CALIFORNIA STATE UNIVERSITY, NORTHRIDGE
Investigation of the depositional environment of clustered boulders,
Nojoqui Valley, Santa Barbara County, California
A thesis submitted in partial fulfillment of the requirements
For the degree of Master of Arts in Geography
By
Danielle D’Alfonso
May 2018
The thesis of Danielle D’Alfonso is approved:
______
Dr. Antony Orme Date
______
Dr. Julie Laity Date
______
Dr. Amalie Orme, Chair Date
California State University, Northridge
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Acknowledgement
I am grateful to Bill Giorgi of Nojoqui Ranch and Art Green of the Alisal Ranch for providing access to their properties, my advisor Dr. Amalie Orme for her guidance and
support, and my field assistant Eiko Kitao.
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Dedication
This paper is dedicated to the long-time residents of Nojoqui, and to people everywhere who wonder why boulders are in their yard.
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Table of Contents
Signature Page ……………………………………………………………………………ii
Acknowledgment ………………………………………………………………...………iii
Dedication …………………………………………………..……………………………iv
List of Tables ……………………………………………………………………………vii
List of Figures ……………………………………….…………………………………viii
Abstract …………………………………………………………………………..………ix
Chapter 1: Introduction …………………………………………………...………………1
1.1 Statement of Purpose …………………………………………………………1
1.2 Location ………………………………………………………………………2
Chapter 2: Scientific Background and Previous Work ……………………...……………5
2.1 Geologic and Tectonic Framework …………………………………………..5
2.2 The Santa Ynez Fault …………………………………………………………6
2.3 Bedrock Geology ………………………………………………………..……8
2.4 Surficial Boulder Deposits ………………………………………………..…10
2.5 Climatic History ……………………….………….…………………………13
Chapter 3: Physical Setting ………………………….……..……………………………14
3.1 Landscape …………………………………………...………………………14
3.2 Anthropomorphic Influence on Landscape ………………………………….16
Chapter 4: Methods ………………………………………………………………...……17
4.1 Developing a Basemap………………….………………………...…………17
4.2 Determining Boulder Locations …………….………………………………17
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4.3 Identifying a Source for the Boulders……….………………………………18
4.4 Distribution of Boulders……………………………………………………..19
4.5 Geomorphic Analysis………………………………………………………..23
Chapter 5: Results ……………………………………………………….………………24
5.1 Lithologic Description ………………………………………………………24
5.2 Boulder Distribution …………...……………………………………………31
5.3 Geomorphology……. …….…………………………………………………54
Chapter 6: Discussion ……………………………...……………………………………60
6.1 Lithology and Bedrock Source………………………………………………60
6.2 Distribution and Depositional Environment ………...………………………61
6.3 Timing of Events……………………………………………………………..65
6.4 Future Work …………………………………………………………………66
Chapter 7: Conclusions …………………………….……………………………………68
References …………………………………………………………………………….…70
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List of Tables
Table 1: Lithologic Comparison of Bedrock in Nojoqui …………………………………9
Table 2: Section A clast count on Nojoqui Ranch ridge…………………...…………….39
Table 3: Section B clast count .………………………………………….………………43
List of Figures
Figure 1: Geographic location of Nojoqui………………………………………...………4
Figure 2: Nojoqui watersheds ……………………………………...……………………15
Figure 3: Boulder distribution …………………………………..………………………21
Figure 4: Grain-size classification ………...…………………………………………….22
Figure 5.1a: Round arkosic sandstone boulder…………………………………………..25
Figure 5.1b: Angular arkosic sandstone boulders…………………………………….….26
Figure 5.1c: Boulder exhibiting both fresh and weathered surfaces……….………...…..27
Figure 5.1d: Varicolored lichens on conglomerate……………………………………....29
Figure 5.1g: Open-framework conglomerate boulder pile…………………………...….29
Figure 5.1e: Large 2 m conglomerate boulder covered with gray and green lichen…….30
Figure 5.1f: Conglomerate boulder with large quartzite and rhyolitic cobbles …...…….31
Figure 6: Boulder Ridge in Section A …………………………………...………………34
Figure 7: Nojoqui Ranch, Section A……………………………………………….…….35
Figure 8a: Section A Group 1……………………………………………………………36
Figure 8b: Section A Group 12……………………………………………..……………37
Figure 8c: Section A Group 13……………………………………………….….………38
Figure 8d: Section A Group 17………………………………………………….…….…38
Figure 9: Section B, Forty Acre Ranch…………………………………………………..41
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Figure 10: Detailed grid survey……………………………………………..…………...44
Figure 11: Section C, Alisal Ranch ……………………….………………….………….46
Figure 12a: Detailed drawings of boulder deposits…………………………...…………47
Figure 12b: Section C Station 18………………………………………………...………48
Figure 12c: Section C Station 17………………………………………………..….……48
Figure 13: Section D the North Slope………………………………………….……...…50
Figure 14: Section D Conglomerate at Station 5………………………………...………51
Figure 15: Section D Station 6 Arkosic boulders pile. ……………………………….…52
Figure 16: Section D Matilija sandstone ……………………………………………..…53
Figure 17: Section B Unconformity …………………………………………….…....…56
Figure 18: Geologic map………………………………………………………..…….…57
Figure 19a: Section D Unconformity …………………………………………..……....58
Figure 19b: Section D Fanglomerate ……………………………………………………58
Figure 19c: Angular Unconformity……………………………………………...………59
Figure 20a: Very large-sized boulder-levee deposits in Montecito, CA………..…….…63
Figure 20b: Medium-sized boulders, Montecito debris flow……………………………63
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Abstract
Investigation of the depositional environment of clustered boulders,
Nojoqui Valley, Santa Barbara County, California
By
Danielle D’Alfonso
Master of Arts in Geography
Large boulder deposits of uncertain origin were mapped in Nojoqui Valley, Santa
Barbara County, California, raising questions concerning the nature of these deposits and their provenance. Using detailed field mapping and remote sensing of the study area, this
investigation seeks to explain the geomorphic history of the region based on the lithology
and distribution of the boulders, and present and past tectonic and climatic conditions.
Field observation shows a large percentage of boulders composed of arkosic sandstone, likely sourced from exposures of the Eocene Matilija Sandstone in the Santa
Ynez Mountains directly adjacent to the study area. A smaller percentage of conglomeratic boulders, originally thought to be derived from the Oligocene Sespe
Formation, contain exclusively felsic clasts, indicating the most likely source to be a previously unmapped basal conglomerate member of the Matilija Sandstone. Distribution of boulders expressed as linear and lobate structures across the landscape, suggest deposition by debris flows with the Santa Ynez Mountains as a plausible source for most, but not all of these boulders.
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Though absolute temporal constraints on the timing of debris flow events cannot be derived directly from this study, a sequence of events for deposition and subsequent displacement along the Santa Ynez fault is interpreted based on locations of groups of boulders. There appears to be an initial deposition of boulders from multiple debris flows followed by a second series of debris flow events that today are found on shutter ridges along the fault. Fluvial transport and removal of material is indicated by boulder deposits perched above the modern valley floor. Incision of the valley floor continues today.
