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U.S. Department of the InteriorU.S. Geological Survey
Examples of MCUI and MUCIII In North-Central Florida and South-Central
Georgia
L.J. WilliamsU.S. Geological Survey
3/15/2012
Middle Confining Units (Miller, 1986)• 8 subregional middle
confining units• The MCUs 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
See full presentation about the MCUs at: http://fl.water.usgs.gov/FASWAM/presentations.html
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
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
The Problem
• MCUI, II, and III all lie in approximately the same stratigraphic position
• Abrupt termination of the units causes “cliffs” in the surfaces
• Lumped units across a fairly thick middle Eocene section sometimes causes confusion when mapping them
III
here
MCUII II
Investigating MCUI and MCUIII in this area
• We are working with nearly the same set of logs and well cutting descriptions that Jim Miller used to construct the original framework
• FGS litho database has been exhausted; no returns over the zones of interest is common
• Some deep exploratory test wells in Georgia and SJRWMD help us to some degree in unraveling the mystery behind these units but we are still lacking data
Key Well Locations in Georgia
GA-COQ2(corehole)
Previously mapped extent of MCUIII
GULF TROUGH
We find “MCUIII” type section in
these two wells, thus extending the
unit to east
USGS Test WellR. E. KrauseOFR-78-117
USGS Test WellS.E. Matthews and R.E. KrauseWRIR 83-4204
GA-WA2: Ware County Test Well Showing MCUI and MCUIII
Focused (guard)ResistivityGamma Ray
MCU I
Logs from files of the USGS
Denotes a geophysical marker bed
Upper unit is Miller’s MCUI
MCU III
My marked up log for GA-WA2 (MP=150’)
• MCUI: easily identified on elog as a highly-resistive zone on elogs; it is a low-porosity dolomitic LS or dolostone; variably fractured; overlain by highly permeable Ocala LS
• MCUIII: less-resistive “spikey” on elog, LS, Dolostone, gypsum– “Gyp Pattern” used for
mapping lower unit
Gyp Pattern
Highly resistive
unit
BASE OF SYSTEM
TOP OF SYSTEM
Normals and Guard Log DT
CAL/ GR
Highly Permeable
MCUI
MCUIII
GA-WA2 (cont’)
• This unit has much significance in coastal areas
• Low-sulfate water above
• High-sulfate water below
Like“MCUIII”
U.S. Gypsum Core hole – Colquitt Co.
• 800 ft deep core hole completed by U.S. Gypsum
• Highly- resistive gypsiferous dolomitic LS interval
• This is underlain by a lower-resistivity “spikey” MCUIII interval
MCUIII
Highly Permeable
Key Well Locations in Georgia
GA-COQ2(corehole)
Previously mapped extent of MCUIII
GULF TROUGH
We find “MCUIII” type section in
these two wells, thus extending the
unit to east
USGS Test WellR. E. KrauseOFR-78-117
USGS Test WellS.E. Matthews and R.E. KrauseWRIR 83-4204
GGS3115
GGS3115(MP=212’)
MCUIII
UPPER FLORIDAN AQUIFER
LOWER FLORIDAN AQUIFER
Less Permeable
(see USGS test well 19E043 in
Valdosta)
Highly Permeable
Lowndes Co.
GGS107(MP=238’)
No middle confining unit was previously mapped here but we clearly find
MCUIII present
Like“MCUIII”
UPPER FLORIDAN AQUIFER
LOWER FLORIDAN AQUIFER
BASE OF SYSTEM
TOP OF SYSTEM
Atkinson Co.
Comparisons
MCUIIIIIU.S. GYPSUM CORE HOLE, COLQUITT CO.
USGS TEST WELL GA-WA2, WARE CO.
