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The Geology of the Florida Land- Pebble Phosphate Deposits ~~cL)i,s~~ • • r.-slON,- V " ~~~-Y<:;
Trace Elements ln'Vestigations Report 265
UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY ( UNCLASSIFIED
Geology - Mineralogy ·
This document consists of 21 pages, Series A
UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY
THE GEOLOGY OF THE FLORIDA LAND-PEBBLE PHOSPHATE DEPOSITS*
By
J. B. Cathcart, L. V. Blade, D. F. Davidson, and K. B. Ketner
September 1952
Trace Elements Investigations Report 265
This preliminary report 's dis tributed without editori 1 and technical review for con ormity with official standards nd no It is not f r ub-
~~This report concerns work done on be f of the Division1 of Raw Materials of the U. S .. Atomic Energy Commission 2
USGS - TEI Report 265
GEOLOGY = MINERALOGY
Distribut ion (Series A) Noo of copies funerican Cyanamid Company~ Watertown 1
Argorme National Laboratoryo o 0· ()> o o " o o o o o o 1 Atomic Energy Commissionj Washington o o o o o o o 2 Carbide and Carbon Chemicals Company, Y-12 Areao l Colorado Raw Materials Office (Fo Ho MacPherson) 1 Division of Raw Materials:~ Grand Junctiono o o 1 Division of Raw Materialsj Grantso o o 1 Division of Raw Materials~ Denvero o o o o o o 1
Division of Raw Materials 2 Hot Springs 1 Division of Raw Materials~ New Yorko o o o o • o o o 6 Divis~on of Raw Materials, Salt Lake Cityo 1 Division of Raw Materials, Richfield o • 1
Division of Rat~ Materials, Butte o o o 1 Division of Raw Materials, Washingtono 3
Dow Chemical Company~ Pittsburgo o o " 1 Technical Inforifi?-tinn Service, Oak Ridge o 6 Ba:ttell.~_ . Memo'f'ial Institut9, ' Goluinbnso . 1
Tennessee Valley Authority, Wilson Dam o • cj " 1
Uo So Geological Survey: Mineral Deposits Branchj Washingtono o o o o • o • 2 Geochemistry and Petrology Branch, Washingtono 1 Geophysics Branch)) Washingtono ., o ., o o 1 Alaskan Geology Branch, Washington " " o 1 Fuels Branch, Washington-- o o o o o • o o 1 Foreign Geology Branch, Cleaves Rodgerso 0 0 1
V. Eo McKelvey, Washington o " o 1
Lo Ro Page, Denver ., 0' o o o o o 2 Ro Po Fischer, Grand Junction. 2 Ao Eo Weissenborn, Spokane o o 1 Jo B. CathcartJ> Plant City o o o o o 4
Jo Fo Smith 9 Jro 2 Denver o o 1 No Mo Denson 2 Denver o o " 0 0 0 1 Ro W• . Swanson, Spokane o o o 4 Lo So Gar.dner~ Albuquerque o 1 Ao Ho Koschmann, Denvero o o 1 Eo Ho Bailey, San Franciscoo o 1 Ao Fo Shride, Tucson o , o 1 David Gallagher; Joplino o o o l
Carl Dutton, liladison " o a " o o " o 1 Ro Ao Laurence, Knoxvil1eo o o o o " o 1
Ho .R" Gould.ll Sarasotao o o 0 0 0 0 1 Ro Jo RobertsJ Salt Lake Cityo o 1 Zo So Altschu1erJ Washington o o 1 TEPCO~ Washington 5 (Including master) "69 3
CONTENTS
Page
Abstract o o o o o o .. o o o o o o o o o o o o .o "o o o o o o o .. o o o o o o o o. o o o o o o o o o o o o o o o o o o 5
Int roduction.oooooooooooooooo ooooooooooooooooooooooo o ooooooooooo 6
General geology o o "o o o o o o o o o o o o o o o o o o .. o o o o o o o o o o o o o o o o o o o ., o o o o o o " 7
Geologic settingooooooooooooooooooooooooooooooooooooooooooo 7
Structureoooooooooooooo oooooooooooo.oooqooooooooooooooooooooo 7
GeologiC historyoooooooooooooooooooooooooooooooooooooooooooooooo 9
