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Geological Evolution of Virginia and the Mid-Atlantic Region
Cross Section L
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Rifting of Pangaea: Initiation of the Atlantic Ocean
(Triassic and Lower Jurassic; 230 my to 175 my
These are slightly more indepth discussions of the rocks, processes, environments, and climates associated with the Atlantic rift lateral graben.
Geology of the Buried Basins and Axial Graben
Faulting and Deposition in the Exposed Basins
Paleoenvironments, Paleoclimates, Paleoworlds
Geology of the Buried Basins and Axial GrabenStanding at Virginia Beach, or any other coastal town today, and looking out toward the Atlantic ocean you would never know there was anything out there but water. And neither did earlier geologists. So much of what is important, even crucial, to understanding the geologic history of the Mid-Atlantic is not visible, nor accesible, and without that information it would be very difficult to construct a history.
It is nice, of course, to have as much information as possible, but some times just knowing that something exists can make all the difference in understanding what happened. So, even though our information is limited it is possible to discover what is deeply hidden. We probe the depths by drilling holes, and bringing up rock cores to study; this has been done up and down the eastern seaboard. We also have geophysical evidence; seismic and gravity studies to give us clues not only to the existence and distribution of features, but also to what is going on down there. One of the results is, even though we cannot see them, several buried lateral graben and an axial rift lie off the coast (map).
The Atlantic axial graben at first seems unusual. The rifting model we use predicts that an axial rift typically remains with one continent, or the other. So, in our mind's eye, on one side of the ocean, we expect to see the axial rift, and on the other side the sharply faulted transition with out axial rift. In the Rift x-section, for example, the axial graben has remained with the west (left) continent while the sharply faulted transition is with the east (right) continent.
For the Atlantic rift, however, the first thing we notice on the map is that the axial rift is not continuous, but broken (map). The axial graben is clearly off the New Jersey coast, but off southern Virginia and northern North Carolina it is absent. Where it is absent the seismic evidence tell us only the sharply faulted transition from continent to ocean is present. At this point the axial graben did not stay with North America but drifted away with Africa. Now, this relationship is in fact typical of all rifting events. Axial rifts are not of a whole; instead they fragment, part remaining with one continent, part going with the other continent.
We learn a couple of things from these observations. The most obvious is that in a rifting event, half of the evidence remains on one side of the ocean, and the other half on the other side of the ocean. Plate tectonics makes it very difficult to understand geology by only looking at what is local. Almost always we are forced to go probing around, even to the other side of an ocean basin, to find missing evidence.
The second thing we learn is that earth processes are often subtler and more complex than we first imagine. The earth is a rich planet, continuously surprising, intriguing, and exciting.
The sediments of the buried basins, what we know of them from well holes, is in some ways typical of what is in the exposed basins, and in other ways surprising. During the Triassic Rift Valley stages of basin filling red, clastic deposits (gravel, sand, and shale) filled the basins. By the Jurassic early DCM stage, however, clastics are replaced by thick sequence of limestones and evaporites (salt) deposits. The evaporites suggest a dry climate with high evaporation, something the exposed basins of the Mid-Atlantic do not give us a lot of clues to. By the lower Cretaceous, when the Atlantic was over 1000 kilometers wide, the rift basins had filled, sunk below sea level, and begun to be buried under divergent continental margin sediments (DCM stage).
But it is the exposed basins that tell us so much about what a rifting is like.
Faulting and Deposition in the Exposed BasinsEach exposed basin in Virginia has it own individual history, determined by its size and location, so the descriptions below are a synthesis. Some features may be missing from some basins, while other basins exaggerate other features. The block diagram to the left captures the general expression of a basin.
Faulting and deposition go on simultaneously, or in alternation with each other, throughout the history of the basin. That is, it is not like a trap door falling; the fault and basin develop in steps, over time. Two kinds of evidence indicate this. First, sediments have in places been broken by faults shortly after deposition and then the fault buried by more sediment at a later time.
Second, the alluvial fan, river, and lake sediments in the basins dip toward the west, toward the border fault, rather than dipping east or being horizontal (block diagram). This is the opposite of what is normal. Take the alluvial fans, for example. They form wedge shaped piles at the base of the horst, and their surfaces dip toward the basin, in this case, east. This can be seen in the drawing of the active fans at the surface of the block diagram. But the buried alluvial fans dip west, and the deeper we go the more west they dip block diagram. This could occur only if, after the beds are deposited, movement along the fault tips them backwards.
A similar process occurs with the lake and river deposits. They are, for the most part, deposited horizontally, but in the block diagram they clearly dip west, toward the horst. This process of deposition, faulting, more deposition, more faulting, must have gone on throughout basin filling since most of the beds have a reversed dip.
Coal in the Triassic BasinsCoal deposits are relatively rare in many of the rift basins related to the opening of the Atlantic, but Virginia seems to have the most with deposits in its Taylorsville, Richmond, Farmville, Briery Creek and Dan River/Danville basins (map). The coals are most common in the lower part of the sedimentary sequence, i.e. early in the basin history, and tend to be thin and discontinuous. The vegetation for the coals may have been low-lying swamps around the lake edges, but some may have been deposited in the floodplain ponds and oxbow lakes of the rivers which fed the low lying lakes. Thus, unlike many coal deposits around the world, the Pennsylvanian coal swamp deposits for example, these are freshwater in origin rather than deltaic/marine. The rift basin coals are thinner, more discontinuous, and more localized than the marine coals of the Pennsylvanian coal swamps in southwest Virginia.
