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Journal of
Marine Science
and Engineering
Article
Upper Pleistocene and Holocene Storm Deposits Eroded from
the Granodiorite Coast on Isla San Diego (Baja California
Sur, Mexico)
Ginni Callahan 1, Markes E. Johnson 2, * , Rigoberto Guardado-France 3and Jorge Ledesma-Vázquez 3
Citation: Callahan, G.; Johnson,
M.E.; Guardado-France, R.;
Ledesma-Vázquez, J. Upper
Pleistocene and Holocene Storm
Deposits Eroded from the
Granodiorite Coast on Isla San Diego
(Baja California Sur, Mexico). J. Mar.
Sci. Eng. 2021,9, 555. https://
doi.org/10.3390/jmse9050555
Academic Editor: Matthew Lewis
Received: 19 April 2021
Accepted: 19 May 2021
Published: 20 May 2021
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Attribution (CC BY) license (https://
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4.0/).
1Sea Kayak Baja Mexico, Nicolas Bravo s/n, Entre Baja California y El Muro de Contención, Colonia Centro,
Loreto 23880, Baja California Sur, Mexico; ginni@seakayakbaja.com
2Department of Geosciences, Williams College, Williamstown, MA 01267, USA
3Facultad de Ciencias Marinas, Universidad Autónoma de Baja California, Ensenada 22800, Baja California,
Mexico; rigoberto@uabc.edu.mx (R.G.-F.); ledesma@uabc.edu.mx (J.L.-V.)
*Correspondence: markes.e.johnson@williams.edu; Tel.: +1-413-2329
Abstract:
This project examines the role of hurricane-strength events likely to have exceeded
119 km/h
in wind speed that entered the Gulf of California from the open Pacific Ocean during Late
Pleistocene and Holocene times to impact the granodiorite shoreline on Isla San Diego. Conglomerate
dominated by large, ellipsoidal to subspherical boulders at the islands south end were canvassed
at six stations. A total of 200 individual cobbles and boulders were systematically measured in
three dimensions, providing the database for analyses of variations in clast shape and size. The
project’s goal was to apply mathematical equations elaborated after Nott (2003) with subsequent
refinements to estimate individual wave heights necessary to lift igneous blocks from the joint-bound
and exfoliated coast on Isla San Diego. On average, wave heights on the order of 3 m are calculated as
having impacted the Late Pleistocene rocky coastline on Isla San Diego during storms, although the
largest boulders more than a meter in diameter are estimated to weigh two metric tons and would
have required waves in excess of 10 m for extraction. Described for the first time, a fossil marine biota
associated with the boulder beds confirms a littoral-to-very-shallow water setting correlated with
Marine Isotope Substage 5e approximately 125,000 years ago. A narrow submarine ridge consisting,
in part, of loose cobbles and boulders extends for 1.4 km to the southwest from the island’s tip,
suggesting that Holocene storms continued to transport rock debris removed from the shore. The
historical record of events registered on the Saffir–Simpson Hurricane Wind Scale in the Gulf of
California suggests that major storms with the same intensity struck the island in earlier times.
Keywords:
coastal erosion; storm surge; hydrodynamic equations; Marine Isotope Substage 5e; Gulf
of California
1. Introduction
Oriented northeast to southwest between mainland Mexico and the Baja California
Peninsula, 40 named islands in the Gulf of California spread out over a sea surface of
160,000 km
2
. These gulf islands range between 1224 km
2
and 22 ha in size [
1
]. Island
development postdates the opening of the gulf to the Pacific Ocean by rifting from the
mainland more than 5 million years ago and many formed as fault blocks influenced
by regional tectonics. Most are composed of Miocene volcanic flows or from intrusive
igneous rocks of yet older Cretaceous origin. Of the 40 islands, 8 islands fall into the
category dominated by granite or closely related granodiorite, and this study looks at
one of the smallest in the lower Gulf of California called Isla San Diego with an area
of 60 ha [
1
]. Survey work conducted through satellite imagery shows that rocky shores
account for nearly half the gulf’s peninsular coastline including related islands [
2
]. At
slightly more than 23%, andesite dominates the region’s total shores, followed by granite
J. Mar. Sci. Eng. 2021,9, 555. https://doi.org/10.3390/jmse9050555 https://www.mdpi.com/journal/jmse
J. Mar. Sci. Eng. 2021,9, 555 2 of 22
or granodiorite at 9%, and limestone at 7.5%, while rock types including other igneous
rocks or metamorphic rocks are less well represented.
This contribution belongs to a series of papers focused on the erosion of coastal
boulder beds from their parent rocks within the Gulf of California. Upper Pleistocene and
Holocene deposits formed by boulders are commonly found along the peninsular shores
of Baja California and around the gulf islands but most studies in coastal geomorphology
seldom compare the results of rock density on an interregional basis as related to different
parent rock types. The application of mathematical formulae to estimate storm wave height
was applied previously to coastal boulder deposits throughout the Gulf of California,
including those formed by limestone, rhyolite, and andesite clasts [3–6]. Extension of this
program now includes the Pleistocene boulder beds eroded from the granodiorite coast
of Isla San Diego, applying the same methodology of systematic size measurements to
calculate volume and weight based on rock density preliminary to the estimation of wave
heights derived from competing equations. The study also newly describes marine fossils
preserved within the Pleistocene conglomerate of Isla San Diego that date the deposits
with reasonable accuracy. Finally, development during the Holocene time of a long marine
ridge off the southwestern tip of the island brings into consideration the ongoing influence
of hurricanes capable of moving large boulders in a shallow, subtidal setting.
Aside from the limited statistics available on the size, geologic origins, and coastal
composition of islands in the Gulf of California [
1
,
2
], barely any literature exists on the geol-
ogy and geomorphology of Isla San Diego except for an early nineteenth-century appraisal
that includes the only previous description of the submarine ridge off the island’s southern-
most end [
7
]. Attention to the phenomenon of coastal mega boulders and their relationship
to major storms or tsunami events is a topic of growing interest [
8
–
11
]. Especially in the
context of rock density, the data from Isla San Diego provide further insight on comparison
of storm beds of Pleistocene and Holocene origins throughout the gulf region [
3
–
6
] with
oceanic basalt in the Azores and Canary Islands of the North Atlantic [
12
,
13
], as well as
rare mantel rocks from storm beds in coastal Norway on the Norwegian Sea [14].
2. Geographical and Geological Setting
Stretching for more than 1000 km in length (Figure 1a), the Gulf of California is a
marginal sea seated over a tectonically active zone that entails spreading centers offset
by a succession of transform faults [
15
]. The central spine of the adjacent Baja California
Peninsula is formed by granodiorite, broadly dated to a Cretaceous origin between 97 and
90 million years ago [
16
]. Upfaulted granodiorite basement occurs on the Baja California
Peninsula at Punta San Antonio north of Loreto, on Isla Catalina east of Loreto, as well
as Isla Santa Cruz and Isla San Diego (Figure 1a). Peninsular and island development
including those areas with granodiorite resulted from Miocene extensional rifting that
began prior to flooding 13 million years ago and lasted for 9.5 million years when a change
in dynamics initiated transform faults connected with the San Andreas Fault on the US
side of the border [
1
,
2
,
15
]. A detachment zone was activated approximately 3.5 million
years ago that resulted in half-graben structures separating the islands from the rest of the
Baja California Peninsula. The detachment zone that extends from Punta San Antonio to
Isla San Diego (Figure 1a) is identified as the ComondúDetachment.
J. Mar. Sci. Eng. 2021,9, 555 3 of 22
Figure 1.
