Modeling of Depth to Base of Last Glacial Maximum 

and Seafloor Sediment Thickness for the California 

State Waters Map Series, Eastern Santa Barbara 

Channel, California

By Florence L. Wong, Eleyne L. Phillips, Samuel Y. Johnson, and Ray W. Sliter

Open-File Report 2012–1161

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Wong, F. L., Phillips, E.L., Johnson, S.Y., and Sliter, R.W., 2012, Modeling of depth to base of Last 

Glacial Maximum and seafloor sediment thickness for the California State Waters Map Series, eastern 

Santa Barbara Channel, California: U.S. Geological Survey Open-File Report 2012-1161, 16 p. 

Cover: Seismic-reflection profile with interpreted horizons of seafloor (green) and base of Last Glacial 

Maximum (bLGM, magenta). See Figure 2 caption.
iii

Contents

Conversion Factors and Datum...................................................................................................................................... v

Conversion Factors......................................................................................................................................................... v

Datum............................................................................................................................................................................. v

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Methods......................................................................................................................................................................... 2

Digitizing Reflection Surfaces .................................................................................................................................... 2

Surface Generation.................................................................................................................................................... 2

Regional Areas and Volumes..................................................................................................................................... 4

Results........................................................................................................................................................................... 4

Summary ....................................................................................................................................................................... 5

Acknowledgments ......................................................................................................................................................... 5

References Cited........................................................................................................................................................... 6

Appendix........................................................................................................................................................................ 6

Figures

Figure 1. Map of Santa Barbara Channel, California. Map blocks are outlines of quadrangles defined for 

the California State Waters Map Series. The data area for this report spans the shelf from Refugio to 

southeast of Hueneme Canyon. Seismic-reflection data were collected by the U.S. Geological Survey in 

2007 and 2008 along the displayed tracklines. The 3-nautical mile limit is the seaward boundary of the 

State waters................................................................................................................................................... 7

Figure 2. SeisWorks® display of a seismic-reflection profile with interpreted horizons of seafloor (green) 

and base of Last Glacial Maximum (bLGM, magenta). Depth to seafloor, depth to bLGM, and sediment 

thickness (all in two-way travel time (TWT)) are available for export. The horizontal scale marks seismic 

shot value; the vertical scale is in TWT (msec). At the right edge of the profile, the seafloor horizon is 

approximately 25 msec (or 0.025 m)/2 × 1,500 m/s = 18.75 m; the sediment thickness is approximately 10 

msec (or 0.010 s)/2 x 1,600 m/s = 8.0 m. The depth to bLGM = 18.8 + 8.0 = 53.5 m.................................... 8

Figure 3. (A) Seismic-reflection tracklines color-coded by survey identification. (B) Points color-coded by 

sediment thickness values as originally exported from SeisWorks®. (C) Enlargement of a subwindow of 

points. Density of data causes “x” symbols on maps to blur together............................................................ 9

Figure 4. Modifications to sediment thickness data. Areas mapped as bedrock exposures are interpreted 

as having zero thickness. Canyon polygons are areas for which thickness data points are excluded before 

gridding. Faults are to be treated as breaklines, disrupting the continuity of the gridding where they occur. 

RM, Red Mountain fault; PP, Pitas Point fault; OR, Oak Ridge fault............................................................ 10

Figure 5. Candidates for data clipping mask. (A) 500-m buffer around tracklines was not used because it 

excludes useable data areas (deep incursions between tracklines). (B) An enclosing polygon (hull) is 

extracted from a triangular irregular network (TIN) constructed from the sediment thickness points. The hull 

is buffered with 100-m and 200-m margins; the 200-m margin was used. (C) For the landward limit, a 100-

m buffer is applied to the coastline. The seaward boundary is the 3-nmi limit line. (D) The final clipping 

mask consists of pieces of the 200-m TIN buffer, 100-m coast buffer, 3-nmi limit, and excludes areas within 

the canyon outlines in the southeast............................................................................................................ 11
iv

