrusive complex () is nonconformably overlain by coastal plain strata. It may have once also have been covered by the North Florida volcanic series () and (or) Suwannee basin rocks, as both units surround the Gaskin intrusive complex (Thomas and others, 1989; Guthrie and Raymond, 1992). However, step-heated ⁴⁰Ar/³⁹Ar analysis of K-feldspar concentrates from the Gaskin “type” granite in borehole W12509 demonstrate cooling below Ar closure (~150 °C) by 540 Ma and confirm this rock was not thermally reset by later burial (Deasy and McAleer, 2022). It is therefore possible that the Gaskin intrusive complex was not completely covered by the North Florida volcanic series () or the Suwannee basin rocks, that is, that inselberg(s) supported by the Gaskin intrusive complex persisted perhaps until coastal plain sedimentation.

The St. Lucie Metamorphic Complex (; Dallmeyer, 1989b; Thomas and others, 1989), also referred to as the “Cowles metamorphic rocks” (Chowns and Williams, 1983), is known from three boreholes in southeastern Florida that penetrated high-grade (amphibolite facies) metamorphic rocks. It is named for St. Lucie County, where two of the three boreholes were drilled. One borehole (W4323) penetrated a diverse range of rock types, including foliated amphibolite, biotite schist, gneissic hornblende-quartz diorite, and the greenschist facies (chlorite zone) retrograde equivalents of those rocks (Bass, 1969; Milton and Grasty, 1969). Hornblende-bearing but otherwise undescribed rocks were also recovered from boreholes W13082 and W14960 (Dallmeyer, 1989b). A massive hornblende-quartz diorite sill and associated biotite hornfels from borehole W1118 were once included in this unit (Applin, 1951). Although available geochronological and thermochronological results (Muehlberger and others, 1966; Bass, 1969) are permissive of this correlation, the identification of a volcanic or volcaniclastic protolith for the hornfels has prompted Chowns and Williams (1983) to include the rocks from borehole W1118 with the younger North Florida volcanic series (), an interpretation that is followed here. The abundances of major-element oxide analyses of granite and diorite from borehole W4323 (Milton and Grasty, 1969) suggest a calc-alkaline affinity for those intrusive rocks (fig. 8). No data on the provenance or depositional history of the metasediments are available; there are also no geochronological constraints on the primary crystallization of the meta-igneous rocks. Step-heated ⁴⁰Ar/³⁹Ar analyses of hornblende concentrates from boreholes W14960 and W13082 yield cooling ages of 513.1±1.8 Ma and 510.8±1.1 Ma, respectively (Dallmeyer, 1989b). Whole-rock K-Ar and Rb-Sr age data (Bass, 1969) are scattered but similarly support a Cambrian post-metamorphic cooling history. The similarity of these cooling ages with those of amphibolites in the Rokelide region of Sierra Leone is the basis of a hypothesized tectonic correlation (Chowns and Williams, 1983; Dallmeyer, 1989b, c). Results of whole-rock K-Ar analyses of dioritic and granitic rocks from borehole W4323 have been reported to represent Late Triassic and Permian crystallization ages, respectively (Milton and Grasty, 1969); however, we interpret these data to reflect a mixture of the cooling ages of multiple magmatic mineral populations and the much later crystallization of clays and other alteration minerals.

The Osceola arc (informal name of Boote and others, 2018) is a late Neoproterozoic to Cambrian volcanic arc. It is composed of the Osceola intrusive complex () and the North Florida volcanic series ().

The Paleozoic Suwannee basin (informal name of King, 1961) has also been called the “North Florida basin” (Duncan, 1998). The Suwannee basin extends from north-central Florida northward into Georgia and westward through the Florida panhandle and into Alabama (fig. 2). Early stratigraphic correlations of Suwannee basin strata across Florida and adjacent areas in Georgia and Alabama are based on petrographic observations of core and cuttings from boreholes (Campbell, 1939; Bridge and Berdan, 1951; Carroll, 1963). Duncan (1998) discriminates proposed formations and identifies a series of northeast-trending folds within the basin, after which the current map contacts have been modified. The Paleozoic sedimentary sequence as described by Duncan (1998) includes the following units in ascending order: (1) continental feldspathic and lithic-rich sandstones of the informal Pumpkin Swamp formation (, Cambrian); (2) feldspathic sandstone of the informal Cooks Hammock formation (, Cambrian to Ordovician); (3) the informal Cherry Lake formation (, Cambrian to Ordovician), which has a lower quartz arenite unit and an upper feldspathic sandstone-shale sequence interbedded with oolitic ironstone; (4) quartz arenite, shale, and quartz wacke of the informal Smith formation (, Middle Ordovician); (5) black shale of the informal San Pedro Bay shale (, middle Silurian to Lower Devonian); and (6) undifferentiated marine and terrestrial strata (, Middle Devonian). Unnamed unconformities are recognized between the Cherry Lake formation () and Smith formation () and between the Smith formation and San Pedro Bay shale (). The intercalation of the lower strata of the Pumpkin Swamp formation () with volcanic rocks of the North Florida volcanic series () indicates overlapping depositional histories of these units and supports a Cambrian age for onset of Suwannee basin deposition. Ordovician to Devonian ages are supported for the rest of the Suwannee sequence by biostratigraphic evidence (Richards, 1948; Howell and Richards, 1949; Kjellesvig-Waering, 1955; Pojeta and others, 1976). Fossil data (Goldstein and others, 1969; Cramer, 1973), along with paleomagnetic studies (Opdyke and others, 1987) and geochronological studies (Chowns and Williams, 1983; Dallmeyer, 1987; Ehrlich and Pindell, 2021) support a Gondwanan or peri-Gondwanan depositional setting for these rocks.