There is a broad scale cyclicity to the coal deposits. They most commonly come at the top of fining-upward sequences, that is, sequences which begin with conglomeratic alluvial fan deposits, which grade into sandy river deposits, and then into silt/clay lake deposits. The conditions favoring coal deposition thus seems to accompany times when tectonic activity was declining, the hills were eroding lower, sediment influx was diminishing, and swamp and marshy conditions could form and exist long enough for plant matter to accumulate.
During the latest Triassic and early Jurassic coal deposition stopped. It is speculated this represents an increasingly arid climate, when there was insufficient rainfall to form lakes, but that evidence needs to be looked at more carefully below.
Paleoenvironments, Paleoclimates, PaleoworldsBy now in the history we should be getting used to the kalidoscope of changes that have affected the Mid-Atlantic region; mountains, to tidal flats, to tropical rain forests, the earth restlessly moving onward. And along with all these geological changes, climate changes too, from glacial, to desert, to tropical, to semi-arid, to tropical rain forest, and now back again to semi-arid and arid.
And through all this we have tried to find modern analogs to fill in for us, in our minds eye, what it would be like to live here at any one point in time. These analogs always have limitations, and not everyone will agree with them, but they can begin to give us perspective.
Virginia by the Jurassic probably looked something like the Basin and Range region of Nevada, southeast California and southwest Arizona today, a hot, dry, starkly beautiful and barren region of elongate horst mountains of bare rock, and elongate graben valleys covered with low, scattered scrub vegetation. During the Triassic lakes floored the valleys, sometimes ringed by narrow gallery forests and marshes, but by the Jurassic the lakes are ephemeral, and soon, salt encrusted playas shimmering in the glaring sunlight.
The major differences between Nevada today and Virginia during the Triassic-Jurassic is that a deep rift graben, soon to be an ocean basin, is present in extreme eastern Virginia, and the plants and animals are conspicuously different. These interpretation are based on several lines of evidence.
One of the distinctive features of Triassic-Jurassic sedimentary rocks, world wide, is that most of them are deep red. Traditionally, this has been interpreted as evidence of arid climates, although the causes of sedimentary rock color are complicated enough such easy interpretations are no longer possible. Yet, other evidence in areas both inside and outside Virginia support an arid climate, such as large fields of wind blown dune sand (eolian deposits), and evaporite deposits. Triassic-Jurassic eolian deposits are found in Nova Scotia, Scotland, Greenland, and England, which were much closer together and closer to Virginia during the Triassic-Jurassic than today.
Evaporites are found in many of the rift graben, and some of the lakes also have minerals, such as analcime, that are reliable indicators of alkaline salt deposits in a playa setting. In the Lockatong lake deposits in New Jersey this has led to an interpretation that playas predominated during repeated desiccation stages of the lake there.
Before plate tectonic theory allowed us to move the continents around the earth, explaining changing climates was difficult. But with modern interpretations of continent positions in the Triassic-Jurassic, and advanced climatic models, Triassic-Jurassic climates can now be modeled with some confidence. During the Triassic the equator lay just south of Virginia. In this climatic belt air currents rise because of the equatorial heat. If moisture is present it rises too, until in condenses and rains - every day, just like clockwork. During the Triassic, however, Virginia lay far inland in Pangaea and the vast land surface had no large body of water to supply moisture. In addition, no prevailing wind existed to bring moisture in on a regular basis so the climate was probably dry most of the year.
Yet it could not be completely desert-like. The lake deposits with coals around their edges during the early history of the basins speak of some moisture, which may have fallen on the high horst block mountains thrown up by the rifting. Such rains were probably seasonal and rare, occurring when major storms crossed the area and dumped torrential amounts of rain in a short time. The mud flow deposits, which typically form in drier climates with infrequent heavy storms, supports this.
The interpretation of unreliable and infrequent rainfall is also supported by the numerous fining upward sequences and cycles seen in the record. A fining upward sequence is when sediment is coarse on the bottom and gets ever finer toward the top. These sequences are found in both the alluvial fan gravels near the border faults and in the lake deposits far away from the fault. Furthermore, these fining upward sequences repeat over and over, that is, cycle.
It is certain that some of the cycles are the result of periodic uplift of the horst mountains bordering the basins. The fining upward occurs because sediment from a high source is eroded rapidly, and is coarse, but as the source erodes ever lower, the sediment being deposited becomes finer and finer. When the fault moves up again the cycle starts over. But some of the sequences and cycles cannot be explained by tectonics alone.
The numerous small fining upward sequences in the lake deposits are too small to be caused by the faulting. Instead, seasonal or multi-year long cycles in rainfall would cause marked fluctuations in lake level and the sediment would enter the lake in short pulses. Similar cycles are seen in modern lakes in regions with unreliable rainfall.
So again, as we have seen in other stages, the record is one of cycles, within cycles, within cycles. That is, a fractal record. And the cycles include tectonic cycles, climatic cycles, depositional cycles, each following their own time table. The result is a record rich with subtlety and complexity, yet one which also has direction, resolutely pushing ahead into the future.