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Form, genesis, and deformation of central wave-cut platforms

W. C. BRADLEY Department of Geological Sciences, University of Colorado, , Colorado 80302 G. B. GRIGGS Division of Natural Sciences, University of California, Santa Cruz, Santa Cruz, California 95064

ABSTRACT nental shelf, but submarine studies have thus far yielded only frag- mentary information, and careful observation of Pleistocene plat- Modern and ancient wave-cut platforms on Ben Lomond Moun- forms within emerged marine terraces is frustrated by a ubiquitous tain in central California are broadly similar in shape. They have a cover of . seaward slope composed of two segments: a steeper, slightly con- The best developed and preserved platforms in central California cave inshore segment, with gradients of generally 0.02 to 0.04 (20 occur on the seaward side of Ben Lomond Mountain between to 40 m/km), and a flatter, planar offshore segment with gradients Santa Cruz and Ano Nuevo Point (Fig. 1). Some of those under of 0.007 to 0.017 (7 to 17 m/km). The flattest inshore and offshore water have been recorded on sparker profiles (Moore and Shum- gradients measured were, respectively, 0.015 (15 m/km) and 0.005 way, 1959; Alpine Geophysical Associates, Inc., 1971) and ex- (5 m/km), suggesting that these are close to minimum gradients for amined locally by engineers (Hyde and Howe, 1925-1926; Helen, erosional platforms in central California. The inshore segments are 1935). Those on land have been reported by Lawson (1893), Wil- generally 300 to 600 m wide and extend to a depth of 8 to 13 m. son (1907), Branner and others (1909), Rode (1930), Page and Platforms are widest in areas where soft sandstone crops out and Holmes (1945), Alexander (1953), Bradley (1956, 1957, 1958, where there has been least uplift. 1965), Bradley and Addicott (1968), Clark (1966, 1970), Jahns Major storm waves now break in water 7 to 12 m deep. We and Hamilton (1971), and Lajoie and others (1972). Most early conclude that inshore platform segments were associated with workers recognized the basic characteristics of these platforms: (1) storm-wave surf zones and that offshore segments were associated they truncate bed and therefore are truly erosional features, with the zone of deep-water wave transformation. A gradient of (2) they slope seaward at a low angle and with a slight concavity, 0.005 for the offshore segment would keep wave at the and (3) they have been deformed since they were cut. Our purposes bottom constant (Zenkovich, 1967). A steeper gradient for the here are to describe the platforms in more detail than has been done inshore segment would enable backwash to counteract heretofore and then to comment on platform genesis and Quater- the strong onshore movement of surf, so that available coarse sed- nary deformation of Ben Lomond Mountain. By the end of this iment could be moved laterally. Slopes less than the minimum paper, it will be evident that we are still deficient in both descriptive would so dissipate wave energy in offshore areas that the detail and genetic understanding. would not be able to provide the needed longshore transport for coarse sediment, and progradation would result. Thus, plat- GENERAL GEOLOGY forms have a shape that allows efficient conversion of wave energy into and longshore transport; their seaward gradient is not Bed Rock used for the downhill transport of sediment. Platform gradients decrease with time, at least until the minimum is achieved. Whether Ben Lomond Mountain is an asymmetric dome with a the offshore segments were eroded at their existing depths or were northwest-southeast elongation, roughly concordant with the eroded by surf zones as level rose remains a matter of con- coastline (Branner and others, 1909). Erosion has exposed a core of troversy. metamorphic and plutonic rocks of Mesozoic and possibly Ben Lomond platforms have been uplifted and progressively Paleozoic age (Leo, 1967), which are unconformably overlain by tilted in a seaward direction, indicating that late Tertiary domical outward-dipping sedimentary rock of Tertiary age. The Tertiary uplift has continued into Quaternary time. Uplift rates have ranged units have been redefined and mapped by Clark (1966, 1970), and from 0.16 m/1,000 yr near Santa Cruz to 0.26 m/1,000 yr near we will use his nomenclature. As far as marine terraces are con- Greyhound Rock. Tilts have varied from 0.001 (1 m/km) for the cerned, the important Tertiary units on Ben Lomond's seaward lowest prominent platform to 0.009 (9 m/km) for the highest plat- flank are the Pliocene Purisima Formation and Santa Cruz - form (which may be as old as 106 yr). Because of uplift, platforms stone (formerly called the Monterey Formation), and the Miocene must have been cut at times of eustatically high . Key Santa Margarita Sandstone, Lompico Sandstone, and Monterey words: , shorelines, features, marine terraces, Formation. Dips are generally toward the at angles of less shallow-marine processes, sea-level changes, Quaternary, deforma- than 20°. This homoclinal structure is only locally complicated by tion, uplift, folds. shallow folds and minor faults until the is reached, where folds and faults are the dominant structural ele- INTRODUCTION ments (Clark, 1970; Alpine Geophysical Associates, Inc., 1971). Angular unconformities are not significant within the younger Ter- Despite their common occurrence in California coastal areas, tiary section, indicating that uplift of Ben Lomond Mountain was a wave-cut platforms have not received the attention they deserve. late Cenozoic event. It evidently had a pre-Tertiary ancestry, how- Inaccessibility and inadequate exposures are the reasons. Modern ever, because planar and linear elements within the crystalline core and premodern platforms are present in many places on the conti- define a steep-sided dome (Rode, 1930).

Geological Society of America Bulletin, v. 87, p. 433-449, 16 figs., March 1976, Doc. no. 60313.

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BEN LOMOND MARINE TERRACES

ANO NUEVO PT. «y

Figure 1. Top: Wave-cut platforms (within marine terraces) on Ben Lomond Mountain, central is shown where it has been identified. Bottom: Locations of platform profiles (except profile 1, California; inner and outer edges are dotted where dissected. Names are correlated with previous which is located off mouth of Santa Cruz Harbor, 2.5 km northeast of Santa Cruz Point). Profile terminologies in Table 1. Outliers of Quarry and Blackrock terrace deposits are taken from Clark data are summarized in Table 2. (1966,1970). Three Santa Cruz platforms are lumped together, although Davenport shoreline (D) Downloaded from gsabulletin.gsapubs.org on September 15, 2015