S t rat igraphy o o o o o o o o o o o ., o o o o o o o o o o o o o o " o o o o o .. o .. o o o o o o o " o o o o o o o " o 11
Hawthorn formationooooooooooooooooooooo 00000000000000000000 1 1
Bone Valley formationo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o "o o o 11
Terrace sands o • o o o o o o , o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o 13
Recent depositSoooooooooooooooooooooooooooooooooooooooooooo 14
The phosphat e depositSoooooooooooooooooooooooooooooooooooooo 14
Mat rixo o o o o o o o o o o o o o o o o o o o " o o " .... o o o o o o o • o o o o o o o o o o o o o o o o o o o 14
The leached zoneooooooooooooooooooooooooooooooooooooooooooo 15
Origin o o o o o o o o o o o o o .. o o " o o o o o o o • " o .. o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o 17
The residual hypothesiSoooooooooooooooooooooooooooooooooooo 17
The depositional hypothesiSooooooooo.ooooooooooooooooooooooo 19
Cqn.clusion. o o o o o o o " o • o o o • o o o o o" o .. o o " o o o o o o o o o o o o o o o o o o o o o o o 20
Bibliog.raphy o o o o o" o o o o. o o o o o o o o o .. o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o.. 21 4
ILLUSTRATIONS Page
Fi gure lo Location of land-pebble phosphate field, Floridao•••••o 8
2o Profiles across the northern part of the land-pebble phosphate field, Fl oridaooooooooooooooooooooooooooooo 10
3. Correlation of stratigraphy, mineralogy, and local terminology land-pebble phosphatefield, Floridao••••• 12 4. Variation in ratio of coarse (.J lo19 mm) and fine (-lol9 f Ool05 mm) phosphate, in the land-pebble phosphate field, Floridaooooooooooooooooooooooooooooo 16 THE GIDLOGY OF THE FLORIDA LAND-PEBBLE PHOSPHATE DEPOSITS
by
Jo B~ Cathcart, L. Vo Blade, Do Fo Davidson, a~d Ko Bo Ketner
ABSTRACT
The land~pabble phosphate district is on the Gulf Coastal Plain of
Floridao The phosphate deposits are in the Bone Valley formation, dated
Pliocene by most writerso These strata overlie the Miocene Hawthorn fo.:r:""' . : . ;.. .~ ... mation and are overlain by unconsolidated sands 3 to 20 feet thicko
The minable phosphate deposits, called "matrix" in the district, range from a featheredge to about 50 feet in thickness and consist of phosphatic pellets and nodules, quartz_ sand, and montmorillonitie clay in about equal proportionso Locally the matrix displays cross-bedding and horizontal laminations, but elsewhere it is structureless. The phosphorite particles, composed largely of carbonate-fluorapatite, range in diameter from less than
Of)l mm to about 60 em and in P.205 ·cori.tAilt from 30 to 36 percento Coarse pebble deposits, containing 30 to 34percent P205 are found mainly on base ment highs; and fine-pebble deposits, containing 32 to 36 percent P2o5, are found in basement lows. Deposits in the northern part of the field contain more phosphate particles and their P205 content is higher than those in the· southern parto
The upper part of the phosphatic strata is leached to an advanced degree and consists of quartz sand and clay-sized particles of pseudowavellite and wavellite. The leached zone ranges in thickness from a featheredge to 60 feeto 6
The origin of the land-pebble deposits is incompletely knowno Possible modes of origin are a residuum of Miocene age, or a reworked residuum of
Pliocene or Quaternary age.
