ABSTRACT
THE SEDIMENTOLOGY, STRATIGRAPHY, AND CHEMISTRY OF PLAYA LAKE DEPOSITS RESULTING FROM HURRICANE NORA IN THE CHAPALA BASIN, BAJA CALIFORNIA, MEXICO
by Liselotte Rachel Shoffner
Laguna Chapala, Baja California, Mexico is a closed basin containing a playa lake and presumably a proxy record of climate. On September 25, 1997, hurricane Nora crossed the Chapala basin, causing flooding in Laguna Chapala and providing the unique opportunity to examine the sedimentological impacts of a hurricane on the playa stratigraphy. Runoff from hurricane Nora formed a lake that locally reached depths up to
1.2 m. The lake did not completely evaporate until February of 1998. The playa was surveyed to determine the lake hypsometry and samples were examined by particle size analysis and x-ray diffraction. The mean grain size was 5.7 phi units. The dominant clay minerals were smectite, illite, and kaolinite. The dominant evaporite mineral was halite.
The evaporite crust, vegetative debris at the flooding boundary, and large-scale mudcracks were unique to flooding and provide the most useful indicators of a storm event in Laguna Chapala’s sediments. THE SEDIMENTOLOGY, STRATIGRAPHY, AND CHEMISTRY OF PLAYA LAKE DEPOSITS RESULTING FROM HURRICANE NORA IN THE CHAPALA BASIN, BAJA CALIFORNIA, MEXICO
A Thesis
Submitted to the
Faculty of Miami University
in partial fulfillment of
the requirements for the degree of
Master of Science
Department of Geology
by
Liselotte Rachel Shoffner
Miami University
Oxford, Ohio
2000
Advisor:______Brian Currie
Reader:______Mark Boardman c.
Liselotte Rachel Shoffner
2001 TABLE OF CONTENTS
Abstract
Title i
Acknowledgements iii
Table of Contents v
List of Figures viii
List of Tables x
1. Introduction 1
1.1 Purpose 2
1.2 Playas 3
1.3 Previous Work 10
1.4 Local Physiographic and Climatic Setting 11
2. Climate 15
2.1 Climate Change 16
2.1.1 Previous Work
2.1.2 Causes of Climate Change
2.2 Regional Climatology 22
2.3 Hurricanes 24
2.3.1 Formation and Processes
2.3.2 Occurrence
2.3.3 Impacts
2.4 Hurricane Nora 32
iii 3. Methods and Data 36
3.1 Hydrological Reconstruction 37
3.1.1 Surveying
3.1.2 Drainage Basin Map
3.1.3 Volume Calculation
3.1.4 Evaporation Model
3.2 Sedimentology and Stratigraphy of the Lakebed 51
3.2.1 High Water Marks and Evaporation Rings
3.2.2 Mudcracks
3.2.3 Grain Size
3.3 Evaporites 60
3.3.1 Previous Work
3.3.2 Modern Facies
3.3.3 Salt Chemistry
3.3.4 Criteria for Describing Salts
3.3.5 Methods of Evaporite Analysis
3.3.6 Evaporite Data
3.4 Clays 85
3.4.1 Previous Work
3.4.2 Clay Mineralogy
3.4.3 Playa Clays
3.4.4 X-Ray Diffraction of Clays
iv 4. Analysis 95
4.1 Hydrology of the Laguna Chapala Flood 96
4.2 Lakebed Features 97
4.3 Evaporites 99
4.4 Clays 100
5. Conclusions 101
References 104
Appendices 113
A. Survey Data 113
B. Hypsometry Data 120
C. Thornthwaite and Hamon Evaporation Calculations 121
D. Field Notes 122
E. Method of Pipette Analysis 131
F. Grain Size Data 140
G. Conductivity Data 145
v LIST OF FIGURES
1.1 Location map of the Chapala Basin 2
1.2 Depositional environments of playa basins 7
1.3 Depositional environments of evaporite minerals 8
1.4 View of Laguna Chapala looking towards the southwest 12
1.5 Geologic map of Laguna Chapala 13
2.1 Track of hurricane Nora 33
2.2 Hurricane Nora on September 25, 1997 34
3.1 Compilation of survey data, topographic map, and air photo 38
3.2 Drainage area of Laguna Chapala 39
3.3 Contour plot of lake 40
3.4 Graph of lake hypsometry 41
3.5 Graph of lake volume with depth 41
3.6 Nomograph 43
3.7 High water marks 52
3.8 Boundary of flooded area 53
3.9 Evaporation rings 54
3.10 July mudcracks 56
3.11 December mudcracks 57
3.12 Saline pan cycle 62
3.13 Flow diagram for brine evolution 68
3.14 Ternary phase diagram for brine evolution 69
3.15 Precipitation sequence of minerals in unsaturated zone 71
vi 3.16 Samples 122498-1 and 122498-2 78
3.17 Graph of conductivity vs. salinity 79
3.18 Playa crust in July 1998 81-82
3.19 Salinity across Laguna Chapala 84
3.20 Joining of tetrahedral and octahedral sheets to form a 1:1 layer 89
3.21 Clay mineral x-ray diffraction pattern 94
vii LIST OF TABLES
2.1 Evidence of regional climatology 23
2.2 Hurricane intensity scale 30
3.1 Results of grain size analysis 59
3.2 Grain size of sand and clay fractions 59
3.3 Primary saline pan features preserved after burial 76
3.4 Evaporite characteristics of Laguna Chapala 83
3.5 Classification of clays based on swelling properties 90
3.6 Classification of clays based on layer type 91
viii ACKNOWLEDGMENTS
This thesis was funded in part by the NASA Jet Propulsion Lab through a grant to
Dr. Larry Mayer. The contents of this thesis reflect the views of the author who is responsible for the facts and the accuracy of the data presented herein. The content does not reflect the official views or policies of JPL.
I am grateful for the field assistance of Ann Thomas, Craig Thomas, Seth Tanner, and Genaro Martinez-Guittierez. Thanks to Dr. John Morton, Dr. Dave Finkelstein, and
Dr. John Rakovan for their advice and technical assistance. Thanks to Dr. John Hughes and Dr. Mark Boardman for their advice and support. Finally, thank you to Dr. Brian
Currie for stepping up when I needed an advisor and seeing me through the final stages of my work.
ix This thesis is dedicated to my parents for their support and belief in my ability to succeed. I love you Mom and Dad.
x Chapter 1
Introduction
Playa lakes are shallow, ephemeral lakes that exist in arid and semi-arid regions.
Playas generally form in the lowest part of an internally drained desert basin following seasonal or episodic storm events (Shaw and Thomas, 1989; Reeves, 1978). Playa lake deposits and evaporites are sensitive indicators of local climate and may contain sedimentologic and geochemical records that are millions of years old (Eugster, 1982).
Playa sediments record local depositional history and evidence for determining paleoclimate (Reeves, 1978; Rosen, 1994). As a result, playa deposits are useful proxies when constructing an environmental and climatic history of a region.
When hurricane Nora swept across the central Baja peninsula in September 1997, it created a shallow lake in Laguna Chapala. Laguna Chapala is located in the eastern half of the Chapala basin in Baja California, Mexico. (Figure 1.1) The lake that formed when hurricane Nora crossed the region was 15.97km2 in area and contained 7,000,000m3 of water. This lake did not completely evaporate until February 1998.
1 Figure 1.1 Location map of the Chapala basin. The insert is a 1995 satellite image of the basin showing Laguna Chapala on the right.
Laguna Salada
Ensenada Baja California
Chapala Basin
Baja California Sur
N La Paz
0 200 Cabo San Lucas kilometers
1.1 Purpose
The purpose of this study is to understand the sedimentology and geochemistry associated with the flooding caused by hurricane Nora in order to identify similar events in the lake’s history. This study seeks to document the stratigraphy of the playa crust related to the Nora event, the chemistry of the sediments and evaporite deposits, and the type of lakebed crust that resulted from the flooding caused by hurricane Nora. The stratigraphy of the lakebed will help determine where flooding occurred and identify the
2 depositional record of a large precipitation event, such as Nora. The chemistry of the sediments and evaporites will help elucidate the evaporative stages of the lake and the mineralogical composition will help determine the salinity and chemistry of the water at the time the sediments and evaporites were deposited (Vance et al., 1992). If the stratigraphy and geochemistry resulting from the Nora flooding are unique, they may serve as a useful tool for identifying other storm events in the playa record.
There are no stream gauges in the Baja peninsula, but the volume of water in a closed basin can be used to determine the runoff resulting from a flood event. By determining the amount of water that was in the Chapala basin after Nora struck, the amount of runoff resulting from that event can be directly measured.
The Chapala basin is closed: only ephemeral streams and precipitation flow into it and its only outflow is evaporation. Because evaporation influences the chemistry of evaporites (Rosen, 1994), understanding the evaporative process within the basin is important for understanding the deposition that occurs. Determining how evaporation occurred at the basin can also be used to determine temperatures during the flooding period.
1.2 Playas
Playas most often form in closed drainage basins (Last, 1984; Last, 1989; Renaut and Long, 1989) in semi-arid and arid environments (Last, 1989). Closed-basin lakes lose most of their moisture by evaporation because they have little to no outflow
(Battarbee, 1999). In order for an arid-zone depression to be considered a playa it must be intercontinental, its outflow must exceed its inflow for more than half of the year and
3 it must have a near surface capillary fringe that allows evaporation to result in water discharge to the surface (Rosen, 1994).
Playas are usually no bigger than a few square kilometers. However, some, like
Lake Eyre, Australia, are upwards of 9000 km2 (Rosen, 1994; Cooke and Warren, 1973).
The size of a playa may be related to the size and morphology of its drainage basin
(Cooke and Warren, 1973). Structural controls, erosion, and ponding can lead to the formation of a playa (Shaw and Thomas, 1989). The processes that occur within the playa may also govern a playa’s shape. Flooding, spring deposition, sheet flow, and wind deposition are processes that can affect the physical character of a playa surface (Cooke and Warren, 1973; Last, 1984).
Structural controls, such as faulting, rifting, intrusions, and differential weathering can produce large regional basins in which playas will form (Shaw and Thomas, 1989).
The Lake Eyre basin in Australia was formed when uplift of the South Australian highlands produced a depocenter that subsided and was filled by sediments eroding from the margin of the basin (Nanson and Price, 1998; Alley, 1998). Rifting in Sudan developed a system of grabens which were ideal for playa formation in the arid climate of northeast Africa (Salama, 1994). Fracturing can lead to the development of playas because fractures may mark basin boundaries and serve as conduits for groundwater flow
(Shaw and Thomas, 1989). Any structural controls in a region merely set up the morphologic conditions necessary for playa formation; a playa will not form unless the climate and the hydrology of an area favor playa formation.
Erosional controls of playa formation include deflation (Magee and Miller, 1998), karstic development, and animal scouring (Shaw and Thomas, 1989). Erosional
4 processes influence the formation of playas because even slight depressions formed by erosional factors in flat arid land have the potential to develop into playas when they collect seasonal rainfall (Shaw and Thomas, 1989). Erosional controls will vary in response to climate, thus they can be as indicative of climate change as the playa deposits they influence (Mulligan, 1998). Erosional controls may act with structural controls to influence playa formation, but are generally more influential in the development of small local or subregional-scale playas (Shaw and Thomas, 1989).
Deflation is the most common erosional control in playa formation (Shaw and
Thomas, 1989). Deflation will cause a playa to form if the material of a basin is susceptible to deflation and the surface is clear of vegetative cover that would protect against erosion (Shaw and Thomas, 1989). If deflation has influenced the formation of a playa, fringing dunes will usually be present (Shaw and Thomas, 1989; Magee and
Miller, 1998).
Playa formation may be hydrologically controlled. A pond may form in an ephemeral channel when processes, such as deflation, block off the flow of water (Shaw and Thomas, 1989). Ponding may also occur between strandlines of paleolakes or between longitudinal dunes. When such ponding occurs in an arid closed basin, playas may form (Shaw and Thomas, 1989). Ponding can affect the appearance of a playa.
Holliday, et al. (1996) suggested that the frequency and duration of ponding could cause variations in the color of a playa surface.
Wood, et al. (1992) hypothesized that basins in the southern High Plains formed in response to the location of the aquifer. In the southern High Plains, caprock is not present and the aquifer is thinner where bedrock highs exist (Wood, 1992). The thinning
5 of the aquifer results in lower transmissivity which causes high water levels over the bedrock highs (Wood, 1992). When the water levels drop in the areas of bedrock highs, the lack of caprock allows deflation to occur resulting in the formation of the High Plains basins (Wood, 1992).
Figure 1.2 shows the subenvironments that may exist within a playa. Playas are generally flat and horizontal in subenvironments regularly flooded by ephemeral waters
(Shaw and Thomas, 1989). Playas that are rarely flooded may have irregular surfaces due to evaporite growth and dune development (Shaw and Thomas, 1989). Rapid weathering of bedrock that outcrops near or at the playa surface may occur, due to salt weathering because of high evaporation rates (Goudie, 1989; Shaw and Thomas, 1989).
The characteristics of the sediments that form a playa surface are controlled by the sedimentary processes acting on the playa (Langer and Kerr, 1966; Last, 1984).
Playas generally contain fine-grained clastic sediments, such as clay or silt, and non- clastic materials, such as evaporites (Cooke and Warren, 1973). Last (1984) defined four main types of deposits found in playas. The first type of deposit is very soluble evaporites, such as magnesium and sodium sulfates, which precipitate (Last, 1984).
Other soluble precipitates, such as gypsum and calcite, may also during evaporation or early diagenesis (Last, 1984). The third type of deposit is allochthonous or clastic inorganic material, that is transported into the playa by runoff, wind, and shoreline erosion (Last, 1984). The fourth type of deposit, organic detritus, may include brine, ostracodes, rootlets, and other vegetation (Last, 1984).
6 Figure 1.2. Depositional environments that may occur in a playa basin. (after Shaw and Thomas, 1989)
Alluvial fans
Ephemeral/Perennial streams and floodplains
Spring deposits and ponds
Ancient evaporites and lake sediments
Ephemeral salt pan with bedded deposits Sand flats
Saline mudflats Dunes
Dry mudflats Shoreline features
Fringing dunes
7 Figure 1.3. Diagram of depositional environments of evaporite minerals. (after Rosen, 1994)
Salt crust Reworked gypsum dune
Bedrock
Bedrock
Subaqueous evaporites Alluvial fan samds (ie halite or gypsum)
Displacive evaporites Alluvial fan boulders, (ie halite or thenardite) cobbles, and gravel
Evaporite nodules in carbonate rock Ancient carbonate rock (ie gypsum, halite, or celestite)
Mudflat sediments Extraclast
Figure 1.3 shows where various evaporite minerals generally form on the surface of playas. The deposition of evaporites is controlled by the hydrology of the basin (Last,
1984; Rosen, 1994). Several chemical and mechanical processes are responsible for the formation of various salts on the playa. The type of evaporites that will form in a playa depends on playa water chemistry (Eugster and Jones, 1979; Hardie, et al., 1978; Li, et
8 al., 1997). For example, waters rich in Na, Cl, and SO4 will precipitate calcite, gypsum, and halite (Li, et al., 1997).
The shape of a playa can cause local variations in the water chemistry and a corresponding distribution of salts across the playa surface (Last, 1989). Efflorescent crusts form during the end stages of evaporation or at the capillary zone where evaporation is occurring (Rosen, 1994). Intrasedimentary saline growth beneath the surface is caused by precipitation from groundwater (Lowenstein and Hardie, 1985;
Rosen, 1994). Salt crystals may form at both the surface of and within a standing body of water (Lowenstein and Hardie, 1985; Rosen, 1994). Previously formed salts may be redeposited by mechanical means (Rosen, 1994). Gypsum dunes may form when a playa surface deflates (Rosen, 1994).
The clastic material in playas is generally fine-grained because of the mechanisms
(such as flooding and wind) that deposit it (Last, 1984). Because playas are the endpoints of drainage, only fine-grained sediments are likely to make it into the basins (Shaw and
Thomas, 1989). In addition, alluvial fans act as fluvial buffers, trapping coarse sediment
(Shaw and Thomas, 1989). Wind transports sediment onto playas which is rarely larger than sand (Shaw and Thomas, 1989).
The existence of clays and silts on playas causes the surface to resist infiltration, resulting in the formation of lakes during times of large runoff (Cooke and Warren,
1973). The clays and silts also encourage the formation of mud cracks, due to shrinkage of the surface (Cooke and Warren, 1973).
9 1.3 Previous Work
A significant amount of research has been conducted on palaeoclimates and how best to determine them. Playa lake dposits are of particular interest to paleoclimate research because even small changes in evaporation or precipitation may cause major changes in lake-level and salinity (Battarbee, 1999). Changes in the hydrologic budget of a closed drainage basin may be indicated by its sediments (Last, 1989). Though various playa deposits such as beach ridges and basin fills have been used in paleoclimate studies, playa evaporites are one of the most common deposits used for climate studies.
Enzel and Wells (1997) studied beach ridges and playa lake deposits of Silver
Lake, California in order to document Pleistocene through Holocene climatic change.
They suggested that laminated couplets in playa cores were associated with shallow lacustrine sedimentation and wet climatic periods. In their study, they used lake-depth calculations and area-volume-elevation curves to calculate lake volumes and estimate the minimum hydrologic change required to fill the Silver Lake basin. They determined that storage capacity, areal extent, and depth of the lake could be derived from geologic and geomorphological evidence, such as the stratigraphy of the preserved shore environment.
Holliday et al. (1996) studied the playa basins of the Southern High Plains in the
United States to determine a paleoenvironmental record for the region. They studied the stratigraphy of the basin fills to determine the depositional history of the playas. They found the accumulation of muds, destruction of playa vegetation, and lack of carbonate in the sediments to be indicative of seasonal flooding of the playas.
Crowley and Hook (1996) used playa evaporites as indicators of short-term climatic variations. By analyzing TIMS data, they were able to use remote sensing to
10 map the evaporites in the Cottonball and Badwater basins of Death Valley, California. A field study and analysis of samples by x-ray powder diffraction were used to verify the conclusions derived from the remotely sensed data. They found that the formation of evaporite minerals was closely linked to the composition of water. For example, salinity of the water influenced what evaporites occurred in the Cottonball basin.
Salt cores from Death Valley were used by Li et al. (1996) to determine paleoclimates. Li et al. studied the stratigraphy of the cores and dated the deposits using the U-series isochron method. By comparing modern deposits in Death Valley to the saline deposits in cores, Li et al. were able to construct a history of flooding events and paleoclimate. They found that depositional environments can be inferred from salt cores based on the distribution of evaporites
1.4 Local Physiographic and Climatic Setting
The Chapala Basin lies in the arid central part of the Baja peninsula, about 400 km south of the California border. The basin is bordered by the Gulf of California to the east and the Pacific Ocean to the west. The basin contains two distinct drainage basins, one in the east and one in the west. Laguna Chapala, shown is figure 1.4, is the eastern drainage basin. The drainage systems trend normal to the coastlines, and rarely contain flowing water (Arnold, 1957). The relative area of the two drainage basins is about the same, but the eastern playa lies about 21m above the western playa (Arnold, 1957). The elevation of the Chapala Basin ranges from 660 meters above mean sea-level (msl) in
Laguna Chapala to 960m msl in the surrounding hills.
11 Figure 1.4. View of Laguna Chapala looking towards the southwest. The western basin can be seen in the background. Sand hills are located in front of the western basin and volcanic hills and lava flows surround the western basin.
The Chapala basin is characterized by hilly to mountainous, rugged, rocky terrain.
Figure 1.5 shows a generalized geologic map of the Laguna Chapala area. The basement of the area is granitic and is overlain by lava flows and sedimentary deposits. Folded metamorphic ridges in the east, and tilted granite blocks in the southwest, as well as fault scars, suggest significant amounts of crustal deformation have shaped the basin (Arnold,
1957).
12 Figure 1.5. Geologic map of Laguna Chapala. (after Arnold, 1957)
13 Laguna Chapala has an arid climate. Mean monthly temperatures stay above
10°C, resulting in a mean annual temperature of 17.1°C. Annual precipitation in the area is about 205mm per year (CNA, 2000). There are two dry seasons in Baja. The first is from April to May, and the second lasts from October to November (Markham, 1972). It is only during periods of heavy rain that the lower parts of the basin contain shallow bodies of water.
14 Chapter 2
Climate
Large-scale geomorphology is most effected by long-term climate, but small- scale geomorphological processes may be just as responsive to climate fluctuation
(Mulligan, 1998). Studies of how geomorphic systems respond to climate change have become increasingly important as scientists realize the potential impact humans have on climate, and the geomorphic hazards that may result from changes in the magnitude and types of climatic processes (Kochel and Miller, 1997). It is particularly important to study the response of water- and salt-fluxes when trying to determine the effects of climate change on landscapes (Eybergen and Imeson, 1989)
15 2.1 Climate Change
Closed lakes and playas are useful indicators of climatic change because the evaporites and sediments they contain can be sensitive to climate-related factors including inflow water volumes, temperatures, regional storm tracks and wind patterns, and evaporation rates (Lowenstein et al., 1999). As a result, playa deposits can be used as indicators of regional, as well as local climatic variability. Evidence from Silver Lake,
Owens Lake, Searles Lake, Great Salt Lake, Death Valley, and several rivers in Utah and
Arizona points to a history of regional climatology in the southwest from the Pleistocene to the present.
2.1.1 Previous Work
Enzel and Wells (1997) found evidence for climatic change in beach ridges and playa lake deposits of Silver Lake, California. The beach ridges revealed the highest stages attained by late Pleistocene and Holocene lakes with the highest stands representing the oldest deposits. The most recent lake stands were dated by McFadden et al. (1992) as middle to late Holocene. Cores from the playa contain laminated couplets, which are associated with a shallow lacustrine environment of deposition. These deposits were dated to 390 ± 70 yr B.P. and 3620 ± 70 yr B.P. In the modern record, even lakes that lasted up to 18 months at Silver Lake were unable to produce enough sediment to be preserved in the sedimentological record. As a result, it is assumed that sedimentary evidence of ephemeral lakes represents lake stands that lasted longer than 18 months.