Mexico’s Baja California Peninsula and Isla San Diego: (
a
) map showing the boundary between the United
States and Mexico as well as the boundary between the northern and southern states of Baja California and Baja California
Sur (dashed lines), together with towns on the Baja California Peninsula and key spots or islands including Ángel de la
Guarda (AG), Punta San Antonio (PA), Carmen (Ca), Santa Catalina (SC), Cerralvo (Ce) and San Diego (box with asterisk);
(b) enlarged
map of Isla San Diego in the lower Gulf of California, showing the location of Stations 1 to 6 where cobbles and
boulders of eroded granodiorite were measured for this study.
Isla San Diego is at the southeast end of the detachment zone 20 km or 11 nautical
miles due east of the closest access point on the Baja California Peninsula. The main
north–south highway is too distant from the peninsula’s eastern shore to make boat
access to the island convenient. Compared to other islands farther to the northwest or
southeast, the relative isolation of Isla San Diego meant that it received little attention
from geographers and geologists. No formal topographic map by the Federal Mexican
government exists for Isla San Diego and its dimensions and topography were appraised
by satellite imagery [
2
]. The island is elongated in shape, approximately 1.5 km in length
and 0.43 km in width with northeast to southwest orientation (Figure 1b). The maximum
elevation is more than 160 m above mean sea level, as attained to the north, but the island’s
central ridgeline tapers gradually downward to the shore at the southwest end.
The island core is composed entirely of granodiorite and the conglomerate beds
eroded from these basement rocks occur exclusively at the southwestern end. A prominent
submarine ridge extends from the tip of the island [
7
], where large boulders of loosely
piled granodiorite are close to the surface (Figure 2a). Granodiorite sea cliffs are well
exposed along the east shore for more than 400 m from the southwest tip of the island to
the northeast, where a series of closely spaced grottos are eroded as a result of spheroidal
weathering between joints in the rock (Figure 2b). Additional weathering along both flanks
of the island is the result of sheeted exfoliation typical of granitoid rocks.
The greater part of Mexico’s Natural Protected Areas (Áreas Naturales Protegida) is
taken up by the Gulf of California Biosphere Reserve protecting all islands in the Gulf of
California, which accounts for roughly 19% of the nation’s total conservation reserves [
17
].
Therefore, Isla San Diego is protected under conservation guidelines due to its biodiversity
and ecological characteristics. All materials and fossils identified in this study were left in
place on Isla San Diego.
J. Mar. Sci. Eng. 2021,9, 555 4 of 22
Figure 2.
South end of Isla San Diego (lower Gulf of California: (
a
) view showing part of the shallow-
water ridge composed of loose cobbles and boulders of eroded granodiorite oriented S 55
◦
W off the
island; (
b
) southwest end of the island showing small sea caves eroded in granodiorite basement
rocks overlain by Pleistocene conglomerate.
3. Materials and Methods
3.1. Data Collection
The raw data for this study were collected in March 2021 from deposits composed
exclusively of granodiorite cobbles and boulders consolidated by a thin limestone matrix.
Individual clasts from six stations were measured manually to the nearest half centimeter
in three dimensions perpendicular to one another (long, intermediate, and short axes).
Differentiated from cobbles, the base definition for a boulder adapted in this exercise was
that of Wentworth [
18
] for an erosional clast equal or greater than 25.6 cm in diameter.
Triangular plots were employed to show variations in clast shape, following the design of
Sneed and Folk [
19
] for river pebbles. In the field, all measured clasts were characterized
as subrounded, and a smoothing factor of 20% was applied uniformity to adjust for the
estimated volume calculated by the simple multiplication of length from the three axes.
Comparative data on maximum cobble and boulder dimensions were fitted to bar graphs
to show size variations in the long and intermediate axes from one sample to the next. The
rock density from a granodiorite sample yielded a value of 2.52 g/cm3.
3.2. Hydraulic Model
Granodiorite is the typical intrusive magmatic rock characteristic of several islands
in the Gulf of California. Herein, two formulas were applied to estimate the size of storm
waves against joint-bound blocks. Equation (1) derives from the work of Nott [
20
] and
Equation (2) is modified from an alternative approach using the velocity equations of
Nandasena et al. [21] applied to storm deposits by Pepe et al. [22].
Hs =ρs−ρw
ρwa
C1
(1)
J. Mar. Sci. Eng. 2021,9, 555 5 of 22
Hs =
2ρs−ρw
ρw.c.[cos θ+(µs. sinθ)]
c1
100 (2)
where Hs = height of the storm wave at breaking point;
ρs
= density of the boulder (tons/m
3
or g/cm
3
);
ρw
= density of water at 1.02 g/mL; a= length of the boulder on long axis in cm;
θ
is the angle of the bed slope at the pretransport location (1
◦
for joint-bounded boulders);
µs
is the coefficient of static friction (=0.7); and C
l
is the lift coefficient (=0.178).
Equation (1)
is more sensitive to the length of a boulder on the long axis, whereas Equation (2) is more
sensitive to the length of a boulder on the short axis. Therefore, some differences are
expected in the estimates of HS.
4. Results
4.1. Base Map and Sample Stations
Isla San Diego is among the smallest named islands formed by granodiorite in the
Gulf of California [
1
]. Its size was conducive to a close coastal survey by kayak that allowed
for the location and appraisal of conglomerate beds on the island’s periphery. Six sampling
stations were chosen from the conglomerate outcrops found only around the southwestern
end of the island (Figure 1b). Between 30 and 35 individual clasts were measured within a
meter’s radius from a position above the source basement rock. Co-ordinates are listed in
Appendix A(Tables A1–A6) for each station recorded by a hand-held device for tracking
by the satellite-based global positioning system (GPS). Sample Stations 1 and 2 (Figure 3a)
are located on the east shore 375 m and 350 m north of the island’s tip, respectively, where
the conglomerate sits directly above a bench of granodiorite approximately 1.5 m above
mean sea level. Station 3 is located at the extreme southwestern tip of the island (
Figure 3b
),
where crude layering in the conglomerate shows a 30
◦
inclination to the northwest. Three
additional stations were established on the west shore (Figure 1b), where the contact is
concealed by talus. Clasts measured at those stations also were limited to a 2 m radius at
points near the bottom of the conglomerate bed but included some samples from the talus
showing evidence of carbonate cement formerly binding the conglomerate. Clean clasts
from the intertidal zone on that side of the island are reworked by coastal currents and
were excluded.
Figure 3.
Sample stations on the southeast side and southern tip of Isla San Diego: (
a
) sample Stations 1 and 2 occur at 375
m and 350 m north of the island’s southern tip (red circles are 2 m in diameter); (
b
) sample Station 3 is located at the far
southwestern tip of Isla San Diego (red oval is 0.5 m wide).
4.2. Comparative Variation in Clast Shapes
Raw data on clast size in three dimensions collected from each of the six sampling
stations are recorded in Appendix A(Tables A1–A6). With regard to shape, points rep-
resenting individual cobbles and boulders were fitted to a set of Sneed–Folk triangular
diagrams (Figure 4a–f). The slope of points is in general agreement among the six plots,
following a uniformly diagonal trend from the middle of the second tier to the lower
J. Mar. Sci. Eng. 2021,9, 555 6 of 22
right-hand rhomboid. Erosional wear on a perfect cube at all four corners results in a clast
with equal values in three dimensions that will plot at the apex of the small triangle in the
topmost tier. Variations that reflect slightly smaller values for the intermediate and short
axis will shift the location more toward the center of that space. Only one or two clasts fall
into this field, which indicates that vertical joints and horizontal fractures in the parent
granodiorite are not evenly spaced in an orderly three-dimensional grid. Any point that
falls into the center of the rhomboid on the right-hand end of the lower tier represents an
individual clast with a long axis twice the length of the intermediate axis perpendicular to
it, which, in turn, measures five times the length of the short axis perpendicular to the other
two. The form of such a clast is initially bar-shaped but becomes more spindle-shaped as
the sharp edges at the corners are worn away by abrasion.
