Late Quaternary Eustatic Sea-Level Changes Along the Malibu Coast, Los Angeles County, California

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln

USGS Staff -- Published Research US Geological Survey

1972

Late Quaternary Eustatic Sea-Level Changes Along The Malibu Coast, Los Angeles County, California

Peter W. Birkeland University of Colorado at Boulder

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Birkeland, Peter W., "Late Quaternary Eustatic Sea-Level Changes Along The Malibu Coast, Los Angeles County, California" (1972). USGS Staff -- Published Research. 492. https://digitalcommons.unl.edu/usgsstaffpub/492

This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES ALONG THE MALIBU COAST, LOS ANGELES COUNTY, CALIFORNIA1'2

PETER W. BIRKELAND Department of Geological Sciences, University of Colorado, Boulder, Colorado 80302; and U.S. Geological Survey, Denver, Colorado 80225

ABSTRACT Late Quaternary relative sea-level changes along the Malibu coast can be deciphered by study of geomorphic features and marine and associated stream deposits. Evidence for such changes includes (a) stream downcutting in large valleys during low stands of sea level, (b) stream aggradation in the same valleys during high stands of sea level, and (c) marine shoreline angles whose altitudes coincide with the tops of valley-fill stream deposits. This evidence, and open-system uranium-series dates for shells from the Corral and Dume terrace deposits, provide a time scale for certain sea-level changes. Evidence from other coasts of the United States suggests that sea level was close to its present position about 30,000 years ago. Such a post-Dume, pre- Holocene relatively high stand of sea level has not been recognized in marine deposits along the Malibu coast. There is evidence, however, for stream aggradation at possibly 30,000 years B.P. Sea-level position at that time cannot be determined, but considerations of tectonic rate of uplift suggest that it was no higher than about -30-foot altitude. Most of the relative sea-level changes identified here are probably due to eustatic variations in sea level. Tectonic movements complicate the interpretation of the stratigraphic and geomorphic record; however, because movements seem to have been consistently upward, relative rises of sea level are interpreted as eustatic and relative falls as partly eustatic. Eustatic origin is also suggested by the similarities in ages of sea-level fluctuations between the Malibu coast and the coasts of Alaska, Barbados, Mallorca, and New Guinea.

INTRODUCTION series shell dates, and then (3) to compare Marine terraces along the California the resulting sea-level variation curve with coast have drawn the attention of many the curves from coasts in other parts of the geologists over the years. Despite this, lit- world. Data on eustatic sea-level variation tle is known of their relation to eustatic must come from detailed study of both ma- and variations in sea level, and little progress rine and nonmarine deposits of coastal has been made in methods used for terrace tectonics. Stream deposits whose tops coincide altitude with shoreline angles correlation. Data presented in recent re- in marine views by King (1965) and by Wahrhaftig can, by their thickness, be used to help dif- laid down and Birman (1965) indicate that the alti- ferentiate marine terrace deposits during a continuously falling sea level, rela- tude of either terrace or shoreline angle is tive to land, from those laid down during a of little use in long-range correlation along fluctuating sea level. This kind of stream- the coast, mainly because of differing Qua- deposit record is best seen in large valleys ternary tectonic movements. that head in the interior of the Santa Moni- The purposes here are (1) to present ca Mountains. Four relatively high stands stratigraphic data for the Malibu coast of sea level during the last 150,000 years are which indicate that some marine terrace recognized. Each high stand is separated deposits relate to eustatic changes in sea from the others by relatively low stands of level, (2) to point out the relation of these sea level. That the swings in sea level are deposits to previously published uranium- mainly of eustatic origin is suggested by 1 Manuscript received August 11, 1971; revised analysis of tectonic movements, as deduced February 8, 1972. from shoreline-angle altitudes and assumed 2Publication authorized by the Director, U.S. altitudes of interglacial sea levels and by Geological Survey. similarity with dated sea-level fluctuations [JOURNALor GEOLOGY,1972, Vol. 80, p. 432-448] for coasts in other parts of the world. @ 1972.The Universityof Chicago.All rightsreserved. Location and geologic setting.-The area 432 LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES 433 of study is the Malibu coast along the south terglacial high stands of sea level, that sea side of the Santa Monica Mountains (fig. 1). level was low between the times of forma- Quaternary deposits between Bass Rock tion of successive marine terraces, and that and Topanga Canyon (fig. 6) were mapped the times of low sea-level position correlate at a scale of 1:20,000. In addition, recon- with times of glaciations elsewhere. He pre- naissance studies were made westward to sented little field evidence, however, to the Oxnard Plain along the coast, eastward demonstrate transgression and regression. to the city of Santa Monica, and at the Pa- He named the two prominent emergent los Verdes Hills. marine terraces Dume and Malibu (the The most recent summaries of the geol- older). He also recognized tilting of the ogy of the central Santa Monica Moun- Dume terrace. Although he correlated ter- tains are by Yerkes and Wentworth (1965) race deposits eastward along the coast to and Campbell and others (1966). Sedimen- Santa Monica with the Dume terrace, it tary and volcanic rocks of Cretaceous to now seems that some of these deposits Miocene age make up most of the bedrock could be pre-Dume in age. In the present along the coast. Movement on some faults, study an effort was made to correlate the which displace many of the bedrock units, Malibu coast terrace deposits with those has continued into the late Quaternary, as near Santa Monica, but no satisfactory ba- evidenced by local displacement of marine sis for correlation was obtained. terrace deposits. The only other detailed study of the Previous work.-Davis (1933) has de- Quaternary deposits in this area is the work scribed the geomorphic relations and Qua- of Yerkes and Wentworth (1965). They ternary stratigraphy of the Malibu coast, recognized a marine terrace intermediate and many of his findings have been verified in position between the Dume and Malibu in this study. His main conclusions were terraces, located shoreline-angle positions, that the marine terraces were cut during in- described upper Quaternary subaerial de-

