Geological Report

March 15, 1995

Proj. 9411.1

6-1

SECTION 6.   GEOMORPHOLOGY

6.1 Playa Geomorphology

Playas are one of the most dynamic of landforms offering daily, seasonal and annual variations in their environment (Neal, 1972; Neal and Motts, 1967).    Individual events can often shape the appearance of a playa for 10s and often 100s or thousands of years. Typical changes to a playa are usually hydrologic and/or biologic in origin.

The Forty Mile Desert (FMD) is a relatively flat alkali playa that occupies a small inter-basinal setting. The FMD is situated along the lower portion of the Humboldt River drainage between the Humboldt and the Carson Sinks.  The FMD basin is separated from the Humboldt Sink by the Humboldt Dike, a natural gravel/pebble point bar barrier.  The Humboldt River enters the basin through a modified natural trough, the Humboldt Drain and exits the basin through the Humboldt Slough, a modified natural trough, to the Carson Sink - the final destination of the Humboldt River.   Surface flow only occurs in the vicinity of the Humboldt Dike and the Humboldt Slough, however, during high run-off years, surface water travels the course of the basin.   During these high run-off years, large subsided areas in the western and southern portions of the basin are inundated to form large ponds and lakes.

We are unsure how the playa looked following the final desiccation of Lake Lahontan, but we can assume it looked much like a typical alkali flat elsewhere in Nevada with an intermittent stream or river through it.

There are three distinct zones to the FMD playa: classical playa surfaces (S/P 3), a transition zone from the classical playa to a playa"ilctive playa zone (P2), and an active playa zone (PJ The topography of the P/S1 surfaces is relatively flat and forms typical playa characteristics, including porous, soft and puffy or crusty surfaces, salt-thrust polygons, mud cracks, and abundant evaporate mineral assemblages.  The P, zone Poner Geotechnical dominantly occurs on the outside rim of the basin.  It is also found along the tops of raised areas within the interior of the basin. The P/5, playas on the outside rim of the basin is being encroached by prograding fans containing coarse grained material (sands, gravels and some cobbles). A sandy apron generally progrades from some of the more active fans. Sand dunes are well established on these surfaces. Most of the moisture for this area comes from precipitation, run-off from adjacent washes, or from ground water from p2 or P1•    these surfaces are believed to represent the original desiccation playa surface.

The p1 and p2   playa surfaces are the active surfaces. The P, surface is the area that receive the most surface flow activity. It includes a mixture of active channels, meander scars, and broad flats that receive seasonal inundation. The Pz surface is a transition surface that is less active than the P, surface, but more active than the S, IP3 surface. It includes a high-water flood surfaces, flood meanders scars, and also acts as an intermediate playa terrace in broad subsided regions. Sand dunes are not common in these areas. Vegetation is more plentiful in the lower P, and P2 zones whereas nearly absent on the S/P3   surface.  Vegetation consists of greasewood, rabbit brush and a variety of salt grasses. Willow and salt cedar grow principally along the banks of the P, channels and abandoned meanders.  Occasional springs attract clusters of vegetation.

6.1.1    Catastrophic Flooding

In the eastern portion of the basin, strong evidence indicates this region has been subjected to periods of catastrophic flooding.  These floods appear to have emanated from the Humboldt Drain region and indicate water from the Humboldt Sink cascaded through the opening in cataclysmic events. The flood waters coarse can be followed by large scours into the once S/P3 surface that has been subsequently vegetated. This course swung northward after it left the Humboldt Drain and swept westward and around the Barren Playa, where it spreads laterally and diminished in erosive strength.  The water rapidly filled the western portion of the basin and eventually, spilled into the Carson Sink.  The P2 surface in the eastern portion of the basin was generally inundated by the flood paths. As the waters receded, meanders were cut and abandoned, and new main channels were formed.  It is believed that some of the Fallon age lakes of Carson Desert described by Morrison (1964) were caused by the breaches in the Humboldt Dike and the subsequent filling of the Carson Sink.

Although  it  is not  clear why  the floods raced northward  after the breach in  the Humboldt  Dike, it is postulated that the massive dune field at the east end of the Barren Playa probably  acted a formidable  barrier against the initial  stages of the release of water just west of the Humboldt  Drain is a large debris apron composed of sand, silt, clay, pebbles and cobbles.  It has an organic rich soil horizon formed on a substantially hummocky topographic surface.  It is heavily dissected by several meander scars. This debris apron represents the initial material that was dispensed from the breach in the Humboldt Dike.

At least two, and possibly three, catastrophic flood events are distinguishable by the freshness of the scouring pattern. The older flood scour appears to have maintained a more central course in the basin, whereas the younger event appears to have taken a more northern route. A very old catastrophic flood is difficult to verify, but appears to have taken a more central route.