Figure 4.
Set of triangular Sneed–Folk diagrams used to appraise variations in cobble and boulder
shapes sampled along the Upper Pleistocene paleoshore on Isla San Diego in the lower Gulf of
California ((a–f), Stations 1–6).
J. Mar. Sci. Eng. 2021,9, 555 7 of 22
The greatest number of points from each of the six samples falls within the third tier
from the top but on opposite sides of the line separating the right side of the diagram from
the center. A particular clast with dimensions in which the intermediate and short axis
are close in value, but roughly half that of the long axis will fall squarely onto the midline
between the two rhomboids at the center of the diagram. As the third dimension (shortest
axis) decreases in length, the point will shift in position across the line to the right and lower
downward in position. Overall, the comparative results of shape analysis indicate that most
of the cobbles and boulders in the Pleistocene conglomerate are elongated in shape but as
relatively fat spindles with an ellipsoidal outline. It is important to distinguish overall size
from shape. That is to say, a smaller cobble with the same ratio of measurements between
long, intermediate, and short axes will plot exactly the same as a larger boulder with the
same ratios. Although clasts were chosen at random, a reasonably large population of clasts
within a limited search radius at any one station assures that the sample is representative.
In this case, the absence of more perfectly spherical clasts, and the dominant trend toward
thickened spindles is evident.
4.3. Comparative Variation in Clast Sizes
Drawn from original data (Tables A1–A6), clast size is treated separately to best effect
on bar graphs as a function of frequency against maximum and intermediate lengths of
the two longest axes perpendicular to one another. The first set of six graphs so plotted
(Figure 5a–f) exhibit trends in the maximum dimension for clast length sorted by intervals
of 15 cm in which the boundary between cobbles and boulders is marked within the range
for clasts between 16 and 30 cm in diameter. The pattern is compared with another six
graphs (Figure 6a–f) based on measurements for the length of the intermediate axis in the
same 200 clasts.
Figure 5.
Set of bar graphs used to contrast variations in the maximum size of clast axes from six samples at Isla San Diego
in the lower Gulf of California: (
a
) bar graphs from Station 1; (
b
) bar graphs from Station 2; (
c
) bar graphs from Station 3;
(d) bar graphs
from Station 4; (
e
) bar graphs from Station 5; (
f
) bar graphs from Station 6. Dashed line (offset to represent
26.6 mm) marks the boundary between large cobbles and small boulders.
J. Mar. Sci. Eng. 2021,9, 555 8 of 22
Based on four of the six graphs (Figure 5a,d–f) representing maximum axial length,
it is shown that boulders outnumber the smaller cobbles at rations between 3:1 and
3:2. The opposite is indicated by two of the graphs (Figure 5b,c) in which the smaller
cobbles outnumber the larger boulders at ratios 2:1 or less. Based on two of the six
graphs (Figure 6b,c) representing the intermediate axial length for the same 200 clasts, it
may be argued that cobbles outnumber the larger boulders in only two of the graphs
(Figure 6b,c), whereas cobbles are slightly outnumbered by boulders in two graphs
(
Figure 6a,d
) and occur at parity in two others (Figure 6a,f). Overall, comparative data
between maximum length and intermediate axial length confirm the results on clast shape
(Figure 4), showing the dominance of ellipsoidal boulder shapes.
Figure 6.
Set of bar graphs used to contrast variations in the intermediate size of clast axes from six samples at Isla San
Diego in the lower Gulf of California: (
a
) bar graphs from Station 1; (
b
) bar graphs from Station 2; (
c
) bar graphs from
Station 3; (
d
) bar graphs from Station 4; (
e
) bar graphs from Station 5; (
f
) bar graphs from Station 6. Dashed line (offset to
represent 26.6 mm) marks the boundary between large cobbles and small boulders.
J. Mar. Sci. Eng. 2021,9, 555 9 of 22
4.4. Clast Imbrication
Clast orientation or imbrication within the boulder deposit on Isla San Diego varies
according to outcrop exposure, as observed on two vertical planes that intersect perpen-
dicular to one another. One is parallel to the island’s long axis along the eastern shore
(
Figures 2b and 3a
). The other crosses through the island’s southern tip (Figure 3b). Both
show direct contact of eroded boulders with underlying granodiorite basement rocks where
the conglomerate attains a maximum elevation of 8 m above sea level. The unconformity
surface traced parallel to the island’s axis is flat lying with a slight gain in elevation rising
to the northeast. In this view (Figure 2b), a change in clast size from boulders to large
cobbles begins above a reactivation surface that follows a minor indentation in the cliffs
obscured by shadows. Shape analyses indicate that a preponderance of clasts from the
basal part of the deposit are ellipsoidal or roughly fusiform in shape (Figure 4), but a
small amount of imbrication is detected only below the reactivation surface (Figure 2b).
The unconformity surface exposed in the plane crossing the tip of the island (Figure 3b)
is more irregular with a dip or swale in the center, but crude layering in the overlying
boulder deposit is increasingly inclined with distance above the unconformity. Based on
photographic evidence supplemental to Figure 3b, evidence for imbrication is detected in
the upper part of the deposit (Figure 7), where this is shown by transfer of outline tracings
(Figure 7a,b) and isolated for clearer viewing (Figure 7c). Due to the partial collapse of
clasts at the island’s tip, relationships among those in the basal part of the deposit are less
clear. However, the orientation of clasts from the upper part of the deposit (Figure 7c)
reveals a pattern of imbrication from northeast to southwest.
Figure 7.
Photographic evidence for clast imbrication: (
a
) view overlooking sample Station 3 at the southern tip of Isla San
Diego; (
b
) upper part of the conglomerate deposit at the same locality, tracing the outline of boulders; (
c
) drawing of the
tracing isolated for clarity.
4.5. Fossil Fauna with Inferences on Age and Water Depth
A mixed fauna of three fossil corals, four bivalves, a single gastropod, and a coral-
boring barnacle is preserved among the granodiorite cobbles and boulders on Isla San
Diego (Table 1). None of the fossils occur as encrustations attached to individual cobbles or
boulders. With possible exceptions among certain bivalves, they qualify as organic clasts
that became secondarily incorporated within the conglomerate.
J. Mar. Sci. Eng. 2021,9, 555 10 of 22
Table 1.
Summary list of marine invertebrate fossils from the Upper Pleistocene boulder beds on Isla
San Diego correlated with Marine Isotope Substage 5e.
Phylum Class Species Phylum Class Species
Coelenterata
Anthozoa Porites panamensis Mollusca Bivalvia Codakia distinguenda
Pocillopora elegans Lyropecten subnodosus
Povona gigantea Ostrea sp.
Spondylus calcifer
Arthropoda Gastropoda
Cirripedia
Turbo fluctuosus
Hexacreusia durhami
Representative fossils are illustrated by field photos (Figure 8). Two kinds of corals are
illustrated: Porites panamensis (Figure 8a) and Pocillopora elegans (Figure 8b), respectively.
