Great depths to pre-Tertiary rocks below Amargosa Valley also suggest that high-angle normal faulting predominated over low-angle detachment in creating the basin. A thick lacustrine section and early cessation would support a detachment model of extension developed by Brian Wernicke. A later, more gradual die-out of extension and high-angle faulting would support a floating-block faulting model proposed by King and Ellis. Northern Chicago Valley, on the other hand, shows a shallow, faulted basin floor, as proposed by Wernicke.

In Pahrump Valley, which Wernicke believes to be a regional extensional boundary, an active stike-slip fault system further complicates tectonic motions. Gordon Shields summarized the 1994 class's work on the fault zone (click for an HTML version of his AGU poster), where shallow ground-conductivity and magnetic methods were able to accurately locate strike-slip fault strands, and perhaps derive their relative ages. The class confirmed fault strand locations with 2-dimensional shallow seismic reflection surveys. A gravity traverse showed some details of the overall basin structure.

This year we will make large-scale potential-field measurements along highways in Stewart Valley, Chicago Valley, Pahrump Valley, and perhaps California Valley. The bedrock is a pre-Cambrian to Cambrian metasedimentary complex overlain by Tertiary volcanics. Sedimentary fill may include Quaternary alluvium, lavas, and ashes. Pluvial lake sediments often cap the alluvial fill, and have since eroded in places. We will make our most detailed electromagnetic and seismic surveys directly on faulted lacustrine sediments. The water table is often at the surface but may be below the bedrock interface near outcrops.

The terrain is flat to gently sloping, occasionally incised by 10 m deep gullies. Vegetation is low, and sparse to nonexistent. Dirt tracks criss-cross the area. All three valleys are almost entirely public land, with few restrictions. Geologic and topographic maps, and some previous publications, are available to the class.

Our task is to profile the structure of lake sediments and the bedrock topography using a variety of geophysical methods. We will try to record a high-resolution 3-d seismic reflection survey across the State Line fault in Stewart Valley, using MSM's new 48-channel seismic recorder, to attempt to image an alluvial channel offset across the fault. We can use source and receiver grid spacings between 1 and 5 meters. Gravity measurements can probably be made of 50-100 stations spaced between 50 and 300 m, for a total profile length of 2.5 to 30 km. Each of these stations must be surveyed in to 1 foot elevation accuracy. Magnetic measurements may be made at twice as many stations over twice the distance, or half the spacing. Electromagnetic soundings may be made at 6-10 locations with 10 to 100 m apertures, and we can record shallow ground conductivities at 10-50 m intervals in grids or over lengths up to 10 km.

• Magnetic
• Gravity
• Electromagnetic
• Seismic reflection

Guidelines for proposing the survey plans are below. Certain questions need to be answered soon, and preparations begun early. These parts of the plans should be prepared by Mar. 18, and will be reviewed and discussed by the class that week. Each team should see J. Louie as soon as possible to begin preparations. The remainder of each plan should be finished by Mar. 27, so we can act on the plans during the week before we go in the field.

Finished plans should also include complete and detailed checklists of every item that will go to the field, data sheets and/or software disks, instrument operation instructions, maps showing proposed survey locations, and schedules for work by each team. Each team should turn in one set of plans on March 27, which I will evaluate and use to affect your final report grades.

This year the Mackay School of Mines will host a gala open house on Friday, April 26, about two weeks after we return from the field. Researchers and classes across MSM will prepare posters for display during the open house, and individual students can compete for cash prizes for the best research posters. In addition, class groups, such as the field teams in this class, can compete for team recognition prizes. The members of any team that presents a merely decent poster on our field results will each have their weakest score among the four parts of this course (midterm, abstracts, discussions, field report) raised by one full grade. The members of any team winning a first or second prize in the poster competition will have two of their weakest scores raised by one full grade.

Teams that decide to compete in the poster competition need to submit the title of their poster to me by March 15. The School will make funds available to registered teams for poster supplies and services such as photographic enlargements.

• **What instruments are needed for the survey?
• **What instruments are available to us?
• **Who oversees the instruments? Are they available?
• **Are the instruments in good order? Have they been recently tested in the lab and on the ground? Can they be calibrated?
• **What supplies are needed to operate the instrument and record data? Batteries? Special paper? How are batteries to be charged, or sensitive materials stored?
• **If the instruments need repair or supplies need to be obtained, can this be done before departure?
• What provision can be made for instrument failure in the field? Would any tools be useful? Spare parts?
• **How and when are the other participants to be trained in the use of the instruments? What manuals are available? Can brief instructions for field use be written? Data entry forms prepared?
• **What items and procedures need to be put on a checklist that can be completed during mobilization?
• **How can the instruments be shipped to the field area? Are they especially sensitive? Do special arrangements need to be made to borrow them from the Bureau of Mines, or other Universities?
• **Are there materials or supplies that can or must be obtained at the field area? Where and when will this be done?