Figure 6. (A) Grid result from applying TopoToRaster to collection of points. (B) Grid calculation includes 

bedrock points (thickness = 0). (C) Final grid calculated from edited isopachs. (D) Location map. ............. 12

Figure 7. Faults (topologic breaklines) that disrupt seafloor sediment, as mapped from seismic-reflection 

profiles. North to south: RM, Red Mountain fault; PP, Pitas Point fault; OR, Oak Ridge fault. (A) Thickness 

interpolated with TopoToRaster, which has no option for breaklines. (B) Thickness interpolated by 

triangular irregular network (TIN) without breaklines. Result is similar to A. (C) Thickness TIN model 

calculated with breaklines, exhibits slight accommodation for Pitas Point fault (middle fault). (D) Final map 

interpolated from manually edited lines of equal thickness.......................................................................... 13

Figure 8. Input data for TIN calculation. SF_type (surface feature type) determines how data layers are 

treated in construction of the TIN. Interpolation takes in all points (masspoints), replaces bedrock with zero 

thickness (hardreplace), recognizes faults (hardline), omits canyons (harderase), and is confined to a clip 

polygon (hardclip). ....................................................................................................................................... 14

Figure 9. Northern (pink) and southern (blue) extents, separated by the Oak Ridge fault, were separately 

gridded and then mosaicked........................................................................................................................ 14

Figure 10. Final grids of (A) sediment thickness, and (B) depth to base of the Last Glacial Maximum for 

offshore eastern Santa Barbara Channel. RM, Red Mountain fault; PP, Pitas Point fault; OR, Oak Ridge 

fault.............................................................................................................................................................. 15

Figure 11. Regional extents for sediment volume calculations. Region A (green) extends from the western 

extent of the data (offshore of Refugio) to the southern strand of the Red Mountain fault zone (RMFZ). 

Region B (pink) extends from RMFZ to the Pitas Point fault (PPF). Region C (brown) extends from PPF to 

the Oak Ridge fault (ORF). Region D (blue) extends from ORF to Hueneme Canyon. Region E (yellow) 

comprises the remainder of the data, on the eastern side of Hueneme Canyon. Faults are shown in heavy 

black lines.................................................................................................................................................... 16

Tables

Table 1. Comparison of grid statistics for the depth to seafloor from multibeam bathymetry data and depth 

to seafloor as digitized from SeisWorks®. ......................................................................................................4

Table 2. Grid statistics for sediment thickness and depth to base of Last Glacial Maximum for entire area of 

study..............................................................................................................................................................5

Table 3. Area, mean sediment thickness, and sediment volume for regional sediment-thickness domains 

(see fig. 11)....................................................................................................................................................5
v

Conversion Factors and Datum

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

mile, nautical (nmi) 1.852 kilometer (km)

SI to Inch/Pound

Multiply By To obtain

meter (m) 3.281 foot (ft) 

meter per second (m/s) 3.281 foot per second (ft/s) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

square kilometer (km2) 0.3861 square mile (mi2)

cubic kilometer (km3) 0.2399 cubic mile (mi3) 

millisecond (msec) 0.001 second

Datum

The World Geodetic System (WGS) 84 is the reference coordinate system used by the Global 

Positioning System.
Modeling of Depth to Base of Last Glacial Maximum 

and Seafloor Sediment Thickness for the California 

State Waters Map Series, Eastern Santa Barbara 

Channel, California

By Florence L. Wong, Eleyne L. Phillips, Samuel Y. Johnson, and Ray W. Sliter

Abstract

Models of the depth to the base of Last Glacial Maximum and sediment thickness over 

the base of Last Glacial Maximum for the eastern Santa Barbara Channel are a key part of the 

maps of shallow subsurface geology and structure for offshore Refugio to Hueneme Canyon, 

California, in the California State Waters Map Series. A satisfactory interpolation of the two 

datasets that accounted for regional geologic structure was developed using geographic 

information systems modeling and graphics software tools. Regional sediment volumes were 

determined from the model. Source data files suitable for geographic information systems 

mapping applications are provided. 

