Though the Gulf of Mexico has been studied for more than a century, the lithology, age, and origin of the basement crustal terranes remain poorly understood. New U-Pb zircon ages of a volcanic sample in the DeSoto Canyon 535 #1 Raptor well were obtained by in situ laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and resulted in a crystallization age of 509 ± 23 Ma, indicating the crustal terrane beneath the northeastern Gulf of Mexico is likely of peri-Gondwanan affinity. This basement may be associated with the felsic volcanism related to Neoproterozoic to Cambrian subduction along the supercontinent’s western margin. This subduction-related volcanism likely represents the last pulses of magmatic activity before the west coast of Gondwana transitioned into a passive margin ca. 500 Ma. The data presented here for the DeSoto Canyon 535 #1 Raptor trachyte represent the oldest radiometrically dated evidence of volcanism in the present-day deep-water Gulf of Mexico.

The depositional history of the Gulf of Mexico Basin has been studied extensively for decades. However, information on the basement crustal units remains scarce. Of the more than 54,000 wells drilled in offshore areas of the Gulf of Mexico (http://www.gomsmart.com/home.php), only 15 wells penetrated into “undifferentiated basement rock” (Gulf of Mexico Basin Depositional Synthesis Project [GBDS], https://ig.utexas.edu/energy/gbds/) with lithologies that are typically igneous or metamorphic. Two of those wells, Deep Sea Drilling Project (DSDP) Leg 77 Sites 537 and 538A (Buffler et al., 1984; Schlager et al., 1984; Dallmeyer, 1984), were drilled in water depths deeper than 152 m (500 ft) and penetrated Paleozoic metamorphic rock. Until this study, these two wells contained the only Paleozoic penetrations confirmed by absolute age determinations in deep-water areas of the Gulf of Mexico (500 ± 8 Ma by whole-rock 40Ar/39Ar and 501 ± 9 Ma by hornblende 40Ar/39Ar, respectively; Dallmeyer, 1984; see Fig. 1, locations 45 and 46; Table 1). Recent work by Erlich and Pindell (2021) presented U-Pb radiometric data from zircon indicating that Cambrian-age (522.9 ± 6 Ma) granites were drilled in relatively shallow water (98 m [324 ft]) on the Florida Platform by the Charlotte Harbor 265 #1 well in 1981 (Fig. 1, location “k”; GBDS ID# 9137). The Charlotte Harbor granite represents the only radiometrically dated record of early Cambrian plutonism in the Gulf of Mexico. Other wells on the Sarasota Arch (Fig. 1, locations d–g) likely penetrated Neoproter ozoic–early Paleozoic igneous rocks (Woods et al., 1991; Erlich and Pindell, 2021), but no other ages are reported.

This study reports analyses on basement rock drilled by the deep-water DeSoto Canyon 535 #1 Raptor well (herein referred to as the Raptor well, API 608234002000, water depth 2489 m [8169 ft]; Fig. 1). Results from our analysis suggest that the basement under the northeastern Gulf of Mexico, along the flank of the Middle Ground Arch–Southern Platform, not only predates the Mesozoic, but is far older and was likely created by Neoproterozoic to early Paleozoic subduction along the western margin of Gondwana (Mueller and Porch, 1983; Mueller et al., 1994, 2014; Heatherington et al., 1996; Heatherington and Mueller, 1999, 2003; Hibbard et al., 2002; Keppie et al., 2003, 2011; Boote and Knapp, 2016). Our data suggest the basement rock penetrated by the Raptor well is both the oldest representation of volcanism and the oldest radiometrically dated volcanic rock in the deep-water Gulf of Mexico. Publicly available data for this well can be found at the U.S. Department of the Interior Bureau of Safety and Environmental Enforcement Web site (https://www.data.bsee.gov/).

