CALIFORNIA STATE UNIVERSITY, NORTHRIDGE
Determining the Relative Age and Correlation of Emergent Marine Terraces,
Vandenberg Air Force Base, Santa Barbara County, California
A thesis submitted in partial fulfillment of the requirements
For the degree of Master of Arts in Geography
By
Eiko Barbara Kitao
August 2018
The thesis of Eiko Kitao is approved:
______
Dr. Antony Orme Date
______
Dr. Julie Laity Date
______
Dr. Amalie Orme, Chair Date
California State University, Northridge
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Acknowledgements
I would like to acknowledge my advisor, Dr. Amalie Orme and my committee members Dr. Antony Orme (UCLA Geography) and Dr. Julie Laity (CSUN Geography and Environmental Studies); Christopher D. Ryan, Cultural Resource Office GS-12
USAF AFSPC 30 CES/CEANC for allowing access and support during this study;
Danielle D’Alfonso, for her time and support; my team of research assistants, Anna
Hilliard, Maximillian Britt, Layia Asakawa-Ekeland, and Christopher Everett; the faculty and staff at Santa Barbara City College Earth Science Department; Dr. Robert S. Gray,
Daniel Muhs USGS, Denver, Colorado, and Danny Morel, UCSB.
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Table of Contents
Signature Page ...... ii
Acknowledgments...... iii
Table of Contents ...... iv
List of Figures…………..…………..………………………………………..…….…....viii
List of Tables ...... xi
Abstract ...... xii
Chapter 1: Introduction ...... 1
1.1 Statement of Purpose ...... 1
1.2 Objectives of the study...... 1
1.3 Significance of the research ...... 1
Chapter 2: Physical Setting of the Study Area...... 3
2.1 Location ...... 3
2.2 Climate ...... 6
2.3 Geomorphic Boundaries ...... 6
2.4 Bedrock Geology ...... 9
2.5 Structural Setting and Features ...... 11
Chapter 3: Scientific Background and Previous Work ...... 15
3.1 General Marine Terrace Morphology ...... 15
3.1.1 Marine Terrace Anatomy ...... 15
3.1.2 Emergent Marine Terraces ...... 17
3.2 Climate Change and Marine Terrace Development……...………...... ….….18
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3.2.1 Sea-Level Fluctuations...... 18
3.2.2 Studies of Relative Sea-level Change in California ...... 19
3.3 Previous Terrace Studies...... 21
3.3.1 Geology and Paleontology of the Santa Maria District ...... 21
3.3.2 Archaeological Investigations on the San Antonio Terrace ...... 24
3.3.3 Late Quaternary Tectonic Deformation in the Casmalia Range ...... 28
3.3.4 Correlation, Ages, and Uplift Rates of Quaternary Marine Terraces:
South-Central Coastal California ...... 32
3.3.5 Age and Deformation of Marine Terraces Between Pt. Conception
and Gaviota, Western Transverse Ranges, California ...... 33
3.3.6 New Terrace Data ...... 34
3.4 Soils and Hardpan ...... 34
3.5 Marine Terrace Deformation ...... 35
Chapter 4: Methods ...... 38
4.1 Equipment ...... 38
4.1.1 GPS Devise ...... 38
4.1.2 USGS Topographic Map & Dibblee Geologic Maps ...... 38
4.1.3 Remote Sensing (DEM, imagery) ...... 39
4.2 Field Observation ...... 39
4.2.1 Geomorphic Expression ...... 39
4.2.2 Stratigraphy of Terrace Deposits ...... 40
4.2.3 Fossils and Other Organic Material ...... 41
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4.3 Correlation ...... 41
4.3.1 Altitudinal Terrace Spacing ...... 41
4.3.2 Correlation of Marine Terrace to Global Sea-level ...... 42
Chapter 5: Results ...... 43
5.1 Casmalia Range ...... 43
5.1.1 Combar Road ...... 44
5.1.2 Point Sal Road...... 54
5.1.3 South Casmalia ...... 58
5.2 San Antonio Terrace ...... 61
5.2.1 Coastal Dunes ...... 62
5.2.2 San Antonio Mesa ...... 63
5.3 North Mesa...... 68
5.4 Burton Mesa ...... 73
5.5 Google Earth and DEM ...... 89
Chapter 6: Discussion ...... 91
6.1 Interpretation of Geomorphic Expression ...... 91
6.1.1 Geomorphic Surface ...... 91
6.1.2 Elevation of Wave-Cut Platforms and Hardpan Layers ...... 92
6.1.3 Erosional Patterns on the Geomorphic Surface ...... 94
6.2 Stratigraphy of Terrace Deposits ...... 94
6.2.1 Marine and Nonmarine Deposits ...... 94
6.2.2 Fossils and Other Organic Materials...... 96
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6.3 Structural Factors in Terrace Deformation ...... 98
6.4 Correlation of Marine Terraces ...... 100
6.4.1 Correlation of Marine Terraces to Paleo-Sea-level Stages ...... 100
6.4.2 Correlation of Terrace Platforms in the Study Area ...... 102
6.4.3 Correlation of Marine Terraces with Past Studies ...... 108
6.5 Future Work ...... 111
Conclusion: ...... 112
Reference: ...... 114
Appendix A ...... 119
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List of Figures
Figure 1: Overview map of Vandenberg Air Force Base ...... 5
Figure 2: Geomorphic boundaries of VAFB ...... 8
Figure 3: Map of major regional structures in the area ...... 14
Figure 4: Generalized diagram of emergent marine terraces ...... 16
Figure 5: View of coastal terraces looking south from Lions Rock ...... 17
Figure 6: Quaternary sea-level curve and emergent terraces on a tectonic-stable coast ...20
Figure 7: Geologic map of VAFB terrace localities by Woodring & Bramlette ...... 24
Figure 8: Geomorphic map of terrace in Johnson’s study on San Antonio Terrace ...... 27
Figure 9: Overview of Casmalia Range section ...... 44
Figure 10: Overview of marine terrace flight on Combar Rd in the Casmalia Range ...... 45
Figure 11: Purisima Point and Minuteman Terrace ...... 46
Figure 12: Stratigraphy of Orion Terrace ...... 48
Figure 13: Hardpan Surface on Orion Terrace ...... 49
Figure 14: Alluvium over Orion Terrace ...... 50
Figure 15: Stratigraphy of Summit Terrace ...... 51
Figure 16: Alluvium over Summit Terrace ...... 53
Figure 17: Hardpan surface of Summit Terrace ...... 54
Figure 18: Cliff exposures along the beach ...... 55
Figure 19: Close up of beach material overlying the wave-cut platform ...... 55
Figure 20: Lower terraces along Pt. Sal Rd ...... 57
Figure 21: Stratigraphy of S. Casmalia ...... 59
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Figure 22: Photos of terrace deposits in drainage north of Shuman Creek ...... 60
Figure 23: Overview of San Antonio Terrace ...... 61
Figure 24: Stratigraphy of El Rancho Oestre ...... 62
Figure 25: Overview of a flight of terrace platforms on San Antonio and North Mesa ....63
Figure 26: Stratigraphy of S. Shuman...... 64
Figure 27: Hardpan and terrace material exposed on south bank of Shuman Creek ...... 65
Figure 28: Stratigraphy on El Rancho near Shuman ...... 66
Figure 29: Hardpan and soil exposed on Rancho Rd ...... 67
Figure 30: Overview of North Mesa section...... 68
Figure 31: View of North Mesa of three platforms on geomorphic surface ...... 69
Figure 32: North Mesa with stations ...... 70
Figure 33: Lompoc-Casmalia Road ...... 72
Figure 34: Hardpan and mottling on Lompoc-Casmalia and Bishop Road ...... 73
Figure 35: Overview of Burton Mesa section ...... 74
Figure 36: Stratigraphy at Firefighter Rd...... 75
Figure 37: Firefighter Road and Hwy 1 ...... 76
Figure 38: Outcrop on Punchbowl Road ...... 77
Figure 39: View of bedrock interface at Santa Lucia Canyon ...... 78
Figure 40: Wave-cut platform and overlying terrace deposits on Monterey Formation ...79
Figure 41: Stratigraphy at Cross Rd. near quarry ...... 80
Figure 42: Exposure on Cross Road near the quarry ...... 81
Figure 43: Stratigraphy at Watt (bridge) ...... 82
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Figure 44: Exposure at Juan Pedro Canyon and Cross Road...... 83
Figure 45: Stratigraphy at Power Rd ...... 84
Figure 46: Terrace deposits on Power Rd ...... 85
Figure 47: Stratigraphy at Washington and 13th ...... 86
Figure 48: Hardpan exposure on 13th and Washington Ave ...... 87
Figure 49: Stratigraphy south of Mira Rd ...... 88
Figure 50: South of Mira Rd near airport ...... 89
Figure 51: Scatter graph of wave-cut platform elevations ...... 93
Figure 52: Scatter graph of hardpan surface elevations ...... 93
Figure 53: Color bar graph of lithology ...... 95
Figure 54: Fossil material collected ...... 98
Figure 55: Correlation of terraces to global sea-level curve ...... 101
Figure 56: Generalized grouping of terraces by similarities ...... 102
Figure 57: Terrace assignment on Casmalia Range ...... 105
Figure 58: Terrace assignment on San Antonio Terrace and North Mesa ...... 106
Figure 59: Terrace assignment on Burton Mesa ...... 107
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List of Tables
Table 1: Terraces in the Woodring & Bramlette (1966) study ...... 23
Table 2: Terrace numbers and Oxygen Isotope Stages (Johnson, 1984) ...... 28
Table 3: Marine terrace data, southwest and west margins of Casmalia Range ...... 31
Table 4: Generalized elevation differences ...... 99
Table 5 Terrace assignments to sea-level highstands ...... 107
Table 6: Comparison of terrace assignment by author ...... 110
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Abstract
Determining the Relative Age and Correlation of Emergent Marine Terraces,
Vandenberg Air Force Base, Santa Barbara County, California
by
Eiko Kitao
Master of Arts in Geography
Active tectonism along the California coast yields distinct and complex
landforms, useful to understanding the geomorphic history of an area. Vandenberg Air
Force Base (VAFB), located in northern Santa Barbara County, California, is located in
the western section of the Santa Maria basin, structurally bounded by fault systems along
the San Rafael Range to the northeast, the Santa Ynez fault along the Santa Ynez Range
to the south, and the Hosgri fault, offshore to the west (Woodring and Bramlette, 1966).
Five terrace platforms were identified and mapped using a combination of methods including, mapping the elevation and distribution of marine terraces, constructing a detailed stratigraphic column for overlying terrace deposits, interpretation
of fossil evidence, and observation of the geomorphic surface expression. A flight of five
terraces on Combar Road were used to correlate with other terraces in the study area.
Relative age was determined by correlating the elevation and uplift rate of
terraces to the global paleo-isotopic sea-level curve derived from oxygen isotope ratios
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(Lajoie, 1986), and paleo sea-level elevations (Shackleton and Opdyke, 1973). Terraces were assigned to MIS 3 (60 ka), MIS 5a (80 ka), MIS 5e (120 ka), MIS 7 (210 ka), and
MIS 9 (330 ka), with no evidence of MIS 5c (105 ka).
Detailed stratigraphic description of overlying terrace deposits, observation of geomorphic surface expression, and elevations of wave-cut platforms and hardpan surfaces were identified, and compared to other terraces in the study area. Fossils were collected and determined by faunal assemblage.
The two lower terraces comprise the broad, coastal platform that extends from
Point Sal to Burton Mesa. The third terrace, present as remnants on Combar Road is the most extensive terrace in the study area, correlating with the broad surfaces of South
Casmalia, San Antonio Terrace, and Burton Mesa. The upper terraces are found in the higher elevations of the study area, along the summit of North Mesa and the Purisima
Hills.
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Chapter 1: Introduction
1.1 Statement of Purpose
The purpose of this study is to map and describe in detail the geomorphic
characteristics of emergent marine terraces on Vandenberg Air Force Base (VAFB). This
research will provide a foundation for a more comprehensive history of climatic and
tectonic changes for this area during the Quaternary Period.
1.2 Objectives of the Study
The objectives of this study will focus on (1) mapping the elevation and
distribution of the marine terraces, (2) description of exposed stratigraphic sections for
the terraces extending from 2 m (6.5 ft) to 330 m (1083 ft) above modern sea-level for
this section of the California coast, and (3) description and interpretation of fossil
evidence found within marine terrace deposits.
