U.S. Department of the Interior U.S. Geological Survey The Middle Confining Units of the Floridan Aquifer System Lester J. Williams 4/21/2011
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U.S. Department of the InteriorU.S. Geological Survey
The Middle Confining Units of the Floridan Aquifer System
Lester J. Williams
4/21/2011
Outline
Describe characteristics of the 8 middle confining units mapped as part of the Floridan aquifer system Extent of each unit Geologic Unit(s) and Lithology Confining properties
Configuration and Thickness Geophysical Log Characteristics Mapping Criteria
Middle Confining Units (Miller, 1986)
8 subregional middle confining units
Divide the system into the Upper and Lower Floridan aquifers
These are named using Roman numerals I to VIII (not 1, 2, 3 ..)
The MCUs are a composite of one or more geologic units
Units overlap in some areas
Middle Confining Unit I
Low-permeability zone mostly within rocks of middle Eocene age extending from South Carolina to Florida Keys
Separates the Upper and Lower Floridan aquifers
Overlaps gypsiferous dolomite of MCU II
MCU I: Geologic Units and Lithology Mostly located in the
middle and upper parts the middle Eocene
Locally lower part of the Late Eocene
Soft micritic limestone and fine-grained dolomitic limestone, both of low porosity
Original porosity not greatly affected by pore filling minerals
mostly here
Very local
MCU I: Confining Properties
Leakiest of all MCUs Lithology similar to
rocks above and below
Minor head differences and water-quality changes suggests acts as a confining bed
Leak-o-meter
MCU I: Configuration and Thickness
Generally dips along stratigraphic horizons
Thickest in southeastern Georgia embayment and east-central Florida
Top of MCU Imodified from Miller, 1986
Focused (guard)ResistivityGamma Ray
MCU I: Geophysical Log Characteristics
High resistivity
Low porosity Multiple beds
may comprise this unit
Waycross, GA
MCU I Mapping Criteria
Lithology Geophysical log
response (low porosity beds)
Position in stratigraphic section
Top of unit is usually picked on the shallowest low-porosity bed in middle Eocene rocks Brunswick, GA
MCUI (Miller, 1986)
Middle Confining Unit II
Low-permeability gypsiferous dolomite and dolomitic limestone
Overlapped by MCU I in part of central Florida
Extensive middle Eocene sabkha or tidal flat
MCU II: Geologic Units and Lithology
Located in the middle part of the middle Eocene
Dolomite and dolomitic limestone
Intergranular gypsum makes it a very low-permeability unit
here
MCU II: Confining Properties
Non-leaky confining bed
Mineralized water contained in the unit suggests poor connection with freshwater in the overlying Upper Floridan aquifer
Leak-o-meter
Configuration and Thickness Generally dips
along stratigraphic horizons
Thickest in the northeast and thins to east
(anomalous thickness in northern Polk)
Top of MCU IImodified from Miller, 1986
MCU II: Geophysical Log Characteristics Identified on elogs by its
“spiky” high and low resistivity zones
Has very Low porosity (<10%)
Citrus Co. FL
900
1300
SonicInduction
E-Log
Upper Floridan
Lower Floridan
MC
U II
Correlation
Oil and gas test well (Citrus Co.) on left
ROMP core hole (Sumter Co.) on right
These wells are 33 miles apart
MCU II Mapping Criteria
Lithology (evaporites) Geophysical log
response (resistivity and porosity logs)
Position in stratigraphic section
Top of unit is usually picked at first occurrence of persistent evaporites
MC
U II
Middle Confining Unit III
Low-permeability dense, fossiliferous, gypsiferous, dolomitic limestone
Occupies area in south-central Georgia and northern Florida (center on Valdosta)
MCU III: Geologic Units and Lithology
Located in the lower and middle part of the middle Eocene
Dolomitic limestone Intergranular
gypsum (rare layers and lenses of gypsum) here
MCU III: Confining Properties
Slightly leaky confining bed
Around edges underlain by permeable strata
Grades downward into low permeability clastic rocks that are part of the aquifer’s lower confining unit Leak-o-meter
Configuration and Thickness Blob shaped
does not conform to any stratigraphic unit
Thickest along northwest edge (near Gulf Trough)
