Chadbourn Formation

Chadbourn Formation
Name	Chadbourn Formation
Type	Formation
Period	Pliocene
Prilliams	North Carolina

Chadbourn Formation is a Neogene stratigraphic unit exposed in southeastern United States, primarily in North Carolina and adjacent portions of South Carolina and Georgia. The formation preserves marine and marginal marine deposits that yield a diverse assemblage of fossils informing studies of Pliocene, Neogene paleoenvironments and biogeographic shifts along the Atlantic Ocean coastline. It has been cited in regional correlations involving units such as the Castle Hayne Formation, Yorktown Formation, and Waccamaw Formation.

Geology and Lithology

The Chadbourn Formation consists predominantly of unconsolidated to semi-lithified sediments including fine- to coarse-grained quartzose sands, glauconitic horizons, shell hash, and interbedded silty clay layers. Field descriptions note abundant carbonate bioclasts, phosphatic concentrations, and sideritic nodules characteristic of transgressive marine deposits. Petrographic and grain-size studies relate its mineralogy to sediment sources traced to the Piedmont and reworking along the Atlantic Coastal Plain. The glauconite-bearing facies are comparable to those described in the Miocene-to-Pliocene stratigraphy of the Coastal Plain by workers from institutions such as United States Geological Survey and regional universities.

Stratigraphy and Age

Biostratigraphic and chronostratigraphic analyses place the Chadbourn Formation within the late Neogene interval, commonly assigned to the Pliocene based on foraminiferal assemblages, molluscan zones, and regional correlation with oxygen isotope stratigraphy. The unit lies stratigraphically above Miocene units in some sections and is overlain by younger Pleistocene terrace deposits and holocene sediments associated with Holocene sea-level fluctuations documented by researchers from Smithsonian Institution, Florida Museum of Natural History, and state geological surveys. Age control has been refined using planktonic foraminifera biozones tied to global events recorded in the International Commission on Stratigraphy frameworks and chemostratigraphic ties to the LR04 benthic stack.

Fossil Content

Fossil assemblages include diverse marine invertebrates—bivalves, gastropods, echinoids—and vertebrate remains such as shark teeth and marine mammal fragments, which have been compared to faunas from the Yorktown Formation and Calvert Formation. Notable invertebrate genera reported in the literature from Chadbourn outcrops include representatives similar to Mercenaria, Chione, and various Scaphopoda taxa; vertebrate material has affinities with Carcharhinus, Galeocerdo, and cetacean remains comparable to specimens curated by the North Carolina Museum of Natural Sciences. Microfossils—planktonic and benthic foraminifera, ostracods, and palynomorphs—provide high-resolution paleoecologic data used in studies by researchers affiliated with Duke University, University of North Carolina at Chapel Hill, and East Carolina University.

Depositional Environment

Sedimentary structures, faunal composition, and mineralogy indicate deposition in a shallow epicontinental shelf setting during a sea-level highstand, with episodes of nearshore reworking and estuarine influence. Interpretations emphasize a gradient from open-marine glauconitic sands to protected bay and estuarine silts influenced by tidal currents and riverine discharge from paleodrainage systems linked to the Cape Fear River and other regional paleorivers. Studies comparing Chadbourn facies to modern analogs in the Gulf of Mexico and North Atlantic continental shelves have been published in regional geological literature and synthesized in syntheses involving researchers from the Coastal Carolina University and the University of Florida.

Geographic Distribution and Thickness

Outcrops of the Chadbourn Formation occur in southeastern North Carolina, extending into northeastern South Carolina and southeastern Georgia where coastal plain erosion and quarries expose the unit. Thickness is variable—ranging from a few meters in distal exposures to over 20 meters in depocenters—reflecting accommodation space controlled by late Neogene subsidence and eustatic changes recorded in regional stratigraphic cross-sections prepared by the United States Geological Survey and state geological surveys. Key exposures are documented near localities such as Bladen County, Columbus County, and along coastal river cuts.

Economic and Scientific Significance

Economically, the Chadbourn Formation yields sand and shell resources exploited for construction aggregate, aquifer materials, and limited phosphate concentrations that have attracted attention from state resource planners and firms in the construction industry and mineral sectors. Scientifically, the unit is a focal point for research on Pliocene climate, sea-level history, and biotic responses to the closing of the Panama Seaway and its downstream effects on Atlantic circulation; it has been included in comparative studies alongside sequences from the Carolinas, Virginia and Florida. Ongoing paleontological surveys and stratigraphic mapping by academic institutions and governmental bodies continue to refine models of southeastern United States Neogene paleogeography and contribute specimens to collections at museums like the North Carolina Museum of Natural Sciences and university repositories.

Category:Geologic formations of North Carolina