1.3 Significance of the research
Studies of marine terraces and sand dune deposits conducted during the twentieth century along the central California coast, have offered a range of interpretations regarding high and low sea-level stands during the Quaternary Period. However, restricted access to VAFB creates a gap in data that separates studies to the north, specifically marine terraces between Santa Cruz and San Simeon and sand dune deposits along the Point Sal coastal segment, from marine terraces between Pt. Conception and
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Ventura to the south of VAFB. Past research on marine terraces conducted on VAFB has
been limited to localized areas and does not provide a comprehensive picture of all
terraces between Pt. Conception to the south and Purisima Point to the north.
This study will attempt to complete a critical part of the climatic and tectonic history of this portion of the California coast.
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Chapter 2: Physical Setting
2.1 Location
The study area on Vandenberg AFB is located along the California’s central coast
in northern Santa Barbara County and along the southern and western portion of the
Santa Maria Basin, a triangular wedge-shaped structure bounded by the San Rafael
Mountains to the northeast and the Santa Ynez Mountains to the south (Figure 1). The
study area on VAFB is divided into two segments: North Base (34°44'25.68"N,
120°34'23.61"W), and South Base (34°35'23.59"N, 120°36'27.83"W), separated by the
Santa Ynez River. North Base, the primary area of study, is a 241 km2 (93 mi2) area that
is bounded by Pt. Sal State Park to the north, the Santa Ynez River to the south, the
Pacific Ocean to the west, and California Highway 1 to the east (Figure 1).
Vandenberg AFB and its surrounding areas are located in a region of complex
geology and geomorphology. The coast, for the most part, is straight and characterized by
bold cliffs and narrow beaches to the south and an extensive, active sand sheet along the
northern part of the study area. Within this area there are three headlands of exposed
bedrock from north to south—Point Sal, Purisima Point, and Point Arguello.
Three major rivers in the study area drain into the Pacific Ocean--Shuman Creek
to the north, San Antonio Creek in the middle, and the Santa Ynez River to the south
(Figure 1). Shuman Creek drains into the ocean southeast of Pt. Sal; San Antonio Creek
drains into the ocean north of Purisima Point; and the Santa Ynez River drains into the
ocean southeast of Purisima Point. The Santa Ynez River forms a boundary between the
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western edge of the Santa Ynez Mountains to the south, and the low-lying hills and
coastal plains of Vandenberg AFB northwards to the Casmalia and Purisima Hills
(Johnson, 1984).
Two broad areas of low topography make up the San Antonio Terrace and Burton
Mesa, with extensive stabilized coastal dune-fields of multiple ages and soils overlying
marine terraces (Johnson, 1984). Channel downcutting in canyons and gullies exposes
bedrock surfaces and overlying sediments on the edges of the two areas.
On the western edge of the Casmalia Hills, broad platforms of coastal terraces
extend to the ocean. A stair-step of terraces have been cut into bedrock on the northern
section of the Casmalia Hills, with overlying alluvium and colluvium eroding into
badland-style topography. Coastal terraces are found throughout the area, extending east
into the Purisima Hills.
The eastern margin of the study area is bounded by the Casmalia Hills and the
Purisima Hills, with the highest elevation of Mt. Lospe, at 502 m (1647 ft), in the northern Casmalia Hills (Figure 1).
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5
Figure 1: Overview map of Vandenberg Air Force Base (modified from Google Earth)
2.2 Climate
The climate at Vandenberg AFB is of Mediterranean type, with mild, moist winters, and warm, dry summers. Owing to its proximity to the coast, there is a strong maritime influence, with coastal fog and predominantly onshore winds.
The average temperature range is from 11oC (52°F) in January to 16oC (61°F) in
September, with an average annual precipitation of about 380 mm (15 in) (Schmalzer,
1988). The greatest rainfall amounts occur between November and April, while summers
experience limited precipitation except as drip from coastal fog and occasional moisture
from tropical storms tracking along the eastern Pacific Ocean. The California current is a
cool ocean current that travels along the California coast from Alaska, helping to
moderate warm summer temperatures.
Vegetation is influenced by maritime conditions and consists predominantly of
coastal dune scrub, sage scrub, and chaparral on established dune surfaces. In the interior
and higher elevations, vegetation consists of grassland, oak woodland, Bishop pine forest
and tanbark oak forest.
2.3 Geomorphic Boundaries
For the purpose of this study, the base is divided into four sections based on
geomorphic characteristics (Figure 2).
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The Casmalia Range makes up the northern section of the base, including the
northern Casmalia Hills and coastal terraces, west to the ocean. This section is bounded
by Shuman Creek to the south, Pt Sal State Beach to the north, the Casmalia Hills divide
to the east, and the Pacific Ocean to the west.
The San Antonio Terrace is the area bounded by Shuman Creek to the north,
San Antonio Creek to the south, Lompoc-Casmalia Road to the east, and the Pacific
Ocean to the west.
The North Mesa is located immediately to the east of San Antonio Terrace, off
VAFB and includes the southern Casmalia Hills. This section is bounded by Shuman
Creek to the north, San Antonio Creek to the south, Lompoc-Casmalia Road to the west,
and the Casmalia Hills to the east.
The Burton Mesa is the broad, flat surface where the main VAFB facilities are located. It is bounded by San Antonio Creek to the north, Santa Ynez River to the south, the Pacific Ocean to the west and the Purisima Hills to the east. The eastern edge of this boundary is off base where it crosses Hwy 1.
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Figure 2: Geomorphic boundaries of VAFB (modified from Google Earth)
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2.4 Bedrock Geology
A comprehensive geologic report of the Southwestern Santa Barbara County was
written by Dibblee (1950) for the California Division of Mines, with a detailed geologic
history of the area. Subsequent detailed maps by Dibblee (1989) provide the foundation
for understanding the lithologic framework of the marine terraces, shore platforms, and
overlying terrestrial and nearshore deposits.
Lithologic descriptions and their mapped distributions include:
The Jurassic Point Sal Ophiolite (opd) is an ultramafic to mafic intrusive igneous
suite composed of gabbro, peridotite, pyroxenite, harzburgite, wherlite, and dunite,
slightly metamorphosed or altered. Exposures located on the north of Lions Head Fault,
west of the Casmalia Range are the only known exposures in the area.
The Oligocene-Miocene Lospe Formation (Tlo) comprises reddish to greenish-
gray claystone, sandstone, and pebbly sandstone or green to reddish conglomerate and
sandstone composed primarily of Franciscan rock detritus. A thick deposit of white,
hard, rhyolitic tuff-breccia is found throughout the unit. Exposures of the Lospe
Formation are found north of Lions Head Fault overlying the ophiolite suite along the
coast and in the Casmalia Range.
Miocene Intrusive Rocks (Td) comprise black fine to medium grained diabase and andesite, composed of calcic or sodic plagioclase and pyroxene.
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The Miocene Point Sal Formation (Tps) is a tan, soft, thin-bedded, silty shale with
thin beds of sandstone and yellow-brown dolomite. Exposures are located on the western
flanks of the Casmalia Range north of Lions Head Fault.
The Miocene Monterey Shale (Tm) is an extensive marine formation found
throughout California, composed mainly of thin-bedded, hard, platy, porcellaneous,
siliceous shale, with thin limestone beds at the base. Exposures are found at the top of
the Casmalia Range, and along the northwest, southwest, and west arroyos of Burton
Mesa.
The Miocene Sisquoc Formation (Tsq) is a white to cream-white, punky, diatomaceous claystone and clayey diatomite to light gray claystone and siliceous clay shale at the base. The Sisquoc Formation is the most extensive exposure of bedrock in the study area. Exposures are found from south of the Lions Head Fault to Shuman
Creek, in arroyos along San Antonio Terrace, and in the northeast, southeast, and east
arroyos of Burton Mesa.
Pleistocene to Holocene Sediments
Pleistocene sediments comprises three units that are dissected and composed of
Orcutt Sand (Qo), a tan to rusty brown, poorly consolidated to locally indurated, wind-
deposited sand, with pebble gravel at base; locally younger dune sands (Qod); and older
stream terrace and alluvial fan deposits of silt, sand, and gravel (Qoa). Exposures are
found overlying marine terrace platforms and perched river terrace platforms, and in low-
lying local basins.
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Holocene deposits comprise surficial sediments of beach sand (Qs), dune sand
(Qds), valley and floodplain alluvial deposits (Qa), and landslide debris (Qls).
2.5 Structural Setting and Features
VAFB is located in the western section of the Santa Maria basin, structurally
bounded by fault systems along the San Rafael Range to the northeast, the Santa Ynez
fault along the Santa Ynez Range to the south, and the Hosgri fault offshore to the west
(Figure 3). At this time, the basin is undergoing north-northeast crustal shortening,
creating west to northwest trending faults and folds. The study area is in a transitional
boundary between the northwest trending San Rafael and Santa Lucia Mountains, and the
east-west trending Santa Ynez Mountains at the western edge of the Transverse Ranges,
with a progressive change in active faults from reverse and right-lateral reverse oblique
motion in the southern Coast Ranges to reverse and left-lateral oblique faulting in the
Santa Maria basin (Eaton, 1984; Clark, 1993).
The Transverse Ranges are part of a north-south compressional area, involving
plate interaction along the San Andreas Fault (Anderson, 1976), with east-west trending faults and folds that extend into the Vandenberg area (Johnson, 1984).
Structural features in the north and east boundaries of the study area include the Purisima
Hills, an anticlinal feature, and the Casmalia Range bounded by the Lion’s Head fault to the south, and the Pezzoni fault to the north and northeast.
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The Lion’s Head fault is a steep, 80° dipping, northwest trending normal fault
that is visible in the sea cliffs south of Lion’s Head, and the Pezzoni fault is a steep,
northwest trending reverse fault that borders the eastern flanks of the Casmalia Range.
Other faults in the range have minor vertical offset of <1 m (3ft), possibly due to
differential erosion of bedrock, and are believed to be inactive (Clark, 1993). A series of
west to northwest trending fold axes are visible on the sea cliffs and are part of the broad,
southwest plunging Casmalia anticline. These folds are usually asymmetric, with long,
gentle dipping south limbs, and steep, sometimes overturned north limbs (Woodring et.
al, 1966). This area is on the structural boundary that is affected by the north to northwest
trending deformation of the San Rafael Mountains.
On the western margin of the study area, the Hosgri fault located offshore is part
of the San Gregorio-Sur-San Simeon-Hosgri fault zone, a major north to northwest
trending, right-lateral strike slip fault system with late Pleistocene and Holocene activity.
The Hosgri fault truncates the Lion’s Head fault ~ 10km (33 feet) west of Point Sal, with horizontal slip and a reverse component, that is transferred from the northern San Simeon fault to the west-northwest trending faults that are located onshore between the Casmalia
Range and Point Arguello (Luyendyk et. al., 1980; Crouch et. al., 1984; Cummings et. al., 1987; Nitchman, 1988; PG&E, 1988). PG&E (1988) mapped several discontinuous, northwest trending faults and folds offshore in this area.
In San Antonio Terrace and North Mesa, east-west trending fold axes are visible in the Tsq and Tm formations, part of the Los Alamos syncline and Purisima anticline, with no faults identified or mapped in the area. This section has crossed over the
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structural transitional boundary into the east-west trending structure of the Santa Ynez
Mountains (Woodring et. al., 1966).
Burton Mesa, similar in structure as the San Antonio Terrace and North Mesa, has
visible fold axes that are part of the broad, shallow, east-plunging, east-west trending
Burton Mesa anticline. The closest fault that has been mapped near Burton Mesa is the
Lompoc-Solvang fault located south of the Lompoc Valley 4.8 km (3 mi.) to the south, and does not intersect or trend towards the study area (King, 1985). Clark (1993) proposes that the break in slope that makes up Burton Mesa may also reflect recent
faulting or folding on a western section of the Los Alamos fault (Guptill et. al., 1980)
and/or the Lompoc Purisima thrust (Namson and Davis 1990). The geomorphic and
structural trends continue into the northern edge of Burton Mesa, and northern Purisima
Hills without any breaks (Dibblee, 1950).
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Figure 3: Map of major regional structures in the area.
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Chapter 3: Scientific Background and Previous Work
3.1 General Marine Terrace Morphology
3.1.1 Marine Terrace Anatomy
Marine terraces are near horizontal, gently sloping wave-cut platforms that are commonly overlain by beach and near-shore sediments (Figure 4). Marine terraces record past shorelines but must be interpreted with caution especially in regions with active tectonic uplift and deformation.
