The Avon Park Formation, its Bombing Range type well, and named microfossils

In December 1944, petroleum geologists Paul L. Applin and Esther R. Applin formally named and defined the Avon Park Limestone (now designated the Avon Park Formation), establishing one of the most prominent lithostratigraphic units in the southeastern United States.[applin-1944-stratigraphy][miller-1986-floridan-aquifer]

The formation was named directly for Avon Park, Florida, from rock cuttings recovered in a deep water-supply test well drilled on the Avon Park Air Force Range (then the Avon Park General Bombing and Gunnery Range).[applin-1944-stratigraphy] Subsequent paleontological study of these borehole samples led to the description of three new fossil micro-organisms named for the community.[applin-jordan-1945-foraminifera]

The Type Locality on the Bombing Range

During World War II, the rapid expansion of the 107,059-acre Avon Park military training range required substantial freshwater supplies for troop barracks, airbase administrative units, and cantonment facilities. Deep test wells were drilled into the subsurface to penetrate the principal artesian water-bearing strata.[applin-1944-stratigraphy]

In Section 31, Township 32 South, Range 30 East—located within the northern Polk County quadrant of the bombing range reservation, roughly ten miles northeast of the city of Avon Park—drilling reached deep Eocene strata.[applin-1944-stratigraphy] Well cuttings examined by the Applins revealed a distinctive sequence of dense, fossiliferous, cream-colored to brownish marine limestones and saccharoidal dolostones between depths of 600 and 930 feet below land surface.[applin-1944-stratigraphy]

Because this borehole provided the first continuous, unweathered subsurface profile of these Middle Eocene beds in central Florida, the Applins designated the bombing range well as the type section and named the rock unit the Avon Park Limestone.[applin-1944-stratigraphy]

Microfossils Named avonparkensis

Esther R. Applin was a pioneer in commercial micropaleontology whose work in the 1920s demonstrated that microscopic fossils (foraminifera) survived the crushing action of rotary drill bits and could reliably date and correlate subsurface strata. In March 1945, Applin and paleontologist Louise Jordan published a monograph in the Journal of Paleontology analyzing the microfauna recovered from Florida borehole cores, including the Avon Park type well.[applin-jordan-1945-foraminifera]

They formally named and described three new foraminiferal species bearing the specific epithet avonparkensis:

1. Valvulina avonparkensis Applin & Jordan, 1945: A conical, agglutinated larger benthic foraminifer with a distinctive triserial chamber arrangement, found abundantly throughout the upper carbonate layers of the formation.[applin-jordan-1945-foraminifera]
2. Flintina avonparkensis Applin & Jordan, 1945: A large, robust, porcelaneous miliolid foraminifer with three visible chambers in the final whorl and a prominent bifid tooth in its aperture.[applin-jordan-1945-foraminifera]
3. Rotalia avonparkensis Applin & Jordan, 1945: A trochospiral rotaliid microfossil with beaded sutures and umbilical plugs, serving as a regional biostratigraphic marker.[applin-jordan-1945-foraminifera]

These three species, alongside associated larger foraminifera such as Coskinolina floridana Cole, Lituonella floridana Cole, and Dictyoconus cookei (Moberg), and the small sand dollar echinoid Peronella dalli, established the biostratigraphic zone defining late Claiborne (Middle Eocene) time across peninsular Florida.[applin-1944-stratigraphy][applin-jordan-1945-foraminifera]

Oldest Exposed Rocks in Florida

Although first identified hundreds of feet underground at the Avon Park Bombing Range, the formation was later discovered at the ground surface. In 1951, Robert O. Vernon of the Florida Geological Survey published a survey of the Ocala Uplift in Citrus and Levy counties.[vernon-1951-citrus-levy]

Vernon documented that regional crustal upwarping along the crest of the structural arch had stripped away younger sedimentary layers, exposing the Avon Park strata in shallow limestone quarries and along the Withlacoochee River.[vernon-1951-citrus-levy] As a result, the rock unit first named from the Avon Park military range represents the oldest geologic formation exposed at the surface anywhere in Florida, dating to approximately 40 to 45 million years ago.[vernon-1951-citrus-levy]

Evolution to the Avon Park Formation and the Floridan Aquifer

In 1986, United States Geological Survey hydrogeologist James A. Miller completed a comprehensive regional investigation of the Floridan Aquifer System.[miller-1986-floridan-aquifer] Miller evaluated the relationship between the Avon Park Limestone and the underlying “Lake City Limestone” (which the Applins had separated in 1944).