CENTRAL CALIFORNIA WAVE-CUT PLATFORMS 435

Marine Terraces garita Sandstone, postplatform alteration has been greatly facili- tated; this accounts for the poor preservation of the Quarry and Figure 1 is a map of the Ben Lomond marine terraces as they Blackrock terraces northwest of Santa Cruz (Fig. 1). would appear if undissected by streams. Each contains a platform with an inner edge, which is a former shoreline ("inner edge" is the STUDY term recommended by Baulig, 1956; it is synonymous with "shoreline angle" of Davis, 1932, 1933, and "back edge" of Rode, Procedures 1930, and Alexander, 1953), and an outer edge created by later erosion. Although terrace remnants survive only on interfluves, Platforms were surveyed by plane table and alidade for their lateral continuity is sufficiently good that Figure 1 is a reasonable shape and inner-edge elevations. Profiles oriented perpendicular reconstruction. Isolated remnants show that higher terraces were and parallel to the shoreline showed that gradients and other shape once more extensive, even though erosion has destroyed their inner elements were broadly similar. This made it possible to adopt an edges. The terrace terminology shown in Figure 1 is new. Previous average gradient for the inner part of each platform, which in turn terminologies have been based on either elevation or number of enabled us to estimate inner-edge elevations. Inner edges are rarely terraces. Both schemes were awkward because terraces locally vary observable. However, one can estimate an elevation by finding a in number, and their inner edges are no longer horizontal. Table 1 point on the platform close to the shoreline, measuring its elevation correlates present and previous terminologies. Terraces that are and distance offshore, and using the average gradient to provide a topographically distinct are given separate names. Some contain correction factor. single platforms, but others have multiple platforms at so nearly Platform points were chosen in two ways. All natural and the same elevation that there may be little or no topographic sep- artificial exposures of authentic platforms were used. Our criteria aration; these different levels are distinguished where we have for authenticity were (1) the presence of holes made by rock-boring enough data to do so, as in the case of the Santa Cruz terrace. mollusks or (2) an undisturbed cover of typical marine deposits High-level sandy deposits and flat topography have been cited as (described by Bradley, 1957). Exposures of bed rock lacking these evidence for a relative stand of the sea higher than the Quarry level criteria were generally ignored. Occasionally this technique led us (Lawson, 1893; Branner and others, 1909; Rode, 1930). However, into error because relief elements on a platform can also be bored sandy deposits collected from elevations above 260 m in the area by mollusks and buried by marine deposits, and these elements may around Bonnie Doon (Fig. 1) have a heavy mineral suite that is rich go unrecognized in a limited exposure; consistency with nearby in mica and poor in amphibole-epidote, similar to the nearby Santa points can be a helpful check. Margarita Sandstone but unlike any of the known marine terrace Where exposures were absent, platforms were located with a deposits. We conclude these deposits are reworked Santa Mar- hammer-powered refraction seismograph. Ben Lomond marine ter- garita Sandston rather than Quaternary marine deposits. We know races lend themselves to this kind of study because their deposits of no evidence that proves a stand of the sea above the Quarry are thin (generally less than 6 m), have low acoustical velocities level. (300 to 800 m/sec), and are separated by a sharp discontinuity from Most platforms have been cut into the Santa Cruz Mudstone. the underlying bed rock, which usually has higher velocities (1,000 This explains both their development and preservation. The unit is to 2,500 m/sec for the Santa Cruz Mudstone and the Purisima thinly bedded and highly jointed, which expedites wave erosion Formation). Where platforms were carved in these units, 70 per- and stream incision, but its siliceous nature retards decomposition cent of the seismograms were unambiguous to interpret, 20 percent once platforms are stranded on the interfluves. Near Santa Cruz required choosing between possible solutions, and only 10 percent and on Ano Nuevo Point, platforms involve other bedrock units as caused interpretive despair. Other bedrock units, however, were well. In areas of rotten crystalline rock and the soft Santa Mar- less cooperative. Velocities in the Santa Margarita Sandstone and

TABLE 1. MARINE TERRACE TERMINOLOGIES, BEN LOMOND MOUNTAIN

Lawson Branner Rode Page Alexander Bradley This paper*' (1893) and others (1930) and (1953) (1965) (1909) Holmes (1945)

Higher Higher Fifth terraces terraces 871f 800 — fourth i 840 Fifth Quarry (1) Fourth 712 500 — third [580 Fourth Blackrock (2) 374 Third 425 Third Wilder (2) 205 250 — second Second 280 Third Second Western (1) Second Cement (1) 96 100 — first First 90 First First Santa Cruz (3) Greyhound level Highway 1 level Davenport level Shore platforms

Numbers in parentheses indicate quantity of separate platforms. Derivations of names: Quarry — named for old quarry road 2 to 3 km north of Davenport, where terrace is well displayed; Blackrock — large abandoned asphalt-sandstone pit 2 km northeast of Hill Bluff, where platform and overlying deposits are well exposed; Wilder — Wilder creek just west of Santa Cruz, where terrace is broadest; Western — Western Drive near western edge of Santa Cruz, where terrace is broad and well preserved; Cement — occurs behind cement plant at Davenport; Santa Cruz — city takes urban advantage of wide part of this terrace; Greyhound — exposed in places in sea southeast of Greyhound Rock; Highway 1 — State Highway 1 faith- fully follows this level; Davenport — Davenport Beach, where sea cliff at northwest end exposes inner edge of platform, f Figures are inner-edge elevations in feet. Downloaded from gsabulletin.gsapubs.org on September 15, 2015

436 BRADLEY AND GRIGGS

Figure 2. Diagramma- in rotted or highly fractured crystalline rocks were so similar to tic cross section of wave- those in the overlying deposits that a discontinuity could not be cut platform surmounted identified. Platforms in these rock types could only be studied in by flat and shore exposures. platform, after emergence Stations located close to old sea cliffs, whether seismic or out- and burial by marine- terrace deposits. Numbers crop, often yielded data that were frustrating to interpret. Relief are explained in text. can be substantial here (5 m or more) because platform develop- ment is incomplete, and this relief may be buried when emergence occurs (Fig. 2). Seismic surveys at stations 1 and 2 (see Fig. 2) may give clear depth solutions, but the platform so determined is not the desired wave-cut platform. If such a point is then corrected to an inner-edge elevation, it will be too high by an amount equal to the relief. A limited exposure can lead one into the same error. Experi- ence showed that we obtained better results with stations that were 100 to 200 m offshore (Fig. 2, station 3). This increased our correc- tion factor but lessened the problem of platform relief (the former was not serious because variations in gradient caused a difference in elevation of only about 1 m at an offshore distance of 175 m). Our best platform profiles came from seismic surveys on the Santa Cruz terrace. One reason was its excellent preservation. Another was the fact that Brussels sprouts were grown there. Farm roads proved to be ideal for seismic surveys because they were flat, well compacted, free of vegetation, and oriented parallel or per- Figure 3. Shore platforms parallel to bedding in Santa Cruz Mudstone. pendicular to the shoreline. Flat area is Santa Cruz terrace. View northwest to Davenport (2 km). To measure distance between a station and an inner edge, the latter was located in the following manner. Where shore platforms are absent, the modern inner edge lies at the base of a nearly vertical sea cliff. Although old sea cliffs are degraded back in their upper part after emergence, their lower part is preserved intact by a bank of colluvium (shown diagrammatically in Fig. 2, and photographically in Bradley, 1957, PI. 1). Accordingly, we used the lowest outcrop on a degraded sea cliff to mark the map position of the inner edge.

Natural Variations

Platforms are rarely flat for any great distance. Local relief of as much as 1 m is so common that it may be accepted as normal for a well-planed platform. Exhumed concretions and strike ridges and channels sometimes cause relief of as much as 2 to 3 m. Still greater relief occurs close to the sea cliff, where it takes the form of shore platforms, stacks, and surge channels (Fig. 2). The foregoing gives some understanding of how far a single determination of elevation Figure 4. Shore platforms that truncate bedding in Santa Cruz Mud- may depart from the local average. One point on an authentic stone. View southeast to Sand Hill Bluff (2 km). platform may have an elevation that deviates from its neighbors by

T 5 00

1000

DISTANCE (m) Figure 5. Profiles perpendicular to shoreline of modern platform (2 and 7) and fathometer trace of sea floor (4; probably also modern platform); see Figure 1 for locations and Table 2 for data. Vertical exaggeration is x5 for profiles 2 and 7, x73 for profie 4. Downloaded from gsabulletin.gsapubs.org on September 15, 2015

CENTRAL CALIFORNIA WAVE-CUT PLATFORMS 437

2 to 3 m. If the top of a stack or shore platform is inadvertently ments of elevation were accurate to within 0.5 to 1 m, varying taken as the wave-cut platform, its elevation may deviate by 5 m or directly with altitude. Seismic accuracy was determined at 5 sites more. It takes many data points to adequately define a local aver- where depth to bed rock was known and was checked at perhaps age. 20 other sites that were close to exposures. From this we conclude that our seismic measurements are also accurate to 0.5 to 1 m, Accuracy of Measurements varying directly with depth. Altogether, we believe our surveying techniques were accurate to within 1 m on the lowest terrace and 2 Instrumental errors lie mainly in measurements of elevation and m on the highest. This means that instrumental accuracy lies within in seismic determinations of depth to bed rock. Closure of alidade the natural variation of the features being studied and therefore can traverses on points of known elevation showed that our measure- generally be discounted.

