Redwall Alluvial Fan Mapping Exercise

Field Methods in Active Tectonics, Spring 2017

Reading: Frankel et al., 2007

An exercise on interactions between active strain and sedimentation, the class will complete an introductory mapping exercise along a spectacularly preserved fault in the northernmost part of Death Valley.

Purpose of the exercise is as follows:

Learn how to identify and map young faults, including surface ruptures formed by the accumulation of Late Quaternary earthquakes

Learn to identify and define the relative chronology of alluvial deposits offset by faults and their relationship to local geomorphology

Identify piercing points that might be used to constrain slip rates on the fault.

Introduction to Redwall Fan The exercise today is an introduction to the methods one might use to determine the rate of slip on an active right lateral fault. Redwall fan is a classic location in which to study an active fault that offsets late Quaternary alluvium. It forms part of the northern Death Valley fault zone in a region sitting just above sea level, north of the lower, more highly extended parts of Death Valley. The area you are mapping includes the southern half of a large alluvial fan (see color bands that mimic topographic contours that trend from lower left to upper right on your basemap). The northwestern part of the fan has had its uppermost deposits eroded by a large channel network that are then buried by younger alluvium. The fault cuts obliquely across topographic contours. The oldest alluvial deposits on the fan surface are ~ 70,000 years old. As in the Baja exercise, the relative ages of alluvial deposits preserved in arid regions can be defined as follows: older fan surfaces are darker in color due to the development of desert patina on cobbles and boulders preserved on their surfaces and are smoother due to the development of desert pavement (making them a lot easier to walk along). In contrast younger surfaces are lighter colored and typified by small braided channels and bars comprised of coarse cobbles and boulders that have not been exposed long enough to develop a desert pavement. A side note is that only clasts comprised of silicic material, such as granite, quartzite and sandstone can develop a patina, in contrast to carbonate rocks that cannot.

A very important aspect of determining slip rates at Redwall is the point that the channel networks cut their way headward from the floor of the valley after the fan surface was abandoned. This is a key point, given you should be using the channel network as a means of confirming the right lateral sense of slip on the fault, and determining the slip rate across it.

Exercise Create a map of the fault zone (the area covered by your basemap) that includes alluvial deposits of relatively different age (i.e. break out alluvial deposits into 4-5 units). Next map the fault zone in as much detail as possible, including separate fault strands. You can label beheaded and deflected streams along the fault using letters keyed to a report (B = beheaded, D = deflected). Most importantly, you need to identify piercing points across the fault; for Redwall these include offset channels that can be matched across it (OC-1 and OC-1’ = Offset channel 1, etc). Three good piercing points have be identified that all show about the same amount of offset and these are associated with the largest channels that are cut by the fault. While these are already identified in Frankel’s paper, it is fun to try and figure it out yourself by just looking at the basemap.

While you are in the field, draw a simplified block diagram of the area you have mapped that illustrates the relationship between the geometry of the fan, and the direction the scarp faces (which switches along the fault surface). Then explain whether the scarp has any actual real (tectonic) vertical (i.e. normal) displacement across it, or whether the vertical displacement is apparent and the result of something else.

Calculate the slip rate on the fault using the average distance between piercing points you identified and an age for the fan of 70,000 years. Please show your calculations and relate them to features on the map. The scale on the map is about 1:3300, or 1 mm = 3.3 meters (1 cm = 33 meters). You can use the scale at the bottom of the page to measure offset. Compare this with Frankel’s results.

Piece of an apple, cut and displaced in an artistic fashion. See my website at dextralfruit.com.