Introduction

The California State Waters Map Series in the eastern Santa Barbara Channel is a series 

of thematic maps covering the offshore area between Refugio and Hueneme Canyon, California 

(fig. 1). The thematic maps of shallow subsurface geology and structure include maps of depth to 

the base of the Last Glacial Maximum (bLGM), and the thickness of the uppermost Pleistocene 

and Holocene sediments deposited over the bLGM. We describe the derivation of these two map 

layers and the calculation of footprint area, thickness, and volume of the deposits, and provide 

results of these calculations for the whole area and for structural subareas (Johnson and others, 

2012a, 2012b). We also provide data files of the original thickness and depth measurements. 
2

Methods

Digitizing Reflection Surfaces

Single-channel seismic-reflection profiles from surveys Z-3-07-SC and S-7-08-SC 

conducted by the U.S. Geological Survey in 2007 and 2008, respectively, were imported into 

SeisWorks® 3D software for interpretation (fig. 1; U.S. Geological Survey 2007, 2008; Johnson 

and others, 2012a). Reflection surfaces representing the seafloor and the bLGM were digitized in 

two-way travel time (TWT) milliseconds (msec; fig. 2. Sediment thickness was calculated as the 

difference between the seafloor and bLGM travel times. Before exporting from SeisWorks®, 

seafloor depths were corrected with a water column velocity of 1,500 m/s (depth = TWT msec/2 

× 1,500 m/s) and sediment thickness with a sediment velocity of 1,600 m/s (thickness = TWT 

msec/2 × 1,600 m/s). The depth to the bLGM is the sum of the corrected seafloor depth and 

corrected sediment thickness (fig. 2). 

The sediment thickness and depth to bLGM were exported with UTM 11 coordinates as 

text files with approximately 925,000 points each. The data points are spaced 1 m apart along 

track (the shot interval of the seismic-reflection data systems) and are spaced 1–2 km apart cross 

track (fig. 3). 

Thickness data were converted to ArcGIS point format with the following original 

attributes plus latitude and longitude (converted from X_UTM1184 and Y_UTM1184) and a 

corrected thickness: 

X_UTM1184 – x coordinate in UTM Zone 11 coordinate system, datum WGS84

Y_UTM1184 – y coordinate in UTM Zone 11 coordinate system, datum WGS84

THK_M0 – thickness in meters as exported from SeisWorks®

LONG84 – longitude, datum WGS84

LAT84 – latitude, datum WGS84

THK_M – adjusted thickness, where any value <= 0.1 m is changed to 0.1 m

Depth to bLGM data have the following original attributes plus latitude and longitude (converted 

from X_UTM1184 and Y_UTM1184):

X_UTM1184 – x coordinate in UTM Zone 11 coordinate system, datum WGS84

Y_UTM1184 – y coordinate in UTM Zone 11 coordinate system, datum WGS84

LONG84 – longitude, datum WGS84

LAT84 – latitude, datum WGS84

BSM_M – depth in meters to base of Last Glacial Maximum

Surface Generation

Many interpolation or gridding methods are available for calculating a continuous surface 

from point data. If, as is the case here, the data area includes discontinuous geologic and 

physiographic features (faults and canyons), the data need to be modified for these features 

before or during the calculation. In this section, we describe adjustments to the sediment 

thickness data and several approaches to a satisfactory regional sediment thickness, from which 

the depth to bLGM was then calculated.

Bedrock exposures from the map of seafloor geology (Ritchie and others, 2012) were 

used to define points of zero thickness (fig. 4). Before adding these points to the dataset, any 

original SeisWorks® value (THK_M0) less than 0.1 m was changed to 0.1 m (THK_M) to allow 

the bedrock points to be the authoritative zero values. Data points were excluded over Hueneme 
3

Canyon, where data were too sparse to adequately image the highly variable changes in sediment 

thickness in the canyon (Ritchie and others, 2012). 

