Vignettes > What's in a fan?: A closer look at the stratigraphy of five Vermont Alluvial Fans

What's in a fan?: A closer look at the stratigraphy of five Vermont Alluvial Fans

Karen Jennings
Coastal Resources, Inc.,
Author Profile

Shortcut URL: http://serc.carleton.edu/48724

Location

Continent: North America
Country: USA
State/Province:Vermont
City/Town: Multiple
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy

Figure 1. A.) This photograph shows fan deposition on the side of a river. The sediment from this depositional event will most likely be washed away rather than preserved in the stratigraphic record. B.) This photograph shows alluvial fan deposition onto a stable river terrace plateau where it has been retained and preserved. The two alluvial fans shown will continue to accumulate sediment from future hillslope runoff events, which will appear as layers in the fan stratigraphy. Details


Figure 2. Location of the five alluvial fans trenched for this study, Vermont, USA. Details


Figure 3. Sub-parallel strata of sand, silt, and fine gravel in the Maidstone Fan. The light-colored sand layers also preserve fluvial structures such as cross bedding. Details


Figure 4. The dashed line traces a scour surface between the sand and gravel/cobble layers in the Bristol fan stratigraphy. We can tell that this is an erosional surface because of the jagged contact between the two units and the abrupt change in grain size. Details


Figure 5. Buried soils in the Bridgewater Corners fan indicate periods of fan stability. The two buried soil layers bracket a gravel unit which would have been deposited on the fan surface during a flood event. Details


Description

An alluvial fan is a cone-shaped accumulation of water-deposited sediment, formed at the interface between steep hillslopes and flat valleys where streams exit confined channels. Alluvial fans act as repositories for the sediment eroded from the contributing watershed, and as such, they can preserve information about upstream hillslope processes and the rates at which they are occurring. In other words, alluvial fans are direct recorders of hillslope activity. Fans directly record the timing of hillslope runoff events in both the depositional strata and the unconformities they preserve. Aggradation on these fans may be the result of increased local storm magnitude, frequency, duration, or landscape disturbance.

Alluvial fans are complex, prominent features of the New England landscape. The glacial history of Vermont provides a valuable setting for alluvial fan research. Because of recent glaciation, alluvial fans in Vermont are geologically young (less than 15,000 years old). This means that most New England alluvial fans are still fairly small in size, allowing us to gain a detailed understanding of the depositional history of a fan within a reasonable amount of time using shallow trenches. Additionally, river terraces preserved within many Vermont valleys act as stable plateaus adjacent to steeper hillslopes, trapping sediment from hillslope runoff and allowing alluvial fan formation. This is important because if the alluvial fan does not deposit sediment onto a stable surface, the depositional record will be lost (Figure 1).

The stratigraphy of five alluvial fans in Vermont was investigated using multiple backhoe trenches and radiocarbon dating of wood and charcoal in order to determine the depositional history of each alluvial fan (Figure 2). Buried wood and charcoal provided the dating control for determining aggradation rates and constraining the age of individual depositional events. The fans range in volume from 1,300 to 14,500 m3 and in age from 200 to 13,320 years before present (BP). Three fans located on river terraces have depositional records whose ages were limited by the age of the terrace on which they are situated. Two other fans, located in glacial valleys, preserve records that extend back nearly to deglaciation.

Trenching of the alluvial fans revealed a characteristic sequence of sub-parallel strata (Figure 3). Most fan units are moderately well sorted, and grain size ranges from silt to cobble. Sediment in all five fans was deposited by flowing water. Sedimentary structures such as cross bedding (inclined layers deposited as ripples) and minor imbrication (overlapping clasts, similar to toppled dominoes) were preserved in two of the fans. The lack of sedimentary structures in the other fans is most likely the result of bioturbation (worm tracks, tree throws, and animal burrows are common).

A closer examination of stratigraphic layers provides more information about how each fan formed. Large cobble units are indicative of larger flood events capable of eroding material from the watershed and transporting it to the fan surface. Finer materials in the fan stratigraphy, such as sand or silt, may indicate smaller flood events or gradual deposition over time. Erosional contacts between alluvial units indicate scour and reworking of alluvial fan sediments during deposition (Figure 4). Patchy, discontinuous deposits of gravel suggest that the flood waters were braiding across the fan surface, while continuous deposits indicate that the entire fan surface was blanketed with sediment during the event. The stratigraphy of all five fans contains evidence of flood activity, including scoured surfaces and layers of gravel and cobbles.

The alluvial fans not only preserve a complicated history of incision and aggradation, but also buried soils indicative of periods of fan surface stability. Soils are the product of processes related to climate, root and animal burrowing, mineral weathering, and time that ultimately change the charateristics of the deposited sediment. Periods of little to no depositional activity on the surface of the fan allowed time for the development of soil horizons on the fan surface. When the next flood occurred, the soil would be buried and a new soil layer would begin to form at the fan surface. The coarse-grained nature of most depositional units suggests that these fans accumulated during a series of flood events that interrupted stable, soil-forming intervals (Figure 5). The preservation of buried soils in the stratigraphy of all five alluvial fans is strong evidence that most sedimentation on the fan surfaces is episodic. Deposition occurs rapidly during geologically short periods of time, rather than gradually and uniformly accumulating sediment over time.

Because the fans are located in separate basins, they show differing depositional patterns and histories. The Maidstone fan was deposited very rapidly in continuous strata. At Hancock, incision alternated with fill events, possibly the result of bedrock outcrops that concentrated flow. The Eden Mills and Bristol fans preserve the most depositional events as continuous beds that cover the entire fan surface, in some cases scouring the underlying units. The Bridgewater fan does not appear to have any scouring, but contains patchy, discontinuous deposits of gravel. Observations indicate that fans fed by perennial streams have large, infilled scour channels. Fans fed by ephemeral streams reveal aggraded contacts with only partially scoured surfaces.

Radiocarbon dating of pieces of wood or charcoal preserved in the fan stratigraphy provides the age of that depositional unit, and allows for the calculation of aggradation rates over time. The youngest fan, located in Maidstone, Vermont, accumulated its entire volume of 4770 m3 within the last 150 to 200 years. An alluvial fan at Bristol, Vermont, shows rapid aggradation events at around 9,300 years BP and 4,000 years BP, with a smaller event at 3,200 years BP. These three depositional events are separated by times of little deposition with minor soil development. In Eden Mills, Vermont, an alluvial fan shows rapid deposition early in the fan's history, from 13,300 to 12,900 years BP, followed by moderate deposition to 9,500 years BP. There is evidence of channel incision followed by rapid filling at 6000 years BP. Small amounts of deposition on the fan ensued until historic clear-cutting of the adjacent hillslope triggered approximately 3000 m3 of material to be deposited on the fan surface; close to a meter of vertical aggradation over the past 100 years. At least three of the fans show the same pattern of rapid aggradation during the past several hundred years in response to land clearance and disturbance.

The alluvial fan in Bridgewater, Vermont, shows the majority of its aggradation between 3000 to 6000 years BP. The Hancock alluvial fan has poor dating control, but also shows a characteristic sequence of rapid aggradation episodes interrupted by periods of fan quiescence as evidenced by large gravel units which overlie buried soil horizons. Comparison of aggradation between the five alluvial fans shows that stable periods are more likely to be synchronous between fans across a large region than flooding events. This indicates that intense localized storms are more influential in triggering deposition than regional storm systems. Additionally, the hillslope lithology, cohesion, and vegetation will influence the threshold for slope failure, causing hillslopes with differing characteristics to respond differently to the same storm event.

Associated References


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