FIG. 1.-Location map of area. Mountainous areas are shaded. 434 PETER W. BIRKELAND posits, soils, and deformation in considerable detail; and provided the shells for the A initial uranium-series dates on the marine terrace deposits. More recently, Szabo and Rosholt (1969) refined the dating methods and reported all age determinationsto date. Detailed geologic maps of the coastal area (1:12,000 scale) have been released to open file by the U.S. Geological Survey (Yerkes and others 1964, 1971; Campbell and others 1970).

SEDIMENTARYDEPOSITS Two kinds of sedimentary deposits- B beach and stream-are essential for recon- struction of the Quaternaryhistory, mainly because their interrelations and altitudes provide control points for the eustatic sea- level variation curve. Gravel roundnessand sorting of <2-mm material are the main criteria used to distinguish one type of deposit from the other (figs. 2, 3). Beach deposits, consisting predominantly of well-sorted sand, rest on marine-abrasion platforms and range in thickness from several inches to 21 feet. Gravel, where present, is generally found at the base of the deposits. The clasts, most of which are pebble FIG. 3.-Roundness of 1-3-inch clasts in marine size, are subrounded to rounded, and their (A) and stream (B) gravel. Determined visually using chart of Powers (1953). surfaces are fairly smooth and display little microrelief. Some beach deposits are unoxi- dized, or only slightly oxidized, and are not indurated, whereas other deposits are brown (7.5 YR 5/4) and are moderately indurated; there are no consistent trends with age, however. (Color notations are from the Munsell Soil Color Charts, 1954 edition.) Stream deposits, consisting typically of interbedded sand and gravel, are common in most valleys and vary in thickness from veneers several feet thick to valley fills more than 100 feet thick. The sand fraction is moderately to poorly sorted, and the gravel clasts generally are subangularto subrounded with slightly irregularmicrorelief. Gravel ranges in size from pebbles to 5-foot boul- ders.

1.0 0.063 0.004 0.002 Alluvial-fan deposits occur at the foot GRAIN DIAMETER(MM) of most steep slopes, and in many places FIG.2.-Cumulative curves for < 2-mm fraction they rest on the beach and stream deposits. of marine terrace and stream deposits. Thicknesses vary from several feet to more LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES 435