Miller established that the two units could not be consistently differentiated across peninsular Florida by rock composition or microfaunal boundaries.[miller-1986-floridan-aquifer] The USGS formally combined both units under the single name Avon Park Formation, abandoning the term Lake City Limestone.[miller-1986-floridan-aquifer] Miller also adopted the term “Formation” rather than “Limestone” because extensive sections of the unit have been altered by geochemical replacement into dense, brown, fractured dolostone (magnesium-rich carbonate).[miller-1986-floridan-aquifer]

Hydrogeologically, the Avon Park Formation forms the thickest and most permeable component of the Upper Floridan Aquifer across central Florida, ranging from 500 to over 1,200 feet in thickness.[miller-1986-floridan-aquifer] Its fractured dolostones yield millions of gallons of artesian water daily to municipal wells in Avon Park and supply the extensive irrigation systems sustaining Highlands County’s citrus groves.[miller-1986-floridan-aquifer]

Role in Lake Wales Ridge Karst and Lake Formation

Beneath the sands of the Lake Wales Ridge, the soluble carbonates of the Avon Park Formation and overlying Ocala Limestone control the physical landscape.[white-1970-geomorphology]

During late Tertiary and Pleistocene sea-level fluctuations, slightly acidic rainwater filtered down through surficial sand dunes into the fracture networks of the Avon Park Formation, dissolving immense subterranean cavities.[white-1970-geomorphology] Subsequent roof collapse and sand subsidence produced the conical sinkhole basins that define Avon Park, including Lake Verona (reaching depths of 60 to 80 feet) and Lake Tulane (reaching 85 feet).[swfwmd-verona-mfl] The geological formation named for the town in 1944 was the very foundation whose dissolution gave Avon Park its signature freshwater lakes.

References

• applin-1944-stratigraphy — Paul L. Applin and Esther R. Applin, Bulletin of the American Association of Petroleum Geologists 28(12), pp. 1673–1753. Regional Subsurface Stratigraphy and Structure of Florida and Southern Georgia. 1 December 1944. https://doi.org/10.1306/3D9336CD-16B1-11D7-8645000102C1865D
• applin-jordan-1945-foraminifera — Esther R. Applin and Louise Jordan, Journal of Paleontology 19(2), pp. 129–148. Diagnostic Foraminifera from Subsurface Formations in Florida. 31 March 1945. https://www.jstor.org/stable/1299169
• vernon-1951-citrus-levy — Robert O. Vernon, Florida Geological Survey. Geology of Citrus and Levy Counties, Florida (Geological Bulletin No. 33). 1951. https://ufdc.ufl.edu/UF00000379/00001
• miller-1986-floridan-aquifer — James A. Miller, United States Geological Survey. Hydrogeologic Framework of the Floridan Aquifer System in Florida and in Parts of Georgia, Alabama, and South Carolina (USGS Professional Paper 1403-B). 1986. https://pubs.usgs.gov/pp/1403b/report.pdf
• white-1970-geomorphology — William A. White, Florida Bureau of Geology. The Geomorphology of the Florida Peninsula (Geological Bulletin No. 51). 1970. https://ufdc.ufl.edu/UF00000397/00001
• swfwmd-verona-mfl — Southwest Florida Water Management District. Lake Verona Minimum Flows and Levels Information and Bathymetry. 2020. https://www.swfwmd.state.fl.us/projects/mfl/lake-verona

Related

• Entity: Avon Park Air Force Range
• Entity: Lake Verona
• Entity: Lake Wales Ridge
• Report: Lake Wales Ridge geology, solution lakes, and frost drainage at Avon Park

Where this comes from

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corroborated — More than one independent source agrees.

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unverified — Drafted from sources but not yet checked by a volunteer.

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Sources

1. Paul L. Applin and Esther R. Applin, Bulletin of the American Association of Petroleum Geologists 28(12), pp. 1673–1753, Regional Subsurface Stratigraphy and Structure of Florida and Southern Georgia (1944)
2. Esther R. Applin and Louise Jordan, Journal of Paleontology 19(2), pp. 129–148, Diagnostic Foraminifera from Subsurface Formations in Florida (1945)
3. Robert O. Vernon, Florida Geological Survey, Geology of Citrus and Levy Counties, Florida (Geological Bulletin No. 33) (1951)
4. James A. Miller, United States Geological Survey, Hydrogeologic Framework of the Floridan Aquifer System in Florida and in Parts of Georgia, Alabama, and South Carolina (USGS Professional Paper 1403-B) (1986)
5. William A. White, Florida Bureau of Geology, The Geomorphology of the Florida Peninsula (Geological Bulletin No. 51) (1970)
6. Southwest Florida Water Management District, Lake Verona Minimum Flows and Levels Information and Bathymetry (2020)

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