Abstract

A phosphate-rich fossil locality along the Wando River in Charleston, South Carolina, exposes the basal unit of the Pleistocene Wando Formation. This study documents the site’s lithology, fossil assemblage, and depositional history, highlighting complex phosphatization processes and stratigraphic relationships. Fossils recovered are exclusively marine, including reworked shark teeth from Oligocene–Miocene deposits, Pleistocene bivalve steinkerns, cetacean remains, and trace fossils, reflecting multiple episodes of sediment reworking and sea-level fluctuations. The site’s phosphorite and marlstone exhibit distinctive yellow-green fluorescence under UV light, indicating variable diagenetic phosphate enrichment under changing redox and depositional conditions. Evidence suggests phosphogenesis at this locality was driven by a combination of upwelling, organic decay, and weathering influenced by Pleistocene sea-level cycles, with possible contributions from older Miocene environments. The site’s geological and paleontological significance is under threat from urban development, unauthorized collecting, and tidal erosion, underscoring the need for conservation measures to preserve this valuable record of coastal marine environments and phosphatic sedimentation.

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Introduction

The Wando dig site is a fossiliferous, phosphate-rich outcrop located along the edge of a tidal creek off the Wando River, within the greater Charleston area of South Carolina. Informally named the McDowell site, it is mapped by the USGS as part of the Wando Formation within the Cainhoy Quadrangle. This report aims to preserve and document key geological and paleontological data from the locality, reconstruct the site's depositional and diagenetic history, assess the stratigraphic context and presence of reworked material, and evaluate the processes of phosphatization observed in both fossils and sediments.

The site is currently threatened by both anthropogenic and natural forces. Portions of the fossiliferous layer have been obscured by gravel and fill due to nearby development, and signs of unauthorized digging are present, likely driven by the high commercial value of shark teeth such as Otodus megalodon. Tidal erosion and vegetation loss also pose risks to the integrity of the exposure.

Geological Overview

The McDowell site exposes the basal unit of the Wando Formation, a Pleistocene marine deposit formed during interglacial sea-level highstands. Auger hole data from the Cainhoy Quadrangle suggest that the Wando Formation unconformably overlies a range of older units, including the Ashley Formation (Oligocene), Marks Head Formation (Lower Miocene), and Goose Creek Limestone (Pliocene). The nearest auger data to the McDowell site indicate that the Ashley Formation directly underlies the Wando. This is likely the case here however the presence of the Marks Head Formation cannot be ruled out.

Just upslope from the site lies the Ten Mile Hill Formation (formalized by Sanders et al. in 2009), an older Pleistocene unit known for its Ice Age terrestrial fauna. Its proximity places the McDowell locality at a critical marine–terrestrial boundary during the Pleistocene. The absence of terrestrial fossils at McDowell reinforces its identity as a dominantly marine deposit.

Fossils reworked from older strata (particularly shark teeth) suggest that the basal Wando layer incorporated material from underlying Oligocene and Miocene formations. Sea-level fluctuations during the Pleistocene, especially during the Pamlico and Talbot highstands, would have exposed and redistributed these older sediments. During the Talbot highstand, the Ten Mile Hill area likely formed a barrier island, with the McDowell site positioned just seaward of it. This paleogeographic configuration is still evident in the modern topography.



Lithologically, the McDowell site consists primarily of fine sand and silt, often grading into muddier or clay-rich textures in lower-lying areas. Among the most common lithified materials recovered are cream to peach-colored marlstone and black phosphorite. The marlstone is fine-grained, weakly calcareous, and frequently displays weathered rinds or desiccation-style cracking. These rocks are interpreted as products of shallow marine deposition followed by early diagenetic cementation. The black phosphorite occurs as small pebbles and larger, irregularly shaped clasts with twisted or pocked surface textures. Many exhibit morphologies reminiscent of hardened, extruded clay. These lithologies, together with the fossil assemblage, provide essential context for interpreting depositional environments, reworking processes, and post-depositional diagenesis at the site.

