We will begin on the left, examining the time column, labeled "Era", and work our way to the right ("Major events").
 

*NOTE: this curve also indicates that when sea level falls, terrestrial environments (dry land, streams, lakes, etc.) develop; whereas, when sea level rises, marine environments encroach upon the land. Remember that a sea level drop is termed a "regression" and a sea level rise is termed a "transgression".

Important: The column labeled "Rock types by region" is a highly generalized summary of the rocks that are present in the state. This includes both outcrops (rocks exposed at the Earth's surface) and subcrop (rocks below the surface). Each of the rock types (basement complex, limestone, sandstone, and shale) has its own symbol. The key to the symbols is located at the base of the sea level and climate curves. I did not use the standard rock symbols (e.g. blocks for limestone) because they weren't available in the software with which I prepared this figure.

The blank areas in the rock column represent gaps in the rock record, i.e. time intervals for which no rocks exist at that location, either at the surface or in the subsurface. This occurs either because during that time interval no rock formed at that location, or because the rock was subsequently eroded away before the overlying, younger rock formed. This leaves only a surface between the older rock below the gap, and the younger rock above the gap. This surface is called an unconformity. The larger the blank area, the longer the time interval that is missing, i.e. the bigger the gap in the rock record.

If you were to actually examine an outcrop with such an unconformity within it, the gap in the rock record might not even be apparent to you, as it could easily just look like a bedding plane between two layers.
 

Take for example the figure to the left. Between the underlying (older) sandstone and the overlying (younger) siltstone there is what appears to be a bedding plane. This sedimentary structure might be nothing more than physical evidence of a relatively rapid decrease in current speed during deposition, resulting in a decrease in the grain size of sediment transported by a stream. As a result, there  really would be no time gap between the cessation of sandstone (sand) deposition and the onset of siltstone (silt) deposition.

However, suppose  this sandstone layer was instead overlain by hundreds or thousands of feet of sediment (or rock) that was later eroded away and THEN the siltstone was deposited -- a scenario implying a gap of hundreds of thousands, or perhaps, millions of years -- then this apparent bedding plane is actually an unconformity.

Based on the duration of some of the gaps in Tennessee's rock record, it is safe to assume that major erosional episodes removed huge volumes of rock and left behind major unconformities. These gaps leave us without any evidence at that location for Tennessee's geologic history during the missing time interval(s). We must look instead at time-equivalent rocks preserved elsewhere in Tennessee -- or if absent statewide, elsewhere in North America -- and make inferences based on our findings.

Precambrian history - Note that the Precambrian appears at the bottom of the column entitled "Era." The Precambrian is actually not an era of geologic time. Eras are the 2nd largest subdivision of geologic time; the largest is the Eon. Although it includes 7/8'ths of Earth history -- from our planet's origin, 4600 m.y.a. (million years ago), to approximately 570 m.y.a. --  the Precambrian is also not an Eon! Rather it is an informal term that we use in place of the three eons contained within it.

There are very few outcrops of Precambrian rocks in TN, and most (all?) are igneous or metamorphic. As a result, our knowledge of Precambrian events is pretty sketchy. In fact, only a few exploratory drilled wells have penetrated into the Precambrian in the subsurface. Keep in mind, however, that Precambrian basement rock underlies the entire state at various depths. In other words, the craton, or continental foundation, underlies the whole state, but is overlain by younger rocks almost everywhere.

Like all cratonic areas, the Precambrian rocks of TN developed -- as we learned in class -- over billions of years by repeated episodes of convergence, rifting, subduction, and obduction. Over time, these tectonic activities caused volcanic island arcs to evolve into microcontinents. Eventually,  multiple microcontinents converged, evolving into protocontinents, and the protocontinents subsequently developed into full-fledged continents. Hence the craton is composed of a complex suite of ancient igneous and metamorphic rocks with long, complex tectonic histories. Any Precambrian sedimentary rocks involved, as well as most igneous and metamorphic rocks, experienced multiple episodes of metamorphism. Note also that erosion removed any evidence of the late Precambrian, as indicated by the unconformity between the Precambrian and the Paleozoic.

As the existing Precambrian rocks are not sedimentary in nature, they tell us little or nothing about what was happening at the Earth's surface. Therefore, we have no direct record of sea level or climatic activity. However, based on the character of the basement rocks in Tenn. and the surrounding states, we can infer that sometime during the Precambrian the proto-Appalachian Mtns. formed, i.e. those that preceded the modern Appalachians. In addition, we also can infer that the Nashville Dome first rose during this time. Finally, late in the Precambrian, we have evidence for continental rifting, sea floor spreading, and the development of the Proto-Atlantic Ocean.

Above the Precambrian craton you see rocks of Paleozoic, Mesozoic, and Cenozoic age. Rocks in Tennessee from these three eras are predominately sedimentary in origin, and constitute most of the rock exposed at the surface in the state. Since sedimentary rocks are fairly likely to contain fossils, and fossils provide us a fairly detailed look into the geologic history of an area, this part of the column contains much more information.