6.1.2   Recent Geomorphic Changes

As part of Newlands Reclamation Project, the Humboldt Sink and the Forty Mile Desert were drained to provide additional acreage for farming and agriculture purposes, as well as provide additional water resources to farmers downstream.  In 1915, a ditch was dug from the Humboldt Sink through the natural breach in the Humboldt Dike, and part way across the eastern portion of the FMD.   The main channel of the Humboldt River through the northern and western portions of the FMD basin were also modified to carry the additional flow expected from this new drain. An additional north trending feeder drain was dug west of the Barren Playa to intercept mid playa flow or perhaps seepage from then active prominent liquefaction eruptive centers. The drain was cleaned and deepened in 1946.   Records show the project was not successful for a variety of no pertinent reasons.

It is beyond the scope of this project to understand the effects of the Newlands drains have had on the basin.  However, it has been documented from many other alkali flats in the western United States that modifications in the natural hydraulic systems can cause substantial surficial effects.  They include: widespread subsidence from ground water withdrawal, drying up of natural springs, changes in vegetation growth patterns, and the formation large desiccation cracks.  All of these changes can be noted in the FMD.   Of particular attention is the formation of large desiccation cracks.

Throughout the basin there are numerous, randomly oriented cracks in the surface. Most of these cracks are on the order of a millimeter to a couple of centimeters; however, others are on the order of 1Os on centimeters in width.  The length of the cracks range from a few dozen feet to about a few thousand feet.  Some of the wider cracks have depths of greater than 10 feet.  In the Ocala Sump area, a wide crack extended as much as + 10 feet into the subsurface (see Figure 1).

Most of the smaller cracks appear to be related to desiccation of the near surface soil. The larger cracks, however, are more perplexing. Neal and Motts (1967) described giant polygon cracks that were developing in alkali plays throughout the United States. These features, which were commonly 10-300 meters across, were caused by unique types of surface soil, usually of high clay content, and   a fluctuating groundwater table at appropriate depths to fadlitate the growth of these cracks. We noticed  on  the  FMD  Basin that  most of  the  cracks follow   zones of  weakness developed  by  faulting, fracturing  or  liquefaction  failures.  Clear examples were observed at the northern portion of the Barren Playa where the development of prominent cracks lined up perfectly with extensive surface rupture traces from faults. Undoubtedly, the desiccation process played a role in their development, but their specific location appears related to pre-established zones of weakness.

6.1.3     Recent Seismic Activity

Within the western Basin and Range, there have been five very strong (M   > 7.0) earthquakes since about 1840: the 1845 north Carson Sink earthquake, the 1872 Owens Valley earthquake, the 1915 Pleasant Valley earthquake, the 1932  Cedar Mountains earthquake, and the Fairview Peak and Dixie  Valley earthquakes (Bell,1984; Bell and Slemmons, 1979; Slemmons and others, 1959; Ryall, 1977).  On the average, western Nevada experiences earthquakes in the magnitude 6.0 range occurring every 11 years, and in the magnitude + 7.0 range every 27 years (Ryall, 1977).

Large magnitude earthquakes in Nevada have caused ground displacements on the order of inches to 1Os of feet. Other physical effects from these events have included differential ground subsidence, Liquefaction and/or lateral spreading, and Liquefaction boils.  These effects are generally restricted too saturated or near saturated soil conditions.  In Nevada, the lacustrine basins have been prime candidates of this kind of physical damage (Siemmons, 1957; Bell, 1984). The earthquake need not be centered in each the basin to cause these physical effects, as powerful  earthquakes send tremors radiating for lOs and even hundreds of miles.

6.1.4    Faults, Fractures and Lineaments

Recent  geologic mapping by photo geologic interpretations  and some field reconnaissance has discovered  a  complex  assemblage of  faults,  fractures and lineaments exist within  the Forty Mile  Desert basin (Plate 3, Geologic Map).   The dominant orientation is in the northeast direction however, other prominent structures are present.  Six distinct orientations are observed: N50-70°E, N20-30°E, N5°W-N10°E, E-W, N40-50°W, and N65-70°W. Numerous minor and subordinate fractures dissect the playa suggesting an appearance like a shattered plate glass window.

Offsets have been observed on many of these lineaments from aerial photographs and field reconnaissance. The most spectacular off-sets were found on the north side of the West Humboldt Range south of the Humboldt Dike.   Field reconnaissance of these structures discovered a set N50-70°E dip-slip faults had an estimated 27 feet vertical offset (north  side down)  with  visible  horizontal  displacements of Lake 4 Lahontan 5 shorelines. Other sub parallel structures were observed along the terrace deposits north of the main rupture trace to contain significant offsets of 8-12 feet. The significance of displacements such as these, hence the magnitude of earthquake producing the rupture, has far reaching liquefaction implications for the saturated sediments of the FMD.

Another structure with substantial vertical displacement is observed just off the northwestern portion of the Geologic Map in the Jessup Wash region.    These prominent sets of northeast trending structures contain displacements on the order of 10s of feet. Limited time did not permit reconnaissance of these structures however they are so prominent they can be seen from Interstate 80 some 2 miles away. Substantial pediment off-setting structures with displacements of several feet were also observed along an access road to the FMD from the Brady’s geothermal field.   Both the Jessup and pediment structures warrant further investigation.