As gridding operations usually produce results that fill a rectangular window or otherwise 

spill beyond the available data extent, a clip mask or polygon was constructed to limit the final 

grid to the available data and to the geographic limits of the map series. A mask can be manually 

sketched around the data or extracted from geographic elements, which was our method. The 

seaward boundary of the mask is defined by the 3-nmi limit for the State of California. The inner 

limit was constructed from the intersection of a 100-m buffer of the coastline and a 200-m buffer 

around the input dataset. The canyon areas also were omitted (fig. 5).

The first sediment thickness grid interpolated from the SeisWorks® point data was 

processed with the ArcGIS TopoToRaster tool (Esri, 2011), an interpolator designed for 

topographic surfaces that often serves well for other types of continuous fields. Figure 6A

displays the results from applying TopoToRaster on the data before the bedrock data were added 

(zero-thickness areas are patchy). Figure 6B displays the same calculation after zero-thickness 

points from bedrock areas were added. This grid provides a rapid overview, but retains some 

patchiness from along-track data variations.

The Red Mountain, Pitas Point, and Oak Ridge faults displace parts of the seafloor 

sediment in this area (Johnson and others, 2012b). TopoToRaster does not have an option to 

include the effect of faults or other topologic breaklines (fig. 7A), so we next tried the triangular 

irregular network (TIN) tool (Esri, 2011). TIN calculations can ingest data points or contours, 

exclude unwanted features, and accommodate breaklines (fig. 8). The TIN results showed some 

breakline (fault) influence, but did not show as sharp a change across the faults as were evident 

in seismic-reflection profiles (figs. 7B-7C; Johnson and others, 2012a).

From the earlier TopoToRaster grid, isopachs (thickness contours) at 0.1, 2, 4, 7 m, and 

then at 5 m intervals from 10 m to the maximum thickness (55 m) were extracted. These 

contours were edited and additional contours were digitized in Adobe© Illustrator to better 

represent the effect of local faults and other geologic structures and to remove gridding artifacts. 

The edited contours alone were gridded by TopoToRaster, using the following parameters. All 

but the grid cell size are default values.

ITERATIONS 50

ROUGHNESS_PENALTY 0.00000000000

DISCRETE_ERROR_FACTOR 0.50000000000

VERTICAL_STANDARD_ERROR 0.00000000000

TOLERANCES 0.50000000000 0.10000000000

ZLIMITS 0.00000000000

CELL_SIZE 50.00000000000

MARGIN 0

This calculation failed to portray the substantial change in sediment thickness caused by 

the uplift associated with the Oak Ridge fault zone (fig. 7C; Johnson and others, 2012a, 2012b;

Ritchie and others, 2012). The most satisfactory approach to accommodate the fault was to 

generate two separate grids–one to the north of the fault and one to the south (fig. 9)–and then 

mosaic the two into one (fig. 7D). This interpolation is used in the final maps of shallow 

subsurface geology and structure (fig. 10A; Johnson and others, 2012b). 

Initially, the depth to the base of Last Glacial Maximum also was calculated from the 

points exported from SeisWorks®. Instead of repeating the sediment thickness adjustments on the 

bLGM data, the final depth to bLGM was calculated by subtracting the modified sediment 

thickness data from the seafloor depth as determined by multibeam bathymetry (fig. 10B; Kvitek 

and others, 2012). At the 50-m resolution of the gridding, the mean difference between the 
4

along-track seafloor depths digitized in SeisWorks® and the multibeam bathymetry was 0.6 m,

with a standard deviation of 5 m, an acceptable uncertainty for the depth of the bLGM (table 1).

Table 1. Comparison of grid statistics for the depth to seafloor from multibeam bathymetry data and depth 

to seafloor as digitized from SeisWorks®.