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Chapter 1: Introduction
1.1 Statement of Purpose
The purpose of this study is to investigate the depositional environment of
surficial boulders in Nojoqui Valley with implications for revealing a more complete
geomorphic history of the region.
The Nojoqui Valley in Santa Barbara County, CA is host to anomalous groupings
of surficial boulders of unknown origin. Depositional environments for very large (>2 m)
to small (20 cm) boulders are typically characterized by high energy and source
proximity. The Santa Ynez Mountains provide a plausible source for most, but not all of
these boulders. A large percentage of boulders are composed of arkosic sandstone, likely
sourced from exposures of the Eocene Matilija Sandstone in the mountains directly
adjacent to the study area; however, a source and transport mechanism for emplacing a
significant number of conglomeratic boulders at Nojoqui are absent in the present
geomorphic setting. The presence of the boulders questions previous workers who have
reported on the geology of this area (Dibblee 1950, Byrd 1983).
This study aims to fill a gap in the literature on the geomorphic evolution of
Nojoqui by examining the source for the boulders and relative timing of boulder deposits based on their lithology and distribution, and the availability of present and past tectonic and climatic conditions. Specific research questions are: 1) What can detailed mapping of the distribution and placement of boulder deposits within the study area reveal about their depositional environment?; 2) What processes are responsible for the unexplained presence of conglomerate boulders?; and 3) What do these observations reveal about the
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tectonic and fluvial history of Nojoqui? A combination of remote sensing and field
observations were performed to answer these questions.
1.2 Location
Nojoqui is located within the Transverse Ranges geomorphic province on the northern boundary of the east-west trending Santa Ynez Mountains, approximately 8 km
(5 mi) north of the Pacific coastline, and 5 km (3 mi) northeast of Gaviota Pass (Figure
1). The Transverse Ranges are unique for their series of east-west striking topographic
and geologic structures and are often divided by these landforms into the Western,
Central, and Eastern Transverse Ranges. The study area is located in the western Santa
Ynez Mountains, a feature of the Western Transverse Ranges (WTR).
The Santa Ynez Fault forms the northern boundary of the Santa Ynez Mountains
and the southern boundary of what is now commonly referred to by ranchers and
travelers as the Nojoqui Valley. A spring-fed waterfall, a Chumash village, a Spanish
rancho, and a small station along the stagecoach route helped establish the place name of
Nojoqui, although no recognized formal boundary of this area exists. For the purpose of
this study Nojoqui will be used to reference an area defined by: the northern boundary of
Nojoqui Ranch where it crosses US Route 101 and meets with Nojoqui Creek; the east
Nojoqui Creek drainage divide and its southern border extending along the ridgeline of
the Santa Ynez Mountains; and the southwest and west Nojoqui Ranch property line. The
area defined here totals 22.5 km2 and ranges in elevation from 871 m (2856 ft) to 190 m
(623 ft) over a short distance of only four kilometers (2.48 mi).
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A small portion of Nojoqui can be accessed by road at Nojoqui Falls County Park, and on foot by hiking through Gaviota State Park to access the Los Padres National
Forest. Three quarters of the study area are owned by private ranchers, accessed only with the permission of the landowner. Owing to the persistent inaccessibility of this area, little previous research has been conducted over recent decades, leaving major gaps in research at a crucial topographic and structural junction in the Western Transverse
Ranges.
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Figure 1: Geographic location of Nojoqui. Study area is shaded yellow.
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Chapter 2: Scientific Background and Previous Work
2.1 Geologic and Tectonic Framework
A review of regional geologic history is necessary in order to place Nojoqui
within the context of its tectonic setting. It is important to understand the topographic
evolution of the study area because timing of uplift along the Santa Ynez fault is likely
the largest factor controlling the source and areal extent of the boulders.
2.1.1 Paleogene to Early Quaternary
The evolution of the North American-Pacific plate boundary over the last 28 Ma
has shaped the geologic history of the Western Transverse Ranges (WTR). Right-lateral
transtension from onset of the San Andreas Fault resulted in the compressive forces
responsible for the broad gentle folds of the Ynezean Orogeny, and further folding during
the Lompocan Orogeny in the early Miocene. These events mark the earliest record of
folding in the Santa Ynez Mountains, causing changes in the paleogeography of the area
(Dibblee 1950) At ~5 Ma the northern edge of the now east-west striking WTR block
began converging with the North American plate forming a compressional regime where
crustal shortening is accommodated by east-west trending fault propagation folds (Keller
1999). After a period of erosion, renewed deformation occurred in the late Pliocene with the start of the Zacan Orogeny. Uplift occurred along areas that are present day major fault systems, and older folds were further compressed (Dibblee 1950). This orogeny
affected the entire mapped area, causing further compression of folds formed by earlier
orogenies, and the development of many other structures.
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2.1.2 Late Quaternary
In the mid to late Pleistocene, deformation of the Coast Range Orogeny occurred in two-stages, separated by two major periods of erosion in this region. The early stage of the Coast Range Orogeny increased the uplift of the Santa Ynez Mountains and formed anticlinal foothills. This was followed by the first period of major erosion, and the deposition of fanglomerates and older alluvium at the base of the Santa Ynez
Mountains (Dibblee,1950).
The late stage of the Coast Range Orogeny caused renewed uplift, elevating old fanglomerate surfaces, which were dissected by downcutting streams. This deformational event is responsible for modern-day topography in this area (Dibblee,
1950). The second period of major erosion started in the late Pleistocene to Holocene.
Only remnants are found in the foothills, after severe downcutting and dissection by stream channels (Dibblee,1950). Lateral erosion by streams formed floodplains in their respective valleys or canyons.
2.2 The Santa Ynez Fault
The Santa Ynez Fault is generally characterized as a steep south-dipping reverse fault. It is the primary geologic structure controlling the topography at Nojoqui, although little detailed work has been performed along this segment. It is one of the largest structures in the WTR, striking east-west for approximately 120 km (75 mi) and uplifting the Santa Ynez Mountains to elevations of more than 2000 m (6562 ft) above sea level.
At Nojoqui Summit the fault splits into a north and south branch, and at least two
6 additional smaller branches (Dibblee 1950), which continue west and south into the
Pacific Ocean. Offset marine terraces on either side of the south branch show an active fault with an uplift rate of ~1.5 m/ka, and sinistral offset of ~500- 800 m (Wampler,
2014). Further age-dating of marine terraces on the Gaviota Coast indicates younger terraces cut during MIS 3, yielding higher uplift rates of 1.7-1.9 m/ka east of the south branch of the Santa Ynez fault (Morel and Keller 2017).
East of Nojoqui Summit, the Santa Ynez Fault is a south-dipping single segment placing Late Cretaceous rocks on the south against Middle Miocene rocks on the north, exhibiting a reverse displacement amounting to approximately 3.2 km (2 mi) (Dibblee
1950). Approximately 2100 m east of Nojoqui Falls the fault has a shallow south dip of ~
15° and similar displacement (Byrd 1983). Dibblee (1950 p. 55) also provides evidence for left-lateral strike-slip movement along the Santa Ynez fault as indicated by “offset canyons emerging from the north slope of the Santa Ynez Range, such as Quiota Canyon, which follow the fault always in a westerly, never an easterly, direction, before resuming their northward course.” Quiota Canyon is located 5 km (8 mi) east of Alisal Canyon and
10 km (6.2 mi) east of Nojoqui, all of which share similar drainage geometry (Figure 1).