TABLE 2. PLATFORM DATA FROM PROFILES

Profile Inshore segment Offshore Depth at no. Width Steepest segment, junction of (see Fig. 1) Bed rock* (m) gradientf gradientf segments (m) Estimates of confidence (%) # ±5-20 ±15-25 ±5-20 ±5-20 Platforms Modern SO. 005 10

(3 profiles) P,SC 610 0.025 0.006 13 SC 396 0.029 S0.008 11 sc 792 0.029 SO.015 19 sc 610 0.028 SI0.018 19 sc 549 0.037 =10.016 21 7 sc >762 0.022 >17 Shore 8 sc Santa Cruz Davenport level 18 sc >213 0.015 19 sc >305 0.010 21 sc >213 0.016 p Highway 1 level 9 0.008 10 sc 396 0.017 0.007 11 sc 427 0.035 0.007 9 12 sc 427 0.017 0.009 8 13 sc 530 0.020 0.010 10 14 sc 305 0.032 0.013 8 15 sc 366 0.029 0.017 9 16 sc 335 0.023 0.010 8 17 sc >287 0.034 >8 18 sc 427 0.019 0.010 19 sc 244 0.034 0.010 20 SC,SI >293 0.029 >6 21 sc 305 0.031 0.017 9 22 sc >30 0.063 23 sc >11 0.054 24 sc >53 0.047 26 P,SC,M >360 0.033 0.007 Greyhound level 25 sc >36 0.080

Western 27 SC,SM 610 0.019 0.010 12 28 SC 396 0.029 0.014 12 29 SC 549 0.023 0.010 12 30 SC >396 0.028 >11 31 SC 366 0.034 0.017 12 32 SC,SI 469 0.028 0.016 13 33 SC 344 0.031 0.016 11 Wilder 34 SC >384 0.033 >13 SC 305 0.030 0.015 9 35 SC >274 0.024 >7 Blackrock 36 SC 152 0.048 0.019 8 37 SC >488 0.046 >21 Quarry 38 SC 335 0.036 0.019 12 39 SC 518 0.028 0.017 16 * Bed rock units: P = Purisima Formation, SC = Santa Cruz Mudstone, SM = Santa Margarita Sandstone, M = Monterey Formation, SI = sandstone intrusives. f Figures are dimensionless and translate as follows: 0.005 = 0.28° = 5 m/km; 0.010 = 0.57° = 10 m/km. # See Study section for discussion of natural variations and errors. § Fathometer profiles; inshore gradients probably pertain to the platform, but it is uncertain whether offshore gradients pertain to the platform or a sediment surface. Downloaded from gsabulletin.gsapubs.org on September 15, 2015

438 BRADLEY AND GRIGGS

PLATFORM SHAPE Platforms of Santa Cruz Terrace

Profiles The Santa Cruz terrace contains three platforms that are well enough developed to have separate names. Most extensive is the The profiles that follow were drawn by connecting points of Highway 1 level, whose inner edge defines the entire terrace (Fig. known elevation. They are representative of true profiles in their 1). Our data pertain chiefly to this platform. gross characteristics but not necessarily in detail, because the seis- Profiles parallel to the shoreline show little relief and are flat to mic technique averages local relief and some irregularities may broadly undulating (Fig. 6). Some undulations, however, are an relate to relief features rather than to the platform itself. This is artifact of changing distance to the shoreline (profile A). Profiles especially important for older platforms; one may accept their gen- perpendicular to the shoreline are presented in Figure 7 and re- eral form but pass over the details as being of uncertain corded in Table 2. The profiles illustrate the difficulty in disting- significance. Additional problems with profile interpretation will be uishing between platform and relief features as the shoreline is discussed when they arise. Profile locations are shown in Figure 1. approached, and they show our technique for making the distinc- tion. The modern platform has a gradient whose inner part can be Shore Platforms approximated by a straight line, anchored at the inner edge (Fig. 5). With ancient platforms, if the innermost profile points fell on a These are narrow bedrock platforms that occur locally along the straight line we accepted them as valid platform points and ex- present-day shoreline (Fig. 1). They range in elevation from ap- tended the line until it intersected the sea cliff (Fig. 7, profiles 12, proximately the level of low to an indefinite height above sea 19, and 21). Profile 13 shows a different situation. The innermost level, although the broad ones are usually found at an altitude of three points suggest a platform whose slope steepens as the less than 8 m. Exceptional platforms have a breadth in excess of shoreline is approached. Favorable exposures, however, revealed 100 m, but most are narrower than 50 m. the existence of steplike shore platforms. We concluded that the Two kinds of platform are present. The majority are parallel to innermost points belonged to the shore platforms and that the true bedding (Fig. 3). They slope seaward except in those areas where wave-cut platform had a shape approximately as shown (profile 10 the homoclinal structure is modified. Inner edges show no eleva- was drawn in a similar fashion). Occasionally points were so scat- tional preference. Less common are shore platforms that cleanly tered that our profile is questionable (profile 11). truncate bedding (Fig. 4). Their distribution is puzzling in that The lower Davenport platform is poorly known, and its inner unambiguous remnants occur chiefly between Davenport and Sand edge cannot be traced at present. It is seen in sea-cliff exposures at Hill Bluff (Fig. 1). These platforms have the usual local relief but Davenport (Fig. 8) and the south side of El Jarro Point (Fig. 1) and are different from wave-cut platforms in being nearly horizontal. in certain Highway 1 profiles (Fig. 7, profiles 19, 21, and possibly Their inner edges show a strong elevational preference for approxi- 10). Its absence on other profiles may be due to inadequate data or mately 2 m. Shore platforms of the first type are present along to the possibility that it is not a continuous topographic feature. emergent shorelines, but poor exposures prevented their study. It is This platform appears to have a lesser gradient than others (Table not known if platforms of the second type are also present. The Greyhound level is the highest of the three Santa Cruz plat- Modern Platform forms. It is recognized only between Scott and Waddell Creeks (Figs. 1, 9) where it is so narrow that its inner edge is mapped with The modern platform is known from bottom surveys in two that of Highway 1. In fact, it would remain hidden by detritus were areas and a number of fathometer traverses (see Fig. 1). Figure 5 it not for exposures in the modern sea cliff. Its steep gradient and shows representative profiles oriented perpendicular to the truncation of bedding indicate that it is not an old shore platform shoreline. This platform has a seaward gradient in two segments: (Table 2). there is a steep inshore segment whose slope is either concave or planar, which gives way at moderate depth to a gentle and planar Platforms of Higher Terraces offshore segment (profile 7 does not extend far enough seaward to include the offshore segment). Vital statistics for both segments are Platforms of higher terraces are shown in profile in Figures 10 summarized in Table 2. One uncertainty with the fathometer data and 11 and are recorded in Table 2. Only a single platform can be is the assumption that platform and bottom everywhere coincide. identified in the Western and Quarry terraces, but multiple plat- The presence of kelp validates the assumption for the inshore seg- forms exist in the Wilder terrace and are strongly suspected in the ment, but the offshore segment remains uncertain; this is why Blackrock. The otherwise excellent western profile (Fig. 10, profile offshore gradients in Table 2 may be too steep. 27) suffers in its nearshore 600 m because bed rock is chiefly Santa

i —r— 1000 soo

-30

-30 t- < > -20 Ul -10 " I' " < -I 500 n U

DISTANCE (m) Figure 6. Profiles subparallel to shoreline of Highway 1 platform (see Fig. 1 for locations); left ends are toward northwest. Points were exposed (solid circles) or seismically determined (open circles). Profile D crosses a stack (S). Vertical exaggeration is x 10. Downloaded from gsabulletin.gsapubs.org on September 15, 2015

