Barnett (1975, p. 138) describes approximately 55 ft of basement penetration beginning at 12,285 ft depth as “metamorphosed volcanic sandstone and granule conglomerate. Cambrian or U. Precambrian (sic)?” and adds “Core 12333 to 12340 rec[overed] 3 [ft] volcanic granule conglomerate and sandstone, 3 ft. slightly metamorphosed red and green shale, 1 ft. pebble conglomerate and sandstone.” This information is also included in the compilation of Lloyd (1985).

Three thin sections were prepared from core chips from each of the following depths:

The sample from 12,335 feet depth is a large, approximately (~) 1.5 centimeter (cm)×~3 cm core chip of feldspathic litharenite. Grains are subangular to well rounded and range in size from clay to granule, with a maximum clast dimension of ~0.3 cm (fig. 2). The chip preserves a bedding contact between coarser and finer grained sand. The sample contains a well-developed bedding-parallel metamorphic foliation defined by white mica (fig. 3). Lithic clasts include fine-grained, rounded sandstone pebbles and feldspar-phyric volcanic clasts (fig. 3). Detrital feldspars are commonly blocky and include perthitic alkali feldspars (fig. 3) and plagioclase with deformation twins (fig. 3, center right). Some feldspathic grains are unaltered and free of inclusions, whereas others are partially to completely sericitized or riddled with inclusions (fig. 2). Some lithic and feldspathic grains have reaction rims of white mica (fig. 3) which are probably coincident with foliation development.

The sample from 12,336 feet depth comprises cuttings ranging in size from submillimeter to ~1 cm in maximum dimension. In hand sample, the fragments have a very fine grained, lustrous foliation. Most fragments are red, but some of the larger fragments have sharp contacts with yellow-green domains and some of the smaller fragments are completely yellow-green in color. The relationship between these color domains suggests redoximorphic structures. In thin section, the phyllitic fragments have strongly hematite-stained matrices and lithic clasts, whereas quartz grains up to 0.1 cm in diameter and phyllitic clasts up to 0.2 cm in diameter with long axes aligned with bedding contain little to no hematite (figs. 4 and 5). Despite the prevalence of detrital feldspars in the litharenitic samples above and below, feldspar clasts appear to be absent in the phyllitic fragments (figs. 4 and 5). Either this sample represents a relatively muddy and more mature sediment intercalated among litharenitic beds, or all feldspars in this layer were replaced by white mica during metamorphism. The high optic density due to the prevalence of hematite in this sample impairs easy estimation of the lithic content of the phyllitic fragments.

The sample from 12,338 feet depth is a large, ~1.5×~3-cm core chip of massive feldspathic litharenite (fig. 6). Grains are subangular to well rounded and range in size from fine sand to granule, with a maximum clast size of ~0.3 cm. The matrix is overprinted by randomly oriented, fine-grained white mica, epidote (fig. 7), and chlorite (fig. 8). The clastic load comprises subangular to well-rounded mono- and polycrystalline quartz grains (figs. 6–8), blocky to subrounded feldspars (fig. 7), rounded fine-grained sandstone clasts (fig. 8, center), and subangular to rounded plagioclase-phyric volcanic clasts (for example, fig. 8, center left).

Hand-picked fragments of red phyllite from 12,336 ft depth were separated and placed in an automated Brinkman grinder fitted with an agate mortar and pestle. The samples were continuously lubricated with acetone during grinding. The resulting powders were further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. Aliquots of the powders were placed in 3×2×0.1-cm cavities of titanium (Ti) “front-pack” mounts and scanned in a Bruker D8 diffractometer with a copper (Cu) anode and point detector from an angular range of 2–90° 2 theta (Θ) at 0.02° steps and a rate of 8 seconds (sec)/step. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in TOPAS (Bruker AXS, 2011).

The broad shape of the 1.0-nanometer (nm) peak in diffraction results prompted re-analysis to characterize clay minerals, as follows. A small amount of the red phyllite was mixed in a slurry with distilled water on a quartz “zero background” plate, was allowed to air dry, and was then scanned on the Bruker D8 from 2 to 100° 2Θ at 8 sec/step. The mounted sample was then placed in a chamber saturated with ethylene glycol (EG) for ~24 hours before being scanned again, immediately upon removal from the EG chamber, from 2 to 40° 2Θ at 2 sec/step.

Upon EG solvation, the faint, broad peak at ~6° 2Θ, corresponding to a d-spacing (d) of ~1.4 nm, disappears (fig. 9), indicating trace amounts of chlorite/smectite. Furthermore, the broad peak at ~9° 2Θ shifts to slightly higher angles (corresponding to a d-spacing shift from 1.005 nm to 0.990 nm), the peak at ~18° 2Θ shifts to slightly lower angles (corresponding to a d-spacing shift from 0.495 nm to 0.505 nm), and the peak at ~27° 2Θ (d=0.33 nm) increases in intensity. This is consistent with trace amounts of illite/smectite. Interference from the illite, (modeled as a one-layer monoclinic [1M] white mica) and muscovite (modeled as a two-layer monoclinic [2M1] white mica) peaks prevent precise estimation of the fraction of smectitic layers in the illite/smectite without further investigation, but the slightness of the EG-solvation effects suggests a low (less than [<]10 percent) abundance. Notably, evidence of feldspars is absent in all XRD scans of phyllite from 12,336 ft depth, corroborating optical evidence (figs. 4 and 5).

Barnett (1975, p. 138) reports 35 ft of basement recovery from 14,480 to 14,515 ft depth, adding that the drill cuttings at 14,510–14,515 ft depth are a “coarse red granite wash and weathered biotite granite pegmatite.”

Summary: Angular to subrounded granitic fragments occur in cuttings as shallow as 14,450–14,460 ft depth and, although they are more abundant with depth, do not compose a majority of cuttings in any interval. Porphyritic rhyolite fragments are also present and compose a similar proportion of the granitic fragments in each interval.

Thin sections were prepared of some of the coarser cuttings from borehole W12309 at 14,490–14,500 ft depth, including representative granitic and porphyritic fragments.

Granitic fragments are comprised of subequal amounts of quartz, plagioclase, and K-feldspar, with accessory chloritized biotite, magnetite, and apatite (fig. 10). Quartz grains have uniform extinction and are mostly free of inclusions. The feldspars are pervasively altered to fine-grained sericite, chlorite, and hematite, particularly along cleavages. Plagioclase grains have deformation twins. Carlsbad twins are common among K-feldspar grains. Graphic intergrowths of K-feldspar and quartz are present in some fragments (fig. 11).

Porphyritic grains have plagioclase and quartz phenocrysts in very fine grained quartzofeldspathic matrices (fig. 12). Dynamic layering at the scale of several micrometers is evident in many fragments and is interpreted as magmatic flow banding. Plagioclase phenocrysts are many times larger than the quartz grains and may exceed the size of the cuttings fragments; therefore, the maximum phenocryst size is unknown. Many plagioclase phenocrysts are euhedral, but broken grains are also present, and zoning is apparent in one grain (fig. 13). The alteration of plagioclase phenocrysts is similar to the alteration in the granitic fragments, with chlorite, sericite, and oxides growing along cleavage planes.

Granitic fragments were handpicked under a binocular microscope to isolate them from the sedimentary and porphyritic fragments and were separated into ~0.7-gram (g) subsamples. Each subsample was ground by hand in an agate mortar and pestle while being continuously lubricated with acetone. The resulting fine powder was allowed to air dry. Aliquots of each subsample were placed in the 2-cm-wide cavities of circular “back-pack” mounts and scanned in a Panalytical X’Pert diffractometer with a Cu anode from an angular range of 3–90° 2Θ. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in HighScore Plus (Malvern Panalytical, 2018). Geochemical analyses were thereafter performed on the same subsamples (Deasy and others, 2024a).