[m, meter]

Minimum (m) Maximum (m) Mean (m) Standard 

deviation (m)

Depth to seafloor (multibeam) -290 -2 -40 25

Depth to seafloor 

(SeisWorks®)

-270 -7 -40 25

Difference: multibeam –

SeisWorks®

-17 55 0.6 4.7

Regional Areas and Volumes

Area, mean sediment thickness, and sediment volume were calculated for regions bound 

by major faults (Oak Ridge fault, Pitas Point fault, and the Red Mountain fault) and Hueneme 

Canyon (fig. 11). The volume of each column of sediment under a grid cell can be determined by 

multiplying the area of the grid cell and the sediment thickness. The ArcGIS “zonal statistics as 

table” tool generates, for each polygon of interest, cell (pixel) count, area, minimum value, 

maximum value, range, mean, standard deviation, and sum. We use the zonal sum (sediment 

thickness of all cells in a zone) and the final grid cell size (50 m) to calculate volume = 

ZONALSUM * 2500.

Results

The thickness of the uppermost Pleistocene to Holocene sediments in the State waters of 

the eastern Santa Barbara Channel has a mean value of 15 m and a maximum thickness of 57 m 

(table 2). This thickness is not evenly distributed across the region, but thinner northwest of the 

Red Mountain fault and thicker southeast of that feature (fig. 10A; table 3; Johnson and others, 

2012b). The depth to the bLGM or bedrock in the area has a mean value of 56 m. Excluding the 

submarine Hueneme Canyon, the value deepens from 12 m to a maximum of 190 m within the 3-

nmi limit. The total volume of sediment in the study area is about 9.6 km3

, distributed over five 

domains, the largest of which lies between the Oak Ridge fault and Hueneme Canyon (table 3; 

Johnson and others, 2012b).
5

Table 2. Grid statistics for sediment thickness and depth to base of Last Glacial Maximum for entire area of 

study.

[m, meter]

Minimum (m) Maximum (m) Mean (m) Standard 

deviation (m)

sediment thickness (m) 0 57 15 16

depth to bLGM: 

multibeam seafloor –

sediment thickness

-190 -12 -56 21

Table 3. Area, mean sediment thickness, and sediment volume for regional sediment-thickness domains 

(see fig. 11).

[km2

, square kilometer, km3, cubic kilometer, m, meter]

Regional sediment-thickness domains Area (km2) Mean sediment 

thickness (m)

Sediment volume 

(km3)

(1) Refugio Beach to northern strand of 

Red Mountain fault zone 357 4 1.27

(2) Northern strand of Red Mountain fault

zone to Pitas Point fault 68 18 1.20

(3) Pitas Point fault to Oak Ridge fault 70 39 2.74

(4) Oak Ridge fault to Hueneme Canyon 75 39 2.90

(5) South of Hueneme Canyon 54 28 1.53

Summary

Several GIS and graphic techniques were combined to produce continuous grids of 

sediment thickness and accompanying depth to the bLGM. Each stage of the calculations was 

evaluated for consistency with the various datasets that informed the interpretation. With a 

variety of tools available, the determination of which tool to use for the individual steps and how 

to minimize data manipulation artifacts was integral to the creation of the final product. Files in 

ArcGIS format of the final grids of sediment thickness and depth to bLGM will be compiled in 

the database accompanying the California State Waters Project. The SeisWorks® point data from 

which the grids were calculated are provided with this report as separate files (sbsedthkpt.zip and 

sbsedbsmpt.zip).

Acknowledgments

This report was greatly improved by reviews from Amy Foxgrover and Pete Dartnell, 

both USGS.
6

References Cited

Esri, 2011, ArcGIS Desktop—Release 9.3.1, service pack 2: Redlands, Calif., Esri, accessed 

June 14, 2012 at http://resources.arcgis.com/content/patches-and-servicepacks?fa=viewPatch&PID=66&MetaID=1620.

Johnson, S.Y., Sliter, R.W., Normark, W.R., and Ritchie, A.C., 2012a, Seismic-reflection 

profiles, Hueneme Canyon and vicinity, California, sheet 8 in Johnson, S.Y., Dartnell, P., 

Cochrane, G.R., Golden, N.E., Phillips, E.L., Ritchie, A.C., Kvitek, R.G., Greene, H.G., 

Krigsman, L.M., Endris, C.A., Clahan, K.B., Sliter, R.W., Wong, F.L., Yoklavich, M.M., and 

Normark, W.R. (S.Y. Johnson, ed.), California State Waters Map Series—Hueneme Canyon 

and vicinity, California: U.S. Geological Survey Scientific Investigations Map 3225, pamphlet 

44 p., 12 sheets, available at http://pubs.usgs.gov/sim/3225/.

