APPENDIX A to FER-250ALLUVIAL FAN STRATIGRAPHY COACHELLA FAN REGION

Alluvial fan stratigraphy and age determination is critical for the evaluation of fault recency of the many strands comprising the Coachella Fan Fault Zone. Mapping by Petra provides a detailed identification of alluvial units across three very large fault rupture investigation sites. These sites, from south to north, include AP-3413 (Petra, 2007a; locally known as the Lomas Del Sol site), AP-3416 (Petra, 2007e; Desert Lakes site), and AP-3477 (Petra, 2007d; Stone Water site) (Plates 1 and 2; Figures 7-9). These sites were investigated over a time span from about 2004 to 2007, with AP 3413 the first and AP-3477 the most recent.

Matti and Cossette (2007) and Matti (2012) also provide a detailed alluvial fan stratigraphy for the Indio-Thermal Canyon (ITC) study area that is relatively consistent with the alluvial fan stratigraphy developed by Petra (2007a) for the Lomas Del Sol site (AP-3413). Petra, in addition to the three large sites investigated, also prepared a preliminary geologic map of the Indio area for the Indio Water Authority that showed alluvial unit designations essentially the same as the AP-3413 unit designations (M. Kenney, written communication, April 2009). The alluvial fan stratigraphy for the Desert Lakes site (AP-3416) is similar to the Lomas Del Sol site, although the report is an un-submitted draft and is subject to change. Significantly, the investigation for the Stone Water site (AP-3477) has modified the alluvial unit designations, based on soil profile assessments by J. Helms (Appendix B in the draft report for the Stone Water site). Most importantly, the designation of the critical early Holocene aged unit Qfo identified in the AP-3413 and AP-3416 sites was modified to the Qf3U unit (upper, younger member of unit Qf3) for the Stone Water report. The Qfo unit’s age was estimated to be latest Pleistocene to early Holocene, based on soil profile analysis by R. Shlemon (Petra, 2007a). The Qf3U unit’s age was estimated to be 12 - 20 ka, based on soil profile analysis by J. Helms as reported by Petra (2007d). It should be noted that Helms did estimate a minimum age of 6.4 ka for the Qf3U unit, based on his soil profile at the Stone Water trench FT-10b.

Additional alluvial fan stratigraphy pertinent to this discussion is found to the south-southwest and to the north of the ITC study area (Figure A-1). The Martinez fan site, described by Gath and Rockwell (2007), is the closest to the study area, located 25 km to the south-southwest. The Silver Lake site, located 110 km north of the ITC study area, is an established soil chronosequence site and is routinely used for comparative soil profile development in arid regions (Wells and others, 1987; Reheis and others, 1989). Both of these sites have a series of alluvial fan surfaces that can be compared with the soil and alluvial fan surface morphology of Petra’s Qf3U unit. Comparative alluvial fan chronology (latest Pleistocene and Holocene) for the sites in the region is presented in Figure A-2.

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SITE-SPECIFIC ALLUVIAL FAN DESCRIPTIONS

AP-3413 (Petra, 2007a; Lomas Del Sol) (Figure 7) This site report was submitted to and approved by the City of Coachella (Petra, 2007a; report dated 1/15/2007, J.N. 4604-04). The site, located south of I-10, is 2,211 acres (3.45 square miles) on which at least 147 trenches were excavated. Lengths of trenches ranged from less than 100 feet to 5,432 feet and were excavated to depths from 10 to 20 feet. The southeastern portion of the site is underlain by Plio-Pleistocene Canebrake Formation, which in turn is overlain by mid-Pleistocene upper Palm Spring Formation. The central portion of the site is underlain by alluvial fans of various ages ranging from the approximately 150 ka Ocotillo Conglomerate to modern wash deposits.