Articulated bivalves are rarely found as fossils within the boulder beds, with the exception
of Codakia distinguenda (Figure 8c), which may have grown in place in cavities among
boulders after their deposition. Very thick but heavily eroded shell fragments belonging to
a species of oyster (Figure 8d) indicate that shell fragmentation normally occurred prior
to burial in the conglomerate. The large and heavily calcified shell of Spondylus calcifer
(Figure 8e
) is preserved intact but disarticulated. The large pecten Lyropecten subnodosus
(Figure 8f) is likewise disarticulated and also broken. The only trace found of fossil
gastropods is the distinctive operculum belonging to Turbo fluctuosus. Of ecological note,
one of the Porites fossils observed in the boulder deposit at Station 4 (Figure 7a) is host to
the boring barnacle Hesareusia durhami (Figure 8g). The same relationship between coral
host and barnacle is known from the Pleistocene reef complex preserved intact on Isla
Cerralvo [23].
Figure 8.
Upper Pleistocene fossils from the granodiorite conglomerate at Isla San Diego: (
a
) filling among boulders that
includes the coral Porites panamensis (ruler for scale); (
b
) broken branch from the coral Pocillopora elegans (approximately
3 cm
in diameter); (
c
) articulated bivalve (Codakia distinguenda) with ruler for scale; (
d
) fragment of a thick-shelled oyster
(pen tip for scale;) (
e
) inside surface of a disarticulated valve belonging to Spondylus calcifer (approximately 7 cm in width);
(
f
) broken shell belonging to the species Lyropecten subnodosus (ruler for scale); (
g
) detail from Figure 7a showing the surface
opening of the barnacle Hexacreusia durhami (approximately 3 mm in diameter).
J. Mar. Sci. Eng. 2021,9, 555 11 of 22
Fossil mollusks from the Gulf of California occupy a wide range of geological ages
through the Pliocene and Pleistocene, but the corals are more nuanced. In a detailed
survey of localities throughout Baja California and associated gulf islands [
24
], 35 out of
47 collection sites include the species Porities panamensis, most of which are dated as Late
Pleistocene in age. Pocillopira elegans is reported previously from only a single locality,
dated as Late Pleistocene [
24
]. A definitive age determination requires radiometric testing
for lead isotopes not within the scope of this paper. Uncertain assignments of P. panamensis
to a mid-Pleistocene age and older Pliocene times are limited to marine terraces or other
inland localities well elevated by tectonic uplift above present-day sea level. The absence
of marine terraces on Isla San Diego supports a Late Pleistocene age for the fossil fauna.
Alone, the fossil corals denote shallow-water conditions, but the related fossil mollusks
add additional support of an intertidal to the shallow subtidal origin of the mixed fauna
that lived nearby.
Global sea level during the last Pleistocene interglacial period (Marine Isotope Sub-
stage 5e) was at its highest, calculated to have stood between 4 m and 6 m higher than
today on the basis of changes in oxygen isotopes of planktonic foraminifera and other
criteria [
25
,
26
]. In that case, the granodiorite rocks at the south end of Isla San Diego
presently exposed above sea level would have been submerged and development of the
overlying boulder deposit would have occurred in very shallow water, where an infusion
of a thin limestone binding matrix insured stabilization.
4.6. Storm Intensity as Function of Estimated Wave Height
Clast sizes and maximum boulder volumes drawn from the six sample stations are
summarized in Table 2, allowing for direct comparison of average values for all clasts, as
well as values for the largest clasts in each sample based on Equations (1) and (2) derived
from the work of Nott [20] and Pepe et al. [22].
Table 2.
Summary data from Appendix A(Tables A1–A6) showing maximum bolder size and estimated weight com-
pared to the average values for sampled boulders from each of the transects together with calculated values for wave
heights estimated as necessary for boulder–beach mobility. Abbreviations: EAWH = estimated average wave height,
EMWH = estimated maximum wave height.
San Diego
Station
Number of
Samples
Average
Boulder
Volume
(cm3)
Average
Boulder
Weight
(kg)
EAWH (m)
Nott [20]
EAWH
(m)
Pepe et al.
[22]
Max.
Boulder
Volume
(cm3)
Max.
Boulder
Weight
(kg)
EMWH
(m)
Nott [20]
EMWH
(m)
Pepe et al.
[22]
1 30 89,079 224 3.2 3.0 780,800 1968 10.6 11.4
2 35 7763 33.8 1.9 1.9 84,816 214 5.1 7.1
3 35 29,884 75.7 2.8 2.8 299,850 756 10.4 9.3
4 35 39,007 98 3.7 3.4 263,516 664 9.3 9.9
5 35 33,456 83.4 3.6 3.3 157,303 396 8.3 8.0
6 30 33,688 85 3.2 2.9 93,960 238 6.2 5.0
Average 33.33 66,536 100 3.1 2.9 280,041 706 8.0 8.5
The Nott formula [
20
], provided in Equation (1), yields an average wave height of
3.1 m
for the extraction of joint-bound blocks from granodiorite sea cliffs exposed at Isla San
Diego, as tabulated for sample stations 1 to 6. A much larger value for a wave height of 8.0
m is calculated from the average of the largest single blocks of granodiorite recorded from
the six stations based on the application of the same equation. The more sensitive to clast
length from the short axis, the more sophisticated Equation (2) applied by
Pepe et al. [22]
yields values that are slightly lower for the estimated average wave height with a difference
of 30 cm. On the other hand, the application of Equation (2) yields a higher value by a half
meter for the average of the largest boulders, compared to that of Equation (1). Notably,
the value for the maximum wave height for the six largest boulders based on Equation (1)
is
2.5 times
higher than the computed average for all 200 clasts. The difference is nearly
3.0 times greater comparing the maximum wave height for the same six boulders with
J. Mar. Sci. Eng. 2021,9, 555 12 of 22
the computed average for all 200 clasts based on Equation (2). Clearly, the pressure of
extreme wave impact against the shore is necessary to loosen and dislodge the largest
joint-bound blocks of granodiorite preserved in the Pleistocene cliff line on Isla San Diego,
as characterized by the enormous block at Station 1 estimated to weigh nearly two metric
tons (Table 1, Table A1)
4.7. Holocene Ridge Offshore Isla San Diego
A distinct feature in the geomorphology of Isla San Diego is the pointed beach at
the island’s southwestern tip that extends offshore along an underwater ridge (Figure 9).
Aerial photos reveal shallow, aquamarine waters above the ridge, indicating its extension
for a distance of 1.4 km on a compass heading of 224 degrees (S 55
◦
W). Exploration by
kayak over the first 250 m offshore confirms that the ridge is formed by loose cobbles and
large boulders. Observed at different times under different sea conditions, it is notable that
the smaller clasts shift in location, whereas the large boulders remain fixed on the ridge.
How far and to what depth the boulder train extends before changing to gravel and sand
toward the distal end is unknown.
Figure 9.
View in the shadow of cliff-forming granodiorite boulders at the tip of Isla San Diego looking to the southwest
across the beach and major extension of a submarine ridge composed of loose cobbles and boulders (see also Figure 2a
viewed from the opposite direction).