The record at Silver Lake shows that the earliest Holocene lake occurred around 9300
B.P. Around 8500 B.P. the lake dried, allowing eolian deposition to take place.
Lacustrine conditions again occurred in Silver Lake at 3620 ± 70 B.P. and 390 ± 70 B.P.
16 Ely (1997) used records of extreme floods in rivers of the southwest to determine the occurrence of climatic variations. By using slackwater deposits, she was able to date flooding that occurred in southwest rivers such as the Little Colorado River, Kanab
Creek, and the San Francisco River Basin. Ely found that the flooding in the rivers clustered around several dates. She determined that the flooding from 3800 to 2200 B.C.,
1000 to 1200 A.D., and 1300 to 1500 A.D. to the present represent wet periods related to increased storms, changing atmospheric circulation patterns, and El Niño events. Periods with a marked lack of flooding, such as 2200 to 400 B.C., were not linked to especially dry, warm periods, but were taken to be periods when storm frequency and temperatures did not favor flooding.
Li et al. (1996) used salt cores from Death Valley to determine paleoclimates.
Because the distribution of evaporites is related to fluctuations in the ratio of inflow to evaporation, depositional environments can be inferred from salt cores. Evaporites accumulate in perennial saline lakes and normally desiccated saline pans. If mud is interlayered with evaporites, deposition likely occurred in a perennial lake or on a normally dry mudflat. In addition, because saline minerals trap fluid inclusions of the brine from which they crystallize, information about the chemical composition and temperature of the waters in which the crystals grew is preserved. Thus, paleoclimates can be determined using modern evaporitic settings as a proxy. Li et al. (1996) compared the stratigraphy and chemistry of saline deposits in cores to that of modern deposits in
Death Valley. This resulted in the construction of a history of flooding events from which paleoclimate could be inferred.
17 Based on a lack of carbonate minerals and an abundance of calcium sulfate minerals, Li et al. (1996) determined that mudflat environments in Death Valley from the period 60 to 100ka were similar to modern dry, low inflow conditions. Deposits from the period 35 to 60ka represented a saline pan environment with interspersed wet periods.
This was indicative of an arid climatealso similar to the present. During the period from
12 to 35ka perennial lakes dominated Death Valley. This indicates that a cooler, wetter climate with average temperatures 6 to 7°C lower than the present and annual precipitation 70% higher than the present (Spaulding et al., 1983). Around 13ka the late
Pleistocene lake began to dry up. During the period from 10 to 12ka the lake was extremely saline, indicating an arid climate with the water table in Death Valley at or below the surface.
Great Salt Lake and its precursor Lake Bonneville have been the focus of several paleoclimate studies. Oviatt et al. (1990) studied the stratigraphic and geomorphic relationships of the Lake Bonneville Stansbury shoreline. They determined that because the altitude of the shoreline did not correlate with any external or internal thresholds of the Bonneville basin, the shoreline could not have formed due to overflow or an increase in the surface area of the lake. This being the case, the authors concluded that the cause of the shoreline had to be hydrologically related and controlled by climate. Oviatt et al. determined the shoreline represented a transgressive phase of Lake Bonneville, and referred to this phase as the Stansbury oscillation. They dated the Stansbury oscillation to 20 to 22ka. This period represents a drop in volume of the lake that could have been triggered by a shift in the jetstream and an overall decrease in storm events.
18 Spencer et al. (1984) used sediment cores of Great Salt Lake to determine the last
30,000 years of history of the lake. Based on sedimentary, biologic, and mineralogical data from their core C, they determined that prior to 32ka an ephemeral lake-playa system was present in the Great Salt Lake basin. From 32ka to the present there has been a perennial lake in the basin, but this lake has changed greatly over the years. Spencer et al. determined that the shift from freshwater to saline conditions occurred over the period from 32 to 25.5ka. This shift was controlled by declining volumes interrupted briefly by increasing lake levels during the period from 29 to 25.5ka. As Oviatt et al. (1990),
Spencer et al. found that the Stansbury shoreline represented a transgressive phase occupying the period from 22.5 to 19.5ka. From 19.5 to about 14.5ka lake levels rose again, reaching the Bonneville level around 16.8ka, when freshwater conditions returned.
From 14.4 to 12ka the lake again experienced a drop in level. From about 8ka to the present the lake has stayed near its current elevation of 1280m, with brief highstands of
1290m and 1295m around 2.3 and 3.5ka respectively. Based on studies like Oviatt et al.
(1990), it is probable that such fluctuations in lake level were precipitated by climatic variation.
Menking et al. (1997) used carbonate content, grain size analysis, and oxygen and carbon isotope values of cores to determine climatic variations at Owens Lake,
California. Because calcium carbonate abundance reflects water residence time in Owens
Lake, it can provide a basis for determining when closed or open-lake conditions existed.
Calcium carbonate values less than 11 wt% indicate periods of overflow, while greater content indicates closed-lake conditions. This criterion suggests that from 155 to 117ka
19 and from 50 to 13ka Owens Lake experienced overflow. At 28ka, 98ka, 92ka, 75ka, and from 117 to 56ka and 13ka to the present the lake experienced closed-lake conditions.
In their grain-size analysis, Menking et al (1997) determined that finer grained sediments were not indicative of deeper conditions. This suggests that factors such as eolian input, changes of grain size input, and turbidity currents may control grain size more than shoreline position. 18O content should be high during periods of closed-lake conditions and low during periods of overflow, mirroring carbonate content. The 18O values did seem to follow a similar trend to the carbonate data, with only slight variations that could have been caused by overflow from other basins or a lag in response time of
18O values to drier climates.
Lowenstein et al. (1999) studied a 186 meter-long core from Death Valley to reconstruct a 200 ka record of paleoclimate in the region. Lowenstein et al. used evaporites dated using the u-series isochron method to determine the sedimentation rate of the salts and muds in the core. They compared modern sedimentary structures and petrographic textures to those in the core to determine that the periods from 200 to 192 ka and 120 to 60 ka were dry and warm, while the periods from 192 to 186 ka and 60 to
35ka were predominantly wet and cold. The presence of less salinity tolerant ostracodes in the 10 to 35 ka and 120 to 186 ka sections of the core confirms that these periods were cold and wet. Fluid inclusions in halites were used to determine the temperatures of the brine and air when the halite crystallized. The temperatures from 35 to 60 ka were determined to be 6 to 11° C lower than modern temperatures. Temperatures at 100 ka and 120 ka were determined to be similar to modern temperatures. The period from 120 to 186 ka was found to be 10 to 15° C lower than modern temperatures
20 2.1.2 Causes of Climate Change
Ely (1997) defines three factors which are responsible for affecting modern climates in the southwest: flood-generating storms, atmospheric circulation patterns, and
El Niño/Southern Oscillation events. Because these factors are evident in the modern record they are assumed to have affected paleoclimates.
Flood-generating storms include winter North Pacific storms, late-summer eastern
North Pacific cyclones, and summer storms, such as convective thunderstorms. Under normal winter conditions, the prevailing westerlies are deflected to the north of the
Pacific high-pressure ridge and enter North America over the Pacific Northwest. During winter storm conditions, however, the pressure center is displaced to the west causing a low-pressure trough to form over the western U.S. (Ely, 1997). This causes storms to track south, centering over the southwest.
When tropical cyclones curve towards Baja California or the western Mexican mainland, a deep low-pressure trough occurs in the mid portion of the westerly upper-air flow, causing moisture to be drawn into the southwest U.S., particularly Arizona and
Utah. The increased moisture causes high precipitation and an increase in flooding.
(Ely, 1997)
In the summer, the Bermuda sub-tropical high-pressure ridge shifts northwestward and the Pacific high-pressure cell shifts northward. These shifts cause an influx of moist tropical air that warms the land surface. As a result of the surficial heating, convective thunderstorms are formed which can contribute to flooding. (Ely,
1997)
21 Ely (1997) found that, in this century, climatic changes correlated with shifts in the Northern Hemisphere atmospheric circulation. A shift in the location of low and high-pressure anomalies in the North Pacific dictates when and where extreme winter floods or cyclone-related floods occur in the southwest. For example, a low-pressure anomaly off of California and a high-pressure anomaly over the Gulf of Alaska shift winter storms into Arizona.
As witnessed in 1997-1998, El Niño sea-surface temperature conditions can have drastically change weather patterns. Though the intensity of El Niño events fluctuates, they generally result in an increase in southwestern precipitation due to increased cyclone and winter storm activity. Positive shifts in the Southern Oscillation Index tend to result in conditions favorable to increased summer flood events (Ely, 1997).
2.2 Regional Climatology
Compilation of the evidence from the sites in the southwest U.S. gives a regional history of the climate (Table 2.1). The period from 155 to 117ka was a wetter period that allowed places like Owens Lake to maintain perennial lake conditions (Menking et al.,
1997). From 100 to 35ka the southwest experienced a dry period, similar to the modern climate, during which gradual increases in the effective moisture paralleled long-term cooling trends coupled with ice sheet buildup (Li et al., 1996).
Beginning around 35ka, Lake Bonneville, Death Valley, Searles Lake, Owens
Lake, and Lake Lahontan experienced high perennial lake levels, indicating a cool, wet climate that lasted until about 10ka. This wet period corresponds to the last glacial maximum (Li et al., 1996 and Oviatt et al., 1990). Brief dry intervals during the 35 to
10ka period may have resulted from periodic retreats of the ice sheets. Retreat of the ice
22 Table 2.1. Summary of evidence for regional climatology. A description of the climate is listed with the time period.
Time Location Evidence Reference Death Valley 200 ka to 192 ka California · halite formed in saline pan Lowenstein et al., ( warm, dry) environment 1999 · mud-flat deposits
Owens Lake1 192 ka to 117 ka California 1 CaCO3 values < 11 wt.% 1. Menking et (cold, wet) 2 low 18O values al., 1997
Death Valley2 2. Lowenstein et · Fluid inclusions in halite al., 1999 Owens Lake1 117 ka to 35 ka California 3 CaCO3 values > 11 wt.% 1. Menking et (warm, dry) 4 high 18O values al., 1997
Death Valley2 2. Li et al., 1996 5 lack of CaCO3 minerals 6 abundance of calcium sulfate minerals 7 interbedded mud and halite saline pan deposits
Owens Lake1 California 8 CaCO3 values < 11 wt.% 1. Menking et 35 ka to 9 low 18O values al., 1997 10 ka (cold, wet) Death Valley2 2. Li et al., 1996 10 structureless mud layers 11 fluid inclusion-banded halite 12 thick halite layers Lake Bonneville Utah 13 Stansbury Oscillation1 1. Oviatt et al., 14 ripple laminae2 1990 15 brine shrimp pellets2 16 tolerant to less tolerant ostracode 2. Spencer et al., assemblage2 1984 17 increasing carbonate content2 18 high organic content2 19 thinning laminae2 20 presence of light then heavier oxygen isotopes2 Silver Lake Playa California 21 shallow lacustrine deposits Enzel and Wells, (laminated couplets) 1997 5.8 ka to 22 eolian deposits present Rivers (wet) Arizona and 23 slackwater deposits Ely, 1997 Southern Utah
23 sheets may have caused a shift in the jet stream producing fewer storms and resulting in temporarily dry climates (Oviatt et al., 1990).
Around 10 to 12ka the climate again began to become increasingly arid resulting in perennial lakes becoming increasingly saline or drying out completely. Until about
6ka a relatively stable climate existed. This was followed by present day climatic conditions which alternate between brief wet and dry periods (Ely, 1997).
Following the warm mid-Holocene, a wet period existed from around 5.8 to 3ka.
During this time precipitation was significant enough and temperatures low enough to allow for perennial lakes in Silver Lake, Death Valley, and Searles Lake, higher lake levels in Mono Lake, tree line shifts in Nevada and California, and recharge of the groundwater aquifer in southern Nevada (Enzel and Wells, 1997). This cool, wet, semi- arid period in the southwest was initiated by increased storm frequency caused by glacial advances in the western U.S. (Ely, 1997). The glacial advances increased the moisture transported into the southwest by changing the circulation pattern in the eastern North
Pacific. The change in circulation resulted in increased sea-surface temperatures off the southern and central coasts of California, which caused increased storm activity (Enzel and Wells, 1997). Although not precipitated by glacial advances, the period from 500
B.P. to the present has seen an increase in the number of flood events due to increased storm frequency, yet the climate in the southwest has remained arid, with little to no recharge of the groundwater aquifers (Broecker, 1996).
2.3 Hurricanes
There are two types of storms that bring heavy rains to Baja: Pacific winter lows and hurricanes. Pacific winter lows occur along the entire Pacific coast. These storms
24 move slowly, and pump large amounts of tropical air from the southwest. Like hurricanes, Pacific winter lows can produce, in a single day, the amount of rain that normally occurs in a year. (Markham, 1972)
2.3.1 Formation and Processes
Hurricanes are defined as large tropical cyclones that have maximum sustained surface winds greater than 74 miles per hour. (Pielke and Pielke, 1997) These storms rotate around a relatively calm center, called the eye, that generally measures between 14 and 20 miles across. (Tufty, 1970) The wind speeds of hurricanes can reach a maximum of around 150 to 200 miles per hour. Each year hurricanes account for more deaths than all other storms combined. (Whipple, 1982)
Hurricanes require very specific criteria for formation. There are six conditions associated with hurricane formation. First, the ocean underlying the storm must have a surface temperature greater than 26 degrees Celsius (Pielke, 1990; Michener et al., 1995).
High sea-surface temperatures result in higher surface evaporation, which leads to a large latent heat flux (Barron, 1989). Second, the vertical wind shear between the lower and upper troposphere should be less than 15 knots (Pielke, 1990). If the wind shear is too high, the cumulus convection that is linked to hurricane genesis will not form because temperature and wind anomolies leading to the convection are dissipated by wind shear
(Barron, 1989). Third, a region of lower tropospheric horizontal wind convergence must exist before the hurricane forms (Pielke, 1990). The low-level convergence promotes hurricane formation by enhancing cumulus convection (Barron, 1989). Fourth, the location of formation should be about 4° – 5° from the equator (Pielke, 1990).
Hurricanes do not form closer to the equator because the vorticity is too low; positive
25 absolute vorticity is necessary for cyclonic rotation to occur (Barron, 1989). Fifth, high humidity generally exists in the middle troposphere (Michener et al., 1995; Barron,
1989). High mid-level humidity results in high latent heat release, and is related to well- coupled lower and upper tropospheric flow, the final condition of hurricane formation
(Barron, 1989). When the lower and upper tropospheric flow patterns are well-coupled, surface pressure may fall increasing low-level convergence and enhancing cumulus convection (Barron, 1989).
The first four of the preceeding criteria must be in place in order for a hurricane to form (Pielke, 1990). When the sun warms the oceans, evaporation and conduction rapidly transfer heat to the atmosphere causing the air and water temperatures to be within two degrees of each other (Whipple, 1982). The water vapor then fuels a tropical storm by condensing into clouds and precipitation, and by pumping heat into a cyclone
(Whipple, 1982; Barron, 1989). In the Atlantic, a westward-moving low-pressure center, known as an easterly wave, bends the low-level winds, while high-altitude winds magnify the wave (Whipple, 1982; Pielke, 1990; Barron, 1989). In the Southern Hemisphere, trade winds create a dent in the equatorial trough creating a wave northward (Whipple,
1982). The coriolus effect is strong enough to cause thunderstorms of such waves to spin cyclonically (Barron, 1989). The Northern Hemisphere trade winds then undercut the wave and carry it away as a tropical storm (Whipple, 1982).
Once the tropical storm has formed, organized circulation is connected and intensifies the thunderstorms (Whipple, 1982). Moist low-level air is then drawn in by a deepening low-pressure center (Michener et al., 1995; Whipple, 1982). Convection then lifts the air, which is pushed outward by high pressure above (Barron, 1989). If there is a
26 strong jet stream, circulation will be accelerated, intensifying the storm (Whipple, 1982).
The storm will also be intensified if cold air sinks through warmer tropical air, causing the hurricane’s exhaust to rush into the resulting void and accelerating the surface winds
(Whipple, 1982; Barron, 1989).
More simply, when a storm seedling spins, thunderstorms organize around the tropical depression. The depression becomes a tropical storm when the wind speeds reach 39 miles per hour. As the pressure falls at the storm center, the ring of maximum wind contracts from around 200 miles in diameter to about 30 miles in diameter. This contraction concentrates the hurricane’s energy, which in turn causes the velocity outside the ring to drop rapidly while hurricane force is maintained within the ring. (Whipple,
1982)
There are four stages in the life of a hurricane. The first is the formative stage.
During this stage there is an organized circulation that ends when the speed of the wind reaches hurricane intensity. The second stage is immaturity. This is the period when the hurricane grows to maximum intensity. The Third stage is maturity. During this time the storm no longer grows, but the greatest area is covered by gale and hurricane force winds.
The final stage is decay. At this time the hurricane starts to dissipate as it moves inland or becomes extratropical as it moves into higher latitudes. (Helm, 1967)
Hurricanes can move at speeds of ten to sixty miles per hour and travel distances of about 300 to 400 miles a day (Tufty, 1970). A hurricane gets its energy from evaporation and stores it as heat (Barron, 1989). Ninety percent of the total energy is released as heat that is equivalent to 16 trillion kilowatt hours per day (Tufty, 1970).
About three percent of the energy is converted to mechanical energy, which is equivalent
27 to about 360 billion kilowatt-hours per day (Tufty, 1970). For comparison, the hurricane that hit Galveston, Texas in 1900, had enough energy to run all the power stations in the world for four years (Tufty, 1970).
A hurricane will only lose energy under one of four conditions. First, if the vertical wind shear becomes to large, the hurricane cannot be sustained. Second, a hurricane will lose energy if it passes over land or relatively cold water, thus losing its source of heat and moisture. Third, dry, cool air transported into the system does not favor the development of thunderstorms and results in a loss of energy. Finally, if cyclonic circulation replaces the high pressure aloft, air is added to the system rather than evacuated from the heat engine, and energy is lost. (Pielke, 1990)
2.3.2 Occurrence
Hurricane incidence is related to climatic variability and involves intensity, occurrence, and landfall frequency. Intensity is a function of a hurricane’s central pressure and speed. The higher the sea surface temperature is, the lower the minimum central pressure will be, and thus, the stronger the storm. (Pielke and Pielke, 1997)
There are about 60 hurricanes a year (Tufty, 1970). The U.S. hurricane season lasts from June 1 to November 30 (Tufty, 1970). However, hurricanes occur year-round across the globe. From January to March, storms occur in the South Indian Ocean, the
South Pacific, North and West Australia, and the western North Pacific (Pielke, 1990).
From April to June, hurricanes occur in the western north Pacific, North Indian Ocean,
South Pacific, east Pacific, and the south Indian Ocean (Pielke, 1990). From July to
September, hurricanes occur in the western North Pacific, West Atlantic, East Pacific, and North Indian Ocean (Pielke, 1990). In October to December, storms occur in the
28 western North, south and east Pacific, North and South Indian Ocean, and west Atlantic
(Pielke, 1990). The highest number of hurricanes occurs from July to September, with the fewest occurring from April to June (Pielke, 1990).
The intensity of a hurricane is ranked by the maximum sustained wind speed and the level of damage that results. Table 2.2 gives the Saffir-Simpson scale used to categorize hurricanes and the damage associated with each level.
Landfalling hurricanes are costly to the U.S. One-third of all U.S. hurricanes that fall on land, hit Florida. More than seventy percent of U.S. landfalling hurricanes that are category four or five hit Florida and Texas. One-half of all landfalling hurricanes that hit southern Florida, the middle Gulf coast, and southern New England are category three or higher. (Pielke and Pielke, 1997)
An average of six hurricanes strike Baja each year (Williams and Williams,
1998). Hurricanes that affect Baja originate in the eastern Pacific. They occur from July to November, but September usually sees the greatest amount of hurricane activity.
Although most hurricanes track across Baja California Sur, the occasional storm will track to the north.
29 Table 2.2. Saffir-Simpson hurricane intensity scale for estimating hurricane intensity and damage associated with each intensity level. (after NOAA, 2001)
Saffir-Simpson Maximum sustained Minimum surface Storm Surge (m) Category wind speed pressure (mb) (m/s) (kts) 1 33-42 64-82 >980 1.0-1.7 2 43-49 83-95 979-965 1.8-2.6 3 50-58 96-113 964-945 2.7-3.8 4 59-69 114-135 944-920 3.9-5.6 5 70+ 136+ <920 5.7+
Category Level Description 1 Minimal Damage mostly to shrubs, trees, unanchored homes. No real damage to structures. Low-lying coastal roads flooded, minor damage to piers and small moored boats. 2 Moderate Considerable damage to shrubs and tree foliage, some trees downed. Major damage to mobile homes. Some damage to structure roofing, windows, and doors. Considerable pier damage, marinas flooded. Small boats torn from moorings. Evacuation of shoreline residences and some low-lying areas required. 3 Extensive Foliage removed from trees. Large trees downed. Structural damage to small buildings. Mobile homes destroyed. Serious coastal flooding. Large coastal structures damaged by waves and debris. Terrain less than 5 feet above sea-level flooded. Evacuation of low-lying residences within several blocks of shoreline required. 4 Extreme Shrubs and trees blown down. Extensive damage to roofing, windows, and doors. Complete failure of roofs on many residences. Complete destruction of mobile homes. Terrain less than 10 feet above sea-level flooded. Major damage to lower floors of near-shoreline structures. Major erosion of beaches. Evacuation of all residences within 500 yards of shore and single- story residences within 2 miles of shore required. 5 Catastrophic Very severe damage to windows and doors, glass shattered. Complete failure of roofs of many residences and industrial buildings. Some complete structure failure. Small buildings overturned or blown away. Major damage to lower floors of all structures less than 15 feet above sea-level and within 500 yards of shore. Massive evacuation of residential areas on low ground within 5 to 10 miles of shore required.
