Field Trip to Turners Falls in the Deerfield Basin

Information for Readers

This is a lab report for my Sedimentology and Stratigraphy class at SUNY Stony Brook. You may use this report as a guideline for any report you may write on this area, but I strongly suggest you read the original sources that I cited.

Introduction

The Deerfield Basin has undergone many sedimentary depositional cycles due to tectonic and climatic changes. Much debate surrounds the causes and extent of these cycles. Some of the controversy surrounding the Deerfield basin includes whether the basin sediment was formed from a playa lake depositional environment or an alluvial plain, what processes were at work, and what was the maximum depth of the lake at its various cycles?

The Deerfield basin is an eastern tilted half graben formed because of lithospheric NW-SE extension due to the rifting apart of Pangaea during the late Triassic. The Deerfield Basin is one basin in a series of basins along the east coast of North America filled by sediments known as the Newark Supergroup. The location where the basin formed was originally located in the tropics and experienced a strong monsoonal climate, but due to continental drift it moved through more arid regions. Sediment deposition of the basin probably began at least 220 million years ago and ended about 175 million years ago (Olsen, "Stratigraphy" 489). Within the Deerfield Basin is the Turners Falls structure that we visited, located in Greenfield, MA near the eastern border fault.

Methods

We began our walkthrough at the vesicular Deerfield Basalt just downstream of the Turners Falls dam. The strata were dipping towards the dam at an angle of about 30°. To measure the length between strata, we used a jacob's staff. The staff had tick marks every ten centimeters. When held perpendicular to uniformly dipping layers and while looking downwards perpendicular to the jacob's staff, we were able to measure the distance between strata as if they were not dipping. There is some error in our measurements due to not looking down the jacob's staff at exactly 90°, but at an angle less than 90°. The section we observed was approximately 120m; however our measurements add up to 102.15m. Also, we did not record every single bed we saw since the cycle between sandstones and mudstones was very consistent in many places.

As we walked up-section, we recorded the distance between beds, the color of the beds, grain sizes, and other sedimentary features. We also looked back down-section so that we could record desiccation cracks and burrows on the tops of lower layers.

Results

The first 120cm consisted of a very flaky, brick red mudstone with mud cracks on the top, then a layer of a brick red, fine sandstone, back to the mudstone, and then back to the sandstone. This cycle continued for the next few measurements; however, at 405cm the sandstone became coarser. At 690cm, some mud cracks were evident in the mudstone layers and some burrows were evident in the sandstone layers, which are shown in Figure 1. Here, the sandstone was a lighter red color than the previous layers and was also speckled with a light colored mineral, perhaps mica.


Figure 1: Burrows in a sandstone

Further up-section, the sandstones become more dominant with smaller layers of mudstone in between them, all of which are a light red color with a tinge of gray (Fig. 2). Many of the sandstones continue to show evidence of burrowing creatures.


Figure 2: Large sandstone layers separated by small mudstone layers

At 13.5m, the sandstone and mudstone layers are about the same length and have returned to a slightly more red color than they were before. However, some of the sandstones in this layer exhibit dark gray patches in them (Fig. 3).


Figure 3: Red mudstone and sandstone layers with some gray patches in sandstone

Continuing up-section, interbedding of the sandstone and mudstone layers continues. There was some evidence of cross stratification over the next 5m. At 21.5m, mud cracks began to appear again. Another 290cm up, the sandstone layers become a grayish, lighter red with burrows and the mudstones have mud cracks. At 28.5m, there is a very prominent gray layer within a brick red mudstone layer (Fig. 4). The gray layer appeared to be of the same facies as the mudstone, it was not one of the gray shales that we see later on in the section. The gray layers could possibly be soil traces or the iron was bleached out in a reducing environment.


Figure 4: Gray layers within mudstone

The section continues to be interbedded with mudstone and sandstone, giving way to larger layers of grayish, lighter red sandstones, then a large mudstone layer surrounded by sandstone layers. At 37m is a conglomerate layer with a grayish, light red matrix (Fig. 5). The clasts are slightly angular and range from 1-10mm.


