Geology and Earth ScienceThe Jurassic As Observed in the Deerfield Basin
By John Bucaro
When the super continent Pangea broke apart a series of rift basins were created along what is now the east coast of the United States and down through Central America. The focus of this work is on the northeastern U.S. along the rift zone and more specifically the Newark super group. The Newark super group formed in the late triassic-early jurassic as a result of sedimentation in the rift basins created by the rifting of Pangea. The outcrop studied in this paper is found in the Deerfield basin in Turner Falls, Massachusetts and is of jurassic age (Rasbury 2005).
Certain cycles can be found in the exposed strata of the rift basins. These strata can be used as clues to determine the history of the area, its climate and tectonic activity. All of these findings give one an insight to the depositional processes of the late triassic-early jurassic time period. The sedimentation appears to be lacustrine in nature and the cycles are of a dry-wet-dry nature (Olsen 1997). These cycles are referred to as Van Houten cycles and they suggest the rising and falling of sea level, seasonality, tectonic activity, uplift and various other processes effecting water levels (Olsen 1997). Observation of the Van Houten cycles and further studies of the strata in general can give evidence to transgressive or regressive shorelines and allow for an interesting look at climate changes, plate tectonics and plate positioning during the late triassic-early jurassic.
“The basins show strong vertical changes in overall facies reflecting an interplay of large-scale tectonic and climatic changes with time.” The vertical progression of a cycle consists of a lower area of fluvial deposits and progresses upward to an area of deeper water facies and then transitions back to shallow water fluvial facies again. The Van Houten cycles suggest a cyclical change in climate. The most compelling theory for the climatic change is that of the milankovich cycles, especially the eccentricity of the Earths orbit (Olsen 1997).
In the Newark Basin the Van Houten cycles suggest the deepening and shallowing of lakes. The laminate organic rich layers are representative of a deep stratified lake environment. To have chemical stratification of the lake the depth must be significant approximately 80+ meters. The other end of the cycle is the shallowing of the lake to a playa lake environment(Olsen 1986). In the playa lake thin red, iron rich mudstone and sandstone should be prevalent. There may be some fluvial structures as well indicative of paleocurrents and other fluvial processes. Mudcracks found in any of the strata are indicative of a periodic drying up of the lake once again showing evidence for climatic changes.
I believe this area was lacustrine in nature. It was probably predominantly a playa lake for most of the time as red mudstones and sandstones are most prevalent. The lake must have been shallow enough to allow for oxidation of the iron minerals present. Also the presence of fluvial features in some of the layers suggests that currents were able to have some effect on the bottom sediment of the lake. We also see a few periods where mudcracks are present. The climate must have at one point been arid enough to dry up the lake, also the lake must have been shallow enough to allow for drying up which leads me to believe it was no more than 5-10 meters deep during the playa stage(Olsen 1997). The presence of burrows in some of the layers also leads one to believe that it wasn’t very deep and fairly low energy because animals were able to live in the environment or plants were able to take root in the environment and the burrows remained after the organisms were gone.
The grey material and especially the black fish beds suggest a period in which there must have been a deep stratified lake. Stratification is significant as it suggests the lake was probably at least 80 meters deep (Olsen 1997). Without the stratification the organic material in the fish beds probably would not have been preserved. Oxidation of iron material is less in the deeper stratified lake and we see less of the red coloration at this point in the section.
The overall Van Houten cycle seen in the section is evidence of the changing of climate and environment through the milankovich cycles, plate movement, and tectonics. The fault found in the section suggests active plate tectonics which cause some local low grade metamorphism. Also as the plate drifted through different latitudes climate changes and environment changes will occur. The milankovich cycles obviously change the climate as they are the primary reason for the Earth’s transition in and out of ice ages (Olsen 1997). The overall Van Houten cycle reinforces these theories but we also see smaller changes within the section.
The transitions from mudstone to sandstone and differences in grain sizes suggest the area went through changes in flow regime. Possibilities include river, glacial melts, turbidites and various others however i think the most important information we can take from this is the idea of seasonality. There must have been some seasonality in the area because of the differing flow regimes we see and there occurrence in cycles. Even if one argues that it was due to river influx the differing flow regimes suggest that the river had different flow rates at different times. I think the sandstones and higher flow regime areas are representative of the spring and summer snow melt and run off from the mountains where as the mudstones and lower flow regime structures represent fall and winter periods of less water influx.
Reference:
Olsen, Paul E. Palisades, NY Science 234, (1986). A 40-Million-Year Lake Record of Early
Mesozoic Orbital Climatic Forcing. Pp 842-848.
Olsen, Paul E. Columbia University, Palisades, NY Science 25 (1997) STRATIGRAPHIC RECORD
OF THE EARLY MESOZOIC BREAKUP OF PANGEA IN THE LAURASIA-GONDWANA
RIFT SYSTEM. Pp 337-401.
Rasbury, Troy. Geo 403/543 FIELD TRIP TO JURASSIC OF DEERFIELD BASIN. Class handout.
