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Offshore Seismic and Geology Survey Portuguese Bend, California • Offshore Seismic and Geology Survey Portuguese Bend, California • • conducted October 20 to 25, 1994 by Robert F. Dill Dill GeoMarine Consultants 610Tarento Drive San Diego.CA 92106 (619)224-5594 and Timothy L.Norall EcoSystems Management Associates, Inc. 2270 Camino Vida Roble. Suitel. Carlsbad,CA 92009 final version submitted June 30, 1995 • r,\. . !/ una t v Table of Contents • Project Summary and Recommendations 1 Background 3 Side-Scan sonar survey 11 Bathymetry 13 Description of Sedimentary Bottom. 13 Description of Rocky bottom. 15 Seismic survey 17 Description of Holocene Sediments blanketing bedrock 17 Sedimentary Thickness (isopach) 19 Geological structures 21 Basalt Sill 21 Folds 22 Portuguese Tuft' 22 Nearshore Seismic Line 24 Representative Profiles of Holocene Sediment over bedrock 26 Nearshore profile 26 Composite profile 26 West side profile 27 East side profile 27 Central profile 27 Profile Summary 35 Comparative Studies 35 Palos Verdes Abalone Cove Survey (1989) 35 Santa Monica, California Castellammare Mesa offshore survey(1993) 38 Acknowledgments 41 References Related to this Study 42 Appendices Field Methods and System Description 45 "High resolution subbottom profiler(HRSBP)." 46 "Boomer"; 46 Sediment Velocity Corrections. 46 Motorola DGPSTM 48 Appendix A-Seafloor Bathymetry off Portuguese Bend Landslide Appendix B -Isopach map of sediment thickness over bedrock seaward of Portuguese Bend Landslide Appendix C-Geology and bedrock topography seaward of Portuguese Bend Landslide Appendix D-Profiles of sediment thickness over bedrock on three NS traverses seaward of the Palos Verdes Landslide. Scale 1 inch to 100 feet. . • Project Summary and Recommendations This report presents the results and recommendations from an offshore geological survey conducted between October 20 and 26, 1994 along the south shore of Palos Verdes Peninsula, California. Continuous line recordings of subbottom reflections from a 3.5KHz and a Boomer seismic system designed by EcoSystems Management, Inc. and a Klein side-scan system were interpreted to provide seismic stratigraphy and possible evidence of present or ancient landslides beyond the present shoreline of Portuguese Bend Landslide. Survey lines were located so as to overlap seismic transects made on an earlier 1989 survey conducted off Abalone Cove by Dill GeoMarine Consultants for the City of Rancho Palos Verdes (Dill, 1989; Dill and Slosson, 1990). Thirty seven north-south lines were made between Abalone Cove on the west and the point of land that forms the eastern boundary of Portuguese Bend Bay. Twelve east-west cross-tie traverses were made, along with 3 diagonal lines, to check for accuracy of soundings at traverse crossings and to check the continuity of features seen in north-south profiles. Seismic traverse lines extended 2 miles offshore to a point where it became clear that the geology was not related to landslides onshore. Traverses parallel to the shore were run from the eastern side of Abalone Cove to an area of hard rock outcrops approximately one mile southeast of Inspiration Point (see Figure 1). A total of 35.6 statute miles of seismic line were made in an area, concentrated off the Portuguese Bend Landslide as defined in maps provided by Dr. Perry Ehlig and Charles Abbott Associates, Inc. Seismic data were used to construct an accurate offshore bathymetric map of the region (Fig. 1 and Appendix A), an isopach map of the thickness of overburden (Fig.2 and Appendix B), chart-profiles showing thickness of Holocene sediments along nearshore lines and a geological interpretation map based on the seismic stratigraphy and side-scan profiles of the region (Fig.3 and Appendix C) and three north-south profiles showing sediment thickness over bedrock at a scale of one inch equals 100 feet. .4, Our basic conclusions and recommendations are: 1. There is no evidence in the seismic profiles or the side-scan records that indicate the presently active Portuguese Bend Landslide has displaced offshore Holocene sediments blanketing the Miocene bedrock, seaward of its present shoreline toe.It is therefore concluded that the southern edge or the toe is at the present shoreline. 2. The Holocene sediments directly off the landslide have lenticular acoustic reflectors that indicate abundant cobble beds,similar to those found on the 1989 seismic and side-scan surveys off Abalone Cove and verified using vibrocores. 3. These cobble beds would make vibrocoring in the area seaward of Portuguese Bend Landslide extremely difficult or impossible. It is therefore not recommended that vibrocoring be used to determine the internal nature of the Holocene sediments covering bedrock on the next phase of this study. These same cobbles could make coring by rotary drilling to obtain rock samples difficult. 