1GEOLOGY 470 FIELD EXERCISE 3, SPRING 2009 -- SKETCHING, DESCRIBING, AND MAPPING EXPOSURES

OBJECTIVES:To develop and refine your field skills in:1. observation and rock and sediment description2. sketching and describing outcrops3. locating yourself and in measuring attitudes, fold axes, and lineations4. simple geologic mapping

LOCATION:Coastal bluffs between the mouth of Redwood Creek and the rocky point approximately 1500 ft north of the mouth - see attachedblowup of topographic map, vertical air photo blowup, composite topo and airphoto, and oblique air photos.

EQUIPMENT NEEDED:Brunton compassHand lensColored pencils

MATERIALS ACCOMPANYING EXERCISE:Oblique 2005 color airphotos of mapping area (2)Black & white vertical airphoto blowups (stereopair)Composite photo and map on Rite-in-the-Rain paper for field use; approx scale 1 in = 156 ftTopographic basemap for office use: scale 1 in = 150 ft (1:1800)

BACKGROUND:Bedrock exposed in the bluffs north of the mouth of Redwood Creek is Redwood Creek schist. This unit, correlative with theSouth Fork Mountain schist, is part of the Franciscan Assemblage. According to Harden et. al (1982) and Cashman et. al (1995),the Redwood Creek schist is typically a light-green to dark grey phyllite or fine-grained schist consisting largely of quartz, chlorite,white mica, and albite. The schist is most commonly metamorphosed mudstone and metagreywacke, but it may locally containmetabasalt and metamorphosed tuff.

1. Preliminary walk-throughWe will start by walking together through the field site so that you all become familiar with what you will be working on. We’llstart at the south end of the bluffs and walk north up the beach looking at the exposures. I’ll point out those that need carefulstudy. When we reach the prominent outcrop at the north end of the beach, you can start working your way south to the bus. Weshould all be back at the bus no later than 1 PM.

2. Field measurement of attitudes and mapping and description of the bedrock exposed along the beach north of the mouth ofRedwood Creek.You will start where we end our walk up the beach. Work back down the beach toward the mouth, keeping track of your locationon the airphotos and the Rite-in-the-Rain composite map/photo.

a. Measurement of bedrock attitudes: At the two exposures I indicate, and also wherever you find an exposure that revealsgeologic structure:1. Locate your position on the airphoto and map. Number your locations sequentially in the notebook beginning with 1 at the

north. Mark your numbered positions on the vertical airphoto.2. At each location examine the bedrock and measure any of the following you can recognize:

• Strike and dip of bedding and/or foliation• Strike and dip of faults, jointing, cleavage, or planar veins filling joints• Trend and plunge of lineations or fold axes

3. Record these neatly in a table in your notebook and plot them on the composite map/photo at the correct location, using theappropriate symbols from Compton (p. 373-374)

2b. Bedrock description and sketch at the two outcrops I indicate:

1. make a careful lithologic description of the bedrock. Descriptions should be clear, concise, and informative.Lithologic descriptions should include:• overall color and appearance• degree of weathering -- how hard is the rock, is it friable, what does it break down to if it's pretty weathered?• particle/grain size• texture (foliated, non-foliated )• mineralogy -- what minerals can you see in it, if any?• any sedimentary features observable: grain size, rounding, sorting, stratification (well-laminated? massive?)• type of rock -- give it a name!

2. describe of the outcrop and its structural features such that a person who has never seen the outcrop could read it andunderstand what you saw

Outcrop descriptions should include (but are not limited to):• attitude (strike & dip) of bedding and/or foliation• how obvious is bedding? how strongly developed is foliation? is there more than one direction of foliation? how are

foliation and bedding related (e.g. parallel, crossing at 90˚, or what?)• is there any folding? how does it relate to foliation? is there more than one set of folding? what is the attitude (trend &

plunge) of any fold axes• how fractured is the rock ? is it massive?• are there any joints? are they closely or widely spaced (give dimensions)? is there more than one joint direction? what

is the attitude of the joints?• are there vein-fillings of joints? if so, what is their orientation (strike & dip)? are they folded or faulted? how

continuous are they?3. make a simple annotated/labeled geologic sketch of the outcrop showing the features you measured/observed

c. Geologic outcrop map -- to be made as you work your way down the beach:There are two geologic units in the bluffs that I want you to map: 1) the outcrops of Redwood Creek schist (KJfr), and 2) thecones of talus (Qtal).1. Locating yourself: At each outcrop or feature to be mapped, carefully locate yourself by inspection of topography, and by

using the vertical and oblique airphotos. You can also use pacing from the outcrop at the north end of the beach.2. Outcrop mapping: On your composite topographic map/photo, draw the boundaries of the schist outcrops in the bluffs and

large blocks on the beach. You can use the vertical and oblique airphotos to help you. You can use pacing to help measurehorizontal dimensions. Vertical extent will have to estimated either by eye, or by the combination of pacing a distancenormal to the bluff and then taking a vertical angle with the Brunton (like you did in estimating the flagpole height oncampus.) On your map lightly color these green.

2. Talus mapping: On the composite topographic map/photo, draw the boundaries of the cones of talus and slope debris.

WHAT YOU WILL NEED TO TURN IN

Due date: All materials are due no later than 5PM on Tuesday, March 241. Field notes: A clear, readable copy of your field notes and sketches. This should be the original, not recopied.2. Observation locations: Vertical airphoto(s) with observation station locations correctly shown and neatly numbered and keyed

to notebook.3. Field map: The map you prepared in the field on the composite topographic map/photo base.4. Geologic map: Transfer your data from the field map/photo to the office copy of the map. You will need to do this by eye since

the scales are slightly different. This geologic map should show: 1) all attitudes correctly plotted and inked at sites where theywere measured; and 2) outcrop and talus boundaries neatly plotted and inked. Label schist outcrops KJfr and lightly color thesegreen. Label the talus cones and slope debris Qtal and color them yellow. Use Compton to be sure you are using correctsymbols (i.e., for bedding, foliation, jointing, cleavage, lineations, fold axes) Note: use a very fine pen for inking.

5. Lithologic and outcrop descriptions For each one of the two outcrops I would like a) a typed, clearly written lithologicdescription (see b1 above) and b) a clearly written outcrop description (see b2 above). These should be single-spaced andshould be a maximum of 2 pages for each outcrop. (That is, no more than 4 pages for the whole works.) I expect that you willuse good English and complete but concise sentences.

6. Evaluation sheet on last page of this lab

3REFERENCES

Cashman, S.M., H.M. Kelsey, and D.R. Harden, 1995. Geology of the Redwood Creek Basin, Humboldt County, California: USGeological Survey Professional Paper 1454-B, p. B1-B13.

Harden, D.R, H.M. Kelsey, S.D., Morrison, and T.A. Stephens, 1982. Geologic map of the Redwood Creek drainage basin,Humboldt County, California: US Geological Survey Water-Resources Investigations Open -File Report 81-496

4EVALUATION OF REDWOOD CR MOUTH MAPPING EXERCISE

Name: ___________________________________________Points

Notebook: organization, neatness & readability (10)

Notebook: completeness & relevant detail, descriptions (10)

Airphoto or composite map/photo: observation station locations: (10)

Map: contact accuracy : (10)

Map: detail & coverage: (10)

Map: attitudes: (10)

Map: presentation, neatness: (10)

Rock unit descriptions (30)

Total ____________

Redwood Cr Mouth Northern Oblique, 2005source: California Coastal Record Project

Redwood Cr Mouth Southern Oblique, 2005source: California Coastal Record Project