Melina BennettSeptember 13, 2016

Mad River Beach and Dunes

Introduction

We characterized Sediment sediment samples were characterized (e.g. size,

color, angularity, sphericity, sorting, and mineralogy) at eight stations to identify

differences in the sediment from the shoreline to the secondary set of dunes. These

observations were taken to see if the differences in the sediment support or oppose our

hypothesis that the sediment is more mature inland and less mature near the surf zone.

These stations were sampled along a transect line that ran from the waterline to

approximately 182 m inland. This experiment took place at Mad River Beach County

Park in Arcata, California (40° 55” 47’ N, 124° 08” 01’ W).

Methods

The first step was to determine everyone’s pace distance. This was

accomplished by measuring out 20.0 m and counting how many steps it took to walk

this distance. This was done three times; the steps were then averaged and divided by

20.0 m to calculate the mean pace. This pace was used to determine the distance of the

transect line and the stations in-between.

Stations were picked mostly at random, although some stations were picked due

to an obviousthe visual difference in the sediment from previous stationsacross the

landscape. At each station sediment was characterized by it’s size, color, angularity,

sphericity, sorting, and mineralogy. We also took notes about each station. Size was

determined using a hand lens and the Wentworth’s size classification system

(Wentworth, 1922). The rest of the characteristics were determined using educated

estimates and figures found in Sedimentary Geology: An Introduction to Sedimentary

Rocks and Stratigraphy by Donald R. Prothero and Fred Schwab (year).

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Results

The first station was located at the wet/dry line about 20.0 m from the surf. Each

station after was spaced on average 26.2 m from each other. The steps between each

station, distance between each station, size, sorting, color, angularity, and

shpericitysphericity for each station are given in Table 1. 28.8 steps were averaged

over 20.0 m making the pace 0.69 steps/m. Sediments from each station were

predominantly composed of mafic granules with some quartz.

Station Steps Distance (m) Size Sorting Color Angularity

SpherictySphericit

y

1 - -

Medium – to

coarse sand

Poorly White, green, black, grey

Sub-rounded

Medium to high

2 35 24.15 Medium Well White, green, black, grey

Sub-angular to

well-rounded

Medium

3 29 20.01 Fine Well White, green, black, grey

Sub-angular High

4 30 20.7 Very fine WellWhite, less

green, black, grey

Sub-angular High

5 42 28.98 Very fine WellWhite, less

green, black, grey

Sub-rounded Medium

6 48 33.12 Fine – coarse Poor

White, dark green, black,

grey

Sub-rounded Medium

7 28 19.32 Fine – coarse Moderate

White, dark green, black,

grey

Sub-rounded to

roundedHigh

8 54 37.26 Fine WellWhite, dark

green, black, grey

Sub-angular to

well-rounded

High

Table 1 Sediment characterizations at each station. Station distances in meters determined by pace distance with a pace of 0.69 meters per step.

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There is a visible pattern that appears from these data. Along the wet/dry line

and the base of the dunes the sediment is poorly to moderately sorted and has a

variable range in sediment size from fine to coarse, whereas along the sides and at the

top of the dunes sediment is well sorted and fine grained. Color, angularity, and

sphericity do not appear to follow this trend.

Discussion

There are many factors that go into determining the maturity level of sediment.

Factors include sphericity, angularity, size, and sorting. The method to determine

maturity is not finite; maturity was determined using educated estimates based on these

factors. Looking at all the factors at every station, there is not a definite answer towe

were challenged to form an hypothesis about whether or not the sediment is more

mature inland. Rather, the data alludes to why the sediment varies from one side of the

dune to the other.

One hypothesis is that Aas the wind blows from offshore it picks up sediment

along the beach. Once the wind encounters an obstacle such as a dune, it deposits the

sediment and over time it builds up. As the dune grows wind blown fine grained

sediment builds up the windward side; wind eddies are generated on the lee side,

keeping fine grained sediment from burying coarser grained sediment at the base of the

dunes. We could test this hypothesis by conducting further experiments that would

include collecting data such as _____.

Conclusion

By looking at the size, color, angularity, sphericity, sorting, and mineralogy we

can get an idea of where the sediment was derived and, to some extent, how it arrived

at it’s current location. From the These data we collected at Mad River Beach, support

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our hypothesis it is clear that fine-grained sediment is the main sediment size to be

transported via aeolian processes. Our hypothesis that These these same processes

keep coarser sediment from being buried on the lee side of the dunes are also

supported by the data.

References

Wentworth, C.K., 1922. A scale of grade and class terms for clastic sediments:

Journal of Geology, v. 30, p. 377-392

Field Notes

Figure 1. Pace Distances, Mapping Area Sketch, and Observation Notes.

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Figure 2. Cross Section and Topographic Map. Station locations are indicated by

numbered dots.

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