2.3.3 Impacts
The hurricane that hit Galveston, Texas in 1900 killed 8,000 people. Thanks to improved prediction and warning systems, modern hurricanes kill far fewer people.
30 Nonetheless, hurricanes kill an average of 24 people and cause about five billion dollars of damage in the U.S. each year. (Parfit, 1998).
The timing, intensity, and frequency of hurricanes affect the hydrology, ecology, and geomorphology of an area (Michener et al, 1997). Large amounts of hurricane- related precipitation may cause erosion, landslides, channel modification, and flooding
(Michener et al, 1997). Hurricanes may also cause tidal overwash, resulting in the deposition of thin sand layers in coastal lakes (Liu and Fearn, 1993).
Storm precipitation can have positive effects, such as recharging water tables or extending a wet season. In the Caribbean, a single tropical storm may contribute 5 to
40% of the annual precipitation; if not for these events droughts might occur. However, storm precipitation usually devastates an area. Storm precipitation can cause wastewater treatment plants to fail because of increased storm-surge runoff. In addition, increased river discharge can contribute to non-point source inputs of sediments and organic material greatly affecting overall water quality. (Michener et al, 1997).
One of the most devastating aspects of hurricanes is a storm surge. A storm surge is a pile up of water caused by prevailing winds of a hurricane. The storm surge can consist of as much as four feet of water along hundreds of miles of coastline
(Whipple, 1982). A surge can be intensified by several factors. If the storm’s onset coincides with high tide, the surge will be increased (Whipple, 1982). The surge will also be intensified if there is a concave coastline that prevents the rising water from moving sideways (Whipple, 1982). Intensification will also occur if the storm is fast-moving, such that the surge water does not have time to spread (Whipple, 1982). Finally, shallow coastal waters will intensify a surge (Whipple, 1982). All these factors can result in a
31 surge of more than 25 feet above normal sea level (Whipple, 1982). Hurricane related storm surges not only claim 90% of a hurricane’s human victims, but can also devestate animal populations and plant communities (Michener et al, 1997).
Hurricanes adversely affect the biodiversity of coastal regions. Hurricanes increase nutrient input, which can cause wetland fires and may enhance denitrification rates. The winds of hurricanes snap tree trunks and uproot coastal trees. Increased salinity in wetlands and soils is caused by surge spray. The increased salinity of soils increases tree mortality. Fish and crustacean populations are reduced by hypoxia, which results from an influx of debris and resuspension of highly reducing bottom sediments during a hurricane. Populations of land animals, such as deer and muskrats, may be noticeably reduced by drownings. Animals that are not killed by hurricane flooding, such as alligators and frogs, are often transported downstream. (Michener et al, 1997).
2.4 Hurricane Nora
Hurricane Nora developed as a tropical storm in the east Pacific on September 16,
1997. Hurricane Nora was the fourth major eastern Pacific hurricane that year. The formation of Nora was related to a tropical wave that had crossed from Africa into the
Atlantic and was related to variations in sea-surface temperatures (HPC, 1997;
Rappaport, 1997). By September 18, Nora was a hurricane with winds up to 168 knots and an eye 45 miles in diameter (CNN, 1997).
Figure 2.1 shows the track hurricane Nora followed from September 16 to
September 25, 1997. Between September 18 and 20, Nora weakened as it moved west- northwest over waters that were 2° to 4°C cooler than those in which the storm formed
(Rappaport, 1997). During this time the eye disappeared. By September 21 Nora had
32 regained its strength, with wind speeds up to 115 knots (NOAA, 1997). The eye of the storm reappeared with a diameter of 15 nautical miles (Rappaport, 1997).
Figure 2.1. Track of hurricane Nora from its formation on September 16 to dissipation on September 26. The track is color coded to represent the strength and classification of the storm as it progressed. (from NOAA, 1997)
From September 21 to 23, Hurricane Nora’s track followed that of Hurricane
Linda, which had occurred earlier in the hurricane season (NOAA, 1997). Nora again lost strength because Hurricane Linda had decreased sea-surface temperatures. (HPC,
33 1997; Rappaport, 1997) Nora’s wind speed dropped to about 70 knots and the diameter of the eye increased to about 50 nautical miles. (Rappaport, 1997)
On September 24 the track of Nora changed north-northwest because of a low pressure system that had developed over the western United States (NOAA, 1997;
Rappaport, 1997). Figure 2.2 shows hurricane Nora during its directional change of
September 24. This new track carried Nora over waters that were 2° C higher than normal (Rappaport, 1997). Travelling over 26° C water for two days allowed Nora to maintain hurricane strength as it approached the coast of Baja California (Rappaport,
1997).
Figure 2.2. Hurricane Nora on September 25, 1997. (from American Weather Concepts, 1997)
34 Nora made landfall over the Chapala basin on September 25 (NOAA, 1997;
Farfan et al, 1997). Hurricane Nora crossed the Baja peninsula at a rate of 20 to 25 knots
(Rappaport, 1997). About 25 cm of precipitation fell on Laguna Chapala when hurricane
Nora crossed (CPC, 1997).
Nora’s strength dropped as it crossed into the United States and was reclassified as a tropical storm (Rappaport, 1997; NOAA, 1997). By September 26, the storm had dropped to tropical depression strength (NOAA, 1997), with a top wind speed of 30 knots
(Rappaport, 1997). By the end of the September 26, Nora had died out (NOAA, 1997).
35 Chapter 3
Methods and Data
The purpose of this study was to understand the sedimentology and geochemistry associated with the flooding caused by hurricane Nora so that similar events can be identified in the lake’s history. Various methods were employed to: 1) reconstruct the hydrology of the Laguna Chapala basin; 2) document the features of the playa lakebed; and 3) determine the grain size, composition, and morphologic features of the storm- related deposits.
36 3.1 Hydrological Reconstruction
The flooding created by hurricane Nora lasted from September 1997 to February
1998. In order to estimate the extent and volume of flooding, a survey of the flooded area was done. A map of Laguna Chapala’s drainage basin had to be constructed in order to make runoff estimates. Additionally, an evaporative model was constructed to aid understanding of the hydrologic setting of Laguna Chapala. The post-hurricane Nora hydrologic reconstruction defines the conditions under which the deposits left by the flooding are found, and aids understanding of the Laguna Chapala system.
3.1.1 Surveying
Enzel and Wells (1997) demonstrated that geomorphological evidence could be used to determine storage capacity, areal extent, and depth of a lake. In order to determine the amount of water in Laguna Chapala after hurricane Nora, it was necessary to survey the area of flooding. A total station (theodolite and Sokkia computer) was used to complete the survey of the lake area. The total station provided both distance and elevation data.
Figure 3.1 is a plot of the December 1998 survey data. The data used to create the plot can be found in Appendix A. The survey data were plotted in Excel then the plotted points were fitted to a compilation of an aerial photo of Laguna Chapala and a topographic map of the region. Comparing the surveyed plot of Laguna Chapala to the actual photo and topographic data served as a check that the survey was correct. The shape of the surveyed data closely mimics that of the aerial photo. The surveyed area is not as large as the lake area in the photo because the perimeter of the survey area was
37 determined by high water marks, or flooding extent, not the actual perimeter of the playa.
The area of flooding was 15.97 km2.
Figure 3.1. Compilation of survey data, topographic map, and air photo. The perimeter of the survey points represents the extent of flooding resulting from hurricane Nora, based on high water marks such as vegetative debris. Because the flood did not cover the entire extent of the the playa, the surveyed area is smaller than the actual area of Laguna Chapala.
38 3.1.2 Drainage Basin Map
The area of the drainage basin was determined by digitizing the drainage boundary represented on a topographic map of the area. The drainage boundary was digitized in MapGrafix, which calculates the area of the digitized polygon. The drainage basin is shown in figure 3.2. A few of the ephemeral streams are shown on the map to indicate the general drainage pattern. The drainage area of Laguna Chapala is 110 km2.
Figure 3.2. Drainage area of Laguna Chapala. Laguna Chapala is outlined within the drainage. (Derived from INEGI, 1995)
N
39 The volume of runoff that resulted from Nora can be estimated by multiplying the drainage area by the amount of precipitation. This calculation serves as a check of the accuracy of the volume calculation produced from survey results.
(1.10 x 108 m2) · (2.50 x 10-1 m) = 2.75 x 107 m3
3.1.3 Volume Calculation
Because the survey data contained area and elevation data, it could be used to construct the hypsometry of Laguna Chapala following the flooding resulting from hurricane Nora. After the survey data was plotted, the elevation data for each point was used to make a contour map of the flooding. The elevation data is part of the survey data listed in appendix A. The contour map shown in figure 3.3 was created in MapGrafix.
Figure 3.3. Contour plot of lake left in Laguna Chapala following hurricane Nora. The contour interval is 20cm. The “0” contour is the perimeter of the flooded area.
40 Figure 3.4. Graph depicting Laguna Chapala hypsometry. The lake left by hurricane Nora was up to 1.2 meters deep and covered an area of 15.97 km2.
Chapala Hypsometry 0 -0.2 -0.4 -0.6 -0.8 -1 -1.2
Area Above Elevation (m2)
Figure 3.5. Graph of the increase in lake volume with depth, based on the hypsometry data.
Volume with Depth
7,000,000 6,000,000 5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 0 0 0.2 0.4 0.6 0.8 1 1.2 Depth (m)
Once the contours were digitized, the area of each polygon was determined using
MapGrafix. The areas of the contour polygons and the elevation data were used to make volume estimates and provided the hypsometry data plotted in figure 3.4. The data used
41 to create the hypsometry graph can be found in appendix B. The total volume of flooding resulting from hurricane Nora was 7,100,000 m3. Figure 3.5 depicts the increase in lake- volume with depth, as determined by the hypsometry data.
3.1.4 Evaporation Model
Evaporation is an important element to consider in the hydrologic budget of a playa lake because it is the only outflow from the playa. Evaporation is affected by factors such as mean daily temperature, solar radiation, surface area and wind. There are several methods for estimating evaporation with varying accuracy, but constraints on time and equipment limit the methods that can be used in the study of Laguna Chapala.
A Class A evaporation pan is one of the easiest ways to estimate evaporation in an area. A Class A evaporation pan is an unpainted galvanized metal cylinder 122 cm in diameter and 25.4 cm deep (Fetter, 1994). The pan is set on a leveled surface and filled with water to within 5.08 cm from the top of the pan. Records of daily precipitation, volume of water added to replace evaporated water, and the daily depth of the water are recorded, and these measurements can used in a simple water budget calculation (inflow
= outflow ± storage) to determine evaporation. The evaporation pan is warmed more readily than surface waters and it can lose or gain heat through its sides and bottom rather than just at the surface (Hemphill, 1996). As a result, the calculated pan evaporation must be multiplied by a pan coefficient in order to yield a valid estimate of evaporation.
42 Figure 3.6. Nomograph for determining the value of daily lake evaporation. Mean daily temperature, mean daily dew point, temperature, solar radiation, and mean daily wind movement must be known in order to estimate evaporation. (From Fetter, 1994)
The use of a nomograph is another relatively easy way to estimate evaporation.
Figure 3.6 shows the nomograph developed by the National Weather Service for shallow lakes (Fetter, 1994). Evaporation is determined by drawing a horizontal line across the chart at the mean daily air temperature. Perpendicular lines are then drawn from the points at which the mean daily temperature intersects the solar radiation and the mean daily dew point temperature. From the point at which the dew point perpendicular intersects the daily wind movement, a horizontal line is drawn to the left. The point at 43 which this line intersects the perpendicular drawn from solar radiation represents the daily lake evaporation.
Mason et al. (1994) proposed a method of calculating evaporation based on the water balance equation. They rewrote the balance equation as:
dV/dt = R - AL(EL - PL) - D + Gi - Go. dV/dt is the change in volume over time. AL, EL, and PL represent the area of the lake, evaporation, and precipitation respectively. D represents the discharge, which for Laguna
Chapala would be 0. Gi and Go are the groundwater seepage into and out of the lake, the clay material of the lake may allow these to be neglected. R is the surface runoff. R can be calculated using the equation:
R = ABPBr where A is the area of the catchment basin (not including the lake), P is the mean precipitation over the basin, and r is the runoff coefficient. The balance equation allows evaporation to be estimated if the change in the area of the lake, precipitation, duration of time and runoff are known.
Because of the difficulty of quantifying the evaporation rate in playa environments due to harsh conditions and low evaporative fluxes, Tyler et al. (1997) used five methods in the calculation of evaporation at Owens Lake: class A pan evaporation, microlysimeters, Bowen ratio energy budget, Eddy correlation, and soil chloride profiling. Microlysimeters were constructed from two lengths of pvc pipe. The outer pipe was placed in the ground and all the soil within it was removed. The inner pipe was driven into the soil near the test site in order to create an intact core. The pipe with the core was then weighed and placed into the outer pipe. The core was removed and
44 weighed daily for up to a week. Evaporation was determined by dividing the weight loss each day by the density of water and the cross-sectional area of the pipe.
The Bowen ratio calculation utilized by Tyler et al. (1997) requires the measurement of thermal and vapor density gradients near the surface, net radiation, and soil heat flux. In order to get these measurements, Tyler et al. (1997) had to use instrumentation such as chromel-constantan thermocouples, a relative humidity- temperature sensor, a net radiometer, and soil heat flux plates.
The Eddy correlation performed by Tyler et al. (1997) also involved a significant amount of instrumentation. In order to measure vertical wind speed and vapor density a sonic anemometer and an open-path hygrometer had to be set up. Instruments similar to those used for the Bowen ratio had to be used for measuring the net radiation and soil heat flux. The major drawback of both the Bowen ratio and Eddy correlation methods for calculating evaporation is the amount of instrumentation required for measuring the many variables.
Tyler et al. (1997) also used soil chloride profiling in their calculation of evaporation from Owens Lake. This method assumes that soluble salts accumulate near the surface because of evaporation occurring on bare soil. In this method cores are collected by hand auger or percussion coring, and the chloride concentration of the soil is analyzed. The chloride concentration of water squeezed from the samples or collected in piezometers is also analyzed. Once the chloride concentration is known, evaporation can be determined from the equation:
[(C(z) - C¥ ) / (Co - C¥ ) = exp[z / (E / nD*)]
45 where Co is the chloride concentration at the land surface, C¥ is the chloride concentration in the groundwater, n is soil porosity, z is the vertical coordinate measured from the soil surface, E is the evaporation rate, and D* is the solute diffusion coefficient in the soil.
Bergonzini et al. (1997) and Laymon et al. (1998) used sets of equations to estimate evaporation. Bergonzini et al. (1997) uses a water budget equation similar to that found in Dunne and Leopold (1978).
(Pc * Sc) + (Pl * Sl) = (Ec * Sc) + (El * Sl) + O
C identifies parameters relating to the catchment area, while l identifies those relating to the lake. P is the precipitation rate in meters per year. E represents evaporation in meters per year. S is the surface area in square meters. O is the surface and groundwater discharge in cubic meters per year.
Bergonzini et al. (1997) refine their calculation of evaporation with the Bowen ratio equation:
El/c = Rl/c / (L * (1 + Bl/c)) where L represents the latent heat of evaporation, B is the Bowen ratio, and R is the net radiation. R comes from the difference between absorbed and emitted radiation. R can be calculated using the equation for absorbed solar radiation and net long-wave radiation.
SW = I * k * (1-(1-n’) * C) * (1-a)
LW = s * ¶ * T4 * (0.39 - 0.058 * Öe) * (1-c’ * C2)
I represents the insolation at the top of the atmosphere in W/m2, k is the atmospheric transmission coefficient, and C represents the ratio of cloud sky cover given in tenths. n’ represents the latitudinal mean coefficient. Albedo is represented by a. In the long-wave
46 equation, s represents the Stefan-Boltzman constant which is 5.67 * 10-8, ¶ is the surface emissivity, and T represents the air temperature in degrees Kelvin. Water vapor pressure in millimeters of mercury is represented by e, while c’ is the mean latitudinal Berliand coefficient. By using the water budget and Bowen ratio equations, Bergonzini et al.
(1997) were able to produce paleoevaporation and paleoprecipitation proxies that agreed with previous global climate models.
Laymon et al. (1998) used an energy balance equation, which resembles those of
Dunne and Leopold (1978) and Bedient and Huber (1988), to determine evapotranspiration in the Great Basin of the United States. In the equation
Rn + G = H + LE
Rn represents the net radiation that is absorbed at the surface, G is the soil heat flux, H is the sensible heat flux into the atmosphere, and LE is the latent heat flux. LE is a product of the evaporation rate and the latent heat of vaporization. Rn, G, and H can be estimated using remote sensing reflectance and emittance data and meteorological data from the surface. Thus, LE can be estimated using the equation
LE = Rn + G - H.
Bedient and Huber (1988) outlined several methods for estimating evaporation.
The mass transport method requires inputs of vapor pressure at the water surface (eo), vapor pressure at a fixed level above the water surface (ea), wind speed (u), and empirical constants for the area being studied (a and B). The mass transfer equation as written in the form of Dalton’s law is :
E = (eo - ea) * (a + bu)
47 Bedient and Huber (1988) also discussed the Penman equation (referred to in
Dunne and Leopold, 1978 as the mass transfer approach), which is a combination of the mass transport method and the energy budget method (Bedient and Huber, 1988 and
Laymon et al., 1998). The Penman equation is
E = (D / (D + g)) * QN + (g / (D + g)) * Ea.
Ea is the value of E given by the equation
E = Nu2(eo-e2) where N is the mass transfer coefficient for the area of study. In the arid southwest, N is calculated by the equation
N = 0.000169A-0.05
where A is the area of the lake. u2 is the wind speed at 2 meters above the water surface, eo is the vapor pressure at the water surface, and e2 is the vapor pressure 2 meters above the water surface. Qn is the net radiation absorbed. g is the psychometric constant, 0.66
P/1000. D is calculated from the slope of a es vs. T curve.
D = (es - esz) / ( Ts - Tz) where es is the saturation vapor pressure at the surface temperature (Ts), esz is the saturation vapor pressure at elevation z given temperature at the elevation (Ts). The
Penman equation is best used on large bodies of water, and is reliable for daily or monthly estimates of evaporation. Dunne and Leopold (1978) simplified the mass transfer approach to
Dh = Eo + S where Dh is the net change of the water surface elevation determined from an elevation- volume curve, and S is the net groundwater seepage in depth units over the lake area.
48 The water budget method described by Dunne and Leopold (1978) relies only on volume measurements. They expand the general budget equation inflow = outflow + change in storage to the form:
Isurf + Isub + P = Osurf + Osub + Eo + (V2 - V1).
Isurf and Isub are the volumes of surface and subsurface inflows from streams and groundwater. P is the precipitation volume onto the lake. Osurf and Osub are the volumes of stream and groundwater outflows from the lake. Eo is the volume of water evaporated.
V1 and V2 are the initial and ending volumes of the lake for the period of study.
The Thornthwaite method requires only temperature data for the estimation of evapotranspiration. The equation used in the Thornthwaite method is:
Et = 1.6[(10 * Ta) / I]a where Et is potential evapotranspiration in centimeters per month and Ta is the mean monthly temperature in degrees Celsius. I is the annual heat index given by the equation
12t 1.5 I = å =1[Tai / 5] . a is derived from the formula
a = 0.49 + (0.0179 * I) - (0.0000771 * I2) + (0.000000675 * I3).
Hamon developed a method similar to that of Thornthwaite, which also uses temperature as the primary factor in determining evapotranspiration (Dingman, 1994). In
Hamon’s equation
PET = 0.042D(r vsat) where PET is the potential evapotranspiration in centimeters per month, D is the day length in hours, and r vsat is the saturation absolute humidity in grams per cubic meter. r vsat is calculated from the equation
49 r vsat = [217 / (T + 273.2)] * [6.11 * exp((17.3 * T) / (T + 237.3))].
Actual evapotranspiration can then be determined based on soil moisture. If the soil moisture is greater than 0, then the actual evapotranspiration (AET) is equal to the potential evapotranspiration. If the soil moisture equals 0 and the PET is greater than the amount of precipitation, then the AET equals the precipitation. If the soil moisture is 0 and the PET is less than the precipitation, then AET equals the PET.
Because temperature recordings from meteorological stations and volume estimates from my survey data were the only data available for my evaporation estimates,
I was limited to using the Mason et al, Thornthwaite, Hamon, and Dunne and Leopold water budget methods.
Once the precipitation data was obtained and the lake and drainage areas were calculated, evaporation was estimated using the methods of Mason et al, Thornthwaite,
Hamon, and Dunne and Leopold. The calculations are given in appendix C. The water balance method of Mason et al used lake area, precipitation, and time duration information in the calculation and produced an evaporation rate of 404,395 m3/month.
The Hamon method is derived from the Thornthwaite method. Both use temperature data to determine evaporation rates. The Thornthwaite method produced an average evaporation rate of 1,101,185 m3/month, while the Hamon method produced and average evaporation rate of 1,118,105 m3/month. The water budget method of Dunne and
Leopold relied only on volume data and produced and average monthly evaporation rate of 1,973,500 m3/month.
50 3.2 Sedimentology and Stratigraphy of the Lakebed
Flooding washes sediments into a lake, influences grain size distribution, and affects the formation of sedimentary structures. As a result, the sedimentology and stratigraphy of a playa is useful to record climatic change. Features such as high water marks and mudcracks are helpful for defining the size of a flood and for determining the feasibility of identifying similar floods in the record of a playa.
