Parachucla Formation

The Parachucla Formation is a geologic formation in the southeastern United States. It preserves fossils from the Aquitanian stage of the early Miocene period. The formation is included in the Hawthorn Group. An exposure at the northern end of the formation has produced fossils estimated to be 19.4 to 20.5 Million years ago (Ma). Another exposure at the southern end of the formation has produced fossils estimated to be 23.9 to 24.7 Ma.

Descriptive history
The Parachucla Formation was defined in 1908 by Earle Sloan as "weakly lithified shales and mudstones, olive-grey to dark-greenish-grey in color, that crop out on the Savannah River". In 1988, Paul Huddlestun added calcareous beds that underlay Sloan's Parachucla Formation to the formation, calling the new addition the Tiger Leap Member, and the original formation defined by Sloan the Porters Landing Member. In 2001, Weems and Edwards removed the Tiger Leap Member from the Parachucla Formation, raising it in rank to the Tiger Leap Formation and dropping the Porters Landing Member name. Other authors continue to use the Huddlestun definition, which includes the Tiger Leap Member in the formation.

Extent and Age
The Parachucla Formation (or the Porters Landing Member of the formation, depending on definition), is a quartz sand and clay structure lacking fossils, formed in the late Aquitanian stage of the early Miocene period. It lies above the Tiger Leap Formation (or Tiger Leap Member of the Parachucla Formation), a chalky, shelly sand structure with phosphate and microfossils, which formed in the Oligocene epoch. The Tiger Leap unit is underlain in part by the Suwannee Limestone, the Lazaretto Creek Formation, and other unnamed units. The Parachucla Formation is overlain in part by the Marks Head Formation. The Parachula Formation trends deeper from north to south. In Effingham County, near the Savannah River, the top of the formation is 163 ft below sea level. On Cumberland Island, it is 420 ft, and next to the St. Marys River, 423 ft below sea level. The formation (or Porters Landing Member) maintains a fairly consistent thickness along most of the north-south line in coastal Georgia, 30 ft thick in Effingham County, then 40 ft to 46 ft from Richmond Hill to Cumberland Island. The formation thins rapidly south of Cumberland Island, being only 10 ft thick near the St. Marys River. The Parachucla Formation in Georgia appears to be narrow. No indication of the formation (or Porters Landing Member) was found in bore holes in Evans County, to the west of Richmond Hill, or at Fort Pulaski, to the east. The siliciclastic beds of the Parachucla Formation resemble those of the equivalent Penney Farms Formation in Florida, but the Penney Farms Formation has carbonate beds which are not found in the Parachucla. Exposed strata along the Suwannee River near White Springs in Hamilton County, Florida, which correlate with the Porters Landing Member strata found along the Savannah River, is the only known occurrence of the Parachucla Formation in Florida. The Parachucla Formation is the lowest unit of the Hawthorn Group in Georgia. The Parachucla Formation also grades into the Chattahoochee Formation in southwest Georgia and the eastern panhandle of Florida. Off the Georgia coast, the Parachucla grades into the Cooper Formation. The Tiger Leap and Porters Landing members appear to be separated by an unconformity.

Clam shells (Ostrea normalis) from two locations were tested for age using Strontium isotope stratigraphy. Shells from the Parachucla Formation exposure at White Springs in northern Florida yielded age estimates of 23.9 to 24.7 Million years ago (Ma). Shells from the formation exposure at Porters Landing in northern Georgia yielded age estimates of 19.4 to 20.5 Ma.

White Springs Local Fauna
The Parachucla Formation (Porters Landing Member) in northern Florida is associated with the White Springs Local Fauna, consisting of fossils of 57 vertebrate species from the Arikareean stage of the late Oligocene epoch. The fossils of the White Springs Local Fauna have been collected from sites on an approximately 10 km long stretch along the Suwannee River where the Parachucla Formation is exposed. The local fauna includes a number of fossils of near-shore marine and land vertebrates, including 9 sharks, 5 rays, 14 bony fish, 6 reptiles, and 27 mammals. The reptiles include a marine crocodile, an alligator, a tortoise, and three snakes (a blind snake and two species of boas). The fossils of three sirenians (Crenatosiren olseni, Dioplotherium manigaulti and a Metaxytherium species) have been found at those sites (White Springs is the holotype site for C. olseni). Rodents in the White Springs Local Fauna include species of the eomyid Arikareeomys and the cricetid Leidymys genera, species the squirrels Protosciurus and Nototamias, and three species of geomyid rodents. Other small land mammals in the White Springs Local Fauna include a lagomorph (Palaeolagus or Megalagus), two bats, and a marsupial. Larger mammals include a horse (Miohippus or Anchippus texanus), an oreodont (Mesoredon floridensis), three different camelids (including a Oxydactylus species), and a small rhinocerus (possibly a species of Diceratherium).

Invertebrate fossils
The dinoflagellates Sumatradinium soucouyantiae, Chordosphaeridium cantharellus, and Exochosphaeridium insigne are typical of the Parachucla Formation in Georgia. Cribroperidinium species are also common.

Sixty-five taxa of invertebrates have been identified in the Porters Landing Member of the Parachucla Formation at the White Springs exposure in north Florida. Included in these are the bivalves Chlamys acanikos and Ostrea normalis and the barnacles Concavus crassostrictola, Balanus reflexus and a species of Solidobalanus.

Most of the land vertebrate fossils in the White Spring Local Fauna are teeth or post-cranial bones of smaller animals. Semi-articulated skeletons of larger mammals have been found at one location along the exposure. It is believed that the land vertebrate bones were carried into a delta or coastal lagoon by a river before burial.