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Fort Thompson Formation
Definition. --The alternating fresh-water and marine marls and limestones exposed at Fort Thompson were initially named the Fort Thompson beds by Sellards (1919, p. 71-72). Cooke and Mossom (1929, p. 211-215) later named this sequence the Fort Thompson formation and indicated that the beds lie unconformably on the Caloosahatchee marl and are overlain by the Lake Flirt marl of Pleistocene and Recent age.
Development. --The Fort Thompson formation at the type locality is about 6 feet thick. In the Miami area it attains a maximum thickness of 80 feet and constitutes the major part of the Biscayne aquifer as described by Parker (1951, p. 820-823). The southern 18 miles of the line of test wells is approximately the western boundary of the Biscayne aquifer. In this area the Fort Thompson formation ranges from 3 to 9 feet in thickness. The strata of Pleistocene age between wells 10 and 33 possibly are transitional beds between the Fort Thompson and Anastasia formations.
Lithology. --The Fort Thompson formation is composed of sand, marl, shell marl, sandstone, and limestone of fresh-water and marine origin. Marl and sand are the predominant constituents along the line of test wells. The occurrence of limestone in the Fort Thompson and Tamiami formations appears to be related to fluctuations of the water table accompanied by cementation with calcium carbonate.
Age. --Parker and Cooke (1944, p. 94-96) correlated the beds at old Fort Thompson with the inferred fluctuations of sea level during the Pleistocene epoch. Fresh-water beds have not been reported in the Pliocene of the Atlantic Coastal Plain, and they do not occur in the Caloosahatchee marl (Pliocene) in the outcrop area, although fresh-water shells are found, in places, mixed with the marine forms. Any sequence of marine and fresh-water beds, or fresh-water beds, older than the Lake Flirt marl is considered as representing the Fort Thompson formation.
Definition. --The Anastasia formation was named by Sellards (1912) from outcrops of coquina on Anastasia Island, near St. Augustine, Fla. Cooke and Mossom (1929, p. 199) expanded this definition to include all the marine deposits of Pleistocene age underlying the lowest plain bordering the east coast of Florida, excluding the Key Largo limestone and the Miami oolite. Parker and Cooke (1944, p. 66) defined the formation as follows: "The Anastasia formation as here defined includes the coquina, sand, sandy limestone, and shelly marl of pre-Pamlico Pleistocene age that lies along both the Florida east and west coasts."
Development. --The pre-Pamlico deposits at the north and south ends of the line of test wells are definitely assigned to the Fort Thompson formation. The deposits of Pleistocene age between wells 10 and 33 have been questionably identified as the Fort Thompson formation. Thin marine sandstones of the Anastasia formation, which are present along the southwest coast, extend as a tongue into Collier and Hendry Counties. In northeast Collier County and southeast Hendry County this marine sandstone has been found within 4 to 6 miles of the line of test wells. The strata of Pleistocene age between wells 10 and 33, tentatively assigned to the Fort Thompson formation, apparently are transitional between the Fort Thompson and Anastasia formations.
Lithology. --The typical coquina of the Anastasia formation in the type locality does not occur in the western part of southern Florida. Sand, shell beds, marl, and calcareous sandstone are the most common materials.
Age. --Fossil evidence is not adequate for determining the age of the materials in the test wells that may be Anastasia but are assigned to the Fort Thompson. The geologic cross sections, however, suggest that the deposits are of Pleistocene age. Elsewhere in southern Florida, molluscan faunas establish a Pleistocene age for the Anastasia formation.
Definition. --The Pamlico sand was extended from the typical locality in North Carolina by Parker and Cooke (1944, p. 74-75). They include in it all the marine deposits of Pleistocene a g e younger than the Anastasia formation. These deposits are referable to terrace materials deposited during a +25-foot stand of the sea during the Pleistocene.
Development. --The Pamlico sand occurs along the test-well line only in the sandy flatlands of Hendry County, where its maximum thickness is about 9 feet.
Lithology. --The Pamlico sand is generally gray or brown. It is composed of quartz.
Age. --The sand that is referred to the Pamlico in southern Florida lies unconformably upon the Miami oolite and Fort Thompson and Anastasia formations, all of Pleistocene age, and upon the Caloosahatchee marl of Pliocene age and the Tamiami formation of late Miocene age. The Lake Flirt marl and deposits of Recent age of peat and muck overlie the Pamlico sand. Cooke (1952, p. 43) refers the Pamlico to a marine shoreline at 25 feet above sea level, which he (1952, p. 51) correlates with the third interglacial stage (Sangamon).
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:03 PM(KP)
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