1.1 Geologic Background of the Suwannee Terrane

Geophysical, paleomagnetic, palaeontologic, and geochronologic data suggest the basement crustal provinces of southeast North America and the Gulf of Mexico are allochthonous Gondwanan or peri-Gondwanan terranes (720–400 Ma), accreted to the Laurentian plate during the Alleghenian orogeny and the final closure of the Rheic Ocean (ca. 300 Ma; e.g., Wilson, 1966; Odom and Brown, 1976; Pojeta et al., 1976; Cook et al., 1979; Chowns and Williams, 1983; Horton et al., 1989; Opdyke et al., 1987; Arthur, 1988; Van Der Voo, 1988; Dallmeyer et al., 1987; Dallmeyer, 1989; Mueller et al., 1994, 2014; Heatherington et al., 1996; Heatherington and Mueller, 1999, 2003; Poole et al., 2005; Steltenpohl et al., 2013). Previous studies (Applin, 1951; Milton and Hurst, 1965; Milton and Grasty, 1969; Bass, 1969; Milton, 1972; Neathery and Thomas, 1975; Barnett, 1975; Chowns and Williams, 1983; Dallmeyer et al., 1987; Winston, 1992; Duncan, 1998) defined the peri-Gondwanan Suwann ee terrane in the subsurface of the Florida Coastal Plain, southern Georgia, and Alabama, and extending east offshore into the Southeast Georgia Embayment (Fig. 1; Horton et al., 1989; Heatherington et al., 1996; Boote and Knapp, 2016). They considered the Suwannee terrane to be the last terrane accreted to Laurentia during this late Paleozoic orogenic phase. The Suwannee suture zone represents the southern boundary of the accreted terranes of Appalachia and the accreted Suwannee terrane (Fig. 1; Williams and Hatcher, 1982; Higgins and Zietz, 1983; Arthur, 1988; Mueller et al., 2014; Parker, 2014; Boote and Knapp, 2016; Hopper et al., 2017).

The oldest igneous rocks in the Suwannee terrane are the Neoproterozoic through early Cambrian felsic volcanic rocks of the North Florida volcanic series (Fig. 1; Heatherington et al., 1996) and the temporally and spatially associated intrusive counterparts (e.g., Osceola Granite; Fig. 1; Applin, 1951; Milton and Hurst, 1965; Milton and Grasty, 1969; Bass, 1969; Milton, 1972; Neathery and Thomas, 1975; Barnett; 1975; Chowns and Williams, 1983; Smith, 1983; Dallmeyer et al., 1987; Winston, 1992; Mueller et al., 1994; Duncan, 1998; Heatherington et al., 1996; Heatherington and Mueller, 1997; Boote et al., 2018; Erlich and Pindell, 2021). Elemental, isotopic, geophysical, and geochronologic data imply these rocks define a single continental margin arc system (Mueller and Porch, 1983; Chowns and Williams, 1983; Milton and Grasty, 1969; Mueller et al., 1994; Heatherington et al., 1996; Heatherington and Mueller, 1999; Boote et al., 2018). Previous studies have confirmed the existence of this continental arc system along the western coast of Gondwana during the late Neoproterozoic (650–551 Ma; Mueller and Porch, 1983; Mueller et al., 1994; Heatherington et al., 1996; Heatherington and Mueller, 1999; Keppie et al., 2003, 2011). One Neoproterozoic-age granodiorite (709 ± 25 Ma, K-Ar feldspar; Fig. 1; Table 1, location 25; Arthur, 1988; Woods et al., 1991) from the subsurface in the Florida Panhandle suggests the existence of plutonism as early as the Cryogenian. Recent work (Erlich and Pindell, 2021), as well as previously published ages of Osceola Granite intrusive units (Mueller et al., 1994; Dallmeyer et al., 1987), suggests that plutonism, and likely arc volcanism, continued into the early Cambrian, implying the arc system in the peri-Gondwana terranes was active from Neoproterozoic through Cambrian times.

