Posts Tagged ‘Trail Ridge’

The Geological and Ecological History of the Okefenokee Swamp (part two)

November 25, 2010

How did titanium-bearing (heavy mineral) sands accumulate in Trail Ridge?

About 1.8 million years ago, a period of time that marks the boundary between the Pliocene and the Pleistocene, there existed an enormous barrier island off the coast of south Georgia and north Florida that was 130 miles long.  A high stand of the Atlantic Ocean combined with wind and wave to create this island which encroached upon and buried a freshwater marsh, then an extension of an early version of the Okefenokee Swamp.  Today, this island is a terrace standing above the surrounding lowlands. Placer deposits of titanium-bearing heavy mineral sands motivated Dupont Corporation to buy large portions of Trail Ridge from lumber companies, but as I noted in my previous blog entry, the controversy over the potential environmental damage was so great that the project was canceled.  Mining heavy mineral sands is a strip mining operation where all vegetation and topsoil is stripped from the ground.  Then big machines eat through vast areas of subsoil, the material churned through pumped-up ground water for 7 days a week and 24 hours a day–an unceasing, hellish operation.  The surrounding area suffers constant heavy truck traffic as well because the separated minerals must be transported to distant factories.  After the land is raped, reclamation efforts turn the mined areas into tree farms and ponds–the former a paltry substitute for the original forest.

Image of a titanium mine in Florida.  The water is necessary in a step that separates the minerals from the sand.  The reclaimed land becomes wetlands and tree farms.  Wetlands are resilient but monocultured tree farms are almost barren of wildlife.

It’s no wonder Dupont Corporation was pressured not to destroy Trail Ridge, but they have other environmentally detrimental projects around the world including some in nearby Florida.

Dupont mines four kinds of coumpounds from heavy mineral soils like those that exist in Trail Ridge.  Ilmenite, or titanium dioxide (FeTiO3), is used as an industrial white pigment; rutile, or titanium oxide (TiO2), is used as a metal alloy; zircon (ZnSO4) is used as a refractory for founding molds and nuclear fuel rods; and monazite ( (Ce, La, Th)PO4) is a radioactive compound consisting of rare earth elements used in high powered magnets, industrial pigment, x-ray screens, fiber optics, and color television tubes.

These heavy mineral sands are black and are usually located under several feet of regular sand, though they occasionally appear on the surface, if erosion occurs.  They are radioactive.  Dr. Gale Bishop of Georgia Southern University found that sea turtles occasionally nest in these radioactive sands (which also occur on St. Catherine’s Island), and the baby turtles suffered a high rate of mutation and mortality.

A complex geological process led to the accumulation of heavy mineral sands in Trail Ridge and other areas along the coast, such as St. Catherine’s Island.  Metamorphic rocks, those formed deep underground, are the most important source of titanium, but igneous rocks from Mesozoic-aged volcanic flood basalts, and even sedimentary rocks are contributing sources of titanium.  Rocks located from what’s now South Carolina to Canada are the original sources of titanium-bearing minerals.  Rain water weathered these rocks into a substance known as saprolite which later eroded into rivers.  Rivers carried these titanium-bearing sands into the ocean.  As the ocean transgressed, these sediments were reworked to the surface where long shore currents and winds concentrated the heavy mineral sands into specific placer deposits.  They concentrated together because they have a certain relative density (specific gravity) that differs from just plain sand, and they tend to deposit in the same places.  Rain further concentrated the minerals by leaching out iron oxides.

The Geological and Ecological History of the Okefenokee Swamp (Part One)

November 19, 2010

Dupont Corporation’s plan to mine titanium from heavy mineral sands in Trail Ridge, which borders the eastern boundary of the Okefenokee Swamp, sparked a crash course study of the geology and ecology of the Okefenokee Swamp.  Scientists and environmentalists feared the impact of the mine could have seriously degraded the swamp.  The potential environmental degradation was so feared, that even conservative politicians opposed the project, and ten years ago Dupont agreed to abandon the proposal and sell the land to the state and the Nature Conservancy.  But the resulting research papers provided a wonderful source of knowledge and excellent fodder for my blog.

The Geological History of the Okefenokee Swamp Basin

The existence of the Okefenokee basin is due to many different geological factors.  It’s located in a region known as the southeast Georgia embayment.  This area was submerged under the Atlantic Ocean during the Cretaceous and for most of the Eocene.  During the Eocene, sediment intermittently began to be deposited, but late in the Eocene, strong ocean currents are believed to have washed these away.  U.S. Geological Survey bore holes near the Okefenokee Swamp reach late Miocene strata about 65-70 feet below the surface.  This strata consists of limestone pitted with the impressions of fossil sea shells–evidence that as recently as 5 million years ago, the southeast Georgia embayment was still periodically inundated with sea water.  Above this limestone layer is an impermeable clay of Pliocene age.  Here we find two reasons for the Okefenokee basin’s existence: the basin sagged because rainwater caused the limestone bedrock to dissolve (albeit unevenly) in a process known as karstification, and the clay layer above this prevents water from draining, thus providing the optimum conditions for wetlands to develop within the basin.

Hardwood hammocks, the islands within the swamp, are elevated above the basin.  These were formed during the late Miocene/early Pliocene when the area was still under sea water.  Ocean currents and river sedimentation built these little islands.  Later, when the basin emerged above sea level, the islands remained in place as dry land humps in the swamp.