Methods

Surface collection was conducted at low tide between March 13 and May 4, 2025. Material was photographed, analyzed for preservation, and identified using field and lab techniques. Acid testing with HCl was employed to evaluate calcium carbonate content, and shortwave UV light was used to detect phosphate fluorescence. Fossils were identified to the lowest possible taxonomic level based on morphological comparison and literature review.

Fossil Inventory

All fossils recovered were marine in origin. No freshwater or terrestrial taxa were found. The fossil assemblage includes:
• Actinopterygii – Bony fish vertebrae and hypural bones
• Cetacea – Vertebrae (including juvenile), tympanic bullae
• Selachii - Lamnoid and Scyliorhinoid vertebrae
• Batomorphi – Ray vertebrae and mouth plates
• Steinkerns – Predominantly bivalves, including Arcidae and Cardiidae
• Echinoidea – Imprint in marlstone (likely irregular echinoid)
• Ichnofossils - Possible crab burrows
• Astrangia – Small corals, dated to ~139 ka, Pleistocene
• Echinarachniidae – Fragmentary remains of sand dollars
• Selachii - teeth, with approximate time ranges:
> o Physogaleus contortus (Oligocene–Miocene)
> o Hemipristis serra (Oligocene–Miocene)
> o Otodus cf. chubutensis or cf. angustidens (Eocene–Miocene)
> o Galeocerdo (Eocene–present)
> o Carcharias (Cretaceous–present)
> o Isurus hastalis (Oligocene–Pliocene)

Many of the shark teeth correspond to extinct taxa from the Oligocene–Miocene, consistent with reworking. The cetacean remains include tympanic bullae that are likely from large odontocetes, possibly sperm whales. Vertebrae likely originate from a similar temporal range but lack definitive age markers. One bony fish hypural bone appears to be a billfish species.

Steinkerns are abundant and taxonomically significant. Many correspond to ark shells (Anadara brasiliana, A. ovalis) and likely represent Pleistocene intertidal to shallow sublittoral habitats. Smaller steinkerns with scalloped edges likely derive from Cardiidae (cockles). These morphologies align with habitat reconstructions for surf zone assemblages.

Discussion: Reconstructing the Past

Fossil Evidence and Timeframe
The fossil assemblage at the McDowell site helps reconstruct a timeline and environmental context for the region. The site exposes the basal layer of the Wando Formation, which, while Pleistocene in age, is known for containing reworked fossils from older formations. The fossil inventory (particularly the shark teeth) includes extinct species such as Hemipristis serra and Physogaleus contortus, which place their original deposition in the Oligocene to Miocene. The presence of these species suggests they were deposited millions of years ago in a warm, shallow marine environment and later reworked into Pleistocene Wando sediments during episodes of sea-level fluctuation and possible subaerial exposure.





The cetacean remains include tympanic bullae likely belonging to large odontocetes, possibly sperm whales. Cetacean vertebrae show juvenile or adult status based on the presence of a fused or detached epiphysis. Bony fish vertebrae indicate species possessing a single pair of ribs (two ribs per vertebra), and one bony fish hypural bone appears to derive from a billfish species. Although these non-dental elements cannot be reliably dated to a specific geologic interval, they nonetheless offer important information on the paleoenvironment and faunal assemblage of the site.

Bivalve steinkerns at the site, particularly those resembling ark shells (family Arcidae), are likely Pleistocene in origin. Their asymmetrical, inflated forms match well with Anadara brasiliana and Anadara ovalis, species identified in auger hole data from the Cainhoy Quadrangle. These taxa prefer shallow sublittoral and intertidal habitats and still persist on modern Charleston-area beaches. Other faunal indicators, such as fragments of sand dollars and coral, also match findings from auger records. The coral specimens were radiometrically dated at 139 ±10 ka BP during auger hole analysis (Weems & Lemon, 1985), placing them within the warm, highstand conditions of the Eemian interglacial.



One notable marlstone specimen features a double imprint that likely represents a burrowing or irregular echinoid. The overprinting suggests the echinoid may have moved slightly. Burrowing echinoids typically prefer stable, well-oxygenated marine environments with low energy and fine sediments, suggesting the marlstone formed under such conditions. This interpretation is supported by shortwave UV fluorescence testing, which revealed yellow-green zones of white phosphate infiltration within the marl.