Note that the climate curve indicates that Tennessee had a relatively warm climate throughout all of the Paleozoic and Mesozoic, and during most of the Cenozoic. That is because our paleolatitude was tropical during much of this time. Over time however, we slowly drifted to our current latitude.  The glacial and interglacial periods of the Pleistocene ("the Ice Age") are shown in the late Cenozoic.

Paleozoic history - Surface outcrops in Tennessee are dominated by Paleozoic age rocks. In addition, Paleozoic rocks compose the bulk of our subsurface sedimentary rocks. Therefore, we have a rich fossil record to examine, as well as abundant sedimentary structures to interpret. We will not treat this information in detail here, but be aware that this provides us with ample evidence to work with in writing Tennessee's Paleozoic geological history.

Nearly all of the Paleozoic rocks in Tennessee were deposited in marine settings. The sandstones generally formed in nearshore environments, such as beaches, or deltas. The shales accumulated in a variety of low energy (i.e. calm water) conditions, both nearshore and offshore. Where marine environments were relatively free from terrigenous input (i.e. sand and mud transported to the ocean by rivers and streams), hard parts of calcareous organisms accumulated to produce limestones. These formed in both low and high energy settings. A few Paleozoic outcrops also host shales, siltstone, or sandstones deposited in non-marine environments, e.g. swamps, lakes, and rivers. However, the preservation potential for these rocks is significantly less than for marine deposits, due to their degree of exposure during marine regressions.

During the 325 m.y. interval of the Paleozoic, TN experienced repeated orogenic events -- i.e. the Taconic orogeny, the Acadian orogeny, and the Allegheny orogeny. The oldest of these -- the Taconic -- probably resulted from convergence of N. America with a volcanic island arc, and the subsequent subduction/obduction of this arc complex. Now if you try to imagine this event for just a moment, perhaps you can picture the tremendous upheaval that this would cause along our eastern continental margin. The resulting orogeny initiated the development of the modern Appalachians. In addition, the accompanying uplift caused a drop in sea level (as reflected by the sea level curve), or regression. When sea level drops, what was sea bottom is exposed on the land surface, and hence, existing deposits are subjected to erosion. As a result, you can see a rough correlation between orogenic activity, regression, and unconformities in this geologic column. (Note: This is one of the major points of this lesson.) You can also see a link between these orogenies and uplift of the Nashville Dome.

The Acadian orogeny is believed to be the result of a hard "bump" between Africa and N. America. Subsequently, the two continents docked causing the Allegheny orogeny. Together, these three orogenies built the Appalachian Mtns., which once stretched all the way from Maine to Texas (they were subsequently eroded and/or buried throughout most of LA, MS, and TX).

Mesozoic and Cenozoic history -  Because of the closing of the Atlantic Ocean (or from a different perspective, the assembly of Pangaea), and the resulting orogenic uplift of the Appalachians and the Nashville Dome, regression left the state of TN exposed to the ravages of terrestrial erosion. And erode it did! However, by the end of the Mesozoic, during the late stages of Pangaea's disassembly, marine environments again developed in TN (note the sea level curve).

In the very late Mesozoic, the Mississippi Embayment formed. This was actually a large inland sea stretching north from the Gulf of Mexico up along the Mississippi River Valley all the way into southern Illinois. Within this embayment, marine sediments accumulated. Due to the depth of the embayment in western TN, these deposits were preserved (i.e. escaped erosion). The embayment grew -- the sea trangressed -- during the early Cenozoic, and began to inundate areas farther east. However, a record of latest Mesozoic to middle Cenozoic marine sedimentation is mostly found in Western TN, since the sea  transgressed into middle TN only briefly (and not at all into eastern TN).

In fact, much of the Cenozoic sediment in middle and eastern TN is terrestrial, not marine, in origin. These non-marine environments were mostly coastal wetlands, lakes, and streams. Sea level fell again during the middle Cenozoic and terrestrial conditions prevailed throughout TN. As a result, much of the Mesozoic and Cenozoic rock record in TN was lost. It was during this interval that most of the modern physiographic features of the state -- e.g. the Highlands Rims and the Central Basin -- developed their characteristic forms.

The youngest Cenozoic rocks (or more accurately -- sediments) in the state are the widespread alluvial deposits formed by numerous rivers and streams. These deposits form at the expense of the older, underlying bedrock. In other words, a widespread unconformity is developing today all over Tennessee. For example, here in Murfreesboro, the youngest bedrock is early Paleozoic in age. Older rocks are present in the subsurface, but no rocks younger than about 450 m.y. are present. However, rivers have deposited sediments on this bedrock surface over the last few thousand years. Hence, an unconformity is present between the alluvium and the bedrock.