Barnett (1975, p. 133) reports 55 ft of granitic basement rock penetration from 12,258 to 12,313 ft depth and includes the following description of a thin section of cuttings from 12,290 to 12,313 ft depth:

A polished thin section was prepared from granitic cuttings from 12,290 to 12,313 ft depth. This granite is coarse grained, with quartz and feldspar grains up to ~1 millimeter (mm) in maximum dimension (fig. 14). Quartz grains are equant, free of inclusions, and display undulose or patchwork extinction in XPL (figs. 14–16). Feldspars are subhedral to euhedral with aspect ratios of ~2:1 and are pervasively altered with fine-grained chlorite, epidote, or sericite, imbuing the grains with a high turbidity. Nevertheless, albite and microcline twinning remain visible in some grains. Some feldspars are broken (fig. 15). Biotite grains are pseudomorphically replaced by chlorite with inclusions of ilmenite (figs. 14 and 16). Fine-grained epidote fills sinuous fractures in some fragments (fig. 16).

Fragments of granite were separated from contaminants by hand under a binocular microscope. The separated fragments were then placed in an automated Brinkman grinder fitted with an agate mortar and pestle. The sample was continuously lubricated with acetone during grinding. The resulting coarse powder was further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. An aliquot of powder was then mounted in the 2-cm-wide cavity of a standard “back-pack” mount and analyzed on a Panalytical Empyrean diffractometer with a Cu anode from an angular range of 3–100° 2Θ. Semiquantitative mineral abundances in this sample (Deasy and others, 2024b) were determined by Rietveld modeling in TOPAS (Bruker AXS, 2011).

This borehole is first mentioned in table 1 of Barnett (1975, p. 136) as “Granodiorite, late Precambrian or early Cambrian?” with no further description or data. A feldspar concentrate from 12,950 to 12,990 ft depth analyzed by the K-Ar method returned an age of 709±25 million years ago (Ma) (Krueger Enterprises, 1981, as first cited in Lloyd [1985, p. 55]); Lloyd (1985) also includes the following note for the age reference: “Unpublished age determination (memo on file at Fla. Bureau of Geology); material submitted by H. Kelley Brooks to Krueger Enterprises, Inc., Geochron Laboratories Division.” A Neoproterozoic crystallization age is confirmed by a U-Pb zircon age of 656±38 Ma (Deasy and others, 2023) and a muscovite ⁴⁰Ar/³⁹Ar plateau age of 653.8±3.4 Ma (Deasy and McAleer, 2022).

This is the type borehole and interval (12,880–13,284 ft depth) of the Gaskin intrusive complex of Winston (1992), a name that has been used by some authors to include all granitoids encountered in the Florida panhandle. However, more recent geochronological and thermochronological work (Heatherington and others, 2010; Deasy and McAleer, 2022; Deasy and others, 2023) demonstrates that some granitoids have Permian crystallization ages and that only the undated rocks in boreholes W12309, W12497, and W14644 may potentially be related to the granite in borehole W12509.

Summary: Red siltstone and mudstone overlie granitic basement rock and the contact occurs at ~12,880 ft depth. The nonconformable nature of the contact is indicated by the presence of arkosic fragments in the basal sediment, a strong oxidation profile in the uppermost granite, and the absence of any fault-related structures. The granite is coarse grained (~4 mm in maximum dimension), pink to red in color, and contains subequal amounts of quartz, plagioclase, and K-feldspar, with accessory muscovite, magnetite, and apatite. The abundances of magnetite and apatite increase with depth. Magmatic muscovite is a minor accessory throughout but is coarser grained and more abundant in a narrow interval from 13,080 to 13,090 ft depth.

Thin sections were prepared of cuttings from the following depth intervals:

The basement lithology here is a massive, coarse-grained, pink to red-orange granite comprised of subequal amounts of quartz, plagioclase, and K-feldspar with minor muscovite, apatite, and opaque minerals, as well as rare zircon. Plagioclase grains are commonly euhedral with polysynthetic and deformation twins and contain ubiquitous fine-grained inclusions of sericite and hematite (figs. 18–21), which impart the red color to the rock fragments. K-feldspar grains are typically anhedral with crosshatch twinning. Compared to plagioclase, K-feldspar grains are relatively unaltered except for muscovite fillings along cleavages (figs. 19–21), although K-feldspar and plagioclase are similarly altered in the topmost sampled interval (fig. 18). A patchy red oxide staining—a deeper, darker red than the pink-orange hue imbued by the alteration of plagioclase—is common along grain boundaries in fragments from the upper intervals (12,960–12,980 ft depth; for example, fig. 19) but is not present in the lower sample (13,080–13,090 ft depth; for example, fig. 21). Graphic intergrowths of quartz and K-feldspar are rare but present in some fragments. Quartz grains are equant, anhedral to euhedral (fig. 18), and commonly display undulose or patchwork extinction. Muscovite occurs as euhedral to subhedral flakes up to 1 mm across and as anhedral inclusions within plagioclase (figs. 19 and 21). These muscovite populations are strongly associated with Ti oxides (for example, on the right side of fig. 19). Muscovite also occurs as fillings along cleavages of K-feldspar grains (figs. 18, 19, 21). Calcite occurs in rare veins 50–100 micrometers (µm) thick and in micropores within feldspars. Opaque grains commonly retain euhedral outlines (fig. 20), but the primary phases are exsolved to mottled symplectic intergrowths of iron (Fe) and Ti oxides. Apatite and zircon grains are commonly euhedral and are strongly associated with opaque grains, within which they commonly occur as inclusions.

Granitic fragments were separated from sedimentary fragments and other contaminating cuttings by hand under a binocular microscope. Five subsamples of granitic cuttings weighing 0.6–0.8 g each were separated from the following depth intervals:

There was sufficient granitic material among the cuttings in each of these intervals to separate and analyze two subsamples from each of the upper two intervals. These subsamples provided an opportunity to test both the effectiveness of the hand-picking method and the representativeness of the sample fractions by revealing compositional variations between small fractions of ostensibly identical coarse-grained rock. The five subsamples were each ground by hand in an agate mortar and pestle while being continuously lubricated with acetone during grinding. The mortar and pestle were thoroughly cleaned between subsamples. The resulting fine powders were allowed to air dry. The powders were placed in 2-cm-wide cavities of circular “back-pack” mounts and scanned in a Panalytical X’Pert diffractometer with a Cu anode from an angular range of 3–90° 2Θ. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in HighScore Plus (Malvern Panalytical, 2018).

Additionally, a sample of K-feldspar concentrate (prepared for ⁴⁰Ar/³⁹Ar analysis) from 12,980 to 12,990 ft depth was placed in an automated Brinkman grinder fitted with an agate mortar and pestle. The sample was continuously lubricated with acetone during grinding. The resulting coarse powder was further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. The powder was then mounted on a silicon “zero background” smear mount and analyzed on a Bruker D8 diffractometer with a Cu anode and point detector from an angular range of 2–100° 2Θ at a rate of 8 sec/0.02° step. Following this analysis, the still-mounted sample was placed in a chamber with an EG-saturated atmosphere for 24 hours before being removed and immediately reanalyzed on the Bruker instrument from an angular range of 2–30° 2Θ at a rate of 2 sec/0.02° step. No change in the diffraction pattern resulted from EG solvation (fig. 22); therefore, the sample is interpreted not to contain expandable clay minerals. Semiquantitative mineral abundances in each subsample (Deasy and others, 2024b) were determined by Rietveld modeling in TOPAS (Bruker AXS, 2011). Major and trace element geochemical analyses of all powders were thereafter obtained by instrumental neutron activation analysis (INAA)/inductively coupled plasma optical emission spectrometry (ICP-OES)/inductively coupled plasma mass spectrometry (ICP-MS) by Activation Laboratories, Inc. (Deasy and others, 2024a).

