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Geology and Archeology - the Research

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RESEARCH

The Johnson Hypothesis argues that many of the geoglyphs mark sources of subsurface water. The hypothesis relies extensively on the fact that faults and structures in the bedrock transmit groundwater to the alluvial valleys.

In order to test this idea scientifically five questions must be addressed:

1) Do the faults exist?
2) Do the faults intersect the valleys?
3) Is there evidence that the faults transmit water?
4) Are the faults marked by geoglyphs?
5) Are there geoglyphs associated with the faults?

Accordingly, we have developed a working hypothesis and a research plan that will address these critical questions. We hypothesize that there are two independent pathways that move water from the source area on the Altiplano to lower elevations where it is used for irrigation and domestic needs. One pathway is water that moves as surface flow and as groundwater flow in the alluvium along the main river channels (hereafter referred to as the river pathway). The other pathway utilizes an interconnected system of fractures, faults, and other transmissive discontinuities that convey water on a regional scale across basins until it discharges from the bedrock along the valley sidewalls into the adjacent alluvium/colluvium (bedrock pathway). The groundwater derived from the bedrock pathway discharges at springs and subsurface seeps along the main river valleys.

Preliminary evidence collected over the last three years suggests that water-bearing conduits in the bedrock supply a reliable, high quality source of fresh water to the east-west river valleys.
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Research Plan

We have developed a multi-faceted research plan to test the Johnson Hypothesis. The plan employs standard geological and hydrogeological techniques in conjunction with customary archaeological methods. A brief description of the research plan follows.
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Remote Sensing and Geologic Mapping - Faults are discontinuities in the bedrock that arise from tectonic forces causing the rocks in the earth's crust to shear and move displacing one side of the fault relative to the other. Once the rocks have been broken, the fault plane becomes a permanent zone of weakness that often enhances the permeability of the surrounding host rock by providing fractures through which water can move. Depending on the type of rock involved and the nature of the fluids moving along the fault plane, some faults can become mineralized and act as a barrier to flow forcing groundwater to seep out at the ground surface whereas other faults remain open allowing water to flow freely along the fault plane. Faults can be a few millimeters to several kilometers in width and range from a single fault plane to a wide zone of highly fractured rock that contains an anastomosing, interwoven network of permeable, interconnected fractures.

There are several clues that provide evidence for faulting in the field. These include offset between rock types on opposite sides of the fault plane, folding of the host rock due to drag along the fault, brecciated rock within the fault zone and striated fault plane surfaces, highly fractured rock adjacent to the fault, and extensive mineralization suggesting past or recent fluid movement.

For this study, we employed remote sensing analysis and geologic mapping to assess the propensity of the bedrock to transmit water.

1. Satellite images and aerial photographs were acquired to look for evidence of faulting. Because faults represent weaknesses in the rock, they are more prone to weathering. As such, they can be more easily eroded than the surrounding country rock and, therefore, often appear as topographic depressions or linear scars across the landscape. These features can often be detected on topographic maps, aerial photographs, and satellite imagery particularly in arid to semi-arid environments where vegetative cover is sparse. These features are referred to as lineaments and very often represent faults. Field checks were made to verify the existence of faults from the lineament analysis.
2. All published geologic maps were gathered for the region and studied.
3. As a final step, traverses were made at specific sites by geologists to verify the published geologic maps and to confirm faults observed on the satellite images and aerial photographs. Any unmapped faults were located and characterized.
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Geophysical Analysis - In order to verify that the faults observed in bedrock exposures and on satellite imagery intersect the major river drainages and lie beneath the surficial materials deposited within the valley, geophysical tools were employed, where feasible, to locate the faults in the subsurface. In addition to locating faults, geophysical tools were also utilized to determine the depth to bedrock below the unconsolidated surficial deposits and the depth to the water table. Although several geophysical methods have been used over the last three years of field work, two were successful and include: 1) seismic refraction to locate the depth to bedrock beneath the surficial soil material and to determine the depth to the water table; and, 2) electromagnetic induction (EM-34) to map the position of buried faults.
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Water Quality Sampling Program - Water samples were collected from springs, seeps, and wells, where feasible, and analyzed for major inorganic constituents and hydrogen and oxygen isotopes. The chemistry of groundwater inherits properties of the material through which it travels and will vary depending on the flow path and length of time the water has resided in the ground. Accordingly, inferences can often be made on the source of the groundwater from a close examination of the geochemistry of the water. In this study, temperature, conductivity, dissolved oxygen, alkalinity and pH were measured in the field. The remaining samples were collected, preserved, refrigerated and shipped back to the U.S. for analysis of other constituents (calcium, magnesium, sodium, potassium, fluoride, chloride, nitrite, bromide, nitrate, phosphate, sulfate, boron, barium, chromium, iron, manganese, strontium, titanium, zinc, silicon and hydrogen and oxygen isotopes).
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Archaeological Analysis - The age and cultural affiliation of archaeological sites was determined by means of differences in the associated ceramic, textile, and tomb styles and in architectural patterns. The function of the component parts of each site was determined by artifact clustering and by differences in the size, composition and distribution of the architectural features. Published archaeological reports on the Río Grande de Nasca drainage were examined as well as unpublished documents. The location, azimuth, length and character of geoglyphs were also noted. Maps were prepared of the geoglyphs along with the location of the nearest water sources and faults.
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