A prominent set of N45°E trending faults was discovered in the southwestern most corner of the FMD basin near the Mule Train Mine and the West Spring.  These structures were previously mapped by Bell (1984); Field reconnaissance indicated variable displacements of shorelines and terrace deposits to be on the order of a few feet, with a right-lateral dip-slip motion.   Inundation of the faulted graven during 52 times may provide a closer approximation to the timing of this event. The West Spring and the other springs have developed along the trace of the faults.  The northern extent of the N45°E faults appears to terminate along a prominent N60-70°W structure. A prominent N65-70°W structure is located along the southern margin of the FMD basin near the Humboldt Slough.  It aligns aerial photograph lineaments including, liquefaction boils, nick points in river channels, the ending of the Carson Basin dune field, the ponding of intermittent surface waters in Carson Basin, and the natural access channel between Carson Lake and the Stillwater reservoir. It also separates the Hot Springs Mountains and the Mopung Hills and perhaps is responsible for periodic opening of the Humboldt Slough. Regional geophysical surveys (Benoit et al, 1982; Irwin and Berg, 1977) and geophysical surveys conducted by this investigation indicate the western portion of the FMD is one of the deepest portions of the basin. The N65-70°W lineament may be a controlling structure to this part of the basin, facilitating the deepening of the basin.  Interestingly, there are few surface thermal manifestations east of this lineament.  It suggests the structure forms a barrier to the thermal source from the Hot Springs Mountains and the Soda Lake-Upsala Hogback thermal trends or along with other structures, provides such a deepening of the basin such that thermal fluids percolate into other permeable sedimentary layers buried deep within the basin.  Another prominent lineament zone composed of several near parallel structures trends N50-70°E and can be traced across the entire playa surface. These structures emanate from the Hot  Springs Mountains  and cross the west-central portion  of the playa, extend across the southern portion  of the east-basin area, and through  the central portion  of the Humboldt  Dike  and into the Humboldt  Sink for a couple of miles. Several photo geologic characteristics define this lineament, including changes in physical feature boundaries, color and textural variations, vegetation and physical feature lineaments and topographic changes. Numerous liquefaction eruption centers are along these lineaments. Field reconnaissance of the Humboldt Dike indicates no clear vertical displacement; however it may contain a left-lateral strike-slip component.   Strong rill seepage is observed on both sides of the dike where the lineaments are projected and a gentle swale with 6-8 inches on vertical variation is seen on the surface of the dike.   Observations of the structure where it crosses the Barren Playa indicate a change in slope that could represent up to 2-3 feet of vertical off-set.

Several structural lineaments appear to cross the Humboldt Dike; however, there are no clear indications of vertical off-set.   There are some indications of left-lateral strike-slip displacement, but further field work is needed to fully comprehend their significance.   It is possible the surface Humboldt Dike may have been modified during one of its many fillings of the Humboldt Sink during the latest Holocene.

Although there are several minor N5°W-N10°E trending lineaments, the more prominent structures appear quite evenly spaced across the playa.  Several distinguishing geomorphic features are observed along the western edge of the playa and identify an extensive fracture system from the southwestern most portion of the playa up into the Jessup Wash. This same fracture orientation was observed along the southeastern most portion of the FMD basin, along the western portion of the Barren Playa, and along the western side of the Humboldt Sink.  Lineaments along the western portion of the Humboldt   Sink are tentatively referred to as the West Humboldt Sink fault zone by Ken Adams (PhD candidate, Center for Neotectonic Studies, University of Nevada, Reno; personal communication, November, 1994). This structure has distinguishable Holocene surface rupture of over a length of 16 km. Notable features include massive liquefaction eruption centers and offset playa surfaces.

One of the most obvious lineaments in the basin are a set of two N40-50°W  parallel features in  the center of  the basin, a couple  miles west of  Highway  95.  The morphology of these features is subdued and discontinuous. They appear to have been cut and displaced by other more recent events, however, have maintained a distinguishable continuity.  They appear to have sustained much erosion, even from catastrophic flooding from Humboldt Sink.  On the ground, these features create elevation changes of several feet.  The origin of these features is not clear; whether they are actual faults that formed during times when the basin had water, or if they are preserved pressure ridges from a powerful local earthquake; the nature of these features is unknown at this time, but may be the oldest structural feature on the playa surface.

Structures in and around the Barren Playa show significant displacement and reveal a complex relationship of events.  A powerful earthquake caused massive liquefaction eruption. The fluid from these centered flowed eastward into the Barren Playa. The western boundary of the playa was subsequently uplifted on the west by a north-south trending structure, which separates the inflowing channel from the basin.  This north-south structure was then cut by an N45ow structure at the north end of the playa, which was subsequently cut by a N40°E structure. The final event also caused significant liquefaction failures along the western portion of the Barren Playa, creating the Ocala Sink.

Not all of the lineaments shown on the map are actual faults. Many are restricted to the playa and may be related to liquefaction features or lateral spreading phenomena from a distant earthquake.  However, many fractures do show substantial displacement over a considerable length and support good continuity of structure and therefore, are faults.   Detailed investigations of these structures, utilizing extensive field and subsurface mapping will bare witness to their potential activity and their contribution to the mineralization history.