Suwannee basin strata probably once covered (nonconformably) some, if not all, of the Gaskin and Osceola intrusive complexes ( and , respectively). However, in every borehole that penetrated granitoid rock, the granitoid is directly overlain by lower Mesozoic rift-related clastic sedimentary rocks and porphyritic rhyolites () or coastal plain sediments. If the Suwannee basin strata were eroded prior to reburial in these locations, the timing of that erosion is coarsely constrained by early Paleozoic cooling ages in the plutonic rocks and early Mesozoic ages of overlying strata. It is also possible that the Gaskin and Osceola intrusive complexes supported topographic highs (for example, drainage divides) throughout the Paleozoic that were never covered by sediments. This hypothesis is compatible with the observation that the youngest (Devonian) strata have terrestrial and marine depositional environments to the northwest and to the east, respectively, of the Gaskin intrusive complex (Pojeta and others, 1976).

The Fox Creek granite () is observed in cuttings from two boreholes, one in northwestern Florida (W16298) and one in southwestern Georgia (G3001, not shown on map). It is a massive, coarse-grained, peralkaline granite (fig. 15) enriched in LREE relative to HREE (La/Yb = 7.5‒16.5) and with strong Eu anomalies (Eu/Eu* = 0.16‒0.38; fig. 16). The latter indicates significant fractionation and loss of plagioclase prior to emplacement. A region of low gravity and high magnetic anomalies south of and including the Florida-Alabama-Georgia border (figs. 4 and 5) is interpreted to represent the subcrop extent of the Fox Creek granite. Zircons extracted from cuttings from both boreholes that penetrate this granite yielded U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages of 296±4 Ma and 294±6 Ma, which are interpreted as the crystallization age of the granite (Heatherington and others, 2010).

A separate, smaller body, represented by cuttings from one borehole in the Florida panhandle (W12498), is a massive, gray to pink, coarse-grained, biotite-bearing granodiorite () containing ~45 percent plagioclase, ~30 percent quartz, and ~18 percent alkali feldspar. The rock is partially altered to chlorite, sericite, and epidote (Deasy and others, 2026b, figs. 14‒16). Step-heated ⁴⁰Ar/³⁹Ar analyses of K-feldspar separates from the granodiorite yield climbing age spectra that indicate Triassic cooling (Deasy and McAleer, 2022). This thermal history is distinct from that of the surrounding Gaskin intrusive complex () but is compatible with that of the nearby Fox Creek granite (). On this basis, a Permian crystallization age is inferred.