4. Sediment eroded from the exposed toe of the landslide by storm waves have been carried both seaward and to the east, forming a lobe of sediment blanketing an irregular eroded bedrock surface. Bedding within the bedrock has been truncated by an erosion surface which forms a distinct boundary between overlying Holocene sediment and underlying bedrock. Although this interface • surface has irregularities, it has a gentle slope to the south. The erosion surface probably formed in the surf zone during both the regressive and transgressive periods of sea level fluctuations during the last ice age. In water depths less than Background SThis study is part of a response to recommendations made in a May, 1994 Initial Project Management Plan (IPMP) by the City of Rancho Palos Verdes, California and the US Corps of Engineers to determine the feasibility of protecting the shoreline and an environmental restoration project at the seaward boundary of the City of Rancho Palos Verdes. For a complete description of project objectives, historical data, and construction details one should refer to the complete reconnaissance report's main text and technical appendices, dated May, 1992 (rev)which is available at the Los Angeles District Office of the Corps of Engineers. Prior to 1956, shoreline erosion and sediment transport from eroding cliffs on the southern shore of Palos Verdes Peninsula had not become an environmental issue or attracted the attention of the general public or local residents. Historically, the Peninsula was utilized primarily for agriculture and had a low population density. Increased urban use following the rapid development of the peninsula after World War II and with the advent of freeways and the southward spread of the populous of Los Angeles created a new network of roads to support housing projects in this previously rural area. Loading from road construction and ground water from septic tanks and runoff from a newly created road III system apparently triggered the reactivation of ancient landslides along the southern terraced shore of Palos Verdes (Ehlig, 1982.; Anderson, J. L. 1987; Slosson, J. E. and Havens, G. W., 1987). The steep cliffed bluffs facing the sea slipped into the surf zone exposing their previously stable seaward margin to wave erosion. Eroded sediment was then suspended and carried seaward and along the coast by wave-generated longshore currents. Continual erosion of the toe had a domino effect and triggered a headward movement of the slide blocks creating unstable land masses unfit for urban utilization. The unique rocky intertidal habitat of the area, particularly within the nearshore parts of ibif • Portuguese Bend east of Inspiration Point, has been severely impacted by landslide debris lr• Li�a a and associated turbid water. Under present conditions, continued seaward movement of _`it the landslide pushes the toe berm into an area of wave erosion, preventing it from i counteracting the landslide's driving force. Erosion of the landslide material also provides a source of sediment to longshore currents that carry it up and down the coast smothering the highly productive rocky habitats of the area. One of the options for prevention of erosion is to protect the toe by a man made structural buttress anchored to bedrock. If successful such a structure would protect the toe of the landslide from wave erosion, stabilizing its • seaward movement so that it no longer is a source of environmentally damaging sediments. • One of the main concerns in stabilization of a landslide by a buttress, is if an unstable subsurface or prior slump zone weakness exists in the underlying bedrock. If a potentially 3 • southern shore of Palos Verdes Peninsula. Dill GeoMarine Consultants and EcoSystems Management had previously conducted a study with similar objectives seaward of the adjacent Abalone Cove Landslide in 1989 (Dill, 1987, Dill and Slosson, 1989, Slosson and Dill, 1990). Records and coring from this prior survey were used in preparing maps of the offshore geology of both areas and presented in the charts,maps and profiles in this report. A summary of the results of previous studies relevant to this one, are presented in the Comparative Studies section of this report. Seismic stratigraphy and evidence of present and ancient landslides were examined by interpreting continuous line recordings of subbottom reflections from 3.5KHz and Boomer seismic systems designed by EcoSystems. Survey lines were located so as to overlap seismic transects made on the previous 1989 surveys conducted off Abalone Cove. During this survey (October 20 to 26, 1994), thirty