Johnson, S.Y., Phillips, E.L., Ritchie, A.C., Wong, F.L., and Sliter, R.W., 2012b, Local 

(Hueneme Canyon and vicinity) and regional (offshore Refugio Beach to Hueneme Canyon) 

shallow-subsurface geology and structure, Santa Barbara Channel, California, sheet 9 in

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

Kvitek, R.G., Greene, H.G., Krigsman, L.M., Endris, C.A., Clahan, K.B., Sliter, R.W., Wong, 

F.L., Yoklavich, M.M., and Normark, W.R. (S.Y. Johnson, ed.), California State Waters Map 

Series—Hueneme Canyon and vicinity, California: U.S. Geological Survey Scientific 

Investigations Map 3225, pamphlet 44 p., 12 sheets, available at 

http://pubs.usgs.gov/sim/3225/. 

Kvitek, R.G., Phillips, E.L., and Dartnell, P., 2012, Colored shaded-relief bathymetry, Hueneme 

Canyon and vicinity, California, sheet 1 in Johnson, S.Y., Dartnell, P., Cochrane, G.R., 

Golden, N.E., Phillips, E.L., Ritchie, A.C., Kvitek, R.G., Greene, H.G., Krigsman, L.M., 

Endris, C.A., Clahan, K.B., Sliter, R.W., Wong, F.L., Yoklavich, M.M., and Normark, W.R. 

(S.Y. Johnson, ed.), California State Waters Map Series—Hueneme Canyon and vicinity, 

California: U.S. Geological Survey Scientific Investigations Map 3225, pamphlet 44 p., 12 

sheets, available at http://pubs.usgs.gov/sim/3225/.

Ritchie, A.C., Johnson, S.Y., and Clahan, K.B., 2012, Offshore and onshore geology and 

geomorphology, Hueneme Canyon and vicinity, California, sheet 10 in Johnson, S.Y., 

Dartnell, P., Cochrane, G.R., Golden, N.E., Phillips, E.L., Ritchie, A.C., Kvitek, R.G., Greene, 

H.G., Krigsman, L.M., Endris, C.A., Clahan, K.B., Sliter, R.W., Wong, F.L., Yoklavich, 

M.M., and Normark, W.R. (S.Y. Johnson, ed.), California State Waters Map Series—Hueneme 

Canyon and vicinity, California: U.S. Geological Survey Scientific Investigations Map 3225, 

pamphlet 44 p., 12 sheets, available at http://pubs.usgs.gov/sim/3225/.

U.S. Geological Survey, 2007, USGS CMG Z-3-07-SC Metadata: accessed June 25, 2012, at 

http://walrus.wr.usgs.gov/infobank/z/z307sc/html/z-3-07-sc.meta.html.

U.S. Geological Survey, 2008, USGS CMG S-7-08-SC Metadata: accessed June 25, 2012, at 

http://walrus.wr.usgs.gov/infobank/s/s708sc/html/s-7-08-sc.meta.html. 

Appendix

Compressed GIS files of data points for depth to base of Last Glacial Maximum and 

sediment thickness used in calculations described in this report are available from 

http://pubs.usgs.gov/of/2012/1161/.
7

Figure 1. Map of Santa Barbara Channel, California. Map blocks are outlines of quadrangles defined for 

the California State Waters Map Series. The data area for this report spans the shelf from Refugio to 

southeast of Hueneme Canyon. Seismic-reflection data were collected by the U.S. Geological Survey in 

2007 and 2008 along the displayed tracklines. The 3-nautical mile limit is the seaward boundary of the 