CENTRAL CALIFORNIA WAVE-CUT PLATFORMS 439

Margarita Sandstone. The Blackrock profile (Fig. 11) was unusual lie within 3 m of most points, although occasionally they deviate by in having an exposed inner edge and an extremely narrow inshore an amount greater than 6 m. Single lines have been drawn through segment. the Wilder and Blackrock points because data are inadequate to resolve their multiple levels. This increases the uncertainty in our Summary quantification of their tilts. Having been horizontal when formed, these inner edges now reveal a pattern of deformation that is inter- Profiles and data (Table 2) show that Ben Lomond platforms are nally consistent and generally progressive with altitude. A broad broadly similar. They slope seaward with a gradient that consists of sag lies just west of Santa Cruz (S in Fig. 13). Inner edges rise a steeper, slightly concave inshore segment and a flatter, planar eastward toward an arch recognized by Alexander (1953). They offshore segment. Common gradients are 0.02 to 0.04 for the in- also rise toward the northwest, gradually at first, then more rapidly shore segment and 0.007 to 0.017 for the offshore segment. Absence after passing an inflection point (I) near Scott Creek, culminating in of any inshore slopes less than 0.015 and any offshore slopes less an arch (A) near Greyhound Rock, thence declining again to Año than 0.005 indicates that these are close to minimum gradients for Nuevo Point. This broad asymmetric arch is in line with the asym- wave-cut platforms in this area. The inshore segment is usually 300 metric crestline of Ben Lomond Mountain (Fig. 13). Using the to 600 m wide, and it generally extends to a depth of 8 to 13 m inner-edge lines, we determined longshore tilts and key elevations (Fig. 12). Platforms cut on the Santa Cruz Mudstone appear to be (Table 3). Tilts become larger with altitude, except for the anomal- similar to those cut on other bedrock units. ous Wilder platform and the essentially similar Quarry and Black- A satisfactory genetic theory must account for the characteristics rock platforms. just summarized. Whatever constituted the erosional environment, A second component of tilting is suggested by a comparison of it recurred several times during the Quaternary Epoch. platform gradients. Only the offshore segments are compared, be- cause their data show the least variation (see Table 2). Despite the DEFORMATION scatter when gradients are plotted, there is an indication that higher platforms are steeper than lower platforms (Fig. 14). Assuming that Inner-edge points, determined by the method described and pro- gradients were broadly similar to start with, we see a suggestion of jected to a plane at today's shoreline, are plotted in Figure 13. The seaward tilting that has been progressive with altitude (the Quarry lines of visual best fit thus represent undissected inner edges. They platform excepted). This is not unreasonable when one recalls the

DISTANCE (m) Figure 7. Profiles perpendicular to shoreline of Highway 1 platform; see Figure 1 for locations and Table 2 for data. Points were exposed (solid circles) or seismically determined (open circles). Vertical exaggeration is X5. Inner edge of Davenport platform (D) appears on profiles 19 and 21 and possibly on profile 10. Profile 21 shows stack (S) whose top is exposed at terrace surface. Profile 12 shows erosional low (E) below platform. Exposures indicate that innermost two points of profile 13 are on shore platforms (SP). Downloaded from gsabulletin.gsapubs.org on September 15, 2015

440 BRADLEY AND GRIGGS

seaward dip of Tertiary sedimentary strata. As a first approxima- been progressive with time, at least since Blackrock time. It has also tion in quantifying the seaward tilting, we have simply averaged the been consistently in a seaward direction because of the dominance gradients for each terrace (Fig. 14); because of uncertainty about of that component. Tertiary sedimentary strata dip in the same the modern platform, we have arbitrarily adopted an average, as direction, only more steeply. Evidently the Ben Lomond dome has shown. continued to grow through late Cenozoic time and may well still be A combination of both components gives the resultant tilting active. Approximately 0.5° has been added to the dip of Tertiary (Table 3). These calculations apply to the region southeast of Scott sedimentary deposits since Quarry or Blackrock time. If we knew Creek; tilting may have been greater between Scott and Waddell their age we could estimate a rate of tilting. Creeks, but data are lacking. The figures indicate that tilting has Platform Ages

Only the Santa Cruz terrace has thus far proved datable. Mol- lusks from the Davenport level give U-series ages of 68,000 to 100,000 yr B.P. (Bradley and Addicott, 1968) and an amino acid age of 130,000 ± 50,000 yr B.P. (Lajoie and others, 1975). Because U-series ages on mollusks are questionable (Kaufman and others, 1971; Broecker and Bender, 1972) and amino acid techniques are still being perfected, the age of the mollusks can only be estimated as approximately 100,000 yr. Figure 8. Inner part of Davenport platform (left) and outer part of Recent studies, especially in Barbados and New Guinea where Highway 1 platform at northwest end of Davenport beach. Shore platform hflnw lianta Crii7 Mndctnnp. uplift is in progress, have identified a number of glacio-eustatic high stands of the sea during the past 150,000 yr (Veeh, 1966; Broecker and others, 1968; Mesolella and others, 1969; Veeh and Chappell, 1970; Konishi and others, 1970; Steinen and others, 1973; Ku and others, 1974; Chappell, 1974; Bloom and others, 1974; Szabo and Gard, 1975). Similarity in data from different areas makes the curve of Bloom and others (1974, p. 203) a reasonable approximation of eustatic sea level during this period. The highest stand occurred about 125,000 yr ago and reached a position approximately 6 m above today's sea level; it is presumed to have been the Sangamon (Eemian) interglacial stand (Matthews, 1973). Several younger interstadial high stands occurred, but none reached the level of today's sea. Growth of Ben Lomond Mountain means that its platforms must Figure 9. Inner part of Highway 1 platform (left) and outer part of have been cut during periods of high sea level. Mollusk ages of Greyhound platform. Shore platforms below. View northwest to Grey- approximately 100,000 yr indicate that the Santa Cruz platforms hound Rock (1 km). Santa Cruz Mudstone. were genetically associated with certain of the high stands observed

> Ul | ,——

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90

80

70

60-I " • 1 • l I i • t l • l l i l i i s 0 400 800 1200 1600

DISTANCE (m) Figure 10. Profiles perpendicular to shoreline of Western platform; see Figure 1 for locations and Table 2 for data. Points were exposed (solid circles) or seismically determined (open circles). Vertical exaggeration is x5. Low point (E) on profile 29 is a postplatform erosional saddle. Downloaded from gsabulletin.gsapubs.org on September 15, 2015

CENTRAL CALIFORNIA WAVE-CUT PLATFORMS 441

Figure 11. Profiles perpendicular to shoreline of Wilder platform (34 and 35; two platforms are present in 34, possibly in 35), Blackrock platform (36), and Quarry platform. (38 and 39); see Figure 1 for locations and Table 2 for data. Points were exposed (solid circles) or seismi- cally determined (open circles). Vertical exaggeration is x5.