INTRODUCTION
The land-pebble phosphate district of Florida is an irregular-shaped area between 27015' and 28015' North latitude and 81°30' and 82°30' West longitude (figol). Mining of the deposits started in 1888j and production has increased stead.ily; in 1951 more than 7 million long tons of phosphate rock was producedo Because about two-thirds of the 185 million tons of phosphate rock produced in the United States has been from the Florida land pebble district, it has been one of the most productive in the worldo
Since 1947 the U. S. Geological Survey has maintained a field party in the district to study the geology in d~tail--to study the distribution, composition, and origin of the phosphate deposits, to determine the re lations of the deposits to the geologic formations of the district, and to revise and bring up to date the estimates of tonnage and gradeo This paper summarizes available information on the origin and distribution of the deposits.
Such a project in an active mining district can not succeed without the wholehearted cooperation of the mine owners and operators of the districto
The writers are happy to acknowledge such cooperation by the active mining companies in the land-pebble district: American Agricultural Chemical Coo,
American Cyanamid Co., Coronet Phosphate Co., Davison Chemical Corpo, International Minerals & Chemical Corpo, Swift & Co., and Virginia-Carolina
Chemical Corp., Uo S. Phosphoric Products Coo, and the Royster Guano Co., who are principally processors of phosphate rock, have furnished information, 7 · as has Mro Wayne Thomas, an independent consultant on Florida phosphate lando
Since 1947, 21 geologists have helped in the investigations of the
Florida phosphat~ deposits, and each of these men contributed to the present
knowledge of the depositso Most of the ideas expressed in this paper have been expressed in frequent discussions among the party members; thus the
writers can be credited only with presenting them hereo This work was done
on behalf of the Division of Raw Materials of the Uo So Atomic Energy, Commissiono
GENERAL GEOLOGY
Geologic setting
The land-pebble phosphate district (figol) is on the east Gulf Coastal
Plaino King (1951) points out, however, that the geology of peninsular Florida differs considerably from that of the remainder of the eastern
Coastal Plain, for peninsular Florida was a positive, relatively stable area
dur~ng a large part of Mesozoic and Genozoic timeo Because of its distance from the main part of the North American Continent, it received relatively
small amounts of land-derived sediments; and because of the Ocala uplift,
the usual gentle seaward dip of the coastal plain formations is not maintainedo The rocks in the land-pebble phosphate district are divided into four
formations by most writerso They are the middle Miocene Hawthorn formation,
which underlies most of the district, the Pliocene Bone Valley formationjl Pleistocene terrace sands, and Recent deposits (Cooke, 1945; MacNeil, 1950)o
Structure
According to published data (Jordan, 1950; Vernon, 1951)1 formations ~------.------,-----,------~--31'
~-=d-,~;ip!~L¥!-+_:__----,;;.....:._:.__.:__If--~r---1-a'
fi ~urt 2 '-,\, ~ INDIAN RIVER
------t--- -- __ ...._ __~ ;' t--r------\ ~~K£ECH08£[ 1 '\\ \ ST.LUCif HIGHLAHDS ' -'"" ~ ------
27' 63'
HENDRY
1 r------L_~~----CO_L_L_·~_-; -_--_--f- -'+i -~--=--~~~:~~~=~=~D=--=- -==~~~~ 82'
Sta tute Mil es 10 20 30 4 0
L______,______-.e~o:O• u• ,p· ;, -·
LOCATION OF LAND -PEBBLE PHOSPHATE FIELD, FLORIDA. 9
older than the Hawthorn formation in the land-pebble district dip gently to the south. The Hawthorn formation in the land-pebble phosphate district is wedge~shaped 9 thinning from several hundred feet in the southern part of the district to a featheredge in the northern parto
The Bone Valley formation blankets the Hawthorn throughout the district
except along the Alafia and Peace ~ivers~ ~here the Hawthorn is exposed in the channelso Recent work by Blade and Ketner in one area near the Peace
River and another area farther west indicates that the Bone Valley formation thickens as the elevation of the present land surface increasesD Whether or not this relationship is general for the district had not yet been determined.
Quartz sands overlie the Bone Valley formation in most of the district 9 but their age and origin are in dispute. In general the structure of these sands and the nature of their contact with the Bone Valley are obscure.