seven north-south lines were made between Abalone Cove on the west and the point of land that forms the eastern boundary of Portuguese Bend Bay. Twelve east-west cross-tie traverses were made along with 3 diagonal lines to check for accuracy of soundings at traverse crossings and to check the continuity of features seen in north-south profiles. Seismic traverse lines extended 2 miles offshore to a point where it became clear that the geology was not related to landslides onshore. Traverses parallel to the shore were run from the eastern side of Abalone Cove to an area of hard rock outcrops approximately one mile southeast of Inspiration Point (see Fig. 1). A total of 35.6 statute miles of seismic line were run in the area , concentrated off the Portuguese Bend Landslide as defined in maps provided by Dr. Perry Ehlig and CAA. Seismic data was used to construct an accurate offshore bathymetric map of the region (Fig.1 and Appendix A) and an isopach map of the thickness of overburden (Fig. 2 and Appendix B). An offshore geological interpretation map,based on the seismic stratigraphy of the region and input from the literature and Dr. Perry Ehlig was also prepared to support the conclusions and recommendations coming from this study (Figure 3 and Appendix C). We also constructed a series of profiles based on seismic lines showing the thickness of sediment in representative key areas related to the Portuguese Bend Landslide. A chart showing seaward extensions of land profiles was also constructed to provide a one inch equals 100 ft. true scale chart of the sediment thickness over bedrock along three profiles seaward to ones constructed by Dr. Perry Ehlig for land areas (Appendix D). • 5 •4168000. ORM 4170000. 4171000 417200 4171(100 4174000. 4175000 x100 417704' 4178000 417917s0 4180000 4181000. 4182000. 4020000 r----.1 ....{-- i .. .1.---.. 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'\\ > } i iJ t:; i rte t 1 • ]. , 50 iP • A012d:0. 7 r, '....._=:1--,..*:,\` 1'N 1-' ._* •'a It. 4.,.; - i' i .\ ••• 4012000 i` �\ + +� � ♦ 'y\, Tt ` :1. \.� ` J 75 Sm4fikr! _i\�\i ' 1. :1- . 0 { \ `\ \ , \ ,1+ ,111,511,mMMnXiSkN • 00111100. - \ +♦ \ -- 100 1990 Shnreline Map by Flrxiznns Surveys f - 4017600 25a �\ i \ ,? Geology provided by Or.Perry[hll9•CAA.Inc. {i• 275 '�^y\ I y. � '�� ♦ \ ��'•,.��.`�� t• `125 ,on �+ 7•, \ \ �.• 1964 Shoreline map provided b9 Chades Abbott - \ {. .I . .\, ,`I and associates,Inc. • 4010000 1- � \'V - ,- \,\ - \' '41 - 4010000 . '--- �.)`\\� �.,Z �„ \ -150 , n.F.4181,ami Tim Nam-Inas. I- ��- 'ham .•1 - \s\ Call GeoMarine ConaAtnMxfind :._,mac \ ;-.- • .• .\17 EcosystemsManaoeme111.hat. III 40090IX1. ------.!...---1- -I. ___1. . .. .. ' ... E _ _____. `I I \y':♦_ __ - - - '--------1. . .1_.._.J 4009000 5 01641,x111 4169070 0,7(4'.4x1 01710(:(1 A172rNN7 417:1000 4174000. --'-- 41757M 4174x)(.0 11771+90. 417810(1 4174050 .1106010 4181000. 4182060. Figure 1.Bathymetry of the seafloor off Portuguese Bend and Abalone Cove Landslides on the southern shoreline of Palos Verdes,Uslifornia.This is a composite map from surveys made for the City of Rancho Palos Verdes in 1989 and a 1994 survey for the City of Rancho Palos Verdes and the U.S.Corps of Engineers.A larger scale map is included in Appendix A.Depth contours are in feet. Locations were by GPS plotted in California Coordinates(ft),Zone 7. datum is NAD-27. 1170IL 4171*. 4172400 4177000. 4174000. 4125000. 4170000 1177000 4171400 117[000. 4100000. 4101000. 1112000 4021144) 1 . I 1 1 I I 1 —1-----Ta ="" ' . --r--- r Tr—7.-----r- I -I / i 4021000 .40 PB 18 - FE1-1•19 4 AB-86 AB-i19 AB-t1 5+ -I 4021:001 , 4.0000. , + + -i. -!- 4 + AB-14P8-17 AB-l434 EH-4 A8141 pB_R2FT05 g P8425 - _ • . AB-HO -. PB-02 Pgi 27 22PB-I27 FT+04 P 1)+21 82+21 FT-1-01 4019000. +- +.--,-/------ .77:::.'-----S-.4\' AB40, _ 4010000 '''. .--A•----*--78-----\.. 1 121 20____16 •40' PB+14 01-01 FT-I-02 -1 I .- :.' .-\_. —..,^'),,Pe\l,1 IC • ' P .. , I '.....1.•• V ,,,... ..L.-Z7-', '',':-. --•).s. s ' i, ''. '`\------, KC/96 1 "--., \ ), ''', ...,.i.r.,,,:k .".•„:.-..,,,,i; ,''''..;,..,,,z-'-i;•.,,,,vol, _,,,,;t,-.-_- , ; 40179X _„I, - ',-- • -:. ::..."4.4-.„---=': ' :14,via,,'''•,,,. _,I. i 4017000 a--- .N‘\- C .2,i -------/\ 1/ -•':--° K.4- ; '‘'.:-\-5.4 ,•%,'-.._'s ",)(ecti1 \ ''•- •• ‹..• 'e,,\,,,\\ ••) 1 - c-----7:---_:r•----=.---------\-\ \\,... '-'-'-.-------- \'.. If 4ei; ' ' : ,..,'"•- "'..N., 1 llun mem I r•-• 1 \--->---,'t.. ..... r-----,.0,17019d. i .. N\ i 7 ri ..) j ..\. '. 4416000 •-- /1/I/. .. ( ) -....__-__- ',... : 4014.014. i 3' 1 " 1 7 1 L.. 50\i \_, ---._.,---.-.„__,.--'"-' oe o'the --.., '''t.„.., :4_ I'- ‘... '.. .. ...,---,•,,,,..> '.1,'..„..... --- ‘ ••••••.