16 Porosity and Sinkholes

16 Porosity and Sinkholes - The Floridan Aquifer The...

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Unformatted text preview: The Floridan Aquifer The Carbonate Deposition For approximately 125 million years the Florida Platform was dominated by carbonate deposition Late Jurassic 150 mya – 24 mya Carbonate Deposition/Sedimentation Marine Calcium and Magnesium Carbonate CaCO3 MgCO3 The Eocene and Oligocene Limestone The Eocene and Oligocene limestone forms the principal fresh water-bearing unit of the Floridan Aquifer, one of the most productive aquifer systems in the world Eocene: 55 – 34 million years ago Oligocene: 34 – 24 million years ago Eocene/Oligocene Limestone Older limestone Isolation of the Florida Peninsula sediments sediments Georgia Channel Suwannee Current (similar to Gulf Stream) Beginning 24 million years ago 1. Raising of the Florida platform 2. Lowering of sea levels, interruption of the Suwannee Current 3. Infilling of the Georgia Channel with sediments derived from Appalachian/continental erosion 4. Sea level rise, lack of Suwannee current. 5. Suspended siliciclastic sediments settle over the peninsula These sediments blanket the underlying limestone forming the upper confining layer for the Floridan Aquifer. The accumulated limestone forming the Florida Platform is overlain by silicon-based deposits of Miocene age. These are typically clayey and called The Hawthorne Formation. Miocene Clays (Hawthorne) Eocene/Oligocene Limestone (55 – 24 mya) Older limestone Overall Picture Surface Siliciclastics (sandy) (highly permeable) Unconfined aquifer is extensive throughout the state of Florida Low Permeability Confining Unit (poor water movement) Miocene Clays (low permeability) 55 – 24 million years ago The Floridan aquifer is a confined aquifer. The water-bearing unit is permeable limestone. Low permeability rock (confining) Low permeability rock The Floridan Aquifer The water-bearing unit of the Floridan Aquifer is a confined, consolidated limestone formation. The Water-bearing Unit is Consolidated Limestone However, it is extremely productive 3 - 4 billion gallons / day Calcium Carbonate CaCO3 Magnesium Carbonate MgCO3 limestone How does this material hold and deliver water? Carbonate Dissolution Acid (H+) dissolves calcium carbonate Carbonates are made porous by acid dissolution Rainfall is naturally acidic Carbon dioxide dissolved in water produces carbonic acid CO + H O = H CO (carbonic acid) 2 H2 CO =2 H3 + HCO > 2 3 Acid + 3- Acidity from rainfall reacts with CaCO3 and dissolves the carbonate rock. CO + H O = H CO H CO => H +2HCO 2 2 3 CaCO3 + H = +HCO3- +- Ca2+ 2 3 3 (solid) (acid) (solution) (solution) + Dissolution Cavities Dissolution Cave Antacids CaCO3 Stomach Acid Stomach acid H+ H+ H+ H+ H+ CaCO3 + H = HCO3- + Ca2+ + Neutralizes the acid (H+) Acid dissolves calcium carbonate Caves and Solution Cavities CaCO3 + H+ = HCO3- + Ca2+ Clayey Deposits Carbonates Channels and Caves Karst Topography Depressions Florida is Dominated by Karst Topography. Characterized by sinkholes, springs, depressions, lakes The thickness of sediments over limestone has a direct bearing on the evolution of karst landscape type Sediment Thickness Alachua County 30 – 200 ft clayey Thin or absent 30 – 200 ft sandy 30 – 200 ft clayey > 200 ft thick Overlying Material Limestone Extra Credit: 1. Rainfall is naturally acidic/basic 2. Landscapes characterized by sinkholes, springs and depressions 3. ____________ dissolves calcium carbonate Sinkholes: A fundamental Component Of Karst Landscapes Sinkhole formation depends on the material and the thickness of the material overlying the carbonate water-bearing unit The Miocene clayey siliciclastic sediments have undergone significant erosion and shaping during and since their deposition. This is due mainly to changes in sea level over millions of years Sediment thickness varies considerably from region to region within the state, ranging from 0 to 500 ft thick, depending on location What is a sinkhole? Sands Clays Limestone The type of sinkhole depends on the type and thickness of the material overlying limestone Sinkhole formation depends on the material overlying the carbonate water-bearing unit Very thick clays > 200ft. Thin, sandy covering Cohesive clays up to 200ft thick Thick sands up to 200 ft thick and some clays Miocene clays have been eroded and shaped throughout their history resulting in extreme variability in thickness across the state. Sinkhole Types Dissolution – chemical erosion of limestone at the surface. Cover Subsidence – gradual infill of overburden into a solution cavity. Cover Collapse – abrupt collapse of overburden Dissolution Sinkhole Thin Sand Carbonates Overlying material is thin and sandy Dissolution chemical erosion of limestone at the surface. Cover Subsidence Sands Carbonates Overlying material of mostly thick sands and perhaps some clays Cover Subsidence Granular sediments Granular spill into secondary openings in the underlying carbonate rocks. carbonate A column of column overlying sediments settles into the vacated spaces (a process termed "piping"). The slow downward erosion eventually forms small surface depressions 1 inch to several feet in depth and diameter. diameter. Dissolution and Dissolution infilling continue, forming a noticeable depression in the land surface. Overburden is generally sandy Cover Collapse Sands Clays Carbonates Thick layer of clay overlying carbonates Cover Collapse Sediments spill Sediments into a cavity. into As spilling As continues, the cohesive covering sediments form a structural arch. structural The cavity The migrates upward by progressive roof collapse. roof The cavity The eventually breaches the ground surface, creating sudden and dramatic sinkholes. Thick, Clayey overburden Cover Collapse sand clay limestone sand clay On May 9, 1981, a large sinkhole collapsed in Winter Park, Florida, swallowing a house, five Porsches at a luxury car dealership, and half of an Olympic-sized swimming pool. The sinkhole collapse occurred when carbonate bedrock had dissolved to the point that it could no longer support the weight of the overlying soil and sediment. The city of Winter Park stabilized and sealed the sinkhole, converting it into a 107-meter-wide (350-foot-wide) urban lake. Devil’s Millhopper (Cover Collapse) 53rd ave. west of 43rd St. The sinkhole is 120 feet deep and 500 feet across. Reported Sinkholes Since 1954 Very thick clays > 200ft. Thin, sandy covering (Solution sinkholes) Cohesive clays up to 200ft (Cover Collapse sinkholes) Thicker sands and some clays (subsidence sinkholes) Human Influences Pumping – Removal of water from the underlying carbonate aquifer, lowering hydrostatic pressure. Water pressure tends to support the structural arch Human Influences Freeze Protection Heavy pumping For agriculture 80 70 60 Sinkholes Reported 50 40 30 20 10 0 Jan Feb Mar Apr May June July Aug sept Oct Nov Dec Sinkhole Types solution Cover Subsidence Cover Collapse Cody Scarp ...
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