A structural relationship that may have a bearing on the origin of the
Bone Valley formation is that topographic higps and lows of the present surface coincide in general with highs and lows on the limestone surface of the Hawthorn formation (fig. 2).
GEOLOGIC HISTORY
The writers' interpretation of the complete geologic history of the land= pebble phosphate district must await resolution of problems of the origin of the deposits. It may be said, however, that after the marine deposition of the middle Miocene Hawthorn formation there followed a period of erosion that may or may not have continued without interruption into the Pleistocene.
During part of the Pleistocene the district .was covered by the sea as 200['EET 0: 150...''" 100 75 so .. Profile alone Ll .. A-A' 0
-
IOO[FEETITO - ~ ' ...110 100 ~ ..7! ., ~~~~~=~--~ 0
Profile alone L1ne 8 - 8'
~ ooo[nn... 150 :;; ... c 100.. .. 0 Profile ••••• Ll•• c-c'
[)(PLANATION
Surface profile; dashed where inferred.
200[F£ET Minable phosphate in which the ratio of '" 150 D - coarse to fine posphot e is less than one. 125 100 75 ~=~~-;_--~~ Profile of the top of the Hawthorn 50 formation Miocene~dashed where inferred. " 0 Profile alone Line D-D'
6 5 • 4 3 2 0 6 MILE S t==:3._ - ~~=- ~ ~
PROFILES ACROSS THE NORTHERN PART OF THE LAND-PEBBLE PHOSPHATE FIELD, FLORIDA. ll recorded by marine terraces (Cooke, 1945; MacNeil 1950) o In Recent time the district has remained above the sea and has undergone onlY modifications produced by weathering and erosiono
STRATIGRAPHY
Stratigraphic relations, local terminology, and mineralogy are summarized in figure 3.
Hawthorn formation The Hawthorn formation is middle Miocene in age (MacNeil, 1947). Work by the Geological Survey fieid party suggests that in the land-pebble dis= trict the Hawthorn may be roughly divided into three lithologic units~ (1) lowermost phosphatic marls and hard limestone containing clay and quarts sands; (2) next a series of clastic beds composed of phosphatic: sands.:~ .: clay~j and sandy clays; and (3) uppermost~ dolomites and dolomitic limestones.
Because of pre.... Bone Valley tilting and erosion, the two middle and upper units occur only in the southern part of the district.
Phosphorite particles ranging from less than 0.1 mm to 10 em in diameter occur within the formation.
Bone Valley formation
The Bone Valley formation generally is said to be of Pliocene age although eviden'ce is not conclusive.
In Polk and Hillsborough Counties--the northern part of the land-pebble district--the Bone Valley formation is divided into a lower and an upper 0 ESCR I PTI ON APPROXIMATE MINERAL OCCURRENCE LOCAL NAMES
Terrace sands Nearly pure quartz sand. Overburden sand
Upper unit: Alumium phosphate Quartz sand and alum inum (wovel ite and pseudowavellite) Le11ched zone z phosphate . 0 (some cloy but no apatite 1- nodules. May be vesicu lar.) <~------4--~~~~~~~~--~~----+------~ ~ Wo::: Lower uni1: zo wLL. (.) o>-w Quartz sand, cloy, and .....J apatite nodules . Matrix .....J.....J (l.
Bed clay n Quartz sand, clay, Qpatite c nodules, and cdlcium CT c ::J carbonate . c Bed rock (upper part soft.) ~-
CORRELATION OF STRATIGRAPHY, MINERALOGY, AND LOCAL TERMINOLOGY, LAND-PEBBLE PHOSPHATE DISTRICT, FLORI OA 13
unito The lower unit, which locally is cross-bedded and horizontally lam= inated but elsewhere is structur'e.less is composed of poorly sorted clay,
silt~ sand~ and gravel. Almost all the gravel~ which includes pebbles as much as 60 em in diameter, and a large part of the sand and clay-sized mater ial in this unit are composed mainly of phosphate~ which has recently been identified as carbonate-fluorapatite by Ze So Altschuler and Eo A.