---- , . • , l.:_,•,,---,___.e.ld,andslide .. ---i-----", , . ',..,..., ...,..t;--.; c3 ht..wean. \, ... ------ \ • . '',,------'' N i,.'i '--- - ° ''''. '25S.i 1 \ •---, k,.. ---- ) .... ‘ • „. , \,;:r • \ •:•-'; imwoo i r-- $ 1 \\ -..:, . 1 ,. g s . 1014004 1. k , / ., 1 : , // Rocky Bottom I \\''' 41""im* ) // . . $ s...„.....! i , • ) (------jc,-T\ , \ . 1r/, • • i \ (------/ q-----------,..7---.-.---ss-s. ) , i (N 'I .\ ) • , , ., .‘,., N,...,LOCh.V BOUOM ' ' R \ '... i c--s''-------j ' j _ . 4 I\ N ... 4012006 \ 0 b., % . .,_ • \ ...11 4012M C. 50C 1000 limmd. ) . :4----- ;; ‘ scale Jr199: . (11. \''''-_'.- 1 ,,,___-, •a . ,--, •'30 ,- 2- 4011000 r . ) 1 \\ ..-____,../..-----------'' N \ --- (Th rr,.,.,":-. 40 -i 4077* I : . s•-s\ _•..-_:._.,....„-----------.:...„.,N. , IC,,s, „--...-S \• -...„, i- . -----17 ...-- _ \ s'N `-----_, -- 40 . .----------/---..---- 4010044 1.- 35 ,..., •‘„, \. c (---., „ \N., • , , ; 9c '1.',., Shoreline and contours in black Iron E 19134 map -1 . 'N provided by Charles Abbott and Associal .In.: '\,..,,, c,_-1-•:,,,13,,,,;, ff_ Land coreours t-red trcrr 1986 map proctrec ty. 44X430011 1- 70 Horizon Su'veys o'Non.-Hotivw00c 1 !speech map ci Holocene sediment thickness Shoreltne geology b‘D• . 00*00 •EhlIE: I (contou,Interval 5 feet) •0 /- RI DI.and limothy ticre.0.1095 Di!04444arrie COIVALUVL 25 EcoSysmnt Wananeenert.Inc 1 , 4ttscoe L ' _ J . I 4170000 4171000 4177009. 4177000 417100 41760/0. 4170004 4177900 4174000. 4179000 4180009 4111101K 4102090 Figure 2. Isopach map of sediment thickness seaward of Portuguese Bend and Abalone Cove Landslides on the southern shoreline of Palos Verdes,California. A larger scale map is presented in Appendix B.Sediment thickness in feet was determined acoustically using a 3.5KHz and"Boomer"(200 to 1100 )seismic system.Computer drawn contours were made using Surface IIIsoftware.Data were from a 1989 survey and one conducted between October,20 to 26, 1994 ethe City of Rancho Palos Verdes.Red lines A.C.and E are offshore extensions of profiles across the Portuguese Bend Landslide prepared by Dr. Ehlig and shown in Appendix D. 7. 4170000 0171000 4177070 4173000 4174000 ---- ---- , . 4178000 4177(00 4170000 4179000 • 419000019000 4019000 _.... _.-_. -_.._-___ _............ ...__.--__..- .-......._ ---_ .--.-..__-_ _ -... ....._.. __...._...-.__-___..- - __-__—. ____ --_.._---___ ..___ 0 4 South Coast of Palos Verdes, California .,\ Ben,II ma rk � �hfrk: ` I, aBedrock topography ra h and Geology 40113400 - 31:1--____31:1--_____,,, < ar 49,99 seaward of the Portuguese Bend 40 35.------ \yQs�a°�h a t + Landslide,Pales Verdes.CA • 1 `�11Vt` ... 4,,r/ dnn1,free B5 _ `` 1l - _ POfil 1990 HCnr1 ++ + 4 + t �� , t-t "�� s n .q 9v r\ 9 mwY�w.k., 70 4 +- ,4- + �-� I �\\v, + !/ mrnln y I edmFk 1- <hhdr. r _. + :�• I rtli 1 ...� 4017000 4017000 _ :"4--"....."-----"4-4 �'t"{�-ti.-+ + + I \I� N / \ + 'R} t '`A•11 ‘�Portuguese i ,Insplrgli9n ®, "�-��'' �',` .. �t,�} ; + lugu. POMt rn�aytawm 1.- +}'t- !1r�,�p d- ''oo1v0a119d< + -+ } = �{+ t `e�`0 .� ift,_... r �.> �V;il`►�I �:.. 6143.17;Intl + 'e • +± +-I�+ # 1 A_! 41% . + +- 4_ �/\ 4016900 1 t + 4-6-4----4 /+� MINO57` i - \ . � � .-••••-74m,.�.3jq �++ + "+ ..�5�. `` + -+ +' a. + f- + -�,• ' ` Sill! _iii: ..+ •11. rt-+4.4 -7, 4 '..ice ..::� rt +-f± t + _ FA- ++ '�''_ �,�rir_ �� itio '�'N ++ m..a - + ++++i:�.+ I _ 2;,� ` Sim `' ,. ++ war Weak R. Nit tkr.,_ + tacky 5 14 *t ,�7 t +t+t+ +'+ + t41,1;11i}a 2 rr4- 14 hbe"19 4015099 i-_Z 4_,\\*, N , , -F 4-+1• 1 ++ a- + + - + +t rt}.- i_ + }r++ �+ .t-t-+ + � - . + + —}+��+ ++ + +tom +}+ Y' 1 + _.4014990 4V 4 + +' f--F _ -a } _ + 'Fault- - •+*+ +� • r litts '1• 1 --..�, 4-14‘ S- 4 + t $ + + + t 4- t + / +.i.+ t -� 45 ,0 4013000 + t + + t++ + ++ + + F�+ t + 6........, 4013900 'ILlitsibi- tt 4: t + I-_ +4 f+ • + 225 r• t t + + +}+ 65 + + 4-4 + + 4- + + + 1a #+�.++++ 7s - t -I- ++i t + i 160 _� ++++ cm "Wit70000 + 1 4171000 ++ + _F 172000------t-1 4173000 4174000 • 4175000 4176000 4177000 '4179000 4I7900o 4110 010 Figure 3. Geological interpretation of the seismic and side-scan surveys seaward of the Portuguese Bend Landslide, Palos Verdes, California (see Appendix C for large scale) Dip and strike symbols are apparent values from seismic track lines. A side-scan sonar survey was conducted to determine the surface topography of the seafloor using an instrument with a 330 ft. horizontal range on either side of the survey vessel. The survey grid was laid out so that all of the bottom between Inspiration Point and the rocky bottom forming the eastern side of the survey would be covered between the surf zone and the 120 foot depth contour. Side-scan records locate and define acoustic reflections from rock outcrops and irregularities on the bottom such as ripple marks and sand channels. Overlapping paths permit construction of an acoustic picture of the bottom, similar to radar