Kinematic data collected regionally along the fault support geomorphic evidence for left- lateral strike-slip offset (Onderdonk 2005). Comprehensive reports performed along the
Santa Ynez Fault (Onderdonk 2005, Byrd 1983, Sylvester and Darrow 1979, Bortugno
1977) agree that previous attempts to quantify lateral offset are together inconclusive and at times contradictory. A constraint on displacement continues to be uncertain.
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2.3 Bedrock Geology
Close attention to the lithology of exposed bedrock at Nojoqui is crucial to
determine the source of surficial boulder deposits. The Western Transverse Ranges
(WTR) is comprised of folded sedimentary rocks ranging from Cretaceous to Miocene in age. A comprehensive stratigraphic sequence of these rocks was mapped and described
by Dibblee (1950). This report accounts for the most detailed information available on
the study area. A geologic report on the Alisal Ranch by Byrd (1983) provides useful
detail on the Oligocene Sespe Formation and Eocene Gaviota Formation where they crop
out on the east end of Nojoqui. Analyzing the lithologic composition of the boulders may
reveal their source, thus providing a path and direction of transport.
Dibblee’s 1950 report for the California Division of Mines and Geology contains the first and only comprehensive geology of the entire area of Nojoqui. A near complete stratigraphic sequence of late Cretaceous to Miocene rock crops out within the study area.
The lower portion of the section is well-exposed in cross-sectional view on the north face of the Santa Ynez Mountains, while the upper section is exposed in the low foothills of
Nojoqui Valley.
Notable formations composed of well-cemented rock capable of weathering to boulders are listed in stratigraphic order from oldest to youngest: the sandstone member of the Jalama Formation, the Matilija Sandstone, the sandstone member of the Gaviota
Formation, the Vaqueros Formation and the Sespe Formation. The lithology of these units described in detail by Dibblee (1950) is provided in Table 1, along with a modified version of the map published with his report.
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Byrd (1983) examined the geology of the Alisal Ranch in order to gain a better understanding of the boundary between the WTR province and the “Santa Maria district” to the north. Formation descriptions developed by Dibblee were followed but more detail was added to the stratigraphic sections of each formation, such as clast counts performed at seven localities in the Sespe formation. A geologic map was produced at a scale of
1:12,000, covering approximately one third of the Nojoqui study area.
Table 1: A comparison of bedrock descriptions by workers in Nojoqui.
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2.4 Surficial Boulder Deposits
2.4.1 Fanglomerates
Previous workers describe surficial boulder deposits in the study area as
fanglomerate composed largely of sandstone boulders (Byrd, 1983; Dibblee, 1950).
Similar deposits have been described in greater detail in the vicinity of Santa Barbara and
Goleta as older alluvium comprised of both debris-flow material and fluvial deposits,
with proximal fan material dominated by debris-flow deposits consisting of poorly sorted matrix-supported sediment that includes boulders up to 3-4 m in diameter (Zepeda 1987).
Similarly, a well- studied fan deposit at the Santa Barbara Mission is easily recognizable by an abundance of large boulders on the landscape, commonly referred to as the
“Mission debris flow” (Selting and Keller 2001) and “Mission diamicton” (Urban 2004).
The terminology used above to describe boulder deposits similar to those in this study is variable and includes: ‘fanglomerate’ (Byrd 1983, Dibblee 1950); ‘older alluvium’ (Zepeda 1987); and ‘diamicton’ (Urban 2004). For consistency this author will use the term ‘fanglomerate’, as it is applied generally to conglomerates and breccias deposited on alluvial fans (Allaby 2013) and is therefore encompassing of the latter two terms.
The mechanism of transport responsible for depositing boulder-size detritus on alluvial fans is typically debris flow, defined by Costa (1984) as a mobilization of poorly sorted rock and soil debris from hillslopes and channels by the addition of moisture.
Surficial deposits of coarse, poorly sorted lateral levees and terminal lobes on fans and bordering channels, coupled with a coarsening-upward subsurface profile, are
10 depositional structures characteristic of debris flows. Geomorphological evidence is presented as gentle slopes and large fan-shaped deposits of open-framework boulders.
Many fan-shaped deposits are mapped and described by previous workers in the foothills above Santa Barbara and Goleta (~40 km east of Nojoqui). Deposits date between approximately 1ka (Urban 2004) and 125 ka (Selting and Keller 2001), with a calculated recurrence interval of ~1 ka (Urban 2004). Keller et al. (2015) describe Santa
Barbara as an example of a debris-flow dominated landscape in a tectonically active setting in which rapid uplift and incision of fan material are major factors (along with episodic intense precipitation, earthquakes, and wildfire) producing a series of high magnitude debris flows such as the event which took place in Montecito on January 9th,
2018.
2.4.2 Fluvial Deposits
There has been no detailed analysis performed on modern fluvial deposits in the study area, although the nearby Santa Ynez River is well documented and is the closest modern depositional environment for large boulders. The Santa Ynez River runs east- to west approximately 8 km (5 mi) north of Nojoqui, and is the largest river in the WTR with a drainage area of 2,320 km2. Many boulders can be seen in the modern channel of the Santa Ynez River north of Nojoqui.
A report by Upson and Thomasson (1951) for the United States Department of the
Interior provides detailed lithologic descriptions of terrace deposits, younger alluvium, and modern channel material adjacent to and in the Santa Ynez River. The ‘Lower
Member of the Younger Alluvium’ was found to contain a ‘lower gravel member’
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consisting of “coarse gravel containing cobbles and boulders, doubtless some sand, but
very little silt or clay” (Upson and Thomasson 1951). Data from well logs near Buellton
record a total thickness of 10 m to 26 m (32 to 86 ft) of Younger Alluvium, and 3 m to 18
m (10 to 60 ft) of the lower gravel member. The report suggests these sediments were
deposited during the global rise of sea level accompanying the retreat of glaciers from
their last glacial maximum approximately 25 ka. This evidence is based on well logs
which show alluvial deposits resting on Tertiary bedrock nearly 61 m (200 ft) to 46 m
(150 ft) below sea level and 8 km (5 mi) upstream from the present shoreline.
Fluvial deposits are generally better-sorted than debris-flow deposits. As stream velocity slows, the ability of a stream to transport sediment decreases and sediments are deposited in order by size. Deposition occurs when the flow velocity falls below the settling velocity of a particle (Wohl 2014, 115, referencing Hjulstrom 1935). The consistency of energy in a depositional environment determines effectiveness and ultimately sorting of sediments. In this region of California rivers tend to have variable flow velocities owing to seasonal effects on discharge, as well as a broad range of available sediment sizes, therefore resulting in deposits that are generally poorly sorted
and include clast sizes from large boulders to silts and clay. Coarser sediment in these
channels may show imbrication or form linear, step, and bar bedforms as described by
Wohl (2014, 108-114). Similar to debris flows, fluvial deposits can form lobate structures
but lack lateral levees (Costa 1984).