Previously correlated by lithology and stratigraphic position and, rarely, geochronologic data, the volcanic rocks of the North Florida volcanic series are known or inferred to exist in 24 onshore wells located throughout Florida, southern Georgia, and Alabama (Fig. 1; Table 1, and references therein). Twenty-one additional wells, including the recently dated offshore Charlotte Harbor 265 #1 well (Erlich and Pindell, 2021), encounter the intrusive equivalents of the North Florida volcanic series, the Osceola Granite in northern Florida, the Gaskin Granite in the Florida Panhandle (Barnett, 1975; Winston, 1992), or the Monroeville and Dothan intrusive complexes in Alabama (Table 1; Boote et al., 2018). We present new, in situ U-Pb radiometric ages on zircon from a trachyte recovered from the Raptor well, providing the first radiometrically dated, deep-water Gulf of Mexico evidence of felsic volcanism, which occurred coeval with the North Florida volcanic series and the formation of the Neoproterozoic to early Cambrian Gondwanan arc system.

2.1 Raptor Well Volcanic Basement

In 2013, Anadarko Petroleum Company and BHP Billiton drilled the Raptor Prospect in Desoto Canyon OCS (Outer Continental Shelf) Block 535 of the Gulf of Mexico, at a water depth of 2499 m (8200 ft; Fig. 1). The original exploration target was a structural high on the Middle Ground Arch containing the Upper Jurassic Norphlet interval. This formation was penetrated at 21,630 ft subsea true vertical depth (SSTVD) (6592 m SSTVD; Fig. 2) and was found to be a non-oil-bearing interval.

The well then drilled into Lower to Middle Jurassic–age Louann Formation evaporites (e.g., Hudec et al., 2013; Rowan, 2018; Snedden and Galloway, 2019; Pindell et al., 2020b), consisting of mostly anhydrite, interspersed with limestone and clastic rocks, beginning at 21,805 ft SSTVD (6646 m SSTVD; Fig. 2).

Immediately below the Jurassic evaporites, at 22,004 ft SSTVD (6706 m SSTVD), the well encountered igneous rock, according to the mud-log report and gamma-ray, density, and resistivity responses on wireline logs (Fig. 2). Mud loggers originally described the igneous rock as a mixture of crystalline rhyolite with plagioclase feldspar, biotite mica, and trace quartz phenocrysts and “friable” granite. Our analytical results below indicate that this is not an accurate description of the basement rock, and we suggest future publications generate their descriptions based on the results presented here.

The igneous basement was sampled at 22,015.3 ft SSTVD (6710.3 m SSTVD) by a 38-mm-long and 25-mm-diameter rotary sidewall core (RSWC); this RSWC was the focus of our study (Fig. 2D). The sample is reddish, with visible feldspar lathes and secondary calcite veins and does not fluoresce strongly under ultraviolet light, suggesting a relative lack of hydrocarbons (Fig. 2D). Due to the extremely limited volume of sample available (19 cm3), we prioritized in situ and nondestructive analyses for this study.

2.2 “Whole-Rock” Geochemistry from Electron Microprobe Quantitative Compositional Mapping

Electron-probe micro-analyzer (EPMA) data acquisition was carried out at the Department of Earth, Environmental and Planetary Sciences at Rice University using a Jeol JXA 8530F Hyperprobe that is equipped with a field emission–assisted thermo-ionic (Schottky) emitter, five wavelength-dispersive spectromet ers (WDS), and one JEOL silicone drift (SD) energy dispersive spectrometry (EDS) detector. Fast identification of mineral phases in all samples was done by EDS analysis, using the SD X-ray detector with 10 mm2 active area and 133 eV resolution.

The WDS element maps were acquired using 15 kV accelerating voltage, 50 nA beam current, and 10–50 ms dwell time, using stage mode scanning. Dead-time correction was applied for every mapped element. Each element map was subsequently quantified by employing the following standards (elements calibrated are in parenthesis): olivine (Si, Mg, and Fe), biotite (K), plagioclase (Na, Ca, and Al), rutile (Ti), chromite (Cr), and rhodonite (Mn).