Wave-cut platforms are usually horizontal or near horizontal benches cut into bedrock by the hydraulic and abrasive action of waves in the intertidal and nearshore beach environment (Clark, 1993). On the platform surface, there may be a layer of polished, rounded boulders, cobbles, and gravels deposited over bedrock. Platforms can contain little to abundant shell and other fossil material, and pholad borings may be found on the bedrock surface. Overlying the wave-cut platform, marine deposits of poor to well-sorted marine sands with occasional layers of well-rounded gravels may be present.
Terrestrial or non-marine alluvial and colluvial sediments may be deposited over
marine sediments as outwash from the upland areas adjacent to the terraces. Terrestrial
sediments in the area often may be recognized by their reddish oxidized color and are composed of loose, poorly consolidated to unconsolidated sands, silts, and clays with interbedded deposits of angular to sub-angular boulders to gravel-size clasts.
Additionally, there may be significant deposits of dune sand blanketing the
15
terraces and present in exposed sections along canyon walls. At the strandline or the
platform back-edge, there may be a steeper paleo-sea cliff that intersects the wave-cut
platform, usually composed of resistant bedrock. This intersection is the shoreline angle,
(Hanson, et. al., 1994). Since older terraces are usually buried under a blanket of
terrestrial deposits, shoreline angles are rarely exposed and difficult to locate, thus
making it necessary to infer their location.
The fore-edge of a marine terrace is the closest seaward exposure of a wave-cut
platform, perpendicular to the downslope direction of the terrace. The fore-edge
indicates the lowest platform elevation above mean sea-level. Similar to the back-edge, this surface is difficult to locate on older terraces.
Figure 4: Generalized diagram of emergent marine terraces (Macias after Muhs, 2012)
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On the surface, terraces may be characterized by broad, gently sloping topography
(Clark, 1993), and may carry multiple wave-cut platforms depending on the deposition of
the overlying sediment (Figure 5).
The platform surface may be traced across a large area if the bedrock interface, or the boundary between bedrock and overlying terrace deposits is continuous.
Figure 5: View of coastal terraces looking south from Lions Rock
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3.1.2 Emergent Marine Terrace
On an emergent coast, a vertical succession of marine terrace platforms rise in
“stair-steps”, with each stair-step indicating a discrete sea-level highstand. A sequence of
stair-steps may record major advances and retreats of global sea-levels or it may reflect
local or regional tectonic uplift, or both forces may occur (Clark, 1993).
The process in which emergent marine terraces form starts with the wave-cut
platform carved into bedrock at the beach during a period of relatively stable sea-level.
The terrace will become elevated and abandoned if sea-level drops or uplift occurs, thus
preserving it. Each terrace must either be elevated rapidly or the next subsequent sea-
level rise is of lower elevation in order for terraces to be preserved. This process occurs
again until a vertical flight of terraces is formed and at least partially preserved.
3.2 Climate Change and Marine Terrace Development
3.2.1 Sea-level Fluctuations
The development of marine terraces reflects the relationship between the tectonic
behavior of a region and the nature and magnitude of sea-level change over time.
However, the complexity of developing a sea-level history for different coastlines globally may reflect glacio-isostatic adjustments (Lambeck, et. al., 2014), tectonic uplift
(Muhs, et. al., 2014), crustal deformation and potential mantle migration away from continental ice sheets (Clark, et. al., 2014), and the volume of sediment delivered from coastal watersheds.
18
Age dating of sea-level fluctuations have relied on a variety of methods including
the uranium-series dates of coral reefs (Veeh and Chappell, 1970; Mesolella, et. al., 1969;
Bender et. al., 1979), deep sea sediment cores (Shackleton and Opdyke, 1973; Chappell
and Shackleton, 1986), cosmogenic radionuclide concentrations (Perg, et. al., 2001),
radiocarbon techniques (Muhs, et. al., 2014), and amino-acid stereochemistry (LaJoie, et.
al., 1982). Recent work by Reynolds and Simms (2015) has used “sea-level index points”
(these have an “established vertical relationship to reference water level” such as known
estuarine mollusk species) and “limiting data” that indicate environmental controls such
as “marine limiting” or “terrestrial limiting” species or geomorphic features such as fossil
barrier beaches.
Sea-level fluctuations during the Quaternary and Holocene are recorded from
radiometrically dated coral reefs in Papua New Guinea and Barbados (Broecker et. al.,
1968; Mesolella et. al., 1969; Veeh and Chappel, 1970; Matthews, 1973; Bloom et. al.,
1974; Bender et. al., 1979; Chappell, 1983), and oxygen-isotope ratios from deep sea sediment cores (Shackleton and Opdyke, 1973; Chappell and Shackleton, 1986). A global paleo-sea-level curve is constructed, with numbers assigned to each major maximum (odd) and minimum (even) stage, and letters designating sub-stages (Figure 6).
3.2.2 Studies of relative sea-level change in California
On emergent coastlines, marine terraces are normally associated with sea-level highstands (Bradley and Griggs, 1976, with terraces formed during sea-level lowstands usually destroyed by subsequent transgressions (Lajoie, 1986).
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On the California coastline, there have been numerous studies that have attempted
to place marine terrace sequences in the context of global sea-level change, but conflicting ages have made it challenging. However, the last major glacial highstand at
120 ka is well-preserved along the California coast and dated extensively, yielding a
common sea-level value of 6 m (20 ft) above present sea-level (Hanson, 1994).
Subtracting the paleo sea-level depth to the current elevation of the 120 ka terrace, an
uplift rate can be derived and compared with other terraces in sequence (Figure 6).
Figure 6: Quaternary sea-level curve and emergent terraces on a tectonic-stable coast. Sea-level curve is modified from Chappell (1980), and the oxygen isotope stages are modified from Shackleton and Opdyke (1973)
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3.3 Previous Terrace Studies
Few studies on VAFB have been conducted, with most restricted to small areas
with no correlation across the study area. By contrast, many studies have been conducted
north and south of VAFB. These studies are discussed below.
3.3.1 Geology and Paleontology of the Santa Maria District
A comprehensive USGS geologic report of the Santa Maria district was written by
Woodring and Bramlette (1966), with the western portion of VAFB included in the study
(Figure 7). This was the first detailed report written on the terraces at VAFB. All paleo-
stream terraces and marine platform surfaces were mapped as Qt without discerning each discrete surface, since they suggested that stream terraces transitioned into marine terraces near the coast. Terrace surfaces below the Pleistocene Orcutt Sand are extensive
and, in 1966, were considered the highest and oldest terrace surfaces. Owing to local
deformation, only remnants of these surfaces are preserved.
In the Casmalia Range, “five marine terrace platforms were identified on the south slope of Mount Lospe, through marine deposits with fossils on three of five of the terraces” (Woodring and Bramlette, 1966). The second terrace in sequence was only found in one small area, and the fourth terrace was identified but could not be recognized elsewhere. The highest, most conspicuous terrace (fifth) had a wave-cut platform exposed at an elevation of ~244 m (800 ft), covered by deposits ~15 m (50 ft) thick. A fossil layer composed mainly of barnacle fragments and calcareous sand, partly cemented into a 1.8
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m (6 ft) thick limestone was located at a platform elevation of 253 m (830 ft). Marine
deposits were found near the same elevation in different localities throughout the
Casmalia Range (Table 1). Southeast of Point Sal Landing, only three extensive terraces
were identified along the coast to which they assigned the terms low terrace (first), the
intermediate terrace (third), and the high terrace (fifth).
The low terrace along the coastline, has varied elevations that increase from 15 m
(50 ft) along the modern sea cliff to 38 m (125 ft) inland. A fossil locality at the rear of
the low terrace with abundant marine fossils and beach gravels, indicated a platform
surface with an elevation of 35 m (115 ft).
A fossil locality near the intersection of south Combar Road and Globe Road, at
an elevation of 183 m (600 ft) was determined to be the surface of the intermediate
terrace. Woodring suggested the intermediate terrace correlated with the highest terrace
in North Mesa above the intersection of Lompoc-Casmalia Road at Bishop Road, and the highest wave-cut platform surfaces on San Antonio Terrace and Burton Mesa.
At the time of this study, San Antonio Terrace and Burton Mesa had not yet been studied, so it was not known if the platform on Burton Mesa was one wide marine terrace, or if there were multiple platform surfaces buried below a cover of alluvium and/or colluvium, depositing seaward during emergence.
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Terrace # Name Comments Elev. m (ft)
1 Low Lowest terrace on coastline 12-35m (40-115 ft)
2 Only one surface on Combar Road
3 Intermediate 183m (600 ft)
4 Extensive on Combar Road only
5 High Highest, oldest terraces 244 m (800 ft)
Table 1: Terraces in the Woodring & Bramlette (1966) study.
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Figure 7: Geologic map of VAFB terrace localities (modified from Woodring & Bramlette, 1966)
3.3.2 Archaeological Investigations on the San Antonio Terrace
D. L. Johnson conducted an archaeological investigation on the San Antonio
Terrace, in connection with MX Facilities Construction in 1984, which included a brief overview on marine terraces located in the study area. Marine terraces were identified and a geomorphic map was constructed, from the San Antonio Terrace into North Mesa,
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the southern end of Casmalia Hills at Shuman Creek, and northern Burton Mesa (Figure
8). He noticed distinct features on the geomorphic surface suggesting the presence of
marine terraces, such as prominent slope breaks accompanied with marine sands, beach
gravels and cobbles, and fossils. He assigned the modern beach as Qt0 and each
successive flight of terraces as Qt2-Qt7, though he did not map the Qt1-Qt2 in the study
area (Table 2) .
He assigned the lowest terrace in the study area as Qt3, at an elevation of ~107 m
(350 ft), located below the large dunefields at San Antonio Terrace, where he found
“windows” through the dunes into the platform surface. At the northeast edge of the
dunefield, a flight of northwest-southeast trending terraces (Qt4-Qt7), was mapped from
San Antonio Terrace into North Mesa, and a vertical profile was constructed.
An exposure of marine gravels and cetacean (whale) bones in a bedrock quarry was assigned the wave-cut platform of Qt4, with an elevation of ~125 m (410 ft), with a rise in the bedrock, the paleo-sea cliff.
Further upslope at an elevation of ~172 m (565 ft), a prominent slope break marked the shoreline angle and platform he assigned as Qt6, a wide terrace that was mapped to the south end of Shuman Creek. At the section of the vertical profile, Qt5 was very subtle and inferred, but widens to the northwest, extending across Shuman Creek into the southern Casmalia Range. The highest terrace Qt7, was determined below the
summit of Lompoc-Casmalia Road where it intersects Bishop Rd, with a wave-cut
platform of beach cobbles and pholad borings at an elevation of 203 m (666 ft).
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Even though the main focus of his study was on the San Antonio Terrace,
Johnson worked into Burton Mesa, where he found exposures of the wave-cut platform throughout the area. Marine deposits and pholad borings were found near Tangair, on the cliffs of Lake Canyon, the northern banks of the Santa Ynez River, and as far east as
Santa Lucia Canyon. He assigned the terrace under Burton Mesa as Qt5, with a small exposure of Qt7 near the intersection of Hwy 1 and Firefighter Road just below the summit.
Though no measurements were calculated, Johnson was confident that the shoreline angle of Burton Mesa would correlate with one or more sea-level highstands associated with either MIS 7, 9, 11, or 13 (Shackleton and Opdyke,1973).
He determined that Burton Mesa was formed during a significant inland marine
incursion into weak, easily erodible shales and diatomite of the Tsq, the main bedrock
that the platform rests on. He went on to discuss how a platform cut into incompetent
bedrock such as the Tsq could be so well-preserved, and came up with two possible
hypotheses.
His first hypothesis involves the large-scale dune deposits that blanket the surface.
Dune deposits are composed of sand that is porous and will soak up rainwater, greatly
reducing erosion from runoff. The second hypothesis involves the hardpan layers that are
located within the soils throughout the study area. Soils form in the dunes and underlying
marine sands and protect the underlying sediment from erosion. As mentioned earlier,
Narlan and Tangair soils are composed of secondary silica and iron, making them highly resistant to erosion.
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Figure 8: Geomorphic map of terraces in Johnson’s (1984) study on San Antonio Terrace (Johnson, 1984). Red dashes indicate terrace boundaries, Q3-Q7 are terraces.