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2.5 Climatic History
There are no long-term paleoclimate indices for the Santa Ynez Valley, however pollen data from ocean sediment cores taken from the nearby Santa Barbara Basin were analyzed by Heusser (1995), and reveal the best chronologically controlled record of climate for the last 160 ka. Pollen assemblages suggest that the region’s climate was much wetter and cooler than current conditions during a glacial period from 71- 57 ka, and shifted to very wet and cold from 29-14 ka (Heusser, 1995).
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Chapter 3: Physical Setting
3.1 Landscape
Topographically the study area is characterized by the steep northern flank of the
Santa Ynez Mountains and low hills dissected by ephemeral streams. Nojoqui Valley runs through the center of the field area. It is broad and relatively flat, composed of approximately 46 m (150 ft) of alluvial fill (personal well log), with rich topsoil well- suited for farming. Gentle rolling hills surround the valley covered with soils that support mostly grasses and chaparral, with large mature oak trees lining the canyons and creeks.
On the steep slopes of the Santa Ynez Mountains the vegetation is extremely dense.
Thick impassible chaparral and manzanita grow along the ridges and south-facing slopes of canyons. Forests of oak and bay trees support an undergrowth of ferns that cover north-facing slopes, characterized by tall cliffs of Matilija Sandstone and deep valleys that experience little to no direct sunlight year-round.
The study area covers 22.5 km2. Approximately one third of this area lies within the Canada de la Gaviota watershed, emptying south into the Pacific Ocean. The remainder of the area lies within the Nojoqui Creek watershed which drains north into the
Santa Ynez River (Figure 1).
Nojoqui Creek is the largest stream running through the study area. It is fed primarily by ephemeral streams along the north slope of the Santa Ynez Mountains, and a few springs along the Santa Ynez Fault. Nojoqui Creek begins in the southeast corner of the study area. It flows north approximately 2 km before reaching the base of the Santa
Ynez Mountains where its course makes a 90 degree turn to flow west. It continues in
14 this direction for another 4 km (2.5 mi) before making another 90 degree turn to flow north again for 8 km (5 mi) where it empties into the Santa Ynez River. The north boundary of the study area cuts across the area at this transition. Alisal and Quiota Creeks directly east of Nojoqui share similar drainage geometries (Figure 2).
Cañada de la Gaviota feeds Gaviota Creek and is the second largest drainage in
Nojoqui. It begins at the head of an east-west striking canyon southeast of Nojoqui
Summit. It flows west for 3 km (1.8 mi) before turning south to reach the Pacific Ocean
7.5 km (4.6 mi) away.
Figure 2: Nojoqui Creek and Cañada de la Gaviota watersheds within Nojoqui study area.
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3.2 Anthropomorphic Influence on Landscape
The most significant anthropogenic change to occur in the Nojoqui valley was the construction of U.S. Route 101 in the mid 1920s. Prior to that time the Old Coast
Highway followed the old stagecoach route, winding its way through the natural topography. The construction of Nojoqui grade carved and filled the drainages south and north of Nojoqui summit, destroying the original channel geometry and, presumably, the presence of boulders. The grading of roads on local ranches has also displaced boulders from their original location, however only slightly. This is accounted for in data collection.
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Chapter 4: Methods
4.1 Developing a Basemap
To initially map the geographic extent of the deposits and define areas of focus, groups of boulders were located remotely using Google Earth Pro (GEP). GEP provided the highest resolution images publicly available for the large Nojoqui area over multiple time periods. Access to high-resolution historical imagery was critical. For example, seasonal variation in the imagery is especially important because the boulders are camouflaged by the dry grass in summer and fall, but stand out and are easily identifiable against the green grass in spring.
Boulder deposits identified using the satellite imagery were outlined for reference using the polygon tool in GEP. Each region was field checked by referencing a printed copy of the satellite imagery and the locations of boulders were mapped with more detail and accuracy. The corrected location of boulders was then re-entered into GEP as part of a new layer.
4.2 Determining Boulder Locations
An iPhone was used to accurately locate myself using the Google Earth app and a
USGS topographic map overlay (http://www.earthpoint.us/). I discovered the GPS location device on my iPhone to be consistently more accurate than a handheld Garmin
GPS device. Both the Garmin device and the iPhone have the ability to acquire GPS information offline, and without cell service; however, the iPhone has access to better imagery and was able to consistently provide a location with an error of less than 0.5m.
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The GPS device gave errors of more than 1m making it difficult to map the precise locations of boulders with a diameter at least that size. The camera in the phone also stores GPS data with each image which proved helpful in organizing and recording important information.
A USGS Solvang 7.5’ Quadrangle topographic map and Brunton compass were used in the field for location purposes when the iPhone was insufficient. The topographic map was additionally used to draft a hardcopy map of the boulder deposits and note stations during field work.
4.3 Identifying a source for the boulders
The lithology of the boulders was analyzed to determine their source. Several outcrops in Nojoqui were analyzed for comparison with the boulders. This was performed using a hand lens to identify grain compositions, and a rock hammer to compare fresh and weathered surfaces on the rocks.
Each formation cropping out in the Santa Ynez Mountains above the boulder deposits in Nojoqui was investigated to account for any oversight by previous workers.
Special attention was paid to any regionally described formation said to contain a basal conglomerate.
Surficial boulders represent either allochthonous deposits or autochthonous products of weathered bedrock. Sandstone outcrops along the ridges and peaks of the
Santa Ynez Mountains commonly exhibit spheroidal weathering. A boulder resting on bedrock that is compositionally different was determined to be transported to that position, and therefore allochthonous.
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4.4 Distribution of the boulders
Field observations were essential because a large portion of the boulder deposits
are located under trees and not visible using remote techniques, therefore requiring a
traverse of the region on foot. For ease of reference, the study area was divided into four
sections and named after relation to respective property or topography: A) Nojoqui
Ranch; B) Forty Acre Ranch; C) Alisal Ranch; and D) the North Slope (Figure 3).
Varying levels of detail were used to record the spatial distribution of boulder size and
lithology in each section.
At Forty Acre Ranch a highly-concentrated 900 m2 group of boulders was gridded
and surveyed by hand drafting a detailed map of the accurate size and orientation of each
boulder. This technique helped decide that a broader survey with less detail was best for
the study. In lieu of the grids, stations were created at locations with notable geomorphic features and distinct depositional patterns were recorded through hand-sketches of
boulder size and orientation.
At Nojoqui Ranch and Forty Acre Ranch, a total of 68,000 m2 of boulders
clustered into groups were delineated and the number of boulders of each size was
counted using the method of Ingram modified from Wentworth (Figure 4). Boulder sizes
are classified as: Small (256-512 mm), Medium (512-1024 mm), Large (1024-2048 mm),
and Very Large (2048-4096 mm). The composition of the boulders was also recorded.