z 35 o 130 I- 4> 120- UJ 110

38

Dl STANCE (m)

elsewhere. But which ones? The highest and most extensive of the dates, although corroboration by a more reliable dating technique younger Barbados reefs grew 125,000 yr ago (Mesolella and is needed. We suggest the following history as a tentative working others, 1969). Because of its pre-eminence among the Santa Cruz hypothesis. The sea transgressed and cut the Davenport platform, platforms, we conclude that the Highway 1 platform was carved at then regressed, then transgressed again about 125,000 yr ago to a this time. slightly higher level, where the Highway 1 platform was cut (the If one accepts the assumptions that (1) the curve of Bloom and upper part of the old Davenport sea cliff would have retreated as others (1974) is correct, (2) the Highway 1 platform is 125,000 yr the Highway 1 cliff). Regression would then have left beach de- old, and (3) Ben Lomond has grown at a constant rate, then one is posits on both platforms, but the outer part of these led to some interesting thoughts concerning the Davenport plat- would have been removed by the transgression —105,000 yr ago form: it may have been cut before the Highway 1 platform, even (which did not reach the level of the preceding high stand). Mol- though its deposits, and possibly mollusks, are younger. An older lusks of this age could be the ones being dated. Subsequent regres- age is suggested by the fact that its inner edge lies ~13 m below sion would have left the younger beach deposits. Any record of the that of the Highway 1 platform. By combining uplift and eustatic high stand of —85,000 yr ago would be found close to today's sea sea-level curves, one can calculate that the elevational difference level (due to uplift), where it has evidently been obliterated by between the two platforms would be larger if the Davenport plat- Holocene erosion. form had been carved during any of the high stands later than If the Highway 1 inner edge was indeed active 125,000 yr ago at 125,000 yr ago (for example, the -105,000- or ~85,000-yr-ago an altitude of 6 m, the following deformation rates can be calcu- stands), but they would be about as observed if erosion occurred lated (Table 4). Our uplift rates are slightly greater than some in during the high stand postulated at —140,000 yr ago. Weathering southern California (Ku and Kern, 1974), slightly less than those at of unstable minerals shows that marine deposits on the Davenport Barbados (Broecker and others, 1968; Matthews, 1973), and much platform are younger than those on the Highway 1 platform (Brad- less than some in New Guinea, , and Japan (Veeh and ley, 1957). Young mollusks are also suggested by the U-series Chappell, 1970; Bloom and others, 1974; Wellman, 1969, 1971a, 1971b; Singh, 1971; Lewis, 1971; Sugimura, 1967; Konishi and others, 1970; Yoshikawa, 1974). Having stepped this far onto uncertain ground, we will now bring the other foot forward by using what data we have, plus an assumption of uniform deformation, to estimate ages for the older • platforms (Table 5). This makes the Quarry terrace older than in M previous estimates (Lajoie and others, 1972), but doubt about uni- X form deformation makes our estimate uncertain. Uplift without • significant tilting evidently occurred between Quarry and Black- : rock times. In addition, if dip has been increased by only 0.5° in —1,000,000 yr, a backward extension of this rate would leave the < Ï Pliocene strata with less dip than is observed. We could try to improve the credibility of our age estimates by comparing them with one of the available climatic curves, but good sense makes us HI We decline the opportunity.

PLATFORM TERRACE DEPOSITS Figure 12. Depth of water at junction between inshore and offshore platform segments. M = Modern; HI = Highway 1; We = Western; Wi = Because of a changing content of unstable minerals, marine de- Wilder; B = Blackrock; Q = Quarry. 1 = minimum depth. posits of the Santa Cruz terrace were believed to have been pro- Downloaded from gsabulletin.gsapubs.org on September 15, 2015

442 BRADLEY AND GRIGGS

o •»

Rock DISTANCE Figure 13. Inner-edge elevations of Ben Lomond platforms: Q = Quarry, B = Blackrock, Wi = Wilder, We = Western, C = Cement, G = Greyhound, HW 1 = Highway 1, D = Davenport. Points were determined by procedure described in text and are based on outcrop data (solid circles) and seismic data (open circles); lines were drawn visually. Vertical exaggeration is x20. Deformational elements, discussed in text, are A, crest of arch; I, inflection point; S, trough of sag. Elevations of these elements and longshore component of tilting are summarized in Table 3. Generalized crestline of Ben Lomond Mountain is shown, but scale is different from rest of diagram; summit elevation exceeds 800 m.

duced in a single regressive cycle (Bradley, 1957). Recognition of Wave Action and Currents the Davenport level indicates that deposits of more than one regres- sive cycle were inadvertently sampled. Sediments on the Davenport Wave transformation consists of a complex series of interrelated platform are younger than those of Highway 1 (even though the changes in both form and orbital motion. Only wave period re- platform itself is believed to be older), but they do not form a mains unchanged. Wave length and velocity decrease, whereas depositional continuum with the other. Sedimentary structures and , steepness, and asymmetry increase. Refraction is a other characteristics continue to support the belief that each unit modifying influence; it may either augment or counteract the other was a prograded beach. factors that affect height. Energy is lost from the system by a variety of mechanisms, prin- NEARSHORE PROCESSES AND SEDIMENT MOVEMENT cipally orbital friction at the bottom (see Teleki, 1972, and his references). Simultaneously, the surviving energy is concentrated Platform erosion occurs within the nearshore zone as defined by because of diminishing depth and wave length. As long as bottom Shepard and Inman (1950, 1951), wherein waves feel bottom with slope is relatively large (in excess of 0.01 to 0.005, according to their orbital motion, deep-water waves are transformed into soli- Zenkovich, 1967), energy concentration exceeds energy loss and tary waves, and wave action generates a distinctive system of cur- the balance goes into an increase in orbital size and velocity. This is rents. This zone begins where depth equals one-half the deep-water why waves become higher. Where slope is low (less than 0.005; wave length (one-fourth suffices for some workers) and extends to Zenkovich, 1967), more energy is lost than concentrated, and shore. It may cover the entire continental shelf when major storm waves may lose height without breaking because orbital size and waves are active, but at other times it is restricted to much shal- velocity are reduced. In both cases, orbital shape changes from lower water. A brief qualitative review of processes and sediment circular to elliptical (very much flattened at the bottom), and orbi- transport within this zone will facilitate discussion of platform tal velocity becomes variable (water particles move faster shore- genesis. Our source materials include Emery (1960), Shepard ward than seaward). Wave transformation progresses until either (1963), Bagnold (1963), Inman and Bagnold (1963), Wiegel the form becomes too steep or orbital velocity overtakes wave (1964), Johnson and Eagleson (1966), Ingle (1966), Zenkovich velocity, at which point the wave breaks. Depth is generally about (1967), Komar (1971), Cook and Gorsline (1972), King (1972), 1.3 times breaker height. papers in Swift and others (1972), Davies (1973), Kulm and others Wave-drift ( transport) is negligible in the open sea, but in (1975), and references contained therein. The review assumes an the transition zone it produces a weak onshore that is exposed coast with a relatively steep submarine slope, appropriate accelerated where waves break. The main longshore current is for much of central California. found in the surf zone, and its velocity is a direct function of Downloaded from gsabulletin.gsapubs.org on September 15, 2015

CENTRAL CALIFORNIA WAVE-CUT PLATFORMS 443

Figure 13 (Continued).

