California State Waters Map Series—Offshore of Refugio 

Beach, California

By Samuel Y. Johnson, Peter Dartnell, Guy R. Cochrane, Nadine E. Golden, Eleyne L. Phillips, Andrew C. Ritchie, 

Lisa M. Krigsman, Bryan E. Dieter, James E. Conrad, H. Gary Greene, Gordon G. Seitz, Charles A. Endris, Ray W. 

Sliter, Florence L. Wong, Mercedes D. Erdey, Carlos I. Gutierrez, Mary M. Yoklavich, Amy E. East, and Patrick E.

Hart

(Samuel Y. Johnson and Susan A. Cochran, editors)

Pamphlet to accompany

Scientific Investigations Map 3319

2015 

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015 

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment—visit 

http://www.usgs.gov or call 1–888–ASK–USGS

For an overview of USGS information products, including maps, imagery, and publications,

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Suggested citation:

Johnson, S.Y., Dartnell, P., Cochrane, G.R., Golden, N.E., Phillips, E.L., Ritchie, A.C., Krigsman, L.M., Dieter, B.E., 

Conrad, J.E., Greene, H.G., Seitz, G.G., Endris, C.A., Sliter, R.W., Wong, F.L., Erdey, M.D., Gutierrez, C.I., 

Yoklavich, M.M., East, A.E., and Hart, P.E. (S.Y. Johnson and S.A. Cochran, eds.), 2015, California State Waters 

Map Series—Offshore of Refugio Beach, California: U.S. Geological Survey Scientific Investigations Map 3319, 

pamphlet 42 p., 11 sheets, scale 1:24,000, http://dx.doi.org/10.3133/sim3319. 

ISSN 2329-132X (online)

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents 

Preface...........................................................................................................................................................................1

Chapter 1. Introduction...................................................................................................................................................3

By Samuel Y. Johnson

Publication Summary..................................................................................................................................................5

Chapter 2. Bathymetry and Backscatter-Intensity Maps of the Offshore of Refugio Beach Map Area (Sheets 1, 2, 

and 3).............................................................................................................................................................................8

By Peter Dartnell

Chapter 3. Data Integration and Visualization for the Offshore of Refugio Beach Map Area (Sheet 4)........................10

By Peter Dartnell

Chapter 4. Seafloor-Character Map of the Offshore of Refugio Beach Map Area (Sheet 5) ........................................11

By Eleyne L. Phillips, Mercedes D. Erdey, and Guy R. Cochrane

Chapter 5. Ground-Truth Studies for the Offshore of Refugio Beach Map Area (Sheet 6)...........................................16

By Nadine E. Golden and Guy R. Cochrane

Chapter 6. Potential Marine Benthic Habitat Map of the Offshore of Refugio Beach Map Area (Sheet 7) ...................19

By H. Gary Greene and Charles A. Endris

Classifying Potential Marine Benthic Habitats ..........................................................................................................19

Examples of Attribute Coding ...................................................................................................................................22

Map Area Habitats....................................................................................................................................................23

Chapter 7. Subsurface Geology and Structure of the Offshore of Refugio Beach Map Area and the Santa Barbara 

Channel Region (Sheets 8 and 9)................................................................................................................................24

By Samuel Y. Johnson, James E. Conrad, Eleyne Phillips, Andrew Ritchie, Florence L. Wong, Ray W. 

Sliter, Amy E. East, and Patrick E. Hart

Data Acquisition........................................................................................................................................................24

Seismic-Reflection Imaging of the Continental Shelf................................................................................................24

Geologic Structure and Recent Deformation ............................................................................................................25

Thickness and Depth to Base of Uppermost Pleistocene and Holocene Deposits ...................................................26

Chapter 8. Geologic and Geomorphic Map of the Offshore of Refugio Beach Map Area (Sheet 10)...........................29

By James E. Conrad, Andrew C. Ritchie, Samuel Y. Johnson, Gordon G. Seitz, and Carlos I. 

Gutierrez

Geologic and Geomorphic Summary........................................................................................................................29

Description of Map Units ..........................................................................................................................................31

Offshore Geologic and Geomorphic Units ............................................................................................................31

Onshore Geologic and Geomorphic Units ............................................................................................................32

Chapter 9. Predicted Distribution of Benthic Macro-Invertebrates for the Offshore of Refugio Beach Map Area and 

the Santa Barbara Channel Region (Sheet 11)............................................................................................................34

By Lisa M. Krigsman, Mary M. Yoklavich, Nadine E. Golden, and Guy R. Cochrane

Acknowledgments........................................................................................................................................................36

References Cited .........................................................................................................................................................37

Figures

Figure 1–1. Physiography of Santa Barbara Channel region......................................................................................... 6

Figure 1–2. Coastal geography of Offshore of Refugio Beach map area ...................................................................... 7

Figure 4–1. Detailed view of ground-truth data, showing accuracy-assessment methodology.................................... 15

Figure 5–1. Photograph of camera sled used in USGS 2008 ground-truth survey ...................................................... 16
iv

Figure 5–2. Graph showing distribution of primary and secondary substrate determined from video observations in 

Offshore of Refugio Beach map area ....................................................................................................... 18

Tables 

Table 4–1. Conversion table showing how video observations of primary substrate, secondary substrate, and 

abiotic seafloor complexity are grouped into seafloor-character-map Classes I, II, and III for use in 

supervised classification and accuracy assessment................................................................................. 14

Table 4–2. Accuracy-assessment statistics for seafloor-character-map classifications.............................................. 15

Table 7–1. Area, sediment-thickness, and sediment-volume data for California’s State Waters in Santa Barbara 

Channel region, as well as in Offshore of Refugio Beach map area......................................................... 28

Table 8–1. Areas and relative proportions of offshore geologic map units in Offshore of Refugio Beach map area .. 30

Map Sheets 

Sheet 1. Colored Shaded-Relief Bathymetry, Offshore of Refugio Beach Map Area, California

By Peter Dartnell, Eleyne L. Phillips, and David P. Finlayson

Sheet 2. Shaded-Relief Bathymetry, Offshore of Refugio Beach Map Area, California

By Peter Dartnell, Eleyne L. Phillips, and David P. Finlayson

Sheet 3. Acoustic Backscatter, Offshore of Refugio Beach Map Area, California

By Peter Dartnell, Eleyne L. Phillips, and David P. Finlayson

Sheet 4. Data Integration and Visualization, Offshore of Refugio Beach Map Area, California

By Peter Dartnell

Sheet 5. Seafloor Character, Offshore of Refugio Beach Map Area, California

By Eleyne L. Phillips, Mercedes D. Erdey, and Guy R. Cochrane

Sheet 6. Ground-Truth Studies, Offshore of Refugio Beach Map Area, California

By Nadine E. Golden, Guy R. Cochrane, Peter Dartnell, and Lisa M. Krigsman

Sheet 7. Potential Marine Benthic Habitats, Offshore of Refugio Beach Map Area, California

By Bryan E. Dieter, H. Gary Greene, Charles A. Endris, Mercedes D. Erdey, and Nadine E. Golden

Sheet 8. Seismic-Reflection Profiles, Offshore of Refugio Beach Map Area, California

By Samuel Y. Johnson, Ray W. Sliter, James E. Conrad, Andrew C. Ritchie, Amy E. East, Patrick E. 

Hart, and Eleyne L. Phillips

Sheet 9. Local (Offshore of Refugio Beach Map Area) and Regional (Offshore from Refugio Beach to Hueneme 

Canyon) Shallow-Subsurface Geology and Structure, Santa Barbara Channel, California

By Samuel Y. Johnson, Eleyne L. Phillips, Andrew C. Ritchie, Florence L. Wong, Ray W. Sliter,

Amy E. East, James E. Conrad, and Patrick E. Hart

Sheet 10. Offshore and Onshore Geology and Geomorphology, Offshore of Refugio Beach Map Area, California

By James E. Conrad, Andrew C. Ritchie, Samuel Y. Johnson, Gordon G. Seitz, and Carlos I. 

Gutierrez

Sheet 11. Predicted Distribution of Benthic Macro-Invertebrates, Offshore of Refugio Beach Map Area and Santa 

Barbara Channel Region, California

By Lisa M. Krigsman, Mary M. Yoklavich, Guy R. Cochrane, and Nadine E. Golden
California State Waters Map Series—Offshore of Refugio 

Beach, California

By Samuel Y. Johnson,1 Peter Dartnell,1 Guy R. Cochrane,1 Nadine E. Golden,1 Eleyne L. Phillips,1 Andrew C. 

Ritchie,1 Lisa M. Krigsman,2 Bryan E. Dieter,3 James E. Conrad,1 H. Gary Greene,3 Gordon G. Seitz,4 Charles A. 

Endris,3 Ray W. Sliter,1 Florence L. Wong,1 Mercedes D. Erdey,1 Carlos I. Gutierrez,1 Mary M. Yoklavich,2 Amy E. 

East,1 and Patrick E. Hart1

(Samuel Y. Johnson1 and Susan A. Cochran,1 editors)

Preface 

In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping 

Program (CSMP), designed to create a comprehensive seafloor map of high-resolution bathymetry, 

marine benthic habitats, and geology within California’s State Waters. The program supports a large 

number of coastal-zone- and ocean-management issues, including the California Marine Life Protection 

Act (MLPA) (California Department of Fish and Game, 2008), which requires information about the 

distribution of ecosystems as part of the design and proposal process for the establishment of Marine 

Protected Areas. A focus of CSMP is to map California’s State Waters with consistent methods at a 

consistent scale.

The CSMP approach is to create highly detailed seafloor maps through collection, integration, 

interpretation, and visualization of swath sonar bathymetric data (the undersea equivalent of satellite 

remote-sensing data in terrestrial mapping), acoustic backscatter, seafloor video, seafloor photography, 

high-resolution seismic-reflection profiles, and bottom-sediment sampling data. The map products 

display seafloor morphology and character, identify potential marine benthic habitats, and illustrate both 

the surficial seafloor geology and shallow (to about 100 m) subsurface geology. It is emphasized that the 

more interpretive habitat and geology maps rely on the integration of multiple, new high-resolution 

datasets and that mapping at small scales would not be possible without such data. 

This approach and CSMP planning is based in part on recommendations of the Marine Mapping 

Planning Workshop (Kvitek and others, 2006), attended by coastal and marine managers and scientists 

from around the state. That workshop established geographic priorities for a coastal mapping project and 

identified the need for coverage of “lands” from the shore strand line (defined as Mean Higher High 

Water; MHHW) out to the 3-nautical-mile (5.6-km) limit of California’s State Waters. Unfortunately, 

surveying the zone from MHHW out to 10-m water depth is not consistently possible using ship-based 

surveying methods, owing to sea state (for example, waves, wind, or currents), kelp coverage, and 

shallow rock outcrops. Accordingly, some of the maps presented in this series commonly do not cover 

the zone from the shore out to 10-m depth; these “no data” zones appear pale gray on most maps. 

This map is part of a series of online U.S. Geological Survey (USGS) publications, each of 

which includes several map sheets, some explanatory text, and a descriptive pamphlet. Each map sheet 

 

1 U.S. Geological Survey

2 National Oceanic and Atmospheric Administration, National Marine Fisheries Services

3 Moss Landing Marine Laboratories, Center for Habitat Studies 4 California Geological Survey
2

is published as a PDF file. Geographic information system (GIS) files that contain both ESRI5 ArcGIS 

raster grids (for example, bathymetry, seafloor character) and geotiffs (for example, shaded relief) are 

also included for each publication. For those who do not own the full suite of ESRI GIS and mapping 

software, the data can be read using ESRI ArcReader, a free viewer that is available at 

http://www.esri.com/software/arcgis/arcreader/index.html (last accessed March 27, 2013). 

The California Seafloor Mapping Program (CSMP) is a collaborative venture between numerous 

different federal and state agencies, academia, and the private sector. CSMP partners include the 

California Coastal Conservancy, the California Ocean Protection Council, the California Department of 

Fish and Game, the California Geological Survey, California State University at Monterey Bay’s 

Seafloor Mapping Lab, Moss Landing Marine Laboratories Center for Habitat Studies, Fugro Pelagos, 

Pacific Gas and Electric Company, National Oceanic and Atmospheric Administration (NOAA, 

including National Ocean Service—Office of Coast Surveys, National Marine Sanctuaries, and National 

Marine Fisheries Service), U.S. Army Corps of Engineers, the Bureau of Ocean Energy Management, 

the National Park Service, and the U.S. Geological Survey. 

 

5 Environmental Systems Research Institute, Inc.
3

Chapter 1. Introduction

By Samuel Y. Johnson

The map area offshore of Refugio Beach, California, which is referred to herein as the “Offshore 

of Refugio Beach” map area (figs. 1–1, 1–2), lies within the western Santa Barbara Channel region of 

the Southern California Bight (see, for example, Lee and Normark, 2009). This geologically complex 

region forms a major biogeographic transition zone, separating the cold-temperate Oregonian province 

north of Point Conception from the warm-temperate California province to the south (Briggs, 1974). 