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Oxygen Isotope Age Terrace # Stage/Substage ka
0 0 Qt0
1 10
3a 40
3c 60
5a 83 Qt1
5c 105 Qt2
5e 120 Qt3
7 215 Qt4
9 320 Qt5
11 430 Qt6
13 480 Qt7 Table 2:Terrace numbers and Oxygen Isotope Stages (after Johnson 1984)
3.3.3 Late Quaternary Tectonic Deformation in the Casmalia Range
In 1993, D. C. Clark worked in the Casmalia Range for his Master’s thesis, to determine the deformation rates of major faults in the Casmalia Hills. A major part of his study was to map marine terraces, to establish slip rates on the Point Sal Fault, the Lion’s
Head Fault, and a few unnamed faults in the area. Marine terraces were determined by measuring the backedge (shoreline angle) elevations with a survey level, and a geologic map was constructed including other data.
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Clark identified a flight of at least nine marine terraces with shoreline angle
elevations from 9 and 264 m (30-866 ft), numbered Q1 to Q9 sequentially from lowest to
highest (Table 3):
Q1: Shoreline angle elevation 7-10 m (23-33 ft) - the lowest and youngest terrace,
exposed almost continuously between the Lion’s Head fault to Point Sal, with possible
exposures southeast of Lion’s Head fault. Multiple marine fossil localities are found on
beach cliffs.
Q2: Shoreline angle elevation 21-26 m (69-85 ft) - is discontinuously exposed from ~ 1 km (0.6 miles) south of Lion’s Head fault past Point Sal. Clark determined that the Q2 terrace was the most important strain gauge for late Quaternary deformation in the
Casmalia Range.
Q3-Q6: - discontinuous, partially exposed terrace remnants between Point Sal and
Lion’s Head fault, with shoreline angle elevations of 58 m (190 ft) + 4m (13 ft), 69 m
(226 ft) + 4 m (13 ft), 75 m (246 ft) + 2m (6.5 ft), and 96 m (315 ft) + 7m (23 ft). Wave-
cut platform surfaces are evident by marine sand on Q3, Q4, and Q5, pholad borings and
marine fossils present on the Q4 platform, and another platform of pholad borings on Q5.
The Q6 platform was identified by small notches or benches in bedrock.
Q7: Shoreline angle elevation 161 m (528 ft) - broad, well preserved wave-cut platform with marine sands and pholad borings on bedrock Clark correlated this terrace with multiple planar surfaces southeast of the Lion’s Head fault, with bedrock surfaces ranging in elevation from 120 m (394 ft) to 170m (556 ft), suggesting multiple platform surfaces that have been eroded away.
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Q8: Shoreline angle elevation 184-192 m (604 ft-630 ft) - a terrace with poor geomorphic expression, with marine sands at the wave-cut platform, located ~ 3km (1.8 mi.) east of Lions Head.
Q9: Shoreline angle elevation 260-268 m (853-879 ft) - the highest terrace in the
Casmalia Range, with excellent terrace remnants south of Mt. Lospe. The wave-cut platform is exposed in arroyos, with marine sands overlain by a thick deposit of alluvium that is capped by a resistant silica and iron-rich hardpan layer, 1-3 m (1.5-10 ft) thick.
Clark identified other marine terraces south of Shuman Creek, with marine sands and/or pholad borings at an elevation of 207 m (679 ft), and 244 m (800 ft), which he suggested was a marine terrace that covered the entire surface.
Another surface that he discussed was the geomorphic surfaces of San Antonio
Mesa, Burton Mesa, and Lompoc Mesa, south of the Lion’s Head fault, with a maximum elevation of ~250-300 m (820-984 ft). Similar to Johnson (1984), Clark suggested the occurrence of a marine incursion, however, the higher elevations of the western Casmalia
Range may have been islands, compared to the eastern sections that were part of the geomorphic surface.
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Marine Shoreline-angle Terrace elevation (m) MIS Age (ka) Comments Q1 7-10 5a 83 Exposed from S of Lion’s Head to Point Sal
Q2 21-26 5e 120 Exposed discontinuously from S of Lions Head to N of Mussel Rock
Q3 58 + 4 9 320 Marine sand on platform. Q3 - Q8 are discontinuously exposed on the SW margin of Casmalia Range
Q4 69 + 4 11 430
Q5 75 + 4 13 560 Pholad borings on platform
Q6 96 + 7 15 630 Alignment of erosional notches on bedrock
Q7 161 + 4 900 Pholad borings on platform
Q8 188 + 4 >999
Q9 264 + 4 >999 Extensive, well preserved terrace on SW margin of Casmalia Range
Table 3 - Marine terrace data, southwest and west margins of the Casmalia Range. (modified from Clark, 1993)
Clark indicated a pattern in the behavior of past sea-level stages along the
California coastline, with the most prominent platforms cut by MIS 5a (80 ka), and MIS
5e (120 ka). The MIS 5c (105 ka) was short lived so this platform is rarely observed, and the MIS 7 (210) terrace was destroyed by a newer highstand, so it is rarely preserved.
The next prominent platform surfaces are the MIS 9 (320 ka) and MIS 11 (430 ka)
(Clark, 1993).
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3.3.4 Correlation, Ages, and Uplift Rates of Quaternary Marine Terraces: South-
Central Coastal California
A study was conducted by K. L. Hanson et. al. (1990) on the marine terraces in
San Simeon and between Morro Bay and Santa Maria basin. In the San Simeon area, four to five terrace platforms were identified and correlated to MIS cycles. Terraces are correlated sequentially with the first terrace assigned to MIS 3 or 5a (60 or 80 ka). The next terraces were correlated with 5a or 5c (80-105 ka), 5e (120 ka), 7 (210 ka), and 9
(330 ka). U-series dating and thermoluminescence (TL) dating were applied to the lowest two terraces, along with soil profile development, geomorphic expression, and terrace altitudinal spacing methods.
Deformation of marine terraces was calculated to determine the slip and uplift rate along the San Simeon Fault Zone. Sea-level height during the 80 ka sea stand was calculated by correlating with the well-established sea-level height +6m (above present sea-level) for the 120 ka terrace. Estimated age and correlation of terraces across the fault zone were determined by lateral correlation of the 120 ka terrace.
Uplift rates were established at a rate of ~0.17 m/ka south, and 0.16 m/ka north of the fault zone, comparable to other parts of California with similar tectonics.
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3.3.5 Age and Deformation of Marine Terraces Between Point Conception and
Gaviota, Western Transverse Ranges, California
Rockwell et. al. (1992), conducted a study on marine terraces south of VAFB, between Point Conception and Gaviota, to determine slip rates along the south branch of the Santa Ynez Fault (SBSYF). Five marine terrace platforms were identified by geomorphic properties and measurements were taken of the shoreline angle and platform elevations. Several methods were applied to date and correlate terraces including: 1) U- series dating of bone and shell fragments on the marine and nonmarine sediments, 2) amino-acid analysis, 3) faunal assemblages were identified for sea temperature analysis, and 4) relative terrace spacing and correlation of terraces to marine oxygen isotope stages
(MIS).
The youngest terrace in sequence was correlated to MIS 5a, during an interstadial within an interglacial. Rockwell et. al., dated a mammal bone, a mollusk, and related the faunal assemblage to cool waters. The results were conducive to the 80 ka age of MIS
5a. By obtaining an age date on the first terrace, Rockwell et. al. were able to correlate the remaining four terraces to MIS 5c (105 ka), MIS 5e (125 ka), and MIS 7 (200 ka).
Results of this study indicate the “marine terrace between Gaviota and Point
Conception are uplifted and warped. “Average uplift rates of 0.15-0.23 m/ka near Point
Conception to 0.30m/ka on the east side of the SBSYF near Gaviota" (Rockwell et. al.,
1992). North of Point Conception, uplift rates vary from 0 mm/yr to .20 mm/yr from the
Santa Maria basin to Cayucos. Rockwell et. al. determined the rate of uplift in near Point
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Conception is only a little higher than the average rate for the majority of coastal
California.
3.3.6 New Terrace Data
During the course of this study, new data have been presented by Morel and
Keller (2017), debating the age of the lowest marine terrace along the Gaviota coast.
Radiocarbon dating and/or optically stimulated luminescence (OSL) dating were applied to marine mollusk fossils at the wave-cut platform of the lowest terrace. Two age dates of 41.1 ka and 44.2 ka indicate the marine terrace correlates to MIS 3 with an age of ~37 ka. These new dates have debated the previously accepted date of 80 ka of the lowest terrace along the Gaviota coast, as well as terraces in other areas. Establishing a better constraint on terrace dates changes estimates of uplift rates significantly.
3.4 Soils and Hardpan
Soil stratigraphy can be useful when correlating marine terrace surfaces across the field area. The most extensive soils in the study area are found in the Older dune (Qos) and ancient dunes that cover South VAFB, Burton Mesa and the Casmalia Range. Three main soil types were identified by Johnson (1984), as the Narlon soils, the Narlon hardpan variant soils, and the Tangair soils. In the B soil horizon, the Narlon soils contain clay, the Narlon hardpan soils contain a silica-cemented subsoil pan (durapan), and the
Tangair series contain iron or silica cemented sand concretions.
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Soil development in the paleo-dune and terrace surfaces are marked by a
conspicuous, erosionally resistant, silica and iron-rich hardpan layer that varies in
thickness. Woodring and Bramlette (1966), described these layers as consolidated or indurated, a few to 15 feet thick, and usually found within 10 feet below the surface.
Multiple, thin depositional layers are cemented together as one, compose of sand and silt with coarser pebble and rock fragments, and can contain clay if the bedrock is composed of it. The color is usually rusty brown from iron-rich cement, but can be mottled
brownish and grayish in color, and cylindrical root casts are abundant. These layers are
easy to locate on many terrace surfaces that Woodring and Bramlette (1966), assigned to
the Intermediate terrace, and Louderback (1914) suggested that this “pseudostrata” are
widespread enough that mapping remnant surfaces may allow reconstruction of the
surface.
3.5 Marine Terrace Deformation
Marine terraces that form on emergent coastlines are ideal for determining
deformation style and uplift rates in a tectonically active area. The only significant study
along the Casmalia Range, was done by Clark (1993).
Clark measured the shoreline angle elevation of the assumed 5e terrace on either
side of the Lion’s Head fault, indicating a late Quaternary, relatively uniform uplift rate
of 0.14 - 0.17 mm/yr (0.006 in/yr), between Point Sal and the fault, with no significant
35
internal faulting, tilting, or folding (Clark, 1993), and Rockwell (1989) reduces it slightly to 0.13-0.18mm/yr.
A sea cliff exposure south of Lion’s Head exposes the Lion’s Head fault and other smaller faults. The Lion’s Head fault places Jurassic Franciscan ophiolite rocks on the northeast against the Tm on the southwest, but there is no 5e marine terrace displacement.
A high-angle normal fault immediately to the northwest of Lion’s Head fault offsets the
5e marine terrace by 1.4 m (4.6 ft) vertically through alluvium to the surface, with gravel beds overlying the wave-cut platform offset 0.6 to 0.7 m (2-2.3 ft) along the fault.
Marine terraces adjacent to the Lion’s Head fault indicate 7-9 m (23-30 ft) of offset or warping along the fault trace, and Clark assumes an MIS Stage 11 terrace with a shoreline angle that appears to be offset 4-5 m (13-16 ft) across the fault, with vertical offset rates of middle and late Quaternary marine terraces to be no greater than 0.012-
0.017mm/yr (0.0005 - 0.0007 in/yr), with a dip-slip faulting rate of 0.12 - 0.18 mm/yr along the 80° dipping fault, assuming the uniform uplift rate.
Though no recent fault displacement has been documented in this area, Clark
(1993) proposed that the difference between late Quaternary uplift rates of the Casmalia
Range at 0.14-0.18 mm/yr, and those at Point Conception at 0.2-03 mm/yr (Rockwell et. al., 1992), and 0.4-0.6 mm/yr (Lajoie et. al., 1982), coupled with the north-side up displacement of the Lion’s Head fault, require late tectonic, vertical motion of the
Casmalia Range to have occurred.
Looking south across Shuman Creek, the wave cut platforms are the same elevation on San Antonio Terrace, with a gradual, decrease in elevation towards the
36
south-southwest or seaward. This could be a product of erosion, although Clark (1993)
suggests the decline in bedrock elevations may have been caused by active faulting
and/or faulting, reflecting active folding on the south limb of the Casmalia-Orcutt
anticlinal trend. South of Lion’s Head fault, the active deformation is poorly constrained and there has been speculation on the continuity of terraces. Identifying individual terraces across San Antonio Terrace, Burton Mesa, and beyond is challenging as much of the surface is covered by Orcutt Sand and shoreline angle elevations are buried.