Barnett (1975, p. 134) reports 360 ft of basement rock penetration from 12,040 to 12,400 ft depth and describes a thin section of cuttings from 12,390 to 12,400 ft depth (TD) as follows:

Summary: Two basement lithologies are penetrated in this borehole: a gabbro and a granodiorite. Red micaceous siltstone and gray micaceous marl overlie gabbroic rock at ~12,120 ft depth. Rounded pebbles of the gabbro are present in the basal sediment, indicating an erosional nonconformable contact. Granodiorite occurs at 12,280–12,400 ft depth. Beyond this basic structural juxtaposition, any relationship between the granodiorite and gabbro is not obvious from the cuttings. Some cuttings of both the gabbro and granodiorite are deeply weathered. Whereas the fresh (partially chloritized but not red from oxide staining) gabbroic fragments are green to gray, the weathered gabbroic fragments are a deep rusty red; sharp contacts between red and gray-green domains are present in rare fragments. Fresh granodioritic fragments are pink and the weathered granodioritic fragments are chalky and white. No contacts between weathered white and fresh pink domains were seen among the cuttings.

Granodioritic and gabbroic fragments were separated by hand from sedimentary fragments and other contaminants under a binocular microscope. A ~10-g composite sample of granodioritic cuttings from 12,350 to 12,390 ft depth was isolated, and additional subsamples of granodioritic cuttings (each weighing 0.9–1.1 g) were separated from the following intervals (the number of subsamples per interval is given in parentheses):

The redundant samples from each depth interval facilitate a test of both the effectiveness of the hand-picking method and the representativeness of the sample fractions by revealing compositional variations between small fractions of ostensibly identical coarse-grained rock.

Separate subsamples of altered and fresh gabbroic fragments (each weighing 0.6–0.7 g) were separated from the following depth intervals:

The larger composite sample was ground in an automated Brinkmann grinder fitted with an agate mortar and pestle. It was continuously lubricated with acetone during grinding. The resulting powder was further ground by hand in a corundum mortar and pestle, again lubricated with acetone. An aliquot of sample was placed in the 2×3×0.1-cm cavity of a Ti “front-pack” mount and analyzed in a Bruker D8 diffractometer with a Cu anode and point detector from 2 to 100° 2Θ at a step size of 0.02° and scanning rate of 9 sec/step. Semiquantitative mineral abundances (Deasy and others, 2024b) were determined by Rietveld refinement modeling in TOPAS (Bruker AXS, 2011). The sample was subsequently analyzed for its major and trace element geochemical composition by Activation Laboratories, Inc. (Deasy and others, 2024a).

All subsamples were ground by hand in an agate mortar and pestle and were continuously lubricated with acetone during grinding. The mortar and pestle were thoroughly cleaned between subsamples. The resulting powders were allowed to air dry. The powders were placed in 1.5-cm-wide cavities of circular “back-pack” mounts and scanned in a Panalytical Empyrean diffractometer with a Cu anode and X’Celerator detector from an angular range of 3–90° 2Θ. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling inHighScore Plus (Malvern Panalytical, 2018). Major and trace element geochemical analyses of four granodioritic subsamples, two altered gabbroic subsample, and two fresh gabbroic subsamples were thereafter obtained by INAA/ICP-OES/ICP-MS by Activation Laboratories, Inc. (Deasy and others, 2024a).

Additionally, to test the variable abundances of alteration minerals within each lithology, we analyzed small amounts of altered and fresh gabbroic and granitic fragments by XRD. Between one and four cuttings fragments were separately ground by hand using the methods described above. Each powder was mounted in a slurry with distilled water on a quartz “zero background” plate, was allowed to air dry, and was then scanned on a Bruker D8 diffractometer from 2 to 90° 2Θ at a minimum of 8 sec/step. The mounted sample was then placed in a chamber saturated with EG for ~24 hours before being scanned again, immediately upon removal from the EG chamber, from 2 to 40° 2Θ at 4 sec/step. Representative diffractograms are given in figures 41–44.

The XRD patterns of fresh gabbro and of some fragments of red, altered gabbro are unaffected by EG solvation (fig. 41), indicating the absence of expandable clays in these samples. In contrast, some fragments of red, weathered gabbro have patterns that are strongly affected by EG solvation (fig. 42). Specifically, under air-dried conditions, these patterns have high low-angle (2–3° 2Θ) intensity and a ~6° 2Θ (d=1.42 nm) peak with a broad, high-angle shoulder. Upon EG solvation, the 1.42 nm peak loses peak intensity and becomes narrower, the intensity from 2 to 3° 2Θ is diminished, and a broad peak centered at ~5° 2Θ (d=1.62 nm) appears (fig. 42). This behavior is consistent with the presence of chlorite/smectite. Although unquantified, the expandable clay accounts for a significant fraction of the 28.8 weight percent (wt. pct.) chlorite estimated by Rietveld modeling (Deasy and others, 2024b). Other alteration minerals include illite, talc, prehnite, actinolite, serpentine and, in the oxidized samples, hematite.

The XRD patterns of the fresh granodiorite are very weakly affected by EG solvation (figs. 43 and 44), with weak, broad peaks appearing at ~6 and ~12° 2Θ, demonstrating the presence of trace amounts of chlorite/smectite. Other alteration minerals identified by XRD include chlorite, muscovite, prehnite, actinolite, and laumontite. The chalky, altered fragments of granodiorite respond weakly to EG solvation, indicating the presence of small amounts of chlorite/smectite (fig. 43), and contain significantly more illitic clay than the fresh rock. Semi-quantitative mineral abundances from Rietveld modeling of the eight aforementioned splits of fresh granodiorite and of one sample of altered granodiorite are available in Deasy and others (2024b). Several minerals that were identified optically or in electron microscopy did not appear in whole-rock powder XRD analyses and were therefore not included in Rietveld modeling. These minerals include, in decreasing order of abundance, ilmenite, apatite, zirconolite(?), baddeleyite, zircon, and thorite.

Barnett (1975) reports that the Eagle Mills Formation is present at 11,753–12,131 ft depth (TD), with diabase sills at 12,060–12,070 ft depth and 12,095–12,131 ft depth.

A description of thin sections of diabase from borehole W12496 at 12,090–12,124 ft depth follows. Rock is diabasic with subhedral to euhedral plagioclase laths enclosed in clinopyroxene oikocrysts (fig. 45). Plagioclase grains have penetration twins and aspect ratios of 2:1 to ~5:1, although the upper limit is unknown due to the small size of the cuttings (~2 mm). Grain size of clinopyroxene is unknown but is larger than cuttings fragments. Some fragments contain subhedral to euhedral orthopyroxene(?), completely pseudomorphically replaced by chlorite+talc+actinolite. Plagioclase and clinopyroxene are mostly unaltered, and a massive magmatic texture is preserved in all fragments. Evidence of deformation is limited to intracrystalline veins of prehnite, chlorite, talc, and actinolite.

Fragments of diabase were separated from drill cuttings by hand under a binocular microscope. The diabase separate was then placed in an automated Brinkman grinder fitted with an agate mortar and pestle. The sample was continuously lubricated with acetone during grinding. The resulting powder was further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. An aliquot of powder was then mounted in the 2-cm-wide cavity of a standard “back-pack” mount and analyzed on a Panalytical Empyrean diffractometer with a Cu anode from an angular range of 3–100° 2Θ. Semiquantitative mineral abundances in this sample (Deasy and others, 2024b) were determined by Rietveld modeling in TOPAS (Bruker AXS, 2011). The major and trace element composition of the powder was thereafter obtained by INAA/ICP-OES/ICP-MS at Activation Laboratories, Inc. (Deasy and others, 2024a).