MCUI
Key Well Locations Florida
KKKK
FFFF
JJ (old)
FF (old)
AA (o
ld)
ZZ
P1033
P1052
P1015
YY P59
Previously mapped extent of MCUIII
Base of Floridan Aquifer System
Hydrogeologic units generally thin across the
structural high and subcrop at
shallower depths exposing them to
greater weathering and
dissolution
Note: this is a draft map of the base of system
Ocala “uplift”
P1033
P1033 (MP=21’)
BASE OF SYSTEM
TOP OF SYSTEM
“MCUII Horizon”
“MCUI Horizon”
MCUIII
Note : measuring point is from NGVD29
This part of the Middle Eocene section is low-
permeability and gypsiferous in
Valdosta but here no evaps. Indicated from well cuttings
This part of MCUIII is much deeper in the
section than seen in Valdosta
AVON PARK
OLDSMAR
P1033 (MP=21’)
Classic“Gyp”
Pattern
BASE OF SYSTEM
TOP OF SYSTEM
“MCUII Horizon”
“MCUI Horizon”(absent)
MCUIII
Madison Co.
P1015 (MP=10’)
UPPER GAMMA RAY MARKER
LOWERGAMMA RAY
MARKER
INDUCTION DTSP AND
GAMMA RAY
MCUIII
UPPER FLORIDAN AQUIFER
LOWER FLORIDAN AQUIFER
“MCUII Horizon”
“MCUI Horizon”
Taylor Co.
Key Well Locations Florida
KKKK
FFFF
JJ (old)
FF (old)
AA (o
ld)
ZZ
P1033
P1052
P1015
YY P59
Previously mapped extent of MCUIII
P1052 (MP= 82’)
INDUCTION DTSP AND
GAMMA RAY
MCUIII
Note: This pick south of previously mapped extent of MCUIII Lafayette Co.
Glauconite Gamma-Ray Marker (see next slide)
Glauconite Marker as a Tool to Help Guide placement of MCU’s
Northern Extent of mappable Glauconite Gamma-Ray Marker
Reese and Richardson’s northern extent
Marker first mapped by Duncan Et. Al. in Brevard and then expanded across central southern Peninsular Florida by Reese and Richardson
Lets look Even Further South at P66
P1052
P66
P1 and FL-DIX4(litho descrips + old elogs)
-500
-600
-200
-300
-400
MCUI
MCUII
P66(MP=14’)
MCUII
UPPER FLORIDAN AQUIFER
LOWER FLORIDAN AQUIFER BASE OF SYSTEM
TOP OF SYSTEM
??What is this
below MCUII?
P62 (cont’)
MCUII
Like “MCUIII”
Further south these rocks form a major confining sequence in the Lower
Floridan aquifer and are represented in the revised
framework
BASE OF SYSTEM
TOP OF SYSTEM
Lower Floridan below MCUII
Unfortunately, gamma-ray log not available to confirm the pick of MCUIII below MCUII
but the section only thickens slightly and the pick can be
supported by way of correlation
Key Well Locations Florida
KKKK
FFFF
JJ (old)
FF (old)
AA (o
ld)
ZZ
P1033
P1052
P1015
YY P59
Previously mapped extent of MCUIII
P59 (MP=134’ msl)
“MCUIII”
“MCUIB”
Permeable Zone
Permeable Zone
“MCUIA”(semi-confining)
Summary• In north-central Florida and south-central Georgia MCUI, MCUIII lie in
the middle and upper parts of the Avon Park Formation• MCUI is a “leaky” fairly massive dolomitic LS or dolostone characterized
by high resistivity on electric logs and low porosity on porosity logs; we find both high and low permeability zones associated with this unit; intergranular gypsum is common in south-central and eastern Georgia
• MCUIII is a highly gypsiferous unit typically mapped on a “spikey” resistivity pattern; this is a “lumped” geologic unit originally mapped in Valdosta where it starts at top of Avon Park, but is much lower in section in Florida; the upper part of MCUIII in Valdosta is stratigraphically equivalent to MCUII much further south
• We have a revision for this part of the system that will allow more uniform structural surfaces to incorporate the subregional low-permeability units into the models of the area