Friday, December 26, 2008

The Geology of Georgia

Notes on Part One of an Occasional Series:
The Appalachian Plateau

Don't be fooled by the quick appearance of a second installment of the occasional "Geology of Georgia" series. This isn't a new entry; rather, it's merely notes on the last entry about the geology of the Appalachian Plateau. A legitimate new entry isn't likely to appear here anytime too soon. However, my research following my little geologic reconnaissance up to northwest Georgia compels me to share some observations about the state of geologic sciences here in Georgia.
For those of you unfamiliar with geology, or confused about the appearance of somewhat academic, geologic discussion in an otherwise unrelated blog, either just skip this entry or listen to the words as one would a new and unfamiliar music (remember the first time you heard reggae? zydeco?). If you like it, seek out some more; if not, then don't bother. For you science types who may have come here via a Google or other search and are bewildered to find science and spirituality discussed side by side: Boo!

Preliminary geologic investigations in the southeast tended to be regional in scope and cover a variety of geological issues. For example, the earliest reports on the geology of the Appalachian Plateau and most of northwest Georgia were done by C.W. Hayes in the 1890s and 1900s. Despite the limitations of his time (no automobile access - he likely visited much of the territory on foot or by horseback), Hayes' work is still quite valid. For example, he correctly interpreted the relationship between the Corbin Gneiss and the overlying metaconglomerate, and was the first to identify the Cartersville Fault as the boundary between the Paleozoic sedimentary rocks of the Valley and Ridge and the older, crystalline rocks of the Blue Ridge and Piedmont. Although much of his work was later modified, the primary contributions of this exceptional pioneer in Georgia geology still remain intact.

In northwest Georgia, Hayes named a thick and persistent sandstone he encountered the "Lookout sandstone" for its conspicuous displays as the rim rock at the top of Lookout Mountain. However, although a perfectly adequate term, the concept of the "Lookout sandstone" has gone through considerable revision since Hayes' time, and the name has subsequently fallen out of common use.

Stratigraphic terminology was treated differently at that time, and there was a particular lack of uniformity in treatment of lithostratigraphic units. The concept of a "formation" during the early 20th Century was not based so much on lithologic content as is the case today, but on stratigraphic association, stratigraphic position and fossil content. Geologic time was therefore inherent in the concept of lithostratigraphic units. This often resulted in ambiguous, confusing, and contradictory assignments of rocks to "formations" based on chronostratigraphic, not lithostratigraphic, concepts.

Case in point: working in the 1930s and 40s, Charles Butts included Hayes' "Lookout sandstone" into the so-called "Pottsville formation," named for similarly-aged rocks near the town of Pottsville, Pennsylvania. Butts' "Pottsville formation" consisted of all rocks of Pennsylvanian age in Georgia; therefore, time, not rock content, was the integral part of the definition of the "Pottsville formation," inconsistent with the modern definition of "formation" in the North American Stratigraphic Code. Today, only rocks that are lithically similar to the Pottsville Formation at its type locality in Pennsylvania and laterally continuous with those beds are considered "Pottsville formation." However, citing Butts, many subsequent geologic reports identified the Pennsylvanian-aged rocks in Georgia and Alabama as the "Pottsville formation."

In the early 1960s, Charles Cressler applied the terms used in Tennessee to these rocks, and divided Hayes' "Lookout sandstone" into a lower Gizzard member and an upper Sewanee member, avoiding association with the Pottsville Formation of Pennsylvania. The Gizzard, named for Fiery Gizzard Creek in Marion County, Tennessee, has been subsequently elevated to formation status in Georgia, and the lower part of what Hayes once called the "Lookout sandstone" is now called the Gizzard Formation. In Georgia, the overlying Sewanee has been designated a member of the Crab Orchard Mountains Formation, so that the upper part of what Hayes called the "Lookout sandstone" is now called the Sewanee Member of the Crab Orchard Mountains Formation. Today, the once perfectly-adequate term "Lookout sandstone" is no longer used.

This problem of confusion over time (chronostratigraphic) and rock (lithostratigraphic) terms was not restricted to the rocks of northwest Georgia. In the Coastal Plain, many preliminary investigators, including W.H. Dall (1890s), T.W. Vaughan (1910s), J.O Veatch and L.D. Stephenson (1908 through 1915), and J.A. Gardner (1920s), set the ground work for subsequent studies. Between 1925 and 1945, C.W. Cooke published a series of comprehensive compilations of the stratigraphic units in the Coastal Plain of South Carolina, Georgia and Florida, based largely on the work of his predecessors. This often resulted in the assignment of Coastal Plain sediments to "formations" based on time (chronostratigraphy), not rock (lithostratigraphy). For example, Cooke called all limestones with a certain fossil assemblage indicative of Oligocene age the "Tampa formation" for similarly aged limestones in Florida, regardless of the lithologic nature of the limestones. It was not until the late 1980s that Paul Huddlestun redefined the stratigraphic units of the Georgia Coastal Plain based on modern formation concepts, resulting in increased stratigraphic resolution and a better understanding of the geologic framework and history of the Georgia Coastal Plain.

Even where the time/rock controversy wasn't an issue, understanding of the geology of Georgia has been hampered by inadequacies of earlier investigations. For example, between northwest Georgia and the Coastal Plain lies a broad area of crystalline bedrock called the Piedmont Province. Geologists working in the adjacent Carolinas have divided these crystalline rocks into poorly defined, locally named belts or accreted "suspect terranes." Each belt or suspect terrane supposedly had a characteristic association of lithologies, metamorphic grade and structural features. However, in Georgia, neither the "belts," nor the "suspect terranes," are internally homogeneous geologic entities with features that contrast sharply with those of nearby "belts" or terranes, despite the efforts of many geologists to extent the boundaries of these units from the Carolinas into Georgia. The belt concept, although now deeply ingrained in southeastern geologic literature, has greatly hindered understanding of the geology and history of Georgia.

Despite these setbacks and despite the current lack of a Geological Survey in the state of Georgia (the Governor disbanded the former Georgia Geologic Survey, my one-time employer, earlier this decade for financial reasons), the efforts of many scientists working at universities and for the federal U.S. Geological Survey have allowed the understanding and modern interpretations of the geology of Georgia to continue.


1 comment:

BruceO said...

Interesting summary. I was even more facinated by your remark at the end since I was also employed by the Ga. Geol. Survey for the last 30 yrs. of it's existence. I'm curious, when were you there?