• What location accuracy is needed for survey stations? Do they need to be surveyed in? Could pace-and-compass locations be adequate?
• What site or geologic factors will contribute to successful bedrock profiling with this technique? Where could these be present in the field area?
• What site or geologic factors will contribute to successful shallow profiling with this technique? Where could these be present in the field area?
• In what parts of the field area would this technique help to constrain the interpretations from another technique?
• In what parts of the field area would the use of this technique be difficult? Are there access or surveying problems?
• How will the instruments be moved to different sites in the field area? When will vehicles be required? Will some stations have to be reached on foot?
• How many people and how much time are needed for each station, or each experiment?
• How much area or how many kilometers of profile can we do while we are in the field?
• How can a schedule be set up so everyone uses each of the instruments in the field?
• Where will the survey stations be? What profiles or areas will they cover? In what order will they be measured?
• How will the data be labeled and stored when it is collected to avoid loss or confusion later?
• What data quality-control procedures can be used? Can data be immediately reduced or plotted in the field to check for accuracy?
• Can initial results be used to guide the other techniques? Or to adjust survey plans on the fly?

• What procedures will be used to reduce and interpret the data? Would any results of the other surveys be needed? Would this survey's results be useful to another's?
• How and when will the data be disseminated to the other students? How will needed accessory information, such as station locations and instrument settings, be provided?

• Are there any items or concerns that need to be added to this list?

References

Compton, 1962, Manual of Field Geology, chapters 2, 3, 4, 11.

Dobrin and Savit, 1988, Introduction to Geophysical Prospecting, on reserve in the Mines library, pages 3-8 and as noted below:

• **What instrument would be easiest to use while providing enough accuracy? Alidade and plane table? Laser transit?
• **What maps, airphotos, and remote-sensing images are available? In what forms will copies be needed before, during, and after field work?
• What horizontal and vertical control has already been established in the field area? Are control points accessible? How will they be tied in?

• **What special precautions need to be taken to assure the stability of the gravimeter? Are extra batteries needed?
• Are any absolute gravity control stations available near the field area? When should they be measured?
• What gravity measurements have been made previously in the field area? How will our survey improve upon that work?
• Where should control stations be established, and how often should they be measured?
• Are any data reduction or modeling packages available? Can they work in the field?
• What sources of local rock density measurements are available?
• What accuracy is needed to make a useful interpretation of basement topography? What procedures will enhance accuracy?

• **Are the instruments working and ready for the field? What can be rented or borrowed?
• **What power sources are required?
• What is the maximum electrode spread, loop area, or time gate? What is needed to sound to our target depths?
• What are the expected depths of penetration for each of the available instruments?
• Are special procedures needed to properly ground electrodes in materials such as dry gravels? What tools and supplies will be needed?
• What will be the effect of the water table?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?

• **What instruments are available and working?
• **Are recording base stations available? Gradient instruments?
• What magnetic surveys have previously been performed in the field area? How will our survey improve on them?
• Where should drift control stations be located, and how often should they be measured?
• What accuracy is needed to detect the target basement topography? What if the bedrock changes from volcanic to metasedimentary?
• What will be the effect of volcanic materials within the sediments?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?

• **What recorders, cables, and geophones are available? Can any be rented or borrowed?
• What seismic surveys have been performed in the region? What were their results?
• What seismic velocity measurements are available for our area? If none, what are the likely velocities, and their contrasts at the basement interface?
• What line lengths are needed to locate basement refractions for different target depths? What are the possible dips?
• Which geophones should be used? Can an S-wave experiment be conducted?
• Will hammer blows provide enough seismic energy? Should another source be considered? At which sites will hammer surveys have the most chance of succeeding?
• What arrivals will likely be observed? How will they be interpreted?
• What 3-d or pseudo-3-d shallow surveys are possible with the available time and equipment?
• What areas are most conducive to getting good high-resolution data? What condition of the water table is helpful?
• **Where are fault offets of buried neogene channels most accessible to reflection surveying?
• How small a hammer source can be used? What tests will need to be done in the field on different sources?
• What interpretation or analysis packages are available? Can analysis be done in the field? How will data be downloaded?