Cisney. The remainder of the sand is mainly qua~tz, with minor amounts of
feldspar and trace amounts of ilmenite9 zircon3 tourmalinej staurolite~ and other minerals; Altschuler and Co Eo Boudreau have identified the remainder
of the clay-sized material as chiefly montmorillonite~ although minor amounts of kaolin are present. They have identified the minerals in the upper unit of the Bone Valley fonnation as chief~ quartz sand, the aluminum phosphate minerals wavellite and pseudowavellite, and kaoliniteo
The lower unit, which ranges in thickness from less than a foot to more than 50 feet, is commonly about 20 feet thick; and the upper unit, which
ranges from a featheredge to about 60 feet in thickness~ is commonly about 6 feet thick. The two units tend to complement one another in thickness in some areas.
In the southern part of the district the Bone Valley formation cannot be divided into an upper and a lower unit on the basis of present informatione
The entire formation is thought to be homogeneous, consisting of fairly well sorted quartz and phosphate sand and sandy clay.
Terrace sands
Loose massive sand, in terraces commonly considered to be .of:.:Pleistocene age, overlies the Bone Valley formationo This sand ranges from 2 feet to 30 feet or more in thicknesso Although Cooke (i945) described seven terraces in peninsular Florida., MacNeil (1950) recognized only three in the land pebble district-one at 30 feet, one at 100 feet, and one at 150 feet above sea levelo
Recent deposits Thin, discontinuous sands, siltJ and muck at the surface are considered
Recent a
THE PHOSPHATE DEPOSITS
Matrix
Phosphate is being mined only in the northern part of the district from parts of the lower zone of the Bone V~lley formation, locally·tenned "matrix". The main f•ctors in determining the minable parts of the lower Bone Valley are~ (1) tonnage of recoverable phosphate per acre, which is determined both by thickness of the lower zone and by the proportion ?f phosphorite nodules within it; (2) P205 content of the phosphorite nodules; and (3) thickness of overburdeno Material now being mined generally consists of about equal parts of quartz sand, clay, and phosphorite noduleso The tonnage of recoverable phosphate ranges from about 500 to 35j)OOO tons per acre and is typically about 5,000 tons per acre. The P205 content of the phosphorite nodules ranges from 30 to 36 percent. In general this content varies inversely with nodule size, probably because the larger nodules have a greater proportion of calcium carbonate. Th~ content also generally increases upward within the lower unit of the Bone Valley. _:_ __ formation 9 probably as a result of removal or replacement of contained car~ bonate in the nodules in the upper part of the zoneo
Coarse phosphorite nodules containing 30 to 34 percent P205 predominate
in the north-central part of the district, whereas finer-grained phqsphorite
containing 32 to 36 percent P205 predominates in peripheral areas (figo 4)o
The grain size of nonphospha.tie constituents is more uniform over the·. dis~ ,
trict. In general fine .phosphorite particles are found on Hawthorn formation
lows and coarser phosphorite is found on Hawthorn formation highs. Although I ' •. I , the lower zone of the Bone Valley formation conforms to irregularities on the
surface of the limestone of the Hawthorn formation, as does the entire
sequence of unconsolida. ted beds, it generallY.. ::tends. :·to' £hicken -:·somewlllat ~'ov:er the major hills on the limestone (fig.2)e.
The leached zone
The upper part of the Bone Valley formation probably represents leached and weathered material originally somewhat similar to the lower zone immediately below. This clayey sand (locally indurated) has been fourid .~ i.:n : all but the southern part of the districto As has been stated, it·s thic.kness va.ries.9 as does its compositiono About 5 to 40 percent of the material occurs in particles smaller than Ool ~ The material greater than Ool mm
includes coarse to fine quartz sand 9 small amounts of tthea.vytt minerals-- including variable amounts of nodular apatite--and variable amounts of
aggregated wavellite and pseudowavellite. Kaolinite, wavellite 1 pseudo- wavellite~ apatite, and quartz make up the bulk of the material smaller
- . .,.,,'/-' • 0 Auburndale
0 Mu ~ berry
1.