The map area lies offshore of the steep, incised, south flank of the Santa Ynez Mountains. The 

crest of the range, which lies about 8 km from the shoreline (north of the map area), has a maximum 

elevation of about 780 m. The coastal zone is largely open space, partly used for livestock grazing, with 

no significant towns or population centers. The most significant developments are the recreational state 

beaches at El Capitan Beach and Refugio Beach. Highway 101 crosses the map area, adjacent to and 

within a few hundred meters of the shoreline. 

The Offshore of Refugio Beach map area lies in the west-central part of the Santa Barbara littoral 

cell (fig. 1–1), which is characterized by west-to-east transport of sediment from Point Arguello on the 

northwest to Hueneme and Mugu Canyons on the southeast (see, for example, Griggs and others, 2005; 

Hapke and others, 2006). On the basis of harbor dredging records, Griggs and others (2005) reported 

east-southeast longshore drift rates that range from about 160,000 to 800,000 tons/yr, averaging 400,000 

tons/yr. At the east end of the littoral cell, eastward-moving sediment is trapped by Hueneme and Mugu 

Canyons (fig. 1–1) and then transported down these canyons into the deep-water Santa Monica Basin 

(Normark and others, 2009). 

Sediment supply to the western and central part of the littoral cell is mainly from relatively small 

coastal watersheds, which have an estimated cumulative annual sediment flux of 640,000 tons/yr 

between Point Arguello and the Ventura River (Warrick and Farnsworth, 2009). Within the Offshore of 

Refugio Beach map area, these coastal watersheds include (from east to west) Cañada del Capitan, 

Tajiquas Creek, Arroyo Hondo, Cañada del Molino, and several unnamed canyons and creeks (fig. 1–2). 

The much larger Santa Ynez and Santa Maria Rivers, the mouths of which are 80 to 120 km northwest 

of the map area, are not considered significant sediment sources because Point Conception and Point 

Arguello provide obstacles to southeasterly sediment transport, and much of their sediment load 

presently is trapped in dams (Griggs and others, 2005). Additionally, the large Ventura and Santa Clara 

Rivers have high sediment yields (Warrick and Farnsworth, 2009) but lie about 70 km to the southeast, 

“downdrift” from the map area, and, therefore, are not sediment sources for the Offshore of Refugio 

Beach map area. Coastal-watershed discharge and sediment load are highly variable, characterized by 

brief large events during major winter storms and long periods of low flow and minimal sediment load 

between storms. In recent history, the majority of high-discharge, high-sediment-flux events have been 

associated with the El Niño phase of the El Niño–Southern Oscillation (ENSO) climatic pattern 

(Warrick and Farnsworth, 2009). 

Narrow beaches with thin sediment (sand and pebbles) cover, fronted by low (10- to 20-m-high) 

cliffs that are capped by a narrow coastal terrace, characterize the shoreline in the Offshore of Refugio 

Beach map area. Notably, a large boulder delta has formed at El Capitan Beach by deposition from 

Cañada del Capitan Creek (Griggs and others, 2005). Beaches are subject to wave erosion during winter 

storms, followed by gradual sediment recovery or accretion during the late spring, summer, and fall 

months during the gentler wave climate. Hapke and others (2006) suggested that essentially no net 

change (accretion or erosion) to the beaches in the map area has occurred over the long term (since the 

mid- to late 1800s); however, the beaches have been eroding at an average rate of about 0.6 m/yr over 

the short term (1976 to 1998). Hapke and Reid (2007) also indicated that coastal bluffs in the map area 
4

are eroding at a rate of about 0.2 m/yr. As with stream discharge and sediment flux, coastal erosion has 

been most acute during the El Niño phases of the ENSO climatic pattern.

The Offshore of Refugio Beach map area consists of relatively flat and shallow continental shelf. 

The shelf dips gently seaward (about 0.8° to 1.0°), and water depths at the 3-nautical-mile (5.6-km) limit 

of California’s State Waters range from about 80 to 100 m. Locally, slopes are steeper (about 2° to 3°) in 

relatively shallow water (10 to 30 m) on the seaward-facing slope of nearshore bars (fig. 1–2, see also, 

sheets 1, 2). This part of the Southern California Bight is relatively well protected from large Pacific 

swells from the north and northwest by Point Conception and from south and southwest swells by 

offshore islands and banks (O’Reilly and Guza, 1993). Fair-weather wave base is typically shallower 

than 20-m water depth, but winter storms are capable of resuspending fine-grained sediments in 30 m of 

water (Xu and Noble, 2009, their table 7), and so shallow (depths of 30 to 60 m) shelf sediments in the 

map area probably are remobilized on an annual basis. As with sediment discharge from rivers, the 

largest wave events and the highest sediment transport rates on the shelf are typically associated with 

ENSO events. The shelf is underlain by bedrock and variable amounts (0 to 12 m) of upper Quaternary 

shelf, estuarine, and fluvial sediments deposited as sea level fluctuated in the late Pleistocene (see Sheet 

9 of this report; see also, Slater and others, 2002; Draut and others, 2009). 

In the map area, the shelf break is at depths of about 90 m and lies about 5.6 to 6.4 km offshore. 

Beyond the shelf break, the slope is steep (as much as about 7°) and unstable. Several submarine 

landslides have been documented offshore of Goleta, a few kilometers east of the map area (Fisher and 

others, 2005; Greene and others, 2006; Lee and others, 2009). One of the largest slides (about 130 km2

) 

is the Goleta landslide complex (fig. 1–1), which is inferred to have been initiated more than 200,000 

year ago but which also includes three recent failure thought to have been generated 8,000 to 10,000 

years ago (Greene and others, 2006); in addition, smaller slides may have occurred as recently as 300 

years ago (Lee and others, 2009). Modeling of the larger events indicates that they could have generated 

a local, 10-m-high tsunami (Greene and others, 2006). 

Seafloor habitats in the broad Santa Barbara Channel region consist of significant amounts of 

soft, unconsolidated sediment interspersed with isolated areas of rocky habitat that support kelp-forest 

communities nearshore and rocky-reef communities in deep water. The potential marine benthic habitat 

types mapped in the Offshore of Refugio Beach map area are directly related to its Quaternary geologic 

history, geomorphology, and active sedimentary processes. These potential habitats lie primarily within 

the Shelf (continental shelf) but also partly within the Flank (basin flank or continental slope) 

megahabitats of Greene and others (2007). The fairly homogeneous seafloor of sediment and low-relief 

bedrock provides characteristic habitat for groundfish, crabs, shrimp, and other marine benthic 

organisms. The bedrock outcrops are potential benthic habitats for rockfish (Sebastes spp.) and other 

groundfish that forage and seek refuge in such habitats. In addition, several areas of smooth sediment 

that form nearshore terraces with relatively steep, smooth fronts may provide interfaces attractive to 

certain species of groundfish. Below the steep shelf break, within the basin flank or continental slope 

megahabitat, the seafloor is composed of soft sediment interrupted by a few carbonate mounds, some 

bedrock exposures, and a gully. Carbonate mounds and the steep shelf break also are good potential 

habitat for rockfish. 

The Offshore of Refugio Beach map area is in the Ventura Basin, in the southern part of the 

Western Transverse Ranges geologic province, which is north of the California Continental Borderland6

(Fisher and others, 2009). Significant clockwise rotation—at least 90°—since the early Miocene has 

been proposed for the Western Transverse Ranges province (Luyendyk and others, 1980; Hornafius and 

others, 1986; Nicholson and others, 1994), and this region is presently undergoing north-south 

 

6 The California Continental Borderland is defined as the complex continental margin that extends from Point Conception 

south into northern Baja California.
5

shortening (see, for example, Larson and Webb, 1992). Regional cross sections (Tennyson and Kropp, 

1998; Forman and Redin, 2005; Redin, 2005) suggest that the south flank of the Santa Ynez Mountains 

is a large, south-dipping homocline that extends beneath the continental shelf. The homoclinal section 

extends upward from the Cretaceous strata exposed high in the mountains to the Pliocene strata 

encountered at shallow depths in offshore wells. Coastal cliffs mainly consist of fine-grained, folded 

strata of the early Miocene Rincon Shale, the Miocene Monterey Formation, and the late Miocene and 

early Pliocene Sisquoc Formation (Dibblee, 1981a,b). Uplift rates in the map area, which are based on 

marine-terrace elevations and geochronology, are estimated to be about 0.25 to 0.5 mm/yr (Muhs and 

others, 1992; Metcalf, 1994). 

Smaller folds (see sheets 8, 9, 10) related to local faulting are superimposed on the regional 

homocline. One of these superimposed anticlines hosts the Molino gas field, which was discovered in 

1962 and subsequently developed through directional drilling from onshore wells (Galloway, 1998). The 

Oligocene Sespe and Vaqueros Formations are the reservoirs in the Molino gas field (Forman and 

Redin, 2005), and the map area includes numerous seafloor hydrocarbon seeps (Ashley and others, 

1977). 

Publication Summary 

This publication about the Offshore of Refugio Beach map area includes eleven map sheets that 

contain explanatory text, in addition to this descriptive pamphlet and a data catalog of geographic 

information system (GIS) files. Sheets 1, 2, and 3 combine data from two different sonar surveys to 

generate comprehensive high-resolution bathymetry and acoustic-backscatter coverage of the map area. 

These data reveal a range of physiographic features (highlighted in the perspective views on sheet 4), 

such as the flat, sediment-covered Santa Barbara shelf interspersed with tectonically controlled bedrock 

uplifts and coarse-grained deltas and sediment lobes associated with coastal watersheds. To validate the 

geological and biological interpretations of the sonar data shown on sheets 1, 2, and 3, the U.S. 

Geological Survey towed a camera sled over specific offshore locations, collecting both video and 

photographic imagery; this “ground-truth” surveying data is summarized on sheet 6. Sheet 5 is a 

“seafloor character” map, which classifies the seafloor on the basis of depth, slope, and rugosity 

(ruggedness), and backscatter intensity and which is further informed by ground-truth-survey imagery. 

Sheet 7 is a map of “potential habitats,” which are delineated on the basis of substrate type, 

geomorphology, seafloor process, or other attributes that may provide a habitat for a specific species or 

assemblage of organisms. Sheet 8 compiles representative seismic-reflection profiles from the map area, 

providing information on the subsurface stratigraphy and structure of the map area. Sheet 9 shows the 

distribution and thickness of young sediment (deposited over the last about 21,000 years, during the 

most recent sea-level rise) in both the map area and the larger offshore Santa Barbara region (offshore 

from Refugio Beach to Hueneme Canyon), interpretated on the basis of the seismic-reflection data. 

Sheet 10 is a geologic map that merges onshore geologic mapping (compiled from existing maps by the 

California Geological Survey) and new offshore geologic mapping that is based on integration of highresolution bathymetry and backscatter imagery (sheets 1, 2, 3), seafloor-sediment and rock samples 

(Reid and others, 2006), digital camera and video imagery (sheet 6), and high-resolution seismicreflection profiles (sheet 8). Sheet 11 uses the ground-truth-survey imagery to develop a statistical 

model and maps that predict the distribution of benthic macro-invertebrates for both the Offshore of 

Refugio Beach map area and the Santa Barbara Channel region.

The information provided by the map sheets, pamphlet, and data catalog have a broad range of 

applications. High-resolution bathymetry, acoustic backscatter, ground-truth-surveying imagery, habitat 

mapping, and maps of predicted species distribution all contribute to habitat characterization and 

ecosystem-based management by providing essential data for delineation of marine protected areas and 
6

ecosystem restoration. Many of the maps provide high-resolution baselines that will be critical for 

monitoring environmental change associated with climate change, coastal development, or other 

forcings. High-resolution bathymetry is a critical component for modeling coastal flooding caused by

storms and tsunamis, as well as inundation associated with longer term sea-level rise. Seismic-reflection 

and bathymetric data help characterize earthquake and tsunami sources, critical for natural-hazard 

assessments of coastal zones. Information on sediment distribution and thickness is essential to the 

understanding of local and regional sediment transport, as well as development of regional sedimentmanagement plans. Documentation of hydrocarbon seepage and tarball accumulation is critical for 

distinguishing “natural” from anthropogenic pollution. In addition, citing of any new offshore 

infrastructure (for example, pipelines, cables, or renewable-energy facilities) will depend on new highresolution mapping. Finally, this mapping will both stimulate and enable new scientific research and 

also raise public awareness of, and education about, coastal environments and issues. 

Figure 1–1. Physiography of Santa Barbara Channel region. Box shows Offshore of Refugio Beach map area. 