Age estimates of sedimentary units by Petra included consideration of soil profile development, relative stratigraphic and morpho-stratigraphic position, and published and unpublished age data (including some OSL dating and volcanic ash correlation). Petra’s classification of late Quaternary deposits for the Lomas Del Sol site includes (from youngest to oldest): Qf1 – (Historical to latest Holocene) Alluvium deposited in modern drainages; lacks

CaCO3 or Fe2O3 cementation and lacks soil development. Qf2 – (late Holocene) Alluvium exists typically as fill terraces 0.5 – 1.5 feet above active

stream channels – age is estimated to be < 3 ka, based on soil profile development.

Qf3 – (early to mid Holocene) Fill terrace deposits 2-3 feet above modern drainage and

incised by relatively younger channels (Qf1, Qf2); age estimates: Qf3 – 4 ka; Qf3+ - 7 ka (Qf3+ age based on soil description by R. Shlemon)

Qfo - (early Holocene to latest Pleistocene) Alluvium is stratigraphically below Qf3 and

above Qf4; age extends beyond Holocene-Pleistocene boundary; unconformable contact with Qf3 delineated by buried Bw soil horizon; distinguishing characteristic of the Qfo unit is multiple weak Bw soil horizons.

Subdivision within Qfo

QfoU (upper) - 8-9 ka, based on soil age assessment by R. Shlemon (in Petra

2007a)

QfoL (lower) - 7-12 ka based on soil development assessment by R. Shlemon; however, exposures of QfoL in trench T-59 (Desert Lakes project, Petra 2007e - AP 3416) suggest soil age for QfoL ranges from 10 ka to 27 ka.

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Qf4 – (latest Pleistocene) – elevated abandoned fan surfaces 5-8 feet above current drainage; subdivided into 3 subunits: Qf4 -; Qf4; Qf4L; Qf4 - thought to be younger than 33 ka; Qf4L 33-52.5 ka.

Qf5 – probably older than 27 ka and could range from 50-77 ka, based on soil profile

development. Qsw – late Quaternary age slopewash deposits found along flanks of bedrock ridges

and interfinger alluvial fan deposits within current drainages. Age of Qsw is contemporaneous with age of fan units within the local drainages and can range from age of units Qf1 through Qf5.

Qo – Ocotillo Conglomerate – coarse-grained fluvial fan deposits; fan surfaces exhibit

stage IV CaCO3 development and soil profiles contain multiple Bk and K horizons; cosmogenic surface age dating suggests age of ~76 – 124 ka for the younger subunit (designated Qoy) and about 145 ka for the Qo unit.

AP-3416 (Petra, 2007e; Desert Lakes) (Figure 9)

The Desert Lakes site report is un-submitted and is in very rough draft form (text of report mostly is the same as the AP-3413 report). At this writing, the alluvial fan stratigraphy is the same as the AP-3413 site. AP-3477 (Petra, 2007d; Stone Water) (Figure 8)

The Stone Water site is an un-submitted fault rupture investigation for an approximately 807-acre site located south of Fargo Canyon Road and east of Dillon Road. Age estimates of sedimentary units by Petra (2007d) included soil profile development (Appendix B to Petra’s report by J. Helms), relative stratigraphic and morpho-stratigraphic position, and published and unpublished age data. OSL and radiocarbon dates were established for a Holocene unit that Petra termed the Fun Valley Wash deposits (see trench FT-10a, Appendix B of this FER). No other direct dating methods were used for the Stone Water site. What is significant is the age estimation of Petra’s Qf3U alluvial unit. This unit is probably equivalent to the Lomas Del Sol and Desert Lakes Qfo alluvial unit, and was considered to be Qfo in their preliminary map of the Indio area (M. Kenney, written communication, April 2009). Qfo unit’s age was estimated to be latest Pleistocene to early Holocene (age range of 8 to 27 ka, Petra, 2007a and 2007e). The age of the Qf3U alluvial unit is estimated to range from 12 ka to 20 ka, based on soil profile analysis by J. Helms (in Petra, 2007d). Petra’s classification of mid to late Quaternary deposits for the Stone Water site include (from youngest to oldest):

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Qf1 - (Historical to late Holocene) – fine to very coarse sand with abundant gravel, light gray (2.5Y 7/1 to 2.5Y 7/2 – dry), no soil, oxide, or carbonate development.