State waters.
8

Figure 2. SeisWorks® display of a seismic-reflection profile with interpreted horizons of seafloor (green) 

and base of Last Glacial Maximum (bLGM, magenta). Depth to seafloor, depth to bLGM, and sediment 

thickness (all in two-way travel time (TWT)) are available for export. The horizontal scale marks seismic 

shot value; the vertical scale is in TWT (msec). At the right edge of the profile, the seafloor horizon is 

approximately 25 msec (or 0.025 m)/2 × 1,500 m/s = 18.75 m; the sediment thickness is approximately 10 

msec (or 0.010 s)/2 x 1,600 m/s = 8.0 m. The depth to bLGM = 18.8 + 8.0 = 53.5 m.
9

Figure 3. (A) Seismic-reflection tracklines color-coded by survey identification. (B) Points color-coded by 

sediment thickness values as originally exported from SeisWorks®. (C) Enlargement of a subwindow of 

points. Density of data causes “x” symbols on maps to blur together.
10

Figure 4. Modifications to sediment thickness data. Areas mapped as bedrock exposures are interpreted 

as having zero thickness. Canyon polygons are areas for which thickness data points are excluded before 

gridding. Faults are to be treated as breaklines, disrupting the continuity of the gridding where they occur. 

RM, Red Mountain fault; PP, Pitas Point fault; OR, Oak Ridge fault.
11

Figure 5. Candidates for data clipping mask. (A) 500-m buffer around tracklines was not used because it 

excludes useable data areas (deep incursions between tracklines). (B) An enclosing polygon (hull) is 

extracted from a triangular irregular network (TIN) constructed from the sediment thickness points. The hull 

is buffered with 100-m and 200-m margins; the 200-m margin was used. (C) For the landward limit, a 100-

m buffer is applied to the coastline. The seaward boundary is the 3-nmi limit line. (D) The final clipping 

mask consists of pieces of the 200-m TIN buffer, 100-m coast buffer, 3-nmi limit, and excludes areas within 

the canyon outlines in the southeast.
12

Figure 6. (A) Grid result from applying TopoToRaster to collection of points. (B) Grid calculation includes 

bedrock points (thickness = 0). (C) Final grid calculated from edited isopachs. (D) Location map.
13

Figure 7. Faults (topologic breaklines) that disrupt seafloor sediment, as mapped from seismic-reflection 

profiles. North to south: RM, Red Mountain fault; PP, Pitas Point fault; OR, Oak Ridge fault. (A) Thickness 

interpolated with TopoToRaster, which has no option for breaklines. (B) Thickness interpolated by 

triangular irregular network (TIN) without breaklines. Result is similar to A. (C) Thickness TIN model 

calculated with breaklines, exhibits slight accommodation for Pitas Point fault (middle fault). (D) Final map 

interpolated from manually edited lines of equal thickness. 
14

Figure 8. Input data for TIN calculation. SF_type (surface feature type) determines how data layers are 

treated in construction of the TIN. Interpolation takes in all points (masspoints), replaces bedrock with zero 

thickness (hardreplace), recognizes faults (hardline), omits canyons (harderase), and is confined to a clip 

polygon (hardclip).

Figure 9. Northern (pink) and southern (blue) extents, separated by the Oak Ridge fault, were separately 

gridded and then mosaicked. 
15

Figure 10. Final grids of (A) sediment thickness, and (B) depth to base of the Last Glacial Maximum for 

offshore eastern Santa Barbara Channel. RM, Red Mountain fault; PP, Pitas Point fault; OR, Oak Ridge 

fault.
16

Figure 11. Regional extents for sediment volume calculations. Region A (green) extends from the western 

extent of the data (offshore of Refugio) to the southern strand of the Red Mountain fault zone (RMFZ). 

Region B (pink) extends from RMFZ to the Pitas Point fault (PPF). Region C (brown) extends from PPF to 

the Oak Ridge fault (ORF). Region D (blue) extends from ORF to Hueneme Canyon. Region E (yellow) 

comprises the remainder of the data, on the eastern side of Hueneme Canyon. Faults are shown in heavy 

black lines.