The site also contains “half-pipe” shaped trace fossils interpreted as burrow remains. These are elongated, tubular features that consistently measure approximately 6 cm in length and 2.5 to 3 cm in width, split longitudinally to expose the interior. The uniformity in size is notable and may reflect constraints of the trace maker or environmental factors influencing their formation. Similar burrows have been observed in other areas of Charleston, suggesting a broader regional occurrence. Most specimens are black or brown in color and often exhibit internal smoothing or a distinct lining. One example from the McDowell site is a lighter gray and contains partial infill along with small shell casts, offering additional evidence of its origin and subsequent diagenetic processes.




Stratigraphy and Formation Relationships
Auger hole data from the Cainhoy Quadrangle indicate that the Wando Formation may overlie the Ashley Formation (upper Oligocene), the Marks Head Formation (lower Miocene), or the Goose Creek Limestone (lower Pliocene), depending on the locality. The closest auger hole to the McDowell site suggests the Wando directly overlies the Ashley Formation in this area.

Each of the three underlying formations shows unique lithological characteristics in the Cainhoy Quadrangle auger hole descriptions:
• Goose Creek Limestone: Calcareous and shelly, though often leached; typically contains scallop fossils.
• Marks Head Formation: Micaceous, semi-calcareous marl; not strongly fossiliferous but present in Georgia outcrops and underground in South Carolina.
• Ashley Formation: Similar to Marks Head but lacks mica; often includes abundant large foraminifera.

The Goose Creek Limestone can likely be ruled out as the underlying unit at the McDowell site. No scallop fossils, which were a diagnostic feature of the unit in auger hole record, were recovered, and the site’s sediment and rock samples show only weak calcareous reactions, inconsistent with the highly calcareous nature of the Goose Creek Limestone. The Ashley and Marks Head formations remain possible candidates; however, neither large foraminifera (typical of Ashley) nor mica flakes (noted in Marks Head within the Cainhoy Quadrangle) were observed in surface materials. It is possible these indicators were present but not recognized during field examination, particularly in the case of foraminifera, which lacked descriptive guidance in the auger records.

There is a recognized stratigraphic hiatus in the Charleston region spanning the middle and upper Miocene. This missing interval may also be represented at McDowell by fossils and rocks reworked into the Wando's basal lag deposits, as evidenced by the fossil taxa whose extinction predates the Pliocene.

Rock Types and Contact Specimens
The McDowell site features two primary rock types: a cream-to-peach marlstone and black phosphorite. Both are significant in understanding depositional and diagenetic history. The marlstone is fine-grained, semi-calcareous, and sometimes bears a desiccation rind with conchoidal cracks. In contrast, the phosphorite appears in nodules, pebbles, and irregular clumps with a pocked or weathered surface. Many rocks have the appearance of compressed and hardened clay.





Fluorescence testing under shortwave UV revealed yellow-green phosphate zoning in many marlstone specimens and within the interiors of certain phosphorite samples, indicating infiltration by phosphate-rich fluids during diagenesis. Two “contact” specimens are particularly notable: each displays a striking contrast between a black phosphorite half and a brown, weathered half. Internal cross-sections show a uniformly black core, suggesting that the brown exterior represents a weathered or leached rind rather than a distinct lithology. This pattern implies that part of each specimen was buried and protected, while the exposed portion experienced erosion and weathering, either in a shallow marine setting or through later subaerial exposure. Another form of contact specimen shows on a broken surface a distinct cream marlstone interior surrounded by and covered on unbroken surfaces by black phosphorite a few millimeters thick.







For both phosphorite and marl acid testing with 5% vinegar showed no reaction, but 10% HCl caused moderate to strong effervescence, particularly in powdered or chipped samples. This suggests low to moderate carbonate content, possibly confined to the outer rind in some specimens. The mineral composition, phosphate zoning, and limited carbonate presence point to complex diagenetic interactions rather than simple sedimentary origin.



Depositional Environments and Phosphogenesis
Phosphogenesis in marine environments typically results from three main processes: (1) upwelling of phosphate-rich deep waters, (2) weathering of phosphatic rock, and (3) decay of organic matter under anoxic conditions. At the McDowell site, all three mechanisms may have contributed to phosphate formation, particularly in response to sea level fluctuations.