Qf2 – (late to mid Holocene) – deposit often is indistinguishable from Qf1 except for soil

development on Qf2 surface. Deposit estimated to be 6-8 ka, based on soil profile estimates by Helms (Petra, 2007d).

Qf3 – important unit for evaluation of fault recency at site. Petra subdivided this alluvial

unit into Qf3 upper (Qf3U) and Qf3 lower (Qf3L)

Qf3U – (latest Pleistocene) - Debris flow and fluvial fan deposits generally composed of loose sand and gravel with minor silt; light gray (2.5Y 7/1 – 2.5Y 7/2) to pale yellow (2.5Y 7/3); typically 1 – 3 feet thick; clasts generally lack carbonate coatings and exhibit no discernable grussification. Soil profile development indicates age of 12-20 ka, based on soil estimates by Helms (Petra, 2007d). However, Helms estimated a minimum age of 6.4 ka for Qf3U soil, based on exposures in Petra trench FT-10b. Helms relied on comparison of soil profile development of this unit with the soil profile development of the Qf2 alluvial fan unit at Silver Lake Playa as described in Wells et al. (1987) and Reheis et al. (1989).

Qf3L - (latest Pleistocene) – alluvial (fluvial) fan deposits, generally well-bedded,

fine to very coarse sand with occasional gravel; light brownish gray (2.5Y 6/2), light yellowish brown (2.5Y 6/3) or pale yellow (2.5Y 7/3); not correlated with any fan surface; soil profile development indicates age of 22 – 38 ka, based on soil estimates by Helms (Petra, 2007d).

Qf4 – (latest Pleistocene) – alluvial (fluvial) fan deposits, generally well bedded fine to

very coarse sands with occasional gravel; general colors for this unit are variations of light yellowish brown: 2.5Y 6/3, 2.5Y 6/4, and 10YR 6/4, but has an overall reddish hue to the deposits; thickness ranges from a few inches to 3 – 4 feet; uppermost member has distinctive Bk soil horizon (Stage I to I+). Soil profile development indicates age of 35-67 ka, based on soil estimates by Helms (Petra, 2007d).

Qf5 – (latest Pleistocene) - Mostly debris flow deposit consisting of massively bedded

pebble to cobble gravels and minor sands; contains reworked carbonate soils from older Qo unit; Qf5 unit considered by Petra to be the basal conglomerate unit of Qf4. Thus, age is same as Qf4. Petra stated that differentiation between Qf5 and the older Qo unit often is difficult due to similarities. However, they also stated that carbonate development typically was more developed in the Qo unit.

Qf6 – (latest Pleistocene) – oldest identified alluvial fan unit incised into Ocotillo Conglomerate (Qo) unit; consists of silt, sand, gravel; color yellowish brown (10YR 5/4) to reddish yellow (7.5YR 6/6); has Bt soil horizon; soil profile

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development indicates age of 73-132 ka, based on soil estimates by Helms (Petra, 2007d).

Qo – (late Pleistocene) – Ocotillo Conglomerate; consists of coarse-grained silty sand

debris flows and some fluvial fan deposits; massively bedded, beds 1 to 4 feet thick; well-developed soil profile with Bt, Bk, and K horizons with clast coatings to 0.25 inches thick; Soil profile development indicates age of 110-230 ka, based on soil estimates by Helms (Petra, 2007d).