Figure 5: Conglomerte with red matrix

Above that are more layers of alternating mudstones and sandstones. At 43m, there is evidence of cross bedding and ripples in a very red sandstone. The next section consists of mudstones with mud cracks. At 46m, there is a grayish, light red sandstone with parallel lamination (Fig. 6).


Figure 6: Sandstone with parallel lamination

Mudstone and sandstone layers begin alternating again. At 50m, mud cracks began forming again. A little further up, there are more burrows in the sandstone layers. At 57m, there's a large sandstone layer capped with ripples directly overlain by a mudstone layer (Fig. 7).


Figure 7: Sandstone with ripples

Shortly further up is a darker red mudstone with rain drop impressions (Fig. 8).


Figure 8: Mudstone with rain drop impressions

At 63m, there is a sandstone layer that is capped by a conglomerate layer (Fig. 9). The clasts are about 1-2mm big and are pretty well-rounded within a grayish, light red matrix.


Figure 9: Conglomerate with grayish, light red matrix

At 69m, there is a black shale layer that is about 70cm thick (Fig. 10). The layers leading up to it were progressively getting grayer and darker and the layers above it become less gray and redder.


Figure 10: Black shale lake bed layer

Large scale cross stratification between a grayish, light red sandstone layer and a dark red sandstone layer is evident at 76m (Fig. 11). At the top of the figure there are also more ripples.


Figure 11: Large scale cross stratified grayish, light red sandstone with ripples

The layers continue to become redder and the sandstone and mudstone layers begin alternating again with some cross stratification noticeable at 92m and 94.5m. As we continue to go up-section, the sands become coarser until we reach the end of our measured section at 101m. Here, there is a conglomerate mass which has some very large angular clasts that are about 3cm big in a dark red matrix (Fig. 12).


Figure 12: Conglomerate with large clasts in dark red matrix

Much further up from the section we measured, there were two more distinct black shale layers, and then along the vertical wall at the dam is a fourth black shale layer with very large mud cracks up to 15cm long filled by conglomerate (Fig. 13). There were also nodules that were precipitated in some time after the black shale layer formed.


Figure 13: Fourth black shale layer with conglomerate filling mud cracks

Figure 14 shows the stratigraphic column that I constructed from our walk up-section as well as a modified legend (Fig. 15) from Gierlowski-Kordesch (1994). The shaded in area near the beginning of column four represents the black shale unit.


Figure 14: Stratigraphic column of Turners Falls structure


Figure 15: Legend for figure 14, adapted from Gierlowski-Kordesch (1994)

Discussion

The Deerfield Basin has undergone many climatic and depositional environment changes. According to Paul Olsen, the basin started off in the late Triassic as a red pebbly to conglomerate fluvial and alluvial arkose, changed to gray and red lacustrine sandstone and siltstone, followed by the 80m Deerfield basalt lava flow early in the beginning of the Jurassic, and then the cyclical sequence of red and black fluvial siltstone interfingering with alluvial conglomerate known as the Turners Falls Formation and the Mt. Toby Conglomerate (Olsen, "Stratigraphy" 488).

The two main contributors to the area surrounding the Turners Falls Formation and the East Berlin Formation (which is age equivalent to the Turners Falls Formation and located just a little further south in Connecticut) disagree mainly on the causes of the lake formations and its depth. They also dispute the occurrences of a playa lake or an alluvial plain.

Paul Olsen and others attribute the dry-wet cycles of dark red mudstone-sandstone-gray mudstone-black shale-gray mudstone-rippled coarse sandstone that we see at 69m and is also evident further up-section to celestial mechanics known as Milankovitch cycles. The change from a dry climate to a wet climate and back to a dry climate corresponds very well to predicted climatic patterns caused by Earth's precession cycle and eccentricity cycles affecting the distribution of sunlight as shown in figure 16 (Olsen, "40-Million-Year" 842).


Figure 16: Milankovitch cycles correlated to lake depth in the Newark Basin (from Newark Basin Coring Project website)

The darker colored layers are evidence of a lacustrine environment and the black shales represent the maximum depth of the lake at that time in that particular area. There are some sections further up-section that do not attain the black shale layer because the lake was not deep enough, but there is still evidence of lake formation such as the darker layers, lamination which would form in a relatively still water environment, ripples and stratification in sand that would be found near the shoreline, mud cracks that would form while the lake is in a regressive mode, burrows, and dinosaur footprints that need to form in mud with a very specific consistency ("Dinosaur Footprints").