What is staggering about these structures, is most or all of them occurred since or at near desiccation of the last Lake Lahontan high-stand or about 7,500 ybp.  If it is assumed that each of the six orientations of structures described above is valid surface rupturing, it suggests a potential reoccurrence interval of 1,250 years.  It is possible to form more than one orientation of displacement during a faulting event. During the 1954 Stillwater earthquakes, orientations of N40-50°E and N45°W developed in the Carson Sink (Bell, 1984; Slemmons, 1957). However, even if we eliminate half of the events, there is still a reoccurrence rate of 2,500 years. This is an exceedingly high surface rupture reoccurrence rate for anywhere in Nevada, and matches rate for such prominent fault zones as the Walker Lane or the Sierra Nevada eastern escarpment fault zone in the Owens Valley.  It also suggests that since 330,000 years, or the time represented in boring H5E5 at 131 feet, there have been on the order of + 130 surface rupturing events in the basin.

6.1.5 Liquefaction Failures and Eruptions

Liquefaction is the condition of soil losing its resistance to shearing.  Saturated fine-grained sands are the soils most subject to liquefaction.   When pore pressure exceeds the internal pressure of grain to grain contact, the soil takes on the characteristic of a dense liquid.  Sufficiently intense or long duration shaking from an earthquake tends to break the cohesion between the grains, increasing pore pressure, and the strength of the soil is lost and the sand/water mixture begins to flow.   Two dramatic features result from liquefaction: 1) overlying soils tend to collapse forming subsidence blocks or unstable hummucky topography; and 2) force a sand/water mixture into fractures that erupt on the surface, sometimes in copious quantities (see Figure 2).

Both of these liquefaction features have been observed on the FMD.  Two principal areas observed have experienced liquefaction ground failures: 1) on the west side of the Barren Playa, near Ocala; and, 2) the other on the southeast side of the Barren Playa. The liquefaction ground failure near Ocala is dramatic. In an area confined by two recent northeast trending surface rupturing faults a distinct hummucky topography has formed, with mounds and valleys typically on the order of 3-4 feet. Some areas have dropped substantially, with an estimated 30 feet of displacement. One area, called the Ocala Sink, shows 30 + feet of displacement, providing excellent exposures of lacustrine   soils and sediments (Figure 3, Ocala Sink).  [Recent groundwater withdrawal from the basin may have facilitated these liquefaction features.

The non-degraded appearance of the Ocala liquefaction failures indicate it happened in the not so distance past. It is possible that this feature may have been related to the powerful North Carson Sink + 7.0 earthquake in 1845, for which the location is not well constrained. The liquefaction ground failures on the southeast side of the Barren Playa have been overrun by the dune field and have a degraded appearance suggesting their older age.

Broad areas in the western portion of the basin that are inundated with seasonal flooding show characteristics on being subsidence blocks. Many of the subsidence blocks are bounded by structural lineaments and have numerous liquefaction boils along their perimeter. These subsidence blocks can also be found in the eastern portion of the basin but are more difficult to identify because of the thick vegetation. It suggests that the much of the broad P1 surfaces are actually a down-dropped 5/P1 surfaces. Subsidence features like these were observed in the Fallon area following the powerful Stillwater and Dixie Valley-Fairview peak earthquakes in 1954. This method of forming prominent troughs throughout the basin will have substantial influence on the sedimentation history and as well as the mineralization record.

Aerial photographs show numerous circular and elliptical features occurring along the strike of recent fractures (see Figure 4).  These features ranged in size from 1Os to hundreds of feet across.  Investigation has determined these features are enormous ground water eruptions or liquefaction boils.  Because of their size, they obviously involved copious quantities of fluid.  As one stands in the middle of these features it is difficult to imagine the scale of the ground water eruption. Several key boils show fluid erosion extending away from the elliptical center, pooling in adjacent lows, or finding its way to an active or abandoned channel.  One such event caused boils to erupt in the Ocala area sending fluid into the Barren Playa, now  evident by the distinct lack of vegetation (probably due to the toxicity of the original groundwater fluids).  These eruptive centers are believed to be a major source of subsurface fluid transport to the surface.

6.2        Playa Geomorphology Summary

Dramatic evidence for faults, subsidence, liquefaction and liquefaction boils have been observed in the FMD basin.  Several surface rupturing fractures and faults are found in multiple orientations throughout the FMD basin. Some show substantial offsets of 1Os of feet, while others display offsets on the order of feet or inches.  Evidence from aerial interpretation and direct ground verification for these recent structures includes offset lacustrine terraces, shorelines, playa surfaces, fan deposits, and stream channels. Possible offsets in the Humboldt Dike and the breach at the southern end is likely due to fault ruptures that caused a catastrophic flood event across the FMD.   Liquefaction features have played a major role in shaping the current playa surface and probably had a considerable effect on the sedimentation history of the basin.  Liquefaction eruptive centers produce massive amount of fluid and are seen as one of the major contributors of transporting solutions from deep within the basin sediments to the surface.