than 100 feet. Characteristic features in- are near Point Dume; elsewhere,deposits of clude poor sorting of both matrix and gravel this age occur only as isolated remnants. sizes, crude layering parallel to fan surfaces, Because all terrace deposits are discontin- the common occurrenceof clay-rich layers, uous east of Malibu Creek, correlationthere and only slight rounding of gravel clasts. is difficult and based only on altitude, position in terracesequence, and on the assump- QUATERNARY STRATIGRAPHY tion that the tectonic style west of Malibu The stratigraphic nomenclature is modi- Creek-uptilting to the east-continues fied from that of Davis (1933), who first east of Malibu Creek. described the Malibu and Dume terraces. Pre-Malibu deposits.-Marine sands and Yerkes and Wentworth (1965) named the stream deposits at altitudes higher than the intermediate terrace near Corral Canyon nearby Malibu terrace are considered to be terrace C; it is here designated the Corral of pre-Malibu age. They are most common terrace. north and west of Point Dume (figs. 4, 5, 6). The relative ages and correlations of Because these deposits are at various alti- Quaternary deposits along the Malibu tudes, several ages probably are included. coast are primarily based on continuity and The marine deposits cannot be related to a altitude. In addition, stratigraphic relations eustatic high stand of sea level because none suggest that for any part of the Malibu of the deposits can be correlatedwith a non- coast the marine and stream deposits are marine valley fill. A 100-foot-thick stream progressively older with increasing altitude. deposit whose top is about 350 feet in alti- Other correlation methods that were tried tude crops out along the valley side just included analysis of soils and heavy-mineral west of Malibu Creek (fig. 5). Although a etching. Soils proved to be of little use in deposit in that position and of that thick- correlation, because in most places alluvial- ness is suggestive of stream aggradation fan deposits covered the marine sediments during a rise in sea level, no marine deposits before much soil formation could take place. with which it might correlate are nearby. Bradley (1957, 1965) has demonstratedthat Malibu marine and stream deposits.- the degree of etching of heavy minerals and Malibu marine terrace deposits are related selective loss of heavy minerals during to a shorelineangle that is between 200 and weathering were effective tools for correlat- 250 feet in altitude near Point Dume (fig. ing marine deposits in the Santa Cruz, Cali- 6). The best exposures are in the highway fornia, area. A similar approach attempted cut north of Point Dume (Davis 1933, p. in this study was not successful. Slightly al- 1064), where 9 feet of nonmarine alluvium tered marine sand, collected several feet rests on 10 feet of marine sand and gravel, above the bedrock platform from many of which in turn rests on the bedrock platform the terrace deposits, was analyzed. In most (loc. M, fig. 4). A 125-foot-thick stream de- samples, pyroxene grains are highly etched, posit along Zuma Creek has a top at an al- whereas hornblende grains are unetched or titude close to that of the nearby Malibu only slightly etched. This relation occurs in shoreline angle (fig. 4). This suggests that Quaternary marine terrace deposits of all Zuma Creek was aggrading just prior to ages along the Malibu coast, including those formation of the Malibu terrace, which in near Santa Monica. turn suggests that the Malibu terrace was Terrace continuity varies along the coast. formed during a transgression. The Dume terraceis nearly continuous from Corralmarine and streamdeposits.-Cor- Little Sycamore Canyon to Malibu Creek, ral marine terrace deposits are related to a whereas the Corral terrace, though recog- shoreline angle that is between about 150 nized near Point Dume, is continuous only and 176 feet in altitude between Point from Corral Canyon to Malibu Creek. The Dume and Malibu Creek. The best expo- largest expanses of Malibu terrace deposits sure in that area is just west of the mouth 436 PETER W. BIRKELAND

FIG. 4.-Quaternary geology of the Malibu Coast near Point Dume of Corral Canyon (loc. C, fig. 5), where as 6). Hence, data from the stream deposits in- much as 20 feet of marine deposits rest on dicate that a major regression occurred be- the marine abrasion platform (Yerkes and tween the formation of the Malibu and the Wentworth 1965, p. 36-40, 157-158). Corral marine terraces. Yerkes and Wentworth (1965) also de- Three open-system uranium-series shell scribed other occurrences of Corral marine dates have been obtained for the Corral ter- terrace deposits that were exposed in arti- race deposits (Szabo and Rosholt 1969). ficial trenches just east of Corral Canyon. They range from 115,000 + 15,000 years Valley cutting and subsequent stream to 154,000 + 30,000 years, and average aggradation preceded formation of the Cor- 131,000 + 15,000 years. ral marine terrace. Deposits of stream grav- Dume marine and stream deposits.-Dume el, as much as 125 feet thick, crop out in marine terrace deposits are related to a several places near the mouth of Malibu shoreline angle that varies from less than 50 Creek. They are though to be of Corral age feet in altitude at the western end of the because their tops are close in altitude to study area to 130 feet in the eastern part that of the nearby Corral shoreline angle. (fig. 6). The best exposures are in the sea In addition, stream deposits more than 150 cliff just east of the mouth of Zuma Creek feet thick crop out along the east side of the (loc. D, fig. 4), where the relations of marine mouth of Las Flores Canyon and contain in and nonmarine deposits can be explained their upper part a sand unit of probable main terms of eustatic sea-level rise. At that rine origin. This sand lies just south of, and locality, 18 feet of marine sand and gravel close to the altitude of, the 175-foot abra- rest conformably on 59 feet of stream de- sion platform of probable Corral age (fig. posits laid down by Zuma Creek. The base LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES 437