The sizes and compositions were tallied and then entered into Google Earth Pro. This
technique was used to see how various sizes were distributed across a sample area of both
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sections, specifically looking for sorting associated with specific depositional environments (refer to sub-chapter 2.3 of this paper).
Distribution of boulders across the remainder of the study area was recorded by mapping the areal extent of surficial deposits on printed satellite imagery and a USGS
Solvang 7.5’ Quadrangle Topographic Map. The locations of boulders over 2 m in diameter were plotted owing to the more specific nature involved in the deposition of such large clasts.
Additional field work was performed in Montecito with special permission from
Santa Barbara County. This was a rare opportunity to observe a modern deposit of boulders with similar size and scale of distribution to those in Nojoqui. A collection of photographs and notes were collected near the source of the debris-flow in Cold Springs
Canyon, and at two locations down-fan along San Ysidro Creek at Glen Oaks Drive and
San Leandro Lane. This information was used to compare and contrast boulder deposits in Montecito with those at Nojoqui.
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Figure 3: Boulder deposit distribution and associated sections. A) Nojoqui Ranch; B) Forty Acre Ranch; C) Alisal Ranch; and D) the North Slope
21
Figure 4: Grain-size chart used to classify clast size (Roy L. Ingram, Modified Wentworth scale).
22
4.5 Geomorphic Analysis
A map of watersheds bordering Nojoqui was traced over a 1:125,000 USGS topographic map. A 1905 edition of this map was used to rule out anthropomorphic influences on drainages, such as the construction of a massive five-lane freeway on either side of Nojoqui Summit. This was performed to assess the shape of the Nojoqui drainage in the context of a tectonically active landscape. A good understanding of this relationship is important for determining why boulders are distributed outside of channels on hilltops and ridges.
23
Chapter 5: Results
5.1 Lithologic Description
The lithology of the boulders was analyzed to determine their provenance. There are two boulder types in the study area: buff-colored arkosic sandstone and reddish- brown pebble-cobble conglomerate. Based on the lithologic descriptions and geologic maps, the Vaqueros Formation, Sespe Formation, Gaviota Sandstone, and Matilija
Sandstone were identified in this study as potential sources (Table 1).
5.1.1 Arkosic Sandstone
The composition of arkosic sandstone boulders is consistent across the study area and is as follows: Sandstones are well cemented; fresh surfaces are a light buff color, weathered surfaces are gray- buff; medium to coarse grains composed primarily of feldspars and some quartz display well to moderate sorting, and are angular to sub- rounded while coarser grains tend to be more angular. The boulders appear to be mostly massive, although pock marks/ bowls and holes appear on the tops and sides of some clasts over 2 m in diameter, likely caused by chemical weathering from pooled water and variable permeability throughout the rock. Boulders appearing partially buried/sunk into the landscape are more spherical, while boulders that are less buried and more exposed at the surface are more angular. The surfaces are spotted with orange, yellow, black, light greenish gray, and chartreuse lichen growths (Figure 5.1a, 5.1b). Exposures of freshly exposed buried surfaces are buff to light yellowish orange, and subrounded (Figure 5.1c).
24
Figure 5.1a: Partially buried round arkosic sandstone boulder with pock marks and varicolored lichen. White square label is 7.62 cm (3 in) wide. Section B, Group 1.
25
Figure 5.1b: Angular arkosic sandstone boulders are less buried. Section A, Group 1, view east.
26
Figure 5.1c: A boulder recently moved from its in situ position exhibits both fresh and weathered surfaces. Section D, Station 4.
27
5.1.2 Conglomerate
The composition of the conglomeratic boulders is consistent across the southwest portion of the field area where they are found. Conglomerates are very well-cemented; weathered surfaces are reddish brown, and spotted with lichen similar to the arkosic sandstone boulders (Figure 5.1d). Conglomeratic boulders shaded by oaks are almost entirely covered with gray and light green lichens (Figure 5.1e). Clasts are composed of well-rounded to rounded quartzite and felsic volcanic fragments, and subangular red and green Franciscan chert (Figure 5.1f). Quartzites are white, gray, and pink to purple. Felsic volcanic clasts are white or pink, and usually porphyritic. Clast sizes range from medium pebbles to large cobbles set in a matrix that is medium to coarse-grained arkosic sandstone. Approximately 50% of boulders appear partially buried/sunk into the landscape to some degree. Their weathered surfaces are subangular-to-subrounded.
28
Figure 5.1d: Varicolored lichens on small conglomeratic boulder. Section B, Group 1.
Figure 5.1g: Open-framework large to very large conglomerate boulder pile in south Section A.
29
Figure 5.1e: Large 2 m conglomeratic boulder covered with gray and green lichen. Section B, group 1.
30
Figure 5.1f: Very large (>1.8 m) conglomeratic boulder with large quartzite and rhyolitic cobbles weathering from its base. Reddish pink color of a separate conglomerate clast seen at bottom of image. Both conglomerates are in contact with arkosic sandstone boulders. Section A
5.2 Boulder Distribution
Boulder deposits are located along the low foothills and base of the Santa Ynez
Mountains forming a southwest-northeast trending strip no more than 2 km wide (Figure
3). Boulders are generally found northwest of the main branch and south branch of the
Santa Ynez Fault. This general pattern holds true across most of the study area except at two locations east and west of Nojoqui Falls Park where the linear trend is interrupted by a large alluvial deposit comprising the valley floor. Arkosic sandstone boulders are found throughout the study area, and conglomeratic boulders are found only west of the creek at
Nojoqui Falls Park. In addition to boulders, peculiar float deposits of well-rounded
31
quartzite cobbles are present in the absence of conglomeritic boulders. Open-framework
boulder piles are associated with the presence of mature oak trees, and widely spaced
deposits are associated with grass fields.
To describe boulder distribution in greater detail, the study area is divided into
four sections: A) Nojoqui Ranch; B) Forty Acre Ranch; C) Alisal Ranch; and D) the
North Slope (Figure 3). Descriptions for each section are outlined below.
5.2.1 Nojoqui Ranch (A)
A low boulder-covered ridge trending east-west (Figure 6) is located at the north end of Section A, Nojoqui Ranch (Figure 7). The east end of the ridge is ~10 m across and widens gradually to 70 m over a distance of 500 m. The top of the ridge is flat and stair steps down to the west three times midway along its strike. Notable depositional patterns (sedimentary structures) are present here and include: linear distribution of very large boulders parallel to strike, medium boulders perpendicular to strike, and medium to large boulders at angles away from the ridge crest in a down slope direction; S-shaped and V-shaped trails of same-size boulders; and nested lobate rings of same-size boulders, with outer rings comprised of larger clasts relative to inner rings (Figure 8a, b, c). Linear structures occur over the entirety of the ridge and nested lobes and sinuous trails over the western half. Boulders here were examined in detail by counting every clast to record a size and lithology in order to determine provenance and any differences from deposits in other sections (Table 2). At the west end and south edge of this ridge, boulders are observed resting unconformably over a bedrock exposure of the Anita Shale.
32
From the west end of the ridge the boulder deposits continue southwest 600 m to form a large fan-shaped deposit. The fan-shaped deposit covers a broad south-facing slope. Very large boulders are deposited along the top of the fan where the slope breaks to the south, and linear and sinuous depositional patterns become less distinctive down slope.