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Chapter 4: Methods
Marine terraces were identified by field observation and with Google Earth Pro.
Once terraces were identified, the following methods were used to correlate terraces
across the field area, and assign relative age dates.
4.1 Equipment
4.1.1 GPS Device
A combination of various GPS style devices were used to determine the location
and elevation of each terrace. The compass application on an iPhone 6 and Commander
Compass Go 3.9.9 were used in combination owing to potential error when cell reception was not available.
4.1.2 USGS Topographic Map & Dibblee Geologic Maps
A series of base maps were used to plot locations of observations and
measurements in the study area. 7.5 minute USGS topographic maps of the Point Sal,
Casmalia, Orcutt, Lompoc, and Surf were used in combination with Dibblee geologic
maps of the same quadrangles, to show bedrock and structural features. A two and ten
foot contour map of the Casmalia Range quadrangles from 30CES VAFB Cultural
Resource Management was used for detailed mapping along the coast.
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4.1.3 Remote Sensing (DEM, imagery)
A Digital Elevation Map (DEM) of Santa Barbara County with hillshade was used to determine slope breaks and rises that were not easily recognizable through field observation. Google Earth Pro was used to help plot location, and pathways and polygons were created to identify distinct features of terrace morphology. Data from topographic and geologic maps were transferred onto Google Earth Pro for a comprehensive digital map of the field area.
4.2 Field Observation
A combination of physical properties determined through field observation were used to identify marine terrace localities and correlate with other localities across the field area. A “how-to” on relative marine terrace dating was written by Muhs (2000), and some of the techniques mentioned were used for this study.
4.2.1 Geomorphic Expression
Initially, field observations across the study area were conducted to identify slope breaks and geometry that might indicate a terrace. Next, a closer, more detailed observation was conducted to identify the various sections of the terrace.
The easiest recognizable feature is the interface between bedrock overlain by marine or nonmarine sediments, exposed on higher, older terraces, and along the sidewalls of arroyos and gullies. The distinct boundary that characterizes the bedrock
39
interface can be observed across the study area, and was thus, plotted on the map with
dashed lines.
The most important feature for this study was to locate the wave-cut platform,
with marine deposits that include pholad borings and/or marine fossils. Once identified,
these were plotted on a map with a line and circles indicating a platform, a “P” for
pholads and “F” for fossils. Elevation of platforms using GPS applications were recorded
and samples of fossils collected.
The backedge of mapped terraces were identified on the surface by a change in
slope, with steeper bedrock exposed higher than the terrace deposits, and a seaward slope
angle measured when exposed (Muhs, 2000).
Muhs (2000) determined the degree of erosion that has occurred on a marine
terrace can be useful to determine the relative age with other terraces in the study area.
Older terraces presumably have been subjected to greater stream dissection, headward erosion, and drainage densities, compared to younger terraces.
4.2.2 Stratigraphy of Terrace Deposits
A detailed lithological description and a stratigraphic column were constructed
detailing non-marine deposits overlying marine terraces, using a hand lens, hydrochloric
acid (HCl), and measuring tape. This included alluvial/colluvial, dune deposits, hardpan
“pseudo” layers and soil development. For relative correlation of terraces, detailed lithology serves to identify distinguishable marker beds in terrace deposits that were traced throughout the study area.
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4.2.3 Fossils and Other Organic Material
On the wave-cut platform of the lower two terraces, abundant marine invertebrate fossils were collected and identified by species. Fossil identification helped determine the distribution of species across the study. Further, species identification helped determine warm or cool temperature climates, which may contribute to broader understanding of sea-level during the time of deposition. Vertebrate fossils and found in non-marine deposits were identified and charcoal fragments were collected.
4.3 Correlation
The scope of this study did not allow for any age dating of fossil material, though datable material was collected. Relative correlation methods were used to identify terraces in the study area through a combination of methods.
4.3.1 Altitudinal Terrace Spacing
Elevations of marine terrace platforms were determined and a method by Bull
(1985) applied, suggesting the altitudinal spacing of each platform on a flight of terraces is unique and can be compared with paleo sea-level highstand elevations. Altitudinal spacing of terraces record the time interval between consecutive sea-level highstands.
This method assumes constant uplift rate, determined by corresponding age and paleo- sea-level to a specific terrace. If the uplift rate is not constant, the irregular spacing between terraces may help to identify which platform surface it is (Muhs, 2000).
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4.3.2 Correlation of Marine Terraces to Global Sea-level
Age correlation was determined by the elevation of marine terraces in the study
area and compared with the paleo sea-level highstand elevations (Figure 6). Uplift can be
calculated by subtracting the height of each highstand (relative to current mean sea-level) at the time of formation, from the elevation of each shoreline angle. Uplift is plotted on a graph with terrace ages, and the slope of the line is the inferred uplift rate. A best-fit line determines the best correlation of elevation and terrace age (Clark, 1993).
This graph was created multiple times with a wide range of ages assigned to
terraces to achieve the best fit along the sea-level curve.
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Chapter 5: Results
The results of this study are presented according to the geomorphic area in which the marine terraces occur: Casmalia Range, San Antonio Terrace, Burton Mesa, and
North Mesa.
5.1 Casmalia Range
The Casmalia Range is divided into three sections: Combar Road, Point Sal Road, and south Casmalia (Figure 9). The center of the Casmalia Range did not contain any visible terraces, therefore, the area was not mapped.
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Figure 9: Overview of Casmalia Range section. Yellow: Combar Road section; orange: Point Sal Road section; red: South Casmalia section. EEG: sloth quarry.
5.1.1 Combar Road
A flight of five terraces are visible on Combar Road on the south slope of Mount
Lospe. Exposures of the lower two terraces are visible on the cliffs of Cow Creek, southwest of Point Sal Road (Figure 10).
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Figure 10: Overview of marine terrace flight on Combar Road in the Casmalia Range. Pink- the lowest platform of Purisima Point Terrace; Purple- the second platform of Minuteman Terrace; Blue- the third platform of Vandenberg Terrace; Orange- the fourth platform of Orion Terrace; Red- highest platform of Summit Terrace.
Purisima Point Terrace (PPT) - the lowest terrace that makes up the cliffs of the
modern beach, with a wave-cut platform exposed ~10 m (20-30 ft ) high, and pholad
borings in ophiolite, the underlying bedrock (Figure 11). The PPT can also be a bench
cut into bedrock on small points along the coast, and contains a rounded cobble to gravel
layer 1 m (2-3 ft) thick, composed of mainly ophiolite material above it. Above this layer
is a white marine sand with interlayers of rounded gravels 3 m (10 ft) thick, overlain by a pale-orange to buff colored alluvium, eroded into badland-style topography. The fore-
45
edge is visible continuously along the beach cliffs and the back-edge is not exposed at this location so it is inferred (Station 9).
Minuteman Terrace (MT)- the foredge is exposed in Cow Creek 24m (80 ft) high,
where alluvium changes abruptly into bedrock. The overlying material is covered with
vegetation and not identifiable. On the terrace surface, boulders of ophiolite “float” are
visible on either side of the creek, indicating bedrock exposure. The backedge is visible
in a drainage on Combar Road at an elevation of 58m (190 ft) (Station 41).
Figure 11: Purisima Point Terrace in the center with outcrops of bedrock into which the Minuteman Terrace is cut.
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Vandenberg Terrace (VT) - the third terrace in sequence is located near
multiple launch facility structures that were built on this surface. A contact between the
Point Sal formation (bedrock) and the terrace deposit was identified at 70m (230 ft) with pholad borings and rounded beach cobbles (Station 11). The terrace surface is covered over and not well exposed at this location so the backedge was inferred at ~109m (360 ft)
from the slope break in the geomorphic surface.
Orion Terrace (OT) - a well pronounced terrace with a bedrock interface 158 m
(520 ft) high (Figure 12). A hardpan surface is exposed at an elevation of 183 m (600 ft),
with a rusty-orange to bright orange colored medium sand, well sorted, sub-rounded, with
white sand filled hexagonal fractures (Station 10). White sand is more resistant and
forms fins on the surface, and some contain dark organics in the center. The orange and
white look mottled and we called them zebra-striped (Figure 12,13; Station 10).
Below the hardpan, a thick 6 m (20 ft) section of alluvium is exposed in a gully, with a base of pale orange to buff (weathered), and rust orange (fresh) colored, medium to coarse sand. The base makes up a steep, cliff former with thin vertical flutes 3 m (10ft)
high. Above the base is a boulder to cobble size layer 1m (3 ft) thick of angular clasts,
with graded beds that coarse upward, and pinch in and out (Figure 14).
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Figure 12: Stratigraphy of Orion Terrace
The top of the alluvium is a buff-brown (weathered), orange-brown (fresh), fine grained sand and silt with pebble clasts 2m (6 ft) thick. The top of this unit is a slope former with rounded flutes. A change in slope on the geomorphic surface is identified and a backedge is inferred at 195 m (640 ft).
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Figure 13: Hardpan surface on Orion Terrace
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Figure 14: Alluvium over Orion Terrace
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Summit Terrace (ST) - The highest terrace in the flight is well-developed, easily
recognizable, with a thick surface of alluvium (Figure 15).
Figure 15: Stratigraphy of Summit Terrace
A wave-cut platform is identified at an elevation of 256 m (840 ft), with pholad borings and rounded beach cobbles and pebbles, with a 1 m (3 ft) thick layer of marine sands above (Station 2). Above the marine sands, a layer of alluvium 17 m (56 ft) thick
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(Figure 16). Above that is 3 m (10 ft) thick unit of rust-red and gray mottled lines and squiggles composed of fine grain expanding soils (Figure 17). Some areas are pure red with no gray color, and the fresh surface is rusty orange-red. Above that is a 1.5m (5 ft)
thick layer of smaller mottled patches of iron red and white. The top unit is a crumbly orange to orange-buff and light gray mottling that is not as red as the units below, with the light gray mottling from organics. The color becomes darker orange towards the base of the unit. In the center of this unit is a hardpan layer of resistant sand that is dark brown and orange with hexagonal infilled cracks at the surface 1.5m (5 ft) thick. Small rounded concretions 1cm (.3 in) in size are scattered at the surface with polished, rounded cobbles.
Continuous exposures of this terrace are visible along the northern Casmalia
Range with southwest facing beaches at the time of formation. A well-defined change in slope is visible on the geomorphic surface and a backedge is inferred at 292 m (960 ft)
(Station 64).
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Figure 16 : Alluvium over Summit Terrace
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Figure 17: Hardpan surface of Summit Terrace
5.1.2 Point Sal Road
The area that is located on Point Sal Road consists of the lower two terraces along
the coastline from Flume Canyon to the north and Shuman Creek to the south.
In the northern section between Flume Canyon and Brown’s Beach, a wave-cut platform is visible on the sea cliff ~6m (20 ft) high (Station 34, 9; Figure 18). Above the platform is an 8 ft thick layer of beach deposits, with shell fragments, pholad borings, and well-rounded boulder to gravel size beach deposits, cemented with caliche at the base
(Figure 19).
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Figure 18: Cliff exposures along the beach cliff.
Figure 19: Close up of beach material overlying wave-cut platform.
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South of Brown Beach, the west facing beach turns southwest past a small point,
and the second terrace is visible (Station 4). The lower terrace is a distinct bench cut into
bedrock with pholad borings (Station 43), or a lower bench with eroded alluvium that is mostly continuous along the coast except along two rocky points (Station 35). The
second terrace is well exposed in the sea cliff above the first bench, with a wave-cut
platform elevation of 24m (80 ft) (Station 40).
On the north side of Lions Head, the Purisima Point Terrace is discontinuous and
Minuteman Terrace is visible with a wave-cut platform at an elevation of 15 m (50 ft),
and a layer of coquina and well-preserved marine fossils 0.3 m (1 ft) thick with rounded
beach cobbles above the platform (Station 1). Gastropod species Olivella and Tegula
were collected and sent to D. R. Muhs at the USGS office in Colorado for age dating
(Figure 20).
On the southside of Lions Head, Minuteman Terrace at the same elevation is
visible with a similar layer of marine fossils. Samples of the same species were collected
along with other species for age dating on the northside of Lion’s Head fault. The
bedrock elevation is noticeably different on either side of the fault, however,
measurements could not be taken along the vertical cliff walls (Station 89).