record is interpreted to mean that while sea EXPLANATION level was rising Zuma Creek aggraded, and that only near the end of the period of transgression did the sea get close enough to the Alluvium Beach sand present shore to deposit marine sand and gravel on previously deposited stream alluvium. Some deposits mapped as stream depos- Post-Dume deposits its may have been deposited in an estuarine Stream sand and gravel environment. After this study was com- pleted, fossils were collected by C. M. Wentworth at locality D (fig. 4) from a Dume deposits position about 8 feet below the top of the Qdm, marine sand and gravel stream deposit. J. E. Hazel (written com- Qds, stream sand and gravel munication, 1970) has identified them as Ostracoda, with Cyprideis beaconensis being abundant and Cypridopsisvidua scarce. Corraldeposits He interprets this assemblage as indicating sand and gravel Marine deposition probably in an estuary that had iO:mm J:::^Qms a salinity in the oligohaline range. Several valleys east of Point Dume also Malibudeposits contain stream deposits related to the Dume omm, marine sand and gravel terrace, either as fill- or strath-terrace de- Qms, stream sand and gravel posits (figs. 4, 5). Age assignment is based either on position directly beneath Dume marine deposits or because the gradient on Pre-Malibu deposits their tops projects close to the position of Qpmm, marine sand and grovel the Dume shoreline angle. Qpms,stream sand and grovel These relations of marine and nonmarine deposits suggest that the Dume and Corral Contact exposed or well expressed marine terrace deposits were separated by in topography a period of major marine regressionand valley deepening. similar mag- Contact concealed or not well expressed A regressionof in topography nitude followed formation of the Dume terrace. Contact buried by thick alluvial fan Seven open-system uranium-series shell or eolian deposits dates have been obtained for the Dume M marine deposits (Szabo and Rosholt 1969). Localitymentioned in text They range from 95,000 _+ 25,000 years to 112,000 _+ 15,000 years, and average 104,000 ± 5,000 years. of the stream deposits is not exposed and Post-Dume deposits.-Stream deposits in extends below present beach level. Up- several major valleys form terraces that ei- stream along Zuma Creek are stream-fill ther are below or their tops project to an terrace deposits. The tops of the terrace altitude below that of the Dume shoreline remnants grade downvalley to the altitude angle (figs. 4, 5). A pre-Holoceneage is sug- of the Dume shorelineangle, thus indicating gested by their position above recognized that the stream terracedeposits and the ma- Holocene alluvium and by their brown color rine and nonmarine deposits in the sea cliff (7.5YR); in contrast, Holocene alluvium all relate to the same transgression. The generally is yellow brown (10YR 3/4- EXPLANATION

Alluvium Beach sand

Post-Dume deposits Stream sand and grovel

Dume deposits Qdm, marine sand and grovel Qds, stream sand and gravel

Corral deposits Qcm, marine sand and gravel Qcs, stream sand andgravel

Malibu deposits Marine sand and gravel

Pre-Malibu deposits Stream sand and gravel

Contact exposed or well expressed in topography

Contactconcealed or not well expressed in topography

Contactburied by thick alluvial fan or eoliandeposits

Fault with late Quaternarydisplacement Bar and bao/llon downthrownside C FIG. 5.-Quaternary geology of the Malibu Locality mentionedin text Coast near the mouth of Malibu Creek.

10YR 5/4). This browner color seems to be partly age dependent. the highest nearby beach sand. Such low Alluvium.-Stream deposits along most terraces could have been formed during valley bottoms are judged to be of Holocene stream incision accompanying Holocene sea- age because either they are graded to pres- cliff retreat. ent sea level or they occur as low terraces Table 1 gives the time scale for sea-level whose tops are no more than 25 feet abov changes in the terrace deposits. LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES 439

TABLE 1 between 109 and 120 feet just west of Point TIME SCALEFOR SEA-LEVEL CHANGES Dume, and then decreasesmore rapidly far- ther west until it lies between 25 and 40 feet

Relative Stream Average Age in altitude just east of Bass Rock. West of Marine Average Age Sea-Level Activity in Bass Rock in Santa Mountains Terrace (Years B.P.) the Monica Position Large Valleys neither Davis (1933, p. 1069-1070) nor I High Aggradation <10,000t located any Dume deposits. At the west end Low Downcutting of the Santa Monica Mountains young al- Rela- makes up Plain. tively luvium the Oxnard These high? Aggradation 230,000f relations suggest that the Dume shoreline Low Downcutting angle is close to or below present sea level High Dume Aggradation 104,000 ±5,000 Low Downcutting at the western edge of the Santa Monica High Corral Aggradation 131,000±15,000 Mountains (Davis 1933, p. 1051-1052). Low Downcutting Large-scale tilting of pre-Dume deposits High Malibu Aggradation Low Downcutting is less well defined, because the deposits are less extensive and more poorly preserved Being cut at present. and because correlation east of Malibu t No absolute dates; ages are estimated and based on corre- Creekis angle lation with events elsewhere. difficult. The Corralshoreline is between 154 and 175 feet in altitude at The thickness of Holocene alluvium in Point Dume and rises eastward to 190 feet the major stream valleys indicates deposi- at Las Flores Canyon. Altitudinal control tion of the alluvium during a major trans- on the Malibu shoreline angle indicates that gression, the Monic transgression of Davis it rises from between 211 and 236 feet at (1933). Few water wells drilled into this al- Point Dume to 318 feet just west of To- luvium reach bedrock; those that do may panga Canyon. In that section of the coast not be located at the deepest part of the for which there is altitudinal control on all valley fill. Davis (1933, p. 1078) reported a three terraces, the data on the Dume and thickness of 76 feet for the alluvium at Corral terraces can be interpreted as indi- Sycamore Creek, and the records of the cating that the terraces are nearly parallel, Marblehead Land Company (oral commu- and hence most of the differential uplift is nication, 1968) indicate minimum thick- post-Dume. In contrast, the Malibu terrace nesses of 89 feet for the alluvium at Zuma appears to rise more rapidly eastward than Creek and 105 feet for the alluvium at the Corral terrace, indicating that some of Malibu Creek. the differentialmovement is pre-Corral. The relatively high altitude above pres- LATE QUATERNARYTECTONIC MOVEMENTS ent sea level of many of the marine deposits The amount of deformationof the marine indicates that much of the Malibu coast terracedeposits is indicated by the variation has been rising in late Quaternary time. in altitude of abrasionplatforms and shore- Guilcher (1969, p. 73) recently reviewed the line angles along the coast (fig. 6). Regional data on the positions of interglacial sea tilting arching, and faulting have all con- levels and concluded that the levels may tributed to this deformation. have been no more than a few meters above The Malibu coast has been tilted in present sea level. If this is correct, then post-Dume time, the overall relative move- most of the marine terrace deposits studied ment being up in the east and down in the here must have been uplifted. This conten- west (Davis 1933, p. 1051-1052). The east- tion is supported by the fact that marine ernmost measured altitude on the Dume deposits in the study area are as high or shoreline angle is 130 feet just west of To- higher in altitude than marine deposits of panga Canyon (fig. 6). Westward, the Dume nearly equivalent ages on other coasts shoreline angle decreases only slightly to (fig. 9). That this upward movement has fig. using 49). FIG.