Barnett (1975, p. 134) reports 36 ft of penetration of felsic porphyry from 14,261 to 14,297 ft depth and includes the following description of thin sections of core:

Thin sections of porphyritic rhyolite were prepared from core chips recovered from the following depths:

All samples are quartz and K-feldspar-phyric, welded crystal tuffs with ~50–70 percent cryptocrystalline matrix and variable minor xenolithic content. Whereas the shallowest sample is massive (fig. 46), a foliation defined by the shape-preferred orientations of phenocrysts and lithic clasts becomes stronger with depth (figs. 47–49). Evidence of compaction can be found in fiamme (fig. 48), the slight embayment of fiamme by adjacent quartz grains, and the in situ brecciation of quartz phenocrysts (fig. 48).

K-feldspar phenocrysts are blocky to elongate and up to 0.4 cm in maximum dimension. All feldspar phenocrysts are faintly reddish, have a high turbidity (compared to quartz phenocrysts), and are partially altered to sericite and, locally, to calcite (fig. 50). A patchy, fine-scale (tens of micrometers), weakly developed perthitic texture is pervasive among K-feldspar phenocrysts (figs. 50–52). Many of the more elongate feldspar grains have Carlsbad twins (figs. 51 and 52). Quartz grains are typically either euhedral phenocrysts or angular fragments. Rare round quartz grains (fig. 49) may be sedimentary xenocrysts. Other xenoliths include angular clasts of felsic porphyry up to 0.5 cm across (fig. 46, lower left), quartzitic sandstone (fig. 49, center right), and opaque grains. Some opaque grains, such as those with dendritic edges (fig. 47, center right), may be authigenic or have authigenic overgrowths. The volcanic clasts have finer grained matrices than the host rock, and many have strong foliations suggestive of flow banding (fig. 46, lower left; fig. 49, center right; fig. 52, right) which are absent in the host rock.

Four chips from three depth intervals (one each from 14,270 and 14,271 ft depth, and two from 14,288 ft depth) were separated from core with a diamond bandsaw and shattered in a steel percussion mortar. The broken pieces were placed in an automated Brinkman grinder fitted with an agate mortar and pestle. The samples were continuously lubricated with acetone during grinding. The resulting coarse powders were further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and were allowed to air dry. An aliquot of each powder was then mounted in the 2.0×3.0×0.1-cm cavities of Ti “front-pack” mounts and analyzed on a Bruker D8 diffractometer with a Cu anode and point detector from an angular range of 2–70° 2Θ, 2–90° 2Θ, or 2–100° 2Θ at a step size of 0.02° 2Θ and step rate of 4, 5, or 6 sec/step. Semiquantitative mineral abundances in this sample (Deasy and others, 2024b) were determined by Rietveld modeling in TOPAS (Bruker AXS, 2011). The presence of elongate phenocrysts with Carlsbad twins and the pervasiveness of weak perthite development suggests that K-feldspar in these samples may be incompletely ordered and that monoclinic and triclinic domains may be extant among the grains. Therefore, K-feldspar in these samples was modeled in Rietveld refinements with a combination of microcline, orthoclase, and sanidine structures.

Duncan (1998) contributes a comprehensive description of cuttings from 4,610 to 10,078 ft depth (TD) and defines major units and depths to important boundaries in Suwannee basin strata, including depth to volcanic rock at 9,960 ft depth.

Summary: Undifferentiated Mesozoic red sandstone overlies Paleozoic Suwannee basin rocks at 4,620 ft depth. Suwannee basin units, all informally named by Duncan (1998), include the Cherry Lake formation (4,620–6,030 ft depth), a transitional stratum (6,030–6,460 ft depth), the Cooks Hammock formation (6,460–9,417 ft depth), and the basal Pumpkin Swamp formation (9,417–9,960 ft depth). Mafic to intermediate porphyritic volcanic rock was encountered from 9,960 to 10,078 ft depth (TD).

Individual cuttings fragments from 4,640 to 4,650 ft were independently powdered by hand in a corundum mortar and pestle. Each sample was continuously lubricated with acetone during grinding. The grinding equipment was cleaned thoroughly between samples. Resulting powders were mounted on quartz “zero background” plates in a slurry with deionized water and analyzed on a Bruker D8 diffractometer from an angular range of 2–70° 2Θ at a rate of 15 sec/0.02° step. Then, the mounted samples were placed in a chamber with an atmosphere saturated in EG for 24 hours. Immediately upon removal from the chamber, the samples were analyzed again from 2 to 40° 2Θ at 4 sec/step. Representative diffractograms under air-dried and EG-solvated conditions for a single cuttings fragment from 4,640 to 4,650 ft are shown in figure 63.

Additionally, volcanic fragments were handpicked under a binocular microscope and separated into subsamples for mineralogical and geochemical analyses. A ~5-g subsample from 10,010 to 10,050 ft was separated for X-ray fluorescence (XRF)/ICP-MS and, to test for variability and control for potential contamination, three ~0.6-g subsamples of pale green and dark gray volcanic rock were separated from 10,020 to 10,040 ft; from 10,040 to 10,050 ft (green); and from 10,010 to 10,050 ft (gray). The larger subsample was ground in an automated Brinkman grinder fitted with an agate mortar and pestle while being continuously lubricated with acetone. This subsample was then ground further by hand in a corundum mortar and pestle. An aliquot of the subsample was placed in the 2×3×0.1-cm cavity of a Ti “front-pack” mount and analyzed on a Bruker D8 diffractometer from 2 to 70° 2Θ at a rate of 8 sec/0.02° step. Semiquantitative mineral abundances (Deasy and others, 2024b) were determined by Rietveld modeling in TOPAS (Bruker AXS, 2011), and the geochemistry of the subsample was determined by XRF/ICP-MS (Deasy and others, 2024a). The three smaller subsamples were each ground by hand in an agate mortar and pestle while being continuously lubricated with acetone and were allowed to air dry. They were mounted in the 2-cm-wide round cavities of standard “back-pack” mounts and analyzed on a Panalytical X’Pert diffractometer from 3 to 90° 2Θ. Mineral abundances were determined with HighScore Plus (Malvern Panalytical, 2018) and the Rietveld method (Deasy and others, 2024b). Alteration minerals identified by XRD include disseminated calcite, chlorite, prehnite, actinolite, as well as minor illite and talc. Major and trace element geochemical analyses were thereafter obtained by INAA/ICP-OES (Deasy and others, 2024a).

Duncan (1998) defines major units and important boundaries in Suwannee basin strata, identifies a nonconformable contact with volcanic basement rock at 7,660 ft depth, and contributes a description of cuttings from 4,840 to 9,073 ft depth (TD). A whole-rock K-Ar age of 552±21 Ma is reported for volcanic rock from 9,073 ft depth (Amoco Corporation unpublished data, as cited in Duncan [1998]).

Summary: Undifferentiated Mesozoic(?) red beds overlie Paleozoic Suwannee basin sedimentary rocks at 4,940 ft depth. Suwannee basin units, per Duncan (1998), include the Cambrian–Ordovician Cooks Hammock formation (4,940–7,510 ft depth) and the Cambrian–Ordovician Pumpkin Swamp formation (7,510–7,660 ft depth). Below this, nearly 1,400 ft of volcanic rock of the North Florida volcanic series were penetrated before a total depth of 9,073 ft was reached. The volcanic rock is interbedded with siliceous, fossiliferous carbonate rock from 8,080 to 8,240 ft (Duncan, 1998; not seen in this study). The volcanic rock contains green to dark aphanitic rock, weakly plagioclase-phyric mafic to intermediate porphyry, and welded lapilli tuff.

