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The Middle Confining Units of the Floridan Aquifer System

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

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The Middle Confining Units of the Floridan Aquifer System

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  1. The Middle Confining Units of the Floridan Aquifer System Lester J. Williams 4/21/2011

  2. 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

  3. 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

  4. 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

  5. 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 Very local mostly here

  6. 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

  7. MCU I: Configuration and Thickness Top of MCU I modified from Miller, 1986 • Generally dips along stratigraphic horizons • Thickest in southeastern Georgia embayment and east-central Florida

  8. MCU I: Geophysical Log Characteristics • High resistivity • Low porosity • Multiple beds may comprise this unit Focused (guard) Resistivity Gamma Ray Waycross, GA

  9. 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)

  10. 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

  11. 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

  12. 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

  13. Configuration and Thickness Top of MCU II modified from Miller, 1986 • Generally dips along stratigraphic horizons • Thickest in the northeast and thins to east • (anomalous thickness in northern Polk)

  14. MCU II: Geophysical Log Characteristics Induction E-Log Sonic • Identified on elogs by its “spiky” high and low resistivity zones • Has very Low porosity (<10%) Upper Floridan 900 MCU II 1300 Lower Floridan Citrus Co. FL

  15. Correlation • Oil and gas test well (Citrus Co.) on left • ROMP core hole (Sumter Co.) on right • These wells are 33 miles apart

  16. 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

  17. Middle Confining Unit III • Low-permeability dense, fossiliferous, gypsiferous, dolomitic limestone • Occupies area in south-central Georgia and northern Florida (center on Valdosta)

  18. 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

  19. 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

  20. Configuration and Thickness Top of MCU II modified from Miller, 1986 • Blob shaped does not conform to any stratigraphic unit • Thickest along northwest edge (near Gulf Trough) Gulf Trough

  21. Middle Confining Unit IV (abbreviated) • Calcareous sand and clay of relatively lower permeability • Limited to a few counties in eastern Panhandle

  22. 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

  23. MCU IV: Confining Properties • leaky confining unit • Lower Floridan lies below this unit Leak-o-meter

  24. Middle Confining Unit V (abbreviated) • Low permeability clay bed • Located in Florida’s western Panhandle

  25. 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

  26. MCU V: Confining Properties • confining unit • Lower Floridan lies below this unit Leak-o-meter

  27. 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

  28. 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

  29. 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

  30. 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 Thin Thickens Top of MCU VI modified from Miller, 1986

  31. MCU VI: Geophysical Log Characteristics Sonic Elogs • Identified by its highly resistive character • Low porosity APPZ 1900 MCU VI Charlotte Co. FL 2300

  32. 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 1900 MCU VI 2300

  33. 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

  34. 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

  35. 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

  36. Middle Confining Unit VIII • Thick regionally extensive confining unit within the Lower Floridan aquifer • Overlies and extends beyond the boulder zone

  37. 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

  38. 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

  39. 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 Thins Thickest Thins Top of MCU VIII modified from Miller, 1986

  40. MCU VIII: Geophysical Log Characteristics • In some logs it is a very low resistivity unit, other logs very high resistivity unit SP Resistivity 2800 MCU VI Boulder Zone Dade Co. FL 3000

  41. 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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