SCAI.E I I 4 I I I 0 6 MILES l 1 l I I I I EXPLANATION ~ c v~l COARSE (+1.2mrn) =
VARIATION IN RATIO OF COARSE TO FINE PHOSPHATIC
PARTICLES IN- THE Fl:ORIDA LAND-PEBBLE Fl ELD 17
An iRdication of the v~riations in proportions of the above minerals is given by the Al20.3 and P205 contentso The A1203 ranges from less than 9o0 to more than .39 percent of the minus 0ol mm fractiono The P205 for the same fraction ranges from less than 2 to mGre than 28 percente
The minerals in the lower unit of the Bone Vrlley formation contain all the elements necessary to produce the minerals found in the upper unit o This permits the interpretation that the upper unit of the Bone Valley was probably formed in place by percolating ground waters from material similar to that in the lower unito
.• ;tif-r{#.?'J;:,;.t: . ORIGIN
The origin of th~ land-pebble deposits is incompletely knowno Possible modes of origin are: (1) a residuum of MioceDf age (hereinafter referred to ~s the residual hypothesis) and (2) a reworked residuum of Pliocene and/or Quaternary age (hereinafter referred to as the dep:>sitional hypothesis)o
The residual hypothesis
The residual hyPothesis of origin of the phosphate deposits in its extreme form ~ttempts to ~ecou~t f~r the entire sequence of unconsolidated material over- lying the limestone of the Hawthorn formation in the land-pebble district as the relatively insoluble re$id~e of the Hawtho~ that has been concentrated in place by weathering:~ This hypothesis is suggested by the following facts~ (1) With some exceptions--the bed-clay, for example, =-minerals present in the limestone persist upward in overlying strata in accordance with their insolubilities • ·• f I
(figo .3) o The calcium ca~'bonate content or the phosphorite nodules in much of the lower unit of the · &>;ne Valley formation decreases upward, whereas they increase upward in the less soluble apatite an:i iron and aluminuinJloxide contentso The bed-clay might be analo~ous to a clay hardpan, formed ·• by (4) Dist~ict-wide cross sections show the unconsolidated material above the limestone conforming in general to the irregularities of the limestone surface rather than tending to fill depressions~ Actuallyj the highest hills on the limestone surface are generally overlain by zones that are somewhat thicker than usual.tJ as is common in residual depQs:itse (5) Paleontologic evidence in dicating different ages for the different zones is open to more than one inter pretationo The/ residual hypothesis requires that the entire sequence of zones be Miocene in ageo Paleontologist ·express uncertainty about the -identification and corr-elation of many of the fossils because of their fragmentary nature or non-diagnostic character~ Simpson (1930), reviewing the evidence afforded by land mammals in th~ Bone Valley formation says: "It indicates approximate equivalence with the Upper Snake Creek or the Republica~ Rivero With t -his ~el ative age .! established, -: it is of litt.le ~ on s equence whether it be called late Miocene or early Plioceneo At present the consensus seems to be that the Re~ publican 'River and eouiva.lents are referable to the Lower Pliocene c 11 19 ·Kello,gg (1924), on the other hand~ says that certain marine mammals found in the Bone Valley formation are clearly earlier than Plioceneo Most fdss:\.ls used in dating the Bone Valley formation were collected by laymen in the prQ cess of miningo The few found in place by geolog-ists have' not settled the question of ageo The depositional hypothesis The depositional hypothesis was set forth in detail b.r Sellards (1915) and has been generally acceptedo It postulates erosion of the limestone of the Hawthorn formation at the end of Miocene time, deposition in Pliocene time of the lower unit of the Bone Vall-ey forination in a shallow marine near- shore environment, the