Conglomerates are not found along the middle portion of this section but appear again at the southern end of the study area as piles of very large boulders up to 4 m in diameter (Figure 5.1f and 5.1g). Large rounded quartzite cobbles are found within the conglomerate and as adjacent float material.
33
Figure 6: Boulder Ridge in Section A. Numbers indicate boulder groups in Table 2.
34
Figure 7: Nojoqui Ranch. Section A.
35
Figure 8a: Nojoqui Ranch. Section A, Group 1, view west.
36
Figure 8b: Section A, Group 12, view west. Nested lobate ring structures step west down slope along the spine of the ridge.
37
Figure 8c: Section A, Group 13, view west.
Figure 8d: Section A, Group 17, view east. V-shaped boulder trail points west down slope.
38
Table 2: Section A clast count on Nojoqui Ranch ridge.
39
5.2.2 Forty Acre Ranch (B)
A north-south trending ridge of boulder deposits form a topographic high that is
surrounded by alluvium on all sides, except on the south where it meets the mountain
front. The distribution of boulders in this section resembles a tear-drop that begins
narrowly and widens north into the valley (Figure 9). The southern area of this section
begins in a shallow strike valley formed by the Santa Ynez Fault, continuing as a narrow
strip that wraps from the ridge top down to the west flank of the ridge and east flank of a
small canyon. At this location the distribution area broadens to encompass all sides of the
ridge, and the tear-drop ends on the north where Nojoqui Creek meanders around its bottom edge. Open framework piles of large- very large boulders, lone very large arkosic sandstone boulders, and conglomerates were found throughout the Forty Acre Ranch section. Sedimentary structures include S-shaped trails, triangles, and nested rings
(Station 2 and 3, Figure 9).
40
Figure 9: Section B, Forty Acre Ranch. Numbers indicate boulder groups in Table 3.
A boulder count and detailed grid survey was performed on the north half of the
Section B to determine differences or similarities to other sections (Table 3). A highly- concentrated 900 m2 group of boulders (Figure 9, Station 6) was gridded and surveyed
and a detailed field map drafted of the accurate size and orientation of each boulder
(Figure 10). The long axis of boulders show a general trend of north-south with the
41 exception of one 4 m boulder with east-west orientation. Conglomerates are clustered in the southern half of the grid and are .5- 2 m in size.
Large rounded quartzite cobbles and medium angular cobble-sized clasts of
Franciscan Chert are found on the hilltop at Station 2.
42
Table 3: Section B clast count
43
Figure 10: Field map from a detailed grid survey of 900 m2 boulder group, Section B, Station 6.
44
5.2.3 Alisal Ranch (Section C)
This section covers a large area of boulders distributed east-west along the mountain front, north of the Santa Ynez Fault. Broad grassy slopes and tree covered ridges are cut by three north-south drainages that flow into Nojoqui Creek (Figure 11).
Boulders deposited on broad gentle slopes show distinct depositional patterns of nested lobate rings and v-shaped, linear, and S-shaped trails (Figure 11, Stations 14, 17, 20, 11,
12). Detailed drawings of the deposits were recorded in the field (Figure 12a). Ridge tops are covered with nested rings and ridge flanks have linear trails angling away from the ridge crest down slope (north). Very large boulders up to 4 m in diameter are located at the slope break between gentle slopes and the steep walls of the modern drainages
(Figure 12b). The drainage at Station 6 has a notable absence of boulders in its channel,
differing from the boulder lined channels of both the drainage at stations 4 and 9. Linear
deposits of small to very large boulders are deposited perpendicular to the channel at
Station 17 (Figure 12c). Open-framework piles composed of small to very large boulders
are located at the toe of north-south ridges that terminate at Nojoqui Creek (Figure 11,
Station 13).
The boulders on Alisal Ranch are composed entirely of arkosic sandstone and are
determined to be different compositionally from sandstone outcrops of the Vaqueros and
Gaviota Formations exposed in this section (Table 1).
45
Figure 11: Section C, Alisal Ranch
46
Figure 12a: Detailed drawings of boulder deposits recorded at Alisal Ranch (Section C), Stations 1, 4, 10, 11, 12, 14, 15, 17, 18, 20, 24
47
Figure 12b: Section C, Station 18. Arkosic sandstone boulders are aligned from lower left to upper right side of picture. View north downslope.
Figure 12c: Section C, Station 17, view southwest upstream.
48
5.2.4 North Slope
This section encompases the steep north slope of the Santa Ynez Mountains south
of the Santa Ynez Fault. Relatively small patches of medium to large, sub-rounded
arkosic sandstone boulders are located on flat ridge tops in an area south of Nojoqui Falls
Park. Deposits appear to continue into dense impenetrable brush at Station 4 (Figure 13).
A single large well-rounded quartzite cobble was located as float along a fire road at Station 2. Upslope at an elevation of 667 m (2200 ft), similar clasts were found, and one buff colored medium-sized conglomerate boulder was also discovered lying in the road at this location (Figure 14). Very large piles of angular arkosic sandstone boulders were found at Station 6 (Figure 15), 91 m (300 ft) below a thick resistant bed of Matilija
Sandstone that forms the ridgeline of the Santa Ynez Mountains. At the ridge, fractured bedrock and chemical weathering form massive sandstone pillars that lean slightly away from the outcrop, north over the vertical face of the mountainside. Boulders at this location are determined to be autochthonous based on observing bedrock in a mature stage of spheroidal weathering in contact with boulders of identical lithology (Figure16).
49
Figure 13: Section D, the North Slope
50
Figure 14: Conglomerate at Station 5, Section D. Pebbles are composed of chert, quartzite, and rhyolite
51
Figure 15: Section D, Station 6. Very large (> 2 m) arkosic sandstone boulders 91 m (300 ft) below the ridge of Santa Ynez Mountains.
52
Figure 16: Section D, Station 7. Matilija Sandstone outcrop forms the ridge of the Santa Ynez Mountains. Spheroidal weathering produces in situ (autochthonous) boulders of arkosic sandstone. View northwest.
53
5.3 Geomorphology
5.3.1 Drainage Geometry
Several tributaries of similar length and catchment size (~1 km2) flow north from the crest of the Santa Ynez Mountains into Nojoqui Creek. Tributaries flowing south into the creek are smaller with the exception of one double in size. At Nojoqui Valley, the west-flowing Nojoqui Creek turns acutely to flow north towards the Santa Ynez River.
Although the study area does not cover the full watershed of Nojoqui Creek, analysis of its basin geometry is important for understanding the morphology within the field area. A drainage map drafted over a USGS 7.5’ quadrangle reveals that the north-flowing segment of Nojoqui Creek has high-order streams connecting to Nojoqui Creek at acute angles pointing upstream (Figure 2). This reverse geometry normalizes 2 km from its mouth at the Santa Ynez River.