Pholad borings mark the wave-cut platform on the edge of Single Tick Creek, and
a sample of a preserved pholad clam was collected and sent to Christopher Ryan, VAFB
Cultural Resource Management office for radiocarbon dating (Station 46).
Walking up Single Tick Creek, the bedrock interface of Minuteman Terrace is
exposed in the creek bank at an elevation of 24 m (80 ft) (Station 45), southwest of Point
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Sal Road, and the Purisima Point Terrace is visible at the mouth of the creek. The two lower terraces trend towards the southeast, as the coastline transitions into the mouth of
Shuman Creek with a large area of active modern dunes and west -acing beaches.
Figure 20: Lower terraces along Pt. Sal Rd. Top left; view looking north across Lion’s Head fault, top right; pholad borings in ophiolite bench of first terrace, bottom left; coquina and fossil layer of second terrace, bottom right; view looking south across Lion’s Head fault.
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5.1.3 South Casmalia
The area designated as south Casmalia is bounded by the Lion’s Head fault to the
north, the ocean to the west, the Casmalia crest to the east, and Shuman Creek to the
south. A large portion of the central Casmalia Range was not mapped in an area called
Dairy Basin, an area with no marine terraces present.
As described in the section above, the lowest two terraces trace to the southeast,
marking the beginning of the major dune field of San Antonio Terrace to the southwest.
Based on the break in slope in the geomorphic surface, the lower terrace appears to cross
the access road to Minuteman Beach (Station 89), before disappearing into the dunefield.
The second terrace can be traced east of Point Sal Road, until it crosses the road at an
elevation of 24m (80 ft) and heads into the dunefield. The coastal terrace in this area crosses over the Lion’s Head fault and an unnamed fault, with visible deformation of the platform surface.
On the northern banks of Shuman Creek, a bedrock interface is exposed at an elevation of 85 m (280 ft), with Tsq below (Figure 21). The section is nicknamed the
“anvils”, named after the shape of drainages here (Station 7).
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Figure 21: Stratigraphy of S. Casmalia
Near the top of the drainage along Globe Road, a wave-cut platform was mapped at an elevation of 85 m (280 ft) (Station 8), with pholad borings and beach cobbles and gravels, overlain by a 3 m (10 ft) thick crumbly, slope forming layer of white to light gray marine sand with dark clay bands near the top. Above the marine sand, a 3m (10 ft)
layer of steep, fluted alluvium, buff and gray (weathered), with a basal member that is
rounded pebbles, coarse gravels, and sand (Figure 22).
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Figure 22: Photos of terrace deposits in drainage north of Shuman Creek. Top left: white marine sand; top right: marine sand with overlying alluvium with hardpan layer near the top; bottom left: basal member of the alluvium; bottom right: an overview of the terrace deposits
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5.2 San Antonio Terrace
San Antonio Terrace is an area that is bounded by Shuman Creek to the north, the southern Casmalia Range to the east, San Antonio Creek to the south, and the Pacific
Ocean to the west. For the purpose of this study, we divided this area into two sections based on elevation differences, Coastal Dunes and San Antonio Mesa (Figure 23).
Figure 23: Overview of San Antonio Terrace. Yellow: Coastal Dunes section; Orange: San Antonio Mesa section
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5.2.1 Coastal Dunes
The Coastal Dunes section consists of the low-lying areas of San Antonio Terrace
covered under a very large and active dunefield, making it extremely difficult to locate
marine terrace surfaces. On the northeast boundary of the Coastal Dunes section, a well-
defined slope is observed in the geomorphic surface in a northwest trend from San
Antonio Creek to Shuman Creek, marking the boundary of San Antonio Mesa.
On El Rancho Oestre Road, an outcrop of Tsq is exposed at an elevation of 100
m (328 ft ) (Station 39, Figure 24). Traveling south off the slope on El Rancho Oestre
Road, an outcrop of marine sand with a base of rounded beach cobbles and a coarse gravel to pebble layer are exposed (Station 38).
Figure 24: Stratigraphy of El Rancho Oestre
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5.2.2 San Antonio Mesa
San Antonio Mesa is a section of higher elevation in the San Antonio Terrace, bounded by Shuman Creek to the north, Lompoc-Casmalia Road to the east, San Antonio
Creek to the south, and the Coastal Dunes section to the west (Figure 25).
As discussed in the previous section, the slope on the northeast edge of Coastal
Dunes marks the boundary of the San Antonio Mesa. A flight of three terraces are visible
in the geomorphic surface, trending in a southeast direction into North Mesa.
Figure 25: Overview of a flight of terrace platforms on San Antonio Mesa and North Mesa. Blue - lowest terrace; Orange- middle terrace; Red- highest terrace. Flags mark field stations and pale yellow line marks bedrock interface.
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In a roadcut near the intersection of El Rancho Road and Orion Road, a wave-cut
platform is exposed at an elevation of 158 m (520 ft), with dune sand overlying the interface (Station 36; Figure 26). Another wave-cut platform with pholad borings is exposed on the southern bank of Shuman Creek at the 164 m (538 ft), with a thin layer of rounded gravels to cobbles of Tsq or Tm, chert, quartzite and lithics, and fluted marine sand above.
Figure 26: Stratigraphy of South Shuman Creek
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A hardpan surface is exposed at an elevation of 181 m (595 ft), with dark orange to light gray and buff mottling, and organics in the center of the light gray. The hardpan has a lower cliff 1.5 m (5 ft) thick and an upper cliff that is 3 m (10 ft) thick. Iron concretions are visible on the bench below the hardpan layer, and hexagonal cracks are visible on the vertical cliff face (Figure 27).
Figure 27: Hardpan and terrace material exposed on south bank of Shuman Creek. Top left: orange hardpan; Top right: terrace material on cliff face; bottom: another hardpan
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A bedrock interface exposed at a roadcut next to the train tracks at an elevation of
35 m (280 ft) (Station 74). El Rancho Road climbs up onto the lower platform surface where a hardpan layer is exposed at an elevation of 122 m (400 ft), next to the road
(Station 29; Figure 28).
Figure 28: Stratigraphy on El Rancho near Shuman
The hardpan layer is gray to pale-orange with mottling and hexagonal infilled cracks with organics in the center. The gray infilled cracks are more resistant to erosion and form small fins at the surface. A layer of resistant, reddish-brown coarse pebbles, gravels, and sand is exposed at road level (Figure 29).
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Figure 29: Hardpan and soil exposed on Rancho Rd. Top left: hardpan with sand; top right: base of dune sand; Bottom left: hexagonal cracks in hardpan; Bottom right hardpan and dune sand
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5.3 North Mesa
North Mesa is a continuation of geomorphic surfaces from San Antonio Mesa,
east of Lompoc-Casmalia Road, bounded by Shuman Creek to the north, San Antonio
Creek to the south, and Lee Road to the east. The majority of this section is off the
VAFB boundary, however, a flight of three terraces in North Mesa is visible on the geomorphic surface, and may prove beneficial for correlating on the base (Figure
30,31,32).
Figure 30: Overview of North Mesa section in red.
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Figure 31: view of North Mesa of three terrace platforms on geomorphic surface. Blue-lowest terrace; Orange-middle terrace; Red-highest terrace.
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Figure 32: North Mesa with stations.
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The lowest terrace on the north bank of San Antonio Creek has a wave-cut
platform with pholads and marine deposits at an elevation of 35 m (280 ft) in multiple
locations (Station 80). An outcrop of Tsq is exposed on the slope at an elevation of 149 m
(490 ft) (Station 3). Above the lower terrace, the slope flattens out to a second platform
surface at 158m (520 ft), and a bedrock interface is exposed in a gully, east of Lompoc-
Casmalia Road near the same (Figure 31) elevation. Cliff exposures on both sides of
Lompoc-Casmalia Road indicate a bedrock interface from 164 to 170 m (540 to 640 ft)
south of the summit and a wave-cut platform with marine sands and pholad borings at
146 m (480 ft) (Station 4). Above the wave-cut platform are rounded beach cobbles and coarse gravels of sandstone and shale clasts 0.3m (1ft) thick overlain by sub-angular colluvium (Figure 33).
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Figure 33: Lompoc-Casmalia Road. Left: wave-cut platform with bedrock below and marine deposits overlying; Top right: overview of terrace exposure on Lompoc-Casmalia Road; Bottom right: wave-cut platform exposure.
The highest terrace platform is visible at the summit of the southern Casmalia
Range near the intersection of Lompoc-Casmalia Road and Bishop Road. The bedrock interface is visible on both sides of the summit at an elevation of 198 m (650 ft) (Station
54, 53), and a thick mottling is exposed on a roadcut at an elevation of 219 m (720ft)
(Figure 34). Below the hardpan is a thick orange to light gray mottled layer, with a rust- red and gray mottled layer below.
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Figure 34: Hardpan and mottling on Lompoc-Casmalia Road and Bishop Road.
As mentioned above, the summit of Lompoc-Casmalia Road near Bishop Road has outcrops of overlying terrace deposits. Heading north down the summit, outcrops of
Tsq bedrock is visible from 198 to 173 m (650 to 570 ft) (Station 53), then terrace deposit at 173m (570 ft), until it reaches the pholad boring wave-cut platform mentioned above at
146 m (480 ft) (Station 4). Below the platform, bedrock is visible to the bottom of the hill.
5.4 Burton Mesa
Burton Mesa is a well-preserved, elevated, planar surface that starts with a steeper slope off the Purisima Hills to the northeast, and flattening out in a southwest direction.
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There is a decrease in elevation in a west and southwest direction, creating a beveled
surface on the edge of Burton Mesa, with a blanket of dune sand over it. Bedrock is only
exposed on the edges of gullies and arroyos that dissect this area, making it very
challenging to locate terrace platform surfaces (Figure 35).
Figure 35: Overview of Burton Mesa section in yellow.
The highest section of Burton Mesa is on the edge of the Purisima Hills, with an excellent exposure of terrace deposits at the intersection of Firefighter Road and Hwy 1
(Station 6; Figure 36).
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Figure 36: Stratigraphy at Firefighter Road
At road level, fluted, white, medium grained marine sand, 3 m (10 ft) thick, with
small rounded gravel and cobble layers at an elevation of 198 m (600 ft), a massive, cliff former of alluvium 6 m (20 ft) thick, overlain by a hardpan layer with orange and gray mottling, becoming orange near the top, at 216 m (710 ft) (Figure 37).
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Figure 37. Firefighter Road and Hwy. 1. Top: hardpan layer with dune sand above and alluvium below. Bottom: fluted marine sand with a basal gravel and clay layer overlain by alluvium.
Across Highway 1, exposures of Tsq is observed at an elevation of 177 m (550 ft), a wave-cut platform exposed on Punchbowl Road (Figure 38), with pholad borings, and large rounded beach cobbles (Station 61,62,63).
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Figure 38: Outcrop on Punchbowl Road. Top left: bedrock at bottom of photo and overlain with marine sand; top right: bedrock exposure with wave-cut platform; bottom: wave-cut platform exposed at road level.
The southern section of Burton Mesa is dissected by numerous drainages that deepen towards the east, from the mouth of the Santa Ynez River to Santa Lucia Canyon.
Santa Lucia Canyon is a large north-south drainage that dissects a major portion of
Burton Mesa, with bedrock exposures along the cliff edge. At the mouth of the canyon, a
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bedrock interface is visible at an elevation of 180 ft with dune sand overlying Tsq
(Station 85).
Santa Lucia Canyon bifurcates with Lake Canyon at Washington Blvd near the
Lompoc base gate. A wave-cut platform is exposed on a road cut with pholad borings at an elevation of 61 m (200 ft), overlain by dune sands (Station 58; Figure 39).
Figure 39: View of bedrock interface at Santa Lucia Canyon
Along the southern edge of Burton Mesa, numerous bedrock exposures are visible along the gullies and tributaries that drain into the Santa Ynez River. Drainages adjacent to 13th St. contain cliff exposures of Tsq near the top of Burton Mesa at an elevation of
116 m (380 ft) (Station 82, 83). The bedrock interface can be traced westward into smaller drainages.
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The next drainage is located at a hairpin turn on New Beach Road, where there are three terrace platforms, including the lower terrace along the Santa Ynez River
(Station 17). The highest terrace is at 85 m (280 ft), the middle is at 24-61m (80-200 ft), and the lowest terrace is at 4m (15 ft). The highest terrace can be traced along the southern edge of Burton Mesa (Station 25), and wraps around the western edge. Breaks in the geomorphic surface can be traced at similar elevation with the lower two terraces
(Station 84, 32). Cliff exposures of the lower terrace along the length of the coast is visible at an elevation of 12 m (40 ft) (Station 24,30,31; Figure 40).