California

6.-Altitudes of Division of EXPLANATION

FOR shoreline MAP Highways angles,

abrasion topographic

maps CROSS platforms,

(5-footand sea TERMINOLOGYSECTION contour cliffs USED of SHOWING interval; various scale of ages 1

inchalong = the 100 Malibu feet) for coast.

control, EXPLANATION andAltitudinal FOR fromdata are Yerkes SECTION

andmainly

from

hand Wentworth

(1965,leveling LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES 441 been going on for a long titnie is indicated served in good exposures (Yerkes and Went- by the presence of older deposits at pro- worth 1965, p. 159-174); maximum vertical gressively higher altitudes. separation, perhaps the result of tectonic Some rough rates of late Quaternaryup- movement, is 17 feet along a north-dipping lift can be calculated from the above data thrust fault. Corralmarine deposits are also and the radiometricages of Szabo and Ros- faulted at locality F3 (fig. 5), where the holt (1969) for the Dume and Corral ter- marine abrasion platform and overlying races. These rates amount to 1-1.2 feet per marine deposits are displaced along two 1,000 years near Point Dume to 1.2-1.5 faults, both down on the north side and with feet per 1,000 years between Las Flores and total vertical separation nearly 47 feet. Tuna Canyons. Pleistocene deposits have been displaced SEA-LEVEL VARIATION AND ITS ORIGIN along faults in several places. Pre-Malibu The stratigraphic and altitudinal rela- nonmarine deposits are vertically separated tions of both marine and nonmarine depos- a minimum of 12 feet at locality Fl (fig. 4) its can be used to construct a curve for rela- north of Point Dume (Yerkes and Went- tive changes in sea level, as discussed by worth 1965, p. 151). Deformation of Cor- Upson (1949) and Alexander (1953). The ral marine deposits just east of Corral Can- effects of the Holocene transgression serve yon (loc. F2, fig. 5) is complex and only ob- as a model (fig. 7). Sea level was low during

T-3: Marine terrace deposits related to old shoreline angle

<--YOUNGER SEA LEVEL

SEA LEVEL VAR/AT/ON DEPOSIT T-6 IT-5 T-4 T-3 T-2 T-l TIME SEA LEVEL VARIATIONCURVE

FIG. 7.-Schematic cross section at lower end of large stream valley showing interrelations of marine and nonmarine deposits and resulting sea-level variation curve. 442 PETER W. BIRKELAND