Arrows show direction of sediment transport in Santa Barbara littoral cell, which extends from Point Arguello 

(PA) to Hueneme Canyon (HC) and Mugu Canyon (MC). Other abbreviations: C, Carpinteria; CC, Calleguas 

Creek; G, Goleta; GL, Goleta slide; O, Oxnard; PC, Point Conception; SB, Santa Barbara; SBB, Santa Barbara 

Basin; SM, Santa Monica Mountains; SMB, Santa Monica Basin; SMR, Santa Maria River; SR, Santa Clara 

River; SYM, Santa Ynez Mountains; SYR, Santa Ynez River; V, Ventura; VR, Ventura River.
7

Figure 1–2. Coastal geography of Offshore of Refugio Beach map area. Dashed line shows boundary of Molino 

gas field (MGF) (from Barnum, 1998). Other abbreviations: AH, Arroyo Hondo; CC, Cañada del Capitan and El 

Capitan Creek; CM, Cañada del Molino; CR, Cañada del Refugio; EC, El Capitan Beach; RB, Refugio Beach; 

TC, Tajiquas Creek. Yellow line is 3-nautical-mile limit of California’s State Waters.
8

Chapter 2. Bathymetry and Backscatter-Intensity Maps of the Offshore 

of Refugio Beach Map Area (Sheets 1, 2, and 3)

By Peter Dartnell

The colored shaded-relief bathymetry (sheet 1), the shaded-relief bathymetry (sheet 2), and the 

acoustic-backscatter (sheet 3) maps of the Offshore of Refugio Beach map area in southern California 

were generated from bathymetry and backscatter data collected by the U.S. Geological Survey (USGS) 

and by Fugro Pelagos for the U.S. Army Corps of Engineers (USACE) Joint Lidar Bathymetry 

Technical Center of Expertise (fig. 1 on sheets 1, 2, 3). The offshore area was mapped by the USGS in 

2008, using a 234.5-kHz SEA (AP) Ltd. SWATHplus-M phase-differencing sidescan sonar. The 

nearshore bathymetry and coastal topography were mapped for USACE by Fugro Pelagos in 2009, using 

the SHOALS-1000T bathymetric-lidar and Leica ALS60 topographic-lidar systems. These mapping 

missions combined to collect bathymetry (sheets 1, 2) from the 0-m isobath to beyond the 3-nauticalmile limit of California’s State Waters, as well as acoustic-backscatter data (sheet 3) from about the 10-

m isobath to beyond the 3-nautical-mile limit.

During the USGS mapping mission, GPS data with real-time-kinematic corrections were 

combined with measurements of vessel motion (heave, pitch, and roll) in a CodaOctopus F180 attitudeand-position system to produce a high-precision vessel-attitude packet. This packet was transmitted to 

the acquisition software in real time and combined with instantaneous sound-velocity measurements at 

the transducer head before each ping. The returned samples were projected to the seafloor using a raytracing algorithm that works with previously measured sound-velocity profiles. Statistical filters were 

applied to discriminate seafloor returns (soundings and backscatter intensity) from unintended targets in 

the water column. Finally, the soundings were converted into 2-m-resolution bathymetric-surface-model 

grids. The backscatter data were postprocessed using USGS software (D.P. Finlayson, written commun., 

2011) that normalizes for time-varying signal loss and beam-directivity differences. Thus, the raw 16-bit 

backscatter data were gain-normalized to enhance the backscatter of the SWATHplus system. The 

resulting normalized-amplitude values were rescaled to 16-bit and gridded into GeoJPEGs using GRID 

Processor Software, then imported into a geographic information system (GIS) and converted to GRIDs. 

During the 2009 Fugro Pelagos coastal airborne-lidar mapping mission that was completed as 

part of the National Coastal Mapping Program of USACE, the Leica ALS60 topographic-lidar and the 

SHOALS-1000T bathymetric-lidar systems were mounted on an aircraft that flew survey lines at an 

altitude of 300 to 400 m (bathymetry) and 300 to 1,200 m (topography), at speeds of between 135 and 

185 knots. The ALS60 system collected data at a maximum pulse rate of 200 kHz, and the SHOALS 

system collected data at 1 kHz. Information on aircraft position, velocity, and acceleration were 

collected using the Novatel and POS A/V 410 systems (SHOALS) and the onboard GPS/IMU system 

(ALS60). Aircraft-position data were processed using POSPac software, and the results were combined 

with the lidar data to produce 3-D positions for each lidar shot. Various commercial and proprietary 

software packages were used to clean the data, to convert all valid data from ellipsoid to orthometric 

heights, and to export the data as a series of topography and bathymetry ASCII files. 

Soundings from the different mapping missions were converted into individual 2-m-resolution 

bathymetric-surface-model grids. The individual bathymetric-surface models were then merged into one 

overall bathymetric-surface model and clipped to the boundary of the map area. Difference calculations 

of the overlapping bathymetry grids showed that there is good agreement between surveys (mean 

difference of 0.15 m, standard deviation of 0.27), even though the surveys were conducted at different 

times using different mapping equipment, and in the dynamic nearshore environment. 
9

An illumination having an azimuth of 300° and from 45° above the horizon was then applied to 

the bathymetric surface to create the shaded-relief imagery (sheets 1, 2). In addition, a modified 

“rainbow” color ramp was applied to the bathymetry data for sheet 1, using reds and oranges to 

represent shallower depths, and greens to represent greater depths (note that the Offshore of Refugio 

Beach map area requires only the shallower part of the full-rainbow color ramp used on some of the 

other maps in the California State Waters Map Series; see, for example, Kvitek and others, 2012). This 

colored bathymetry surface was draped over the shaded-relief imagery at 60-percent transparency to 

create a colored shaded-relief map (sheet 1).

Bathymetric contours (sheets 1, 2, 3, 5, 7, 10) were generated from a modified bathymetric 

surface of California’s State Waters within the Santa Barbara Channel. This surface was generated by 

merging all of California Seafloor Mapping Program’s bathymetry data for the region into one surface 

model. After merging, the surface model was resampled to 10-m resolution, and then a smooth 

arithmetic mean convolution function that assigns a weight of one-ninth to each cell in a 3-pixel by 3-

pixel matrix was applied iteratively to the surface ten times. Following smoothing, contour lines were 

generated at 10-m intervals from -10 to -100 m and then clipped to the boundary of the map area. 

The acoustic-backscatter imagery from each different mapping system and processing method 

were merged into their own individual grids. These individual grids, which cover different areas, were 

displayed in a GIS to create a composite acoustic-backscatter map (sheet 3). On the map, on which 

brighter tones indicate higher backscatter intensity, and darker tones indicate lower backscatter intensity. 

The intensity represents a complex interaction between the acoustic pulse and the seafloor, as well as 

characteristics within the shallow subsurface, providing a general indication of seafloor texture and 

sediment type. Backscatter intensity depends on the acoustic source level; the frequency used to image 

the seafloor; the grazing angle; the composition and character of the seafloor, including grain size, water 

content, bulk density, and seafloor roughness; and some biological cover. Harder and rougher bottom 

types such as rocky outcrops or coarse sediment typically return stronger intensities (high backscatter, 

lighter tones), whereas softer bottom types such as fine sediment return weaker intensities (low 

backscatter, darker tones).

The onshore-area image was generated by applying an illumination having an azimuth of 300° 

and from 45° above the horizon to the coastal airborne topographic-lidar data, as well as to publicly 

available, 3-m-resolution, interferometric synthetic aperture radar (ifSAR) data, available from National 

Oceanic and Atmospheric Administration (NOAA) Coastal Service Center’s Digital Coast (National 

Oceanic and Atmospheric Administration, 2011). 
10

Chapter 3. Data Integration and Visualization for the Offshore of 

Refugio Beach Map Area (Sheet 4) 

By Peter Dartnell

Mapping California’s State Waters has produced a vast amount of acoustic and visual data, 

including bathymetry, acoustic backscatter, seismic-reflection profiles, and seafloor video and 

photography. These data are used by researchers to develop maps, reports, and other tools to assist in the 

coastal and marine spatial-planning capability of coastal-zone managers and other stakeholders. For 

example, seafloor-character (sheet 5), habitat (sheet 7), and geologic (sheet 10) maps of the Offshore of 

Refugio Beach map area may assist in the designation of Marine Protected Areas, as well as in their 

monitoring. These maps and reports also help to analyze environmental change owing to sea-level rise 

and coastal development, to model and predict sediment and contaminant budgets and transport, to site 

offshore infrastructure, and to assess tsunami and earthquake hazards. To facilitate this increased 

understanding and to assist in product development, it is helpful to integrate the different datasets and 

then view the results in three-dimensional representations such as those displayed on the data integration 

and visualization sheet for the Offshore of Refugio Beach map area (sheet 4). 

The maps and three-dimensional views on sheet 4 were created using a series of geographic 

information systems (GIS) and visualization techniques. Using GIS, the bathymetric and topographic 

data (sheet 1) were converted to ASCIIRASTER format files, and the acoustic-backscatter data (sheet 3) 

were converted to geoTIFF images. The bathymetric and topographic data were imported in the 

Fledermaus software (QPS). The bathymetry was color-coded to closely match the colored shadedrelief bathymetry on sheet 1 in which reds and oranges represent shallower depths and greens represent 

deeper depths. Topographic data were shown in gray shades. The acoustic-backscatter geoTIFF images 

were also draped over the bathymetry data. The colored bathymetry, topography, and draped backscatter 

were then tilted and panned to create the perspective views such as those shown in figures 1, 2, 4, 5, and 

6 on sheet 4. These views highlight the relatively low-relief, natural outcrops along the outer shelf in the 

Offshore of Refugio Beach map area, as well as sediment-transport features within the nearshore area.

Video-mosaic images created from digital seafloor video (for example, fig. 3 on sheet 4) display 

the geologic complexity (rock, sand, and mud; see sheet 10) and biologic complexity (see sheet 11) of 

the seafloor. Whereas photographs capture high-quality snapshots of smaller areas of the seafloor (see 

sheet 6), video mosaics capture larger areas and can show transition zones between seafloor 

environments. Digital seafloor video is collected from a camera sled towed approximately 1 to 2 meters 

above the seafloor, at speeds of less than 1 nautical mile/hour. Using standard video-editing software, as 

well as software developed at the Center for Coastal and Ocean Mapping, University of New 

Hampshire, the digital video is converted to AVI format, cut into 2-minute sections, and desampled to 

every second or third frame. The frames are merged together using pattern-recognition algorithms from 

one frame to the next and converted to a TIFF image. The images are then rectified to the bathymetry 

data using ship navigation recorded with the video and layback estimates of the towed camera sled. 

Block diagrams that combine the bathymetry with seismic-reflection-profile data help integrate 

surface and subsurface observations, especially stratigraphic and structural relations (for example, fig. 6 

on sheet 4). These block diagrams were created by converting digital seismic-reflection-profile data 

(Sliter and others, 2008) into TIFF images, while taking note of the starting and ending coordinates and 

maximum and minimum depths. The images were then imported into the Fledermaus software as

vertical images and merged with the bathymetry imagery.
11

Chapter 4. Seafloor-Character Map of the Offshore of Refugio Beach 

Map Area (Sheet 5)

By Eleyne L. Phillips, Mercedes D. Erdey, and Guy R. Cochrane

The California State Marine Life Protection Act (MLPA) calls for protecting representative types 

of habitat in different depth zones and environmental conditions. A science team, assembled under the 

auspices of the California Department of Fish and Game (CDFG), has identified seven substrate-defined 

seafloor habitats in California’s State Waters that can be classified using sonar data and seafloor video 

and photography. These habitats include rocky banks, intertidal zones, sandy or soft ocean bottoms, 

underwater pinnacles, kelp forests, submarine canyons, and seagrass beds. The following five depth 

zones, which determine changes in species composition, have been identified: Depth Zone 1, intertidal; 

Depth Zone 2, intertidal to 30 m; Depth Zone 3, 30 to 100 m; Depth Zone 4, 100 to 200 m; and Depth 

Zone 5, deeper than 200 m (California Department of Fish and Game, 2008). The CDFG habitats, with 

the exception of depth zones, can be considered a subset of a broader classification scheme of Greene 

and others (1999) that has been used by the U.S. Geological Survey (USGS) (Cochrane and others, 

2003, 2005). These seafloor-character maps are generalized polygon shape files that have attributes 

derived from Greene and others (2007). 

A 2007 Coastal Map Development Workshop, hosted by the USGS in Menlo Park, California, 

identified the need for more detailed (relative to Greene and others’ [1999] attributes) raster products 

that preserve some of the transitional character of the seafloor when substrates are mixed and (or) they 

change gradationally. The seafloor-character map, which delineates a subset of the CDFG habitats, is a 

GIS-derived raster product that can be produced in a consistent manner from data of variable quality 

covering large geographic regions.

The following four substrate classes are identified in the Offshore of Refugio Beach map area:

• Class I: Fine- to medium-grained smooth sediment 

• Class II: Mixed smooth sediment and rock 

• Class III: Rock and boulder, rugose 

• Class IV: Anthropogenic material (rugged) 

The seafloor-character map of the Offshore of Refugio Beach map area (sheet 5) was produced 

using video-supervised maximum-likelihood classification of the bathymetry and intensity of return 

from sonar systems, following the method described by Cochrane (2008). The two variants used in this 

classification were backscatter intensity and derivative rugosity, which is a standard calculation 

performed with the National Oceanic and Atmospheric Administration (NOAA) benthic-terrain modeler 

(available at http://www.csc.noaa.gov/digitalcoast/tools/btm/index.html; last accessed March 4, 2013), 

using a 3-pixel by 3-pixel array of bathymetry.