The early Mesozoic undifferentiated clastic sedimentary rocks and porphyritic rhyolites () that subcrop in northwestern Florida are extensions of the South Georgia rift system (fig. 2) and include the Conecuh half-graben, the Valdosta basin, and other unnamed outlying rift basins. They contain arkosic sedimentary rocks and felsic volcanic rocks of the Newark Supergroup, including the Eagle Mills Formation, and are commonly intruded by dikes and sills of the North Florida tholeiites (). The subbasins have dominantly half-graben structures bounded by northeast-trending normal faults. The subbasins are truncated by northwest-trending faults that have been interpreted as Late Triassic to Early Jurassic transfer faults coeval with subbasin formation (Heffner, 2013) and as younger normal faults related to Early to Late Jurassic tensional stresses (Hutley, 1985); these interpretations are not mutually exclusive, as reactivation of faults is common. Across Georgia and South Carolina, the polarity of the detachment faults bounding the subbasins alternates across the transfer zones between northwest dipping and southeast dipping (Heffner, 2013). The extension of that pattern into Florida is compatible with geophysical and borehole data. The Conecuh half-graben (Hutley, 1985) subcrops in the northwestern corner of Florida on the northwest flank of the Pensacola-Chattahoochee arch (refer to map) and is bound on the southeast by the Blackwater fault. The Valdosta basin (refer to map) (refer to Barnett, 1975, and Heffner, 2013), also known as the Tallahassee half-graben (Duncan, 1998), subcrops in the northern part of the State and extends into Georgia. An unnamed half-graben within the Pensacola-Chattahoochee arch, north-northwest of Panama City (refer to map), was first identified in seismic data by Arden (1974). Barnett (1975) identifies the Eagle Mills Formation in cuttings from three boreholes in the northern part of the half-graben (refer also to Frederick and others, 2020). Felsic volcanic rocks recovered from three boreholes in the southern part of this structure, including borehole W12309 (Deasy and others, 2026b, figs. 12‒13), are correlated with other early Mesozoic volcanic rocks on the basis of similar textures, compositions, and degrees of alteration. Therefore, the half-graben structure is interpreted as another early Mesozoic subbasin of the South Georgia rift system (fig. 3). The large unnamed basin that subcrops under Apalachicola (refer to map) may once have extended to the east to include the smaller unnamed and irregularly shaped basin to the southeast of the Valdosta basin which, if so, may have been uplifted, dissected, and eroded prior to coastal plain sedimentation.

The North Florida tholeiites () are a group of basaltic dikes, sills, and lava flows (Arthur, 1988; Heatherington and Mueller, 1999, 2003). Major-element compositions of these rocks define a tholeiitic fractionation trend (fig. 17; Arthur, 1988). Samples of the North Florida tholeiites () define systematic variations in the abundances of high field strength elements (HFSE) versus the magnesium number (Mg#, which is the molar ratio of magnesium oxide and combined iron and magnesium oxides [expressed as a percent] and is calculated as follows: 100×MgO/(FeO*+MgO)) (fig. 18). (The term “FeO*” indicates Fe₂O₃ abundances that were recalculated as FeO for petrologic reasons.) The olivine gabbro of borehole W12497 plots along these trends at higher Mg# values and lower HFSE concentrations than the finer grained rocks. Chondrite-normalized (_N) REE plots of the gabbro of borehole W12497 have slight LREE enrichment relative to HREE (La_N/Yb_N = 1.4‒1.9) and no Eu anomaly (fig. 19). Tholeiites from near the Jay fault in peninsular Florida have higher REE concentrations but similar La/Yb and Eu/Eu* values (fig. 19). Previous work on these rocks revealed a petrogenetic history similar to that of other early Mesozoic tholeiites in eastern North America (Arthur, 1988). Geochemical characteristics, cross-cutting relations, and available geochronology (Grasty and Wilson, 1967) allow the correlation of these rocks with the central Atlantic magmatic province (Marzoli and others, 2017). Isotopic evidence supports derivation from ~1‑Ga (giga-annum, billion years before present) continental lithospheric mantle and crust similar to magma sources in the Carolina terrane (Heatherington and Mueller, 1999). These data have been invoked to correlate the Suwannee terrane with the Sunsas-Rondonian or Orinoquian provinces of the South American parts of Gondwana, rather than with African sources.

The Southwest Florida volcanic province () subcrops across much of southern Florida and extends west into the Gulf Coastal Plain. Rocks attributed to this unit are represented by samples from 20 boreholes and include mafic to felsic volcanic rocks. Petrographic descriptions and thin section photomicrographs of many of these rocks are available in Applin (1951) and Milton (1972). The three easternmost boreholes were drilled through the Southwest Florida volcanic province into the St. Lucie Metamorphic Complex (). Underlying units elsewhere are inferred from seismic profiles to include the Osceola arc in south-central Florida and sedimentary rocks of the Suwannee basin in the unit’s northwestern extent (Ehrlich and Pindell, 2021).

A largely bimodal distribution of basaltic and rhyolitic compositions (with one intermediate sample) is demonstrated in a total alkalis versus silica diagram (fig. 20), and a calc-alkaline fractionation trend is suggested by whole-rock major-element compositions (fig. 21). Chondrite-normalized REE plots of felsic rocks show moderate LREE enrichment relative to HREE concentrations and slight Eu anomalies (Eu/Eu* = 0.60 and 0.83) showing loss of plagioclase (fig. 22).