? : Z u o c S u

- 200 o S

Wi —a» Wi

-100 _o We >r

HW 1 S < >-«-o m » . «» —»—iy-«iD-a>-go—«-e» n » ^ »» n « niBo^cfli-c o—•&»—w-o-8- -e—o—•

I 15 10 Santa Cruz

(km) breaker height and obliquity. Rip currents result where longshore head of a submarine , (2) burial in a prograding shoreline, currents turn seaward and are the primary means by which water tidal delta, or washover fan delta, (3) loss to inland , or (4) and sediment are moved through the breaker line. Backwash and comminution to a finer . undertow assist but only within the surf zone. Altogether these are the nearshore currents. They are the pre-eminent currents in shal- Marine Erosion low water, but their velocity and importance diminish outside the breaker line where they overlap with currents of other origin ( Mechanisms of marine erosion include quarrying, abrasion, so- drift, tidal, density). lution, and biological activity. All are facilitated if rock is weath- ered. This is clearly important on the sea cliff, where salt wedging and wetting and drying cause crumbling above some level of per- manent saturation. Weathering may also occur underwater, but its Directly or indirectly, waves are responsible for sediment trans- port in the nearshore zone. They initiate particle movement by their orbital motion, and they create currents that maintain the move- ment. Wave energy at the bed increases as water because of the increasing size and velocity of bottom orbits. Energy within the nearshore currents also increases as depth decreases. Both reach a maximum in the surf zone, where the greatest of sediment movement occurs. Fine-grained sediment (fine sand and mud) may remain more or less continually in suspension until rip currents move it seaward of the breaker line; thereafter it settles out or is moved elsewhere by other currents or by diffusion. It is relatively

444 BRADLEY AND GRIGGS

TABLE 3. INNER-EDGE ELEVATIONS AND PLATFORM TILTS

Platform Inner-edge Tilts (dimensionless) elevations Parallel to shoreline Perpendicular Resultant m/km (m) Northwest AI IS East to shoreline (segment IS) A I S of A of S (segment IS)

Quarry 0.0017 0.0085 0.0087 8.7 Blackrock 0.0021 0.0091 0.0093 9.3 Wilder 117(?) 0.0033 0.0016 0.0057 0.0066 6.6 Western 112 96 80 0.0028 0.0049 0.0010 0.0022 0.0038 0.0039 3.9 Greyhound 50 44(?) 0.0023 Highway 1 39 36 26 0.0020 0.0014 0.0006 0.0006 0.0009 0.0011 1.1 Davenport 23(?) 12(?) Note: A = crest of arch; I = inflection point; S = trough of sag; see Figure 13.

importance cannot be judged at present (Mackenzie and Garrels, their role more protective than destructive. The activity of or- 1965; Mackenzie and others, 1967; Rex and Martin, 1966; Martin ganisms has been best studied in the , where there is and Emery, 1967). little doubt about its contribution to rock destruction. Little is Quarrying refers to the disaggregation of bedrock particles, known, however, about in deeper water. which can range in size from mineral grains to large joint blocks. Because of the uncertainties mentioned above, the depth to Fluid drag and changes in pressure within pore fluids (water and which significant erosion of bed rock extends remains a matter of air) are responsible. Both are most effective in the surf zone and on dispute. The extreme estimates have been 200 m (Johnson, 1919) the cliff face. Drag and changes in pore pressure decrease rapidly and 10 m (Dietz and Menard, 1951; Dietz, 1952; Bradley, 1958). seaward of the breaker line. Zenkovich (1967, p. 168) favored 45 to 60 m. Abrasion in a broad sense includes all processes of mechanical wear (Kuenen, 1956). It depends on traction transport, and its WAVE CONDITIONS ALONG CENTRAL importance in the surf zone needs no defense because of the volume CALIFORNIA COAST and coarse grain size of material moved therein. Unfortunately, not enough is known about its importance in deeper water where fine If Ben Lomond platforms were cut at times of eustatically high sand is the abrasive material. The problem is not a lack of energy to sea level, then modern wave conditions should be a good analogue transport sand on the continental shelf; known or suspected trans- of past wave conditions. Wave data are available from two sta- port agents include major storm waves, internal waves, and tidal tions: one at Point Sur, 72 km south of Santa Cruz, the other off the currents (Chamberlain, 1964; Draper, 1967; Gorsline and Grant, coast at 37.6°N, and 123.5°W, 121 km to the northwest (U.S. 1972; Komar and others, 1972; Smith and Hopkins, 1972; Stern- Army Corps of Engineers, 1956; National Marine Consultants, berg and McManus, 1972; Southard and Cacchione, 1972; Stride, 1960). The following summary has been distilled from these 1972). However, some of this transport involves sand in suspen- sources, with supplemental help from Hyde and Howe sion, where it is incapable of abrasion. In addition, there are two (1925-1926), Scripps Institution of (1947), John- other problems for which data are inadequate. One is the thickness son (1953), Wiegel and Fuchs (1955), and Sorensen (1968). of the layer of activated sand. No abrasion of bed rock can occur Waves (both sea and ) reaching the Ben Lomond coast come unless the entire cover of sand is moved (C.A.M. King, 1951). The from all sectors of the northwest and southwest quadrants, al- other problem is grain size. Twenhofel (1945) and Kuenen (1955, though the great majority arrive from the northwest and west- 1956, 1958) have argued from field and laboratory evidence that northwest. The latter are extensively refracted by the time they fine sand is not an important abrasive in water transport. This reach shore, which means their breaker heights are less than they conclusion is open to question, however, where silicate sand is would be otherwise, but they retain sufficient obliquity to transport dragged over soft or friable bed rock (Kuenen, 1955). sand southeastward toward Monterey . Periods for all waves Solution is restricted to rocks with a carbonate content, and it is range from 5 to 20 sec. The largest waves are caused by winter impressive above the level of low tide (Fairbridge, 1950; Revelle and Emery, 1957; Kaye, 1959). It does not appear to be important TABLE 5. ESTIMATED AGES OF OLDER PLATFORMS in deeper water. (FOR AREA SOUTHEAST OF SCOTT CREEK, GREYHOUND PLATFORM EXCEPTED) Marine animals and plants carry on a variety of activities that cause weathering and erosion of bed rock (see Neumann, 1966; Platform Age* Healy, 1968; and their references). These include wedging by hold- From uplift ratef From tilt rate fasts, boring by mollusks and other organisms, and grazing by (Table 4) animals who don't mind ingesting a little rock with their algae. Plants sometimes serve as a baffle to fluid motion, which makes Quarry 1,200,000 1,000,000 Blackrock 900,000 1,050,000 Wilder 700,000 750,000 TABLE 4. DEFORMATION RATES DURING PAST 125,000 YR Western 450,000 450,000 Uplift Tilt Cement 260,000 Location (m/1,000 yr) (per 1,000 yr) Greyhound 160,000

Crest of arch 0.26 * Rounded to nearest 50,000 yr (20,000 yr for Cement and Greyhound platforms). Inflection point 0.24 ^ x f Using inner-edge elevations near Laguna Creek and uplift Trough of sag 0.16 (segment IS) rate of 0.186 m/1,000 yr. Downloaded from gsabulletin.gsapubs.org on September 15, 2015

