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LAB 1: FIELD TRIP TO McKINLEYVILLE AND MOUTH OF THE MAD RIVER

OBJECTIVES:

a. to look at geomorphic and geologic evidence for large scale thrust-faulting of young sediments in the Humboldt Bay region b. to look at topographic and geologic evidence for uplifted marine terraces in the McKinleyville-Trinidad area c. to investigate the geology and tectonic history of the Mad River Mouth area

ROAD LOG AND FIELD TRIP GUIDE

1. Mad River Fault Zone: Proceed north on Highway 101 from HSU; follow along on the attached map when we get onto it near the Mad River bridge. As we approach the Mad River, note the steep hillslope forming the north margin of the valley. This slope is cut by a series of thrust faults which dip about 30˚ to the north. These faults, which continue inland to the SE for at least 50 km, are part of the Mad River Fault Zone (MRFZ). The MRFZ is about 15 km wide and contains five principal thrusts (Trinidad, Blue Lake, McKinleyville, Mad River, and Fickle Hill faults) and three folds (the Fickle Hill and Blue Lake anticlines, and the Jacoby Creek syncline).

When Highway 101 turns NW after crossing the Mad River, we will be roughly paralleling the fault scarp. The faults cross the highway just north of School Road.

2. Stop 1: School Road Fault

a. Locate yourself on the map; then look around and try to pick out the scarps. The School Road fault consists of two main traces here, one above the other. The yellow house to the NE sits at the top of the uppermost scarp. The Baptist church is near the base of the lowermost scarp. Do your best to draw in the fault traces on the topographic map. Each fault should be a line running along the base of its scarp.

The flat surface that is offset here is part of the 83,000 year-old McKinleyville terrace, the lowest marine terrace in this area. Across the highway, the faults flatten out and become nearly horizontal; hence they do not form obvious scarps.

b. The topographic expression of thrust fault scarps in this region is very distinctive. A broad depression typically occurs in the flat just above the scarp, and a sharper depression occurs below the scarp base. This basal depression is commonly wet, and may contain ponds or water-loving vegetation. The diagram below shows these features schematically.

depression above scarp scarp

depression at scarp base; may be wet

conjugate bedding shears

fault

depressions are exaggerated

c. As we proceed north on Highway 101, we cross another fault along the N side of Widow White Creek. If you look east just before reaching the Airport turnoff, you can see the scarp of the McKinleyville fault forming a rise behind the daffodil fields. We will turn off and stop before we reach the airport.

2 2. Stop 2: McKinleyville Fault (Airport Road)

a. Locate yourself. The linear feature running NW-SE here is the scarp of the McKinleyville fault., one of the five major thrusts in the Mad River Fault Zone. It offsets the 83,000 year-old McKinleyville terrace. At the airport, the fault includes two main traces defined as scarps and sharp warps of the terrace. To the east, these traces converge to form a single steeper scarp.

b. The fault runs under the fill at the end of the runway and continues to the southeast, separating the fields from the airport buildings. The fault trace shows as a distinct step in the topography (scarp). To the best of your ability, draw in the trace of the McKinleyville fault on your map. Extend it as far as you can see it.

c. As we proceed east along Airport Road to Central Ave., much of the way we will be running along the top edge of the fault scarp.

3. Stop 3: McKinleyville Fault (Central Ave.)

a. The hill that Central Ave. climbs to the north is the scarp of the McKinleyville Fault. If you look east along Norton Ave., you can see where it also climbs up the scarp face. Draw in the fault trace on your map.

b. From this point, use topography to trace the McKinleyville fault SE on the map as far as you can recognize it.

4. Central Ave En Route to Highway 101

a. As we go north on Central Ave., note the dip in the highway behind the crest of the scarp; this is an example of the broad sag typically found behind the scarp.

b. Look west, and note the gentle warps of the terrace near the airport. Small fault scarps run right next to the terminal.

c. As we cross Dows Prairie, look for fault scarps east of Central Ave.

d. As the road dips down into the valley of Strawberry Creek, we run along another fault scarp. Sandy rocks cropping out in roadcuts here are the Crannell sands, marine sediments about 400,000? years old. The much younger McKinleyville Terrace is bevelled across them.

5. Stop 4: South End of Clam Beach

As we face south along Highway 101, the McKinleyville terrace lies at the top of the abandoned sea cliffs to our left. Before we start up the hill, note the great width of Clam Beach on the right It is a late Holocene marine terrace, which has been uplifted by several meters over the last 1200 years. This uplift probably occurred in association with large earthquakes on the Little Salmon Fault south of Eureka.

6. Stop 5: McKinleyville Vista Overlook -- Mouth of Mad River

a. Find your location on the attached map. What is the elevation of the point where we left the bus?

b. The mouth of the Mad River has moved since the USGS map was printed in 1972. A map included in this lab shows the positions of the river mouth since 1942.

c. Why did CalTrans have to emplace rock slope protection below and to the north of us in 1992?

d. From here we will drop down the hillside to the south to Stop 6 -- the cliffs along the Mad River.

3 7. Stop 6: Cliffs Along Mouth of Mad River

At this site we will look at a variety of geologic features -- young sedimentary deposits, fossils, and small faults-- exposed in a cliff face along the right bank of the Mad River. This cliff is undergoing rapid erosion and is subject to frequent small and not-so-small landslides. For your own safety and the safety of the rest of the group do not climb or stand close to the cliff. You can see everything by standing back on the beach. We may be able to examine fossils that have fallen from the cliff in landslides.

Enter your answers to the questions below on Part II of the attached answer sheet.

a. Look at the geologic units exposed in the cliff face. Describe two different units exposed in the cliff face. In your description you should include grain size (clay, sand, gravel, etc.)., average thickness of layers, color of unit, and any special features such as presence of fossils.

b. What do the contrasting sediment types in the cliff face indicate about sea-level changes during the period of deposition of these sediments?

c. The McKinleyville (airport) fault cuts these deposits in this vicinity; it’s trace is shown by a network of small intersecting offsets in the sand and pebble layers.

When you look at the pebble or sand layers offset by one of the small faults, what direction of offset do you see on the fault? That is, how have the layers on the left (north) side of the fault moved relative to the layers on the right (south) side?

d. Sketch a fault and an offset pebble or sand layer. What kind of fault (normal, thrust, or strike-slip) does it appear to be?

e. Since these faults offset young unconsolidated deposits, it would be very interesting to date the age of this faulting. Can you think of any way that the age of the faulting might be determined?