Geochronological data for this unit include U-Pb ages of zircon in rhyolite from borehole W12838 which indicate Early Jurassic (Toarcian) crystallization (Ehrlich and Pindell, 2021). The occurrence of North Florida tholeiites () within the unit necessitates that some rocks of the Southwest Florida volcanic province were deposited in the Late Triassic. This is supported by whole-rock data from three boreholes (W966, W1005, and W1655) which yielded a Rb-Sr isochron age of 240±20 Ma (Heatherington and Mueller, 1991). Whole-rock ⁴⁰Ar/³⁹Ar analyses of mafic rock from borehole W1655 yielded ages of 192±7 Ma and 196±6 Ma (Mueller and Porch, 1983), whereas samples from the same borehole analyzed by the K-Ar method yielded ages of 143±7 Ma and 147±3 Ma (Milton and Grasty, 1969). The Early Jurassic ages are close to but slightly older than the results from borehole W12838 and may represent a different eruption within the same volcanic package. The Late Jurassic ages, however, are of “highly altered” material (Milton and Grasty, 1969, p. 2,487) and probably represent mixed ages that have no geologic significance. Likewise, an amygdaloidal basalt from within the volcaniclastic sequence in borehole W4323 yielded a K-Ar age of 89.3±2.2 Ma (Milton and Grasty, 1969); because of the reported abundance of secondary minerals, we suspect this rock is older and include it and the volcaniclastic rocks with the Southwest Florida volcanic province (). Whole-rock K-Ar analyses of granite and diorite underlying the volcaniclastic rocks in borehole W4323 yielded Permian and Triassic ages, respectively (Milton and Grasty, 1969). The Triassic ages have been invoked to correlate the diorite with the Southwest Florida volcanic province. However, we interpret both the Permian and Triassic ages as mixtures of cooling ages and alteration ages and include the plutonic rocks with the St. Lucie Metamorphic Complex (). Together, these data constrain the age of the Southwest Florida volcanic province () to the Late Triassic to Early Jurassic.

The Pensacola-Chattahoochee arch (figs. 1, 4, and 5) is an uplifted block of Neoproterozoic and early Paleozoic Suwannee terrane rocks. It subcrops under much of the Florida panhandle and is bounded by the Blackwater and Tallahassee faults. The Wiggins-Hancock arch (figs. 1, 4, and 5) is a Mesozoic horst that subcrops in the panhandle west of the Jay fault; it is also known as the Wiggins arch (Montgomery, 2000), Wiggins block (fig. 2) (Mueller and others, 2014), and Wiggins uplift (Dallmeyer, 1989a). Hornblende, biotite, and phyllite samples from basement rocks in the Wiggins-Hancock arch in Alabama have late Pennsylvanian ⁴⁰Ar/³⁹Ar cooling ages (Dallmeyer, 1989a) interpreted as cooling from metamorphism in the early stages of the Alleghanian orogeny. Basement rocks of the Wiggins-Hancock arch have not been sampled in Florida.

Several basins and uplifts in the Gulf Coastal Plain south of the Jay fault are inferred from geophysical data. These include, from northwest to southeast, the Apalachicola-Desoto Canyon basin, the Middle Ground arch/Southern platform, the Tampa basin, the Sarasota arch, and the South Florida basin (figs. 1, 4, and 5). Upper Jurassic or Lower Cretaceous carbonates have been penetrated at depths between ~11,975 and 17,716 ft in the South Florida basin (Florida Geological Survey, 2023); the depth and nature of basement rock in the South Florida basin is unknown. The West Florida terrane (proposed name of Ehrlich and Pindell, 2021) comprises the pre-late Jurassic basement underlying the South Florida basin and the other structures south of the Jay fault. It is distinguished from the Suwannee terrane by the presence of post-Cambrian Paleozoic zircons and is perhaps related to the Maya block (Ehrlich and Pindell, 2021).

The Central Florida uplift, also known as the Peninsular arch, subcrops in the Florida peninsula and brings rocks of the Osceola arc ( and ) up against Suwannee basin strata across an unnamed north-northeast-striking normal fault. This uplift is truncated to the south by the Hatchineha fault and to the north by the Lake City fault. The timing of latest uplift is mapped as early Mesozoic, but this fault may have reactivated a structure potentially as old as early Paleozoic (Duncan, 1998).

Duncan (1998) employed stratigraphy and dipmeter logs (a dipmeter is a device for measuring the orientations of planar structures intersecting a borehole) to identify several broad, open, upright folds in the Paleozoic sedimentary rocks of the Suwannee terrane. The northeasterly trends of these folds run approximately parallel to the Higgins-Zietz line, which may represent the northern extent of the Suwannee terrane (fig. 2; Horton and others, 1984, 1989, 1991; Thomas and others, 1989), suggesting that folding occurred during late Paleozoic assembly of Pangaea. Quantitative constraints on the timing of folding are not available.