5. Discussion
5.1. Inclined Boulder Beds and Imbrication Pattern as Mitigating Factors
Conglomerate beds at the southern terminus of Isla San Diego (Figure 3b) suggest
crude layering with a 30
◦
dip to the northwest. The contact with underlying granodiorite
makes it difficult to know for certain if the entire island has been tilted due to tectonics or
if the fracture pattern in the granodiorite conforms to a pattern with horizontal fractures at
right angles to vertical joints, as implied by the small sea caves or grottos eroded under
present-day conditions along the island’s eastern shore. The more complicated
Equation (2)
applied to storm deposits by Pepe et al. [
22
] requires input on the slope value of the
J. Mar. Sci. Eng. 2021,9, 555 13 of 22
granodiorite bench under attack by storm waves. Introduction of the 30
◦
angle observed in
the crude layering of the conglomerate generates negative wave height values. As shown
in the original fieldwork by Peppe et al. [
22
], the inclination of platform rocks is very small
(less than 2.5
◦
) on which blocks are potentially subject to plucking by storm waves. In the
case of previous work on andesite rocky shores from Isla San Luis Gonzaga in the Gulf
of California [
6
] and basalt rocky shores from Gran Canaria in the Canary Islands [
11
],
it is assumed that the angle of bed slope is only 1
◦
at the pretransport location for joint
bound blocks. Herein, the same assumption is made for the granodiorite on the eastern
flank of Isla San Diego, which yields results from the application of Equation (2) based on
Pepe et al. [
22
] that are similar to the wave heights obtained by application of Equation (1)
following the work of Nott [20], as summarized in Table 1.
In this case, the 30
◦
angle of repose observed in the upper part of the conglomerate is
interpreted as the slope acquired by the superimposed deposit spilling over a rocky ridge in
shallow water and not related to an earlier structural tilting of the island. The conglomerate
layers may be interpreted as the result of overwash at the southern end of the island by
storm waves. Today, the maximum elevation between Stations 2 and 6 on opposite shores
is 8 m at the top of the conglomerate (Figure 1b). During the Late Pleistocene time (Marine
Isotope Substage 5e), the bedrock spur at the island’s south end was easily vulnerable to
impact by exceptionally large waves. Although limited in scope, the available evidence for
clast imbrication within the Isla San Diego conglomerate is consistent with the arrival of
wind-driven waves from the northeast or east related to the shifting northward passage of
a hurricane. In further consideration of the tilted layering in its upper part, a geometric
solution for the true thickness of the deposit perpendicular to the dip angle amounts to
about 4 m with the reactivation surface located roughly in the middle. The implication of
the reactivation surface is that a second phase took place in storm energy, or that a separate
storm event occurred sometime after the passage of the first event.
5.2. Significance of the Fossil Fauna from Isla San Diego
A strong latitudinal bias is reported in a comprehensive survey of the stony corals
now living in the Gulf of California with 38 species occurring in the southern (or lower)
part of the gulf, out of which only 16 have an extended range into the northern (or upper)
part of the gulf [
27
]. In contrast, only three stony corals are limited to the upper Gulf of
California with no representatives in the lower gulf. The disparity in today’s geographic
distribution is strongly related to the north–south temperature gradient. Pleistocene coral
reefs are well preserved at several locations throughout the Gulf of California as far north
as latitude 28
◦
(Figure 1). The species Porites panamensis is ubiquitous in Upper Pleistocene
coral reefs throughout the peninsular shores and gulf islands of Baja California Sur [
24
],
and this coral species is the primary component. For example, P. panamensis is the principal
component of Upper Pleistocene reefs on Isla Cerralvo [
23
], located 120 km farther to the
southeast from Isla San Diego in the lower Gulf of California (Figure 1a). The north–south
distance between the two islands is a full degree of latitude. In addition to P. panamensis
(Figure 7a), the occurrence of Pocillopora elegans (Figure 7b), together with Pavona gigantea
(Figure 7c), suggests all three were part of a former reef community that no longer persists
around the island.
5.3. Maine Circulation and Recent Hurricanes in the Region of Isla San Diego
On an annual basis from November to May, strong winds capable of generating
large-scale wave trains travel from north to south over the entire length of the Gulf of
California, but lighter winds typically blow in the opposite direction under the influence
of a semi-monsoonal pattern of atmospheric circulation during the spring and summer
times [
28
]. Winds out of the north set up long-shore currents that may be responsible for
delivering granodiorite gravel and perhaps even some cobbles from the flanks of Isla San
Diego to the linked underwater ridge. In contrast, the lighter southerly winds probably are
cable of shifting only pebbles.
J. Mar. Sci. Eng. 2021,9, 555 14 of 22
The threat of hurricanes affecting the lower Gulf of California is reviewed in prior
studies on storm deposits at Ensenada Almeja north of Loreto [
4
], Arroyo Blanco on Isla
del Carmen west of Loreto [
3
] Puerto Escondido south of Loreto [
5
], and especially on Isla
Cerralvo east of La Paz [
23
] (see Figure 1a for geographic relationships). Between September
1996 and September 2019, six named hurricanes (Fausto, Marty, Ignacio, John, Odile, and
Lorena) entered the Gulf of California after originating farther south off the mainland coast
of Mexico around Acapulco (Figure 1a). Storm rotation is counter-clockwise and as winds
intensify during the northward passage, the greatest impact is expected to come from
wind-driven waves pushing east to west out of the storm’s northeast quarter. Eye-witness
accounts of coastal wave surge during hurricane events in the Gulf of California are not
common, but the published account of the Holocene storm beds at Ensenada Almeja
includes a video clip of the 9 m storm surge against the rocky shore on nearby Ensenada
San Basilio during Hurricane Odile [
4
]. Storm waves pushed by winds streaming from
east to west during the passage of the same event farther south at Isla San Diego can be
expected to have caused a surge that topped over the low-lying southern end of the island.
The 2015 hurricane season was unusually active [
29
], and Hurricane Patricia was a
Category 5 event with wind speeds of 346 km/h that took an unexpected easterly turn
north of Acapulco to strike the village of Cuixmala below the opening to the Gulf of
California (Figure 1a). It remains one of the largest storms recorded in the eastern Pacific
basin and the strongest yet to strike western Mexico. With a diameter of 2400 km, the
storm’s outer wind bands already swept across the tip of the Baja California Peninsula
before the center veered eastward. Had the storm continued onward in its expected track
to the northeast and gained strength from warmer waters in the Gulf of California, major
damage from storm surge was certain to have occurred. Isla San Diego is one of those
remote spots in the lower Gulf of California that would have experienced the full impact of
wave shock against its east-facing shores and likely overwash of eroded materials across
the bedrock at the island’s southern tip.
Research based on the tagging of larger boulders from the conglomerate on Isla San
Diego and its related submarine ridge would be instrumental in documenting the role of
future hurricanes as a source of ongoing erosion. As part of a potentially larger project, the
same monitoring program could be undertaken for the Pleistocene and Holocene storm
deposits at Isla del Carmen [
3
], Ensenada Almeja [
4
], Puerto Escondido [
5
], and Isla San
Luis Gonzaga [
6
]. The relevance of such a program is underscored by the record-breaking
early start to the eastern Pacific hurricane season with the formation of tropical storm
Andres located 960 km south of the tip of the Baja California Peninsula on 9 May 2021 [
30
].
The head start of the hurricane season in this part of the world portends the consequences
of increased global warming.
5.4. Comparison with Storm Deposits Elsewhere in the Gulf of California
Variations in density among rock types studied so far from Pleistocene and Holocene
boulder beds around the Gulf of California range from 1.86 g/cm
3
for limestone,
2.16 g/cm3
for the banded rhyolite, and as much as 2.55 g/cm
3
for andesite based on different locali-
ties [
3
–
6
]. Estimates for the average wave height based on the average weight of sampled
boulders taking rock density into account vary between 4.3 m to 5.7 m from locality to
locality. However, when the largest boulders from each of four localities previously studied
are taken into account, wave heights necessary for dislodgment from joint-bound basement
rocks fall between 9.8 and more than 13 m. By comparison, the granodiorite samples from
Isla San Diego register average wave heights rather smaller, between 2.9 and 3.1 m, based
on the average of average estimates from six sample stations. However, the largest single
granodiorite boulder measured on the island is estimated to weigh nearly two metric tons
and to have required a wave height between 10.6 and 11.4 m to achieve dislodgement
(Table 1). In this regard, the maximum wave shock that affected Isla San Diego during
Pleistocene time is not out of the ordinary for the localities studied elsewhere in the Gulf of
California. In terms of future hurricane events certain to strike the lower Gulf of California,
J. Mar. Sci. Eng. 2021,9, 555 15 of 22
an interesting prospect would be to tag some of the largest granodiorite boulders on the
shallow ridge extending southwest of the island to see if movement occurs following the
next major storm. Likewise, it could be interesting to tag some of the cobbles at the top of
the crudely layered conglomerate beds currently at an elevation of 8 m to see if the next
major storm creates waves capable of washing over that part of the island.