DISCUSSION OF ALLUVIAL FAN CHRONOSEQUENCE

Matti and Cossette (2007) developed an alluvial fan chronosequence for the Inland Empire region, based on soil descriptions for the Coachella Valley region by the U.S. Department of Agriculture, as well as lithologic correlations and alluvial fan surface morphology. That classification was used for Matti’s (2012) mapping of Quaternary deposits for the 1:100k Palm Springs Quadrangle, which includes the ITC study area. Only Matti and Cossette’s Young Series will be considered in this discussion. They break down the fan deposits like this:

Qyf5 – 0.2 – 2.3 ka Qyf4 – 1.8 – 3.5 ka Qyf3 – 3.0 – 6.2 ka Qyf2 – 5.8 – 8.3 ka Qyf1 – 8.0 – 13.1 ka

Qof3 – >13.0 ka

Matti and Cossette’s unit Qyf1 is equivalent to Petra’s unit Qf3U. This is based on the similar alluvial fan morphology, but, most convincingly, where the two maps overlap. Petra’s (2007d) Qf3U unit and Matti’s (2012) Qyf1 unit are the same. In addition to mapping by Matti (2012), Gath and Rockwell (2007) developed an alluvial fan chronology for the Martinez fan, located about 25 km south-southwest of the ITC study area (Figure A-1). The fan surface is complex and clast population of the fan is composed of leucocratic granitic rocks and a suite of metamorphic rocks including schist, gneiss and limestone/marble. Gath and Rockwell estimated ages of alluvial fan surfaces using degree of rubification and rock varnish development on fan surfaces, limited soil descriptions [CaCO3 development, comparison of soil development with known dated sites, such as Silver Lake, Coyote Mountains (Imperial County)

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(Goodmacher and Rockwell, 1990), and New Mexico (Gile et al., 1981)], as well as degree of preservation of constructional surfaces. Their alluvial fan classification and age estimates are: Q1 – 0 – 0.5 ka Q2 – 0.3 – 2 ka Q3 – 2 - 3 ka Q4 – 3 - 9 ka, but could be older – they were unable to excavate adequate soil

pits; fan surface morphology was compared to the Coyote Mountains chronosequence. Gath and Rockwell estimated that the Q4 surface is intermediate between the Coyote Mountains (CM) 2-3 ka and 9-20 ka surface, which corresponds to the CM Q4 and Q5 deposits. The Q4 deposit lacks Bw/Bt soil horizons, has Stage I CaCO3 development, has 10YR colors and is classified as Torrifluvent soil. – this deposit is characterized by distinctly developed rock varnish, with 10YR color developed over 25-90% of clast bottoms with distinctly developed small patches of 7.5YR color; well-developed pavement; disintegration of granitic and carbonate clasts

Q5 – 9 - 20 ka, but could be older. The Q5 surface has up to 19 cm thick

Bw/Bt horizon, Stage I+ CaCO3 development, and 10YR color and is classified as a Calciorthent soil.

Based on fan surface morphology and relative height above active channels, I would place the Qf3U unit at Stone Water between the Q4 and Q5 unit of Gath and Rockwell, probably closer in age to the Q4 unit (refer to photos A-1 and A-2 for comparison of surface morphology between ITC study area and Martinez alluvial fan). The Martinez fan is the closest to the Stone Water fan complex and is subject to similar climatic conditions. Refer to the description of the Qf3U surface in the discussion of the Qf2 fan surface at Silver Lake below.

The Qf2 alluvial fan unit at Silver Lake is a critical unit in the relative age comparison for the Qf3U unit at the Stone Water site (Photos A-1 and A-3). Wells et al. (1987) estimated an age of between 8 kybp (thousand years before present) and about 10.5 kybp for the Qf2 fan. They base this estimate on the relationship between dated Lake Mojave high-stand shoreline A (10.5 to 15.5 kybp) and the cessation of deposition of the older Silver Lake alluvial fan unit Qf1. Age control of the shoreline is based on radiocarbon dates of pelecypod shells (11.86 ± 0.095 kybp) and lithoid tufa (10.85 ± 0.075 kybp) reported by Ore and Warren (1971). Based on their mapping, unit Qf2 postdates shoreline A and grades to a younger shoreline, identified as shoreline B by Wells et al. (1987). Shoreline B was determined to be between 8 ka and 10.5 kybp, based on radiocarbon age dates of shells as reported by Wells et al. (1984). Therefore, Wells et al. (1987) assigned a maximum age of 10.5 kybp for the Qf2 fan deposit.