Fragments of volcanic rock were separated by hand from sedimentary fragments and other contaminants by hand under a binocular microscope. A sample of volcanic rock from 9,010 to 9,040 feet depth was obtained by combining subsamples from the interceding intervals. The sample was ground in an automated Brinkmann grinder fitted with an agate mortar and pestle. It was continuously lubricated with acetone during grinding. The resulting powder was further ground by hand in a corundum mortar and pestle, again while lubricated with acetone. The final powder was allowed to air dry before it was placed in the 1.5-cm-wide cavity of a circular “back-pack” mount and scanned in a Panalytical Empyrean diffractometer with a Cu anode and X’Celerator detector from an angular range of 3–90° 2Θ at 0.02° steps and a scan rate of 50 sec/step. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in HighScore Plus (Malvern Panalytical, 2018). A major and trace element geochemical analysis was thereafter obtained by INAA/ICP-OES/ICP-MS by Activation Laboratories, Inc. (Deasy and others, 2024a).

To evaluate the sample for its expandable clay content, an aliquot of powder was mounted in a slurry with distilled water on a quartz “zero background” plate, was allowed to air dry, and was then scanned on the Bruker D8 diffractometer from 2 to 90° 2Θ at 8 sec/step. The mounted sample was then placed in a chamber saturated with EG for ~24 hours before being scanned again, immediately upon removal from the EG chamber, from 2 to 40° 2Θ at 4 sec/step. There was no change in the diffraction pattern of the sample upon EG solvation (fig. 68), demonstrating the absence of expandable clays in this rock.

Applin (1951, p. 21) reports 19 ft of basement rock penetration of “volcanic ash and tuff” from 3,873 to 3,892 ft depth (TD). A whole-rock major element geochemical analysis of core from 3,881 ft depth returns a rhyolitic composition (Milton and Grasty, 1969). Bass (1969) presents a petrographic report of cuttings from 3,766 to 3,892 ft depth, wherein the identification of minerals and mineral compositions is supported by XRD analysis. Milton (1972) summarizes Bass (1969) and adds several photomicrographs. Two samples from 3,889 to 3,890 ft depth and from 3,890 to 3,892 ft depth are also rhyolitic (Mueller and Porch, 1983). Heatherington and others (1996) report additional whole-rock geochemical and isotopic data of samples from 3,885 to 3,892 ft depth.

A thin section of a core chip from 3,883 to 3,885 ft depth shows tan-gray porphyritic rhyolite tuff (fig. 69). The tuff contains dispersed subhedral to euhedral quartz and plagioclase phenocrysts up to 100 µm long and round ash lapilli up to 1 mm in diameter in a massive, fine-grained matrix. This texture is overprinted by red-orange Liesegang banding and dendritic Mn(?)-oxide stylolites(?).

Core chips of rhyolitic rock from 3,879 to 3,883 ft depth were prepared for XRD and geochemical analysis as follows. Contaminants were first removed by hand with tweezers under a binocular microscope. The chips were then placed in an automated Brinkman grinder fitted with an agate mortar and pestle. The sample was continuously lubricated with acetone during grinding. The sample was then further ground by hand in a corundom mortar and pestle, again lubricated with acetone, and allowed to air dry. An aliquot of the resulting fine powder was placed in the 2×2×0.1-cm cavity of a Ti “front-pack” mount and analyzed on a Bruker D8 diffractometer from 2 to 90° 2Θ at a rate of 5 sec/0.02° step. Mineral abundances (Deasy and others, 2024b) were then determined by the Rietveld method using TOPAS (Bruker AXS, 2011). Major and trace element concentrations in the same powder were thereafter determined by XRF/ICP-MS analysis (Deasy and others, 2024a).

Barnett (1975) mentions penetration of volcanic rock from 4,426 to 5,572 ft depth. In the ~1,150 ft of basement rock penetration, Winston (1992) distinguishes 630 ft of an upper rhyolitic lithology, including shaley beds at 4,500 and 4,570 ft depth, from an underlying 490 ft of andesite.

Summary: The contact between Cretaceous(?) limestone and basement rhyolite occurs between 4,420 and 4,440 ft depth. The first appearance of intermediate (andesitic) volcanic fragments occurs at 4,700 ft depth, and the first appearance of intermediate volcanics dominate cuttings occurs from 4,800 to 5,572 ft depth (TD). The contact between the volcanic rocks may be interleaved or reworked. Of the two basement lithologies, only the lower, andesitic unit was sampled in this study.

Petrographic evidence shows complex textures, mixed tephra, and detrital quartz grains at the millimeter scale (that is, within individual cuttings fragments) such that each fragment potentially contains multiple unrelated magmatic and sedimentary provenances. Moreover, cuttings from any interval are susceptible to contamination from overlying rocks and anthropogenic sources. Therefore, no combination of cuttings fragments can be said unequivocally to belong to a single stratum, let alone a single eruption. These factors complicate efforts to obtain a representative sampling of the volcanic rock(s) present in this borehole. Therefore, redundant fractions of basement rock from multiple depths were separated for geochemical and mineralogical analyses to test for compositional variability and control for possible contamination.

Volcanic fragments were separated from sedimentary fragments and other contaminating cuttings by hand under a binocular microscope. Subsamples of volcanic cuttings weighing 0.6–1.0 g each were separated from each of the following depth intervals:

Each subsample was ground by hand in an agate mortar and pestle while being continuously lubricated with acetone during grinding. The mortar and pestle were thoroughly cleaned between subsamples. The resulting fine powders were allowed to air dry. The powders were placed in the 2-cm-wide cavities of circular “back-pack” mounts and scanned in a Panalytical X’Pert diffractometer with a Cu anode from an angular range of 3–90° 2Θ. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in HighScore Plus (Malvern Panalytical, 2018). Major and trace element geochemical analyses of six of these powders were obtained by INAA/ICP-OES/ICP-MS by Activation Laboratories, Inc. (Deasy and others, 2024a).

Additionally, larger, ~5-g fractions of volcanic rock for XRF/ICP-MS analyses were attained from the following intervals by combining hand-picked cuttings from the interceding 10-ft intervals:

These fractions were placed in an automated Brinkman grinder fitted with an agate mortar and pestle. The samples were continuously lubricated with acetone during grinding. The resulting coarse powders were further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. Aliquots of powder were mounted in Ti “front-pack” mounts and analyzed on a Bruker D8 diffractometer from an angular range of 2–100° 2Θ with a Cu anode and point detector. Semiquantitative mineral abundances (Deasy and others, 2024b) were determined by Rietveld modeling in TOPAS (Bruker AXS, 2011). The major and trace element geochemistry of these powders was then determined by XRF/ICP-MS by Activation Laboratories, Inc. (Deasy and others, 2024a).

Applin (1951, p. 21) reports 44 ft of penetration of “tuff and volcanic agglomerate of rhyolitic composition” beginning at 4,588 ft depth and quotes F.F. Grout’s remarks, “Mixed tuff derived from an igneous complex. Origin, sedimentary or explosive igneous action.” Milton (1972) contributes downhole core descriptions from 4,560 to 4,575 ft depth of white to buff to red, friable to indurated sandstone and from 4,601 to 4,632 ft depth of metamorphosed volcanic agglomerate containing rhyolitic and basaltic clasts, as well as detrital quartz and devitrified glass. Alteration products include epidote, chlorite, and amphibole; veins are filled with zeolite(s) and calcite. Heatherington and others (1996) provide whole-rock geochemical analyses and isotopic data for “rhyolitic breccia” (n=1) and “epidotized mafic volcanic” (n=3) from 4,618 to 4,624 and from 4,624 to 4,639 ft depth, respectively.