3.2.1 High Water Marks and Evaporation Rings
High water marks were used to determine the extent of flooding resulting from hurricane Nora. Vegetative debris was the most useful indicator of flooding boundaries.
Figure 3.7 depicts some of the high water marks found at Laguna Chapala. The concentration of debris, such as that in figure 3.7a, occurred only on the perimeter of
Laguna Chapala and was often associated with a large evaporation ring, suggesting that this concentration was due to flooding alone. Additionally, concentration of vegetative debris sometimes occurred in stages that mimicked the wave swash found on beaches, further indicating these features were associated with flooding boundaries. Figure 3.7b was taken in December, 1998, and indicates that the high water marks were preserved long after the lake had dried.
51 Figure 3.7. High water marks at Laguna Chapala.
A) Concentration of vegetative debris representing a high water mark.
B) High water marks present in December 1998. Such marks resembled wave swash seen on beaches.
Lack of vegetation was as useful an indicator of flooding extent, as was the presence of vegetative debris. Figure 3.8 dramatically displays the contrast between the
52 flooded surface and unaffected areas. No vegetation was found in either July or
December where flooding had been present. Observation of other playas throughout
Baja, such as Laguna Salada and Seco playa, suggest that vegetation is slow to recover on playa surfaces. For example, Seco playa, which has not been flooded in at least ten years, had very small and sparse vegetation on the playa surface similar to that found at the boundaries of Laguna Chapala.
Figure 3.8. Boundary of flooded area. The area that was flooded by hurricane Nora was completely void of vegetation, while the non-flooded surface had small brush and grasses present.
Evaporation rings form as the lake area shrinks due to evaporation. As the lake evaporates, sediments fall out of solution and different minerals precipitate due to changing lake chemistry, causing evaporation rings to form (Cooke and Warren, 1973).
The variations in color indicate variability in duration and frequency of ponding
(Holliday et al, 1996). The rings are useful for determining the area and volume of a shrinking lake. Figure 3.9 shows some of the evaporation rings that were present in July, 53 1998. The evaporation rings that resulted from the hurricane Nora flooding event were not present in December because another wetting event occurred that provided enough precipitation for new evaporation rings to occur.
Figure 3.9. Evaporation rings associated with the hurricane Nora flooding event.
A) Playa surface showing multiple evaporation rings.
B) Close-up of the center of an evaporation ring.
54 3.2.2 Mudcracks
Mudcracks form at Laguna Chapala because of the clay content in the playa.
Mudcracks are a feature that may be preserved over time under the right conditions. If mudcracks resulting from a particular wetting event are preserved at Laguna Chapala, they may help identify storm events in the playa record.
The mudcracks present in July 1998 were well defined with sharp edges and fresh, clean surfaces (Fig. 3.10). The mudcracks on the northern side of the playa were 4 to 6 cm deep and up to 15 cm across. The mudcracks on the southern side of the playa were 7 to 10 cm deep and 25 to 50 cm across.
On the surfaces of the mudcracks associated with hurricane Nora flooding, smaller cracks were present. These cracks were poorly defined and very small, less than a millimeter deep and up to 5 cm across. The hurricane Nora mudcracks cut across these smaller mudcracks, indicating that the small mudcracks were from a previous wetting event. Also present were even smaller, very faint mudcracks. These were cut by both the small cracks and the Nora mudcracks, indicating a second older wetting event prior to hurricane Nora.
The mudcracks present in December 1998 were readily apparent, but somewhat less defined than those in July. Figure 3.11 shows the mudcracks found in December.
On the northern side of Laguna Chapala the mudcracks were partially filled in and the surface was covered by a very fine layer of sediment. The mudcracks from previous wetting events were very faint and in some cases not present. Small mudcracks were present that cross-cut the hurricane Nora mudcracks, suggesting they were from a more recent wetting event. The edges of the mudcracks were rounded yet still defined. In
55 cross-section it was apparent where the original boundary of the mudcrack had been before it was filled in with sediment.
Figure3.10. Mudcracks in July, 1998.
A) Mudcracks from northern side of Laguna Chapala
B) Mudcracks from southern side of Laguna Chapala.
56 Figure 3.11. Mudcracks in December 1998.
A) Mudcracks from northern side of Laguna Chapala.
B) Mudcracks from southern side of Laguna Chapala
57 Mudcracks on the southern side of the playa in December did not show the degree of weathering present in those on the northern side. The southern mudcracks had sharp edges and the surfaces were only slightly covered by sediment. The mudcracks did not appear to be filled in as those on the northern side were. There was no evidence of newer mudcracks on top of the hurricane Nora mudcracks and the mudcracks from previous events were still apparent.
3.2.3 Grain Size
Grain size analysis was performed on the 14 samples collected from the playa surface. Samples were treated with H2O2 and heat to remove organic material. Samples were triple rinsed with deionized water and sonicated. The dissolved sample was passed through a 300 mesh sieve to separate the sand sized fraction; this fraction was then dried, weighed, and sieved to further define the grain size of the sand fraction. Pipette analysis was performed on the sample fraction that passed through the 300 mesh sieve after the procedure of Singer and Janitzky (1986) and Black et al (1965). Appendix E gives the procedure for grain size analysis. Table 3.1 presents the results of the grain size analysis.
Table 3.2 presents the detailed grain size of the sand and clay size fractions. Grain size calculation data is presented in appendix F.
58 Table 3.1. Results of grain-size analysis.
Sample # % Total Sand % Clay % Silt 071498-1 1.31 13.51 85.18 071498-2 0.68 18.28 81.05 071498-3 0.11 18.21 81.68 071498-4 0.07 18.97 80.96 071498-5 0.25 19.73 80.02 122298-1 0.27 16.49 83.23 122298-2 0.51 15.28 84.21 122298-3 0.07 17.59 82.34 122298-4 0.08 18.66 81.25 122398-1 0.99 16.90 82.10 122398-2 0 19.43 80.57 122498-1 0.19 15.34 84.47 122498-2 0.04 16.88 83.07 122498-3 0.10 14.31 85.59
Table 3.2. Breakdown grain size of sand and clay fractions.
Sample # 1.0 mm 0.5 mm 0.25 0.180 0.125 0.063 20 µm 5 µm 2 µm (%) (%) mm mm mm mm (%) (%) (%) (%) (%) (%) (%) 071498-1 0.08 0.08 0.14 0.08 0.17 0.55 5.17 4.50 3.84 071498-2 0.11 0.07 0.08 0.04 0.06 0.14 6.59 6.59 5.09 071498-3 0 0.01 0.02 0.01 0.02 0.06 6.95 6.19 5.07 071498-4 0.01 0 0.01 0.01 0.01 0.04 8.29 6.08 4.60 071498-5 0 0.01 0.03 0.02 0.03 0.09 7.93 6.45 5.35 122298-1 0 0.01 0.03 0.03 0.03 0.11 7.18 5.63 3.69 122298-2 0 0.01 0.06 0.05 0.07 0.21 6.69 4.98 3.60 122298-3 0 0 0.01 0.01 0.01 0.04 6.71 5.26 5.62 122298-4 0 0 0.02 0.01 0.01 0.04 8.48 5.84 4.34 122398-1 0.02 0.08 0.17 0.09 0.10 0.37 6.87 5.76 4.27 122398-2 0 0 0 0 0 0 8.14 6.81 4.48 122498-1 0 0.01 0.01 0.01 0 0.02 6.84 4.99 3.51 122498-2 0 0 0.01 0 0.01 0.03 7.64 5.87 3.38 122498-3 0 0 0 0.01 0.01 0.03 6.18 4.06 4.06
59 3.3 Evaporites
Evaporites have been used extensively in the study of playa lakes. They are useful for monitoring evaporative concentration, for determining brine evolution, and for studying paleoclimates. Studying modern playa salts provides an analogue for ancient playa depositional processes.
3.3.1 Previous Work
Li, et al. (1996) studied evaporites in Death Valley, California. From December,
1992, to February, 1993, the Badwater Basin contained a 0.5 m deep saline lake. Li, et al. sampled the saline pan in April, 1993. They determined the characteristics of evaporite deposits resulting from the flooding that were analogous to characteristics found in salt cores from the Badwater Basin. They used this information to construct a 100 ka record of climate in Death Valley.
Handford (1982) studied modern evaporites in Bristol Dry Lake, California in order to analyze Permian playa facies. Handford found that evaporites in the lake form in concentric zones and have a cyclic stratigraphy. The cycles apparent in the stratigraphy of Bristol Dry Lake reflect the processes of the saline pan cycle identified by Lowenstein and Hardie (1985).
Friedman, et al. (1976) studied Owens Lake, California from 1969 to 1971 after large amounts of runoff produced a lake 2.4 m in depth. Owens Lake experienced similar flooding in 1938. With the previous event in mind, Friedman, et al. determined that up to a year may pass before dissolution of old salt crusts ceases and that seasonal variation in
60 temperature may cause temporary crystallization of salts or the dissolution of salts that crystallized earlier in the flood event.
3.3.2 Modern Facies
There a three types of playa crusts: dry crusts, wet crusts, and compound crusts.
Wet crusts are surfaces which are always wet. They consist of salt, clay, and silt masses that are plastic. Wet crusts may occur where springs are present, but are often associated with puffy crusts. Dry crusts can be subdivided into compact floors, puffy floors, salt pavements, and carbonate surfaces. Compact floors consist of hard, dry, densely packed, fine-grained clay materials. Compact floors are not easily penetrated by precipitation.
Although compact floors may contain significant amounts of water after flooding, they quickly return to their normal dry state. Puffy floors consist of loosely packed, coarse- grained clay materials that readily absorb water. Puffy floors are dry, soft, porous, and have a “crusty” texture in the summer, and are wet in the winter. Such surfaces tend to swell when wetted. Salt pavements are formed by brine deposition of salts, which results in a crystalline surface. Salt pavements tend to be broken, due to the pressure resulting from salt crystallization, but may display a variety of characteristics. Carbonate surfaces also display a variety of physical characteristics. They consist of tufa crusts which result from deposition of carbonate minerals by springs. Compound crusts display features of more than one crust type. In a compound crust, one crust type may grade into another type. (Langer and Kerr, 1966)
61 Figure 3.12. The stages of the saline pan cycle. The insets, show the associated depositional features expected with each phase. (after Lowenstein and Hardie, 1985 and Thomas, 1989)
1. Flood Stage (brackish lake)
Processes: floodwaters - dissolution of saline crust saline - deposition of mud mudflat deposits Sediment Characteristics: layered salts of - smooth dissolution surface saline pan - truncated halite crystals - fine layer of detrital mud suspended sediment over salts mud layer - coarser gypsum etched
dissolution surface on crust dissolution vugs and pipes
2. Evaporative Concentration (saline lake) evaporation Processes: - subaqueous crystallization and growth of salts in the saline lake
Sediment Characteristics - flat crystals - cloudy crystals
surface nucleated rafts and hoppers syntaxial overgrowth on rafts and hoppers settled on lake bottom 3. Dessication (dry saline pan)
evaporation crust broken into polygons Processes: - diagenetic growth of salts within saline and mud layers beneath pan surface - breaking of saline crust into polygons
Sediment Characteristics: puffy efflorescence growth - clear crystals, sharp on polygon pressure ridges boundary separates from saline cement growth stage 2 crystals in voids and dissolution - polygonal surface cracks vugs
62 . The major salts in modern saline pans are halite (NaCl), gypsum (CaSO4
. . 2H2O), mirabilite (Na2SO4 10H2O), thenardite (Na2SO4), epsomite (MgSO4 7H2O), and
. . trona (NaHCO3 Na2CO3 2H2O) (Lowenstein and Hardie, 1985). These salts are found in layers of crystalline salts which alternate with siliciclastic-rich mud (Lowenstein and
Hardie, 1985). The layering is due to the saline pan cycle.
The saline pan cycle consists of three phases: flooding, evaporative concentration, and desiccation. During flooding, the salt crust is dissolved and mud is deposited
(Lowenstein and Hardie, 1985). During the evaporative concentration phase, crystallization and growth of salts takes place in the waters of the saline lake (Lowenstein and Hardie, 1985). Desiccation is the normal state of a saline pan (Lowenstein and
Hardie, 1985). During desiccation, diagenetic growth of salts occurs within the mud and salt layers beneath the pan surface (Lowenstein and Hardie, 1985). In addition, cementation of the salt crust takes place, and the salt crust shrinks into polygons (Casas and Lowenstein, 1989 and Lowenstein and Hardie, 1985). Figure 3.12 shows the stages of the saline pan cycle and the depositional features associated with each stage.
3.3.3 Salt Chemistry
The most common solutes found in saline lakes are SiO2, Ca, Mg, Na, K, HCO3,
CO3, SO4, and Cl (Hardie, et al, 1978). Na is the most abundant cation (Hardie, et al,
1978). There are five types of solute behaviors, as determined by Eugster and Jones
(1979). A solute may remain in solution for the duration of the concentration process.
Such perfectly conserved species may form crusts which are redissolved by runoff, as in
63 the case of Na and Cl. The conserved species can be used to determine concentration factors and to track increases in concentration.
A cation and an anion may combine to precipitate a mineral phase. In this system, solute A increases, while solute B is rapidly depleted. For example, in waters where the concentration of HCO3 is greater than that of Ca and Mg, calcite precipitation will cause CO3 enrichment and Ca depletion. (Eugster and Jones, 1979)
A solute may also be gradually removed from solution due to the action of several mechanisms. In this scenario, the removal of the solute has a primarily linear relationship to evaporative concentration. At Lake Magadi, Kenya, degassing and decreasing solubility, resulting from increasing salinity, work to gradually decrease the concentration of HCO3 and CO3 in the water. (Eugster and Jones, 1979)
Some solutes are removed only during the middle portion of the concentration range of the solution. This behavior is dependent on concentration and is usually due to ion exchange, surface adsorption, or biogenic reduction. K and SO4 follow this pattern of behavior. (Eugster and Jones, 1979)
The final type of behavior results when a solute is unchanged when saturation of a corresponding solid phase is reached. The unchanged solute will remain constant after the saturation point. SiO2 is a solute which tends to behave in this fashion. (Eugster and
Jones, 1979)
Solutes which are conserved over all or at least a portion of the concentration range of a solution are considered conservative elements. Conservative elements are useful because they can be used for monitoring concentration. For example, Cl stays in 64 solution until halite saturation occurs, so it is a good measure of the degree of evaporative concentration up to that point. After halite saturation, concentration must be measured using solutes such as Mg, K, or Br. (Eugster and Jones, 1979)
Reeves (1978) defined the most common playa deposits and the conditions under which they may form. Calcite, aragonite, and dolomite are the most common carbonates found in playas. Calcite and aragonite will always precipitate first. Dolomite only occurs when the solution has a high Mg/Ca ratio. Trona may be deposited if the sulfate
+ - - concentration is low, if there are high concentrations of Na2 , HCO3 , and CO3 , and if the pH of the solution is greater than 9.
Gypsum is the most common sulfate found in playas. Bloedite, epsomite, and thenardite are other common sulfates. Sulfates are able to form when salt-resistant vegetative debris is incorporated into the playa sediment and reduced through bacterial decay to produce sulfides. Sulfate is then produced when the sulfides are oxidized.
(Reeves, 1978)
Common borates found in playas include borax, kernite, searleasite, and colemanite. Nitrates, such as soda nitre and nitre, are also found in playas. Borates and nitrates usually occur in playas that are surrounded by volcanic areas. (Reeves, 1978)
The devitrification of volcanic ash that occurs in highly alkaline environments generally results in the formation of silicates. Silicates may be found in semi-arid to arid environments. (Reeves, 1978)
Lithium is deposited during late evaporative stages because it is highly soluble.
As a result, it is associated with potassium, boron, chloride, or sulfate brines and 65 montmorillonite clays. The lithium-bearing clays often occur with calcite, dolomite, sepiolite, and attapulgite. (Reeves, 1978)
Zeolites, such as clinoptilolite, erionite, and phillipsite, commonly occur in alkaline playa sediments. Potassium feldspars will form in highly alkaline, or saline, environments. Erionite and phillipsite primarily form in saline lacustrine deposits.
Clinoptilolite and mordenite can form in fresh or saline environments. Volcanic glass and alkalic zeolites will only form in fresh environments. (Reeves, 1978)
There are four major brine types found in playas. These are Na-CO3-Cl-SO4 brines, Na-Cl-SO4 brines, Na-Mg-Cl-SO4 brines, and Ca-Mg-Na-Cl brines (Hardie, et al,
1978 and Reeves, 1978). The chemical composition of a playa surface and brines depends on the composition of inflow waters, mineral precipitation, formation and dissolution of efflorescent crusts, exchange and sorption reactions on active surfaces, degassing, diagenetic reactions, and redox reactions.
The composition of inflow water is controlled by the bedrock geology. Because different types of bedrock result in different weathering products, they generally cause different mineralogical and geochemical lake deposit compositions (Schütt, 1998).
However, very different bedrocks may produce similar chemical compositions. For
2+ - example, as much Ca and HCO3 can be contributed to inflow waters from the weathering of plagioclase-rich igneous or metamorphic rock as can be contributed by the dissolution of limestones (Hardie, et al, 1978).
Four types of weathering reactions influence water chemistry: congruent dissolution of non-silicates, congruent dissolution and hydrolysis of alumino-silicates, 66 incongruent dissolution and hydrolysis of alumino-silicates to produce clay, and oxidation of metal sulfides to produce metal oxides and sulphate ions (Hardie, et al, 1978). The
- dominance of HCO3 in the inflow water of closed basins and the abundance of clay minerals in fine sediments that are washed into desert basins demonstrates that carbonic acid weathering is a major chemical process in arid closed basins (Hardie, et al, 1978).
Climate will affect the amount and type of weathering that takes place. Processes of chemical weathering will be impeded by subarid conditions, resulting in low concentrations of metal oxides. Arid conditions will result in increased concentrations of
Na and K and precipitation of sodium and potassium minerals. (Schütt, 1998)
Precipitation of salts affects the evolution of solution compositions. Figure 3.13 demonstrates that the composition of the water at the dilute inflow stage determines the composition of the water during evaporative concentration (Eugster and Jones, 1979).
For example, if the inflow water contains a much greater concentration of Ca and Mg than
HCO3 and there is more Ca than Mg, then gypsum will be precipitated and the resulting brine will be enriched in alkaline earth carbonates and depleted in CO3 and HCO3 (Hardie, et al, 1978).
