LATER TERTIARY (MIOCENE AND PLIOCENE). 809 



appears to have been formed by the replacement of the calcareous portion of a sandy limestone 

 by silica which was probably derived from organic remains such as sponge spicules and diatoms. 



"The exposures of the limestone of the Jacksonville formation seldom exceed 5 or 6 feet in 

 thickness, but there is a maximum exposure of about 15 feet at a locality 2 miles above Middle- 

 burg, on Black Creek." 



The maximum thickness of the formation is indicated by well borings to be about 500 feet, 

 according to Matson and Clapp. Miocene strata have been identified in well borings as far south 

 as Key Vaca, at depths between 375 and 400 feet.^^^^ 



The Choctawhatchee marl is described by Matson and Clapp ^^"^ as follows : 



"The Choctawhatchee marl includes the 'Ecphora bed' and 'aluminous clay' of DaU. 

 It comprises a grayish sandy sheU marl and gray plastic sandy clay of Miocene age. It lies 

 stratigraphically between the Oligocene and Pliocene beds and contains characteristic species 

 of Miocene fossils. 



" The Choctawhatchee marl rests unconformably upon the Alum Bluff formation at Alum 

 Bluff, where the contact shows a wavy surface marked by shallow channels due to erosion. 

 At this locality there is also a sharp change from the coarse light-gray sands of the Alum Bluff 

 formation, which contain few fossils, to the bluish-gray shell marl of the Choctawhatchee, 

 with its abundant fauna. Several years ago Dr. Vaughan .noted similar evidence of an uncon- 

 formity between the Oligocene and Miocene at Jackson's Bluff, on the Ocklockonee River. 

 This fact is well shown by his section at that locality. * * * 



"The paleontologic evidence indicates a stratigraphic break between the Oligocene and 

 Miocene as is shown by the following quotation : " 



" 'As I have on various occasions insisted, the faunal gap between the uppermost Oligocene 

 (Oak Grove) and the Chesapeake (Choctawhatchee) or Miocene is the most sudden, emphatic, 

 and distinct in the whole post-Cretaceous history of our southeastern Tertiary and indicates 

 physical changes in the surrounding region, if not in Florida itself, sufficient to alter the course 

 of ocean currents and wholly change the temperature of the waters on our southern coast.' 



" Lithologically the two members of the Miocene in Florida are very unlike, both in charac- 

 ter of the material and state of aggregation. The Choctawhatchee marl varies from greenish 

 gray to light gray in color and consists of quartz sand containing very large admixtures of shells 

 and shell fragments and a smaller percentage of calcareous sand. In some parts of the forma- 

 tion the shells comprise a large percentage of the whole, and they are often in an excellent state 

 of preservation. Elsewhere the organic remains form a very subordinate part of the whole 

 or they may be entirely wanting. One phase of this formation is distinctly plastic and was 

 called 'aluminous clay' by DaU and Stanley-Brown. 



"When examined with a microscope the marl is found to consist of clear quartz sand, of 

 medium fineness, coated and partially cemented with calcium carbonate mixed with more or 

 less dark clay. The presence of calcium carbonate may be detected by the effervescence when 

 treated with dilute hydrochloric acid. There is also a certain amount of dark-colored clayey 

 material which appears in the form of a flocculent sediment in water. This material varies 

 from light gray to nearly black and is, without doubt, organic matter. In addition, hydrous 

 iron oxide may be detected, usually in the form of a coating about the sand grains or as a 

 stain along the cracks and on the exposed surfaces of the beds. It is this iron compound which 

 sometimes gives the exposed surfaces a rusty color. 



"The Choctawhatchee marl attains a thickness of over 50 feet in the vicinity of Redbay, 

 Walton County, where it is exposed in some small ravines and on the banks of Mill Creek; near 

 Holland post office, Leon County, its thickness exceeds 30 feet. However, from observations 

 elsewhere it appears probable that the average thickness is not more than 25 to 30 feet." 



West of Florida Miocene beds have been recognized in the well of the MobUe Oil Co., at 

 Mobile, at a depth of 700 feet, and probably continued to a depth of about 1,100 feet, where 

 upper Oligocene fossils representing the Alum Bluff formation were encountered. Beds of 



oDall, W. H., Trans. Wagner Free Inst., vol. 3, p. 1594. 