on land. Rock structures protruding above the bottom form patterns that can be related to the acoustic topography seen in seismic records. Orientation and size of microtopography can provide a record of oceanographic conditions and an indication of the type of sediment forming the seafloor. For example coarse grained sand forms larger ripples than fine grained sand and muddy bottoms are usually smooth with little relief. Navigation was controlled using a modified Motorola "six-gun" Differential Global Positioning System (DGPS) with shore and vessel satellite receivers to provide a differential signal allowing an navigational accuracy of ± 6 feet. Positions were automatically recorded on a floppy disk in the ECO-NAV computer aboard the survey vessel. Simultaneous to each DGPS position recorded, the navigation software sent an event mark to the seismic record that automatically annotated seismic and side-scan records with a navigation ID number,time and date. A dual frequency seismic system designed by EcoSystems Management was used for determining the acoustic stratigraphy of the region. High resolution 3.5 KHz and lower frequency 200 to 2000 Hz "Boomer" acoustic signals were plotted simultaneously on the same record so that features seen on the high resolution shallow penetration 3.5 KHz record could be compared with the low frequency deep penetration "Boomer" records. Our survey equipment included the following instrumentation: Motorola "six gun" DGPS 3.5 kHz Sub-bottom High Resolution Seismic System EPC dual channel Mod. 9600 Digital Recorder 1000 Joule "Boomer" Sub-bottom Seismic System, filters set for 270-1100 Hz. A 100/500 KHz Kline Mod. 590 side-scan sonar system 200 KHz echo sounder for depth measurements on the side scan survey All equipment was mounted on our 25 ft. survey boat(Fig.4) which was launched from the Cabrillo Park boat ramp in San Pedro Harbor. The DGPS differential shore station was placed directly over the County bench mark "Shields," (Fig. 5) identified by Dr. Ehlig on the hill north of the survey area (located at Northing 4018302.94 , Easting 4179107.25 : M33° 44' 29.11024 "N; 118° 21' 33.07934" W). Survey lines were preplotted using EcoSystem navigation software and loaded into the computer system aboard the boat. 9 Preplot presentations were used to guide the vessel on track lines with a north-south and east-west track separation of approximately 250 feet. All traverse data and records have a time designation every 15 seconds and a location identification number printed on the original record so that they can be related to their position on traverses plotted on a regional chart for correlation. Because of conflicting frequencies between the seismic and side-scan systems it was necessary to make two survey passes over the area. The side-scan sonar survey was done first,followed by the seismic survey. Precise navigation allows us to superimpose the two charts to compare seafloor surface features found on the side-scan survey with sub-seafloor geology seen on the seismic records to prepare the geological map of the survey area. Side-Scan sonar survey The side scan-survey covered all of the area seaward of the landslide out to a depth of 120 ft. (Fig.6 and Appendix C). Bedrock is exposed on the sea floor on the eastern side of the surveyed area. This rocky area supports a thick kelp forest. Irregular elevations of individual beds within the rock formations show that there is considerable variability in rock resistance to erosion. Rock exposures have relief of up to 6 feet. Folding and fault displacement are evident in the reflections from outcrop bedding planes in this eastern margin of the survey area. The zones of tectonic deformation can be projected onto land and tie in with deformed areas exposed in the cliffs along the shore to the east of the landslide.The trend of this deformation of the bedrock to the west, is approximately 2000 feet seaward of the landslide and is probably not related to any present movements. West of the rocky bottom forming the eastern boundary of the survey area, sediment blankets a relatively smooth bottom with little to no relief. There are no rock outcrops seaward of the 20 foot contour, only a relatively smooth sedimentary plane. This type of bottom extends all the way to the western boundary of the Abalone Cove survey of 1989. Dives over a similar seafloor topography during the 1989 survey showed the bottom to be composed of a silty, fine-grained sand with little or no relief. In shallow water directly seaward of the landslide toe at depths between the shoreline and the 20 foot contour, there are a few rock outcrops protruding through the Holocene overburden. These are colonized by kelp which is visible from the surface although the rocks themselves are not visible due to the turbidity of the water. The seismic profiles near these outcrops indicate they are part of the irregular eroded surface of the bedrock of the region and not eroded remnants of a previously active landslide. As will be discussed later the seismic lines verify this relationship in areas we could survey. 