5.5. Comparison with Storm Deposits Elsewhere in the North Atlantic Ocean
Studies on boulder beds from the Pleistocene of Gran Canaria in the Canary Is-
lands [
11
] and the Pleistocene and Holocene of Santa Maria Island in the Azores [
10
], as
well as the Holocene of north Norway [
12
], follow the same format as those in Mexico’s
the Gulf of California using the triangular plots after Sneed and Folk [
16
] to appraise
boulder shape and the same equations after Nott [
20
] and Peppe et al. [
22
] to estimate wave
heights. Basalt boulders from El Copnfital Beach on Gran Canaria register a rock density of
2.84 g/cm3
, whereas those from Santa Maria Island in the Azores were treated as having
an even higher rock density of 3.0 g/cm
3
. Holocene beach cobbles and boulders on Leka
Island in the subarctic of Norway were assigned an even higher rock density of
3.32 g/cm3
associated with low-grade chromite ore [
12
]. All these rock densities from island localities
in the North Atlantic Ocean surpass those for limestone, banded rhyolite, and andesite
found at localities in the Gulf of California. Other things being equal in terms of volume,
it requires a larger wave to extract a block of denser material such as basalt from a rocky
shoreline than for material much less dense such as limestone. Based on the average weight
and rock density of all basalt clasts measured from Gran Canaria, wave heights were
4.5 m
,
whereas based only on the largest boulders from each sample station, the maximum wave
height was more than twice that value at 11 m. The results for basalt clasts from Santa
Maria Island in the Azores were significantly less in both categories with an average weight
from all sample stations, amounting to 2.6 m, whereas the maximum wave height based
on the single largest boulder from each sample station amounted to 5.1 m. For storm
deposits from Norway’s Leka Island, the average results for cobbles and boulders derived
from chromite ore from three sample stations estimated wave heights between 3.6 and
4.3 m
. However, the average wave heights based on the single largest boulders from those
stations yielded values for wave heights between 5.1 and 6.7 m. A comparison with the
North Atlantic data on this basis puts the results from Isla San Diego closest in the range of
values obtained from the Canary Islands. Essentially, storms of hurricane strength reaching
the Gulf of California are no less severe in terms of their erosional effect than those in the
northeastern Atlantic Ocean.
6. Conclusions
Study of the cobble–boulder deposits from Isla San Diego in Mexico’s lower Gulf of
California offers the following insights based on mathematical equations for estimation of
Late Pleistocene wave heights from major storms in the same region:
•
Consolidated cobbles and boulders studied from six sample stations with Upper
Pleistocene conglomerate exhibit evidence of high-energy erosion from granodiorite
exposed along the rocky shoreline of Isla San Diego in Mexico’s lower Gulf of Cali-
fornia. Evidence of clast imbrication indicates that a major storm had an impact with
wave surge against the island’s eastern shore;
•
The average estimated volume at 66,536 cm
3
and the average weight of individual
granodiorite cobbles and boulders at 100 kg from a total of 200 samples suggest that
wave heights of 3 m are responsible for their derivation from the adjacent and joint-
bound body of parent rock. However, the largest igneous boulder from among all
six sample sites is estimated to weigh two metric tons and may have been moved by
a wave of extraordinary height around 10 m. Alternately, smaller waves may have
gradually loosened this block from its parent body until the force of gravity entrained
it within the conglomerate;
J. Mar. Sci. Eng. 2021,9, 555 16 of 22
•
Compared to other localities in the Gulf of California where sea cliffs composed
of igneous rocks such as andesite or banded rhyolite shed Holocene boulders, the
granodiorite from Isla San Diego includes a larger fraction of elongated boulders that
were more bar-like in form when originally loosened from the parent sea cliffs;
•
At a higher rock density than local limestone or rhyolite at 1.86 g/cm
3
and 2.16 g/cm
3
,
respectively, granodiorite at 2.52 g/cm
3
required more wave energy for shore erosion.
However, the difference between the measured rock density of local andesite only
slightly exceeds that for local granodiorite and made little difference;
•
Fossils recovered from granodiorite conglomerate on Isla San Diego expand the range
distribution of reef-dwelling corals such as Pocillopoora and Povona farther northward
than previously known. Otherwise, fossil representatives among the mollusks are
typical of faunas more widely attributed Marine Isotope Substage 5e throughout the
Gulf of California.
Author Contributions:
M.E.J. was responsible for conceptualization and methodology of the project
as a contribution to the Special Issue in the Journal of Marine Sciences and Engineering devoted to
“Evaluation of Boulder Deposits Linked to Late Neogene Hurricane Events.” G.C. was responsible
for validation during field work at the site in May 2020 and March 2021. R.G.-F. was responsible for
software applications and formal analysis. J.L.-V. was responsible for compilation of resources. All
authors have read and agreed to the published version of the manuscript.
Funding: This project received no outside funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: All data are available in the published manuscript.
Acknowledgments:
M.E.J. is grateful to Jay Racela (Environmental Lab, Williams College) for help
with the experimental calculation of density for the granodiorite sample from the lower Gulf of
California. Three readers contributed peer reviews with useful comments that led to the improvement
of this contribution.
Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
Table A1.