Geological Report

March 15, 1995

Proj. 9411.1

6-1

SECTION 6.   GEOMORPHOLOGY

6.1 Playa Geomorphology

Playas are one of the most dynamic of landforms offering daily, seasonal and annual variations in their environment (Neal, 1972; Neal and Motts, 1967).    Individual events can often shape the appearance of a playa for 10s and often 100s or thousands of years. Typical changes to a playa are usually hydrologic and/or biologic in origin.

The Forty Mile Desert (FMD) is a relatively flat alkali playa that occupies a small inter-basinal setting. The FMD is situated along the lower portion of the Humboldt River drainage between the Humboldt and the Carson Sinks.  The FMD basin is separated from the Humboldt Sink by the Humboldt Dike, a natural gravel/pebble point bar barrier.  The Humboldt River enters the basin through a modified natural trough, the Humboldt Drain and exits the basin through the Humboldt Slough, a modified natural trough, to the Carson Sink - the final destination of the Humboldt River.   Surface flow only occurs in the vicinity of the Humboldt Dike and the Humboldt Slough, however, during high run-off years, surface water travels the course of the basin.   During these high run-off years, large subsided areas in the western and southern portions of the basin are inundated to form large ponds and lakes.

We are unsure how the playa looked following the final desiccation of Lake Lahontan, but we can assume it looked much like a typical alkali flat elsewhere in Nevada with an intermittent stream or river through it.

There are three distinct zones to the FMD playa: classical playa surfaces (S/P 3), a transition zone from the classical playa to a playa"ilctive playa zone (P2), and an active playa zone (PJ The topography of the P/S1 surfaces is relatively flat and forms typical playa characteristics, including porous, soft and puffy or crusty surfaces, salt-thrust polygons, mud cracks, and abundant evaporate mineral assemblages.  The P, zone Poner Geotechnical dominantly occurs on the outside rim of the basin.  It is also found along the tops of raised areas within the interior of the basin. The P/5, playas on the outside rim of the basin is being encroached by prograding fans containing coarse grained material (sands, gravels and some cobbles). A sandy apron generally progrades from some of the more active fans. Sand dunes are well established on these surfaces. Most of the moisture for this area comes from precipitation, run-off from adjacent washes, or from ground water from p2 or P1•    these surfaces are believed to represent the original desiccation playa surface.

The p1 and p2   playa surfaces are the active surfaces. The P, surface is the area that receive the most surface flow activity. It includes a mixture of active channels, meander scars, and broad flats that receive seasonal inundation. The Pz surface is a transition surface that is less active than the P, surface, but more active than the S, IP3 surface. It includes a high-water flood surfaces, flood meanders scars, and also acts as an intermediate playa terrace in broad subsided regions. Sand dunes are not common in these areas. Vegetation is more plentiful in the lower P, and P2 zones whereas nearly absent on the S/P3   surface.  Vegetation consists of greasewood, rabbit brush and a variety of salt grasses. Willow and salt cedar grow principally along the banks of the P, channels and abandoned meanders.  Occasional springs attract clusters of vegetation.

6.1.1    Catastrophic Flooding

In the eastern portion of the basin, strong evidence indicates this region has been subjected to periods of catastrophic flooding.  These floods appear to have emanated from the Humboldt Drain region and indicate water from the Humboldt Sink cascaded through the opening in cataclysmic events. The flood waters coarse can be followed by large scours into the once S/P3 surface that has been subsequently vegetated. This course swung northward after it left the Humboldt Drain and swept westward and around the Barren Playa, where it spreads laterally and diminished in erosive strength.  The water rapidly filled the western portion of the basin and eventually, spilled into the Carson Sink.  The P2 surface in the eastern portion of the basin was generally inundated by the flood paths. As the waters receded, meanders were cut and abandoned, and new main channels were formed.  It is believed that some of the Fallon age lakes of Carson Desert described by Morrison (1964) were caused by the breaches in the Humboldt Dike and the subsequent filling of the Carson Sink.

Although  it  is not  clear why  the floods raced northward  after the breach in  the Humboldt  Dike, it is postulated that the massive dune field at the east end of the Barren Playa probably  acted a formidable  barrier against the initial  stages of the release of water just west of the Humboldt  Drain is a large debris apron composed of sand, silt, clay, pebbles and cobbles.  It has an organic rich soil horizon formed on a substantially hummocky topographic surface.  It is heavily dissected by several meander scars. This debris apron represents the initial material that was dispensed from the breach in the Humboldt Dike.

At least two, and possibly three, catastrophic flood events are distinguishable by the freshness of the scouring pattern. The older flood scour appears to have maintained a more central course in the basin, whereas the younger event appears to have taken a more northern route. A very old catastrophic flood is difficult to verify, but appears to have taken a more central route.