The watershed of Cañada de la Gaviota Creek begins 100 m higher and to the south of Nojoqui Creek. It deviates from its east-west trend in alignment with Nojoqui
Creek, but flows south in the opposite direction towards the Pacific Ocean. The south- flowing section is assumed to have been greatly disturbed by the late 1920’s construction of U.S. Route 101 which forms a wall along the right bank of the channel. A 1:125000 scale USGS topographic map published in 1905 is the only indication of the natural stream path.
5.3.2 Erosional Surfaces
Nojoqui Creek flows along a contact between resistant sandstone beds in the
Gaviota Formation and alluvium comprising the broad, flat floor of Nojoqui Valley. West
54 of Nojoqui Park the creek begins to incise several meters into its channel, exposing in cross section a view of the boulder deposit at Forty Acre Ranch and the alluvium which fills the valley. At the north end of Forty Acre Ranch, an angular unconformity between the Sespe Formation and a fanglomerate deposit is exposed in the left bank of the creek
(Figure 17). The fanglomerate is composed of angular to subangular boulders and cobbles of arkosic sandstone. A few meters downstream the fanglomerate ends and alluvium composed of silts and pebble lenses overlies the same green siltstone of the
Sespe Formation. The top of the alluvium forms the broad surface of Nojoqui Valley
(Figure 18, “Qa”). Chip samples observed at drill sites show sediments up to 60 m (200 ft) thick.
Above the alluvium another surface is observed. At Nojoqui Ranch, Forty Acre
Ranch, and the areas north of Nojoqui Creek on Alisal Ranch, boulder deposits tend to be most concentrated on flat-topped ridges and hills ~ 24 m (80 ft) above adjacent stream channels and valley bottoms.
South of the Santa Ynez Fault in the canyon above Nojoqui Falls, an angular unconformity between the Anita Shale and overlying fanglomerate is exposed at an elevation of 363 m (1200 ft.) (Figure 19a,b,c). The fanglomerate is slightly finer grained and contains more matrix material than the deposit exposed in Nojoqui Creek. In Section
A a similar unconformity is observed between boulder deposits and the Anita Shale.
55
Figure 17: Section B, Station 5, left bank of Nojoqui Creek. Unconformity between shale member of Oligocene Sespe Formation and boulder deposits interpreted to be fanglomerate.
56
Figure 18: Geologic map after Dibblee (1950) showing boulder deposits in yellow.
57
Figure 19a: Section D, Station 4, view east at road-cut. Unconformity between middle Eocene Anita Shale and boulder deposits interpreted as fanglomerate.
Figure 19b: Section D, Station 2. Cross-section of boulder- cobble deposits interpreted as fanglomerate. View south.
58
Figure 19c: Section D, Station 3, view west at road-cut. Angular unconformity between south-dipping middle Eocene Anita Shale and small boulder- cobble deposits interpreted as fanglomerate.
59
Chapter 6: Discussion
6.1 Lithology and Bedrock Source
The arkosic sandstone boulders are interpreted to be weathered from the Matilija
Sandstone based on a comparative analysis of local outcrop and boulder composition.
The Matilija Sandstone was observed in outcrop along the ridge of the Santa Ynez
Mountains. A combination of angular blocks and sub-rounded boulders match with the size and shape of allochthonous deposits across the study area, further evidence that the
Matilija Sandstone is the source.
The source of the conglomerate boulders remains unsolved. Initially the
conglomerates were hypothesized to be derived from the basal member of the Sespe
Formation due to a similarity in composition and color, and proximal exposure to the
study area. Compositionally, the Sespe Formation in the vicinity of Nojoqui differs from
its regional characterization as reddish cobble to pebble conglomerates composed of well-rounded quartzite, volcanic rock, and Franciscan chert. Detailed lithologic descriptions at multiple locations on the Alisal Ranch by Byrd (1983) show clasts composed primarily of mafic material. Outcrops along Alisal Road and in cut-banks of some drainages on Alisal Ranch crumble and are greenish-gray. Clasts in the conglomeratic boulders of Nojoqui are felsic and boulders are well-cemented.
Conglomeratic outcrops in the Vaqueros Formation were observed in Nojoqui Valley along the east side of U.S. Route 101, and on the Alisal Ranch at the reservoir. It is greenish tan, poorly consolidated, and contains a high percentage of mafic material.
These observations indicate a poor source match for the conglomerate boulders.
60
The presence of felsic conglomeratic material in a dirt road which cuts through
outcrop of Matilija Sandstone (Figure 13, Station 5) may be evidence that a basal
conglomerate exists locally within the formation, although no convincing outcrop has
been observed. Dibblee (1950 p. 27) briefly mentions a similar deposit in San Lucas
Canyon, 17 km east of Nojoqui: “The basal portion [of the Matilija] locally contains an
algal reef and some rounded cobbles of quartzite and granitic rocks”. If a basal
conglomerate in the Matilija Sandstone is the source for conglomerate boulders in
Nojoqui, it must be very localized and ~3 m thick to account for boulders of that size.
The clast composition described by Dibblee in addition to observations of float-material composition, conglomerate boulder composition, and distribution among arkosic sandstone boulders derived from the Matilija Sandstone, point to the Matilija as the most likely source for conglomerate boulders.
Future field work across the difficult terrain of the north slope of the Santa Ynez
Mountains above Nojoqui is needed to confirm the existence of a basal conglomerate in
the Matilija Sandstone. It is important to understand that the structural complexity of this
area makes plausible the idea of terrain rafted from the east along left lateral motion on
Santa Ynez Fault.
6.2 Distribution and Depositional Environment
The distribution of boulder deposits exclusively at the base of the Santa Ynez
Mountains and along the margins of Nojoqui Valley and Cañada de la Gaviota, is
evidence that the orientation of canyons and ridges contributed to the transport and
deposition of the boulders. Boulder deposits located high above the valley floor in the
61
Santa Ynez Mountains, but absent high in the hills north of Nojoqui, suggest that deposition and transport came from the south in the mountains or east through the valley.
Conglomeratic boulders are not deposited east of the drainage at Nojoqui Falls
Park. Approximately six large conglomeratic boulders were observed in the creek
draining from the waterfall, but no other conglomerates were observed in the modern creek bed upstream from waterfall. The only sign of conglomeratic debris is the large single quartzite cobble described at Station 2 in the lower half of Section D. If a basal conglomerate in the Matilija Sandstone does exist, it must begin west of this location as
there is no sign of it being part of the present drainage.
The Nojoqui boulders are interpreted to be deposited by debris flows based on the presence of sedimentary structures associated with such events. Nested rings and lateral boulder trains described in this paper are interpreted to be lateral levees and terminal lobes as described by Costa (1984).
The catastrophic debris flow event which took place in Montecito on January 9th,
2018, provides the strongest evidence supporting the deposition of Nojoqui boulders by a
similar process. Aerial footage taken hours after the event show depositional patterns
identical to those observed in Nojoqui. Special access to debris flow deposits in
Montecito one week after the event revealed further evidence confirming strong
similarities to the Nojoqui deposits (Figure 20a and 20b).
62
Figure 20a: Very large (>2 m) boulder levee deposits in Montecito, CA. These deposits are similar to those found in Section C, Station 18.