Classes I, II, and III values were delineated using multivariate analysis. Class IV (rugged 

anthropogenic material related to oil pipeline) values were determined on the basis of their visual 

characteristics and the known location of man-made features. The resulting maps were cleaned by hand 

to remove data-collection artifacts (for example, the trackline nadir).

On the seafloor-character map (sheet 5), the four substrate classes have been colored to indicate 

the California MLPA depth zones and the Coastal and Marine Ecological Classification Standard 

(CMECS) slope zones (Madden and others, 2008) in which they belong. The California MLPA depth 

zones are Depth Zone 1 (intertidal), Depth Zone 2 (intertidal to 30 m), Depth Zone 3 (30 to 100 m), 

Depth Zone 4 (100 to 200 m), and Depth Zone 5 (greater than 200 m); in the Offshore of Refugio Beach 
12

map area, only Depth Zones 2, 3, and 4 are present. The slope classes that represent the CMECS slope 

zones are Slope Class 1 = flat (0° to 5°), Slope Class 2 = sloping (5° to 30°), Slope Class 3 = steeply 

sloping (30° to 60°), Slope Class 4 = vertical (60° to 90°), and Slope Class 5 = overhang (greater than 

90°); in the Offshore of Refugio Beach map area, only Slope Classes 1 and 2 are present. The final 

classified seafloor-character raster map image is draped over the shaded-relief bathymetry for the area 

(sheets 1 and 2) to produce the image shown on the seafloor-character map on sheet 5.

The seafloor-character classification is also summarized on sheet 5 in table 1. Fine- to mediumgrained smooth sediment (sand and mud) makes up 91.2 percent (104.3 km2

) of the map area: 11.3 

percent (12.9 km2) is in Depth Zone 2, 79.3 percent (90.7 km2) is in Depth Zone 3, and 0.6 percent (0.7 

km2

) is in Depth Zone 4. Mixed smooth sediment (sand and gravel) and rock (that is, sediment typically 

forming a veneer over bedrock, or rock outcrops having little to no relief) make up 8.5 percent (9.7 km2) 

of the map area: 0.4 percent (0.5 km2) is in Depth Zone 2, 3.9 percent (4.5 km2) is in Depth Zone 3, and 

4.1 percent (4.7 km2

) is in Depth Zone 4. Rock and boulder, rugose (rock outcrops and boulder fields 

having high surficial complexity) makes up 0.1 percent (0.2 km2) of the map area: less than 0.1 percent 

(<0.1 km2) is in Depth Zone 2, 0.1 percent (0.2 km2) is in Depth Zone 3, and less than 0.1 percent (<0.1 

km2

) is in Depth Zone 4. Rugged anthropogenic material (a pipe that traverses the entire width of 

California’s State Waters in the map area) makes up 0.2 percent (0.2 km2

) of the map area: less than 0.1 

percent (<0.1 km2) is in Depth Zone 2, 0.2 percent (0.2 km2) is in Depth Zone 3, and less than 0.1 

percent (<0.1 km2

) is in Depth Zone 4. 

A small number of video observations were used to supervise the numerical classification of the 

seafloor. All video observations (see sheet 6) are used for accuracy assessment of the seafloor-character 

map after classification. To compare observations to classified pixels, each observation point is assigned 

a class (I, II, or III), according to the visually derived, major or minor geologic component (for example, 

sand or rock) and the abiotic complexity (vertical variability) of the substrate recorded during groundtruth surveys (table 4–1; see also, chapter 5 of this pamphlet). Class IV values were determined from the 

visual characteristics and known locations of man-made features.

Next, circular buffer areas were created around individual observation points using a 10-m radius 

to account for layback and positional inaccuracies inherent to the towed-camera system. The radius 

length is an average of the distances between the positions of sharp interfaces seen on both the video 

(the position of the ship at the time of observation) and sonar data, plus the distance covered during a 

10-second observation period at an average speed of 1 nautical mile/hour. Each buffer, which covers 

more than 300 m2

, contains approximately 77 pixels. The classified (I, II, III) buffer is used as a mask to 

extract pixels from the seafloor-character map. These pixels are then compared to the class of the buffer. 

For example, if the shipboard-video observation is Class II (mixed smooth sediment and rock), but 12 of 

the 77 pixels within the buffer area are characterized as Class I (fine- to medium-grained smooth 

sediment), and 15 (of the 77) are characterized as Class III (rock and boulder, rugose), then the 

comparison would be “Class I, 12; Class II, 50; Class III, 15” (fig. 4–1). If the video observation of 

substrate is Class II, then the classification is accurate because the majority of seafloor pixels in the 

buffer are Class II. The accuracy values in table 4–1 represent the final of several classification 

iterations aimed at achieving the best accuracy, given the variable quality of sonar data (see discussion 

in Cochrane, 2008) and the limited ground-truth information available when compared to the continuous 

coverage provided by swath sonar. Presence/absence values in table 4–1 reflect the percentages of 

observations where the sediment classification of at least one pixel within the buffer zone agreed with 

the observed sediment type at a certain location. 

The seafloor in the Offshore of Refugio Beach map area is mainly flat, with small, local 

sedimentary-bedrock exposures (Class III) on the outer shelf and slope. The seabed is predominantly 

covered by Class I sediment composed of soft, unconsolidated sand and mud. Class II sediments varies

from gravel to sediment-covered tar flows. Differentially eroded sedimentary-bedrock outcrops (Class 
13

III) are present mainly further offshore on the outer shelf and slope, including outcrops near and just 

outside the 3-nautical-mile limit of California’s State Waters. Exposed rock is covered intermittently by 

varying thicknesses of fine- (Class I) to coarse-grained (Class II) sediment (coarse sand and gravel). One 

rugged anthropogenic feature (Class IV), an oil pipeline, cuts across both the shelf and slope. 

The classification accuracy of Classes I and II (81 and 62 percent accurate, respectively; table 4–

2) is determined by sediments are found to be accurate based on comparing the shipboard video 

observations and the classified map. The weaker (10 percent) agreement in Class III likely is due to the 

distribution of small, localized rock outcrops and also the relatively narrow, intermittent nature of 

transition zones from sediment to rock, as well as the size of the buffer. The bedrock outcrops in this 

area are composed of sedimentary rocks exhibiting differential erosion (Cochrane and Lafferty, 2002). 

Erosion of softer layers produces Class I and II sediments, resulting in patchy rugose rock and boulder 

habitat on the seafloor. A single buffered observation locale of 78 pixels, therefore, is likely to be 

interspersed with other classes of pixels, as well as with Class III. Percentages for presence/absence 

within a buffer also were calculated as a better measure of the accuracy of the classification for patchy 

rock habitat. Within the coverage of the 2-m-resolution seafloor-character map, the presence/absence 

accuracy was found to be significant for Classes I and II (88 percent for Class I, and 73 percent for Class 

II). Only a limited number of observations (16 total) were made over Class III rugose rock and boulder 

substrate, likely because the rock outcrops in the map area are characterized by linear, narrow ridges; 

therefore, the presence/absence is somewhat lower (25 percent) for Class III. Even though the pipe was 

observed on the video footage, it was not part of the 10-second period used for recording the sediment 

type therefore no video observations were available for accuracy assessment of Class IV substrate.

 
14

Table 4–1. Conversion table showing how video observations of primary substrate (more than 50 percent seafloor 

coverage), secondary substrate (more than 20 percent seafloor coverage), and abiotic seafloor complexity (in 

first three columns) are grouped into seafloor-character-map Classes I, II, and III for use in supervised 

classification and accuracy assessment in Offshore of Refugio Beach map area. 

[In areas of low visibility where primary and secondary substrate could not be identified with confidence, recorded 

observations of substrate (in fourth column) were used to assess accuracy]

Primary-substrate component Secondary-substrate component Abiotic seafloor complexity Low-visibility observations

Class I

mud cobbles low

mud gravel low

mud sand low

mud mud low

sand mud low

sand sand low

sediment

ripples

Class II

cobbles mud low

mud rock low

mud boulders low

mud boulders moderate

rock mud low

rock sand low

Class III

boulders cobbles moderate

boulders mud moderate

boulders sand moderate

cobbles rock moderate

rock boulders moderate

rock cobbles high

rock mud moderate

rock rock moderate
15

Table 4–2. Accuracy-assessment statistics for seafloor-character-map classifications in Offshore of Refugio Beach 

map area. 

[Accuracy assessments are based on video observations (N/A, no accuracy assessment was conducted)]

Class Number of observations % majority % presence/absence

I—Fine- to medium-grained smooth sediment 342 80.9 88.0

II—Mixed smooth sediment and rock 22 62.4 72.7

III—Rock and boulder, rugose 16 10.3 25.0

IV—Rugged anthropogenic feature (pipe) 0 N/A N/A

Figure 4–1. Detailed view of ground-truth data, showing accuracy-assessment methodology. A, Dots illustrate 

ground-truth observation points, each of which represents 10-second window of substrate observation plotted 

over seafloor-character grid; circle around dot illustrates area of buffer depicted in B. B, Pixels of seafloorcharacter data within 10-m-radius buffer centered on one individual ground-truth video observation.
16

Chapter 5. Ground-Truth Studies for the Offshore of Refugio Beach Map 

Area (Sheet 6)

By Nadine E. Golden and Guy R. Cochrane

To validate the interpretations of sonar data in order to turn it into geologically and biologically 

useful information, the U.S. Geological Survey (USGS) towed a camera sled (fig. 5–1) over specific 

locations throughout the Offshore of Refugio Beach map area to collect video and photographic data 

that would “ground truth” the seafloor. This ground-truth surveying occurred on two separate cruises in 

2007 and 2008. The camera sled was towed 1 to 2 m above the seafloor, at speeds between 1 and 2 

nautical miles/hour. Ground-truth surveys in this map area include approximately 7.85 trackline 

kilometers of video and 838 still photographs, in addition to 457 recorded seafloor observations of 

abiotic and biotic attributes. A visual estimate of slope also was recorded.

Figure 5–1. Photograph of camera sled used in USGS 2008 ground-truth survey.

During the 2008 cruise, a USGS camera sled was used that housed two standard-definition

(640×480 pixel resolution) video cameras (one forward looking and one downward looking), as well as 

a high-definition (1,080×1,920 pixel resolution) video camera and an 8-megapixel digital still camera. 

During this cruise, in addition to recording the seafloor characteristics, a digital still photograph was 

captured once every 30 seconds.

The camera-sled tracklines (shown by colored dots on the map on sheet 6) are sited in order to 

visually inspect areas representative of the full range of bottom hardness and rugosity in the map area. 

The video is fed in real time to the research vessel, where USGS and National Oceanic and Atmospheric 

Administration (NOAA) scientists record both the geologic and biologic character of the seafloor. While 

the camera is deployed, several different observations are recorded for a 10-second period once every 

minute, using the protocol of Anderson and others (2007). Observations of primary substrate, secondary 
17

substrate, slope, abiotic complexity, biotic complexity, and biotic cover are mandatory. Observations of 

key geologic features and the presence of key species are also made. 

Primary and secondary substrate, by definition, constitute greater than 50 and 20 percent of the 

seafloor, respectively, during an observation. The grain-size values that differentiate the substrate 

classes are based on the Wentworth (1922) scale, and the sand, cobble, and boulder sizes are classified 

as in Wentworth (1922). However, the difficulty in distinguishing the finest divisions in the Wentworth 

(1922) scale during video observations made it necessary to aggregate some grain-size classes, as was 

done in the Anderson and others (2007) methodology: the granule and pebble sizes have been grouped 

together into a class called “gravel,” and the clay and silt sizes have been grouped together into a class 

called “mud.” In addition, hard bottom and clasts larger than boulder size are classified as “rock.” 

Benthic-habitat complexity, which is divided into abiotic (geologic) and biotic (biologic) components, 

refers to the visual classification of local geologic features and biota that potentially can provide refuge 

for both juvenile and adult forms of various species (Tissot and others, 2006). 

Sheet 6 contains a smaller, simplified (depth-zone symbology has been removed) version of the 

seafloor-character map on sheet 5. On this simplified map, the camera-sled tracklines used to groundtruth survey the sonar data are shown by aligned colored dots, each dot representing the location of a 

recorded observation. A combination of abiotic attributes (primary- and secondary-substrate 

compositions), as well as vertical variability were used to derive the different classes represented on the 

seafloor-character map (sheet 5): on the simplified map, the derived classes are represented by colored 

dots. Also on this map are locations of the detailed views of seafloor character, shown by boxes (Boxes 

A through E); for each view, the box shows the locations (indicated by colored stars) of representative 

seafloor photographs. For each photograph, an explanation of the observed seafloor characteristics 

recorded by USGS and NOAA scientists is given. Note that individual photographs often show more 

substrate types than are reported as the primary and secondary substrate. Organisms, when present, are 

labeled on the photographs. 