6.1.2   Recent Geomorphic Changes

As part of Newlands Reclamation Project, the Humboldt Sink and the Forty Mile Desert were drained to provide additional acreage for farming and agriculture purposes, as well as provide additional water resources to farmers downstream.  In 1915, a ditch was dug from the Humboldt Sink through the natural breach in the Humboldt Dike, and part way across the eastern portion of the FMD.   The main channel of the Humboldt River through the northern and western portions of the FMD basin were also modified to carry the additional flow expected from this new drain. An additional north trending feeder drain was dug west of the Barren Playa to intercept mid playa flow or perhaps seepage from then active prominent liquefaction eruptive centers. The drain was cleaned and deepened in 1946.   Records show the project was not successful for a variety of no pertinent reasons.

It is beyond the scope of this project to understand the effects of the Newlands drains have had on the basin.  However, it has been documented from many other alkali flats in the western United States that modifications in the natural hydraulic systems can cause substantial surficial effects.  They include: widespread subsidence from ground water withdrawal, drying up of natural springs, changes in vegetation growth patterns, and the formation large desiccation cracks.  All of these changes can be noted in the FMD.   Of particular attention is the formation of large desiccation cracks.

Throughout the basin there are numerous, randomly oriented cracks in the surface. Most of these cracks are on the order of a millimeter to a couple of centimeters; however, others are on the order of 1Os on centimeters in width.  The length of the cracks range from a few dozen feet to about a few thousand feet.  Some of the wider cracks have depths of greater than 10 feet.  In the Ocala Sump area, a wide crack extended as much as + 10 feet into the subsurface (see Figure 1).

Most of the smaller cracks appear to be related to desiccation of the near surface soil. The larger cracks, however, are more perplexing. Neal and Motts (1967) described giant polygon cracks that were developing in alkali plays throughout the United States. These features, which were commonly 10-300 meters across, were caused by unique types of surface soil, usually of high clay content, and   a fluctuating groundwater table at appropriate depths to fadlitate the growth of these cracks. We noticed  on  the  FMD  Basin that  most of  the  cracks follow   zones of  weakness developed  by  faulting, fracturing  or  liquefaction  failures.  Clear examples were observed at the northern portion of the Barren Playa where the development of prominent cracks lined up perfectly with extensive surface rupture traces from faults. Undoubtedly, the desiccation process played a role in their development, but their specific location appears related to pre-established zones of weakness.

6.1.3     Recent Seismic Activity

Within the western Basin and Range, there have been five very strong (M   > 7.0) earthquakes since about 1840: the 1845 north Carson Sink earthquake, the 1872 Owens Valley earthquake, the 1915 Pleasant Valley earthquake, the 1932  Cedar Mountains earthquake, and the Fairview Peak and Dixie  Valley earthquakes (Bell,1984; Bell and Slemmons, 1979; Slemmons and others, 1959; Ryall, 1977).  On the average, western Nevada experiences earthquakes in the magnitude 6.0 range occurring every 11 years, and in the magnitude + 7.0 range every 27 years (Ryall, 1977).

Large magnitude earthquakes in Nevada have caused ground displacements on the order of inches to 1Os of feet. Other physical effects from these events have included differential ground subsidence, Liquefaction and/or lateral spreading, and Liquefaction boils.  These effects are generally restricted too saturated or near saturated soil conditions.  In Nevada, the lacustrine basins have been prime candidates of this kind of physical damage (Siemmons, 1957; Bell, 1984). The earthquake need not be centered in each the basin to cause these physical effects, as powerful  earthquakes send tremors radiating for lOs and even hundreds of miles.

6.1.4    Faults, Fractures and Lineaments

Recent  geologic mapping by photo geologic interpretations  and some field reconnaissance has discovered  a  complex  assemblage of  faults,  fractures and lineaments exist within  the Forty Mile  Desert basin (Plate 3, Geologic Map).   The dominant orientation is in the northeast direction however, other prominent structures are present.  Six distinct orientations are observed: N50-70°E, N20-30°E, N5°W-N10°E, E-W, N40-50°W, and N65-70°W. Numerous minor and subordinate fractures dissect the playa suggesting an appearance like a shattered plate glass window.

Offsets have been observed on many of these lineaments from aerial photographs and field reconnaissance. The most spectacular off-sets were found on the north side of the West Humboldt Range south of the Humboldt Dike.   Field reconnaissance of these structures discovered a set N50-70°E dip-slip faults had an estimated 27 feet vertical offset (north  side down)  with  visible  horizontal  displacements of Lake 4 Lahontan 5 shorelines. Other sub parallel structures were observed along the terrace deposits north of the main rupture trace to contain significant offsets of 8-12 feet. The significance of displacements such as these, hence the magnitude of earthquake producing the rupture, has far reaching liquefaction implications for the saturated sediments of the FMD.

Another structure with substantial vertical displacement is observed just off the northwestern portion of the Geologic Map in the Jessup Wash region.    These prominent sets of northeast trending structures contain displacements on the order of 10s of feet. Limited time did not permit reconnaissance of these structures however they are so prominent they can be seen from Interstate 80 some 2 miles away. Substantial pediment off-setting structures with displacements of several feet were also observed along an access road to the FMD from the Brady’s geothermal field.   Both the Jessup and pediment structures warrant further investigation.