2005.
When the super continent Pangea broke apart a series of rift basins were created along what is now the east coast of the United States and down through Central America. The focus of this work is on the northeastern U.S. along the rift zone and more specifically the Newark super group. The Newark super group formed in the late triassic-early jurassic as a result of sedimentation in the rift basins created by the rifting of Pangea. The outcrop studied in this paper is found in the Deerfield basin in Turner Falls, Massachusetts and is of jurassic age (Rasbury 2005).
Certain cycles can be found in the exposed strata of the rift basins. These strata can be used as clues to determine the history of the area, its climate and tectonic activity. All of these findings give one an insight to the depositional processes of the late triassic-early jurassic time period. The sedimentation appears to be lacustrine in nature and the cycles are of a dry-wet-dry nature (Olsen 1997). These cycles are referred to as Van Houten cycles and they suggest the rising and falling of sea level, seasonality, tectonic activity, uplift and various other processes effecting water levels (Olsen 1997). Observation of the Van Houten cycles and further studies of the strata in general can give evidence to transgressive or regressive shorelines and allow for an interesting look at climate changes, plate tectonics and plate positioning during the late triassic-early jurassic.
“The basins show strong vertical changes in overall facies reflecting an interplay of large-scale tectonic and climatic changes with time.” The vertical progression of a cycle consists of a lower area of fluvial deposits and progresses upward to an area of deeper water facies and then transitions back to shallow water fluvial facies again. The Van Houten cycles suggest a cyclical change in climate. The most compelling theory for the climatic change is that of the milankovich cycles, especially the eccentricity of the Earths orbit (Olsen 1997).
In the Newark Basin the Van Houten cycles suggest the deepening and shallowing of lakes. The laminate organic rich layers are representative of a deep stratified lake environment. To have chemical stratification of the lake the depth must be significant approximately 80+ meters. The other end of the cycle is the shallowing of the lake to a playa lake environment(Olsen 1986). In the playa lake thin red, iron rich mudstone and sandstone should be prevalent. There may be some fluvial structures as well indicative of paleocurrents and other fluvial processes. Mudcracks found in any of the strata are indicative of a periodic drying up of the lake once again showing evidence for climatic changes.
I believe this area was lacustrine in nature. It was probably predominantly a playa lake for most of the time as red mudstones and sandstones are most prevalent. The lake must have been shallow enough to allow for oxidation of the iron minerals present. Also the presence of fluvial features in some of the layers suggests that currents were able to have some effect on the bottom sediment of the lake. We also see a few periods where mudcracks are present. The climate must have at one point been arid enough to dry up the lake, also the lake must have been shallow enough to allow for drying up which leads me to believe it was no more than 5-10 meters deep during the playa stage(Olsen 1997). The presence of burrows in some of the layers also leads one to believe that it wasn’t very deep and fairly low energy because animals were able to live in the environment or plants were able to take root in the environment and the burrows remained after the organisms were gone.
The grey material and especially the black fish beds suggest a period in which there must have been a deep stratified lake. Stratification is significant as it suggests the lake was probably at least 80 meters deep (Olsen 1997). Without the stratification the organic material in the fish beds probably would not have been preserved. Oxidation of iron material is less in the deeper stratified lake and we see less of the red coloration at this point in the section.
The overall Van Houten cycle seen in the section is evidence of the changing of climate and environment through the milankovich cycles, plate movement, and tectonics. The fault found in the section suggests active plate tectonics which cause some local low grade metamorphism. Also as the plate drifted through different latitudes climate changes and environment changes will occur. The milankovich cycles obviously change the climate as they are the primary reason for the Earth’s transition in and out of ice ages (Olsen 1997). The overall Van Houten cycle reinforces these theories but we also see smaller changes within the section.
The transitions from mudstone to sandstone and differences in grain sizes suggest the area went through changes in flow regime. Possibilities include river, glacial melts, turbidites and various others however i think the most important information we can take from this is the idea of seasonality. There must have been some seasonality in the area because of the differing flow regimes we see and there occurrence in cycles. Even if one argues that it was due to river influx the differing flow regimes suggest that the river had different flow rates at different times. I think the sandstones and higher flow regime areas are representative of the spring and summer snow melt and run off from the mountains where as the mudstones and lower flow regime structures represent fall and winter periods of less water influx.
Reference:
Olsen, Paul E. Palisades, NY Science 234, (1986). A 40-Million-Year Lake Record of Early
Mesozoic Orbital Climatic Forcing. Pp 842-848.
Olsen, Paul E. Columbia University, Palisades, NY Science 25 (1997) STRATIGRAPHIC RECORD
OF THE EARLY MESOZOIC BREAKUP OF PANGEA IN THE LAURASIA-GONDWANA
RIFT SYSTEM. Pp 337-401.
Rasbury, Troy. Geo 403/543 FIELD TRIP TO JURASSIC OF DEERFIELD BASIN. Class handout.
2005.
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