11 Bathymetry 0 The bathymetry of the area was measured in two ways. During the side-scan survey of the inner Cove, we utilized a high resolution 200 KHz echo sounder to continuously record depths. Navigational ID positions were keyed and printed on the record every 15 seconds. This continuous depth record was later read at the marked 15 second intervals using a digitizing board. Depth values were recorded in an Microsoft ExcelTM spread sheet along with their ID station number and the Northing and Easting positions recorded at the same 15 second time interval using a Motorola Global Positioning System. Depths were then computer plotted using Surface IIITM and SurferTM software. A "bar check" was taken at the beginning and end of each day's work to provide a correction for changes in sound velocity due to water density variations in the region and minor changes in the sweep rate of the sounding instrument.This is done by lowering a flat metal target attached to a calibrated wire to depths of 20, 30, and 40 feet below the transducer as the acoustic sounder is recording depth. Any deviation from the sounder tape grid lines is then applied along with a tidal correction to reduce the depth soundings to Mean Lower Low Water for constructing our final map of the bottom topography. Depth soundings from our ExcelTM spreadsheet were saved and read as a text file by Surface IIITM V. 2.5 the software used for plotting positions and contouring depth. • Geological interpretation of the side-scan records which show micro features of the sediment bottom and rock outcrops with macro-algal growth (kelp beds) are shown in Figure 6. Bottom types fell into two categories, sedimentary and rock. Sedimentary Bottom. Most of the sea floor seaward of the active Portuguese Bend Landslide is blanked by Holocene sediment deposited as sea level flooded the area following the last Ice Age and subsequent sedimentation from the erosion of the shoreline and carried seaward as a prograding fill of bedrock irregularities. In general, the bottom has a gentle seaward sloe all th py e waout to the Shelf Break at a _ _depth of about 250 feet, approximately L5 miles offshore. There, the slope steepens and drops to the deep San Pedro Basin that separates Palos Verdes from Catalina Island and the Offshore California Borderland Basins and Ridges ( Emery, K. 0., 1960). From shore to the drop-off at the shelf break there is very little micro relief or bed forms on the surface of this sedimentary blanket except near rock outcrops forming Inspiration Point and the rocky bottom at the extreme south eastern part of the survey area (see Figs.2 and 3 ). In areas near rock outcrops there is a halo of coarse sand extending a short distance seaward from the steep rock walls that protrudes up thorough the Holocene overburden. Turbulence and increased swell induced currents in these areas winnow out the fine grains illand leave a coarser sediment with large rippled surfaces as a sediment halo usually with a 13 Rocky bottom. There is a narrow exposure of rocky bottom around Inspiration Point and isolated outcrops along the eastern edge of the landslide. Contact between the resistant basalt wall defending Inspiration Point and the sediment is sharp. At the Point, basaltic bedrock abruptly rises from a coarse-grained rippled sand bottom that blankets the base of the rock terrace surrounding the point (Fig. 7). A much larger area of exposed bedrock forms the eastern boundary of the study area. Sediment-rock contacts in most areas are sharp except near the low rock ridges that extend across the shoreline at the cobble beach east of the landslide (see Fig.6). In this area the ridges form natural groins that are covered and exposed by seasonal cut-back and accretion of the beach. Offshore bottom contours near projections of the outcrops seaward, reflect a damming of sediment moved to the east by littoral currents The rock bottom along the eastern margin has an irregular surface that reflects the bedding within the formations cropping out at the seafloor. Folds and faults can be seen in the side-scan records of reflections from the beds facing the sonar transducer. Bottom reflections from both echo sounder and side-scan records show that most of the rock exposures are covered with brown algae and sea grass. There are scattered rock outcrops nearshore that protrude through the sediment cover most of which are capped by the giant kelp (Macrocystic ) and ribbon kelp (Egregia) (Fig. 7). The gas filled floats on the algae form excellent acoustic reflectors that provide a record on both the echo sounder and side- scan records. Previous diver observations provide a ground truth comparison with acoustic echoes that allows us to identify the type of algae growing on rock surfaces (Dill, et. al., 1989). The low standing algae which stands approximately 3 to 4 feet above the bottom is probably Laminaria and the high standing algae which reaches the surface forming a visible canopy is Macrocystis. 