The ground-truth survey is designed to investigate areas that represent the full spectrum of highresolution multibeam bathymetry and backscatter-intensity variation. Figure 5–2 shows that, in the 

Offshore of Refugio Beach map area, the seafloor surface is covered predominantly by sediment with 

two areas of low-relief rock outcrops toward the edge of the shelf. The seafloor in both areas consists of 

mixed sediment and rock, as well as flat rock outcrops. 
18

Figure 5–2. Graph showing distribution of primary and secondary substrate determined from video observations in 

Offshore of Refugio Beach map area.
19

Chapter 6. Potential Marine Benthic Habitat Map of the Offshore of 

Refugio Beach Map Area (Sheet 7)

By H. Gary Greene and Charles A. Endris

The map on sheet 7 shows “potential” marine benthic habitats of the Offshore of Refugio Beach 

map area, representing a substrate type, geomorphology, seafloor process, or any other attribute that may 

provide a habitat for a specific species or assemblage of organisms. This map, which is based largely on 

seafloor geology, also integrates information displayed on several other thematic maps of the Offshore 

of Refugio Beach map area. High-resolution sonar bathymetry data, converted to depth grids (seafloor 

DEMs; sheet 1), are essential to development of the potential marine benthic habitat map, as is shadedrelief imagery (sheet 2), which allows visualization of seafloor terrain and provides a foundation for 

interpretation of submarine landforms. 

Backscatter maps (sheet 3) are also essential for developing potential benthic habitat maps. High 

backscatter is further indication of “hard” bottom, consistent with interpretation as rock or coarse 

sediment. Low backscatter, indicative of a “soft” bottom, generally indicates a fine sediment 

environment. Habitat interpretations are also informed by actual seafloor observations from ground-truth 

surveying (sheet 6), by seafloor-character maps that are based on video-supervised maximum-likelihood 

classification (sheet 5), and by seafloor-geology maps (sheets 10). The habitat interpretations on sheet 7 

are further informed by the usSEABED bottom-sampling compilation of Reid and others (2006). 

Broad, generally smooth areas of seafloor that lack sharp and angular edge characteristics are 

mapped as “sediment;” these areas may be further defined by various sedimentary features (for example, 

erosional scours and depressions) and (or) depositional features (for example, dunes, mounds, or sand 

waves). In contrast, many areas of seafloor bedrock exposures are identified by their common sharp 

edges and high relative relief; these may be contiguous outcrops, isolated parts of outcrop protruding 

through sediment cover (pinnacles or knobs), or isolated boulders. In many locations, areas within or 

around a rocky feature appear to be covered by a thin veneer of sediment; these areas are identified on 

the habitat map as “mixed” induration (that is, containing both rock and sediment). The combination of 

remotely observed data (for example, high-resolution bathymetry and backscatter, seismic-reflection 

profiles) and directly observed data (for example, camera transects, sediment samples) translates to 

higher confidence in the ability to interpret broad areas of the seafloor. 

To avoid any possible misunderstanding of the term “habitat,” the term “potential habitat” (as 

defined by Greene and others, 2005) is used herein to describe a set of distinct seafloor conditions that in 

the future may qualify as an “actual habitat.” Once habitat associations of a species are determined, they 

can be used to create maps that depict actual habitats, which then need to be confirmed by in situ

observations, video, and (or) photographic documentation. 

Classifying Potential Marine Benthic Habitats

Potential marine benthic habitats in the Offshore of Refugio Beach map area are mapped using 

the Benthic Marine Potential Habitat Classification Scheme, a mapping-attribute code developed by 

Greene and others (1999, 2007). This code, which has been used previously in other offshore California 

areas (see, for example, Greene and others, 2005, 2007), was developed to easily create categories of 

marine benthic habitats that can then be queried within a GIS or a database. The code contains several 

categories that can be subdivided relative to the spatial scale of the data. The following categories can be 

applied directly to habitat interpretations determined from remote-sensing imagery collected at a scale of 

tens of kilometers to one meter: Megahabitat, Seafloor Induration, Meso/Macrohabitat, Modifier, 

Seafloor Slope, Seafloor Complexity, and Geologic Unit. Additional categories of Macro/Microhabitat, 
20

Seafloor Slope, Seafloor Complexity, and Geologic Attribute can be applied to habitat interpretations 

determined from seafloor samples, video, still photographs, or direct observations at a scale of 10 meters 

to a few centimeters. These two scale-dependent groups of categories can be used together, to define a 

habitat across spatial scales, or separately, to compare large- and small-scale habitat types.

The six categories and their attribute codes that are used on the Offshore of Refugio Beach map 

are explained in detail below (note, however, that not all categories may be used in a particular map 

area, given the study objectives, data availability, or data quality); attribute codes in each category are 

depicted on the map by the letters and, in some cases, numbers that make up the map-unit symbols: 

Megahabitat—Based on depth and general physiographic boundaries; used to distinguish 

features on a scale of tens of kilometers to kilometers. Depicted on map by capital letter, listed first in 

map-unit symbol; generalized depth ranges are given below. 

F = Flank; continental slope, basin and (or) island flanks (200 to 3,000 m) 

S = Shelf; continental and island shelves (0 to 200 m) 

Seafloor Induration—Refers to substrate hardness. Depicted on map by lower-case letter, listed 

second in map-unit symbol; may be further subdivided into distinct sediment types, depicted by lowercase letter(s) in parentheses, listed immediately after substrate hardness; multiple attributes listed in 

general order of relative abundance, separated by slash; queried where inferred.

h = Hard bottom (for example, rock outcrop or sediment pavement) 

m = Mixed hard and soft bottom (for example, local sediment cover of bedrock) 

s = Soft bottom; sediment cover

(g) = Gravel

(s) = Sand

(m) = Mud, silt, and (or) clay 

Meso/Macrohabitat—Related to scale of habitat; consists of seafloor features one kilometer to 

one meter in size. Depicted on map by lower-case letter and, in some cases, additional lower-case letter 

in parentheses, listed third in map-unit symbol; multiple attributes separated by slash. 

b = Beach, relic (submerged) or shoreline 

(b)/p = Pinnacle indistinguishable from boulder 

c = Canyon

c(b) = Bar within thalweg

c(c) = Curve or meander within thalweg 

c(f) = Fall or chute within thalweg

c(h) = Canyon head

c(m) = Canyon mouth 

c(t) = Thalweg

c(w) = Canyon wall

d = Deformed, tilted and (or) folded bedrock; overhang 

e = Exposure; bedrock 

f = Flat; floor

g = Gully; channel

h = Hole; depression 

l = Landslide; mass movement; rubble 

m = Mound; linear ridge 

o = Overbank deposit; levee 

p = Pinnacle; cone 

r = Rill (linear depression on surface formed by subterranean winnowing of sediment) 

s = Scarp, cliff, fault, or slump scar 

t = Terrace 
21

v = Vegetated (grass- or algae-covered) sediment or rock 

w = Dynamic bedform

w(w) = Sediment wave (amplitude, 10 cm to a meter; wave length, tens of meters) 

w(d) = Sediment dune (amplitude, tens of meters; wave length, hundreds of meters) 

y = Delta; fan

Modifier—Describes texture, bedforms, biology, or lithology of seafloor. Depicted on map by 

lower-case letter, in some cases followed by additional lower-case letter(s) either after a hyphen or in 

parentheses (or both), following an underscore; multiple attributes separated by slash. 

_a = Anthropogenic (artificial reef, breakwall, shipwreck, disturbance) 

_a-c = Cable

_a-dd = Dredge disturbance

_a-dg = Dredge groove or channel 

_a-dp = Dredge potholes 

_a-dm = Dredge mound (disposal) 

_a-dp = Dredge pothole 

_a-f = Ferry (or other vessel) propeller-wash scour or scar 

_a-g = Groin, jetty, rip-rap

_a-m = Marina, harbor 

_a-p = Pipeline

_a-s = Support; dock piling, dolphin 

_a-td = Trawl disturbance 

_a-w = Wreck, ship, barge, or plane 

_b = Bimodal (conglomeratic, mixed [gravel, cobbles, and pebbles]) 

_c = Consolidated sediment (claystone, mudstone, siltstone, sandstone, breccia, or 

conglomerate) 

_d = Differentially eroded

_e = Effusive pit; pockmark 

_f = Fracture, joint; faulted

_g = Granite

_h = Hummocky, irregular relief

_i = Interface; lithologic contact

_k = Kelp

_l = Limestone or carbonate rock or structure 

_l(a) = Alive reef

_l(d) = Dead reef

_l(l) = Linear reef

_l(p) = Patch reef

_l(pr-a) = Aggregated patch reef

_l(pr-i) = Individual patch reef 

_l(r) = Reef rubble 

_l(s-g) = Spur and groove 

_m = Massive sedimentary bedrock

_o = Outwash

_p = Pavement

_r = Ripple (amplitude, greater than 10 cm) 

_s = Scour (current or ice; direction noted) 

_u = Unconsolidated sediment 

_v = Volcanic rock
22

_w = Wall

 Seafloor Slope—Denotes slope, typically calculated from XYZ high-resolution bathymetry 

data. Depicted on map by number, listed after modifier. 

1 = Flat (0o–5o)

2 = Sloping (5o–30o) 

3 = Steeply sloping (30o–45o) 

4 = Vertical or near vertical (45o–90o) 

5 = Overhanging (more than 90o) 

6 = Unknown 

Geologic Attribute—Describes additional geologic features seen in video, still photographs, or 

other types of direct observations. Depicted on map by lower-case letter(s) in parentheses, preceded by 

an asterisk.

*(a) = Anthropogenic (for example, cable, pipeline, disturbance) 

*(a-d) = Dredge track, pit, or mound 

*(b) = Boulder 

*(d) = Deformed, faulted, or folded

*(e) = Exposure, bedrock (sedimentary, igneous, or metamorphic) 

*(e-r) = Rough bedrock surface 

*(f) = Fan or apron 

*(g) = Gravel

*(j) = Joint, crack, crevice, overhang (differentially eroded) 

*(l) = Limestone, carbonate deposit 

*(m) = Mud, silt, or clay 

*(q) = Coquina (shell hash) 

*(r) = Rubble 

*(s) = Sand

*(t) = Flat, terracelike seafloor, including sedimentary pavement

*(u) = Undulating surface, hummocky 

*(u-r) = Ripple 

*(u-s) = Scour 

*(u-w) = Sediment wave

*(y) = Barnacle or plate

Examples of Attribute Coding

To illustrate how these attribute codes can be used to describe remotely sensed data, the 

following examples are given: 

Ssc(h)_u2/4 = Canyon head that indents shelf and has smooth, soft, gently sloping, sedimentary 

walls, locally cropping out as steep (near vertical) scarps (10 to 100 m). 

Ssf_u1 = Flat to gently sloping shelf that has soft, unconsolidated sediment (10 to 150 m). 

Fhe_m/c = Continental slope that has hard sedimentary (sandstone) bedrock exposures locally 

and smooth to moderately irregular relief (less than 1 m to 3 m high); exposures often covered with 

sediment (200 to 2,500 m). 

Ssm_a/u*(q) = Soft, unconsolidated sediment and shell-hash mound, adjacent to oil platform 

(anthropogenic). 
23

Map Area Habitats

Delineated in the Offshore of Refugio Beach map area are 16 potential marine benthic habitat 

types within two megahabitat settings, “Shelf” (continental shelf) and “Flank” (basin flank or 

continental slope). On the shelf, these habitat types range from predominantly soft, unconsolidated 

sediment (sand and mud) to areas of hard bedrock exposures (differentially eroded, well-bedded 

sedimentary outcrops). Some sedimentary-bedrock outcrops are partly covered with sediment to produce 

a hard-soft mixed habitat type. Pockmarks and carbonate mounds complete the variety of habitats 

identified on the continental shelf. Minor anthropogenic features associated with oil production, such as 

pipelines, cut across both the shelf and slope. The narrow band of basin-flank or continental-slope 

megahabitat that has been mapped contains macro- and mesohabitats of predominantly soft, 

unconsolidated sediment that has some well-layered sedimentary bedrock and carbonate mounds 

exposed. Along the distal edge of the continental shelf, stringers and mounds of carbonate outcrops 

provide good potential habitat for rockfish (Sebastes spp.), lingcod (Ophiodon elongatus), and other 

demersal fish.

The soft, unconsolidated sediment habitat on the continental shelf, which includes pockmarks 

and nearshore bars, covers 86.9 km2 of the map area, representing 75.8 percent of all the potential 

habitat types identified. Sediment-covered bedrock on the continental shelf, which includes the hard-soft 

mixed habitat type, covers 17.5 km2 (15.2 percent). Hard bedrock exposures on the continental shelf 

cover 2.6 km2

 (2.3 percent). Mixed substrate on the continental slope covers 2.3 km2 (2 percent). 

Unconsolidated soft sediment on the continental slope covers 4.9 km2 (4.3 percent) and hard bedrock 

exposures, including carbonate mounds, at the top of the continental slope cover 0.16 km2 (0.1 percent). 

Anthropogenic features on both the shelf and slope cover nearly 0.23 km2

 (0.2 percent). 

Fluid flowing up to the seafloor from petroleum reservoirs at depth has resulted in the formation 

of locally exposed, hard, carbonate mounds and pockmarks. These carbonate mounds and hard rock are 

locally covered with sediment, providing potential habitat for sessile organisms. Sedimentary-bedrock 

exposures in the nearshore provide hard, irregular substrate for rockfish and other groundfish habitat. 