In a gully north of Mira Road, exposures along the banks were observed from the beach to the train tracks. A terrace platform was located on the beach cliff (Station 32), and an outcrop of Tsq at the base of the train tracks (Station 33).
Figure 40: Wave-cut platform and overlying terrace deposits on Monterey Formation on a beach cliff exposure.
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In the northwest section of Burton Mesa near Tangair, is a quarry west of the train tracks with excellent exposures of a wave-cut platform at an elevation of 61 m (200 ft), with Tsq below (Station 49; Figure 41). Pholad borings in bedrock with pebble to small boulder size beach deposits composed of round to sub-rounded chert and porcellanite .
Figure 41: Stratigraphy at Cross Road and quarry
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Above is a layer of white marine sand with rounded gravels, followed by a
hardpan layer with orange and gray mottling, and dark-orange to rust colored iron
concretion pebbles at the surface. A pale pink-orange to buff dune sand is above the hardpan to the surface.
Across the train tracks from the quarry is another exposure along Cross Road with similar deposits at the same elevation (Station 48) (Figure 42).
Figure 42: Exposure on Cross Road near the quarry.
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Juan Pedro Canyon crosses Watt Street, exposing a wave-cut platform at an
elevation of 61 m (200 ft) with pholad borings and a hardpan layer similar to the quarry mentioned above (Station 12), and another wave-cut platform is visible where Cross
Road intersects Juan Pedro Canyon, at an elevation of 24 m (280 ft) (Station 47; Figure
43). Pholad borings on bedrock are overlain by white marine sand, and a hardpan layer of
orange and gray with hexagonal cracks and mottling (Figure 44).
Figure 43: Stratigraphy at Watt Street (bridge)
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Figure 44: Exposure at Juan Pedro Canyon and Cross Road. Top left: hardpan layer in cross section; Top right: orange and gray mottling; Bottom left: hardpan from a different angle; Bottom right: dark orange hardpan
A wave-cut platform at an elevation of 61 m (200 ft) is visible on Power Road, located off 13th Street, near San Antonio Creek, with pholad borings in Tsq and gravels
(Station 21; Figure 45). A 16 m (5 ft) thick layer of white, marine sand is above the
gravel layer, with pale orange-pink dune sand above. (Figure 46).
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Figure 45: Stratigraphy at Power Road
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Figure 46: Terrace deposits on Power Rd. Top left: beach cobble layer; Top right: wave-cut platform with overlying material; Bottom left: finer gravel layer; Bottom right: pholad borings in Tsq.
Near the intersection of 13th St. and Washington Avenue, a large, flat area of hardpan surface is exposed at an elevation of 109 m (360 ft), with white marine sand below (Station 22; Figure 47).
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Figure 47: Stratigraphy at Washington Avenue and 13th Street
The hardpan is ~1.5 m (5 ft) thick, with small stair-step cliffs of orange and gray hexagonal cracks and mottling (Figure 48)
From this location, a broad platform is downcut and Tsq is visible on the cliff,
with a wave-cut platform at 100 m (330 ft), with pholad borings and beach (Station 23)
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Figure 48: Hardpan exposure on 13th and Washington Ave. Top left: beach deposits with pholad bored clasts; Top right: stair step exposure of hardpan; Bottom left: hexagonal crack infilling; Bottom right: rust color layer of hardpan with iron concretions.
Near the airport, south of Mira Road, a broad hardpan surface is exposed at an elevation of 106 m (350 ft), and a wave-cut platform exposed in an adjacent drainage at
88 m (290 ft) (Station 26), with pholad borings, overlain by beach cobbles and pebbles
(Figure 49). In a gully, a thick layer of colluvium, with pebble to gravel size layers and lenses of coarse cobbles and occasional boulders interlayered. The hardpan is a dark
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orange to pale-orange and buff, cracking and mottling, with root casts sticking out at the surface. Iron concretions are scattered at the surface with pure orange hardpan of similar texture above the mottling. Above the hardpan, wind-blown dune sand covers the surface sporadically (Figure 50).
Figure 49: Stratigraphy south of Mira Road
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Figure 50: South of Mira Road near airport. Left: colluvium deposits; top Top right: hardpan exposed at surface; Bottom right: lower layers of marine terrace deposits.
5.5 Google Earth and DEM
Owing to the challenging terrain in the study site, Google Earth was used to tie stations together by identifying bedrock interfaces and inferring the contact. DEM hillshade was used to identify slope changes that were difficult to see in the field, the
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degree of downcutting along drainages, and surfaces that were too hard to see owing to overlying sediment.
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Chapter 6: Discussion
Characteristics of the five marine terrace platforms on Combar Road were used to
compare terrace surfaces across the study area. On North Mesa, three terraces were
identified in the geomorphic surface that could be traced into San Antonio Terrace. On
Burton Mesa, a flight of three younger terraces were identified on the coast, and two
higher terraces at the front of the Purisima Hills. Each flight of terraces can be compared
with another flight in the study area and terrace sequences can be determined by
similarities.
6.1 Interpretation of Geomorphic Expression
6.1.1 Geomorphic Surface
There are many locations in the study area that require interpretation of the
geomorphic surface for slope breaks and paleo-sea cliffs when there were no cliff
exposures or outcrops. This method was used to infer a terrace platform from one known
location to another.
The terraces on North Mesa were interpreted by geomorphic expression, and
traced into San Antonio Terrace by the stair-step topography on the surface. This method was also applied on the western edge of Burton Mesa.
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6.1.2 Elevation of Wave-Cut Platforms and Hardpan Layers
Recording the elevation of specific features on a marine terrace is one of the most
important tools in correlation. Elevations of the wave-cut platform and the hardpan layer were identified and plotted on a scatter graph, with linear clusters indicating a common elevation throughout the study area (Figure 51, 52).
The plot of wave-cut platform and hardpan elevations suggests there are four common clusters in the study area at 12 to 24 m (40-80 ft), 48 to 61 m (160-200 ft), 79 to
116 m (260-380 ft), and 146 to 183 (480-600 ft). The lowest two clusters include the first two terraces along the coast in the study area, extending south to include the lowest platforms of Purisima Point, and the coastal platforms on the west edge of Burton Mesa.
The third cluster includes the broad platform the extends from South Casmalia into San
Antonio Mesa, and the elevated section of Burton Mesa . The last cluster includes the higher terraces located in all three sections of the study area. We did not cluster the highest terrace because of so few points.
We identified five terrace platforms on Combar Road, and the graph agrees with the number of platforms and the elevation. Google Earth and DEM hillshade were used to locate the interface between bedrock and terrace deposits, with data that agree with the elevation clusters, and geomorphic expression of terrace surfaces.
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Figure 51: Horizontal clusters of plots may indicate a common elevation throughout the study area, and four clusters are marked by dashed color ovals. Vertical scale is elevation in feet, and horizontal scale is relative distance of localities.
Figure 52: Hardpan elevations. Color ovals mark linear clusters of common hardpan elevations in the study area. Vertical scale is elevation in feet and horizontal scale is elative distance of localities.
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6.1.3 Erosional Patterns on the Geomorphic Surface
Geomorphic patterns on the marine terrace surface can be applied to relative
dating from one terrace to another. In general, “older” terraces at higher elevations show
greater development of gullying (Muhs, 2000).
Summit Terrace, the highest and oldest terrace with a well-dissected surface from
deep-gully incision, reveals an almost complete sequence of terrace sediments. In
comparison, the next oldest terrace, Orion, has also been eroded, however, the alluvium is
exposed in a small area and the bedrock has not been exposed yet. We observed similar
erosion occurring in the alluvium near the summit of Lompoc-Casmalia Road. on North
Mesa suggesting that the ages are similar.
6.2 Stratigraphy of Terrace Deposits
The overlying material on a marine terrace can be useful for correlation if there is a distinct marker bed(s) that is (are) unmistakable, or a unique package of sediment that is easily recognized.
6.2.1 Marine and Nonmarine Deposits
A color bar chart with generalized lithology of each platform surface was constructed to find similarities on the package of sediments overlying a marine terrace, resulting in three different sediment groups observed. (Figure53)
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Figure 53: Color bar graph of lithology. Vertical axis indicates the percent of each lithology.
With a few exceptions, thick, cliff formers of alluvium were only found on the highest terraces in the study area. Younger terraces may have alluvium if they are located along a mountain front, and one exception in Burton Mesa with colluvium. The highest terraces in the study area are correlated to either the Summit Terrace or Orion Terrace on
Combar Road
The hardpan layer on Summit Terrace contains the thickest layers of mottling, located above and below the resistant sandstone layer. Other terraces have mottling, however, they are only found below the hardpan layer. The highest terrace near Bishop
Road, North Mesa, and Firefighter Road on Burton Mesa are correlated with Summit
Terrace, based on the package of sediments, and the resistant sandstone layer with mottling above and below.
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Numerous locations were observed where white marine sand was overlain by a
hardpan layer with mottling at the base, punky, friable, rust colored cliff formers with
iron concretions, with a slope forming yellow-pink sand dune deposit at the top. This is
the only hardpan layer that decreases in mottling as you go up-section. All of the
locations with this package were found with elevations from 85 m (280 ft) to 121 m (400
ft), suggesting one marine terrace that covers most of Burton Mesa.
6.2.2 Fossils and Other Organic Materials
In the sea cliff on both sides of Lions Head, a fossiliferous layer was observed on
the wave-cut platform. Samples were collected and a faunal assemblage was identified
(eol.org). Species collected included Niveotectura pallida, Fusitriton oregonensis,
Acmaea mitra, Crepidula sp.?, and Amphissa versicolor (Figure 54) .
Temperatures during the global ocean minimum and maximum range from -2.072 Cº to
29.546 ºC (eol.org). The temperature ranges for species are listed below.
Acmaea mitra 9.215 ºC-10.345 ºC
Amphissa versicolor 6.89 ºC-10.15 ºC
Crepidula sp. All temperatures
Fusitriton oregonensis 3.8 ºC-10.15 ºC
The temperature range of species collected, indicate a cool water faunal
assemblage (eol.org). MIS 3 (60 ka) and MIS 5a (80 ka) water temperatures were much cooler, while the MIS 5c (120 ka) was warmer, similar to modern ocean temperatures
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(Clark, 1994). The fossiliferous layer is interpreted to correlate with either MIS 3 or MIS
5a.
A complete pholad shell in situ was collected on a wave-cut platform south of
LHF. An active quarry on Brown’s Beach has yielded articulated fossil material that has been identified as Paramylodon harlani (ground sloth), part of the Rancholabrean megafauna. Samples from both locations were given to Christopher Ryan, VAFB 30 CES
Cultural Resource Management Office, for future age dating.
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Figure 54: Fossil material collected.
6.3 Structural Factors in Terrace Deformation
Major faults in the Casmalia Range include the Lion’s Head Fault (LHF), and an unnamed adjacent fault to the north. The LHF is a steep, north dipping, reverse fault, and the unnamed fault is also a reverse fault but it is south dipping, indicating the block between the two fault traces has experienced a greater degree of uplift and deformation
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(Clark, 1994). Previous studies indicate that the Casmalia Range has experienced
average Quaternary uplift at a rate of 0.15mm/yr. with only minor movement in the late
Quaternary (Clark, 1994).
Measurements were taken from wave-cut platforms closest to the seaward edge to provide a relative measure of offset. Elevation differences between terraces north of LHF and the remainder of the study area are seen on Table 4.
Terrace N. of LHF S of LHF Purisima 6m (20ft) 2.4m (8ft) Point Minuteman 18m (60ft) 12m (40ft) Vandenberg 85m (280ft) 61m (200ft) Orion 165m (540ft) 146m (480ft) Summit 250m (820ft) 195m (640ft) Table 4: Generalized elevation differences on either side of the Lions Head Fault.
An area located on Globe Street in the South Casmalia section indicates
three remnant platform surfaces that appear to be offset by the two faults. The LHF is a
north-side up reverse fault and the unnamed fault is a south-side up normal fault, creating
a structural high point. Past studies suggest these remnant terraces are older since the
back-edge is assumed at 247 m (810 ft), and it steps down across the trace of both faults.