Top of MCU IImodified from Miller, 1986
Gulf Trough
Middle Confining Unit IV (abbreviated) Calcareous sand
and clay of relatively lower permeability
Limited to a few counties in eastern Panhandle
MCU IV: Geologic Units and Lithology
Equivalent to Lisbon and Tallahatta
Dolomite and dolomitic limestone
Think of it like a tongue of low permeability rocks down dip from the permeable part of the Upper Floridan
here
MCU IV: Confining Properties
leaky confining unit Lower Floridan lies
below this unit
Leak-o-meter
Middle Confining Unit V (abbreviated)
Low permeability clay bed
Located in Florida’s western Panhandle
MCU V: Geologic Units and Lithology
Bucatunna Formation
Sandy massive, dark gray calcareous soft clay
Thickness more uniform than other MCUs
Readily identified on electric logs
here
MCU V: Confining Properties
confining unit Lower Floridan lies
below this unit
Leak-o-meter
Middle Confining Unit VI
Low-permeability argillaceous rocks of various lithologies
Extends below MCU II in the northern part of the unit
Overlapped by MCU I in the southeastern part of the unit
MCU VI: Geologic Units and Lithology
Located mainly in the lower part of the middle Eocene
Interbedded finely to coarsely crystalline dolomite and finely pelletal micritic limestone
mostly here
locally
MCU VI: Confining Properties
Considered an “effective” confining unit based on lithologic character
No hydraulic head data were available to confirm this
Leak-o-meter
Configuration and Thickness
Follows stratigraphic horizons in some places but not others
This surface represents a composite of several different geologic units
Thickens to southwest Top of MCU VI
modified from Miller, 1986
Thickens
Thin
MCU VI: Geophysical Log Characteristics Identified by its highly
resistive character Low porosity
Charlotte Co. FL
1900
2300
Sonic Elogs
APPZ
MC
U V
I
MCU VI Mapping Criteria
Lithology (dolomite) Geophysical log
response (resistivity and porosity logs)
Position in stratigraphic section
Top of unit is usually picked at high resistivity zones near the base of middle Eocene
1900
2300
APPZ
MC
U V
I
Middle Confining Unit VII (abbreviated) Narrow northeast
trending strip of low-permeability rocks in west-central Georgia
Contiguous and southeast of Gulf Trough
It is suggested that fault grabens have juxtaposed rocks into this position
MCU VII: Geologic Units and Lithology
Southwest boarder: middle Eocene
Central: middle to late Eocene
Micritic to finely crystalline limestone, dolomitized
Lenses, pods, beds of gypsum
here
MCU VII: Confining Properties
The southern part of this unit it grades vertically downward into calcareous glauconitic clastic rocks that are part of the lower confining unit
The northern part of this unit it is thin and underlain by permeable limestone
Leak-o-meter
Middle Confining Unit VIII
Thick regionally extensive confining unit within the Lower Floridan aquifer
Overlies and extends beyond the boulder zone
MCU VIII: Geologic Units and Lithology
Middle part of rocks of early Eocene age
Micritic finely pelletal limestone and lesser amounts of interbedded finely crystalline dolomite
here
MCU VIII: Confining Properties
Thick regionally extensive confining unit
Extensive hydraulic testing of this unit from deep well injection projects over the past 20+ years
Leak-o-meter
Configuration and Thickness In southern part
surface seems to be representing more of a horizontal “bed” over BZ
Several different geologic units comprise this unit
Thickest in Palm Beach Co, FL
Thins to north and south
Top of MCU VIIImodified from Miller, 1986
Thickest
ThinsT
hins
MCU VIII: Geophysical Log Characteristics In some logs it is a
very low resistivity unit, other logs very high resistivity unit
Dade Co. FL
2800
3000
ResistivitySP
Boulder Zone
MC
U V
I
Summary
8 MCUs were originally defined and mapped when constructing the RASA framework for the Floridan aquifer system
Each of the MCUs is mapped based on lithology, geophysical characteristics, and stratigraphic position
One or more geologic units comprise each unit, therefore some generalizations were made when mapping the tops of these units
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