Recovered materials are core chips and cuttings, where indicated.

Summary: Loosely consolidated, mature quartz arenite overlies bimodal volcanic basement rock. Basement rock recovery includes an upper rhyolitic unit and lower crystal/lapilli tuff of mafic to intermediate composition.

Volcanic rock from 4,639 to 4,640 ft depth is a massive, green to black, intermediate to mafic welded tuff with angular to well-rounded tephra 0.1–1.5 mm in diameter (fig. 85). Clastic components include amphibole grains, epidote grains, opaque mineral grains, polycrystalline quartz clasts, subhedral to euhedral epidotized plagioclase crystals, micrographic K-feldspar grains, and porphyritic tephra (figs. 86 and 87). The porphyritic bits are plagioclase-phyric in a dark, massive, fine-grained matrix. The sample contains several submillimeter quartz veins and one chlorite vein with a maximum aperture of ~0.5 mm.

A large core chip from 4,639 to 4,640 ft depth was shattered to pea-sized and smaller fragments in a steel percussion mortar. These were ground by hand with an agate mortar and pestle. The sample was continuously lubricated with acetone during grinding. The resulting fine powder was allowed to air dry. An aliquot of the powder was placed in the 2-cm-wide cavity of a circular “back-pack” mount and scanned in a Panalytical X’Pert diffractometer with a Cu anode from an angular range of 3–90° 2Θ. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in HighScore Plus (Malvern Panalytical, 2018). Major and trace element geochemical analysis powder was thereafter obtained by INAA/ICP-OES/ICP-MS by Activation Laboratories, Inc. (Deasy and others, 2024a).

Applin (1951, p. 21) notes 22 ft of “volcanic agglomerate or tuff of rhyolitic composition” beginning at a depth of 4,615 ft. Carroll (1963) contributes a petrographic report on the overlying Paleozoic sandstone (coarse-grained quartzite and feldspathic quartzite), including some photomicrographs. Bass (1969) presents a petrographic analysis of core and cuttings and notes the presence of small amount of volcanic material in the basal conglomeritic sandstone. Milton (1972) summarized work to date and notes that available samples seem incompatible, suggesting some may be mislabeled. Milton also notes that nearby wells of similar depth penetrated Ordovician(?) sandstones but not volcanic rock. Mueller and Porch (1983) present two major element whole-rock geochemical analyses for dacite at 4,618–4,623.5 ft depth. Heatherington and others (1996) present a U-Pb SHRIMP age for zircon of 552±8 Ma from this sample, as well as two trace element whole-rock geochemical analyses on powders also analyzed by Mueller and Porch (1983).

A thin section of a core chip from 4,618 to 4,624 ft shows an altered felsic porphyry (fig. 88) in which the gray matrix is thoroughly sericitized. A moderately well-developed preferred orientation of sericite grains is approximately parallel to primary layering. Subhedral to euhedral feldspar phenocrysts are also completely sericitized (fig. 89). Angular to subrounded quartz grains have uniform or sweeping extinction, suggesting multiple sources. Opaque-rich lithic clasts up to ~0.5 cm across are feldspar-phyric tephra (figs. 90 and 91). Several thin quartz veins cut across the sample.

A single large (~2×~5 cm) core chip recovered from depth interval 4,618–4,623.5 ft was first broken into smaller pieces. The largest remaining piece was retained for thin section preparation, and the smaller pieces were collected and ground to a powder in an automated Brinkman grinder fitted with an agate mortar and pestle. The sample was continuously lubricated with acetone during grinding. The resulting powder was further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. An aliquot of the powder was placed in the 3×2×0.1-cm cavity of a Ti “front-pack” mount and scanned in a Bruker D8 diffractometer from an angular range of 2–100° 2Θ with a Cu anode and point detector at 0.02° steps and a rate of 13 sec/step. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in TOPAS (Bruker AXS, 2011).

The strongly asymmetrical shape of the 8.8° 2Θ (d=1.0 nm) peak in diffraction results (fig. 92) prompted re-analysis to characterize clay minerals, as follows. A small amount of the sample was mixed in a slurry with distilled water on a quartz “zero background” plate, was allowed to air dry, and was then scanned on the Bruker D8 from 2 to 56° 2Θ at 5 sec/step. The mounted sample was then placed in a chamber saturated with EG for ~24 hours before being scanned again, immediately upon removal from the EG chamber, from 2 to 40° 2Θ at 2 sec/step.

The asymmetrical ~9° 2Θ (d=~1.0 nm) peak becomes more symmetrical upon EG solvation by both a decrease in lower angle intensity and an increase in higher angle intensity, and a small peak at ~17° 2Θ (d=~0.515 nm) grows slightly more intense (fig. 92). These effects are attributed to the presence of a small amount of illite/smectite. The position of the Ilt-Sme₀₀₂ reflection at ~17° 2Θ indicates that the smectitic component composes 10–20 percent of the illite/smectite (see Moore and Reynolds 1997, table 8.3), although strong interference from other phyllosilicate peaks limits the precision of this estimate.

Applin (1951, p. 9) first documented this basement rock as “Rhyolitic(?) volcanic rock.” Milton (1972) includes a brief description of cuttings at 5,424.5 ft depth (TD) and an ordinary light thin-section photomicrograph.

Summary: The contact between rhyolitic basement rock and sandstone occurs between 5,400 and 5,410 ft depth. The basal sedimentary unit is nearly pure quartz arenite with a minor regolith component. Rhyolite is porphyritic with conspicuous cleavage faces up to 1 mm of plagioclase feldspar, deeply colored in reds and purples. Rhyolite fragments are weakly magnetic and lack conspicuous veining.

A polished thin section of rhyolitic cuttings fragments 1–6 mm in diameter from 5,410 to 5,420 ft depth was prepared (fig. 94). Clasts are red-orange in ordinary light and PPL and are optically dense with a fine-grained, finely disseminated red oxide alteration overprint (figs. 94 and 95). Although pervasive, the red oxide is concentrated in the K-feldspar domains, as is a moderate sericite overprint (fig. 95).

This rock is porphyritic with randomly oriented, euhedral albitic plagioclase phenocrysts in a K-feldspar+quartz matrix, with minor magnetite (fig. 96). Plagioclase phenocrysts range in size from 0.05 to 1 mm and have aspect ratios ranging from 1:1 to 8:1. Plagioclase grains commonly have deformation twins, and the crystals are commonly bent. Zoning profiles are not apparent in EDS spectra of plagioclase grains. Interstices are mostly intergrowths of quartz and K-feldspar.

Accessory phases include, in decreasing order of abundance, magnetite, apatite, rutile, monazite, titanite, chlorite, and xenotime. Magnetite occurs in grains up to 500 µm in diameter, commonly with apatite or rutile inclusions; brecciated magnetite grains have fractures filled with K-feldspar. Apatite is mostly free of inclusions and is commonly euhedral. Rutile grains contain many inclusions including quartz, K-feldspar, and zircon; amoeboidal rutile is closely associated with sericite at magmatic grain boundaries. Fine-grained monazite (5–10 µm) is disseminated throughout, with one large (~1 mm) amoeboidal aggregate grain filling a fracture with titanite and xenotime. Additionally, some clasts are cut by thin, sinuous alteration seams filled with chlorite, titanite, xenotime (fig. 97), and REE-carbonates.