Figure 3.14 demonstrates that the fundamental control in brine evolution is the early precipitation of insoluble minerals. If the amount of HCO3 and CO3 is greater than the amount of Ca at the time of carbonate precipitation, then a sodium carbonate brine that is rich in CO3 and HCO3 will be produced. If the HCO3 and CO3 amount is less than that of Ca, the brine will be rich in Ca and depleted in HCO3 and CO3, after calcite
67 Figure 3.13. Flow diagram for brine evolution in a closed basin as evaporative concentration progresses. Evaporation progresses in direction of arrows. Chemical composition of waters shown in green. (after Hardie, et al, 1978)
Undersaturated inflow
Ca+Mg » HCO3 HCO3 ³ Ca+Mg HCO3 » Ca+Mg Ca » Mg Mg ³ Ca Ca » Mg
Low Mg calcite precipitation
Ca rich Mg > Ca HCO3 rich HCO3 poor Cl+SO4 > HCO3 Ca poor
Gypsum High Mg calcite precipitation Na-CO3-SO4-CL Precipitation (± protodolomite) brine or aragonite precipitation
Ca rich SO4 rich
SO4 poor Ca poor Ca+Mg »» HCO3 HCO3 »» Ca+Mg Mg » Ca
Ca-Na-Cl Na-SO4-Cl brine brine gypsum Na-Cl-SO4-(CO3) precipitation
Ca »» SO4 SO4 »» Ca
Na-Mg-(Ca)-Cl Na-Mg-SO4-Cl brine brine
precipitation and evaporative concentration takes place. If the waters contain Ca and
SO4, gypsum precipitation will occur. If the SO4 concentration is greater than the Ca concentration, gypsum precipitation causes the brine to be depleted in Ca and enriched in
SO4 resulting in a Na-Cl- SO4 brine. However, if the SO4 concentration is less than that of
Ca, a Na-Ca-Cl brine will result that is rich in Ca and depleted in SO4. At Death Valley,
68 Na-Cl-SO4 mixed inflow waters resulted in the mineral assemblage calcite, gypsum, halite, glauberite, and thenardite, with calcite formed first and thenardite formed last. (Li et al,
1997)
Figure 3.14. Ternary Ca- SO4-HCO3 phase diagram demonstrating how brines evolve. The lines from calcite to SO4 and from calcite to gypsum anhydrite represent two chemical divides. Area 1 waters have HCO3 + CO3 > Ca and evolve into Na- HCO3- SO4 brines. Area 2 waters have HCO3 + CO3 < Ca and SO4 > Ca at the time of gypsum saturation, and evolve into Cl- SO4 rich brines. Area 3 waters have HCO3 + CO3 < Ca and SO4 Ca Ca-Cl 3 Gypsum Calcite Anhydrite 2 Cl-SO4 world river water seawater 1 Na-HCO3-SO4 SO4 HCO3 - alkalinity Hunt (1975) holds that salts are deposited in concentric zones on the playa surface based on solubility and in orderly layers at depth reflecting transition from highest to lowest salinity. Hunt gives an evaporative sequence of salt precipitation based on solubility. In his scheme, carbonates, such as CaCO3 and MgCO3, will precipitate in the 69 most dilute waters, resulting in their deposition around the edge and at the bottom of the salt pan. Sulfates will precipitate next. The proportion of salts will influence which . sulfates are deposited. If there is an abundance of Ca, gypsum (CaSO4 2H2O) will precipitate. If there is little Ca, thenardite (Na2SO4) and sodium carbonate will be precipitated. Chlorides, such as NaCl, will be the next to precipitate, followed by small quantities of magnesium sulfate, Ca, Mg, and potassium chlorides. Sanchez-Moral, et al (1998) found four stages of brine evolution resulting from precipitation of various mineral phases. In the first stage, the precipitation of gypsum and calcite causes a decrease in Ca2+ concentration and a slight increase in alkalinity. During the second and third stages, precipitation of magnesium sulfate salts and sodium 2- + chloride salts causes a decrease in the SO4 and Na concentrations and an increase in the Cl- and Mg2+ concentrations. During the last stage, precipitation of magnesium sulfate and sodium chloride salts causes an increase in Cl- , Mg2+ , and K+ concentrations and a 2- + decrease in SO4 and Na concentrations. Rapid precipitation of minerals in the unsaturated zone occurs in response to evaporation at the soil-atmosphere interface (Yechieli and Ronen, 1997). Figure 3.15 demonstrates the expected sequence of salt precipitation: gypsum, halite, carnallite and sylvite, then bischofite (Yechieli and Ronen, 1997). Increasing salinity will increase the Mg/Ca ratio in the water because CaCO3 precipitation is related to evaporative concentration (Eugster, 1982). The increasing ratio is revealed by sequences such as smectite-zeolite-anaclime-potassium feldspar (Eugster, 1982). However, zeolites will not form in Cl or SO4 dominated waters because their pH is too low (Eugster, 1982). 70 Figure 3.15. Model illustrating the precipitation sequence of minerals in the unsaturated zone as a function of increasing salinity of interstitial water. The blue dashed line represents the total ion content of the water and the minerals. The bold orange line represents the ion content in the water. (after Yechieli and Ronen, 1997) sediment surface Mg2+ + K ß1 = MgCl2·6H2O (bischofite) + depth Na ß3 = KMgCl3·6H2O (carnallite) and KCl (sylvite) 2- SO4 ß2 = NaCl (halite) and evaporation ß1 = CaSO4·2H2O (gypsum) upward solution transport groundwater Ci Efflorescent crusts are formed by capillary action resulting from the evaporation of subsurface brines or groundwater and by evaporation of ephemeral surface water. The resulting crust will contain the same proportion of solutes as the water from which the crust was precipitated (Eugster and Jones, 1979). Evaporation from the surface due to capillary action will stop if the level of the potentiometric surface decreases sufficiently (Yechieli and Ronen, 1997). Upon contact with dilute runoff, partial dissolution of the crust occurs which results in fractionation of the crust constituents (Eugster and Jones, 1979). The fractionation occurs because the more soluble components dissolve first, leaving behind the less soluble components, such as silica and alkaline earth elements 71 (Eugster and Jones, 1979). It is this mechanism by which closed basin ground and surface waters pick up most of their solutes (Eugster and Jones, 1979 and Friedman, et al, 1976). Ion exchange and sorption affects the chemistry of dilute waters, in turn affecting the sediments in a playa. Preferential solute loss from solution is due to uptake and differential sorption and exchange of cations on clays. For example, in the vadose zone of the high plains of west Texas, sorption of sulfate by Holocene lacustrine and continental sediments takes place. (Eugster and Jones, 1979) Differential solute loss can also result from degassing. Degassing of playa waters results from a decrease in solubility, organic activity, an increase in temperature, or equilibration of the solution with the atmosphere. For example, an increase in salinity results in a decrease in solubility of CO2. This leads to CO2 degassing which causes precipitation of Ca-Mg carbonate. (Eugster and Jones, 1979) Redox reactions primarily affect sulfates (Eugster and Jones, 1979). For example, sulfate reduction combined with Cl enrichment may cause low concentrations of SO4 in surface waters (Camur and Mutlu, 1996). Oxidation of ferrous iron or ammonia can influence the pH of the solution (Eugster and Jones, 1979). The pH, in turn, can impact fractionation mechanisms such as degassing (Eugster and Jones, 1979). 3.3.4 Criteria for describing salts A common method of describing playa evaporites is by their surficial texture. Lowenstein and Hardie (1985) described the features and textures of halite produced during the stages of the saline pan cycle. During the flooding stage, water is initially undersaturated with respect to halite so dissolution of the crust occurs. This dissolution 72 results in a smooth surface in which the tops of vertically oriented chevron and cornet crystals are sharply truncated. The ponded waters dissolve the internal portions of the halite layers causing pipes to develop along grain boundaries. This results in fluid inclusions within chevrons and cornets being truncated on the sides which are against dissolution voids. Skeletal spines of halite also form due to preferential dissolution along the original crystal growth banding. The horizontal cavities appear as tubular vugs in plan view or cave-like openings in sectional view. The ponded water also results in the rounding of the top crystal faces on chevrons that are vertically oriented. It also causes dissolution of halite which formed at the water surface then settled to the bottom. This dissolution results in horizontal cavities occurring beneath rafts which have settled; rounded off plates, hoppers, and rafts; and a vuggy crystalline halite layer that has a porosity of up to 50%. During flooding, deposition of mud and salts occurs. Gypsum is reworked and redeposited as a lag of corroded and etched gypsum sand. During this stage only fine material should reach the pan; coarser material found in the pan is generally deposited by wind. Casas and Lowenstein (1989) noted that halite crusts do not remain pristine because they experience dissolution during each flood stage and cementation during desiccation. They determined that modern halite crusts which are “mature” are excellent analogs for ancient saline pan deposits, and defined three textural features of modern saline pan halite. A horizontal truncation surface forms where surface halite crusts are dissolved by floodwaters. Cavities form by dissolution where undersaturated waters 73 percolate through the halite crust. Finally, mud partings form when the muds settle from suspension. Casas and Lowenstein (1989) found three types of early-stage halite crystals at Saline Valley, California. The first type is skeletal, inverted-pyramidal hopper crystals that are a millimeter to a centimeter in diameter. A second type of halite crystals includes square to rectangular plates that have a diameter of about a millimeter. The third type of initial halite crystals is coalesced aggregates of hoppers and plates which form rafts. Bottom-precipitated halite overgrowth on the rafts results in millimeter to centimeter long, vertically oriented, elongate chevron halite crystals. The chevrons contain alternating bands of halite rich in fluid-inclusions and clear halite which has little to no fluid-inclusions. During the saline lake stage, evaporation and dissolution occur and saturation is reached. As a result, halite crystals form as pyramidal hoppers, square and rectangular plates, and skeletal hoppers that contain fluid inclusions. The skeletal hoppers form during rapid growth and look like cloudy laminae under a microscope. The pyramidal hoppers and square and rectangular plates may be flattened and horizontally elongated. These plates and hoppers sink to the bottom when they grow larger. Rafts which settle to the bottom become nuclei for syntaxial growth into the evaporating brine. Because this growth is preferentially upward, the crystals are vertically oriented and elongated. Chevrons and cornets form as the crystals continue to grow. The chevrons and cornets are zoned with cloudy bands which were rapidly formed during intense evaporation and clear bands that formed during low evaporation. (Lowenstein and Hardie, 1985) 74 During desiccation of the playa, the residual brine evolves in response to evaporation and is sustained by perennial groundwater inflow (Lowenstein and Hardie, 1985). The new crust and any underlying crusts experience diagenetic growth of halite from the residual brine (Lowenstein and Hardie, 1985). Displacive and void-filling halite forms in the vadose and upper phreatic zones (Lowenstein and Hardie, 1985). Displacive halite within interlayered muds is predominantly clear and often incorporates millimeter or smaller inclusions of the host sediment (Casas and Lowenstein, 1989). The displacive halite occurs as isolated euhedra and interlocking aggregates of randomly oriented cubes (Lowenstein and Hardie, 1985). The diagenetic halite is clear with no preferred orientation or direction, and grows inward, lining solution cavities and intergranular voids. There is a sharp, curved dissolution boundary that separates the clear halite cement from cloudy halite which formed earlier (Lowenstein and Hardie, 1985). Growth of the halite cement results in lateral expansive growth of the surface crust. This growth causes meter-scale polygons which are rimmed by pressure ridges. A spongy efflorescence of halite forms in the desiccation cracks, and will continue to grow while evaporation occurs. Because of the buckling of the polygons, the efflorescent halite forms dished lenses. (Lowenstein and Hardie, 1985) In Death Valley, Li et al. (1996) determined that the salt pan has a three centimeter thick layer of mud overlain by a three centimeter thick halite crust. The crust contains crystal cumulates and vertically-oriented chevrons. An efflorescent salt crust that is about one centimeter thick formed on top of the thicker halite crust. In addition, the surface is cracked due to desiccation. Li et al. found mature halite crusts underlying 75 the mud layer which reveal important aspects of syndepositional dissolution that occurs when the pan floods. Immediately below the crust, extending four to eight centimeters down, cumulate cubes and rafts as well as chevrons are disrupted by vugs, pipes, and horizontal truncation surfaces which resulted from dissolution. Ten to twenty centimeters below the surface there is a halite crust that contains dense pipes and vugs, but no primary textures. Table 3.3. Primary saline pan features that are preserved after burial. These features were found in the Permian Salado Formation of West Texas. (after Lowenstein and Hardie, 1985) Flood Stage Evaporative Dessication Stage (dissolution features) Concentration Stage (diagenetic growth (crystal growth features) features) · Dissolution surfaces · Hopper cubes and · Mud layers contain that are smooth and connected rafts are displacive halite horizontal truncate randomly oriented in cubes. halite chevrons. cumulates. · Syntaxial regrowth of · Chevrons and cornets · Chevrons and cornets partially dissolved, are internally rounded. are elongated and rounded chevrons. vertically oriented. · Fingers of clear halite · Thin bedded halite penetrate the margins contains meter-scale of cloudy chevrons dish structures. In and cornets. Cloudy cross-section, dish bands are separated structures appear as from clear halite mud-rich layers of cement by smooth buckled polygons. dissolution boundaries. · Clear halite overgrowths. 76 Lowenstein and Hardie (1985) also found that mature saline pan halite contains few primary textures. They found that chevrons and cornets are truncated, rounded, or corroded. Mud and fine grained gypsum prisms are found as internal sediment in vugs between halite crystals. Dissolution cavities contain clear, diagenetic halite. Table 3.3 lists the features of modern halite deposits which are found in ancient halite beds. At Bristol Dry Lake, California, Handford (1982) found concentric zonation of evaporite minerals. He found that halite is the principle evaporite mineral in the center of the playa. The halite consists of chaotic mud-halite, which contain interlocking crystals separated by pockets of green clay, and isolated to interconnected giant hopper cubes. The hopper cubes are about twenty centimeters in diameter. The hoppers indicate displacive intrasedimentary growth from supersaturated brines. Handford suggested that the hoppers probably precipitated in the capillary fringe or phreatic zone just beneath the playa surface from evaporating, discharging brines, and that they may have grown slowly because they are very large and clear. In addition to the halite, Handford (1982) found three types of gypsum. White, finely to coarsely crystalline gypsum is found interbedded with mud in the saline mud flat. White, very finely crystalline anhydrite nodules exist in the saline mud flat as well. Spear-like gypsum blades can be found throughout the playa. The smallest abundance of gypsum and anhydrite occurs in the center of the playa. Truc (1978) found zonation in the French Mormoiron basin similar to that found by Handford. Truc determined that halite is concentrated in the basin center due to evaporative concentration and related the growth of gypsum within mud to the saturation 77 of interstitial waters during desiccation. Gypsarenites and corroded gypsum crystals suggest that undersaturated surface water entered the basin during a humid phase. 3.3.5 Methods of Evaporite Analysis Samples of playa crust were taken in July and December 1998. Sample sites were selected based on location within the playa and evidence of variation in crust composition. Samples were collected by lifting polygons of crust from the playa surface. This method preserved features of the crust surface and features found between polygons in the mudcracks. Lateral and vertical aspects of the crust were analyzed. Figure 3.16 shows a typical sample, which parted along two planes. Figure 3.16. Samples 122498-1 and 122498-2. These samples were taken from a single location in the eastern portion of the basin. The top of the upper segment (122498-1) was the surface of the playa. The sample parted along two planes beneath the playa surface. The first parting is the top of the lower segment (122498-2). These samples were analyzed to determine if the playa characteristics varied with depth. 78 The samples were observed under a microscope to identify the morphology of the evaporites in the crust. Due to the fragility of the crust thin sections could not be made, so evaporites were observed in place on the bulk samples. The morphology of the evaporites suggests the stage of the saline cycle during which they formed. Figure 3.17. Graph showing relation of conductivity to salinity. (after Hesse, 1971 ) 6000 4000 2000 1000 800 600 400 -3 200 100 Salinity in g dm 80 60 40 20 10 1 2 4 6 8 10 20 40 60 80100 Conductivity in mS cm-1 at 25ºC The salinity of the floodwaters is related to evaporative stage and depth of flooding. Salinity is directly proportional to conductivity, as shown in figure 3.17 (Hesse, 1971). In order to measure the conductivity of the crust 1:10 dilutions of sample 79 in deionized water were made. An Horiba U-10 water quality checker was used to measure the conductivity of the dilutions. Samples were ground and powder mounts were made. Evaporite minerals were identified by x-ray diffraction of the samples. The qualitative analyses provide information on the types, but not relative quantities of evaporite minerals present. 3.3.6 Evaporite Data In the field, the reflectance of the crust varied across the playa. The variation in reflectivity was related to abundance of evaporites. Figure 3.18 shows photos demonstrating the variation in the crust. The crust on the northern side of the basin was dull and patchy in July. This crust was very soft and quiet to walk on. The crust in the central portion of the basin was continuous, somewhat reflective, and crunched slightly when walked on. The July crust in the southwestern portion of the basin was highly reflective and continuous. The southwestern crust was so highly reflective that it interfered with the surveying instruments. The southwestern crust was very hard and crackled when walked on. In December the playa crust was covered with a small amount of fine sediments. Despite the overlying material, the evaporite crust formed after the hurricane Nora flooding was still observable at close range. The southwestern crust was reflective, though slightly less than in July. 80 Figure 3.18. Photos of the playa crust in July. Photos A and B were taken in the northern portion of Laguna Chapala, facing north towards the quartzite dome. The crust in this area was dull and patchy. Photo C was taken on the southern side of the playa facing west. The crust in this area was highly reflective. Photo D was taken at an intermediate point between photos A and C and shows a close-up of the progression of evaporite accumulation moving from north to south. A) Playa crust on northern side of Laguna Chapala B) Close-up of crust on northern side of playa. 81 C) Playa crust on southern side of Laguna Chapala D) Close-up of crust in the central part of Laguna Chapala. Note patches of highly reflective crust. 82 Microscopic observation was performed on July and December samples to determine the evaporite characteristics. Table 3.4 summarizes the microscopic observations of the bulk samples. Table 3.4. Evaporite characteristics in the playa crust of Laguna Chapala. Sample ID Collection Season Evaporite Characteristics 071498-1 July <1mm rafts, sparse, no secondary growth 071498-2 July 0.5 mm cubes, 1 mm rafts and hoppers, clear crystals, chevrons not truncated 071498-3 July µm-scale individual crystals, µm- scale rafts, secondary growth in cavities and cracks 071498-4 July all crystals and rafts µm scale, sparse rafts, primarily clear crystals, some cloudy overgrowth 071498-5 July 1 mm clear chevrons, very few crystals, sheen on crust 122298-1 December 1 mm cloudy cubes with rounded edges, <1mm hoppers and chevrons truncated and rounded, µm scale cloudy rafts, secondary growth 122298-2 December <0.5 mm rafts preserved, secondary growth on large (up to 1 mm ) hoppers and rafts 122298-3 December no primary features, 1 truncated chevron, 1 mm hoppers with secondary growth 122298-4 December 0.5 – 1 mm cloudy hoppers 122398-1 December very sparse evaporites, 2 mm cloudy hoppers with dissolved edges 122398-2 December µm scale hoppers, sheen on crust 122498-1 December truncated chevron, partially dissolved cubes and hoppers 122498-2 December 1 mm rounded cloudy hoppers, clear 0.5 mm rafts, partially dissolved µm scale cubes and rafts 122498-3 December µm scale hoppers with secondary growth 83 Conductivity measurements of the samples were indicative of the salinity of the lake that formed due to flooding by hurricane Nora. The conductivity measurements are presented in appendix G. Figure 3.19 shows the general trend in salinity across the playa. Figure 3.19. Salinity across Laguna Chapala, based on conductivity measurements of samples. The red dots represent sample locations. The blue arrows indicate increasing salinity. Because of the limited amounts of evaporites on the samples, it was difficult to create good powder samples for x-ray diffraction. The results from the evaporite x-ray diffraction were rather convoluted making it difficult to narrow down the most common evaporites. However, x-ray diffraction of crust samples did reveal that halite was the 84 primary evaporite present in Laguna Chapala. Small amounts of other evaporites, such as epsomite, potassium chloride, and gypsum, were also found. 