Wisconsin time and valleys were deepened ent position (some 200 feet below the Dume (T-4, fig. 7). As glaciers melted, sea level shoreline angle), and it subsequently has rose to its present position (T-6); this rise risen to its present position and has caused led to stream aggradation of the major val- widespread valley aggradation. leys (T-5). Because the Wisconsin shoreline Several processes may account for the lay seaward of the present shoreline, the al- major relative sea-level fluctuations rec- titude of the base of the Holocene stream ognized here. These include tectonic move- deposits at the present shoreline is at or ments of continents and ocean basins, eu- above the altitude of the Wisconsin shore- static variations in sea level related to line. Stream-fill terrace deposits (T-2) changes in glacial-icevolumes, and isostatic whose top coincides with an adjacent ma- adjustments resulting from the loading and rine shoreline angle (T-3), or whose surface unloading of continental shelves by rising gradient projects downvalley to a marine and falling sea level (Nansen 1921; Hig- shoreline angle, also indicate stream aggra- gins 1965). The likelihood of late Quater- dation during a rising sea level. The alti- nary upward tectonic movements along the tude of the base of the filled valley (T-l) is Malibu coast has been discussed. Provided at or above the preceding low stand in sea the direction of tectonic movement has not level. The difference in altitude between the changed with time, regressioncould be due, base of the filled valley (T-l) and the marine in part, to consistent upward movement of shoreline angle that formed at the end of the coast. The probable rate and magnitude the transgression (T-3) thus provides a min- of many of the regressions,however, suggest imum figure for the relative change of sea that local tectonic movements probably are level. Major stream incision of the marine not the major cause of the regressions; eu- and stream deposits (T-4) is taken as evi- static sea-level loweringseems a more plaus- dence for a subsequent lowering of sea level. ible explanation. Transgressions must be Finally, the difference in altitude between the result of eustatic rise in sea level along present sea level (T-6) and an old shoreline a tectonically rising coast. Higgins (1965, angle (T-3) may be partly due to coastal 1969) and Bloom (1967) have tried to de- uplift. cipher the effect of isostatic adjustments Four cycles of relative sea-level variation caused by water loading and unloading are recognized (fig. 9). Sea level was low along coasts far removed from areas for- prior to formation of the Malibu terrace. merly glaciated. As they pointed out, a As sea level rose to the altitude of the Mali- coast may respond isostatically by going bu shoreline angle, streams aggraded and down during a high stand of sea level, deposited 125 feet of sediment, a minimum whereas the same coast may respond by go- figure for that rise in sea level. Sea level ing up during a low stand of sea level. The then lowered to a position at least 125 feet overall history one reads from the strati- below the Corral shoreline angle. Sea level graphic record is a rise and fall in sea level then rose to the Corral shoreline angle, and that may be eustatic, but some fraction of the transgression was recorded in one place the relative rise and fall would result from by at least 125 feet of stream deposition. isostatic adjustment. Both Bloom (1967, After formation of the Corral terrace, sea p. 1490) and Higgins (1969, p. 143) sug- level lowered to at least 100 feet below the gested that isostatic adjustments may ac- level of the Dume shoreline angle, as de- count for vertical movements of as much termined by the thickness of Dume stream- as one-third the average thickness of the fill deposits along Zuma Creek. The rise overlying water column. in sea level that followed resulted in aggra- It is concluded that the major marine dation of major stream valleys and forma- transgressionsand regressionson the Mali- tion of the Dume terrace. Sea level then bu coast are mostly eustatic in origin and lowered to at least 105 feet below its pres- that this recordis imprinted on a tectonical- LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES 443 ly rising coastline. Isostatic adjustments rine terrace deposits since the reviews by theoretically should have occurred, and King (1965) and Wahrhaftig and Birman they probably account for part of the rela- (1965). Despite the serious problems of tive fluctuations observed. It is not possi- contamination and the various ways of ble, however, to separate the eustatic from calculating the radiometric ages from shell the isostatic component. material, there is a considerable amount of age agreement from widespreadlocalities CORRELATIONWITH OTHER DATED MARINE (fig. 8). For more precise correlation, how- TERRACE DEPOSITS IN CALIFORNIA ever, it would be desirable to have data on Some absolute ages that aid in correla- eustatic sea-level variation for all dated tion have been obtained for California ma- portions of the coast.

TERRACE AGE.YEARS 8.R LOCATION OR (Numberin parenthe REFERENCE si is FORMATION. .. age determinotion)number of

68,000-10,000 Lowest i too. Brodleyand Santo Cruz *r t 100,000 7,000 B and terrace (5) (1968) Oneyounger date is anomalous

Lowest 130,000-30,000 h and Cayucos emergent 140,000 30,000 Valentine terrace (2) (1967)

95,000±25,000 to trrc 112,000+15,000(7) Santa Avg:104,000t5,000 Szbo Monico Rosholtand Mountain-s 1l5,oo0+15,o00to (1969) rra4,000± 30,000 Av1031,000-15,000 V i Av000,O t9)00to TerraCorral e 29 of >'2OpOO*-t2,000(mt)200000 (min.)to V 3,000_12,000(min) to Vale Son Nicolas Norris(1963) 120,00o12,000(mo, and Island (I) Veeh (1969) Terrace9 of 200,000 (m.)000to Vedderand 20,000 (r.) Norris(1963) 200,000 (max.) Polo ddr and 95.0001500070,000+10,000 Szabo to and 110000±15,000 Rosholt

Verdes Ave: 86,000±9,000 95,000± VerdestoPalos Sand 15,000 Hills 130,000±20,000 Fonale (6) and 330,000±50,000 Trroc at Schaeffer965) 1,230 feet 420,000±60,000 alt itude