CENTRAL CALIFORNIA WAVE-CUT PLATFORMS 445 storms, and a significant number arrive in the arc from northwest not the direction of bottom slope. Even though gradient does not to southwest. Waves with deep-water heights of 4.5 m can be directly guide the transport of coarse sediment, there are good expected five times a year and those with 6-m heights every 8 to 10 reasons for believing it is intimately related to the functions of yr. Waves as high as 7.5 m are known. Refraction will be slight if erosion and transportation. One is the similarity in shape of Ben they come from the southwest quadrant, and transformation will Lomond platforms. Another is the fact that we have measured no cause an increase in height of 15 to 30 percent, depending on gradients less than 0.005. We believe 0.005 is close to a minimum period (Sorensen, 1968, after Wiegel, 1964). The resulting breakers gradient for wave-cut platforms in central California. Minimum will be 5 to 9.5 m high, in water approximately 7 to 12 m deep. The gradients predicted by other workers have been 0.010 (King, 1963; modern platform out to a depth of about 10 m is sufficiently bare Fleming, 1965), 0.005 (Zenkovich, 1967, p. 163), and 0.0025 to provide a good substrate for kelp and rock-boring mollusks; (Zeuner, 1952). If these are valid, they indicate that erosion would outcrops, kelp, and mollusks are all scarce in deeper water. One be negligible on a flatter platform. reason for the small quantity of sand in shallow water is that Many people have had the notion that a flat platform dissipates littoral drift potential exceeds sand supply (see Sorensen, 1968, p. . It can be found early in the writings of Gilbert 659; Anderson, 1971). (1885) and Fenneman (1902), and more recently in quantified form Maximum on the Ben Lomond coast is about 2 m. in Keulegan and Krumbein (1949), Zenkovich (1967, with help Little is known about the strength of tidal currents on the continen- from V. Longinov), and others. A decreasing gradient means an tal shelf. expanding transition zone and an increasing loss of wave energy due to bottom drag. Zenkovich (1967) claimed that when gradient PLATFORM GENESIS reaches 0.005, bottom energy in the transition zone stays constant, and wave height will either stay the same or decrease. On a flatter Shore Platforms gradient, bottom energy and wave height would both decrease toward shore. To this, we add that wave refraction would be so Shore platforms on Ben Lomond Mountain are different from great that breaker height and obliquity would be too low to pro- the larger wave-cut platforms but are similar to shore platforms vide adequate longshore transport for available coarse sediment. If found in many parts of the world (see Mii, 1962; Bird and Dent, sand moves laterally at a slower rate than it is being supplied (from 1966; Healy, 1967, 1968; McLean and Davidson, 1968; Bird, and offshore areas, as well as from local erosion), it will clog 1969; Phillips, 1970; Russell, 1970, 1971; Hills, 1971, 1972; Gill, the surf zone and cause shoreline progradation, incapacitating ero- 1972, 1973; Davies, 1973; and references therein). Those parallel sional processes. A minimum gradient would leave the surf zone to bedding are easy to explain. Those that horizontally truncate with just enough vigor to keep up with the supply of sand. bedding are more of a problem. The ones near Davenport are Zenkovich's (1967) model seems to best explain the range in backed by a sea cliff that is extremely crumbly, presumably related platform gradients we have measured. It is not satisfactory, how- to frequent wetting and drying, whereas the platform itself, wet ever, for the platform concavity, because it predicts a convex shape most of the time, is noticeably more coherent (C. Wahrhaftig, (although Zenkovich's Fig. 33 disagrees with the prediction). This 1972, personal commun.). This supports the view of others that may be due to an assumption of no longshore movement of water such platforms are produced by preferential erosion of rock above or sediment, both invalid for central California. His explanation a level of saturation or secondary induration. What we can't ex- for concave beach profiles seems applicable to platforms: a steeply plain is their local distribution or consistent 2-m altitude. Local sloping bottom is needed so that backwash and undertow can uplift is not indicated on the plot of inner-edge elevations (Fig. 13); counteract the onshore movement of surf, the combination being a relationship to a former water table (Russell, 1971) merits further important in moving sand laterally. investigation. The offshore segment with its planar slope is not explained by any literature we have read. We will return to it later. Wave-cut Platforms Width. Platform width is influenced by at least two factors. One is bed rock. The Highway 1 platform is widest at Santa Cruz and Major storm waves from south of west break in water that is 7 to Año Nuevo Point, where moderately soft Purísima sandstones are 12 m deep. With some exceptions, which we do not understand, involved, and the Western and Wilder platforms are broadest near the junction between inshore and offshore platform segments oc- Santa Cruz, where they are carved in the friable Santa Margarita curs at an observed or estimated depth of 8 to 13 m (Fig. 12). We Sandstone. Bed rock is not' the whole answer, however, because conclude that the steep inshore segment of each platform corre- platforms become narrower to the northwest irrespective of rock sponded to the surf zone of major storm waves, and the flat off- type (Fig. 1). A second influence is suggested by an evident relation- shore segment was associated with the seaward part of the near- ship between width and gradient of inshore segments (Fig. 15). shore zone. We concur with Zenkovich (1967, p. 153) that plat- This indicates that platforms become flatter as they are extended forms are shaped by infrequent large waves. landward (at least until a minimum gradient is achieved), as was Gradient. Why do platforms slope seaward? Older literature proposed by Johnson (1919, p. 211) but rejected by Zenkovich offers two principal explanations: (1) they slope because their (1967, p. 154). An interplay between tectonism and eustacy is offshore part is oldest and therefore most eroded, and (2) a gra- indicated because platforms are widest where uplift has been least dient is needed for the removal of sediment (Davis, 1909; Johnson, — at Santa Cruz and Año Nuevo Point. Marine erosion during 1919; Twenhofel, 1939; Von Engeln, 1949; Kuenen, 1950; L. C. eustatic high stands of the sea would be less disrupted by slow King, 1951; Thornbury, 1954; Cotton, 1958). An explicit or im- uplift than by fast uplift. The occurrence of soft sandstones in both plicit part of this latter view is that sediment is removed in a sea- areas would have been an extra dividend for the erosional proces- ward direction. ses. We conclude that present variations in platform width are to Fine sediment capable of being suspended is transported to some extent a reflection of original variations in width, rather than offshore areas, but its movement is not dependent on a bottom purely a product of subsequent erosion. gradient — rip currents, wind-drift currents, tidal currents, and Question of Sea Level. It has been proposed that platform ero- diffusion function independently of a bottom gradient. Coarse sed- sion occurs chiefly in the storm-wave surf zone and that wide plat- iment only moves offshore when it is suspended. At other times, forms can only be cut if sea level rises (Bradley, 1958). This idea any net movement tends to be toward shore or along shore. Most has been supported (Fleming, 1965; King, 1963, 1972), questioned of it moves within the surf zone in a longshore direction, which is (Sorensen, 1968), and rejected (Zenkovich, 1967), and it remains a Downloaded from gsabulletin.gsapubs.org on September 15, 2015