This mix of potential marine benthic habitat types provides the varied relief, rugosity, and substrate 

hardness that contribute to the concentration of a diverse marine ecosystem within an otherwise 

homogeneous, soft, unconsolidated sediment habitat. 
24

Chapter 7. Subsurface Geology and Structure of the Offshore of 

Refugio Beach Map Area and the Santa Barbara Channel Region 

(Sheets 8 and 9)

By Samuel Y. Johnson, James E. Conrad, Eleyne Phillips, Andrew Ritchie, Florence L. Wong, Ray W. Sliter, Amy 

E. East, and Patrick E. Hart

Seismic-reflection profiles presented on sheet 8 provide a third dimension, depth, to complement 

the surficial seafloor-mapping data already presented (sheets 1 through 7) for the Offshore of Refugio 

Beach map area. These profiles, which are collected at several resolutions, extend to varying depths in 

the subsurface, depending on the purpose and mode of data acquisition. The seismic-reflection profiles 

(sheet 8) provide information on sediment character, distribution, and thickness, as well as potential 

geologic hazards, including active faults, areas prone to strong ground motion, and tsunamigenic slope 

failures. The information on faults provides essential input to national and state earthquake-hazard maps 

and assessments (see, for example, Petersen and others, 2008). 

The maps on sheet 9 show the following interpretations, which are based on the seismicreflection profiles on sheet 8: the thickness of the uppermost sediment unit; the depth to base of this 

uppermost unit; and both the local and regional distribution of faults and earthquake epicenters (data 

from Heck, 1998; Minor and others, 2009; Jennings and Bryant, 2010; Southern California Earthquake 

Data Center, 2010). 

Data Acquisition

Most profiles displayed on sheet 8 (figs. 1, 3, 5, 6, 7, 8, 9, 10) were collected in 2008 on U.S. 

Geological Survey (USGS) cruise S–7–08–SC (Sliter and others, 2008). Single-channel seismicreflection data were acquired using the EdgeTech 512 chirp subbottom-profiling system consisting of a 

source transducer housed in a 500-lb fish towed at a depth of several meters below the sea surface. The 

swept-frequency chirp source signal was 500 to 4,500 Hz and 50 ms in length, and it was recorded by 

hydrophones located on the bottom of the fish. The data were digitally recorded in standard SEG-Y 32-

bit floating-point format, using Triton Subbottom Logger (SBL) software that merges seismic-reflection 

data with differential GPS-navigation data. After the survey, a short-window (20 ms) automatic gain 

control algorithm was applied to the data. These high-resolution data can resolve geologic features that 

are a few meters thick (small-scale features) to subbottom depths of as much as a few hundred meters. 

Figures 2 and 4 on sheet 8 show deep-penetration, migrated, multichannel seismic-reflection 

profiles collected in 1985 by WesternGeco on cruise W–40–85–SC. These profiles and other similar 

data were collected in many areas offshore of California in the 1970s and 1980s when these areas were 

considered a frontier for oil and gas exploration. Much of these data have been publicly released and are 

now archived at the U.S. Geological Survey National Archive of Marine Seismic Surveys (U.S. 

Geological Survey, 2009). These data were acquired using a large-volume air-gun source that has a 

frequency range of 3 to 40 Hz and recorded with a multichannel hydrophone streamer about 2 km long. 

Shot spacing was about 30 m. These data can resolve geologic features that are 20 to 30 m thick, down 

to subbottom depths of about 4 km. 

Seismic-Reflection Imaging of the Continental Shelf 

Sheet 8 shows seismic-reflection profiles in the Offshore of Refugio Beach map area that 

document a relatively flat (less than 1°), moderately deep (less than 80 m) wave-cut shelf. This shelf is 

underlain by variably thick (0 to about 15 m; mean thickness, 2.7 m) upper Pleistocene and Holocene 
25

marine, deltaic, and alluvial sediments (Draut and others, 2009; Sommerfield and others, 2009) 

deposited in the last about 21,000 years during the about 125-m sea-level rise that followed glaciation 

and the last major sea-level lowstand. Sea-level rise after the Last Glacial Maximum (LGM) was rapid 

(about 10 to 11 m per thousand years) until about 7,000 years ago, at which time it slowed considerably 

(to about 1 m per thousand years) (Fairbanks, 1989; Fleming and others, 1998; Lambeck and Chappell, 

2001; Peltier and Fairbanks, 2006). Local relief on the shelf is associated with bedrock uplifts (sheet 10). 

In the high-resolution seismic-reflection profiles on sheet 8 (figs. 1, 3, 5, 6, 7, 8, 9, 10), the 

sediments deposited during this post-LGM sea-level rise (shaded blue in profiles on sheet 8) commonly 

are “acoustically transparent,” that is, lacking internal reflections. The presence and continuity of 

seismic reflections in this upper, post-LGM unit on many profiles is also obscured by interstitial gas 

within the sediment. This effect has been referred to as “gas blanking,” “acoustic turbidity,” or “acoustic 

masking” (Hovland and Judd, 1988; Fader, 1997). The gas scatters or attenuates the acoustic energy, 

preventing penetration. Not surprisingly, this effect is especially prevalent near the Molino gas field 

(see, for example, Kunitomi and others, 1998) and near the crests of other anticlines. 

Because the shelf was partly emergent during the postglacial period of rising sea level, the lower 

part of the post-LGM unit may, in places (especially where the unit is thickest), consist of alluvial plain 

and estuarine deposits. These nonmarine to marginal-marine strata were covered by nearshore and shelf 

sediments as sea level rose and the shoreline migrated both landward and upward. The upper part of this 

unit likely consists of a mix of shelf and deltaic deposits that are similar to the sediment being deposited 

on the shelf today (see sheet 6).

On most profiles on sheet 8, the base of the post-LGM depositional unit is a flat to concave 

angular unconformity, characterized by an obvious change in reflectivity. The post-LGM unit commonly 

thins or pinches out over local uplifts (for example, figs. 7, 8). Sediment-covered wave-cut platforms 

and risers (see, for example, Kern, 1977) are imaged at the base of the unit on some profiles most 

commonly at depths of about 71 m, 50 m, and 41 m (see, for example, figs. 1, 7, 8). Given uplift rates 

from nearby coastal terraces (about 0.3 to 0.5 m/ka; Muhs and others, 1992), these depths are most 

consistent with formation during the post-LGM transgression. Post-LGM sea-level rise and landward 

shoreline migration were not steady but, rather, were characterized by periods of relative stability and 

rapid submergence (see, for example, Peltier, 2005; Stanford and others, 2011). For example, sea level 

rose about 20 m in 200 to 500 years during meltwater pulse 1a, about 14,000 years ago (Stanford and

others, 2011). Such pulses rapidly submerge wave-cut platforms, shorelines, and shoreline angles (Kern, 

1977), thereby increasing the potential for their preservation. The shallower submerged shoreline angles 

and platforms could have a similar post-LGM origin, or they could have formed at similar sea-level 

stands earlier in the late Pleistocene (for example, Wright, 2000). 

Geologic Structure and Recent Deformation

High-resolution seismic-reflection profiles in the Offshore of Refugio Beach map area (sheet 8) 

primarily show south-dipping Neogene strata within a large homocline that extends from the south flank 

of the Santa Ynez Mountains (fig. 1–2) into the offshore. Regional cross sections (Tennyson and Kropp, 

1998; Forman and Redin, 2005; Redin, 2005) and industry seismic-reflection profiles (figs. 2, 4 on sheet 

8) indicate that the homocline formed above the blind North Channel Fault. The fault tip is inferred to be 

buried about 2 km deep (about 1.5 sec TWT), about 5 to 6 km offshore, beneath the upper slope and just 

outside California’s State Waters.

The North Channel Fault Zone is inferred to include several splays that are structurally above the 

main fault, on the basis on the irregular pattern of shallow folds in the map area. Closely spaced seismicreflection profiles reveal that these shallow folds display variable geometry, length, amplitude, 

continuity, and wavelength (see sheets 8, 10). Smaller scale, variably continuous folds that have 
26

wavelengths of about 300 to 1,000 m (for example, figs. 3, 5 on sheet 8) are superimposed on the larger 

homocline. The most notable of these smaller scale folds is the anticline that hosts the Molino gas field 

in the southwestern part of the map area (fig. 1–2). This anticlinal trend appears to be truncated to the 

east by an east-northeast-striking fault (fig. 5 on sheet 8; see also, sheet 10). 

Ashley and others (1977) extended the east-west-striking (~270°) Eagle Fault (mapped onland by 

Dibblee [1966] and Minor and others [2009]) offshore for more than 7 km across the shelf, with a 

significant bend to the southwest (~230°). Although their seismic-reflection data did not image the Eagle 

Fault, they inferred its presence on the basis of offset fold trends. However, the high-resolution seismicreflection data collected for our investigation (sheet 8) suggest that fold axes continue across their 

projection of the Eagle Fault; therefore, we infer that this fault either abruptly dies out offshore or 

continues its westerly strike north of the northern end of our seismic-reflection profiles (sheet 8). 

The regional pattern of faults and earthquakes occurring between 1932 and 2010 that have 

inferred or measured magnitudes greater than 2.0 are shown on Map C on sheet 9. Although locations 

have been provided by the CalTech network since 1932, significantly greater precision began in 1969 

with installation of a USGS seismographic network (see, for example, Lee and Vedder, 1973; Sylvester, 

2001; Southern California Earthquake Data Center, 2010). Epicentral data indicate that seismicity in the 

eastern and central Santa Barbara Channel is characterized by earthquake swarms, relatively frequent 

minor earthquakes, and infrequent major earthquakes. 

Three significant earthquakes affected the Santa Barbara Channel area in 1812, 1857, and 1925, 

prior to the time covered by the Southern California Earthquake Data Center (2010) catalog; however, 

locations in the northern Santa Barbara Channel have been reported (Sylvester and others, 1970) for 

both the 1925 event (M6.3) and the largest earthquake (~M5.5, 7/1/1941), which is shown on Map C 

(sheet 9). In addition, Sylvester and others (1970) documented a swarm of 62 earthquakes (M2.5–M5.2) 

that occurred between 6/26/1968 and 8/3/1968, which also were located 10 to 15 km south (offshore) of 

Santa Barbara. The largest event in the Offshore of Refugio Beach map area (~M4.4) occurred on 

5/9/2004 about 7 km south of El Capitan Beach. 

Thickness and Depth to Base of Uppermost Pleistocene and Holocene Deposits

Maps on sheet 9 show the thickness and the depth to base of uppermost Pleistocene and 

Holocene (post-LGM) deposits both for the Offshore of Refugio Beach map area (Maps A, B) and, to 

establish regional context, for a larger area (about 115 km of coast) that extends from the vicinity of 

Hueneme Canyon northwest to the Refugio Beach area (Maps D, E). To make these maps, water bottom 

and depth to base of the LGM horizons were mapped from seismic-reflection profiles using Seisworks 

software. The difference between the two horizons was exported from Seisworks for every shot point as 

XY coordinates (UTM zone 11) and two-way travel time (TWT). The thickness of the post-LGM unit 

(Maps B, E) was determined by applying a sound velocity of 1,600 m/sec to the TWT, resulting in 

thicknesses as great as 65 m. The thickness points were interpolated to a preliminary continuous surface, 

overlaid with zero-thickness bedrock outcrops (see sheet 10), and contoured (Wong and others, 2012). 

Data within Hueneme Canyon were excluded from the contouring because the seismic-reflection data 

are too sparse to adequately image the highly variable changes in sediment thickness that characterize 

the canyon (Maps D, E). 

Several factors required manual editing of the preliminary sediment-thickness maps to make the 

final products. The Red Mountain Fault Zone, Pitas Point Fault, and Oak Ridge Fault disrupt the 

sediment sequence in the region (Maps D, E on sheet 9). The data points also are dense along tracklines 

(about 1 m apart) and sparse between tracklines (1–2 km apart), resulting in contouring artifacts. To 

incorporate the effect of the faults, to remove irregularities from interpolation, and to reflect other 

geologic information and complexity, the resulting interpolated contours were modified. Contour 
27

modifications and regridding were repeated several times to produce the final regional sedimentthickness map (Wong and others, 2012). 

The depth-to-base data available from Seisworks were similarly processed and contoured; 

however, this preliminary dataset was set aside in favor of a surface determined by subtracting the 

modified thickness data from multibeam bathymetry collected separately (see sheet 1) and using 1,500 

m/sec for TWT in the water column. The depth of this surface in the Hueneme Canyon to Refugio 

Beach area ranges from 12 to 190 m (Map D on sheet 9; see also, Wong and others, 2012). 