Quantification of cobble and boulder sizes, volume, and estimated weight from Station 1 near the south end of Isla
San Diego. The density of granite at 2.52 g/cm
3
is applied uniformly in order to calculate wave height for each boulder on the
basis of competing equations. Abbreviation: EWH = estimated wave height. Coordinates 25.11.7009 N and 110.42.0833 W.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al. [22]
(m)
1 118 75 31 274,350 219,480 553 9.7 5.8
2 24 20 7 3360 2688 6.8 2.0 1.3
3 79 53 32 133,984 107,187 270 6.5 6.0
4 39 34 21 27,846 22,277 56 3.2 3.9
5 40 30 28 33,600 26,880 68 3.3 5.2
6 68 53 23 82,892 66,314 167 5.6 4.3
7 68 43 16 46,784 37,427 94 5.6 3.0
8 120 110 70 924,000 739,200 1863 9.9 13.1
9 65 43 29 81,055 64,844 163 5.4 5.4
10 43.5 32 18 25,056 20,045 51 3.6 3.4
11 32 21 12 8064 6451 16 2.6 2.2
12 29 26 14.5 10,933 8746 22 2.4 2.7
13 68 55 23 86,020 68,816 173 5.6 4.3
14 41 34 14 19,516 15,613 39 3.4 2.6
15 64 33 15 31,680 25,344 64 5.3 2.8
J. Mar. Sci. Eng. 2021,9, 555 17 of 22
Table A1. Cont.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al. [22]
(m)
16 90 67 26 156,780 125,424 316 7.4 4.9
17 17 14 8 1904 1523 3.8 1.4 1.5
18 31 26 11 8866 7093 17.9 2.6 2.1
19 15 11 2.5 413 330 0.8 1.2 0.5
20 12 12 4.5 648 518 1.3 1.0 0.8
21 10.5 10.5 3 331 265 0.7 0.9 0.6
22 128 125 61 976,000 780,800 1968 10.6 11.4
23 72 68 67 328,032 262,426 661 5.9 12.5
24 16 14 3 672 538 1.4 1.3 0.6
25 50 48 27 64,800 51,840 131 4.1 5.0
26 18 16 7 2016 1613 4.1 1.5 1.3
27 13 12 4 624 499 1.3 1.1 0.7
28 30 16.5 14 6930 5544 14 2.5 2.6
29 20 12 6 1440 1152 2.9 1.7 1.1
30 18 16 6.5 1872 1498 3.8 1.5 1.2
Average
47.96 37.66 20.13 111,349 89,079 224.0 3.2 3.0
Table A2. Quantification of cobble and boulder sizes and volume with estimated weight from Station 2 near the south end of
Isla San Diego. The density of granite at 2.52 g/cm
3
is applied uniformly in order to calculate wave height for each boulder on
the basis of competing equations. Abbreviation: EWH = estimated wave height. Coordinates 25.11.6713 N and 110.42.1023 W.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al. [22]
(m)
1 23.5 12 12 3384 2707 6.8 1.9 2.2
2 15 11 9 1485 1188 3 1.2 1.7
3 31 29.5 11 10,060 8048 20 2.6 2.1
4 19 13 7 1729 1383 3.5 1.6 1.3
5 16 9.5 4.5 684 547 1.4 1.3 0.8
6 13.5 9 7 851 680 1.7 1.1 1.3
7 16.5 14.5 8 1798 1438 3.6 1.4 1.5
8 18 14 8 2016 1613 4 1.5 1.5
9 26 21 9 4914 3913 10 2.1 1.7
10 21 10 8 1680 1344 3.4 1.7 1.5
11 34 27 15.5 14,229 11,383 28.7 2.8 2.9
12 43 32 17 23,392 18,714 476 3.6 3.2
13 17 10.5 8.5 1517 1214 3.1 1.4 1.6
14 20.5 13 9 2399 1919 4.8 1.7 1.7
15 16 8 3 384 307 0.8 1.3 0.6
16 34 33 30 33,660 26.928 67.9 2.8 5.6
17 33 31 16 16,368 13,094 33 2.7 3.0
18 29 24 9 6264 5011 12.6 2.4 1.7
19 40 36 36 51,840 41,472 105 3.3 6.7
20 62 45 38 106,020 84,816 214 5.1 7.1
21 59 28 24 39,648 31,714 80 4.9 4.5
22 16.5 13 4 858 686 1.7 1.4 0.7
23 15.5 13 5.5 1108 887 2.2 1.3 1.0
24 12 11 5 660 528 1.3 1.0 0.9
25 26.5 19 15 7553 6042 15 2.2 2.8
26 9 8 3 216 173 0.4 0.7 0.6
27 14 12.5 7 1225 980 2.5 1.2 1.3
28 10 9 7 630 504 1.3 0.8 1.3
29 16 9.5 4 608 486 1.2 1.3 0.7
J. Mar. Sci. Eng. 2021,9, 555 18 of 22
Table A2. Cont.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al. [22]
(m)
30 50 39 12.5 24,375 19,500 49 4.1 2.3
31 31 19.5 10 6045 4836 12 2.6 1.9
32 28 15 6 2520 2016 5.1 2.3 1.1
33 8.5 5.5 4 182 150 0.4 0.7 0.7
34 12.5 9 7 788 630 1.6 1.0 1.3
35 23 19 5 2185 1748 4.4 1.6 0.9
Average
24.5 18.0 11.0 10,665 7763 33.8 1.9 1.9
Table A3.
Quantification of cobble and boulder sizes, volume, and estimated weight from Station 3 at the south end of Isla San
Diego. The density of granite at 2.52 g/cm
3
is applied uniformly in order to calculate wave height for each boulder on the basis
of competing equations. Abbreviation: EWH = estimated wave height. Coordinates 25.11.6426 N and 110.42. 2518 W.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al. [22]
(m)
1 68 61 30 124,440 99,552 251 5.6 5.6
2 19 12.5 6 1425 1140 2.9 1.6 1.1
3 20 16 6.5 2080 1664 4.2 1.7 1.2
4 52 36 26 48,672 38,938 98 4.3 4.9
5 18.5 16 6.5 1924 1539 3.9 1.5 1.2
6 63 54 34 115,668 92,534 233 5.2 6.3
7 52 36 26 48,672 38,938 98 4.3 4.9
8 61 25 22 33,550 26,840 68 5.0 4.1
9 26.5 20 13 6890 5512 14 2.2 2.4
10 32.5 16.5 12 6435 5148 13 2.7 2.2
11 27 20 10 5400 4320 10.9 2.2 1.9
12 101 38 37.5 143,925 115,140 290 8.3 7.0
13 35 33 14.5 16,748 13,398 34 2.9 2.7
14 62 45 43.5 121,365 97,092 245 5.1 8.1
15 51 38 23 44,574 35,659 90 4.2 4.3
16 18.5 13.5 10.5 2622 2098 5.3 1.5 2.0
17 15 12 10 1800 1440 3.6 1.2 1.9
18 126 59.5 50 374,850 299,850 756 10.4 9.3
19 20.5 19 13 5064 4051 10 1.7 2.4
20 71 54 28 102,352 85,882 216 5.9 5.2
21 49 22 17 18,326 14,661 37 4.0 3.2
22 17.5 13 6 1365 1092 2.8 1.4 1.1
23 28 27 17 12,852 10,282 26 2.3 3.2
24 10.5 8 5 420 336 0.8 0.9 0.9
25 21 18 12.5 4725 3780 9.5 1.7 2.3
26 12.5 7.5 5.5 516 413 1.0 1.0 1.0
27 13 11.5 8 1196 957 2.4 1.1 1.5
28 11.5 7 4 322 258 0.7 1.0 0.7
29 16.5 14 8 1848 1478 3.7 1.4 1.5
30 11 6 3.5 231 185 0.5 0.9 0.7
31 56 29 23 37,352 24,882 75 4.6 4.3
32 19 13 4.5 9182 7346 19 1.6 0.8
33 14 13 6.5 1183 946 2.4 1.2 1.2
34 15 12 8 1440 1152 2.9 1.2 1.5
35 28 25.5 13 9282 7426 19 2.3 2.4
Average
36.0 24.32 16.0 37,391 29,884 75.7 2.8 2.8
J. Mar. Sci. Eng. 2021,9, 555 19 of 22
Table A4.
Quantification of cobble and boulder sizes, volume, and estimated weight from Station 4 at the south end of Isla San
Diego. The density of granite at 2.52 g/cm
3
is applied uniformly in order to calculate wave height for each boulder on the basis
of competing equations. Abbreviation: EWH = estimated wave height. Coordinates 25.11.6463 N and 110.42. 2586 W.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al.