446 BRADLEY AND GRIGGS matter of dispute because of uncertainty concerning the effective- sion, uplift of Ben Lomond Mountain leaves little doubt that plat- ness of erosion seaward of the breaker line. Our data do not resolve forms were cut by eustatically high . Stillstands occurred the matter, but they do better define certain aspects of the con- whenever uplift was cancelled by eustatic rise, and therefore each troversy. stillsand came at the end of a period of slowly rising sea level. If the concept of a minimum gradient is valid, then any platform Clearly, platform erosion by a rising sea was both feasible and in possession of one would have to be lowered uniformly while it is expectable. The question thus becomes, Did erosion at a time of being extended landward (as depicted by Zenkovich, 1967, p. stillstand consume all features cut while sea level was rising, or did 154). Bed rock at a depth of 30 m or more would have to be eroded any of the earlier features survive? Is the planar offshore segment of just as rapidly as bed rock at a depth of 12 m, an idea that needs platforms such a relict, somewhat chewed by organisms but other- bolstering by evidence before it can be accepted. Erosion by a rising wise still intact? Time may be an important consideration. Eustatic sea would circumvent this problem. However, the argument may high stands of the sea evidently were quite brief (Bloom and others, be more academic than practical because few platforms have the 1974). The time available for erosion would be considerably minimum gradient. As long as slope exceeds approximately 0.15 lengthened by utilization of the final phase of sea-level rise. inshore and 0.005 offshore, modification is presumably still possi- Genetic Summary. We propose the following tectonic-eustatic ble. Figure 15 shows that platform width and gradient are unre- model for development of the Ben Lomond marine terraces, stimu- lated for the offshore segment. This could be taken as evidence in lated by the ideas of Curray (1964), Wellman (1971a), and Bloom support of uniform platform lowering in deep water. However, use and others (1974). It assumes a uniform rate of uplift and a simple of minimum values for width makes interpretation of the data eustatic sea-level curve. questionable. Sea level on Ben Lomond Mountain depends on the relative If significant erosion occurs well outside the storm-wave surf importance of tectonic and eustatic influences. This is shown dia- zone, one may ask why the platform concavity does not do grammatically in Figure 16, where five stages in a resultant sea- likewise. Erosion by a rising sea seems better able to explain the level curve are indicated: stage 1 — sea level rising rapidly; eustatic planar character of the offshore segment. rise greatly exceeds uplift; stage 2 — sea level rising slowly; eustatic Kelp and rock-boring mollusks flourish at depths less than about rise slightly exceeds uplift; stage 3 — sea level constant; eustatic 10 m but are discouraged in deeper water by a continuous cover of rise balances uplift; stage 4 — sea level falling slowly; eustatic rise sand. Still, this does not disprove stirring of sand clear to bed rock slows to a stop and reverses; and stage 5 — sea level falling rapidly; by exceptional storm waves. Ancient platforms show mollusk holes eustatic fall reinforces uplift. at all depths, but it is hard to judge the significance of this because a Topographic features produced during stage 1 would be rela- variety of interpretations is possible. tively minor because submergence would keep marine erosion or Regardless of what one believes about the depth of marine ero- deposition in an embryonic condition. Platform cutting would commence in stage 2 and be completed in stage 3 (provided the land being submerged had a slope in excess of the minimum amount). Erosion would be at a maximum in the storm-wave surf 0.080- zone whose outer limit would be at a depth of approximately 13 m. INSHORE SEGMENT Nearshore processes would move fine suspended sediment out onto the continental shelf for dispersal by other agents. Coarse sediment would be moved alongshore toward Monterey Bay, with a proba- 0.060 ble ultimate destination in the Monterey or foredunes along the bay's southeastern shore. Platforms would flatten with time toward a minimum gradient of approximately 0.015 in shallow water and 0.005 in deeper water. This shape 0.040- would make wave energy at the bottom nearly constant (Zenk- • O • • ovich, 1967) and would keep sufficient vigor in the surf zone to I maintain the lateral transport of available coarse sediment. Stage 3 * o would last the longest at both ends of Ben Lomond Mountain, where uplift was least, allowing broad platforms to be carved in the a * soft sandstone of those areas. Erosion would be inconsequential in stage 4, because nearshore a< slopes would be so flat that wave energy would be dissipated well

200 400 600 offshore. Coarse sediment, partly from offshore sources, would clog the enervated surf zone and cause beach progradation. Thus

OFFSHORE 0.020- SEGMENT

• • 1—/ 1600

WIDTH (m) Figure 15. Platform gradient versus width, plotted by inshore and offshore segments. Gradients are those in Table 2, corrected for seaward tilting (Table 3). Some widths, especially those for offshore segment, are Figure 16. Diagrammatic sketch of uplift curve (U) and eustatic sea- minimum values because of inadequate data or subsequent erosion. Open level curve (E) during interglacial high stand of the sea (adapted from an circles = modern platform; triangles = Davenport platform; solid circles = idea of A. L. Bloom, 1973, personal commun.). Numbers refer to stages Highway 1 and Greyhound platforms; stars = higher platforms. discussed in text. Downloaded from gsabulletin.gsapubs.org on September 15, 2015

CENTRAL CALIFORNIA WAVE-CUT PLATFORMS 447 deposition and a relatively falling sea would both contribute to strandlines, in Bishop, W. W., and Miller, J. A., eds., Calibration of regression. The surf zone would migrate across the platform, but a hominoid evolution: Edinburgh, Scottish Academic Press, p. 19-36. lack of time and energy would leave it erosionally impotent. Mol- Broecker, W. S., Thurber, D. L., Goddard, John, Ku, Teh-Lung, Matthews, lusks, however, could continue their rock-boring activities. Topo- R. K., and Mesolella, K. J., 1968, Milankovitch hypothesis supported graphic features produced during stage 5 would again be minor by precise dating of coral reefs and deep-sea sediments: Science, v. because of the speed of emergence. Additional stages would follow, 159, p. 297-300. Chamberlain, T. K., 1964, Mass transport of sediment in the heads of associated with the ensuing low stand of the sea, but they will not Scripps submarine canyon, California, in Miller, R. L., ed., Papers in be discussed because their features have not been studied. (Shepard commemorative volume): New York, Mac- The tectonic-eustatic model visualizes broadly similar conditions millan Pub. Co., p. 42-64. during each high stand of the sea. This is consistent with the results Chappell, J.M.A., 1974, Geology of coral terraces, Huon , New of recent studies of late Quaternary changes in and sea level Guinea: A study of Quaternary tectonic movements and sea level (see Bloom and others, 1974, and references therein), and explains changes: Geol. Soc. America Bull., v. 85, p. 553-570. why wave-cut platforms on Ben Lomond Mountain are so similar. Clark, J. C., 1966, Tertiary stratigraphy of the Felton-Santa Cruz area, Santa Cruz Mountains, California [Ph.D. thesis]: Stanford, Calif., ACKNOWLEDGMENTS Stanford Univ., 184 p. 1970, Geologic map of the southwestern Santa Cruz Mountains be- tween Ano Nuevo Point and Davenport, California: U.S. Geol. Survey Surveying was competently handled by W. E. LeMasurier, R. R. Open-File Map, scale 1:24,000. Curry, and D. A. Coates, with able assistance from T. E. Silver, Cook, D. O., and Gorsline, D. S., 1972, Field observations of sand trans- A. J. Tamburi, and C. C. Bradley, Jr. The city of Santa Cruz gen- port by shoaling waves: Marine Geology, v. 13, p. 31-55. erously released detailed elevation data, and valuable information Cotton, C. A., 1958, Geomorphology: Wellington, New Zealand, Whit- from bedrock surveys was made available to us by the Pacific Ce- combe and Tombs, Ltd., 505 p. ment and Aggregates Company at Davenport, the Wm. Wriggley Curray, J. R., 1964, Transgressions and regressions, in Miller, J. L., ed., Jr. Company of Santa Cruz, and Coast Ways Ranch on Año Nuevo Papers in marine geology (Shepard commemorative volume): New Point. Our thanks to these colleagues for many fruitful discussions: York, Macmillan Pub. Co., p. 175-203. Davies, J. 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Form, genesis, and deformation of central California wave-cut platforms

W. C. BRADLEY and G. B. GRIGGS

Geological Society of America Bulletin 1976;87, no. 3;433-449 doi: 10.1130/0016-7606(1976)87<433:FGADOC>2.0.CO;2

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