3.4 Clays In a closed basin, such as Laguna Chapala, small changes in precipitation or evaporation will cause significant changes in lake-level and salinity, which are recorded in the basin sediments. (Battarbee, 1999). Clay minerals form by the chemical weathering of rocks related to climate change (Pardo et al., 1999). Clay minerals may also result from tectonic activity and continental structure associated with margin evolution (Pardo et al., 1999). Changes in basin sediments, particularly clay minerals, provide evidence of changes in the weathering and erosion of sediment sources and of changes in climate or lake-level (Pardo et al., 1999; Battarbee, 1999). 3.4.1 Previous Work Droste (1961) studied playas in the Mojave desert to determine the possible effect of diagenesis on clay minerals in continental saline environments. Droste found that the primary clay minerals in Mojave desert playas were montmorillonite, illite, chlorite, and kaolinite. Of the four main clay minerals in the playas, only chlorite was found to be somewhat unstable. Montmorillonite, illite, and kaolinite were the product of their source rocks and underwent little, if any, diagenetic change. Horiuchi et al (2000) studied Lake Baikal clays as a proxy for paleoclimate in central Asia. Horiuchi et al linked the clay minerals illite and chlorite to cold-dry conditions. The clay minerals smectite and kaolinite were linked to strong hydrolysis in the area. 85 Pardo et al (1999) studied clay mineralogy to determine paleoenvironmental changes across the Cretaceous-Tertiary boundary in Kazakhstan. Pardo et al found that a decrease in phyllosilicates corresponded to a rise in sea level. They also found that overall high mica content was associated with a cool climate that had low humidity but high physical weathering. Fluctuating high kaolinite/smectite ratios corresponded to seasonal warm and humid conditions, while decreasing kaolinite/chlorite ratios corresponded to cooler conditions. Ruffel and Worden (2000) utilized clay mineralogy in their analysis of paleoclimate in southern England and southern France. They found that kaolinite/illite ratios were related to the grain-size of sediments. They determined that increased kaolinite was caused by an increase in detrital clay content that showed evidence of clay diagenesis. Ruffel and Worden found that study difficulties caused by the effect of short- term bathymetry on diagenesis could be negated by studying only kaolinite/illite ratios from thick clays developed at flooding surfaces. Nolan et al (1999) studied the climatic significance of the magnetic properties of clay minerals. They found that mineral assemblage and magnetic properties were sensitive indicators of climate change in carbonate lakes in northwest Europe. Ingles et al (1998) studied the relationship of clay mineralogy to depositional environment. They found that clay mineralogy related to source area was preserved in alluvial fans, flood plains without saline influence, and some saline lakes. Diagenesis occurred in saline flood plains and marginal saline lakes, altering the original mineral assemblage. 86 Hillock (1965) studied clays in order to reconstruct the depositional environment of a High Plains playa lake. Hillock found that illite, montmorillonite, and kaolinite were the primary clays present in the playa core. The illite and montmorillonite were associated with moderate rainfall and alternate periods of wetting and drying. 3.4.2 Clay Mineralogy Clay minerals are hydrous layer silicates that are part of the broader group of phyllosilicates (Chamley, 1989; Moore and Reynolds, 1989; Bailey, 1980). Clays are fine-grained and their particle size is less than 2µm (Velde, 1995; Moore and Reynolds, 1989). The primary components of clay minerals are silicon, aluminum or magnesium, oxygen, and hydroxyl (Chamley, 1989). Clays may be detrital or authigenic in origin (Hillier, 1995). Detrital clays derive from clays that have formed in another area and provide information on source area weathering, provenance, transport processes, and deposition processes (Hillier, 1995). Authigenic clays form in situ as a result of precipitation from solution, reaction of amorphous soils, or by transformation of precursor material (Hillier, 1995). Authigenic clays provide information on the geochemistry of the sedimentary environment (Hillier, 1995). Clays may form through the transformation of clays that were formed in other environments (Hillier, 1995). Transformed clays provide information on weathering, provenance, transport processes, deposition processes, and geochemistry of the environment (Hillier, 1995). Clay minerals most commonly have a platy morphology (Moore and Reynolds, 1989), but can exhibit flake, lath or needle shapes (Velde, 1995; Rae and Parker, 1998). The morphology of clay minerals is related to their atomic structure. Two basic units 87 form Clays: tetrahedral sheets and octahedral sheets (Moore and Reynolds, 1989; Chamley, 1989; Bailey, 1980; Velde, 1985). The dominant cation in tetrahedral sheets is Si4+. Al3+ often substitutes for the Si4+, while occasionally Fe3+ will act as a substitute (Moore and Reynolds, 1989). Each Si4+ is surrounded by three oxygens lying in the same plane and a fourth, apical, oxygen (Moore and Reynolds, 1989; Velde, 1995). The individual tetrahedra share their corner oxygens extending the sheet infinitely in two dimensions (Moore and Reynolds, 1989). The most common cations in the octahedral sheet are Al3+, Mg2+, Fe2+, and Fe3+ (Moore and Reynolds, 1989). The cations in the octahedra form polyhedra with six oxygens (Velde, 1995). Octahedra may have two or three cations. A dioctahedral sheet contains octahedra with two cations, while trioctahedral sheets contain octahedra with three cations (Velde, 1995; Moore and Reynolds, 1989). An octahedral sheet can be visualized as cations sandwiched by two planes of closest-packed oxygens (Moore and Reynolds, 1989). The layer structure of clay minerals consists of the linkage between both tetrahedral and octahedral sheets (Velde, 1995; Moore and Reynolds, 1989; Bailey, 1980). Clay layers are classified as 1:1 or 2:1 layer structures. Because the tetrahedral and octahedral sheets have roughly the same dimensions, the apical oxygens of the tetrahedral sheet replace two of the three hydroxyl ions in the octahedral sheet when they are joined (Moore and Reynolds, 1989). A 1:1 layer results from the linkage of one tetrahedral sheet and one octahedral sheet (Velde, 1995; Moore and Reynolds, 1989). In a 1:1 layer, the octahedral sheet contains an unshared plane of hydroxyl ions (Bailey, 1980). Figure 3.20 demonstrates how a 1:1 layer is formed. A 2:1 layer results from the 88 linkage of two tetrahedral sheets to one octahedral sheet (Velde, 1995; Moore and Reynolds, 1989). There are no unshared ions in the 2:1 assemblage (Bailey, 1980). Figure 3.20. Joining of tetrahedral and octahedral sheets to form a 1:1 layer. Octahedral Sheet Tetrahedral Sheet 1:1 Layer The structure of clays is directly responsible for the properties they posses (Moore and Reynolds, 1989). As previously mentioned, clays may be flake-, lath-, or needle- shaped (Velde, 1995; Rae and Parker, 1998). Because of the shape of clay minerals and their small size (< 2µm), they have a large surface area to volume ratio (Moore and Reynolds, 1989). Additionally, clays have layer charges due to unsatisfied bonds on their edges (Moore and Reynolds, 1989). As a result of these factors, ions and molecules, particularly water, can be adsorbed onto the surface of the clays or absorbed into the internal cation sites (Velde, 1995; Moore and Reynolds, 1989). Cations can be exchanged when the clays come in contact with a solution rich in other cations (Moore and Reynolds, 1989). The cation exchange capacity of clay minerals neutralizes layer charge, influences physical characteristics, and influences the chemistry of the minerals. Low charge on clays allows hydrated ions to be absorbed (Pusch, 1998). Attached water in the interlayer space of expandable clays causes them to swell; 89 subsequent removal of the water will cause the clays to shrink (Moore and Reynolds, 1989). The shrinking of expandable clays causes the soils they are in to form mudcracks like those discussed in section 3.2.2 (Pusch, 1998). Attached water molecules can change the volume of clay particles by as much as 95% (Velde, 1995). Clay minerals may be classified by their swelling properties, i.e. swelling or non- swelling, as demonstrated in table 3.5 (Pusch, 1998), but it is more common to classify them by layer type, layer charge, and interlayer type (Bailey, 1980). Table 3.6 gives the classification of some clay minerals by layer type. Table 3.5. Classification of clay minerals based on swelling properties and layer distance. (after Parker and Rae, 1998) Non-swelling clays 7 Å minerals (kaolinite) No water sorption 14 Å minerals (chlorite) 10 Å minerals that are neutral (talc) 10 Å minerals with high charge (illite) Sepiolite-palygorskite minerals Water sorption (palygorskite) Swelling clays Mixed-layer minerals (corrensite) No water sorption 10 Å minerals with low charge Water sorption (montmorillonite) 90 Table 3.6. Classification of clay minerals based on layer type and layer charge. (after Bailey, 1980) Layer Type Group Sub-group Example Species (x = formula unit charge) 1:1 Serpentine-kaolin (x~0) Serpentines chrysotile, antigorite Kaolins kaolinite, dickite, nacrite 2:1 Talc-pyrophyllite (x~0) Talcs talc, willemseite Pyrophyllites pyrophyllite Smectite (x~0.2-0.6) Saponites saponite, sauconite Montmorillonites montmorillonite, beidellite Vermiculite (x~0.6-0.9) Trioctohedral trioctohedral vermiculite vermiculites Dioctahedral dioctahedral vermiculite vermiculites Mica (x~1) Trioctohedral biotite, lepidolite micas Dioctahedral muscovite, paragonite, micas illite Brittle mica (x~2) Trioctohedral clintonite, anandite Brittle micas Dioctahedral margarite Brittle micas Chlorite (x is variable) Trioctohedral clinochlore, nimite chlorites Dioctahedral donbassite chlorites Di,trioctahedral cookeite, sudoite chlorites 2:1 Sepiolite-palygorskite Sepiolites sepiolite, loughlinite inverted (x is variable) Palygorskites palygorskite ribbons 3.4.3. Playa Clays Clay minerals are important in the study of playas because variations in clay mineralogy indicate changes in source area, changes in weathering, and changes in climate (Pardo et al, 1999). Smectite and illite are the most common clay minerals found in playas, but kaolinite and chlorite are also common. 91 Smectites have a 2:1 layer type and may be dioctahedral or trioctahedral (Brindley, 1980). The interlayers of smectites are occupied by cations, such as Na, K, or Ca, and water resulting in widely variable interlayer spacings (Chamley, 1989). Smectites have a low layer charge and possess the ability to maintain their crystollographic integrity while they expand or contract (Moore and Reynolds, 1989). Smectites are the clay minerals primarily responsible for soil shrinkage (Dixon, 1998). Smectites generally form in volcanic areas, deriving from the weathering of basalt (Stanley et al, 1998; Tanner, 1994). Smectites may also form from weathered shales and marls (Stanley et al, 1998). Under alkaline dry conditions smectites may form from micas or illite (Inoue et al, 1998). Other origins of smectites include diagenesis and in situ formation in a magnesium rich environment (Martinez-Ruiz et al, 1999). Smectites are generally associated with a warm climate that has alternating humid and arid seasons (Pardo et al, 1999). Illite refers to micaceous clays that resemble muscovite, but generally contain more Si, Mg, and water and less K than muscovite (Chamley, 1989). Illite is part of the 2:1 layer type group of micas (Moore and Reynolds, 1989). Illite is most commonly dioctahedral, but may be trioctahedral (Moore and Reynolds, 1989). Illite occurs as a weathering product of igneous and magmatic rocks (Stanley et al, 1997; Tanner, 1994). Illite formation occurs where there is moderate rainfall, where there are alternate periods of wetting and drying, where evaporation exceeds precipitation, where there are stagnant water conditions, and where poor leaching and alkaline conditions prevail (Hillock, 1965). A narrow 10 Å x-ray peak of illite has been linked to cold-dry environments (Horiuchi et al, 2000). 92 Kaolinite is 1:1 layer type dioctahedral clay mineral (Chamley, 1989). Kaolinite is resistant to transport and weathering (Martinez-Ruiz et al, 1999). It forms as a residual weathering product in warm humid climates, through diagenesis, or by hydrothermal alteration of other aluminosilicates (Pardo et al, 1999; Moore and Reynolds, 1989). In the presence of high effective moisture kaolinite may form by the weathering of potassium-rich volcanics (Pederson et al, 2000). An increase in kaolinite in a sediment sequence is related to an increase in detrital clay content with evidence of clay diagenesis (Ruffel and Worden, 2000). Chlorites are made up of a 2:1 layer that is negatively charged and a positively charged hydroxide interlayer (Moore and Reynolds, 1989; Chamley, 1989). Chlorites are detrital in origin and do not resist transport and weathering (Martinez-Ruiz et al, 1999). Chlorites primarily originate from the alteration of volcanic rocks and weathering of metamorphic and crystalline igneous rocks (Chamley, 1989). Chlorites may form in soils, in shales that are at the highest grades of diagenesis, in porous sandstones on the surfaces of sand grains, in carbonate rocks as replacement of carbonate grains and matrix, or as a product of alteration of odinite in warm shallow marine waters of recent to Pleistocene age (Moore and Reynolds, 1989). Chlorite may also form as a product of the diagenetic process that converts illite to smectite (Moore and Reynolds, 1989). 3.4.4 X-ray Diffraction of Clays The clay fraction of the crust samples was x-rayed to determine the types of clays present in the playa. Initially, the sample was washed to remove salts, then a slurry of clays was used to make smear mounts for x-ray diffraction analysis. This method was refined using particle size analysis to yield a sample of the 2µm fraction of clay in each 93 crust sample. Smear mounts of the 2µm fraction were then analyzed. Analysis of the sorted and non-sorted clay samples yielded similar results. Analysis was performed from 0 to 40 degrees 2q, because most clays will have peaks in the range 2 to 35. Analysis was also done at 60 to 66 degrees, because a peak in this region will indicate alkaline clay. The analysis indicated that the clays in the playa are primarily smectite and illite, with small amounts of kaolinite. Peaks occurring in the 0 to 8 degree 2q range indicate a smectite phase in the sample. A peak at 2q of 9 indicates the 001 orientation of illite. A peak at 2q of 18 is the 002 orientation of illite. Figure 3.21. Typical clay x-ray diffraction pattern of samples. 94 Chapter 4 Analysis Analyses of the data are presented in this chapter. The analyses are organized into separate sections for hydrology of the Laguna Chapala flood, features of the lakebed, evaporites, and clays. 95 4.1 Hydrology of the Laguna Chapala Flood An important aspect of this study is documenting the hydrology of the flood associated with hurricane Nora. In the absence of actual floodwaters, surveying provided the best estimate of lake hypsometry and flood volumes. An attempt was made to minimize errors by carefully focusing on the stadia rod while surveying, and by manually writing down station heights and readings. The plot of the survey data was checked against air photos and topographic maps to verify accuracy. Although the data matched the photos and maps relatively well, some errors did occur due to changes in station height and rod height. Errors in total station calculations were adjusted by manually recalculating problem points. Hypsometry data obtained by the survey of the lake provided an estimated maximum flood depth of 1.2 m. Though no actual measurements of the flood depth exist, the value of 1.2m corresponds with personal accounts of Chapala area residents. The flood volume estimated from the hypsometry data is 7,100,000 m3. The accuracy of this value was checked against a simple calculation using drainage area and amount of precipitation. Multiplying the drainage area times precipitation produced a volume estimate of 27,500,000 m3. The calculated estimate is nearly four times greater than that obtained through survey data. The calculated estimate may be so much higher because it does not account for infiltration of precipitation into the soil. Considering the aridity of the area, it seems likely that most precipitation soaks into the soil. Although hurricane Nora was a large event of short duration, the total precipitation that resulted occurred over a three day period possibly allowing enough time for precipitation to infiltrate the soil. The volume estimated by the survey data is not unreasonable if most of 96 the precipitation that fell over the Laguna Chapala drainage soaked into the soil and only 25% of the precipitation became runoff into the playa. Of the evaporation estimates calculated, only that of Mason et al (1994) was not reasonably similar to the others. The Mason et al estimate of 404,395 m3/month would result in only 28% of the total lake volume being evaporated by the end of February 1998. The Thornthwaite evaporation estimate of 1,101,185 m3/month would result in about 78% of the flood volume being evaporated by the end of February 1998. The Hamon estimate of 1,118,105 m3/month would result in a total evaporated volume of 5,590,525 m3 by the end of February 1998, still 21% less than the total estimated flood volume. The Dunne and Leopold evaporation estimate was 1,973,500 m3/month; this value would result in the total flood volume being evaporated by the end of January 1998. It is known that the flooding caused by hurricane Nora was completely evaporated by February 1998. An exact date or approximate time in February is not known, however, only the Dunne and Leopold calculation produced an estimated evaporation sufficient to eliminate all the floodwaters before March 1998. 4.2 Lakebed Features Concentrations of vegetative debris occurred only at the playa boundary and often correlated with the edge of an evaporation ring. Because the concentration of vegetative debris was unique to the playa edges and because it often mimicked wave swash found on a beach, it was considered a useful high water mark. The concentration of vegetative debris was preserved in December, suggesting it may be preserved with burial providing a useful indicator of the extent of previous flooding events. 97 Evaporation rings were easily defined and useful for studying flooding extent in July. In December, despite relatively low amounts of precipitation and the lack of any major storm event in Laguna Chapala, survey data and comparison with photographs revealed that the evaporation rings present on the playa were not the same as those present in July. This suggests that the deposits that make up evaporation rings are easily reworked by minor amounts of precipitation, thereby eliminating the possibility of evaporation rings being preserved with burial. The mudcracks associated with desiccation of the playa following flooding by hurricane Nora were much larger than those found from previous wetting events. Personal accounts suggested that no flooding of the scale of that associated with hurricane Nora had occurred in at least 20 years; this suggests that large-scale mudcracks at Laguna Chapala would be a rare occurrence, only indicative of large storm events. The mudcracks were relatively well preserved from July to December, particularly in the southern portion of the playa. Additionally, even the small-scale mudcracks of pre- hurricane Nora wetting were evident in December. It is apparent that mudcracks, especially large mudcracks associated with large storm events, are preserved in the playa lakebed and can be useful indicators of storm events in the playa record. Grain size of the playa sediments did not vary across the playa or with depth. Additionally, average grain size did not vary from July to December, despite the addition of aeolian sediments on the crust associated with hurricane Nora flooding. Considering the nature of the Laguna Chapala basin, it is not surprising that the grain size distribution following hurricane Nora was not unique to a flooding event. 98 4.3 Evaporites Halite was the primary evaporites present in both July and December samples. Unfortunately, because of the small amount of sample available, x-ray mounts were not pure evaporite and the relative abundance of evaporites present could not be determined, so it could not be compared in July and December samples. Microscopic observation of the playa crust indicated that some primary growth features, such as chevrons and rafts, were preserved, but that they were usually truncated or partially dissolved in the December samples. Secondary growth occurred in both July and December samples. In the December samples overgrowth on primary features was evident, but did not completely camouflage the primary features. Dissolution surfaces were evident in July samples taken from areas that experienced the shallowest flooding. The July dissolution surfaces suggest that salts from previous events were slightly reworked, but complete evaporation in that area occurred before new salts could form in solution. Dissolution surfaces were present in the December samples in locations where deeper flooding had been, as the deeper flooding occurred in small dips in the lakebed, small puddles could form in these areas as a result of only minor amounts of precipitation. The December samples give evidence that minor precipitation can rework the evaporites deposited by a large flooding event, but such reworking occurs only in isolated areas of the playa. Evaporite crusts were found on buried samples. The primary evaporite in buried samples was halite. Partially truncated and dissolved primary features, as well as secondary growth features were present in the buried samples. The buried samples parted along planes where evaporite crusts occurred. Analysis of the bulk of these samples 99 (areas between the partings) revealed an absence of evaporites. This suggests that evaporite crusts can be identified in the record of Laguna Chapala. The amount of primary and secondary growth features at the partings suggests that the evaporite crusts at these places may be related to large flooding events. Age dating of the samples and correlation to known storm events would be necessary to verify that the partings are related to storm events. However, based on the small sedimentation rate in Laguna Chapala (~1mm/year) it is not unlikely that a parting at a depth of 17 mm represents the last large flooding event in the playa. 