upper unit of the Bone Valley in a marine offshore environment 9 subsequent erosion, and finally, deposition in Pleistocene tim~ of surficial terrace sandso The following facts support this hypothesis~ (1) Vertebrate fqssils indicate different ages for different formationso Tpe HaWthorn formation has been dated middle Miocene on the basis of abundant ip vertebrate remainso Vertebrate remains support this ~geo The following fo~- ~ sils have all been collected from the lower unit of the Bofte Valley formation (matrix) and have been identified by Jean Hough and Co Lo Gazin, of 'the Uo S~ National Museumo The fossils dated Pliocene and Quaternar.y have, witp one exception, been found in the central area of the northern part of the land-pebble district, whereas the Miocene fossils were found in the peripheral areas~ Teleoceras proterus Pliocene 2 . i -~- .·. ' Na~~ppus ingenuus lower Pliocene 3 Nannippus minor lower Pliocene 49 5 Neohipparion pho~phorum lower Pliocene 4, 5 . .., - [; -~ ~ Tayassuid of Desmathyus lower to middle Pliocene(?) .M&tthew possibly Miocene 6 ? Aphelops middle Mio~ene 7 and 6 Mer,ychipp~s westoni middle lij.ocene 6 ~ . Bison Quatern~ry West bf 3 Ant~locaprid, generically 1Jpper Tertiary 6 undeto (2) Conglomerates and irregular contacts .immediately above and below the Bone Valley formation are evidence of unconfor.mitieso (3) There is a rather distinct bedding, including graded and cross-bedded forms~ in places in the lower and upper units of the Bone Valley formationo "Such bedding is not likely to have been preserved as relics of bedding of th~ Hawthorn formation in a residual deposit because of slumpingo Conclusion A combination of the two working hypotheses might best explain the origin of the Bone Valle.y formationo In the north-central part of the district the . . depositional hypothesis is applicable, whereas in the periphery the residual hypothesis is more suitableo However~ excellent bedding is found in a few places in the periphery o The lower part of the lower unit of the Bone Valley formation may be residual in places, whereas the upper part and all overlying strata may be transportedo Although the fact that the general sequence of strata. illustrated in figure .3 is nearly universal in ~P,e distr~ct, militates :, - - :~ ; ~ 1 ~ . ~ : • ; 21 against the idea of diverse modes of originjl it is possible that the whole district was more or l~ss uniformly zoned by ~ubssquent intense weathering .. . Evidence is lacking on which to establis~ either hypothesis as a fact at any specific place because many field relations are subject to two or more interpretationso BIBLIOGRAPHY Cooke 1 C.. W.,j 1945, Geology of Florida~ Florida Geolo Survey Bullo 29j 339 PPo Jordanjl Ro Ho, 1950, An interpretation of Floridian karst~ Jouro GeologyJ volo 58, noo 3 .. PPo 261-268o Kellogg, Ao RoJ 1924, Tertiar.y pelagic mammals of eastern North America: Geolo Soc., America Bullo volo 35, nO., 4, ppo 755-766o . Kingjl R~ B.. , 1951, The tectonics of Middle Ro:rth America: Middle North America east of the Cordilleran system: Princeton University Press, 203 ppo MacNeil, F .. So, 1947, Correlation chart for the outcropping Tertiary formations of the eastern Gulf region: Uo So Geolo Surveyj Oil and Gas Invso Prelim., Chart 29o ------, 1950, (1951), Pleistocene shore lines in Florida and Georgia~ u~ So Geolo Survey Prof., paper 221-F, ppo 95-107o Mansfieldjl Go Ro, 1942, Phosphate resources of Florida: Uo So Geolo Survey Bullo 934, iv, 82 po, 8 plsojl 1 figo inclo ind~x mapso Sellards, Eo Ho, 1915, The pebble phosphates of Florida~ Florida Geolo Survey Anno Repto 7, PPo 25-116o Simpson, Go Go, 1930, Tertiary land mammals of Florida: . Amo Mus o Nato History · Bulla volo 59, PPo 149-2llo Vernon, Ro Oo, 1951, Geology of Citrus and Levy Counties, Florida~ Florida Geol., Survey Bullo 33, 246 PPo