The local bulk composition of each homogeneous domain on the mapped area (yellow outline in Fig. 3) was estimated by taking the average of several rectangular area analyses (e.g., Carpenter et al., 2013, 2019; Lanari et al., 2019). This method is used in situations where small domains in the rock representing local chemical domains cannot be separated and/or anal yzed using other methods. This also allowed for compositions to be calculated spatially throughout the thin section, which became necessary after petrographic analysis. If whole-rock X-ray fluorescence (XRF) analysis had been done instead, those compositional differences may have been overlooked. Refer to the Supplemental Material for analytical conditions and EMPA methods.1

2.3 Geochronology

Zircon grains in the thin section were chemically identified and imaged by the EPMA at Rice University (n = 11; Fig. 3). A portion of the imaged zircons (n = 4) was then analyzed by laser-ablation–inductively coupled pl asma–mass spectrometry (LA-ICP-MS) at the University of Houston, using an Analytik Jena PlasmaQuant MS Elite quadrupole ICP-MS, coupled to a Photon Machines Excite laser-ablation instrument. The zircon was analyzed with a laser set to a 15 µm spot and a repetition rate of 8 Hz. Instrumental fractionation was monitored with the 337.1 ± 0.4 Ma Plešovice zircon (Sláma et al., 2008). Data reduction methods followed those of Shaulis et al. (2010). The 207Pb/206Pb, 238U/206Pb, 207Pb/235U, and U/Th ratios are reported in Table 2.

3.1 Thin Section Petrography and Mineralogy

Based on the description of the Raptor RSWC above, the sample presented in this study now represents the only recovered, presalt volcanic rock sample from the northern Gulf of Mexico. There is no evidence suggesting that the formation from which the core was sampled is allochthonous or reworked volcanic material. Without additional well penetrations in the area, we cannot confirm the lateral extent of the igneous rocks beneath the Louann-age evaporites extending beyond the Raptor well.

Figure 4 shows thin section photographs of the Raptor well volcanic rock. It is a crystal-rich tuff with a fine-grained, felty (pilotaxitic) groundmass, displaying some trachytic texture. The phenocrysts consist of feldspar and altered mafic phases. The feldspars are predominantly albitized plagioclase (100% albite [Ab]), exhibiting albite twinning with no chemical zonation. They occur as euhedral or slightly resorbed lathes, except when appearing broken, presumably during eruption.

Clinozoisite and calcium-bearing Fe-Ti oxides (Fig. 5) represent the altered and oxidized mafic phases, likely amphiboles and biotites. These secondary phases are pseudomorphs of the primary minerals. The clinozoisites have calcium-rich cores and iron-rich rims (see Fig. 6G). Calcium-rich Fe-Ti oxides occur with the accessory minerals fluorapatite and zircon (Fig. 5B). The fluorapatite occurs as euhedral inclusions in feldspar, as well as within the groundmass (Fig. 5C). Table 3 summarizes the geochemistry of spot analyses for accessory minerals, with reference to locations in Figure 5B. The sample contains secondary veins consisting of pyrite, calcite, and anhydrite.

The thin section and sidewall core in this study showed evidence of an additional igneous fragment similar in mineralogy, but texturally different (holocrystalline), from the fine-grained tuff (Figs. 3A and 4). All zircons in this study were found in the fine-grained tuff of the sample. No zircons were found in the holocrystalline fragment.

Zircons exist throughout the fine-grained tuff and are generally small (<10–20 μm). They are often found as inclusions in the calcium-rich Ti-Fe oxides or in plagioclase phenocrysts.

3.2 Geochemical Analyses

Backscattered-electron (BSE) and WDS compositional maps defined the chemical composition of the crystal-rich tuff, as well as the included holocrystalline fragment in the Raptor RSWC (Fig. 6; Tables 4 and 5). The crystal-rich tuff imaged by BSE (Fig. 6A) is a trachyte according to the geochemical classification scheme of Le Maitre et al. (2002), and it falls within the calc-alkaline series of the alkalis-iron-magnesium (AFM) ternary diagram (Fig. 7; Irvine and Barager, 1971). The holocrystalline fragment is a monzonite (Table 5) as described by the classification scheme of Middlemost (1985).