Rising and falling sea levels can influence thermocline depth and ocean circulation patterns, potentially driving upwelling events. These changes may allow phosphate-rich bottom water to migrate upward and flow over newly deposited sediment, or permit nutrient-enriched deeper waters to shift landward into more shallow marine settings. Sea level fluctuations also expose phosphatic sediments to erosion and reworking. As previously buried material is brought into high-energy nearshore zones, or even subaerially exposed, it becomes susceptible to mechanical weathering and chemical alteration. The resulting phosphate-laden runoff can re-enter the water column, fueling further phosphogenesis.

Anoxic conditions in deeper subtidal zones or in restricted estuarine and lagoonal settings with high organic input can also produce dissolved phosphate through microbial decay. These conditions are conducive to the formation of dark, organic-rich phosphorite deposits.

At McDowell, phosphatization appears to have occurred in multiple episodes, reflecting dynamic shifts in sea level, redox conditions, and sedimentation rates. The presence of both black and white phosphate suggests a range of phosphogenic environments over time. Many samples show evidence of post-depositional weathering: for example, phosphorite that fluoresces under shortwave UV typically does so only on freshly broken surfaces, indicating that surface weathering has obscured or altered the original phosphate layer. Some black phosphate specimens display light brown areas where the phosphate color and material have been visibly stripped away.





Both black and white phosphorite are present at the site. Black phosphorite forms in anoxic, organic-rich environments and often incorporates hydrocarbons, bitumen, or iron sulfides, consistent with the strong petroliferous odor released by some samples during hydrochloric acid testing. In contrast, white phosphate tends to form in well-oxygenated waters with little to no organic matter or iron oxides. While black phosphate is more common at McDowell, white phosphate zones occur within marlstone, and certain cetacean bone fragments and lighter-colored shark teeth appear to reflect this mode of preservation. White phosphate may result from an influx of phosphate-rich water, either from upwelling or weathering, without concurrent organic sedimentation, and may also form through biologically mediated processes such as cyanobacterial activity.

Although many phosphatization events in the Charleston region are linked to Pleistocene sea level changes, the phosphate at McDowell may have a more complex history. Miocene conditions, particularly those involving enhanced upwelling and high biological productivity, are also conducive to phosphate formation. It is therefore plausible that some of the phosphorite at McDowell originated in Miocene marine environments, with subsequent Pleistocene transgressions and regressions contributing to further exposure, reworking, and diagenesis.

Several phosphatic rocks at the site exhibit fine internal laminations, likely recording quiet-water deposition under alternating organic-rich and organic-poor conditions. These laminated rocks also show features such as vertical jointing, soft sediment deformation, and banding, suggesting that post-depositional processes played a significant role in shaping their present textures. Phosphatization may have occurred after these deformation events, potentially during renewed marine incursions.





Overall, the mineralogical and fossil evidence indicates that the McDowell site experienced repeated marine transgressions and regressions, accompanied by shifting energy levels, redox conditions, and sedimentation regimes. The phosphorite nodules and phosphate-rich marlstone preserve a complex record of these environmental changes, reflecting a dynamic interplay of depositional, diagenetic, and weathering processes across geologic time.

Conservation Concerns

The McDowell Dig is at risk from human interference and natural degradation. The ridge exposure has been partially buried by fill material from adjacent development, leaving only a small portion accessible. Evidence of illicit digging, including sifting screens, shovel pits, and discarded fossil material, suggests collectors are targeting commercial-grade specimens, particularly megalodon teeth.

Tidal erosion, compounded by vegetation loss and slope destabilization from digging, is accelerating the site's degradation. This location preserves an irreplaceable record of marine Pleistocene environments and reworked Neogene faunas. As urban development in Charleston intensifies, the adoption of formal review protocols for geologically and historically significant land could help safeguard important scientific data before it is lost. States like Virginia have implemented such measures, requiring archaeological and geological assessments prior to construction, ensuring that sites are documented even if development proceeds. South Carolina could benefit from similar policies, including stronger enforcement and penalties for unauthorized excavation.