A prominent set of N45°E trending faults was discovered in the southwestern most corner of the FMD basin near the Mule Train Mine and the West Spring.  These structures were previously mapped by Bell (1984); Field reconnaissance indicated variable displacements of shorelines and terrace deposits to be on the order of a few feet, with a right-lateral dip-slip motion.   Inundation of the faulted graven during 52 times may provide a closer approximation to the timing of this event. The West Spring and the other springs have developed along the trace of the faults.  The northern extent of the N45°E faults appears to terminate along a prominent N60-70°W structure. A prominent N65-70°W structure is located along the southern margin of the FMD basin near the Humboldt Slough.  It aligns aerial photograph lineaments including, liquefaction boils, nick points in river channels, the ending of the Carson Basin dune field, the ponding of intermittent surface waters in Carson Basin, and the natural access channel between Carson Lake and the Stillwater reservoir. It also separates the Hot Springs Mountains and the Mopung Hills and perhaps is responsible for periodic opening of the Humboldt Slough. Regional geophysical surveys (Benoit et al, 1982; Irwin and Berg, 1977) and geophysical surveys conducted by this investigation indicate the western portion of the FMD is one of the deepest portions of the basin. The N65-70°W lineament may be a controlling structure to this part of the basin, facilitating the deepening of the basin.  Interestingly, there are few surface thermal manifestations east of this lineament.  It suggests the structure forms a barrier to the thermal source from the Hot Springs Mountains and the Soda Lake-Upsala Hogback thermal trends or along with other structures, provides such a deepening of the basin such that thermal fluids percolate into other permeable sedimentary layers buried deep within the basin.  Another prominent lineament zone composed of several near parallel structures trends N50-70°E and can be traced across the entire playa surface. These structures emanate from the Hot  Springs Mountains  and cross the west-central portion  of the playa, extend across the southern portion  of the east-basin area, and through  the central portion  of the Humboldt  Dike  and into the Humboldt  Sink for a couple of miles. Several photo geologic characteristics define this lineament, including changes in physical feature boundaries, color and textural variations, vegetation and physical feature lineaments and topographic changes. Numerous liquefaction eruption centers are along these lineaments. Field reconnaissance of the Humboldt Dike indicates no clear vertical displacement; however it may contain a left-lateral strike-slip component.   Strong rill seepage is observed on both sides of the dike where the lineaments are projected and a gentle swale with 6-8 inches on vertical variation is seen on the surface of the dike.   Observations of the structure where it crosses the Barren Playa indicate a change in slope that could represent up to 2-3 feet of vertical off-set.

Several structural lineaments appear to cross the Humboldt Dike; however, there are no clear indications of vertical off-set.   There are some indications of left-lateral strike-slip displacement, but further field work is needed to fully comprehend their significance.   It is possible the surface Humboldt Dike may have been modified during one of its many fillings of the Humboldt Sink during the latest Holocene.

Although there are several minor N5°W-N10°E trending lineaments, the more prominent structures appear quite evenly spaced across the playa.  Several distinguishing geomorphic features are observed along the western edge of the playa and identify an extensive fracture system from the southwestern most portion of the playa up into the Jessup Wash. This same fracture orientation was observed along the southeastern most portion of the FMD basin, along the western portion of the Barren Playa, and along the western side of the Humboldt Sink.  Lineaments along the western portion of the Humboldt   Sink are tentatively referred to as the West Humboldt Sink fault zone by Ken Adams (PhD candidate, Center for Neotectonic Studies, University of Nevada, Reno; personal communication, November, 1994). This structure has distinguishable Holocene surface rupture of over a length of 16 km. Notable features include massive liquefaction eruption centers and offset playa surfaces.

One of the most obvious lineaments in the basin are a set of two N40-50°W  parallel features in  the center of  the basin, a couple  miles west of  Highway  95.  The morphology of these features is subdued and discontinuous. They appear to have been cut and displaced by other more recent events, however, have maintained a distinguishable continuity.  They appear to have sustained much erosion, even from catastrophic flooding from Humboldt Sink.  On the ground, these features create elevation changes of several feet.  The origin of these features is not clear; whether they are actual faults that formed during times when the basin had water, or if they are preserved pressure ridges from a powerful local earthquake; the nature of these features is unknown at this time, but may be the oldest structural feature on the playa surface.

Structures in and around the Barren Playa show significant displacement and reveal a complex relationship of events.  A powerful earthquake caused massive liquefaction eruption. The fluid from these centered flowed eastward into the Barren Playa. The western boundary of the playa was subsequently uplifted on the west by a north-south trending structure, which separates the inflowing channel from the basin.  This north-south structure was then cut by an N45ow structure at the north end of the playa, which was subsequently cut by a N40°E structure. The final event also caused significant liquefaction failures along the western portion of the Barren Playa, creating the Ocala Sink.

Not all of the lineaments shown on the map are actual faults. Many are restricted to the playa and may be related to liquefaction features or lateral spreading phenomena from a distant earthquake.  However, many fractures do show substantial displacement over a considerable length and support good continuity of structure and therefore, are faults.   Detailed investigations of these structures, utilizing extensive field and subsurface mapping will bare witness to their potential activity and their contribution to the mineralization history.