FIG. 8. -Dated marine terrace localities along the California coast 444 PETER W. BIRKELAND Deposits of the lowest emergent terrace emergent marine terrace, and evidence for at Santa Cruz appear to have been laid eustatic origin of the terrace was not rec- down during the youngest identified pre- ognized either by Woodring and others Holocene high stand of sea level (Bradley (1946) or by me during a 1-day reconnais- 1957, 1965). Bradley attributed at least part sance study. Radiometric ages of Palos of the origin of the terrace to eustatic sea- Verdes Sand determined by Szabo and level variation. Alexander (1953) worked Rosholt (1969) should be used for compari- just south of Bradley and recognized the son with the Malibu coast, because they same marine terrace. The presence of a have dated shell material from both areas stream-fill terrace graded to the lowest by the same methods. The Palos Verdes emergent marine terrace suggests formation Sand is about the same age as the Dume of the latter during a eustatic high stand of terrace deposits, or slightly younger. Other sea level (Alexander 1953, p. 23). Thus, the low terraces in the Palos Verdes Hills have lowest emergent terrace at Santa Cruz not been dated. The age of the 1,230-foot- seems correlative with the Dume terrace high terrace as determined by Fanale and on both geologic evidence and radiometric Schaeffer (1965) is questioned, because the ages. Although Bradley (1965) reported method used (based on He-U ratios) also progressive weathering of heavy minerals yielded a date of 155,000 + 30,000 years with increasing age of marine terrace de- for the Lomita Marl. This date is not com- posits, this weathering was not found along patible with either the geologic age of the the Malibu coast; hence, there is no basis Lomita Marl (earliest Pleistocene [Wood- for correlating the older deposits in the two ring and others 1946]), or its K.Ar age on areas. authigenic glauconite of 3 m.y. (Obrado- Two radiometric ages for marine terrace vich 1968). deposits at Cayucos suggest correlation To summarize, deposits of the lowest with the Corral terrace or older deposits. emergent marine terrace in several places According to Veeh and Valentine (1967) along the California coast have somewhat the younger age given in figure 8 is for similar radiometric ages. In addition, some coral and is the more reliable of the two, of these deposits show evidence of deposi- although both should be considered mini- tion during the last eustatic pre-Holocene mum. high stand of sea level. This terrace is about Terrace 2 on San Nicolas Island has a 100,000 years old. Deposits of the next-older shoreline angle near 100 feet in altitude and eustatic high stand of sea level (Corral ter- is the lowest well-preserved marine terrace race) are more difficult to correlate. More (Vedder and Norris 1963). Although no stratigraphic- and radiometric-agedata are evidence is given for a possible eustatic needed to separate these two youngest high origin for the terrace, the age determina- stands of sea level. Still older deposits are tions suggest correlation with either the difficult to correlateby stratigraphicmeans, Dume or the Corral terrace. Terrace 1 has and some may be too old to date by the ra- a shoreline angle at about 25-30 feet in al- diometric methods in use at present titude, but it is not well preserved, nor has (Broecker 1965, p. 737-740), it been dated. The age of terrace 9 is only a minimum, and therefore cannot be used PROBLEM OF RECOGNIZING MID-WISCONSIN for correlations with the mainland. EUSTATIC HIGH STAND OF SEA LEVEL The flight of 13 marine terraces in the IN CALIFORNIA Palos Verdes Hills (Woodring and others Many workers in other areas recognize 1946) makes the area important with re- one or more eustatic high stands of sea level gard to marine terrace origin, because of during the Wisconsin or Wiirm Glaciation the number and altitudes of the terraces. (Guilcher 1960, p. 85-86). Most work on The Palos Verdes Sand lies on the lowest this problem, for coastal areas of contermi- LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES 445 nous United States far removed from for- per 1,000 years. If we assume a uniform rate merly ice-covered areas, has been done on of uplift, mid-Wisconsin marine deposits- the east and Gulf coasts. Estimates vary, if laid down at the position of present sea however, on the altitude and precise age of level-should now be some 30 feet above such sea-level maxima. Curray (1965) spec- present sea level. The lack of such marine ulated that sea level rose to slightly below deposits may indicate that the mid-Wiscon- its present level sometime between 22,000 sin high stand of sea level was no higher and 35,000 years B.P., and Milliman and than about 30 feet below present sea level. Emery (1968) believed that it was close to This is supported by recent data from New its present position 30,000-35,000 years Guinea which suggest that the mid-Wis- B.P. In contrast, Hoyt and others (1968) consin high stand of sea level (between inferred that sea level was slightly above 35,000 and 50,000 years B.P.) could have its present position at about 25,000-30,000 been as much as 100 feet below present sea years B.P., and again at 40,000-48,000 level (Veeh and Chappell 1970; see also fig. years B.P. Schnable and Goodell (1968, p. 9). 50-51, 59) also suggested that sea level was Some post-Dume, pre-Holocene stream close to its present position, or slightly deposits may be related to a mid-Wisconsin above it, sometime between 24,000 and high stand of sea level. Both strath-terrace 40,000 years B.P. Osmondand others (1970) and fill-terrace deposits are present. The reported on a high stand of sea level dated strath-terracedeposits could, however, have at about 30,000 years ago. The postulated been laid down during the downcutting that high stands of sea level generally are be- accompanied the post-Dume regression. lieved to be mid-Wisconsin in age. M6rner Fill-terrace deposits, however, record a (1971) has questioned the existence of such change in stream regime from downcutting high stands at that time. to aggradation, perhaps caused by a rise in Little evidence exists for a mid-Wiscon- base level; evidence for this is best seen at sin high stand of sea level along the Cali- the mouth of Trancas Creek (fig. 4). There, fornia coast. Marine deposits of that age Dume marine deposits rest on a bedrock have not been recognized, the only possi- platform both to the northwest and south- ble exception being deposits dated at east, but on approaching the valley axis, > 50,000 years B.P. in Tomales Bay, north the marine deposits and platform are trunc- of San Francisco (Richards and Thurber cated by post-Dume stream-fill deposits of 1966). However, Alexander (1953, p. 37-38) Trancas Creek. The stream deposits extend found stream terrace deposits at altitudes from below present highway level (14 feet) below the lowest emergent marine terrace to an altitude above that of the marine de- in several valleys south of Santa Cruz, and posits (about 40 feet); they record aggrada- he ascribed their origin to deposition during tion that is interpreted to have been con- a high stand of sea level during the Wis- tinuous, inasmuch as no unconformities consin. are recognizedin the outcrop. The relations Post-Dume, pre-Holocene marine de- between the stream deposits and the Dume posits have not been found along the Mali- marine deposits and the inferred altitude bu coast, suggesting that the mid-Wiscon- and position of the Dume shoreline angle sin sea level did not rise above present sea suggest that the fill-terrace deposits may level. That it probably was somewhat be- record stream aggradation related to a mid- low present sea level is suggested by the Wisconsin transgression. If so, the altitude present altitude of the Dume shoreline an- of the top of the fill-terrace deposit in the gle. Data presented here indicate that parts sea cliff is a maximum altitude for any mid- of the Malibu coast have been uplifted per- Wisconsin high stand of sea level. haps as much as 130 feet since Dume time, The suggestion of a mid-Wisconsin high a rate of uplift on the order of 1-1.5 feet stand of sea level along the California coast 446 PETER W. BIRKELAND