[22]
(m)
1 66 49 30 97,020 77,616 196 5.5 5.6
2 46 36 14 23,184 18,547 47 3.8 2.6
3 41 26.5 14 15,211 12,169 31 3.4 2.6
4 30 21 14.5 9135 7308 18 2.5 2.7
5 41.5 36 11.5 17,181 13,745 35 3.4 2.1
6 70.5 52 29 106,314 85,051 214 5.8 5.4
7 57 21 20 23,940 19,152 48 4.7 3.7
8 33.5 28 13.5 12,663 10,130 26 2.8 2.5
9 30 29.5 14 12,390 9912 25 2.5 2.6
10 50 36.5 14 25,550 20,440 52 4.1 2.6
11 60.5 33 19.5 38,932 31,145 78 5.0 3.6
12 52 37 22 42,328 33,862 85 4.3 4.1
13 15 12.5 8.5 1594 1275 3.2 1.2 1.6
14 113 55 53 329,395 263,516 664 9.3 9.9
15 55 32 32 56,320 45,056 114 4.5 6.0
16 34 28 9 8568 6854 17 2.8 1.7
17 43 36.5 23 36,099 28,879 73 3.6 4.3
18 19 13.5 7 1796 1436 3.6 1.6 1.3
19 63 49 35 108,045 86,436 218 5.2 6.5
20 16 12.5 9.5 1900 1520 3.8 1.3 1.8
21 91.5 74 17.5 118,493 94,794 239 7.6 3.3
22 65 42.5 19 52,488 41,990 106 5.4 3.5
23 89 35 32.5 101,238 80,990 204 7.4 6.1
24 55 42 22.5 51,975 41,580 105 4.5 4.2
25 31 15.5 11.5 5526 4421 11 2.6 2.1
26 40.5 15.5 9.5 5,964 4,771 12 3.3 1.8
27 64 44 35 98,560 78,848 199 5.3 6.5
28 28 19 18.5 9842 7874 20 2.3 3.5
29 56.5 35 13 25,708 20,566 52 4.7 2.4
30 13 9 6.5 761 608 1.5 1.1 1.2
31 16 10 7 1120 896 2.3 1.3 1.3
32 19.5 12 9.5 2223 1778 4.5 1.6 1.8
33 98 47 26 119,756 95,805 241 8.1 4.9
34 19.5 17 11 3647 2917 7.4 1.4 2.1
35 57.5 56 44 141,680 113,344 286 4.8 8.2
Average 48.0 32.0 19.31 48,758 39,007 98.0 3.7 3.4
Table A5.
Quantification of cobble and boulder sizes, volume, and estimated weight from Station 5 at the south end of Isla San
Diego. The density of granite at 2.52 g/cm
3
is applied uniformly in order to calculate wave height for each boulder on the basis
of competing equations. Abbreviation: EWH = estimated wave height. Coordinates 25.11.6494 N and 110.42.2551 W.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al.
[22] (m)
1 48.5 40.5 20 39,285 31,428 79.8 4.0 3.7
2 64 33 23.5 49,632 39,706 100 5.3 4.4
3 69 31 28.5 60,962 48,769 123 5.7 5.3
4 36 35 20.5 25,830 20,664 52 3.0 3.8
5 73 43 25 78,475 62,780 158 6.0 4.7
6 25 22.5 13 7313 5850 15 2.1 2.4
7 33 23 12 9108 7286 18 2.7 2.2
J. Mar. Sci. Eng. 2021,9, 555 20 of 22
Table A5. Cont.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al.
[22] (m)
8 36 36 11.5 14,904 4923 30 3.0 2.1
9 13 7.5 7 683 546 1.4 1.1 1.3
10 21 7.5 6.5 1024 819 2 1.7 1.2
11 14 13 8.5 1547 1238 3.1 1.2 1.6
12 49.5 43 19.5 41,506 33,205 84 4.1 3.6
13 19.5 17 10.5 3481 2785 7 1.6 2.0
14 49.5 29 22 31,581 25,265 64 4.1 4.1
15 31 30 5 4650 3720 9.4 2.6 0.9
16 11 9 6.5 644 515 1.3 0.9 1.2
17 9.5 8 7 532 426 1.1 0.8 1.3
18 31.5 30 16 15,120 12,096 30 2.6 3.0
19 42 38 14.5 23,142 18,514 47 3.5 2.7
20 45 45 22 44,550 35,640 90 3.7 4.1
21 63 40 26 65,520 52,416 132 5.2 4.9
22 67 36 22 53,064 42,45 107 5.5 4.1
23 63 53 29.5 98,501 78,800 199 5.2 5.5
24 40 30 16.5 19,800 15,840 40 3.3 3.1
25 28 19.5 14 7644 6115 15 2.3 2.6
26 84.5 58.5 28 138,411 110,729 279 7.0 5.2
27 63 39 28 63,504 50,803 128 5.2 5.2
28 78.5 62 25 121,675 97,340 245 6.5 4.7
29 65 46 38 113,620 90,896 229 5.4 7.1
30 29 28 9.5 7714 6171 16 2.4 1.8
31 100.5 45.5 43 196,628 157,303 396 8.3 8.0
32 37 32 16.5 19,536 15,629 39 3.1 3.1
33 66 29.5 28 54,516 43,613 40 5.5 5.2
34 58 45.5 24 63,336 50,669 128 4.8 4.5
35 32 23 8.5 6256 5005 13 2.6 1.6
Average 45.58 32.24 18.73 43,391 33,456 83.4 3.6 3.3
Table A6.
Quantification of cobble and boulder sizes, volume, and estimated weight from Station 6 at the south end of Isla San
Diego. The density of granite at 2.52 g/cm
3
is applied uniformly in order to calculate wave height for each boulder on the basis
of competing equations. Abbreviation: EWH = estimated wave height. Coordinates 25.11.6802 N and 110.42.2417 W.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al. [22]
(m)
1 75 58 27 117,450 93,960 238 6.2 5.0
2 69 33 14 31,878 25,502 64 5.7 2.6
3 50 45 35 78,750 63,000 159 4.1 6.5
4 64.5 39 35.5 89,300 71,440 180 5.3 6.6
5 30 24 13.5 9720 7776 20 2.5 2.5
6 25.5 14 14 4998 3998 10 2.1 2.6
7 60 38 26 59,280 47,424 120 5.0 4.9
8 67 42 19 53,466 42,773 108 5.5 3.5
9 53.5 28 25 37,450 29,960 75 4.4 4.7
10 19.5 18 8 2808 2246 5.7 1.6 1.5
11 73 35 20.5 52,378 41,902 106 6.0 3.8
12 63 54 25 85,050 68,040 171 5.2 4.7
13 85 56.5 19.5 93,649 74,919 189 4.7 3.6
14 76 43.5 30 99,180 79,344 200 6.3 5.6
15 63 40.5 19 48,479 38,783 98 5.2 3.5
16 27 26.5 11.5 8228 6583 17 2.2 2.1
17 35.5 22 14 10,934 8747 22 2.9 2.6
18 49 41 15.5 31,140 24,912 63 4.0 2.9
J. Mar. Sci. Eng. 2021,9, 555 21 of 22
Table A6. Cont.
Sample
Long
Axis
(cm)
Intermediate
Axis (cm)
Short
Axis
(cm)
Volume
(cm3)
Adjust
to 80%
Weight
(kg)
EWH
Nott [20]
(m)
EWH
Pepe et al. [22]
(m)
19 20.5 12.5 12 3075 2460 6.2 1.7 2.2
20 64 53 24.5 83,104 66,483 168 5.3 4.6
21 20.5 15.5 8.5 2701 2161 5.4 1.7 1.6
22 11 10 6 660 528 1.3 0.9 1.1
23 33.5 22 20 14,740 11,792 30 2.8 3.7
24 57 41 36.5 85,301 68,240 172 4.7 6.8
25 43 38.5 19 31,455 25,164 63 3.6 3.5
26 22.5 22 6.5 3218 2574 6.5 1.9 1.2
27 53 41 12 26,076 20,861 53 4.4 2.2
28 38 22.5 16 13,680 10,944 28 3.1 3.0
29 41 19.5 12.5 9994 7995 20 3.4 2.3
30 59 49 26 75,166 60,133 152 4.9 4.9
Average
48.3 33.5 19.0 42,110 33,688 85.0 3.2 2.9
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