At times when there is no black shale layer evident, the dry-wet cycles still hold to the Milankovitch cycles, only the lake is shallower. When the lake is shallow, the anoxic bottom does not form, so the organic matter is not preserved as well, leaving the layer a more grayish color than black. Olsen believes that the lake was relatively deep many times, even as much as 100m deep, because he believes the black shales could not have been deposited above the wave base (Olsen, "40-Million-Year" 843). He also believes that these lakes completely dried out many times as is evident by the extensive mud cracking we found in the section.

Olsen believes that there are other factors that contribute to the geology of the Turners Falls structure; however, they are not as significant as the climate controlling Milankovitch cycles. The Turners Falls structure is near the eastern fault border margin and it is believed that the fault system was active during sedimentation (Olsen, "Stratigraphic Record" 346). It would seem that the conglomerate layers that we observed would indicate that there was a level of relief that allowed for a high velocity, alluvial transport of pebble to cobble sized material; however, Olsen sites evidence that increased faulting actually causes alluvial fans to retreat and during dry lake times they prograde.

Beth Gierlowski-Kordesch does believe that climatic events controlled by Milankovitch cycles does play a part in the geology of the area, but she feels that tectonic events are more prominent. She attributes the small-scale cycles of water level changes to climatic influences, but the large-scale cycles that affect open vs. closed drainage patterns are as a result of tectonic influences (Gierlowski-Kordesch, 261). Fluctuations in the cycles could be attributed to tectonic activity along the border fault which controls subsidence in the basin (Drzewiecki). Generally, the more tectonic activity there is, the deeper and wider the basin will become most likely resulting in lower lake levels and more playa conditions (Schlische). Like Olsen, Gierlowski-Kordesch also believes that there was active faulting during sedimentation of the basin, but she admits that the extent of tectonic activity is unknown because of erosion of a thick upper sequence of the basin (Gierlowski-Kordesch, 261).

Gierlowski-Kordesch describes three continental environments in the East Berlin Falls section as: alluvial plain to sandflat, playa, and perennial saline lake. The red sandstone layers we observed are attributed to sheetfloods and are interstratified with the red playa mudstones with extensive mud cracking (Gierlowski-Kordesch 256).

Gierlowski-Kordesch has a different interpretation than Olsen's for the depth of the lake. She compares the earlier black shales with irregular lamination to modern shallow saline lakes with similar black shales and the later finely laminated black shales as evidence of less salinity. She cites the lack of bioturbation near the black shales as additional evidence of high salinity since many burrowing creatures have a low tolerance for high salinity. The high salinity creates greater density stratification in the lake which could possibly cause anoxic conditions at a much lower depth than what Olsen postulates. The occurrence of evaporites could further Gierlowski-Kordesch's hypothesis; however evaporites do not preserve well in the rock record. There were some nodules within the black shales that may have contained magnesite that could be an indicator to high salinity (Gierlowski-Kordesch 260).

Conclusion

It is evident by the persistant changes in mudstone and sandstone layers that we observed that there were many fluctuations in climate, particularly between aridity and humidity caused by monsoonal climates, during deposition of the Turners Falls structure. The layers leading up to and following the black shale layer we observed accurately described the transition from a playa-like landscape to a lake with anoxic conditions to a lake with shoreline deposits and alluvial deposits.

Olsen and Gierlowski-Kordesch seem to agree that there are alluvial deposition environments, playa conditions, and non-marine lake conditions which are all supported by the observations we made. Their debate mainly surrounds the lake conditions, particularly the depth of the lake and the salinity of it, and what caused fluctuations in its depth. Gierlowski-Kordesch does not seem to have enough evidence to fully support her claims; however that does not mean they are not true. The rock record is imperfect; the extent of the black shales is not known and the preservation of evaporites is poor. The correlation between Milankovitch cycles and lake cycles is much stronger, particularly for the short-term Van Houten cycles.

Works Cited

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