GEOLOGY OF FLORIDA
ORIGIN AND PALEOGEOGRAPHI SIGNIFICANCE
OF FLOIDA'S PHOSPHORITE DEPOSITS
John Compton
Chapter 12
I. Origin and Paleogeographic Significance of Florida’s Phosphorite
Deposits (Chapter 12)
A. Introduction
1. P is a common element
at low levels
2. Apatite, the principal
mineral, is the tenth most abundant mineral and occurs in a rock types
3. There are practically
no phosphate deposits that contain 37% P2O5; most are in the 10-20% range
or lower (Florida is typically around 7% in the ground)
4. SEUS province contains
about 10 billion tones of resources and reserves
5. Florida’s economic reserves
are estimated at 1-4 billion tons
6. Preservation of deposits
depends on tectonic setting (includes burial), climate, sea level, and
oceanic circulation
7. Miocene was a time of
major climatic, tectonic, and oceanographic changes
8. Organic cycling of P
presented in book in only one model, and is highly disputed because of
the difficulty in explaining away all the organic matter ( Redfield C:P
ratio is about 60:1). Where is it now? How did it get there?
9. This model links the
global carbon cycle with their model of phosphate deposition (high stands
with C&P deposition and low stands with oxidation of C and reworking
of P)
10. Total model based on
one core from southwest Florida
11. There are several other
models that are discussed in Bentor’s SEPM volume on Phosphorites
12. Recent work on the Sr
dating of the West Florida Shelf phoshorites has shown that Sr isotope
ratios may be changed after deposition
13. Francolite is the main
phosphate mineral in sedimentary phosphate rocks
B. Phosphorite in the Hawthorn Group of Florida
1. Environments
a) High and low energies
b) On-shore and on-shelf
c) All in between, but generally shallow water
d) Phosphate and siliciclastics seem to be antipathetic
2. Lithology
a) Sand
b) Limestone
c) Dolomite
d) Silicified units (porcellanites and opalline); chert
e) Mixtures
3. Mineralogy
a) Francolite
b) Weathered phosphates (crandallite and wavellite)
c) Palygorskite/sepiolite (inosilicates-double chain like pyroxenes)
d) Smectites
e) Kaolinite
f) Non-stochiometric calcite (High Mg)
g) Non-stochiometric dolomite (Hi Ca)
h) Opal ( A and CT)
i) Zeolite (clinoptilolite??)
j) Feldspars (K-spar microcline and orthoclase)
k) Organic matter (two or more types)
4. Age is not well constrained
a) Base is Latest Oligocene based on microfossils
b) Top is Early Pliocene based on Bone Valley fossils
c) Phosphogenesis seems to be early to mid-Middle Miocene
5. Structural setting
a) Paleotopographic highs and lows
(1) Highs
(a) Ocala
(b) Brevard
(c) Sanford
(d) St. Johns Platform
(2) Lows
(a) Jacksonville Basin (SE Georgia Embayment)
(b) Florida Platform
(c) Okeechobee Basin
b) Particle size
(1) Coarser sizes (pebbles of land pebble phosphate) may include dolomite
with phosphate
(2) Sand sizes are mostly phosphate and quartz
(3) Calcite occurs in coarse silt fraction
c) Weathering
(1) Beneficial vs. non-beneficial
(2) Depends on the environment (CO3 Vs non-CO3)
(3) Clay minerals also altered
(4) Uranium may be redistributed
(5) May release radon
6. Origin of the Phosphorite
a) Grain types
(1) Organic grains
(a) Bone and skeletal fragments
(b) Fecal pellets
(c) Replacement textures (molds and casts)
(2) Inorganic grains
(a) Cobbles and pebbles
(b) Intraclasts (lithic fragments)
(c) Peloids
(d) Silt sized
(e) Clay sized
b) Origin of grains
(1) Fecal
(2) Rip up clasts
(3) Pore fillings
(4) Replacements
7. Isotopic Evidence
a) C and S signatures indicate formation near the sediment –water interface.
b) Also may be related to Eh
c) C vs. O isotopes depends on alteration of the apatite; C may be more
mobile than generally thought
Question: How and why would the O and C isotopes be altered and the Sr/Sr
not be???
8. Physical Reworking of
the Phosphorites
a) Evidence of reworking
(1) Peloids (thought to be derived from microsphorite which does not form
in this model); it is hard to collect and concentrate disseminated pellets
of the current C-P model
(2) Intraclasts (form from previous phosphorite and cemented rocks)
(3) Polishing?????
(4) Phosphate crusts
(5) Hard grounds
9. Chemical Alteration of
Phosphorite
a) Sulfate loss
b) Decarbonation
c) Color changes (real or not)
II. Age of the Phosphorite
A. 87Sr/86Sr—assumes no “resetting” which may not
be the case
B. sediment age based on forams
C. results
1. phosphate pellets are
(10-22 My) older than included forams (5-15 My), sometimes by millions
of years in the same samples
2. may vary systematically
in the same core
III. Paleoceanographic Significance of Phosphogenic Episodes on the
Florida Platform
A. Hypothesis based on the C-P cycling model
1. Rising sea level is associated
with phosphate deposition and a positive 13C shift of about 1/mil
2. Falling sea level is
associated with reworking and a negative 13C shift
B. Reworking explains the inclusion of older phosphatic
peloids (15-17 My) in younger calcareous sediment (5 My)
C. Correlation with global C cycle is far from perfect.
Many “ifs and buts” seem to be needed.
D. Carbon cycle shifts depend on which is the cause
and which is the effect used in making the interpretation.
E. Global cooling seems to correlate with late Paleozoic
and Paleogene phosphogenesis at least to a first approximation.