Rhyolitic cuttings fragments from the interval between 5,410 and 5,420 ft depth were handpicked under a binocular microscope. The purified rhyolite fragments were placed in an automated Brinkman grinder fitted with an agate mortar and pestle for initial grinding. The sample was continuously lubricated with acetone during grinding. The resulting coarse powder was further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. An aliquot of the powder was placed in the 3×2×0.1-cm cavity of a Ti “front-pack” mount and scanned in a Bruker D8 diffractometer from an angular range of 2–80° 2Θ with a Cu anode and point detector. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in TOPAS (Bruker AXS, 2011). The major and trace element geochemistry of the powder was then determined by XRF/ICP-MS (Deasy and others, 2024a).

The strong alteration overprint evident in thin section prompted further analysis to characterize the nature and abundances of clay minerals, as follows. A small amount of the powdered sample was mixed in a slurry with distilled water on a quartz “zero background” plate, was allowed to air dry, and was then scanned on the Bruker D8 from 2 to 90° 2Θ at 6 sec/step. The mounted sample was then placed in an EG-saturated chamber for a minimum of 24 hours before being scanned again, immediately upon removal from the EG chamber, from 2 to 40° 2Θ at 4 sec/step. Solvation in EG effected no change in the diffraction pattern (fig. 98), indicating the absence of expandable clays in this sample. A relatively weak 0.307 nm white mica (Ilt₁₁₂/Mu₁₁₄) peak suggests a higher abundance of 2M1-type muscovite than 1M-type, illitic mica. Other alteration minerals identified by XRD include small amounts of calcite, dolomite, and kaolinite.

Barnett (1975, p. 136) reports 202 ft of penetration of “deeply weathered basic igneous?” rock from 5,195 to 5,397 ft depth. This information can also be found in Arthur (1988).

Summary: Weathered metavolcanic rock of mafic to intermediate composition is overlain by limestone.

Volcanic fragments are porphyritic with acicular to lath-shaped plagioclase phenocrysts in a glassy to fine-grained matrix (figs. 99–105). Phenocryst density ranges from sparse to very dense. Plagioclase phenocrysts define a magmatic foliation that ranges from weak (fig. 99) to strong (fig. 100). Contacts between the porphyritic rock and sedimentary country rock are preserved in some fragments (figs. 101–103). Rare miarolitic cavities are lined with quartz and euhedral chlorite and filled with randomly oriented quartz and chlorite (fig. 104). Although magmatic textures are well preserved, the magmatic assemblage has been pervasively replaced by albite, chlorite, epidote, titanite, prehnite, magnetite, and apatite, with rare clinoamphibole. Magnetite grains are euhedral to amoeboidal and contain exsolved lamellae of ilmenite/rutile(?) and hematite(?) (fig. 105).

Volcanic fragments were separated from sedimentary fragments and other contaminating cuttings by hand under a binocular microscope. Subsamples of volcanic cuttings weighing 0.6–1.0 g each were separated from each of the following depth intervals:

Each subsample was ground by hand in an agate mortar and pestle while being continuously lubricated with acetone during grinding. The mortar and pestle were thoroughly cleaned between subsamples. The resulting fine powders were allowed to air dry. The powders were placed in 2-cm-wide cavities of circular “back-pack” mounts and scanned in a Panalytical X’Pert diffractometer with a Cu anode from an angular range of 3–90° 2Θ. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in HighScore Plus (Malvern Panalytical, 2018). Major and trace element geochemical analyses of these powders were obtained by INAA/ICP-OES/ICP-MS by Activation Laboratories, Inc. (Deasy and others, 2024a).

Additionally, a larger ~5-g fraction of volcanic rock for XRF/ICP-MS analysis was obtained from 5,290 to 5,350 ft depth by combining picked cuttings from the interceding intervals. The hand-picked fragments were placed in an automated Brinkman grinder fitted with an agate mortar and pestle. The sample was continuously lubricated with acetone during grinding. The resulting coarse powder was further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. An aliquot of the powder was placed in the 3×2×0.1-cm cavity of a Ti “front-pack” mount and scanned in a Bruker D8 diffractometer from an angular range of 2–80° 2Θ with a Cu anode and point detector. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in TOPAS (Bruker AXS, 2011). The major and trace element geochemistry of the powder was then determined by XRF/ICP-MS by Activation Laboratories, Inc. (Deasy and others, 2024a).

Barnett (1975, p. 136) described basement rocks in this borehole as “Weathered igneous (drill cuttings)” and included the following remarks: “drill cuttings 5770-80: very coarsely crystalline, 70-75% orthoclase, 5-10% biotite and pyroxene 20% quartz.” A U-Pb age from zircon of 551±6 Ma is given in Heatherington and others (1996), citing unpublished data.

Summary: The contact between granitic basement rock and overlying limestone occurs at ~5,730 ft depth. The bottom ~50-ft interval of sediment contains some lithic/feldspathic input, presumably from granite immediately below.

The observations below apply to samples from the following depth intervals:

The basement rock here is a medium-grained (0.2–2 mm), idiomorphic granite. Feldspars are penetratively altered to sericite (illite), chlorite, and fine-grained oxides (fig. 106). K-feldspar grains include graphic quartz intergrowths. Carlsbad and penetration twins are common in plagioclase, but they are partially obscured by sericitization. Quartz grains are inclusion-free, and most have uniform extinction; undulose extinction is uncommon. Magnetite is a common accessory, occurring in most fragments as anhedral, commonly brecciated grains or as clusters of grains ranging from ~0.1 to 0.5 mm. Biotite is uncommon, occurring in only a few fragments as anhedral interstitial grains, and is pseudomorphically replaced by chlorite and stained with hematite.

Granitic cuttings fragments from 5,750 to 5,770 ft depth were handpicked under a binocular microscope to isolate the basement lithology from contaminants. The fragments were placed in an automated Brinkman grinder fitted with an agate mortar and pestle for initial grinding. The sample was continuously lubricated with acetone during grinding. The resulting coarse powder was further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. An aliquot of the powder was placed in the 3×2×0.1-cm cavity of a Ti “front-pack” mount and scanned in a Bruker D8 diffractometer from an angular range of 2–80° 2Θ with a Cu anode and point detector. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in TOPAS (Bruker AXS, 2011).

Because of the strong alteration that is evident in thin section, further analysis was performed to characterize the nature and abundances of clay minerals that are extant in the K-feldspar concentrates that were prepared for ⁴⁰Ar/³⁹Ar analysis; this analysis is described here. A small amount of the powdered sample was mixed in a slurry with distilled water on a quartz “zero background” plate, was allowed to air dry, and was then scanned on the Bruker D8 from 2 to 90° 2Θ at 6 sec/step. The mounted sample was then placed in an EG-saturated chamber for a minimum of 24 hours before being scanned again, immediately upon removal from the EG chamber, from 2 to 40° 2Θ at 4 sec/step.

Upon EG solvation of powdered K-feldspar concentrate, the broad, low-angle shoulder of the 6° 2Θ (d=1.4 nm) chlorite peak disappears and is replaced by a broad peak centered at ~5° 2Θ (d=1.74 nm), indicating the presence of a small amount of chlorite/smectite in the sample (fig. 107). There is no corresponding change in the shape or intensity of the ~9° 2Θ (d=1.0 nm) peak.