3.2.3 U-Pb Geochronology

We used 238U/206Pb and 207Pb/206Pb ratios (Table 2) in zircon to calculate crystallization ages on a Tera-Wasserburg regressi on plot (Fig. 8). Only zircons found in the crystal-rich, fine-grained trachyte were used for U-Pb geochronology. In total, four zircons were analyzed by LA-ICP-MS. A coherent population of in situ zircon analyses (nanalyses = 5; nzircons = 3) from the Raptor well volcanic rock yielded a Tera-Wasserburg regressi on intercept age of 509 ± 23 Ma (2 standard deviation; mean square of weighted deviates [MSWD] = 0.44) and a 207Pb/206Pb intercept of 0.484 ± 0.042, indicating the population’s initial common lead composition (Fig. 8). One of the four zircons analyzed (Zr #10; Fig. 3) was large and provided three results that contributed to formation of the regression line. The fourth zircon, zircon #8, yielded a concordant 207Pb/206Pb, 206Pb/238U, and 207Pb/235U age of 600 ± 22 Ma (Fig. 8; Table 2). The concordant data were not used to calculate the Tera-Wasserburg regressi on intercept and may represent a population of inherited zircon hosted in the 509 Ma main igneous lithology.

In eastern North America, Neoproterozoic to Cambrian volcanic rocks are considered to be exotic to Laurentia (e.g., Dallmeyer, 1989; Heatherington et al., 1996). The Raptor well concordant age of 600 Ma and regression intercept age of 509 Ma define a Neoproterozoic to Cambrian crustal age for the eastern flank of the Middle Ground Arch–Southern Platform in the Gulf of Mexico (Fig. 1) and therefore indicate its peri-Gondwanan affinity. Similar conclusions of peri-Gondwanan affinity have been made for late Neoproterozoic volcanism on other terranes accreted to Laurentia (e.g., Avalon and Carolina terranes; Kozuch, 1994; Dallmeyer and Gibbons, 1987). The Raptor well age results are also coeval with other Neoproterozoic through Cambrian reported ages of peri-Gondwanan magmati c arc activity in the region, specifically the North Florida volcanic series (Fig. 1; Table 1; Mueller and Porch, 1983; Chowns and Williams, 1983; Arthur, 1988; Dallmeyer et al., 1987; Woods et al., 1991; Mueller et al., 1994, 2014; Heatherington et al., 1996; Duncan, 1998; Keppie et al., 2003, 2011).

The Raptor results also indicate ~340 m.y. of missing stratigraphic section from the Raptor well, with Jurassic evaporites lying unconformably above the Neoproterozoic to early Cambrian igneous basement (Fig. 2). The entire section of post-Cambrian to Paleozoic Suwannee basin strata and early Mesozoic Eagle Mills Formation (Scott et al., 1961; Bishop, 1967; Moy and Traverse, 1986; Raymond, 1989; Dawson, 1995; Frederick et al., 2020) is absent in the Raptor well. This substantial unconformity between the Jurassic evaporites and the Cambrian igneous rock is corroborated by major dip changes along this boundary, as seen in the dip meter log (Fig. 2B). Similar results are seen on the Sarasota Arch in the Charlotte Harbor 265 #1 well, with Lower Jurassic sandstone overlying early Cambrian granite (Erlich and Pindell, 2021).

Neoproterozoic, subd uction-related arc magmatism has been recognized in multiple peri-Gondwanan terranes (Figs. 1 and 9). Specifically, samples from the Mayan block (Keppie et al., 2003, 2011; Fig. 1), the Carolina terrane (Kozuch, 1994; Hibbard et al., 2002, 2007), and the western and eastern Avalonia terranes (Dallmeyer and Gibbons, 1987; Bevier and Barr, 1990; Tucker and Pharaoh, 1991; Barr et al., 1994; Murphy et al., 1999, 2004; Keppie et al., 2003) have been associated with arc magmatism along Gondwana’s subducting western margin (Keppie et al., 2011). This subduction and related magmatism are thought to have ceased between 570 Ma (Newfoundland) and 540 Ma (Carolinian), with a switch from a convergent to transform margin (Murphy and Nance, 1989; Keppie and Dostal, 1991; Murphy et al., 1999; Nance et al., 2002; Keppie et al., 2003).

