U.S. Department of the Interior U.S. Geological Survey Examples of MCUI and MUCIII In North-Central Florida and South- Central Georgia L.J. Williams U.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:

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 MCUI I 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 Well R. E. Krause OFR USGS Test Well S.E. Matthews and R.E. Krause WRIR

GA-WA2: Ware County Test Well Showing MCUI and MCUIII Focused (guard) Resistivity Gamma 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 Well R. E. Krause OFR USGS Test Well S.E. Matthews and R.E. Krause WRIR 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 MCUIIIII U.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 (old) 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 LOWER GAMMA RAY MARKER INDUCTIONDT SP 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 (old) ZZ P1033 P1052 P1015 YY P59 Previously mapped extent of MCUIII

P1052 (MP= 82’) INDUCTIONDT SP 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) 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 (old) ZZ P1033 P1052 P1015 YY P59 Previously mapped extent of MCUIII

P59 (MP=134’ msl) “MCUIII” “MCUIB” 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