The ~14 ft of basement rock penetration in this borehole was first described in the scientific literature as an “altered and veined biotite granite” by F.F. Grout, as cited in Applin (1951, p. 19). A major element geochemical analysis of a sample from 8,034 to 8,042 ft depth is reported in Milton and Grasty (1969); a black-and-white, PPL photomicrograph of the sample is also included in the publication. Bass (1969) provides a detailed petrographic analysis of core from 8,042 to 8,044 ft depth, noting strong vein development and cataclastic zones, and estimates a crystallization age of ~530 Ma from rubidium-strontium (Rb-Sr) isotopic analyses of feldspar concentrates (n=4, with high scatter). A summary of the above, a brief core description, and a pair of black-and-white photomicrographs in PPL and XPL are included in Milton (1972). Dallmeyer (1989) inferred a cooling age of 527–535 Ma from ⁴⁰Ar/³⁹Ar analysis of biotite. U/Pb analyses by SHRIMP of six separated zircon grains returned two Archean ages and four Neoproterozoic ages and, from this, a somewhat older (~600 Ma) crystallization age for the granitoid was inferred by Mueller and others (1994). U/Pb analyses of zircon by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) returned an age of 554±13 Ma (n=15; Deasy and others, 2023), an age that is equivalent to the ages of other granitoids in the Osceola intrusive complex (unit of Mueller and others [2014]).

Summary: The contact between granitic basement rock and overlying arkose occurs at ~8,030 ft depth. Weathered granitic regolith is a major component of the bottom ~10 ft of basal sediment. The sand grades upward into more evolved, quartz-dominated detritus. Recovered granitic material includes several bags of cuttings (fragments range from ~6 mm in maximum dimension to dust sized) and core chips.

Hand-picked core chips from two intervals (8,034–8,042 and 8,043.5–8,044.5 ft depth) were ground to a powder in an automated Brinkman grinder fitted with an agate mortar and pestle. The samples were continuously lubricated with acetone during grinding. The resulting powders were further ground by hand in a corundum mortar and pestle, again lubricated with acetone, and allowed to air dry. Aliquots of each powder were placed in 3×2×0.1-cm cavities of Ti “front-pack” mounts and scanned in a Bruker D8 diffractometer from an angular range of 2–70° 2Θ with a Cu anode and point detector. Semiquantitative mineral abundances (Deasy and others, 2024b) were obtained by Rietveld refinement modeling in TOPAS (Bruker AXS, 2011).

The strongly asymmetrical shape of the ~9° 2Θ (d=1.0 nm) peak in diffraction results from the sample from 8,034 to 8,042 ft depth (fig. 115) prompted re-analysis to characterize the nature and abundances of clay minerals, as follows. Small amounts of each powdered sample were mixed in a slurry with distilled water on quartz “zero background” plates, were allowed to air dry, and were then scanned on the Bruker D8 from 2 to 70° 2Θ at 6 sec/step. The mounted samples were then placed in an EG-saturated chamber for a minimum of 24 hours before being scanned again, immediately upon removal from the EG chamber, from 2 to 50° 2Θ at 2 sec/step.

The asymmetrical ~9° 2Θ (d=1.0 nm) peak in the sample from 8,034 to 8,042 ft depth becomes more symmetrical upon EG solvation by both a decrease in lower angle intensity and an increase in higher angle intensity, and there is a corresponding slight increase in the intensity of the peak at ~15.8° 2Θ (fig. 115). This is attributed to the presence of a small amount of illite/smectite. Although interference from other phyllosilicate peaks is strong, the positions of the illite/smectite peaks suggest ~50 percent smectite in the illite/smectite (Moore and Reynolds, 1997, table 8.3).

Ethylene glycol solvation has no effect on diffraction results from the sample from 8,043.5 to 8,044.5 ft depth (fig. 116), demonstrating the absence of expandable clays in this interval. To constrain the abundance of the smectitic component in the sample from 8,034 to 8,042 ft depth, an aliquot was spiked with corundum and reanalyzed on the Bruker D8 from 2 to 80° 2Θ. Modeling in TOPAS estimates the abundance of an “amorphous” component—interpreted here as the abundance of illite/smectite—of ~0.5 wt. pct.

Applin (1951) reports 58 ft of rhyolite beginning at 8,740 ft depth. A sample from 8,781 ft depth returned a whole-rock K-Ar age of 173±4 Ma (Grasty and Wilson, 1967). Milton (1972) provides a core description and several photomicrographs.

The basement lithology in this borehole is a rhyolite porphyry dominated by a moderately indurated microcrystalline groundmass with randomly oriented quartz, plagioclase, and K-feldspar phenocrysts up to 2.5 mm across that compose 10–15 percent of the rock by volume (figs. 117 and 118). Ellipsoidal chloritic and opaque-rich lithic clasts are also randomly oriented and make up another ~2–5 percent volume. One lithic clast contains relatively coarse-grained chlorite and epidote. Opaque grains, making up another ~1 percent of total volume, include subhedral magnetite with ilmenite lamellae and round ilmenite-rutile symplectites (fig. 119). Zircon grains are rare, small (≤40×≤20 µm), subhedral to euhedral, commonly have broken edges, and have oscillatory zoning in BSE. The massive matrix comprises equant quartz and elongate feldspar grains up to 50 µm in diameter and 200 µm in length, respectively (figs. 119–122). Much of the rock is overprinted by Liesegang rings, leaving sharp contacts between red-orange hematite- and goethite-bearing domains and tan non-oxidized domains (figs. 117, 118, 121). All quartz grains have uniform extinction and many are equant and round, although some grains retain crystal faces (fig. 120). The round grains are uniformly dim in CL with quartz overgrowths that are bright in CL (fig. 123), whereas the quartz grains with subhedral to euhedral shapes have bright, oscillatory zoning in CL and lack overgrowths (fig. 124). It is likely, then, that at least some of the rounded grains are sedimentary xenocrysts. K-feldspar phenocrysts are subhedral to euhedral, are pervasively mottled with quartz lamellae (fig. 125), and are partially altered to illite (figs. 120 and 121). Although fine-grained, lath-shaped plagioclase composes a significant fraction of the groundmass (fig. 122), large plagioclase phenocrysts are uncommon. Rare large plagioclase grains are up to 2 mm in length, are euhedral with slightly rounded or resorbed corners, and have oscillatory zoning (figs. 122 and 126). In contrast to K-feldspar, plagioclase is unaltered except along fractures and contains only patchy domains of exsolved quartz+albite+K-feldspar (fig. 126). Vugs in the groundmass are rare, small (<20 µm across), lined with euhedral quartz, and locally contain REE-rich carbonates or other unusual minerals (fig. 127). Evidence of deformation is limited to the apparently in situ brecciation of many quartz grains (figs. 121 and 123).

Representative core chips from the following depth intervals were shattered in a steel percussion mortar to pea-sized or smaller fragments:

These were placed in an automated Brinkmann grinder fitted with an agate mortar and pestle. The sample was continuously lubricated with acetone during grinding. The sample was then ground further by hand in a corundum mortar and pestle, again continuously lubricated with acetone. An aliquot of the resulting fine powder was loaded into the 2×3×0.1-cm cavity of a Ti “front-pack” mount and analyzed on a Bruker D8 diffractometer with a Cu anode and point detector from an angular range of 2–100° 2Θ at a scanning rate of 8–14 sec/0.02° step.

In order to characterize the clay populations in this sample, an aliquot of the sample from 8,750 to 8,753 ft depth was mounted on a quartz “zero background” plate in a slurry with distilled water and allowed to air dry before being analyzed on the Bruker D8 from 2 to 70° 2Θ at 9 sec/step. The mounted sample was then placed in a chamber with an atmosphere saturated with EG. After 24 hours, the sample was again scanned, immediately upon removal from the EG chamber, on the Bruker D8 from 2 to 40° 2Θ at 2 sec/step. There was no change in the diffraction pattern as a result of EG-solvation, indicating the absence of expandable clays in this sample.

Semiquantitative mineral modes in each sample were determined using TOPAS (Bruker AXS, 2011) and the Rietveld method and are available in Deasy and others (2024b). K-feldspar populations were modeled with a combination of microcline, orthoclase, and sanidine models. A major and trace element geochemical analysis of core chips from 8,744 to 8,756 ft depth is available in Deasy and others (2024a).