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The Silver Lake Qf2 fan age reported in Reheis et al. (1989) differs somewhat

from that estimated by Wells et al. (1987). Reheis et al. assigned an age of 11 kybp with a range from > 8 kybp to 13.1 kybp. The minimum age is based on field relationships reported by Wells et al. (1984, 1987) and the maximum age is based on cation-ratio age estimates of rock varnish supplied by Dorn (p.c. 1986). The cation-ratio age dating method was experimental at the time of publication and has since been discredited (K. Kendrick, personal communication, June 2013). Thus, other than field correlations with shoreline features with age dates determined from 14C dating of shells, the Qf2 fan lacks independent direct age-dating determinations. What does seem to be well-established, however, is that the Qf2 fan deposits post-date the shoreline A feature and may be contemporaneous with the shoreline B strand line deposits. Thus, the Qf2 deposit and surface are early Holocene in age and the time span is greater than 8 ka, and less than 13.1 ka.

Qf2 unit surface, as described by Wells et al. (1987), is as much as 5 m (16 feet) above all younger units in the proximal and medial piedmont areas. In the distal areas unit Qf2 is >1.5 m (> 5 feet) above younger units. In comparison, the Qf3U alluvial unit at Stone Water is between 3 and 5 feet above the modern drainage. Much of unit Qf2 is characterized by well-developed stone pavements with nearly 50% reduction in depositional relief of original bar and swale topography (Photo A-3). The calcareous Bwk horizon is several decimeters thick and the secondary carbonate morphology is Stage II. Secondary gypsum occurs in the Bky horizon. The older Qf1 unit has almost no bar and swale topography and soils have B horizons as thick as those on Qf2 deposits, but the upper 15 cm contains significantly more clay and silt and the carbonate morphology is Stage II and locally Stage III (Photo A-4). In comparison, the Stone Water Qf2 fan surface can be described as follows (from Helms’ soil description at about sta 250 in FT-6 (I presume FT-6a, but only identified as FT-6 in Helms description)): “The alluvial fan surface is characterized by coarse bar and swale morphology with lithology of crystalline granitic rocks, gneiss, and massive quartz; weakly developed desert pavement; spotty and thin dorsal varnish accretions; 10YR clast rubification, with some 2.5Y color. The Qf3U alluvial fan surface can be described as follows (based on description of test pit T-6 by Helms – T-6 is just east of FT-2b): Surface displays subdued bar and swale topography. Surface has disturbed and degraded desert pavement with moderately interlocked to slightly well-embedded clasts. Clast composition is predominantly crystalline granitic. Dorsal rock varnish is very spotty and moderately thick with moderate rubification value on clast bottoms. Max hues 10YR to low chroma 7.5YR value. Surface is 4 to 6 feet (1.2 to 1.8 meters) above active channel”.

Figure A-2 provides a graphic comparison of the age assignments (latest Pleistocene and Holocene) of the different alluvial fan units developed by Matti and Cossette (2007), Petra (2007a, 2007d), Gath and Rockwell (2007), Wells et al. (1987), and Reheis et al. (1989). The units depicted in yellow show what I consider to be equivalent alluvial units. Of principal concern regarding this change in age estimation

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between the Lomas Del Sol and Stone Water sites is the way the alluvial chronosequence of Petra (2007d) is constructed for the Stone Water site: Qf1 is late Holocene, but is considered to span from modern time to about 4 ka Qf2 is designated as 6 to 8 ka Qf3U is designated as 12 to 20 ka Qf3L is designated as 22 to 38 ka