Exposures in gullies south of the LHF indicate a continuous exposure of bedrock
interface from 85 m (280 ft) to the fault contact, and again along cliff exposures at the
anvils on the banks of Shuman Creek. The results of this study agree that there is a
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remnant back-edge above the faults, however, it is debatable which terrace surface is on the upturned block between the faults. The Vandenberg terrace may well extend to the trace of the fault The section north of the LHF was not mapped.
Burton Mesa appears to be a flat terrace surface, although one continuous platform decreases from 85 m to 122 m (280 ft to 400 ft), far steeper than the average seaward slope of a terrace platform. A gradual downhill slope marks the edge of Burton
Mesa from the Purisima Hills, but once it reaches Burton Mesa, the highest elevation is located in the center along a linear, north-south trending rise that gradually decreases to the edges of the mesa. The highest elevation is at 134 m (440 ft), with a downward slope to 85 m (280 ft), with no evidence of multiple terrace platforms. This gentle warping may be attributed to the Purisima Syncline, located to the east along the Purisima Hills.
6.4 Correlation of Marine Terraces
6.4.1 Correlation of Marine Terrace to Paleo-Sea-level Stages
A graph of global sea-level curves was used to correlate marine terraces with paleo-sea-level stages. Using the altitudinal terrace spacing method (Muhs, 2000), a vertical profile was constructed of the flight of terraces on Combar Road, and lines were plotted against a global sea-level graph to determine the best fit. This method correlates the unique spacing of terraces with the timing and spacing of each sea-level highstand.
Since we did not acquire any age dates in the study area, this method was applied multiple times, each using different ages for platforms until a best fit was determined. It
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was interpreted the best fit to correlate with the MIS 3 (60 ka) terrace as the lowest and
the MIS 9 (330 ka) terrace as the highest (Figure 55).
Figure 55: Correlation of terraces to global sea-level curve with no tectonic influence. Sea-level elevations measured in meters from modern sea-level.
The behavior of each sea-level highstand was also applied when determining the correlating sea-level stage. The MIS 5a was a prominent highstand, which correlates to the broad lower platform along the coast that is Minuteman Terrace. Since Purisima Point
Terrace is younger and lower, it correlates with MIS 3 (60 ka). The next prominent highstand is marked by MIS 5e (120 ka), which correlates with the large expanse that the
Vandenberg Terrace covers in the study area.
Though MIS 7 is rarely preserved, the Orion Terrace may correlate since it is found in such small, thin remnants. The MIS 9 was another prominent highstand, which
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would seem to correlate better with Summit Terrace, at the top of all the hills in the study area.
6.4.2 Correlation of Terrace Platforms in the Study Area
Analysis of data collected in the field, coupled with relative terrace age and correlation methods, a comprehensive interpretation can be determined and mapped across the study area (Figure 56, Table 5).
Figure 56: Generalized grouping of terrace by similarities. Pink: Purisima Point T.; purple: Minuteman T.; Blue: Vandenberg T.; Orange: Orion T.; Red: Summit T. Numbers indicate stations.
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Purisima Point Terrace - the lowest and youngest platform identified on the coast. Cliff exposures are nearly continuous in the Casmalia Range, except on rocky points, and the wave-cut platform elevation remains constant at 9m (30ft), often cut into bedrock to form a slight notch with pholad borings. South of the Lion’s Head Fault, the terrace platform is noticeably lower in elevation 6 m (20ft) but continues south into the
Shuman floodplain towards the edge of San Antonio Terrace (Figure 57). Based on
DEM hillshade, the Purisima Point Terrace can be traced along the beach cliffs into San
Antonio Terrace (Figure 58), inferred when there is no bedrock exposure. Around
Purisima Point, the dunefield gives way to beach cliffs south of Purisima Point, and becomes the broad platform surface on the west edge of Burton Mesa (Figure 59).
Minuteman Terrace - mapped nearly continuously by Combar Rd., except for the Brown’s Beach area, where the fore-edge of Minuteman Terrace is inferred until it is visible again south of Brown’s Beach past the rocky point. Minuteman terrace remains continuous along the coastline, except along rocky points, and south of Lions Head Fault, turning to the southeast towards Shuman Creek where it transitions into stream terraces of similar elevation (Figure 57). The terrace platform is part of the Coastal Dunes section (Figure 58), and a wide section on the northwest and west portion of Burton Mesa
(Figure 59).
Previous studies indicated the coast was a single terrace platform, and Clark
(1993), was the only person who thought there might be a second terrace. The field work from this present study distinguished two distinct wave cut benches along the coast.
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Vandenberg Terrace – only exposed as remnants on Combar Rd., with an
elevation of 85-103 m (280-340 ft). The Vandenberg Terraces correlates with multiple
platform surfaces south of Casmalia Range (Figure 57),, into San Antonio Terrace, and
most of Burton Mesa.
Orion Terrace – on Combar Road, erosion exposes an almost complete package
of sediment with a wave-cut platform at an elevation of 164 m (540ft), and 183 m (600 ft)
at the hardpan surface (Figure 57). Orion Terrace is one of the older terraces with
alluvium deposited below a well-developed hardpan layer with mottling under the hardpan layer.
Orion Terrace - is correlated with South Shuman, both ends of El Rancho Rd. on
the San Antonio Terrace (Figure 58), and west of Firefighter Rd. on Burton Mesa (Figure
59). All three locations have similar elevations, and contain similar deposits, with
mottling below the hardpan surface. This study interprets the wave-cut platform to be
west of the intersection between Hwy 1 and Firefighter Rd., as a thin remnant where the
landslide is located, inferred based on elevation towards the Purisima Hills.
Summit Terrace– the well-developed terrace platform on Combar Road (Figure
57) with the resistant calcareous sand layer that is observed just below the summit of
North Mesa (Figure 58), forming a cliff that can be traced along drainages on Bishop Rd.
The marine sand on Firefighter Rd. is at an elevation correlated with Summit Terrace that
is traced up to the summit (Figure 59).
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Figure 57: Terrace assignment on Casmalia Range. Pink: Purisima Point; Purple: Minuteman; Blue: Vandenberg; Orange: Orion; Red: Summit. Numbers indicate stations.
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Figure 58: Terrace assignment on San Antonio Terrace and North Mesa. Pink: Purisima Point; Purple: Minuteman; Blue: Vandenberg; Orange: Orion; Red: Summit Numbers indicate stations.
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Figure 59: Terrace Assignment on Burton Mesa. Pink: Purisima Point; Purple: Minuteman; Blue: Vandenberg; Orange: Orion; red: Summit.
Terrace MIS Age Locations Foredge elev. (ka) avg. m(ft) Purisima 3 60 Exposed on modern beach cliff faces, makes up the lower (10-30 ft) Point surface of west Burton Mesa
Minuteman 5a 80 Low coastal terrace along Pt. Sal Rd., in the dunes at San 40-60 Antonio Terrace, northwest and west margin of Burton Mesa4 Vandenberg 5e 120 Remnants on Combar Rd., extensive terrace surfaces 200-340 across S. Casmalia, San Antonio Terrace, and Burton Mesa Orion 7 210 Remnants on Combar Rd., higher section of San Antonio 500-600 Mesa, North Mesa, and near the landslide off Firefighter Rd. Summit 9 330 Highest terrace on Combar Rd., the summit of N. Mesa, 640-840 and the summit near Firefighter Rd. in the Purisima Hills. Table 5: Terrace assignments to sea-level highstands
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6.4.3 Comparison of Marine Terraces with Past Studies
Data from this study indicates conflicting information from previous studies
conducted on VAFB. A table was constructed with the ages assigned by each study
(Table 6).
Woodring and Bramlette (1966), assigned three terraces in the study area, north of
LHF, as Low, Intermediate, and High terraces. They assigned the coastal terrace
platform as one surface (Low). The intermediate terrace was assigned to all surfaces on
San Antonio Terrace and Burton Mesa, including the summit of the hills, and they
assigned a high terrace but did not mention it at all. The research in this current study
disagrees with Woodring and Bramlette (1966) based on the number of sequential marine
terrace flights observed in multiple locations together with the description of each terrace.
Johnson (1984), was the closest in terrace localities, although he assigned the
highest terrace to be either MIS 9,11, or 13, which is vague, and he did not assign age
dates. This current study correlates the marine terrace surfaces on San Antonio Terrace
and North Mesa with the same surfaces he observed, though not in the same sequential
number.
Clark’s (1993) study was the most ambitious as he interpreted nine terraces in the
Casmalia Range, and assigned an age to the highest terrace over a million years old,
though he did mention it was unlikely. He cited Woodring and Bramlette (1966) and the
five terrace platforms on Combar Road, but mapped more. He was identifying terrace
surfaces to known landslide areas, or on any planar surface without any evidence, and it
appears that he correlated multiple ages on the same surface.
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Studies conducted outside of the field area include Hanson et. al. (1994), who identified terraces along the San Simeon coast with the lowest terrace identified as MIS
3, and a flight of five terraces. The width, terrace spacing, and lithology in the San
Simeon area are similar to the terraces in the study area. In the south, Rockwell and
others (1992) were working on or near the South Branch of the Santa Ynez Fault along
the Gaviota coastline.
The terraces on VAFB suggest similar ages, number of terraces, lithology, and
spacing with previous studies along the California coast. The development of emergent
coastlines in central California, may indicate uplift rates similar to the study area.
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MIS Age Terrace # Terrace Terrace # Terrace # (ka) Johnson # Woodring Kitao Clark
0 0 Qt0 modern
1 10 Qt1
3a 40 Qt2 Q1 1 low
3c 60 Qt3 Q2 1 low Purisima Point
5a 83 Qt4 Q3 2 Minuteman
5c 105 3
5e 120 Qt5 Q4 4 interm Vandenberg
7 210 Qt6 5 high Orion
9 330 Qt7 Q5 Bryce
11 430 Q6
13 480 Q8
15 560 Q9
Table 6: Comparison of marine terraces assigned by author.
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6.5 Future Work
Identifying terrace platforms on VAFB is just a beginning for the vast amount of work that still needs to be conducted here. Acquiring true age dates on terrace platforms is critical in the study area and should be a priority moving forward. At the time of thesis submission, samples were collected in the study area and mailed to Dr. Daniel Muhs at the USGS Headquarters in Denver, CO, for 14C and amino stereochemistry age dating.
The northern section of VAFB was studied extensively, but the southern section has not been studied between Point Conception and the Lompoc Valley. New information from this study may allow for terraces to be traced across the broad Santa
Ynez River Valley into the Lompoc Mesa.
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Conclusion
Five terrace platforms were identified and mapped on Vandenberg Air Force Base
using a combination of methods including mapping the elevation and distribution of
marine terraces, constructing detailed stratigraphic columns for overlying terrace
deposits, interpretation of fossil evidence, and observation of the geomorphic surface
expression. A flight of five terraces on Combar Road was used to correlate with other
terraces in the study area, based on altitudinal terrace spacing and geomorphic
expression.
Relative ages were determined by correlating the elevation and uplift rate of
terraces to the global paleo-isotopic sea-level curve derived from oxygen isotope ratios
(Lajoie, 1986), and paleo sea-level elevations (Shackleton and Opdyke, 1973). Terraces were assigned to MIS 3 (60 ka), MIS 5a (80 ka), MIS 5e (120 ka), MIS 7 (210 ka), and
MIS 9 (330 ka), with no evidence of MIS 5c (105 ka) in the study area.
Detailed stratigraphic description of overlying terrace deposits, observation of geomorphic surface expression, and measurement of elevations of wave-cut platforms and hardpan surfaces were applied to determine the correlation with other terraces across the study area.
The two lower terraces comprise the broad, coastal platform that extend from
Point Sal to Burton Mesa. Although the third terrace is observed in small surfaces on
Combar Road, it is the most extensive terrace in the study area, correlating with the broad surfaces of South Casmalia, San Antonio Terrace, and Burton Mesa. The upper terraces
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are found in the higher elevations of the study area, along the summit of North Mesa and the Purisima Hills.
Prior to this study, past research on marine terraces conducted on VAFB has been limited and restricted access has created a gap in data separating studies from marine terraces to the north and south. A comprehensive map of marine terraces in the study area may bridge the gap to studies conducted along the coast from Santa Cruz to San
Simeon to the north, and allow the terraces on Burton Mesa to be extended onto the
Lompoc Mesa for future studies to the south.
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Appendix A
Oversize Map One: Marine Terrace Localities, Vandenberg Air Force Base, Santa Barbara County, California
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