The North Florida volcanic series likely represents the last magmatic pulses along the Gondwanan subduction margin during Neoproterozoic to Cambrian times (Heatherington et al., 1996). The penecontemporaneous intrusive and extrusive igneous rocks of the North Florida volcanic series were initially identified only in northeastern Florida (Table 1; Heatherington et al., 1996). A later study combined deep seismic reflection profiles with additional well data and inferred that the magmatic series extended significantly further north and west, beneath the coastal plain throughout northern Florida and southern Georgia and Alabama, terminating toward the Brunswick suture zone (Fig. 1; Boote et al., 2018).

The Raptor well trachyte is located in proximity, both spatially and temporally, to the North Florida volcanic series (Fig. 1). The location and scarcity of coeval samples in the area make this sample critically important. The Raptor trachyte geochemically overlaps with the North Florida volcanic series, suggesting it is a part of this volcanic series (Fig. 7). This would significantly extend the distribution of this late Neoproterozoic volcanism within the Suwannee terrane. The Raptor well trachyte would mark the westernmost encounter of the North Florida volcanic series and the only affiliated sample retrieved from offshore Gulf of Mexico areas. It could mark the final pulses of arc-related volcanism associated with subduction, prior to complete cessation, and the formation of a passive margin along the western coast of Gondwana. The large spatial extent of Neoproterozoic–Cambrian volcanic samples may illuminate dynamic processes of amalgamation of peri-Gondwanan terranes into a single composite terrane. Further geochemical analyses (e.g., trace elements) of the Raptor well trachyte and other North Florida volcanic series samples would advance the understanding of a revised, more extensive North Florida volcanic series.

The DeSoto Canyon 535 #1 Raptor well in the deep-water area of the Gulf of Mexico penetrated a late Neoproterozoic to Cambrian–age trachytic tuff, directly beneath the Jurassic-age evaporites. We report a Tera-Wasserburg regressi on intercept age of 509 ± 23 Ma based on a population of five U-Pb analyses and one concordant 600 ± 22 Ma age on in situ zircons within the trachyte, suggesting the basement beneath the Middle Ground Arch–Southern Platform is peri-Gondwanan in origin. Its compositional and chronological affinity to the North Florida volcanic series suggests the Raptor well trachyte was also created by Neoproterozoic to early Cambrian subduction along the western coast of Gondwana.

The Raptor well trachyte represents the first radiometrically dated, deep-water penetration of Neoproterozoic to early Cambrian felsic volcanism in the Gulf of Mexico stratigraphically underlying the Louann Salt. Our study could extend the distribution of the Neoproterozoic North Florida volcanic series in southeastern North America, and it provides important constraints on the presence of peri-Gondwanan terranes south and west of the Bahamas Fracture Zone.

1Supplemental Material. Contains information on EMPA analytical conditions and tabulated data of previously published geochemical analysis of samples referenced in text. Please visit https://doi.org/10.1130/GEOS.S.24669279 to access the supplemental material, and contact editing@geosociety.org with any questions.
Science Editor: David E. Fastovsky
Associate Editor: G. Lang Farmer

We are grateful to the management and several colleagues at Occidental Petroleum Corporation for their support of this project, especially Bill Raatz, Jake Ramsey, Lisa Linder, and Ryan Morgan. We thank the anonymous reviewers and Geosphere editors who provided comments that improved this paper. We also thank Occidental management and our partners at BHP Billiton for permission to publish this study.

Regional data for this work were compiled from The Gulf of Mexico Basin Depositional Synthesis (GBDS) Project at the University of Texas–Austin, as well as Lexco Offshore Well and Lease Databases (OWL). We kindly acknowledge the use of the electron-probe micro-analyzer facility at the Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, Texas, under the guidance of Gelu Costin, as well as the use of the laser-ablation–inductively coupled pl asma–mass spectrometer at the University of Houston, Houston, Texas, under the guidance of Thomas J. Lapen.

Gold Open Access: This paper is published under the terms of the CC-BY-NC license.