Five different “domains” of sediment thickness, which are bounded either by faults or by 

Hueneme Canyon, are recognized on the regional maps (Maps D, E on sheet 9): (1) north of the south 

strand of the Red Mountain Fault Zone; (2) between the south strand of the Red Mountain Fault Zone 

and the Pitas Point Fault; (3) between the Pitas Point and Oak Ridge Faults; (4) between the Oak Ridge 

Fault and Hueneme Canyon; and (5) south of Hueneme Canyon. Table 7–1 shows the area of these five 

domains, along with estimates of their mean sediment thickness and total sediment volume. These data 

highlight the contrast among three general zones of sediment thickness: (1) the uplifted, sediment-poor 

Santa Barbara shelf (domain 1; mean sediment thickness of 3.5 m); (2) a transitional zone (domain 2; 

mean sediment thickness of 18.0 m); and (3) the subsiding, sediment-rich delta and shelf offshore of the 

Ventura and Santa Clara Rivers and Calleguas Creek (domains 3, 4, and 5; mean sediment thicknesses 

of 39.2, 38.9, and 28.3 m, respectively). 

In the Offshore of Refugio Beach map area, thickness data (Map B on sheet 9) reveal that the 

post-LGM section has a mean thickness of just 2.7 m. Sediment is either notably lacking or it forms only 

a thin veneer over bedrock outcrops on the midshelf to outer shelf areas. Post-LGM sediment is thickest 

in three elongate, shore-parallel, coalescing, nearshore to inner shelf bars. These bars, which have 

positive seafloor relief and delta-mouth-bar morphology, are characterized by a relatively flat (about 

0.7° to 1.4°) upper surface and a steeper (about 4.0°) seaward-dipping front. Local, steep, small coastal 

watersheds (Duvall and others, 2004) are the obvious predominant sediment source, and the width of the 

bar may, in part, reflect both sediment supply and the size of the adjacent watershed.

In the western part of the map area, the sediment-thickness map (Map B) reveals an elongate, 

shore-parallel bar that extends 800 to 1,000 m offshore and contains as much as 10 m of post-LGM 

sediment; the seismic-reflection profile in figure 1 on sheet 8 crosses the southern margin of this feature. 

This bar, which extends to the west for several kilometers (beyond the mouth of Gaviota Creek, 3 km 

west of the map area), and lies offshore of the mouths of steep, small (about 3 to 12 km2

) coastal 

watersheds (for example, Cañada del Molino; fig. 1–2). These local watersheds and the much larger 

Gaviota Creek (about 52 km2

; Warrick and Mertes, 2009) are the obvious predominant sediment source. 

Sediment thickness in this bar increases west of the map area to as much as 25 m offshore of Gaviota 

Creek.

Another bar is present in the central nearshore to inner shelf area (Map B) offshore of the mouths 

of Tajiquas Creek and Cañada del Refugio (fig. 1–2; both watersheds are about 12 km2); its sediment 

thickness is as much as 7 m, and it similarly extends about 1,000 m offshore. The seismic-reflection 

profile in figure 6 on sheet 8 crosses the southern margin of this feature.

The third, easternmost bar is present in the inner-shelf area offshore of El Capitan Beach and the 

west and east forks of El Capitan Creek (each about 12.5 km2

; Duvall and others, 2004), and it hosts 

sediment as much as 12 m thick. The east-west-trending axis of sediment thickness associated with this 

bar lies about 800 to 1,000 m offshore (Map B), and the area where thicker sediment is present is 

noticeably broader than that which is associated with similar bars in the central and western parts of the 

map area. This increased breadth may partly reflect the relative subsidence associated with an east-westtrending syncline (fig. 8 on sheet 8; see also, sheet 10). Similar sediment thickening (at most, a few 

meters) in the axes of synclines also is apparent on a few other seismic-reflection profiles (for example, 

fig. 1 on sheet 8), consistent with active folding. 
28

Table 7–1. Area, sediment-thickness, and sediment-volume data for California’s State Waters in Santa Barbara 

Channel region, between Refugio Beach and Hueneme Canyon areas (domains 1–5), as well as in Offshore of 

Refugio Beach map area.

[Data from within Hueneme Canyon were not included in this analysis]

Regional sediment-thickness domains in Santa Barbara Channel region

Area (km2) Mean sediment 

thickness (m)

Sediment volume 

(106 m3) 

(1) Refugio Beach to south strand of Red 

Mountain Fault Zone 357.8 3.5 1,266

(2) South strand of Red Mountain Fault Zone to 

Pitas Point Fault 67.1 18.0 1,205

(3) Pitas Point Fault to Oak Ridge Fault 68.6 39.2 2,688

(4) Oak Ridge Fault to Hueneme Canyon 75.4 38.9 2,933

(5) South of Hueneme Canyon 53.9 28.3 1,527

Sediment thicknesses in Offshore of Refugio Beach map area

Offshore of Refugio Beach map area 97.3 2.7 258
29

Chapter 8. Geologic and Geomorphic Map of the Offshore of Refugio 

Beach Map Area (Sheet 10)

By James E. Conrad, Andrew C. Ritchie, Samuel Y. Johnson, Gordon G. Seitz, and Carlos I. Gutierrez

Geologic and Geomorphic Summary

Marine geology and geomorphology were mapped in the Offshore of Refugio Beach map area 

from approximate Mean High Water (MHW) to the 3-nautical-mile limit of California’s State Waters. 

MHW is defined at an elevation of 1.33 m above the North American Vertical Datum of 1988 (NAVD 

88) (Weber and others, 2005). Offshore geologic units were delineated on the basis of integrated 

analyses of adjacent onshore geology with multibeam bathymetry and backscatter imagery (sheets 1, 2, 

3), seafloor-sediment and rock samples (Reid and others, 2006), digital camera and video imagery (sheet 

6), and high-resolution seismic-reflection profiles (sheet 8). 

The onshore geology was compiled from Dibblee (1981a,b,c,d) and Minor and others (2009). 

Unit ages, which are largely from Minor and others (2009), reflect local stratigraphic relations.

The offshore part of the map area largely consists of a gently offshore-dipping (less than 1°) 

shelf underlain by sediments derived primarily from relatively small coastal watersheds that drain the 

Santa Ynez Mountains (fig. 1–2). Nearshore and shelf deposits are primarily sand (unit Qms) at water 

depths less than about 45 m. More fine-grained sediment (very fine sand, silt and clay) of unit Qmsf are 

mapped between depths of about 45 m and the shelf break, which occurs at about 90 m. The boundary 

between units Qms and Qmsf is based on observations and extrapolation from sediment sampling (see, 

for example, Reid and others, 2006) and camera ground-truth surveying (see sheet 6). It is important to 

note that the boundary between units Qms and Qmsf should be considered transitional and approximate 

and is expected to shift as a result of seasonal- to annual- to decadal-scale cycles in wave climate, 

sediment supply, and sediment transport. Fine-grained deposits that are similar to unit Qmsf also are 

mapped at water depths greater than 90 m, below the shelf break on the upper slope; however, here they 

are identified as a separate unit (unit Qmsl) because they are present at or below the distinct shelf-slope 

geomorphologic break. 

Coarser grained deposits (units Qmsc, Qmscl, and Qsc), which are recognized on the basis of 

their high backscatter (sheet 3) and, in some cases, their moderate seafloor relief (sheets 1, 2), have three 

modes of occurrence. In the nearshore (10 to 30 m water depth), coarse-grained strata (sand and gravel) 

of unit Qmsc underlie laterally coalescing and discontinuous bars at the mouths of steep coastal 

watersheds. Coarser grained deposits of unit Qmscl form several distinct lobes at water depths of 25 to 

70 m, about 600 to 3,000 m offshore; these lobes, which range in size from about 0.1 km2 to about 1.9 

km2, are mapped on the basis of their high backscatter and subtle positive seafloor relief. These coarsegrained strata clearly were derived from fluvial point sources in the adjacent, steep Santa Ynez 

Mountains. An isolated patch of clast-supported cobbles (unit Qsc), which rests on bedrock southwest 

of Refugio State Beach at a water depth of about 76 m, also may have been deposited at the mouth of a 

coastal watershed when sea level was lower during the late Pleistocene.

Offshore bedrock exposures near El Capitan State Beach (fig. 1–2) are assigned to the Miocene 

Rincon Shale (Tr) and the Miocene Monterey Formation (Tm) on the basis of their proximity to coastal 

outcrops mapped by Dibblee (1981a,b). Much of the outer shelf (water depths of more than 70 m) is 

underlain by the undivided Tertiary bedrock unit (Tbu) or the undivided Tertiary and Quaternary 

bedrock unit (QTbu). On the basis of regional cross sections that are constrained by deep seismicreflection data and borehole logs (Heck, 1998; Tennyson and Kropp, 1998; Forman and Redin, 2005; 

Redin, 2005), as well as on high-resolution seismic-reflection data (see sheet 8) coupled with proprietary 

oil-industry dartcore data (Ashley and others, 1977), these outer shelf outcrops consist of the late 
30

Miocene and early Pliocene Sisquoc Formation and the overlying Pliocene and Pleistocene Pico 

Formation (both of which are mapped here as the undivided QTbu unit). These undivided rocks have 

been uplifted in a large, warped, south-dipping regional homocline that formed above the blind, northdipping North Channel Fault. The fault tip is inferred to be about 2 km deep (that is, at two-way travel 

time of about 1.5 sec) about 6 to 7 km offshore, beneath the slope and just outside the limit of 

California’s State Waters (see sheet 8). 

This area has a long history of petroleum production (Barnum, 1998), and pockmarks (unit 

Qmp) caused by gas seeps are common features in the Offshore Refugio Beach map area. In 1962, Shell 

Oil Company discovered the Molino gas field (see fig. 1–2 in pamphlet) 4 km offshore, in the 

southwestern part of the map area. Natural gas production, achieved by onshore directional drilling of an 

offshore anticlinal trap, has been underway since the 1960s (Galloway, 1998). 

Table 8–1. Areas and relative proportions of offshore geologic map units in Offshore of Refugio Beach map area.

Map Unit Area (m2) Area (km2) Percent of total area

Marine sedimentary units

Qmp 494,483 0.49 0.42

Qms 24,783,678 24.78 20.83

Qmsf 43,440,452 43.44 36.52

Qmsl 4,795,686 4.80 4.03

Qmsc 2,090,630 2.09 1.76

Qmscl 6,705,794 6.71 5.64

Qsc 26,299 0.03 0.02

Total, sedimentary units 82,337,022 82.34 69.21

Marine bedrock and (or) shallow bedrock units

Qms/QTbu 8,915,979 8.92 7.49

Qms/Tbu 7,194,073 7.19 6.05

Qms/Tm 1,701,328 1.70 1.43

QTbu 8,775,763 8.78 7.38

Tbu 6,850,735 6.85 5.76

Tm 3,154,004 3.15 2.65

Tr 35,717 0.04 0.03

Total, bedrock units 36,627,599 36.63 30.79

Total, Offshore of Refugio Beach map area 118,964,621 118.96 100.00
31

DESCRIPTION OF MAP UNITS

OFFSHORE GEOLOGIC AND GEOMORPHIC UNITS

[Note that, where older units (typically, bedrock) are overlain by thin (<1 m thick) Quaternary deposits, 

composite units are mapped. These composite units, which are shown with gray stipple pattern on older 

unit, are designated by composite label indicating both overlying sediment cover and lower (older) unit, 

separated by slash (for example, Qms/Tbu indicates that thin sheet of Qms overlies Tbu)]

Qms Marine nearshore and shelf deposits (late Holocene)—Mostly sand; ripples common. 

Found on gently seaward-dipping (about 1°) surface that extends from nearshore to 

water depths of about 45 m 

Qmsc Coarse-grained marine nearshore and shelf deposits (late Holocene)—Ranges from 

coarse sand and gravel to boulders. Found in laterally coalescing to discontinuous 

nearshore (10 to 30 m water depth) bars at mouths of steep coastal watersheds. 

Recognized primarily on basis of high acoustic-backscatter intensity and moderate 

positive relief (see sheets 1–4) 

Qmsf Fine-grained marine shelf deposits (late Holocene)—Mostly clay, silt, and very fine sand; 

commonly bioturbated; found on gently seaward-dipping (about 0.7°) surface at 

depths greater than about 45 to 60 m and less than about 90 m 

Qmp Marine pockmarks (late Holocene)—Sand and mud, in circular to elliptical pockmarks. 