The southeast Georgia embayment emerged above sea level during the Pliocene, and the resulting geological uplift caused the Suwanee River to backflow and reverse course, providing the source of all the fresh water accumulated there.

Map of Georgia’s Pliocene shoreline.  The present day location of the Okefenokee Swamp was submerged under the Atlantic Ocean.  The Suwannee River flowed from west to east.  After the emergence of the Okefenokee Swamp basin above sea level, geological uplift caused the Suwanee River to back flow into the basin and reversed its course, so that it flows west instead of east.  Note the islands which later became hardwood hammocks. Illustration from ‘The Geological and Natural History of the Okefenokee Swamp and Trail Ridge’ by Rich and Bishop.

Some scientists hypothesized that the early Okefenokee Swamp basin was a massive estuary and salt marsh, but there’s no evidence of this.  Instead, evidence from the nearby Trail Ridge to the east of the swamp suggests the basin has been a freshwater swamp since early in its existence.  Trail Ridge  was originally a massive barrier island 130 miles long which is far longer than any current barrier island off the Atlantic Coast, the longest in Georgia being Cumberland Island which is a mere 16 miles long.  Cape Hatteras off the North Carolina Coast is only 48 miles long.  Trail Ridge runs from south Georgia to North Florida as the following map will show.  This geological feature too contributes to the back flow of water into the swamp but is considered a minor influence compared to geological uplift, limestone sinkage, and the impermeable clay layer.  Scientists have discovered fossil freshwater swamp vegetation buried under Trail Ridge sands–evidence that a swamp already existed in the area late in the Pliocene when the barrier island that is now Trail Ridge first formed during a high stand of the Atlantic Ocean.

Trail Ridge is the oldest terrace on Georgia’s coastal plain.  Terraces run parallel to the Atlantic Ocean and were all formerly barrier islands that formed during warm interglacials or interstadials when sea level was higher than that of today’s.  Logically, the oldest terraces are further west because subsequent high stands of water would have washed younger ones away.

Map of Pleistocene terraces on the Georgia coastal plain.  Terraces were barrier islands formed during warm climate cycles when sea level was higher than that of today.  From west to east they are aged  from oldest to youngest.  Scientists  name  these terraces.  The following is a list of the terraces and their estimated ages.  (Note: scientists don’t always agree with these age estimates.  I’m using those from a college thesis that I list in my references.)  The Wicomicico Shoreline (Trail Ridge)–1.8 million years; the Penholoway Shoreline–780,000 years; the Talbot shoreline–240,000 years; the Pamlico shoreline–130,000 years; the Princess Anne shoreline–80,000 years; and the Silver Bluff shoreline–40,000 years.  The Talbot shoreline formed during the Yarmouthian interglacial.  The Pamlico shoreline formed during the Sangamonian Interglacial.  The Princess Anne shoreline formed during the early Wisconsin Ice Age when sea level was dropping but paused during an interstadial.  The Silver Bluff shoreline also formed during a pause in sea level drop caused by an interstadial.  Eventually, during glacial cycles, the shoreline could be located as much as 50 miles to the east of where it is today due to a lowering of sea level when much of the planet’s water was locked in ice.  Pollen and fossil wood has been recovered from Grays Reef, 11 miles off Sapelo Island, showing that forest and prairie grew in areas deep under water today.  It’s interesting to note that the Silver Bluff and Princess Anne shorelines formed when ice core data suggest temperatures were cooler than those of today.  I think this shows that a certain threshold must be met before sea level fell drastically during glacial cycles.  The sea level had been falling during the Wisconsin Ice Age but paused when the climate warmed up, even though it wasn’t as warm as that of today.  It also shows that sea level lags behind climate change by thousands of years.

The Ecological History of the Okefenokee Swamp Basin

The Okefenokee Swamp apparently has existed since the late Pliocene, but it has been an intermittent existence.  During the Pleistocene whenever there was a cycle of glacial expansion, the climate became cool and arid.  Sea level retreated many miles to the east and the water table fell, causing the Okefenokee Swamp to become a relatively dry basin.  Instead of hosting primarily wetland vegetation, oaks and pines and grasses grew in the basin.  Wetland species such as cypress, tupelo, and water lily, still existed but were relegated to small scale marshes bordering the rivers and streams that still incised the basin.

Some time during a glacial cycle, wind and remaining water eroded 4 elliptically shaped Carolina Bays into the swamp basin.  The bays, along with the remaining rivers, provided relic habitat that allowed wetland plants to rapidly recolonize the basin when climatic conditions once again became favorable to swamp development.  Scientist have taken cores of the peat in the swamp and discovered that the peat in today’s Okefenokee has only been accumulating for 6600 years.  It’s likely the Okefenokee basin was dryland habitat from about 36,000 years BP to about 7,000 years BP.  The last glacial maximum, the coldest and dryest climate cycle of the late Pleistocene, lasted from 28,000 BP-15,000 BP.  So it took thousands of years for the water table to rise to swamp level again, following the drying out of the swamp during the Ice Age.

References:

Rich, Frederick, and Gale Bishop

Geology and Natural History of the Okefenokee Swamp and Trail Ridge, southeastern Georgia and northern Florida.

Georgia Geological Society Field Guides 1998

http://etd.lsu.edu/docs/available/etd-03292006-161251/unrestricted/Willis_thesis.pdf