LAB 11: FIELD TRIP TO BIG LAGOON

OBJECTIVES:

1. The chief objective of this trip is to look at evidence for recent coastal erosion near the community of Big Lagoon. We will look at the processes responsible for retreat of the bluffs, calculate historic erosion rates, and discuss what might control the timing and rate of erosion here.

2. We will also examine the materials exposed in the bluffs and speculate on how they may have originated and

what they tell us about local geologic history.

3. We will discuss the origin and significance of Big Lagoon.

I. EROSION RATES AT BIG LAGOON The aerial photograph below shows the town of Big Lagoon in 1947. Use the roads and buildings to find your location on the photograph.

What changes have occurred since the air photo was taken in 1947? Consider changes in shoreline of the lagoon: cliff and beach: houses: roads:

The table below shows measurements from Park Road and Ocean View Drive to the edge of the coastal bluffs based on 1931, 1941, 1942, and 1974 aerial photographs. The Feb 16, 1942 photo is shown above. The work was done by Don Tuttle, Humboldt County Resource Analyst ("Investigation of Methods for Determining Coastal Bluff Erosion”, March, 1981). In addition, on-the-ground measurements were made on line 7 by the Geology 350 class in April, 1998, 2003, and 2004.

distance from base line to edge of bluff, ft

Line Aug. 1931 Nov. 1941 Feb. 1942 Aug. 1974 Apr. 1998 Apr. 2003 Nov. 2004 1 840 810 791 772 -- -- -- 2 805 780 773 762 -- -- -- 3 733 691 689 664 -- -- -- 4 720 681 667 639 -- -- -- 5 648 590 582 565 -- -- -- 6 585 540 525 495 -- -- -- 7 430 396 384 374 310 294 294

From 1931 through 1974 the erosion at stations 1 through 9 ranged from 3 to 90 feet, averaging about 56 feet. During December 1982, a combination of storm waves and very high tides caused further retreat of between 20 and 40 feet

a. How many feet of bluff retreat occurred on line 7 (along the road on the north side of the northernmost line of

houses), and what was the average rate in ft/yr for each of the following periods?

between 1931 and 1941?__________________ ft rate:_______________ ft/yr between 1941 and 1942?__________________ ft rate:_______________ ft/yr between 1942 and 1974___________________ ft rate:_______________ ft/yr between 1974 and 1998?__________________ ft rate:_______________ ft/yr between 1998 and 2004?__________________ ft rate:_______________ ft/yr b. What was the total amount of bluff retreat on line 7 over the period for which we have records? ___________________ ft c. What was the average rate of bluff retreat on line 7 for this period? ________________ ft/yr

II. MATERIALS IN THE BLUFFS AND RETREAT PROCESSES

As we walk down the beach, look carefully at the materials making up the bluffs. Fill in the table below:

property upper unit lower unit

grain size

rounding

sorting

stratification (good, poor, none?)

cross-bedding/sed. structures?

cementation (weak? strong?)

probable origin (env of deposition)

Look also for evidence that might indicate the mechanisms by which the bluff is retreating. a. Is there undercutting of parts of the cliff? ________________________ b. Are there landslides? _______________________________________ If so, what kind? ___________________________________________ c. Are there joints (fractures) that might facilitate erosion? ____________ If so, describe their orientation:________________________________ d. Consider the energy environment of the shoreline: 1. Do the waves break close to the shore or farther out from the shore? _________________________ 2. Are the waves close to shore relatively large or relatively small? ____________________________ 3. Overall, is this a high energy or low energy beach?_______________________________________ e. Consider the protection provided by the beach: 1. Is the beach generally relatively narrow or relatively wide? ________________________________ 2. Is the beach generally flat or generally steep? ___________________________________________ 3. Is there evidence for active beach erosion or change in beach elevation? ______________________ If so, what is it?___________________________________________________________________ 4. Overall, does the beach provide much protection for the cliffs? _____________________

III. ORIGIN OF BIG LAGOON The community of Big Lagoon is located on the lowest widely preserved marine terrace in this area. This flat surface is approximately 83,000 years old. The terrace extends from Big Lagoon to the south, rising gradually to over 200' elevation two mile south of Big Lagoon in the vicinity of Patricks Point State Park. Marine terraces are formed at sea level by erosion of a wave-cut platform and deposition of beach sediments on the platform, followed by uplift of the coast relative to sea level. What does the variable elevation of this terrace tell us about coastal uplift in the Big Lagoon - Patricks Point area?

Crustal deformation in the last hundred thousand years included both uplift of Patricks Point and subsidence of the area now overlain by Big Lagoon. WHAT YOU ARE EXPECTED TO KNOW 1. Be able to calculate the average rate of retreat of the bluffs at Big Lagoon from the air photo and table of

measurements. 2. Be able to discuss factors that influence how fast the cliffs retreat. 3. Be able to describe grain size, sorting, and degree of lithification of sedimentary deposits such as those exposed in

the bluffs at Big Lagoon. Know how these features are used to interpret the environment of deposition of the sediments.

4. Know what a marine terrace is, and be able to describe what we can tell about the geologic history of the Patricks

Point – Big Lagoon area from the marine terraces exposed here.