Pockmarks are solitary or grouped, and range in size from 50 to 150 m along their 

long axis; they typically are 20 cm to 40 cm deep and ringed with a convex rim as 

high as 150 cm 

Qmsl Marine upper slope deposits (late Holocene)—Predominantly clay, silt, and very fine sand; 

commonly bioturbated. Occurs below shelf break on seaward-dipping (about 5°) 

surface at water depths more than 90 m 

Qmscl Marine shelf-sediment lobes (Holocene)—Predominantly sand to boulders(?). Found as 

five distinct, irregularly shaped lobes, at water depths of 25 to 70 m, about 600 to 

3,000 m offshore. Lobes, which range in size from about 0.1 km2

 to about 1.5 km2, 

are mapped on basis of high backscatter intensity and subtle positive seafloor relief 

Qsc Marine or nonmarine coarse-grained deposits (late Pleistocene?)—Isolated outcrop of 

consolidated or semiconsolidated, flat-lying sedimentary strata, at least partly

conglomeratic. Unconformably overlies folded marine sedimentary rocks of the 

undivided Pliocene and Miocene bedrock unit (Tbu). Deposit is inferred to be 2 to 3 

m thick

QTbu Bedrock, undivided (Pleistocene to Miocene)—Predominantly mudstone and siltstone; 

includes conglomeratic sandstone. Consists of Pliocene and Pleistocene Pico 

Formation and late Miocene and early Pliocene Sisquoc Formation. Stippled areas 

(composite unit Qms/QTbu) indicated where thin sheets of Qms overlie unit 

Tbu Bedrock, undivided (Pliocene and Miocene)—Mainly mudstone and siltstone. Possibly 

consists of the early Pliocene and Late Miocene Sisquoc Formation, the Miocene 

Monterey Formation, and the early Miocene Rincon Shale. Stippled areas (composite 

unit Qms/Tbu) indicate where thin sheets of Qms overlie unit 

Tm Monterey Formation (Miocene)—Predominantly well-bedded, siliceous and calcareous 

mudstone and shale. Stippled areas (composite unit Qms/Tm) indicate where thin 

sheets of Qms overlie unit 

Tr Rincon Shale (early Miocene)—Mostly mudstone, with subordinate dolomite, siliceous 
32

shale, sandstone, and tuff 

ONSHORE GEOLOGIC AND GEOMORPHIC UNITS

 [Units are compiled from from Dibblee (1981a,b,c,d) and Minor and others (2009); unit ages, which are 

derived from these sources, reflect local stratigraphic relations]

af Artificial fill (late Holocene)—Engineered and (or) nonengineered 

Qb Beach deposits (late Holocene)—Unconsolidated, loose, fine- to coarse-grained sand; well 

sorted. Mapped in coastal band from shoreline to highest elevation of swash zone 

Qyf Alluvium and colluvium, undivided (Holocene and late Pleistocene)—Poorly 

consolidated silt, sand, and gravel deposits, in modern drainages and piedmont 

alluvial fans and floodplains 

Qc Colluvium (Holocene and late Pleistocene)—Poorly consolidated, poorly stratified, and 

poorly sorted deposits that mantle gentle to moderate slopes; formed by weathering 

and downslope movement of nearby bedrock

Qls Landslide deposits (Holocene to middle Pleistocene)—Deposits of diverse slopemovement processes, ranging from poorly sorted, disrupted mixtures of rock 

fragments and soil to relatively intact bedrock slump blocks 

Qomp Marine-terrace deposits (late Pleistocene)—Basal, about 1 m thick, weakly to moderately 

well consolidated, variably stratified, fossiliferous gravel, sand, and silt; overlies 

nonmarine eolian, alluvial, and colluvial deposits; deposited as marine intertidal, 

beach, and estuarine deposits. Unit, which rests on elevated, marine wave-cut platforms, 

forms single terraces or flights of terraces that, in Santa Barbara coastal region, range in 

elevation from 10 to 130 m and in age from about 120 ka (oxygen-isotope stage 5) to 40 

ka (substage 3) (Trecker and others, 1998; Gurrola and others, 2014)

Qoa Alluvial deposits (late and middle Pleistocene)—Weakly consolidated, stratified silt, sand, 

gravel, conglomerate, breccia, and rare interbeds of clay, silt, and mudstone that form 

low, rounded, moderately well dissected terraces and piedmont alluvial fans. Found at 

elevations higher than the modern coastal-piedmont surface 

Tsq Sisquoc Formation (early Pliocene and late Miocene)—Marine, tan- to white-weathering, 

diatomaceous mudstone and shale, conglomerate, and subordinate dolomite. Unit 

distinguished by thick beds of conglomerate that contain angular clasts (commonly as 

much as 1 m across; some blocks as large as 10 m) derived from the Monterey 

Formation. Both top and base are erosional unconformities 

Tm Monterey Formation, undivided (Miocene)—Marine, predominantly well-bedded siliceous 

and calcareous mudstone and shale, with subordinate porcelanite and dolomite. 

Contains abundant microfossils. Unit deposited at water depths ranging from upper to 

lower bathyal (150–2,000 m; Isaacs, 2001). Maximum composite thickness in this 

region estimated to be about 830 m. In map area, the Monterey Formation is divided 

into the following subunit 

Tml Lower calcareous unit (middle and early Miocene)—Calcareous, siliceous, and 

phosphatic, white- to tan-weathering mudstone and shale, with subordinate dolomite, 

porcelanite, breccia, glauconitic sandstone, and tuff. In places, unit contains 

intraformational deformation (including breccia) that may have formed by 

gravitational slumping shortly after deposition 

Tr Rincon Shale (early Miocene)—Marine, primarily massive or thick-bedded, light-brownweathering mudstone, with subordinate dolomite, siliceous shale, sandstone, and tuff. 
33

Mudstone is bioturbated and massive, is pervasively hackly fractured, and locally 

contains abundant microfossils. Single or multiple white-weathering tuff layers 

limited to upper 10 m 

Tv Vaqueros Formation (late Oligocene)—Shallow-marine, massive, and bioturbated, 

resistant, light-tan-weathering sandstone. Uppermost part consists of thinly 

interbedded sandstone, siltstone, and mudstone; base typically marked by 50- to 150-

cm-thick, thinly bedded, calcareous conglomerate that contains abundant fossil-shell 

fragments

Tspu Sespe Formation, upper sandstone and mudstone unit (Oligocene)—Interbedded 

sandstone, siltstone, and mudstone; weathers to various shades of maroon, buff, pale 

green, tan, and gray. Proportions of different sedimentary rock types vary both 

laterally and vertically throughout the section. Sandstones commonly are broadly 

lenticular, laminated, and thin to thick bedded 

Ta Alegria Formation (Oligocene)—Shallow-marine, arkosic sandstone and greenish-gray 

siltstone; locally fossiliferous; interfingers to east with the Sespe Formation (Dibblee, 

1981a)

Tg Gaviota Formation (Oligocene and Eocene)—Shallow-marine, thick-bedded arkosic 

sandstone and some gray siltstone; locally fossiliferous (Dibblee, 1981a) 

Tgsl Lower unit (Oligocene and Eocene)—Shallow-marine, gray, concretionary siltstone and 

claystone (Dibblee, 1981b) 

Tg-sa Gaviota Formation and Sacate Formation, undivided (Oligocene and Eocene)—Marine 

sandstone and siltstone; occurs in northwest corner of map area (Dibblee, 1981d) 

Tcw Coldwater Sandstone (late? and middle Eocene)—Shallow-marine, thin- to thick-bedded 

sandstone that weathers to distinctive pale shades of buff, yellow, tan, and brown, 

with subordinate interbeds and thin intervals of gray, olive-gray, and greenish-gray 

siltstone, shale, and mudstone. Sandstone beds are resistant and form hogbacks where 

steeply dipping. Upper part is locally conglomeratic and rich in fossil oyster shells

Ts Sacate Formation (Eocene)—Marine, dark-gray micaceous siltstone and shale, with 

interbeds of hard arkosic sandstone (Dibblee, 1981d)

Tsa Sandstone subunit (Eocene)—Marine, light-gray to tan arkosic sandstone (Dibblee, 

1981d) 

Tcd Cozy Dell Shale (Eocene)—Marine, dark-gray argillaceous to silty micaceous shale with 

minor light-gray to tan arkosic sandstone 

Tcds Sandstone interval (Eocene)—Marine, light-gray to tan arkosic sandstone, with minor 

interbeds of gray micaceous shale 

Tma Matilija Sandstone (Eocene)—Marine, thick-bedded, arkosic sandstone 

Tmas Sandstone and shale subunit (Eocene)—Interbedded sandstone and shale 

Tj Juncal Formation (Eocene)—Marine shale

Tjs Sandstone unit (Eocene)—Marine sandstone 

Tan Anita Shale (Eocene and Paleocene)—Marine, medium- to dark-gray micaceous shale, with 

thin sandstone layers (Dibblee, 1981c) 

Jalama Formation (Late Cretaceous)—Predominantly micaceous clay shale, with minor 

sandstone interbeds 

Kjs Sandstone unit (Late Cretaceous)—Marine sandstone (Dibblee, 1981d) 

Kj Shale unit (Late Cretaceous)—Marine shale (Dibblee, 1981d) 
34

Chapter 9. Predicted Distribution of Benthic Macro-Invertebrates for the 

Offshore of Refugio Beach Map Area and the Santa Barbara Channel 

Region (Sheet 11)

By Lisa M. Krigsman, Mary M. Yoklavich, Nadine E. Golden, and Guy R. Cochrane 

Modeling the distribution of ecologically and economically important species provides managers 

and conservation planners with information on a broad spatial scale that is useful to coastal 

management, ocean energy, marine protected areas, and marine spatial planning. Sheet 11 displays 

predictive models of occurrence for common benthic macro-invertebrate taxa and maps the probability 

of occurrence of these taxa in the Santa Barbara Channel region (Krigsman and others, 2012). These 

models are based on real-time biological observations of all macro-organisms made during ground-truth 

surveys (sheet 6) conducted in 2008 and 2009; the observations were made during a 10-second interval 

every minute along video transects, which were approximately 1 km in length (see sheet 6; see also, 

chapter 5 of this pamphlet). These transects produced a total of 923 observations from Refugio Beach 

(34.5° N., 120.1° W.) to Hueneme Canyon (34.1° N., 119.2° W.). 

Five invertebrate taxa—cup corals, hydroids, short sea pens, tall sea pens, and brittle stars (which 

protrude out of the sediment)—were selected for modeling purposes because of their frequent 

occurrence in the Santa Barbara Channel; all are structure-forming components of valuable habitat for 

groundfish species (Krigsman and others, 2012). Presence-absence data for the selected invertebrates 

were fit to multiple generalized linear models using a combination of three covariates—geographic 

location, seafloor character (sheet 5), and shaded-relief bathymetry (sheet 2)—as well as relevant 

interaction terms. Geographic locations for the five observed invertebrates were derived from analysis of 

the video data from an area along the mainland coast of the Santa Barbara Channel; the Offshore of 

Carpinteria map area was excluded because of insufficient data. Three statistically different locations 

were identified on the basis of a community-structure analysis: (1) the Hueneme Canyon and Vicinity 

and Offshore of Ventura map areas; (2) the Offshore of Santa Barbara and Offshore of Coal Oil Point 

map areas; and (3) the Offshore of Refugio Beach map area. Best-fit models were selected for each 

invertebrate on the basis of Akaike’s Information Criterion (AIC) (Akaike, 1974), a best-fit model being 

defined as the one that has the fewest parameters within two AIC points of the minimum score.

The seafloor in the Offshore of Refugio Beach map area has reefs made up of Class II (mixed 

habitat) and Class III (rugose rock) that are surrounded by large areas of Class I (unconsolidated 

sediment) (see sheet 5; see also, chapter 4 in this pamphlet). Cup corals (Map D on sheet 11), benthic 

cnidarians typically found on rocky habitats, have a very high (about 90 percent) probability of 

occurrence at depths deeper than 60 m where suitable habitat is present. The probability of observing 

cup corals also was very high on nearshore reefs. Hydroids (Map C on sheet 11), another benthic 

cnidarian found on rocky habitats, have a low probability of occurrence on nearshore reefs; however, as 

depth increases so does the probability of occurrence in areas of mixed sediment and rugose rock. 

Sea pens, also members of the phylum Cnidaria, are divided into two groups—short and tall— 

on the basis of their size. Sea pens less than 60 cm in height are identified as short sea pens (Stylatula

spp. and Virgularia spp.); those taller than 60 cm are identified as tall sea pens (Halipteris spp.) (Maps 

B and A, respectively, on sheet 11). Sea pens typically are associated with unconsolidated and mixed 

sediment because their rootlike base anchors them to the seafloor. Short sea pens have the highest 

probability of occurrence on unconsolidated sediment at depths between 30 and 60 m, whereas tall sea 

pens have the highest probability of occurrence at depths greater than 60 m. 

Brittle stars (Map E on sheet 11) can occur in such high densities in the sediment that they create 

a thick carpet on the seafloor. Like sea pens, they typically are associated with unconsolidated and 
35

mixed sediment into which they burrow; however, they also are found in cracks and crevices within 

rugose rock. In the Offshore of Refugio Beach map area, brittle stars have a moderate probability of 

occurrence (about 70 percent) in unconsolidated sediment at depths between 30 and 60 m; however, at 

depths less than 30 m and also greater than 60 m, the probability of occurrence is low.

These predictive maps are based on data available from the California Seafloor Mapping 

Program (location, habitat type, and bathymetry). Other factors such as ocean currents (Cudaback and 

others, 2005), water temperature (Bingham and others, 1997), larval distribution (Grantham and others, 

2003), and recruitment and mortality (Keough and Downes, 1982) also can significantly influence the 

distribution and abundance of these benthic macro-invertebrate taxa.
36

Acknowledgments

This publication was funded by the California Ocean Protection Council and the U.S. Geological 

Survey (USGS) Coastal and Marine Geology Program. Eric Grossman, Jon Warrick, and Scott Starratt

(all USGS) provided constructive reviews. The publication benefitted from the significant effort Taryn 

Lindquist put into refining the design of the maps and pamphlet, and from editing by Sarah Nagorsen. 
37

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