What is staggering about these structures, is most or all of them occurred since or at near desiccation of the last Lake Lahontan high-stand or about 7,500 ybp.  If it is assumed that each of the six orientations of structures described above is valid surface rupturing, it suggests a potential reoccurrence interval of 1,250 years.  It is possible to form more than one orientation of displacement during a faulting event. During the 1954 Stillwater earthquakes, orientations of N40-50°E and N45°W developed in the Carson Sink (Bell, 1984; Slemmons, 1957). However, even if we eliminate half of the events, there is still a reoccurrence rate of 2,500 years. This is an exceedingly high surface rupture reoccurrence rate for anywhere in Nevada, and matches rate for such prominent fault zones as the Walker Lane or the Sierra Nevada eastern escarpment fault zone in the Owens Valley.  It also suggests that since 330,000 years, or the time represented in boring H5E5 at 131 feet, there have been on the order of + 130 surface rupturing events in the basin.

6.1.5 Liquefaction Failures and Eruptions

Liquefaction is the condition of soil losing its resistance to shearing.  Saturated fine-grained sands are the soils most subject to liquefaction.   When pore pressure exceeds the internal pressure of grain to grain contact, the soil takes on the characteristic of a dense liquid.  Sufficiently intense or long duration shaking from an earthquake tends to break the cohesion between the grains, increasing pore pressure, and the strength of the soil is lost and the sand/water mixture begins to flow.   Two dramatic features result from liquefaction: 1) overlying soils tend to collapse forming subsidence blocks or unstable hummucky topography; and 2) force a sand/water mixture into fractures that erupt on the surface, sometimes in copious quantities (see Figure 2).

Both of these liquefaction features have been observed on the FMD.  Two principal areas observed have experienced liquefaction ground failures: 1) on the west side of the Barren Playa, near Ocala; and, 2) the other on the southeast side of the Barren Playa. The liquefaction ground failure near Ocala is dramatic. In an area confined by two recent northeast trending surface rupturing faults a distinct hummucky topography has formed, with mounds and valleys typically on the order of 3-4 feet. Some areas have dropped substantially, with an estimated 30 feet of displacement. One area, called the Ocala Sink, shows 30 + feet of displacement, providing excellent exposures of lacustrine   soils and sediments (Figure 3, Ocala Sink).  [Recent groundwater withdrawal from the basin may have facilitated these liquefaction features.

The non-degraded appearance of the Ocala liquefaction failures indicate it happened in the not so distance past. It is possible that this feature may have been related to the powerful North Carson Sink + 7.0 earthquake in 1845, for which the location is not well constrained. The liquefaction ground failures on the southeast side of the Barren Playa have been overrun by the dune field and have a degraded appearance suggesting their older age.

Broad areas in the western portion of the basin that are inundated with seasonal flooding show characteristics on being subsidence blocks. Many of the subsidence blocks are bounded by structural lineaments and have numerous liquefaction boils along their perimeter. These subsidence blocks can also be found in the eastern portion of the basin but are more difficult to identify because of the thick vegetation. It suggests that the much of the broad P1 surfaces are actually a down-dropped 5/P1 surfaces. Subsidence features like these were observed in the Fallon area following the powerful Stillwater and Dixie Valley-Fairview peak earthquakes in 1954. This method of forming prominent troughs throughout the basin will have substantial influence on the sedimentation history and as well as the mineralization record.

Aerial photographs show numerous circular and elliptical features occurring along the strike of recent fractures (see Figure 4).  These features ranged in size from 1Os to hundreds of feet across.  Investigation has determined these features are enormous ground water eruptions or liquefaction boils.  Because of their size, they obviously involved copious quantities of fluid.  As one stands in the middle of these features it is difficult to imagine the scale of the ground water eruption. Several key boils show fluid erosion extending away from the elliptical center, pooling in adjacent lows, or finding its way to an active or abandoned channel.  One such event caused boils to erupt in the Ocala area sending fluid into the Barren Playa, now  evident by the distinct lack of vegetation (probably due to the toxicity of the original groundwater fluids).  These eruptive centers are believed to be a major source of subsurface fluid transport to the surface.

6.2        Playa Geomorphology Summary

Dramatic evidence for faults, subsidence, liquefaction and liquefaction boils have been observed in the FMD basin.  Several surface rupturing fractures and faults are found in multiple orientations throughout the FMD basin. Some show substantial offsets of 1Os of feet, while others display offsets on the order of feet or inches.  Evidence from aerial interpretation and direct ground verification for these recent structures includes offset lacustrine terraces, shorelines, playa surfaces, fan deposits, and stream channels. Possible offsets in the Humboldt Dike and the breach at the southern end is likely due to fault ruptures that caused a catastrophic flood event across the FMD.   Liquefaction features have played a major role in shaping the current playa surface and probably had a considerable effect on the sedimentation history of the basin.  Liquefaction eruptive centers produce massive amount of fluid and are seen as one of the major contributors of transporting solutions from deep within the basin sediments to the surface.


GEOLOGICAL REPORT

INVESTMENTS