4.4 Clays The primary clay minerals identified at Laguna Chapala were smectite, illite, and kaolinite. The x-ray diffraction patterns of all the July samples were very similar. Additionally, the diffraction patterns of the December samples were similar to those of the July samples. The diffraction patterns suggest that the types and relative abundance of clay minerals does not vary across the playa, with time, or with burial. Because of the similarity of the samples, x-ray analysis of the clay minerals also suggests that the clay mineralogy present after hurricane Nora is not unique to unique to a flooding event in Laguna Chapala. 100 Chapter 5 Conclusions Hurricane Nora offered the unique opportunity to study an extreme storm event and the stratigraphy and chemistry of playa sediments and evaporites that resulted. Documenting the sedimentology, stratigraphy, and chemistry of playa lake deposits resulting from a recent event is the key to identifying similar events in the record of Laguna Chapala. 101 The presence of vegetative debris at the boundary of the playa provides evidence of flooding extent. The unique concentration of vegetative debris at the flooding boundary was preserved over time and may be preserved with burial. Evaporation rings, which also provide evidence of flooding extent, were not preserved. Mudcracks are not unique to large flooding events at Laguna Chapala, but the scale of the mudcracks that occurred after hurricane Nora flooding was unparalleled by the mudcracks of small wetting events. The mudcracks resulting from the large scale flooding were fairly well preserved over time and could be identified with burial. Due to the location and closed nature of Laguna Chapala, grain size distribution and clay mineralogy were not affected by hurricane Nora. Grain size distribution and clay mineralogy did not vary across the lake, over time, or with burial. Evaporites formed on the playa surface due to the flooding caused by hurricane Nora. The presence of evaporites was unique to a flooding event. Slight alteration of primary growth features did occur over time. Dissolution surfaces present in December suggest that evaporites in some areas of the playa may be reworked by slight amounts of precipitation. Samples of evaporite crusts at depth evidence the preservation of evaporite mineralogy and primary and secondary growth features. Several features identified after the flooding caused by hurricane Nora may be useful in the identification of similar events in the playa record of Laguna Chapala. Bands of vegetative debris occur only at flooding boundaries and are preserved, thus they may be useful for defining the extent of previous flooding events. Large-scale mudcracks are unique to an extreme storm event so their identification in the playa record would be indicative of an event similar to hurricane Nora. Evaporites form on the playa only when 102 flooding has occurred, so the existence of an evaporite crust is a useful indicator of a flooding event. 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(1997). “Early diagenesis of highly saline lake sediments after exposure.” Chemical Geology 138: 93-106. 112 Appendix A. Survey Data shot # Easting Northing Elevation Label 1001 9998.6893 10015.0909 -0.0354 L 1002 10111.9769 9923.1092 0.0397 N 1003 10045.0322 9891.1949 -0.0071 A 1004 10163.4475 9398.4127 -0.0266 P 1005 10260.1155 9771.5399 -0.1284 T 1006 10101.4385 9723.5335 -0.0509 A 1007 10367.3815 9660.2865 -0.0508 T 1008 10260.0679 9391.0035 -0.0014 P 1009 10147.406 9571.2477 -0.1037 A 1010 10505.4078 9525.5402 -0.1027 T 1011 10354.0904 9328.8351 -0.1245 P 1012 10183.4265 9402.9879 -0.1202 A 1013 10418.6137 9277.5846 -0.1201 P 1014 10608.7926 9418.2933 -0.1593 T 1015 10353.4562 9393.201 -0.1555 A 1016 10478.9555 9246.5388 -0.059 P 1017 10727.1436 9300.2753 -0.1393 T 1018 10497.0805 9247.0643 -0.0942 P 1019 10848.0958 9182.3468 0.0081 T 1020 10521.3236 9257.9323 -0.1804 P 1021 10379.7398 9401.0333 -0.1477 A 1022 10300.5104 9546.3232 -0.0711 A 1023 10175.9576 9404.312 -0.0303 P 1024 10181.3453 9401.4043 -0.0554 T 1025 10071.0336 9408.9064 -0.0391 P 1026 10226.2904 9664.7144 -0.0992 A 1027 10153.566 9505.2393 -0.1107 N 1028 10022.2829 9438.7237 -0.0642 P 1029 9982.9157 9425.1887 0.0749 P 1030 10110.4949 9845.4118 0.0103 A 1031 10129.7757 9590.4289 -0.1271 N 1032 10018.1663 10007.3564 -0.0084 A 1033 9756.8691 9440.7496 -0.0524 P 1034 10078.8972 9774.3649 -0.0898 N 1035 9629.6174 9392.3441 -0.0762 P 1036 9555.3041 9358.182 -0.1821 P 1037 10021.4143 9981.8916 -0.0147 N 1038 9555.5439 9358.7593 -0.1048 P 1039 9459.7706 9376.1398 -0.0959 P 1040 9930.4423 9897.3185 -0.1023 T 113 shot # Easting Northing Elevation Label 1041 9904.2574 9899.2148 -0.0973 A 1042 9856.9043 9759.2759 -0.1523 T 1043 9818.1743 9776.4049 -0.1363 A 1044 9783.3458 9620.9682 -0.122 T 1045 9717.0458 9651.9094 -0.2183 A 1046 9697.8065 9460.1183 -0.1346 T 1047 9557.3589 9443.2421 -0.2576 A 1048 9485.6445 9500.7795 -0.2904 T 1049 9512.8675 9375.6682 -0.132 A 1050 9374.7146 9400.4135 -0.2242 T 1051 9437.5701 9383.8579 -0.2109 P 1052 9406.7759 9377.9486 -0.1442 P 1053 9385.0556 9324.1964 -0.1792 A 1054 9439.8206 9330.1414 -0.1288 P 1055 9338.7539 9352.7717 -0.2613 A 1056 9408.3328 9361.2563 -0.1325 P 1057 9322.2317 9374.5847 -0.2644 A 1058 9289.1877 9407.3514 -0.3084 T 1059 9258.6904 9441.1395 -0.1697 L 1060 9396.1189 9499.115 -0.3399 T 1061 9425.4646 9482.0021 -0.3103 A 1062 9360.8476 9532.3721 -0.2565 L 1063 9547.9217 9600.4213 -0.2863 A 1064 9533.5233 9617.7079 -0.2275 T 1065 9510.6268 9651.7105 -0.2628 L 1066 9659.5386 9726.1079 -0.2247 T 1067 9668.4688 9718.7241 -0.2274 A 1068 9637.6598 9750.8758 -0.2451 L 1069 9727.7027 9787.3882 -0.1428 T 1070 9830.8961 9879.1217 -0.1354 A 1071 9813.7233 9898.4145 -0.1414 L 1073 9721.5541 9786.9852 -0.1467 base 1074 9721.452 9787.0241 -0.1795 base 1075 9399.3997 9361.5054 -0.1461 P 1076 9344.1606 9284.2715 -0.0586 P 1077 9268.4444 9386.8301 -0.1583 L 1078 9165.4863 9268.5898 -0.013 L 1079 9272.1096 9184.6218 -0.0272 P 1080 9253.9656 9129.1913 -0.0226 P 1081 8996.7856 9065.4868 -0.2165 L 114 shot # Easting Northing Elevation Label 1082 9137.9962 9053.9413 -0.0677 P 1083 9138.0788 9053.9711 -0.0678 P 1084 8888.1644 8927.8319 -0.289 L 1085 8856.6997 9091.038 -0.2978 L 1086 8621.4318 9697.8399 0.1286 S 1087 8813.3224 9227.1162 -0.3829 L 1088 8760.4576 9412.704 -0.4085 L 1089 8834.2368 9371.1273 -0.2548 L 1090 8805.582 9439.6629 -0.3983 L 1091 8834.1536 9371.2467 -0.3504 L 1092 8909.4571 9482.9499 -0.3163 L 1093 8924.5977 9428.3756 -0.2672 L 1094 8967.4722 9482.6334 -0.3289 L 1095 8540.6452 9844.0606 -0.0681 S 1096 8994.0257 9517.2457 -0.2531 L 1097 8811.5288 9551.918 -0.3832 L 1098 8956.0466 9554.6972 -0.3422 L 1099 8681.6846 9619.7392 -0.4951 L 1100 8879.1021 9659.1757 -0.3306 L 1101 8642.4248 9639.3668 -0.3131 P 1102 8757.3356 9753.7848 -0.4096 L 1103 8620.3489 9726.2314 0.1276 P 1104 8622.5923 9802.4979 -0.3591 L 1105 8578.5944 9773.7385 0.0072 P 1106 8729.7698 9870.9434 -0.4345 L 1107 8539.8249 9845.2542 0.048 P 1108 8588.7661 10035.9479 -0.3716 L 1109 8503.8817 9905.4683 0.1486 P 1110 8734.8618 10017.3135 -0.224 L 1111 8916.515 9989.4312 -0.3522 L 1112 9153.428 9935.9651 -0.3 L 1113 8625.339 9975.0487 -0.4038 L 1114 8642.8621 9940.4962 -0.3129 L 1115 8717.3355 9905.8804 -0.3244 L 1116 8824.1067 9885.9047 -0.3696 L 1117 9571.2885 9841.9339 -0.191 L 1118 8940.844 9869.3576 -0.3957 L 1119 9038.0076 9856.8031 -0.2554 L 1120 9159.6642 9842.2786 -0.3051 L 1121 9306.7636 9825.4909 -0.2817 L 115 shot # Easting Northing Elevation Label 1122 9428.4327 9813.9511 -0.2306 L 1123 9532.2705 9805.5752 -0.2017 L 1125 9746.5851 9799.3021 -0.1446 L 1126 10342.8532 10013.0764 -0.3051 L 1127 10263.2875 9979.3311 -0.3629 L 1130 9232.7403 9845.3453 0.1207 S 1131 9232.7375 9845.3442 0.0945 P 1132 9610.2604 9190.0473 -0.6018 P 1133 9547.4091 9210.1331 -0.6096 S 1134 9114.9456 9962.1819 0.0223 P 1135 9438.4744 9209.3187 -0.5813 S 1136 9007.0925 10206.337 -0.1053 P 1137 8991.9782 10229.511 -0.0706 P 1138 9296.7374 9248.3171 -0.428 S 1139 9021.6836 10405.1819 -0.2071 S 1140 9035.657 10527.3894 -0.2141 P 1141 9225.1376 9289.3878 -0.5592 P 1142 9079.4907 9406.7027 -0.4233 S 1143 9512.5245 11113.7467 -0.8239 P 1144 9061.5277 9501.7782 -0.2243 P 1145 9156.7616 9560.8546 -0.4484 P 1146 9508.7847 11165.567 -0.8461 L 1147 9503.472 11230.8742 -0.8728 L 1148 9534.1297 11155.142 -0.8631 L 1149 9444.2754 11112.3436 -0.8544 L 1150 9477.0614 11317.3491 -0.912 L 1151 9601.6861 11240.2195 -0.9411 L 1152 9325.6743 11100.6718 -0.647 L 1153 9457.6982 11414.0294 -1.0055 L 1154 9642.8212 11299.121 -0.9741 T 1155 9211.43 11089.0951 -0.7507 L 1156 9432.8128 11564.6637 -1.1091 L 1157 9711.0659 11406.1354 -0.9729 L 1158 9406.1605 11676.9614 -0.8913 L 1159 9263.9052 11199.7714 -0.6904 L 1160 9782.6719 11493.53 -1.0504 L 1161 9376.8287 11158.128 -0.8256 L 1162 9842.6323 11591.7692 -1.0471 L 1163 9427.4745 11169.5197 -0.8629 L 1164 9889.2241 11641.373 -0.9524 L 116 shot # Easting Northing Elevation Label 1165 9275.4565 11282.8456 -0.7768 L 1166 9257.9587 11381.0412 -0.8975 L 1167 9393.8019 11823.4163 -1.2708 L 1168 9257.8669 11381.0078 -0.9511 S 1169 9383.0037 11885.6631 -0.7634 S 1170 9936.2142 11647.0343 -0.9393 L 1171 9366.6597 11264.5159 -0.857 S 1172 9442.8575 11946.7789 -0.7696 L 1173 9450.3729 11276.624 -0.9341 S 1174 9506.9623 12031.5553 -0.8404 L 1175 9968.9419 11801.2885 -1.2637 L 1176 9371.4741 11463.2002 -1.0234 L 1177 10031.6022 11776.611 -1.2924 L 1178 9428.6179 11680.4402 -1.1695 S 1179 9936.2422 11773.0516 -1.2179 L 1180 9452.0671 11513.7651 -1.0793 L 1181 9937.8404 11800.934 -1.2054 L 1182 9937.8365 11800.8642 -1.208 L 1183 9952.3899 12019.784 -1.3229 S L 1184 9980.2327 12185.4377 -0.8907 L 1185 9980.2364 12185.4725 -0.8705 L 1186 10039.8638 12232.3373 -0.8427 S 1187 10039.8686 12232.3587 -0.797 L 1188 10070.4072 11704.0098 -1.2191 S 1189 10220.1577 11686.6562 -1.3227 L 1190 10018.1349 11802.181 -1.3308 L 1191 10078.6919 12273.033 -0.8465 S 1192 10220.1278 11686.6954 -1.3085 L 1193 10078.6507 12272.9803 -0.8718 S 1194 10078.6581 12273.0062 -0.8466 S 1195 10364.3849 11662.3043 -1.4003 L 1196 10522.19 11632.4081 -1.5276 L 1197 10522.261 11632.4211 -1.5567 L 1198 10138.9014 11858.7972 -1.4044 L 1199 10138.9272 11858.8081 -1.4043 L 1200 10101.0849 12353.4447 -0.8539 S 1201 10618.84 11606.2315 -1.6447 L 1202 10289.4549 11928.0807 -1.4538 L 1203 10124.5549 12367.8888 -0.8665 S 1204 10618.7772 11606.1767 -1.6107 L 117 shot # Easting Northing Elevation Label 1205 10289.4811 11928.031 -1.4725 L 1206 10205.7986 12387.8469 -0.9236 S 1207 10665.2812 11760.3969 -1.6216 L 1208 10488.4019 12060.0796 -1.4735 L 1209 10278.3101 12372.0181 -0.9203 S 1210 10488.457 12060.1082 -1.4734 L 1211 10315.9501 12350.7964 -0.8837 S 1212 10376.0898 12301.9108 -0.924 S 1213 10687.4694 11753.6662 -1.6296 L 1214 10687.456 11753.6703 -1.6303 L 1215 10727.8485 11789.6132 -1.6365 L 1216 10757.4365 11708.981 -1.6397 L 1217 10873.8723 11852.0224 -1.6314 L 1218 10901.0576 11588.1329 -1.685 L 1219 10741.1969 11930.6177 -1.6265 L 1220 10955.3097 11531.1635 -1.644 L 1221 11017.5705 11915.9587 -1.6768 L 1222 10828.4909 12085.8068 -1.5357 L 1223 11025.6979 11450.7543 -1.611 L 1224 11152.205 11976.1436 -1.6148 L 1225 11123.3185 11389.1914 -1.6472 L 1226 10923.7243 12239.5984 -1.5918 L 1227 11212.8285 11313.784 -1.5857 L 1228 11437.5245 12102.1603 -1.5012 L 1229 10964.5114 12299.1134 -1.5422 L 1230 11318.7586 11248.7721 -1.6417 L 1231 10986.2113 12275.3248 -1.5592 L 1232 11414.6048 11172.7348 -1.7704 L 1233 10996.7276 12250.709 -1.5286 L 1234 11021.2249 12227.3581 -1.5944 L 1235 11532.9122 12042.4469 -1.5861 T 1236 11017.5 12204.0791 -1.5491 L 1237 10994.3078 12169.4205 -1.5787 L 1238 11192.4417 11912.5817 -1.5836 L 1239 11039.8998 11863.6623 -1.7057 L 1240 10884.305 12008.3215 -1.557 L 1241 10847.6582 11962.7893 -1.5777 L 1242 10890.9586 11820.9843 -1.6507 L 1243 10733.924 11821.0499 -1.6351 L 1244 11415.336 11172.8071 -1.6314 L 118 shot # Easting Northing Elevation Label 1245 11449.12 11125.7146 -1.6466 L 1246 11474.9864 11148.6395 -1.6459 L 1247 11450.7319 11220.1601 -1.7327 L 1248 11518.675 11037.6453 -1.668 L 1249 11462.9816 11239.2218 -1.6903 L 1250 11583.6855 11119.8075 -1.6317 L 1251 11589.4808 10925.3302 -1.6731 S 1252 11541.713 11400.7192 -1.6685 L 1253 11712.9128 11141.6618 -1.6405 L 1254 11572.5717 10857.7089 -1.6341 S 1255 11613.4769 11565.2487 -1.7249 L 1256 11743.9621 11216.5417 -1.6449 S 1257 11546.4828 10794.0776 -1.631 P 1258 11679.916 11700.7777 -1.6965 L 1259 11566.9939 10705.821 -1.6892 P 1260 11840.2728 11368.1315 -1.7571 P 1261 11706.5912 11732.7009 -1.8764 L 1262 11588.5309 10593.2732 -1.791 P 1263 11819.5403 11565.2691 -1.7673 P 1264 11593.7878 10493.9302 -1.7906 P 1265 10703.3649 11445.7113 -1.7172 P 1266 11487.7401 10776.686 -1.6848 L 1267 10722.6936 11550.9273 -1.6467 P 1268 11453.8522 10910.5685 -1.6262 L 1269 11640.1862 10627.6547 -1.7551 L 1270 11434.1154 11043.1988 -1.5937 L 1271 11831.0807 10422.269 -1.7678 L 1272 11886.5289 10327.591 -1.5623 P 1273 10837.1909 11598.231 -1.5974 P 1274 11853.7835 10302.8382 -1.6967 P 1275 11757.9721 10284.8447 -1.7646 P 119 Appendix B. Hypsometry Data Elevation Area of Polygon Area Above Total Volume Cummulat (m) (ha) Elevation Elevation (m^3) ive (m^2) Area (m^2) Volume (m^3) 0 1597.2113 15972113 4412042 441204.2 0 -0.2 1156.0071 11560071 4470486 1341145.8 1782350 -0.4 708.9585 7089585 2348842 1174421 2956771 -0.6 474.0743 4740743 1729441 1210608.7 4167379.7 -0.8 301.1302 3011302 1944229 1749806.1 5917185.8 -1 106.7073 1067073 1067073 1173780.3 7090966.1 -1.2 0 0 0 0 7090966.1 total area = 15972113 m^2 15.972113 km^2 120 Appendix C. Evaporation Calculations Thornthwaite Method Month Temperature monthly I Et (cm/month) Et (m3)/month) September 23.1 9.93 9.16 1,462,970 October 18.2 6.94 S I 7.22 1,152,643 November 14 4.69 53.96 5.55 886,648 December 10.9 3.22 4.32 690,319 January 10.1 2.87 4.01 639,653 February 11.7 3.58 a 4.64 740,985 March 12.7 4.05 1.34 5.04 804,317 April 15.5 5.46 6.15 981,646 May 16.9 6.21 6.70 1,070,311 June 20.8 8.48 8.25 1,317,306 July 25.7 11.65 10.19 1,627,633 August 25.7 11.65 10.19 1,627,633 average evaporation = 1,101,186 m3/month Hamon Method Month Temperature # Daylight pvsat PET PET hrs (cm/month) (m3/month) September 23.1 12.3 17.23 8.90 1,421,605 October 18.2 10.8 13.40 6.08 970,704 November 14 10 10.73 4.50 719,433 December 10.9 9.5 9.06 3.62 577,387 January 10.1 9.6 8.67 3.50 558,269 February 11.7 10.5 9.47 4.18 666,795 March 12.7 12.3 10.00 5.17 824,857 April 15.5 13.7 11.62 6.69 1,067,997 May 16.9 14.9 12.52 7.83 1,250,938 June 20.8 15 15.33 9.66 1,542,308 July 25.7 15 19.62 12.36 1,974,295 August 25.7 14 19.62 11.54 1,842,675 average PET = 1,118,105 m3/month 121 Appendix D. Field Notes Laguna Chapala Field Notes: July and December 1998 7-13-98 There is little vegetation on the basin floor. The vegetation that exists is basically just scrub. There is no standing water. Mudcracks are present on the basin floor and look fairly new. “Fresh” gullies may give evidence of standing water. Patterning on the eastern basin floor suggests where standing water existed as evaporation took place. Sand hills on the southwest edge of the basin indicate a constant wind direction (high pressure to North). Local people indicated that the lake has contained water several times. They also told us that the east basin contained about 2’ of water from September 1997 to February 1998, and that by February the water was left only in small patches. Around the edges of the flooded area you can see very small brown scrub and coarser, pebble size material that probably resulted from wave wash, and indicates the edge of the lake created by Nora. At the edge of the waterline there is a .5” rise over a distance of 1.62 meters. Two samples were taken from the sand hills on the southwest edge of the basin. Sample 071398-1 was from the bottom of a sand hill, located between two erosional gullies on the outcrop surface. Sample 071398-2 was from the top of a sand hill, and shows some evidence of bedding. Sample 071398-3 was taken west of the other samples, at the base of the sand hills. To the west of the sample area there is bedding and cross- bedding on a cliff. The bedding seems to be coarse and poorly sorted to be aeolian, but there is a lack of other evidence, such as tufa, that would suggest that it resulted from lake deposits. 122 7-14-98 The total station was set up next to a tree, at the base of the quartzite hill on the northern side of the basin. We were only able to sight about half way across the lakebed. We were not able to sight all the way across the lakebed, because heat reflections made it difficult to see the prism. As we surveyed, five samples were taken at locations where the basin crust changed. Sample 071498-1 was taken at site 3. Sample 071498-2 was taken at site 7, which was at the first color change encountered. The color at this point was dark red. Sample 071498-3 was taken at site 10. This site was at the center of a depression where the color changed to a light red. Sample 071498-4 was taken at site 13. Site 13 was located in the center of an area of color change. The color in this area was light yellow, and the surface had become shiny from the evaporites present. Sample 071498-5 was taken about 200 meters south of site 14. The surface in this area had become very shiny from evaporites. Sites 16 and 17 represent high water marks. Site 16 was .077 m higher than station 1. Site 17 was .092 m higher than station 1. Site 17 was the best watermark. It had a line created by vegetation and small debris. 12-21-98 Looking at the playa from atop the quartzite hill, you can see evaporation rings and high water marks. The evaporation rings do not look identical to the summer ones, but occur in the same areas. This might indicate that they are from a different event, but the geometry of the basin may dictate where water will pool. The surface of the southwest side of the playa still looks shiny; indicating evaporites may still exist on the 123 crust. The surface of the playa still looks free of vegetation, though up-close inspection indicates that some vegetation has returned. Vegetation in the Laguna Chapala area includes cardones cactus, barrel cactus, charro cactus, “elephant trunk trees,” and agave plants. Because it is winter, the area is slightly greener than it was in the summer, but there are no blooms or flowers as there are in the summer. There has been a very strong wind blowing to the southwest all day. If such winds are common this time of year, they may be responsible for significant amounts of deposition of the surface. There was wind in the summer, but it was not as strong or persistent. Vegetative debris that marked high water is no longer present in many places. There are still some sticks, but the finer material that made a sharp line is no longer present. Where water comes down from the quartzite hill, coarse material sticks out farther than the rest of the high water marks. There are marks on granite boulders in the northwest portion of the playa that may represent high water marks. Mudcracks are no longer sharp. The edges are rounded, cracks are partially filled in with sediment, and evaporites are not readily evident. Channels where evaporites used to be thick are no longer shiny. In the southwest, mudcracks are still deep and sharp. This preservation of the mudcracks seems to occur in areas where more evaporites occurred. Perhaps the degree of degradation of the mudcracks is related to composition of the deposits. The southwest surface of the playa still more reflective than northeast surface, but it is not as shiny as it was in the summer. Sediment deposited near an old dune feature in 124 the southwest looks like a spit. Samples 122198-4 and 122498-5 were taken from the dune. Material in the samples seems to match what one would expect to see in a dune, but it is cemented. 12-22-98 Lisa Ely stopped by the playa to talk to us. She did a core in the middle of the playa. She says the core contained mostly playa type layers, but there were a couple clay lake layers. She did not find any shorelines. We were able to survey some of the lake. Seth’s first shot is at a color transition; unfortunately, this shot is not labeled with a T. Samples were taken from the approximate area of the summer surface samples. Sample 122298-1 was taken from the approximate location of summer site 7 (071498-2). There is no longer a color change at this location. The color of the sample is 7.5 YR 7/6. Sample 122298-2 was taken from the area of sample 071498-3. There is no longer a color change or depression in this area. The sample color is 7.5 YR 5/8. Sample 122298-3 was taken in the area of sample 071498-4. There is still a slight color change, but none of these areas show any of the shininess that was present in the summer. The mudcracks in this area are mostly filled in. The color of sample 122298-3 is 5 YR 7/8. Sample 122298-4 was from the area of sample 071498-5. The mudcracks in this area are very filled in. There is no evidence of salt crust or color change. The color is 7.5 YR 7/8. Walking across the lake from southeast to northwest, you can see color transition areas (red to yellow), but these are probably new because such transitions do not occur where they were in the summer. 125 12-23-98 More surveying! Sample 122398-1 was from site 1182. This area was very shiny in the summer, but now it is covered with a lot of dust and the mudcracks are filled in. About 200 m from station 4, mudcracks are still deep and sharp. The surface on this side of the playa is much harder than the eastern side. It does not crack easily when walked on and it is hard to rub or break it apart. This area had a lot of salt in the summer. In areas where mudcracks are filled in, you can no longer see old mudcracks on the surface because the surfaces have been smoothed. Sample 122398-2 was taken in the northwest part of the playa near the northernmost part of the sandhills. There is still some noticeable salt crust. The color of the sample is 7.5 YR 6/6. The surface in this area is easily rubbed off and broken. It tends to crack and crumble when walked on. 12-24-98 Samples 122498-1, 122498-2, and 122498-3 taken from crust near old dunes in the southwest portion of the playa. The cracks in this area are about 7.5 cm deep, are very sharp, are not filled in very much, and do not have a shiny surface (probably due to rubbing by wind-blown sediments). The color of the surface is 7.5 YR 7/6. Some stratigraphy can be seen. Sample 122498-1 is the top 3 cm of crust. Sample 122498-2 is 4 cm thick, and came from directly below sample 122498-1. It represents the first parting. Such partings may represent separate storm events. Sample 122498-3 is from directly below sample 122498-2, and represents the second parting. The sample is 3 cm thick. It shows light laminations on the top and undifferentiated darker material on the bottom. 