Figure 20b: Medium-sized (.5 m- 1 m) boulders aligned perpendicular to flow direction and ring-shaped lobes deposited from the Montecito debris flow.
63 Flow direction can be determined from terminal lobe deposits. Terminal lobe deposits identified in Nojoqui indicate material was moving north away from the mountain front in multiple fingers similar to deposits in Montecito, which began as channelized deposits and then fanned out over the flat coastal plain. In sections A, B, and
C where terminal lobes are found, they are deposited on the tops and over the noses of ridges, in addition to being present in relatively flat areas. Fanglomerate has been mapped on ridges above Santa Barbara and interpreted as remnants of uplifted and dissected fan deposits. I apply this interpretation to deposits in section D on the south side of the fault, however I suggest debris-flow deposits were draped over shutter ridges at the base of the mountains owing to left-lateral motion on the Santa Ynez Fault, to explain deposits on noses and ridges. Drainages with a catchment capable of producing the volume of material deposited as levees (Figure 11, Station 15 and 19) are offset from the boulder deposits on ridges in a direction consistent with previous work detailing left lateral movement on the Santa Ynez fault. Linear deposits of boulders over 2 m in diameter are located parallel to the strike of modern drainages and interpreted to be levee deposits from more recent flows which occurred prior to offset of drainages. Open framework boulder piles near the base of the mountains at Nojoqui Creek are interpreted as terminal deposits which accumulated in the paleo valley where they could no longer move across the landscape. The deposit on the low ridge in section A is long and narrow with several sedimentary structures indicating transport of boulders from east to west. I propose this narrow ridge of debris flow deposits represents inverted topography of a channelized flow that is now more resistant to weathering than the surrounding bedrock of Anita
Shale. At the end of the ridge the channel opened up and fanned out down Cañada de la
64
Gaviota. I interpret a gap in boulder deposits between the bottom of the ridge flow and boulders in the far south end of the field area to be from left lateral offset on the Santa
Ynez Fault.
6.3 Timing of Events
The boulder deposits can be dated relative to their offset along the Santa Ynez fault and depositional relationship with bedrock and alluvium. I propose a timeline of events in Nojoqui Valley with supportive evidence as follows:
1. Erosion of Anita Shale and Sespe Formation surfaces exposed in cross section at
Section A, Section B (Station 5), and Section D (Station 2, 3, and 4).
Stratigraphically, these are the lowest surfaces observed in the study.
2. Deposition of boulders in Sections D, B, and A. All three deposits contain
conglomeratic material either in float or as surficial boulder deposits, and show
~400-500 m of left lateral displacement. This amount of offset tentatively
correlates with the findings of Wampler (2014), who suggests a rate of motion
along the south branch of the Santa Ynez Fault at 1.5 mm/yr with ~500- 800 m of
displacement.
Deposition of boulders at Section C. Two events are suggested to have occurred
in this area. Boulder levees along margins of major drainages are deposited prior
to lateral offset of ~ 200 m (Station 15 and 18). The second event is responsible
for boulders that appear to have been deposited on shutter ridges blocking
drainage mouths.
3. Fluvial transport and removal of material connecting deposits north and south of
Nojoqui Creek. Boulder deposits are perched above the valley floor on either side
65
of Nojoqui Creek which suggests erosion of larger sediments and incision of the
valley.
4. Aggradation of sediment in Nojoqui Valley. In Section B, alluvium rests against
debris flow boulders along the same unconformity between bedrock and surficial
deposits. Rapid uplift along the Santa Ynez fault is proposed to have occurred at
this time based on the presence of a thick, broad deposit of alluvium within a
valley whose geometry suggests a reverse flow of Nojoqui Creek to the south.
Uplift is suggested to have blocked the creek from its original course through
Cañada de la Gaviota, to explain the accumulation of sediment in Nojoqui Valley.
5. The study area is continuing to experience further erosion as evidenced by active
incision of Nojoqui Creek into the valley alluvium.
6.4 Future Work
The complex tectonic setting of the Nojoqui area requires further investigation to determine a more detailed geomorphic history and accurate timing of debris flow events.
The most important work should be to continue investigation of the conglomeratic source with detailed geologic mapping of the Matilija Sandstone. Boulder deposits mapped as fanglomerate east and west of Nojoqui should be studied for comparison to deposits discussed in this study.
Future quantitative analysis of Nojoqui should include:
1. Dendrochronology study of oak trees which grow over and within boulder deposits.
This work could provide a temporal baseline for the timing of debris flow deposits
66
following catastrophic fire similar to that experienced in Montecito, CA on January 9,
2018.
2. Employment of stream morphometric indices on the north-south trending segment of
Nojoqui Creek to gather additional supporting evidence of stream reversal. Further, a key
observation from initial morphometric work has revealed the left lateral offset of
Nojoqui, Alisal, and Quiota Creeks. With continued fieldwork and tighter constraints on
possible rates of motion along the Santa Ynez fault, the timing of debris flow deposits
and their truncation in the area may be better understood. This direction of research could
help explain the apparent headward erosion and stream capture for these drainages ~2 km
south of the Santa Ynez River.
3. Trenching of valley alluvium for datable material including charcoal from past fires
which may have created conditions conducive to debris flow events. Other possible
datable materials in valley alluvium could include palaeosoils and freshwater ponded
organics which potentially set a minimum age for abandonment of debris flow deposits and the onset of valley fill.
67
Chapter 7: Conclusion
Large boulder deposits of uncertain origin in Nojoqui Valley, Santa Barbara
County, California. were studied to explain a more complete geomorphic history of the
region based on provenance and distribution and relative timing of deposits.
Boulder composition of arkosic sandstone and conglomerate indicate the source
as the Eocene Matilija Sandstone, with outcrops and exposures found along the ridges of
the Santa Ynez Mountains adjacent to the study area. Distribution patterns of linear and
lobate structures within the boulder deposits suggest transport of boulders during
catastrophic debris-flow events.
A relative timing of events can be determined by comparing the data observed in
the study area and correlating with previous studies of the regional geologic history
during the Late Quaternary to Holocene of Santa Barbara and the Santa Ynez Mountains.
The Nojoqui boulders were deposited in multiple debris flow events. Fluvial processes
removed boulders that once were deposited in the present valley of the east-west portion
of the Nojoqui Creek drainage. Continued erosion and motion along the Santa Ynez Fault
has resulted in incision and offset of drainages along the mountain front, and deposition of at least 50 m of alluvium in Nojoqui Valley since boulder deposition. This paper suggests that uplift on the south side of the Santa Ynez fault blocked Nojoqui Creek from
original drainage into the Pacific thus causing damming in Nojoqui Valley and eventual
reversal along the north-south branch of Nojoqui Creek into the Santa Ynez River.
It is recognized that the timing and dates of these debris flow deposits are
unresolved at this time, and that the provenance of all the boulder deposits found at
68
discrete locations within the study area is only partially resolved. However, better
understanding of the geomorphic evolution of this region, which reflects the complex
tectonics associated with the Western Transverse Ranges and the Santa Ynez fault, is revealed by the provenance, distribution, and disjunct nature of these deposits.
69
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