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

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 I modified from Miller, 1986

Focused (guard) Resistivity Gamma 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 II modified 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 Sonic Induction E-Log Upper Floridan Lower Floridan MCU 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 MCU 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 II modified 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 SonicElogs APPZ MCU VI

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 APPZ MCU VI

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 VIII modified from Miller, 1986 Thickest Thins

MCU VIII: Geophysical Log Characteristics  In some logs it is a very low resistivity unit, other logs very high resistivity unit Dade Co. FL Resistivity SP Boulder Zone MCU VI

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