15 Seismic survey 0 Data from the two seismic systems were interpreted to show shallow stratigraphy within the survey boundaries. High resolution 3.5 KHz records were used primarily to record the thickness over bedrock and internal sedimentary structures within the Holocene sediment blanket. Our maximum penetration with the 3.5 KHz system in Holocene sediments was approximately 100 feet. However, in areas of what we interpret as cobble lenses and traverses over the basalt sill, high frequency acoustic energy is rapidly attenuated and acoustic penetration was much less. For this reason our system uses two acoustic sources the higher frequency 3.5 KHz and a lower frequency (1000 Hz) deeper penetrating but less definitive "Boomer" system for recording the structure of the bedrock underlying the sediments. Acoustic reflections from within the bedrock using the "Boomer" system at the power levels permitted by California State Law can define rock structures to depths of approximately 400 feet. In some areas the Holocene/bedrock boundary was indistinct or beyond the penetration range of the 3.5 KHz echoes, in those areas the deeper penetrating lower frequency "Boomer" records were used to determine sediment thickness. Sediment thickness and bottom depth were measured on seismic records at each station ID and recorded onto a spreadsheet. Seismic data from the seismic records were read using • a CalComp digitizing board. Water depth and sedimentary thickness over bedrock were then used to construct an isopach map of the Holocene sediments (Fig. 2 Appendix B). At the same time the records were being digitized geological features relating to this study were recorded on a separate log for correlation with the side scan features especially in areas of rock outcrops where formation structure could be interpreted from acoustic echoes. Holocene Sediments In general the Holocene sediments which overlay bedrock, increase in thickness towards the south all the way to the Shelf Break. There, at a depth of approximately 250 feet, sediment thickness decreases markedly with indications that in some places bedrock is exposed. Nearshore in depths of less than 25 feet, thickness is variable but thin (usually less than 10 feet). Nearshore sediment thickness decreases to the southeast, away from the toe of the landslide. Projections seaward of linear shoreline basalt outcrops show that the bottom has exposed rock or is an area with only a thin sediment cover. Most nearshore Holocene sediments have horizontal or gently, seaward-dipping acoustic reflectors, indicating coarse sand or cobble lenses within the sediment that blankets the bedrock(Fig. 8). Sediments are thin nearshore and are most probably derived from the toe 0 of the landslide although there is undoubtedly some contribution from gully erosion by the 17 0 • 0 Schooling Fish - .. • ii vi '`.^A-+nl'9R:. -...•,.. e..i.a.,-.. mac:,.::.: :,. r... ....ab»Sssvtaru:aucmYgn� • ... T..,•,.........,.. �. .rr ._� .. -... ^.-.-- a .,.... ... ......, ''"""'"'''''''''-'`'''''''''''-77.;;."--- .... .. '�.. •.. .........v.... VAr• ".. ...wv .m.n'e✓.wM'r<+M.�.. 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Reflective beds are seen all the way to the shelf break just above linear bedrock elevation changes that could represent ancient shore lines and low shoreline cliffs. It is important to stress that none of the horizontal lens of cobbles and coarse sand (?) show any displacement or tectonic folding which &ouk indicate the_sediments_containing them has been stable since their emplacement as sealevel flooded the region in the olocene, even though they are offshore of an unstable landslide area. This strongly indicates that Holocene sediments have not been affected by movements of the Portuguese Bend Landslide. This stability would represent a period of at - i 1 - ie flooding of the Continental Shelf following the last ice age ( mP , K O 1960• Emery, Sedimentary Thickness (isopach) An isopach map showing the sediment thickness above the bedrock is presented in Fig. 2 and Appendix B and in profiles along characteristic seismic tracks in Figs. 9 to 15. The station ID numbers can be located on Chart#1 in Appendix D. • In general the Holocene sediment thickness