There are significant apparent depositional hiatuses with this chronosequence. The largest is the 4 ka time span between Qf2 and Qf3U. Also, it seems unusual that the maximum age for Qf1 extends to 4 ka. As mapped by Petra, Qf1 typically is confined to modern drainages. As constructed by Petra, there is at least a 2 ka hiatus between Qf1 and Qf2. However, in several trench exposures Petra shows combinations of some units, such as Qf2+3U, Qf1+2, so the time gaps seem artificial. Bull (1991) reports that climate change during latest Pleistocene to early Holocene time resulted in maximal monsoonal rains during his Q3a time, which is constrained between 8-12 ka, based on 14C dates of plant fossils in packrat middens. Wells et al. (1987) reported that their field relationships, specifically with respect to Qf2 deposition, supports the Bull and Schick (1979) model that emphasizes an initial increase in sediment load in response to climatic change that favors transport of hillslope deposits, followed by a decrease in sediment load as the hillslope supply decreases. The Silver Lake Qf2 unit records a period of hillslope destabilization – colluvial wedges and runoff reduction at the base of the wedges would explain increased sediment yield from bedrock source areas during the early Holocene. Time-transgressive changes in plant communities and decreases in vegetation densities on hillslopes increased both sediment yield and runoff in spite of reduction in effective moisture. Given these observations, it does not seem correct that the beginning of the Holocene at the Stone Water site would be characterized by a 4 ka depositional hiatus (Bird and Kirby, 2006; Harvey et al. 1999). Neither Matti and Cossette (2007), Matti (2012), nor Gath and Rockwell (2007) indicate that a significant depositional hiatus has occurred in their alluvial fan chronosequences.

I think it is reasonable to conclude that the minimum age of the Qf3U alluvial fan unit is younger than estimated by Helms (in Petra, 2007d), based on the discussions outlined above. Matti and Cossette (2007) considered their Qyf1 unit to be between 8 and 13 ka and mapping by Matti (2012) used the stratigraphic nomenclature developed by Matti and Cossette. Mapping by Matti (2012) and Petra (2007d) overlapped in the study area and the Qf3U unit designation of Petra is the same as the Qyf1 unit designation of Matti. One soil age from Helms indicated an age range of 6.4-29 ka for Qf3U (Petra, 2007d). Shlemon (for the AP-3413 site just to the south) described a soil for the Qfo unit that probably is equivalent to the Qf3U unit, based on soil profile development; his age estimate is 7-12 ka. The Qf3U fan surface morphology (degree of preservation of constructional fan morphology, desert pavement and rock varnish development) is similar to the Q4/Q5 alluvial fan age estimates of Gath and Rockwell

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(2007) at the Martinez fan. Lastly, the significant early Holocene depositional hiatus in the alluvial fan chronology developed by Petra (2007d) is inconsistent with other observations of early Holocene depositional sequences (Bull and Schick, 1979; Bull, 1991; Harvey et al. 1999; Bird and Kirby, 2006).

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Qf3u alluvial fan surface

Photo A-1 (to FER-250). Oblique photo showing Qf3U alluvial fan surface at the northernend of the Stone Water site (Petra, 2007d). Compare the alluvial fan surface morphologywith the Martinez Qf4 and Qf5 fan (photo A-2) and Silver Lake Qf2 fan (photo A-3) surfaces.

Dillon Road

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Photo A-2 (to FER 250). Martinez alluvial fan described by Gath and Rockwell (2007). Fan unit Q4 estimated to be 3 – 9 ka and fan unit Q5 ranges in age from 9 – 20 ka by Gath and Rockwell (2007).

Q4 fan surface

Q5 fan surface

Q3 fan surface

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Qf2 alluvial fan

Photo A-3 (to FER-250). Alluvial fan unit Qf2 from Silver Lake site. Age of Qf2 estimated to be 8 - 13 ka by Reheis et al. (1989). SL85-2c is location of key soil profile description in Reheis et al. (1989).

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Photo A-4 (to FER-250). Alluvial fan unit Qf1 from Silver Lake site. Age of Qf1 estimatedto be > 15.5 ka by Reheis et al. (1989). Note the lack of bar and swale topography.

Qf1 alluvial fan

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