comes only from evidence from stream deposits. Marine deposits of that age are ab- sent, perhaps because uplift has not been great enough to raise these deposits above present sea level. It is quite doubtful if sea level came very close to its present position then because evidence from the midconti- nent suggests that a considerable amount of ice was on land during the mid-Wisconsin (Frye and others 1965; Goldthwait and others 1965).

COMPARISON WITH OTHER DATED COASTLINES Flint (1966) has reviewed the problems of correlating Quaternary marine deposits between distant coastlines and has concluded that the best approach is to decipher relative sea-level variations from the stratigraphic record. He then used similarities in the relative sea-level curves to suggest correlation between the Virginia, Alaska, and Mallorca coasts. Since publication of his report, radiometric dates have become available for some of these and other coasts. The similarities in relative sea-level variations between widely separated coastlines strongly suggest a common eustatic control, whereas the variation in altitudes of shoreline angles probably reflects variation in rates of uplift (fig. 9). There seem to have been two to three eustatic high stands of sea level between about 75,000 and 150,000 years B.P. This agrees with recent work of M6rner (1971). Before about 150,000 years FIG. 9.-Sea-level variation curve for the B.P., sea level was low, and the next finite Malibu coast compared with curves for several other dated coasts. Circles indicate age determina- dates for a relatively high stand of sea level tions (open for C and solid for D), triangles indicate fall between about 170,000 and 230,000 average ages, and horizontal lines show the standard years B.P. Deposits older than about error on age determinations. For the Malibu coast: D = Dume, C = Corral, and M = Malibu; for 250,000 years B.P. are too old for the dat- Alaska: W = Woronzofian, P = Pelukian, and ing methods presently in use. Lack of more K = Kotzebuan; for Mallorca the symbols refer precise similarities between the coastlines to Tyrrhenian IIa, Ib, and III; and for Barbados in figure 9 may be due to some combination the symbols refer to Barbados I, II, and III. At the suggestion of B. J. Szabo (personal communication, of tectonic and eustatic effects, preserva- 1971), the dates for the Pelukian and Kotzebuan tion of stratigraphic units, and problems transgressions in Alaska are those given by Karl- inherent in the various dating methods. strom (1968, table 2) based on a U-234/238 ratio of 1.15. Marine terrace altitude variations between the Malibu, Alaskan, Mallorcan, and Barbados has been removed and the curve depicts sea-level coasts probably are mainly due to tectonic uplifts. variation relative to present sea level (radiometric For New Guinea, however, the tectonic component age control not shown). LATE QUATERNARY EUSTATIC SEA-LEVEL CHANGES 447

ACKNOWLEDGMENTS.-Helpfuldiscussions conducted the particle-size distribution analy- with R. H. Campbell, J. T. McGill, C. M. Went- ses, and S. McKee helped with the heavy- worth, and R. F. Yerkes of the U. S. Geological mineral separations. The work on which this Survey as well as their assistance in the field- report is based is part of a Los Angeles County- work and report preparation are acknowledged. U.S. Geological Survey cooperative program of J. T. Andrews, W. C. Bradley, and K. R. geologic mapping of the central Santa Monica Lajoie also reviewed the manuscript. R. H. Kihl Mountains.

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