The Taylor syncline (Duncan, 1998) is cored by the San Pedro Bay shale () and plunges gently to the southwest. The Dixie anticline (Duncan, 1998) is cored by the Cooks Hammock formation. The apparent closure of this fold in the northeast implies a northeastward plunge of the fold axis, in contrast to other folds. The Otter Creek syncline (proposed name), cored by undifferentiated Devonian rocks and San Pedro Bay shale, lies southeast of the Dixie anticline and plunges gently to the southwest. Other smaller, unnamed folds are interpreted to subcrop in the northern part of Florida (contacts modified from Duncan, 1998).

Several brittle faults are recognized to cut through Florida basement rocks. Two orientations predominate: northeast-trending normal faults (which define the margins of South Georgia rift system subbasins) and northwest-trending sinistral transfer zones. Subbasins of the South Georgia rift system have dominantly half-graben structures, the polarity of which alternates across transfer faults in Georgia and South Carolina (Heffner, 2013). The continuation of this pattern through Florida is supported by borehole data, by reinterpretation of available seismic studies (Arden, 1974), and by reprocessed geophysical data (figs. 4 and 5). All mapped faults are therefore considered to be the products of oblique rifting in the early Mesozoic related to the breakup of Pangaea, although some may follow inherited structures. Reactivation of these faults after the onset of coastal plain sedimentation in the Middle Jurassic is possible but unknown. Historical records of seismicity in Florida are extremely limited (Reagor and others, 1987; U.S. Geological Survey, 2024). Hypothesized associations between proposed faults and historical earthquakes are suggested below.

The Jay fault (Smith 1983, 1993) follows a major lineament that is traceable in geophysical surveys from the Bahamas to Oklahoma; it is also known as the Florida transfer zone (Ehrlich and Pindell, 2021), Bahamas fracture zone (Klitgord and others, 1984), Bahamas transfer zone (Nemčok and others, 2016), and Florida-Bahamas lineament (Beaman and others, 2017). The Jay fault separates the Suwannee and West Florida terranes (fig. 2) and may be the reactivated boundary which originally sutured those terranes (Ehrlich and Pindell, 2021). This major structure may also have originated as a transform fault in the Iapetus Ocean (Thomas, 2010). These hypotheses are not mutually exclusive. Most displacement along this fault is probably Late Triassic to Middle Jurassic (Ehrlich and Pindell, 2021) and broadly contemporaneous with the northeast-striking faults responsible for the South Georgia rift system and related subbasins. Several historical earthquakes near the intersection of the trace of the Jay fault and the Florida-Alabama border (U.S. Geological Survey, 2024) may be associated with recent motion along the Jay fault, but this relationship has not been confirmed.

Another transfer fault northeast of and approximately parallel to the Jay fault truncates the southwestern margin of the Valdosta basin (Heffner, 2013) and is here called the Hatchineha fault. Gravity and aeromagnetic data (figs. 4 and 5) support the projection of this fault through peninsular Florida where it juxtaposes the Osceola intrusive complex () and the Southwest Florida volcanic province (). The Hatchineha fault has northeast-side-down motion in the panhandle and southwest-side-down motion in central and southern Florida, indicating rotational faulting. The Lake City fault (Duncan, 1998) is a right-lateral transfer fault inferred from the truncation of the Taylor syncline and Dixie anticline by a large area of San Pedro Bay shale (). Approximately 215 m of down-to-the-northeast normal motion is estimated, as well as an unquantified dextral strike-slip component (Duncan, 1998). The Cordele transfer zone (Heffner, 2013) is another northwest-trending sinistral transfer fault in the northeastern part of the State. Normal motion on this fault is possible but unknown. The Cordele transfer zone subcrops approximately 20 km southwest of Jacksonville. Its association with the October 31, 1900, magnitude 3.5 Jacksonville earthquake (U.S. Geological Survey, 2024) is possible but not confirmed.

The Blackwater fault (Duncan, 1998) defines the boundary between the Conecuh half-graben to the northwest and the Pensacola-Chattahoochee arch to the southeast. The Tallahassee fault (Duncan, 1998) defines the southeastern boundary of the Pensacola-Chattahoochee arch as well as, for part of its length, the northwestern margin of the Valdosta basin. Several unnamed, northeast-trending faults in northern and central peninsular Florida, identified by strong linear magnetic and gravity anomaly contrasts (figs. 4 and 5), separate inferred horsts (low gravity, low magnetic response) and grabens (high gravity, high magnetic response; Coleman and Stewart, 1982).