126 A trench was dug in a channel off of the playa. The trench is 50 cm tall, 1.4 m wide and 80 cm deep. There seems to be some evidence of in-filled cracks. Finer material tends to e found and cemented in cracks. There are two zones of color. The top layer is about 8 cm thick. The color of this layer is 10 YR 6/6. The peds are angular and vary in size from 1 cm to 4 cm. This layer contains some sand, possibly some carbonate, vegetative material, feldspar, some salts, and lots of silt. The vegetative material includes new roots and some material that looks carbonized. The soil is slightly sticky, but will not roll very far indicating that some clay is present (perhaps about 30%). Sample 122498-4 was taken from this top layer, about 10 cm below the surface. The bottom layer of the trench is about 42 cm thick. The soil is very sticky (it will make about a 2” roll), indicating it has a higher clay content than the top layer. The color of the layer is 7.5 YR4/4. The peds are angular, range from less than 1 cm to 10 cm in diameter, and are difficult to break. You cannot crush them between two fingers. The soil contains silt, but has more clay. The soil has more sand in it than the top layer. The soil also contains quartz, some mica, and vegetative debris. The vegetative debris consists of a few new roots and a lot of old carbonized material. Sample 122498-5 was taken from this layer, about 40 cm below the surface. 127 Seth’s Field Notes: December 1998 12-21-98 Playas are rock “incinerators” because the heat will quickly disintegrate rocks, but quartzite is highly resistant to this kind of weathering. Vesicles in the top layers of the playa can represent that the playa was hot; when rain fell on it, there was gas in the silt. Because of expansive clays, mud cracks form which can then be filled with sediments. This process is repeated and sometimes leads to non-distinct horizontal layering. Piles of debris, which represents the high water level, are visible in areas of the SW side of the playa, near granitoid rocks. “Beaches” of sand to pebble size granitoid sediment surround the playa adjacent to granitoid rocks. 12-24-98 Trench in arroyo north of Laguna Chapala. The boundary between the upper and lower horizons is not sharp. There are vertical cracks ranging from 2 to 30 mm are visible throughout the trench in both layers. Most of the cracks cut both horizons, but some are continued only to the top or bottom of a layer. There are former cracks in the lower horizon that have been filled in with finer material. There are living roots in both layers, but roots larger than 5 mm are confined to the lower horizon. The color of the upper 8 cm is 10 YR 8/6. The top layer contains abundant organic material, which is mostly roots. The soil is almost entirely silt. It contains fine grains of other undistinguishable minerals. The soil breaks into equant angular peds ranging from less than 1 cm to 5 cm across. The clay cement is slightly sticky, maybe 30% clay. The bottom layer is about 40 cm thick. It’s color is 7.5 YR 4/4. The soil is silty with about 25% sand sized grains. It is very sticky and can be rolled, thus contains about 40-50% clay. The soil also contains fine to medium sized grains of quartz and feldspar and fine grains of undistinguishable minerals. The peds are angular and range from 2 cm to 20 cm. 128 Ann’s Field Notes: December 1998 12-21-98 Dried lake has dried at different rates: the chemistry of the deposits change as a function of depth, volume, and salinity. Lake took 5 months to evaporate completely. Shiny area/high reflectivity mark evaporite deposits. Rock types present: Granite – rounded/ridges in the horizon, weathering controlled by jointing Basalt – young cones/flows, some areas high vertical relief of flows from denudation of area à less resistant so lower than granite, cones line up along a fault Gneiss – granite in place weathering à mono-mineralic grains splitting from salt expansion in broken spaces Quartzite – blocky weathering/ lichen and nasty vegetation Recon of lake: Coming down the quartzite transition from vegetated blocky quartz to mudcracks. Evaporite sheen much more evident but has been weathered. Borders of cracks are not sharp, they have become round and smooth since the summer. Mud contains fragments of quartz; on the way out they decrease in size toward the center. Small gully was once shiny, now has a red hue. Contains smaller mud with no clasts, therefore water sat here longer. Two generations of mudcracks represented in one area. Around the backside of quartzite hill more vegetation, older plants suggest this area has been dry for a long time. On side near granitic material there are small, rounded sediments (feldspar and quartz) which covers mudcracks. 12-24-98 Trench in campsite gully Layer 1: 8cm Peds are angular, silty, and hard (cemented). They are 1-4cm. Color is 10 YR 6/6. Soil contains vegetation/organic material, quartz, and feldspar. It is slightly sticky ~ 30% clay. Layer 2: 40cm Darker and less silty than layer 1. Large cracks present and not infilled. Peds are angular and hard with diameters of <1cm to 10cm. Color is 7.5 YR 4/4. Contains more clay, quartz and feldspar than layer 1. Super-saturation of salt around roots due to partial pressure of CO. Soil is very sticky (can make 2-3 inch rolls) ~ 40% clay. 129 Craig’s Field Notes: December 1998 12-21-98 We climbed up a quartzite hill, dodging cacti the whole way. It’s windy but beautiful up here. We are looking over the playa lake. After Nora there were two feet of water. By the end of February all the water was gone. The playa looks sandy. There are tans ranging from dark to light. Notice cinder cones. The strip of trees and shrubs is a ravine where our camp is. We’re sitting on quartzite, to the south is granite, to the west is basalt flows. The granite weathers into relatively large and rounded boulders. The quartzite is angular and blocky. Quartzite is vegetated with sage and small bushes. The border zigzags. Orange (darker) material extends out and blends into the sandy color beyond the vegetation. This could be the drainage from the eastern mountains. On lake, directly below where we were sitting on the quartzite, there are mudcracks. Brown and white quartz gravel. The gravel sticks out further straight down from drainage area. The mud is vesicular, probably signifying hot (warm) surface when flooding took place. Water comes in and forces out gas. Near backside of quartzite the gravel appears to be volcanic à source area? It is on the opposite side of the lake from the volcanics. Granite is the bedrock and it makes up the gravel covering the mud. 12-24-98 Examining a small trench by our campsite, just to the north of the playa. We have about 0.5 m deep, 1.3 m wide. We have two distinct colors. The first layer is about 8cm thick. Layer 1: 8cm thick Organic material, silt, and clay Slightly sticky Well sorted Mineralogy undistinguishable 1-4cm peds small cracks are present (0.5cm) color: 10 YR 6/6 ~ 30% clay Layer 2: Less sorting – fine sand, clay, silt, quartz, feldspar, muscovite grains angular 1-10cm peds, hard and angular large cracks (10-2cm thick) pervades through this layer, but cease at the border with layer 1 New cracks, finer grain, better sorted material Color: 7.5 YR 4/4 ~ 40-50% clay 130 Appendix E. Pipette Method for Grain Size Analysis Grain size analysis is widely used as a method of describing soils. The pipette method is commonly used for separating soils into particle size fractions. Sand is separated through wet sieving. Clay and silts are separated into fractions by extraction of a known volume from a known depth below the surface of a solution in a sedimentation cylinder after a sufficient settling time. The pipette method is based on the Law of Stokes, which holds that spherical particles settle in a solution at a rate proportional to their radii. 2 Law of Stokes: V = [2r (dp-dw) g] / 9h where: V= rate of settling (cm/s) of particles falling through a liquid r = radius (cm) 3 dp = density of particles (g/cm ) 3 dw = density of water (g/cm ) h = viscosity of the liquid (g/cm/s, poise) g = acceleration due to gravity (981cm/ s2) * dp , dw , and h are constants for a given temperature. [2(dp - dw)g] / 9h = K This reduces Stokes’ Law to V = Kr2. Because V is a function of the radius, larger particles will settle faster than smaller particles. For example, sand will settle out of solution after only 40 seconds, and only clay will remain in suspension after 2 hours (Tan, 1996). The pipette method was chosen for efficiency, accuracy, and ease of subsampling. The pipette method is faster, less complicated, and more accurate than the hydrometer 131 method of particle size analysis (Black, 1965 and Singer and Janitzky, 1986). The pipette method also allows for subsampling for clay and silt mineralogy (Singer and Janitzky, 1986). By using the pipette method, the clays are already pretreated and separated for mineralogical analyses, such as x-ray diffraction. Reagents: Distilled Water Hydrogen Peroxide 30% [H2O2] Isopropyl Alcohol Sodium Hexametaphosphate solution [7.94g Na2CO3 + 35.7g (NaPO3)6 per L of H2O] Equipment: 250-mL Beakers Watchglasses Analytical Balance Tweezers Hot Plate Oven Shaker Bottles Teflon Policeman Reciprocating Shaker Large Mouthed Funnel 1-L Sedimentation Cylinders 300-mesh Sieve Rubber Stoppers (large enough to fit the cylinders) Dessicator Sieve Nest (1.0, 0.5, 0.25, 0.177, 0.105 mm) Mechanical Shaker for Sieves Weighing Jars Plunger Thermometer 25-mL Pipette 10-mL Pipette Rubber Hand Pump Dichromate Spatula 132 Procedure: A. Sample Preparation 1. Label the 250-mL beakers with the sample numbers. Weigh the empty beakers and record the weights. 2. Add approximately 10g of sample to the appropriate beaker. Cover each beaker with a watchglass. 3. Removal of Organic Material a. Remove any visible organic material with a pair of tweezers. b. Wet each sample with distilled H2O. Add a few mLs of H2O2. Recover with the watchglass. c. Every 5-10 minutes add 3-5 ml of H2O2 to each sample and stir gently by making a slow circular motion with the hand. Use distilled H2O to rinse foam from the sides of the beaker. *If foaming must be rapidly reduced, add 1-2 drops of alcohol directly to the foam. d. After most foaming subsides, heat the sample to about 70°C (a hot plate, oven top, or water bath may be used) for 1 hour, continuing H2O2 additions. *Samples high in organic material should be left to react overnight, without heat, before proceeding to the heating stage. e. H2O2 treatment should be stopped when organics have been removed. Criteria that suggest treatment should stop are: 133 1) Light-brown foam no longer appears around the solution surface after addition of H2O2. Instead, rapid “self-oxidation” of H2O2 occurs (the reaction exhausts itself within 5 minutes). 2) The appearance of bleached fragments of roots floating on the surface. 3) Time (there is a 1 hour heating limit). 4. Filter the sample to remove dissolved material. a. Wash the sample with distilled H2O. Allow to settle for 24 hours. b. Siphon off or decant the fluid. c. Repeat steps a and b until the sample has been washed 3 times. *Instead of allowing the solution to settle overnight, the solution may be centrifuged to remove excess liquid. d. After the final washing, cover the beakers with the watchglasses and place in an oven for 24 hours at 105°C. e. Allow the beakers to cool in a dessicator. When cool, weigh the beakers containing the dried samples. THIS IS THE WEIGHT USED FOR CALCULATIONS. 5. Dispersion a. Add 10 ml hexametaphosphate solution and 10-20 ml of distilled H2O to each sample and a blank. Gently swirl solution to assure uniformity. b. Label shaker bottles (including one for the blank). 134 c. Carefully clean the beakers with the teflon policeman, being sure to remove baked-on residue from the sides. d. Transfer the suspended soil to the appropriate shaker bottle. Fill each bottle two-thirds full with distilled H2O. Seal the bottles. e. Save the tared beakers for sand analysis. f. Shake the solutions on a reciprocating shaker for 14 to 16 hours. *Do not shake for more than 16 hours because abrasion will cause small segments to break off, skewing the particle size analysis. g. Label the sedimentation cylinders and fill them with distilled water, to allow the water to equilibrate to room temperature. (Water may be kept in neoprene bottles instead). B. Wet-Sieving (Sand Separation) 1. Place a large-mouthed funnel in an empty 1-L sedimentation cylinder. Place a 300-mesh sieve in (or on) the funnel. 2. Without shaking or swirling the solution, transfer the soil solution from the first bottle through the sieve, into the cylinder. 3. Add more water to the bottle, rinsing the sides. Stir the mixture. Allow the sand to settle for 10-20 seconds, then pour the solution through the sieve. Repeat 5-6 times. 4. Transfer the remaining soil into its appropriate beaker. 5. Suspend the soil in the beaker and decant as in step 3. 135 6. Using a wash bottle, gently rinse the inside of the sieve, so that any trapped silt and clay particles pass into the cylinder. 7. Rinse the outside of the sieve, so that clinging particles go into the cylinder. 8. Fill the cylinder to the 1-L mark. Cover with a large rubber stopper. 9. Wash the sand on the inside of the sieve back into the beaker. 10. Cover the beaker with a watchglass and dry at 105°C for 24 hours. 11. Repeat for all samples (including the blank). C. Dry Sieving 1. Cool the sand in a dessicator. 2. Weigh the sand fraction. 3. Place the sand fraction into the top sieve of the sieve nest, using a spatula to complete the transfer. 4. Put the nest on the shaker and shake the sands for 3 minutes. 5. Weigh the sand fractions. Compare the cumulative weight with that previously determined. D. Pipette Sampling 1. Tare the weighing jars. Record the jar number, sample number, and tare weight. 2. Use the plunger to suspend the silt and clay in the sedimentation cylinders at 2.5 minute intervals. a. Upward strokes should be rapid, downward strokes slow. 136 b. Frequent short strokes near the bottom of the cylinder with occasional long strokes is best for mixing. c. RECORD the EXACT TIME the sample is mixed. 3. When removing the plunger, gently tap it against the lip of the cylinder to remove solution on its stem and head. 4. Rinse the plunger with distilled water before moving to the next cylinder. 5. If the cylinders are not in a constant temperature environment, record the temperature of the blank solution every 2-4 hours. a. Draw a plot of time vs. temperature. b. The average temperature for the settling period is at the point on the vertical axis crossed by a horizontal line that bisects the area defined by the temperature-time curve. c. Use the average temperature to determine the proper sedimentation time based on the chart below. ______Temperature 2µm 5µm 20µm (°C) (hour:min) (hour:min) (min:sec) 20 8:00 1:17 4:48 21 7:49 1:15 4:41 22 7:38 1:13 4:35 23 7:27 1:11 4:28 24 7:17 1:10 4:22 25 7:07 1:08 4:16 26 6:57 1:07 4:10 27 6:48 1:05 4:04 28 6:39 1:04 4:00 29 6:31 1:03 3:55 30 6:22 1:01 3:49 137 6. Prepare a 25-mL pipette with a rubber hand pump attached. The pipette must be cleaned with dichromate and rinsed with distilled H2O. 7. When it is time to draw a sample, carefully sample at the 10cm depth using the 25-mL pipette. *it may be useful to mark the cylinder at the appropriate depth to assure proper sampling. a. Try to sample over a 10-12 second period. b. Empty the pipette into a labeled weighing jar. c. Rinse the pipette into the jar with distilled H2O. d. Repeat this procedure on the remaining samples. 8. Evaporate the samples to dryness in an oven at 105°C (generally requires 24 hours). a. Cool the jars (lids open) to room temperature in a dessicator. Seal the jars with the lids before removing from the dessicator. b. Wipe off fingerprints and measure and record the weights of the jars with samples. c. After each jar has been weighed, return it to the dessicator. After all the jars have been weighed, reweigh the first jar. * A weight change greater than 0.0005g means that water has been absorbed and all the samples must be redried and reweighed. 9. Save the suspension until after the clay fraction calculations are completed, so that resampling can be done if necessary. 138 10. In order to determine clay mineralogy, resuspend the samples and extract a sample, which will supply a minimum of 50 mg of <2 µm material. Calculations: A. Clay 1. [weight jar + oven-dry sample] - [weight jar] - [average weight hexametaphosphate] = weight of particle fraction 2. weight of clay particle fraction * 1000 ml solution * 100 weight of mineral fraction 25 ml sample = percent clay-particle fraction in sample B. Sand weight sand * 100 = percent sand in sample total weight of mineral fraction C. Silt silt % = 100 - (clay % + sand %) 139 Appendix F. Grain Size Data Calculation Weights (g) Sample # Beaker Initial Sample Beaker w/ Dry Sample Oven Dry Weight 071498-1 109.82 10.772 119.9 10.08 071498-2 96.86 9.85 106.15 9.29 071498-3 97.64 9.53 106.55 8.91 071498-4 98.61 10.493 108.6 9.99 071498-5 97.85 11.338 108.45 10.6 122298-1 93.51 10.132 103.07 9.56 122298-2 106.42 10.744 116.51 10.09 122298-3 100.98 9.988 110.28 9.3 122298-4 96.87 9.91 106.1 9.23 122398-1 98.34 9.935 107.67 9.33 122398-2 110.71 10.292 120.42 9.71 122498-1 98.73 10.168 108.24 9.51 122498-2 102.61 10.624 112.53 9.92 122498-3 104.3 9.582 113.18 8.88 Sample # Total % Organics Weighing Jar # Weight (g) Organics (g) 071498-1 0.692 6.42 1 49.84 071498-2 0.56 5.69 5 50.65 071498-3 0.62 6.51 7 51.32 071498-4 0.503 4.79 12 51.46 071498-5 0.738 6.51 4 51 122298-1 0.572 5.65 19 53.06 122298-2 0.654 6.09 21 50.31 122298-3 0.688 6.89 71 51.51 122298-4 0.68 6.86 72 53.89 122398-1 0.605 6.09 73 51.42 122398-2 0.582 5.65 86 51.61 122498-1 0.658 6.47 94 50.17 122498-2 0.704 6.63 90 50.87 122498-3 0.702 7.33 99 51.68 92 50.75 Hexameta. 93 50.96 0.025 47 53.8 9 52.59 64 50.14 140 Sand Fraction (g) Sample # Beaker w/Sand Total Sand 1.0 Sand 0.5 Sand 071498-1 111.390 1.570 0.090 0.097 071498-2 97.580 0.720 0.120 0.07 071498-3 97.760 0.120 0.005 0.013 071498-4 98.690 0.080 0.011 0.003 071498-5 98.120 0.270 0.002 0.006 122298-1 93.790 0.280 0.005 0.009 122298-2 107.020 0.600 0.001 0.011 122298-3 101.060 0.080 0.003 0.005 122298-4 96.960 0.090 0.003 0.004 122398-1 99.410 1.070 0.025 0.091 122398-2 110.710 0.000 0.000 0.000 122498-1 98.940 0.210 0.001 0.007 122498-2 102.660 0.050 0.001 0.004 122498-3 104.410 0.110 0.000 0.000 Sample # 0.25 Sand 0.180 Sand 0.125 Sand 0.063 Sand 071498-1 0.168 0.094 0.206 0.658 071498-2 0.085 0.043 0.059 0.145 071498-3 0.020 0.015 0.017 0.065 071498-4 0.008 0.008 0.008 0.039 071498-5 0.033 0.019 0.028 0.095 122298-1 0.035 0.030 0.035 0.114 122298-2 0.075 0.058 0.079 0.248 122298-3 0.012 0.011 0.013 0.047 122298-4 0.020 0.009 0.012 0.045 122398-1 0.185 0.092 0.112 0.402 122398-2 0.000 0.000 0.000 0.000 122498-1 0.008 0.007 0.004 0.022 122498-2 0.006 0.005 0.007 0.031 122498-3 0.005 0.006 0.008 0.029 141 % Sand Fraction Sample # % Total Sand % 1.0 % 0.5 % 0.25 071498-1 1.31 0.08 0.08 0.14 071498-2 0.68 0.11 0.07 0.08 071498-3 0.11 0.00 0.01 0.02 071498-4 0.07 0.01 0.00 0.01 071498-5 0.25 0.00 0.01 0.03 122298-1 0.27 0.00 0.01 0.03 122298-2 0.51 0.00 0.01 0.06 122298-3 0.07 0.00 0.00 0.01 122298-4 0.08 0.00 0.00 0.02 122398-1 0.99 0.02 0.08 0.17 122398-2 0.00 0.00 0.00 0.00 122498-1 0.19 0.00 0.01 0.01 122498-2 0.04 0.00 0.00 0.01 122498-3 0.10 0.00 0.00 0.00 Sample # %0.180 %0.125 %0.063 071498-1 0.08 0.17 0.55 071498-2 0.04 0.06 0.14 071498-3 0.01 0.02 0.06 071498-4 0.01 0.01 0.04 071498-5 0.02 0.03 0.09 122298-1 0.03 0.03 0.11 122298-2 0.05 0.07 0.21 122298-3 0.01 0.01 0.04 122298-4 0.01 0.01 0.04 122398-1 0.09 0.10 0.37 122398-2 0.00 0.00 0.00 122498-1 0.01 0.00 0.02 122498-2 0.00 0.01 0.03 122498-3 0.01 0.01 0.03 142 Clay Fraction (g) Sample # Weight Jar 20 µm Fraction Weight Jar w/ Sample 5 µm Fraction w/ Sample 071498-1 50.02 0.155 51.62 0.135 071498-2 50.85 0.175 53.26 0.175 071498-3 51.53 0.185 50.5 0.165 071498-4 51.25 0.225 50.33 0.165 071498-5 50.41 0.215 51.88 0.175 122298-1 50.86 0.185 50.92 0.145 122298-2 50.06 0.195 51.13 0.145 122298-3 51.08 0.185 52.76 0.145 122298-4 50.39 0.225 52.77 0.155 122398-1 51.72 0.185 51.79 0.155 122398-2 54.16 0.245 54.03 0.205 122498-1 51.67 0.185 51.84 0.135 122498-2 51.66 0.215 53.25 0.165 122498-3 50.51 0.175 51.46 0.115 Sample # Weight Jar 2 µm Fraction w/ Sample 071498-1 51.65 0.115 071498-2 54.05 0.135 071498-3 51.77 0.135 071498-4 51.57 0.125 071498-5 51.17 0.145 122298-1 51.44 0.095 122298-2 51.59 0.105 122298-3 50.49 0.155 122298-4 50.31 0.115 122398-1 50.79 0.115 122398-2 50.91 0.135 122498-1 49.96 0.095 122498-2 50.99 0.095 122498-3 51.14 0.115 143 % Clay Fraction Sample # % 20 µm % 5 µm % 2 µm Total % Clay 071498-1 5.17 4.50 3.84 13.51 071498-2 6.59 6.59 5.09 18.28 071498-3 6.95 6.19 5.07 18.21 071498-4 8.29 6.08 4.60 18.97 071498-5 7.93 6.45 5.35 19.73 122298-1 7.18 5.63 3.69 16.49 122298-2 6.69 4.98 3.60 15.28 122298-3 6.71 5.26 5.62 17.59 122298-4 8.48 5.84 4.34 18.66 122398-1 6.87 5.76 4.27 16.90 122398-2 8.14 6.81 4.48 19.43 122498-1 6.84 4.99 3.51 15.34 122498-2 7.64 5.87 3.38 16.88 122498-3 6.18 4.06 4.06 14.31 % Silt Sample # % Silt 071498-1 85.18 071498-2 81.05 071498-3 81.68 071498-4 80.96 071498-5 80.02 122298-1 83.23 122298-2 84.21 122298-3 82.34 122298-4 81.25 122398-1 82.10 122398-2 80.57 122498-1 84.47 122498-2 83.07 122498-3 85.59 144 Appendix G. Conductivity Data Sample Weight (g) Conductivity (µmhos) 71498-1 10.50 80 71498-2 11.02 89 71498-3 10.21 72 71498-4 10.24 81 71498-5 10.70 99 122298-1 11.20 70 122298-2 10.44 65 122298-3 10.26 72 122298-4 11.64 110 122398-1 10.03 72 122398-2 10.75 90 122498-1 11.17 80 Sample Temperature (ºC) Measured Conductivity (mS/cm) 71498-1 16.6 0.164 71498-2 16.4 0.168 71498-3 16.8 0.208 71498-4 16.8 0.172 71498-5 17.1 0.183 122298-1 15.8 0.160 122298-2 15.1 0.156 122298-3 15.5 0.170 122298-4 15.7 0.165 122398-1 16.2 0.172 122398-2 16.3 0.186 122498-1 16.4 0.170 122498-2 16.6 0.174 122498-3 16.8 0.152 145