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