in the survey area is less than 50 foot thick (Fig.2). Nearshore sediment over bedrock is less than 10 feet with some areas showing almost no cover. Near Inspiration Point bedrock forms the bottom with no sediment cover. The eastern side of the survey area is also anis area with little to no sediment cover out to water depths of 20 feet. Seaward toward the south Holocene sediments thicken out to near the shelf break where sediment cover thins to less than 10 feet. This type of sediment thickness pattern indicates that the sedimentary wedge over bedrock, is gradually prograding seaward as erosion of the cliffs and terraced coastal areas provides sediment to fill in offshore bedrock irregularities and topographic lows. A section of the east-west profile closest to shore is shown in Fig. 9. Note the exposure of bedrock at the western boundary of the survey area. Holocene sediment laps up and covers bedrock in the central part of the profile and represents sediment and coarse gravels derived from the erosion of the toe of the Portuguese Bend Landslide. At the eastern edge of the area,just seaward to the active landslide, the basalt and dolostone outcrops seen on the beach extend seaward to the south forming a low relief linear rocky bottom. There are rock outcrops protruding through the nearshore sediments. Most of these rocky areas are covered with kelp. 19 0 • s Fay, West — 5ft Bedrock Holocene Rock bottom probably eastward Contactl Of t extension of basalt sill exposed at Inspiration Point Holocene sediment — 15 ft.contact ` �•�- 20 ft . .. Tj'T• :••. ` '.'Y+k' A'�"..k x'�4 P�1.t{e� .` 4-4 T V.:-.:'• ST _4i• • s:�- ao n 1 ?' ,. �. 7tt `. �v.;C '+ '$.�. u..yam. -.--:6..,.....,,,,E ,, .. •. .5. .. �, , _ SOh i?'�,, •••4.4,.-r._� „ Y., k«. /,..•�.: • �" .ICF '4 �, n, '.^',n *✓ ..._-.."., 't .*ID D:. 'E'P`` 1-0,1,"fi". Atc - . 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Station ID No. 5043 is at the contact with what we interpret to be an extension of the basalt sill exposed at Inspiration Point on the west side of the Portuguese Bend Landslide. Note how the acoustic energy is attenuated beneath the basalt layer. Holocene sediment thickness at Station ID No. 5049 is 37 feet. Note the depth scale change at the seafloor that reflects the increased sound velocity in the Holocene sediment. Geological structures Although the Holocene sediments appear to have been undisturbed for at least the last 2000 years, the underlying Miocene bedrock show evidence of considerable tectonic movement and structural deformation. A well sieves fault zone in the bedrock, strikes diagonally in a southwest direction across the surve area. It can be traced to a zone of contorted beds on land seen in the cliffs that form the southeastern boundary of the survey area. East of the landslide, beyond the survey area, a major fault system on land associated with resistant hydrothermally altered sediments trends seaward into the survey area. Along this trend seismic records show contorted bedding with an irregular topography on the bedrock erosional surface that could be attributed to faulting. Strata adjacent to the fault exhibit gentle folds as would be expected with tectonic movement along a fault. The important implication to this study is that none of the Holocene acoustic lenses or bedding overlying this fault zone are displaced,indicating the areahasben stable for at least the last 2,000 years. The southern part of the survey area near the shelf break, has well defined bedding dipping to the south. This probably represents well developed and acoustically definable beds of the Miocene Monterey Formation. These beds would be geologically younger than the beds seen under the Holocene cover near shore. Basalt Sill The basalt sill that protects the seaward margin of Inspiration Point extends to the east as exposed bedrock on the seafloor, forming a resistant structural restraint to landslide movement. Seismic profiles indicate_that_a_layered bedrock formation_underlies the basalt sill and gently dips to the southeast. The seaward faces of both Portuguese and Inspiration Points are defended by a resistant basalt sill (Fig. 10). One of the objectives of this study was to determine if this sill extended to the east seaward of the Portuguese Bend Landslide. Seismic records are not definitive,but there is a hard rock body with out bedding extending to the east under the Holocene sediments. This could be a eastward extension of the basalt sill that outcrops at Inspiration Point. Bedrock with distinct bedding underlies this massive rock body that we interpret to be basalt (?). 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' • c I Y 1 r!1 i.1 / tv NC Iii I. /14;1 :1 I"' C i{ i r'i 1 • if i • IBIS 0 1.... m e rf n," ` �•>� , I .1 • ++ � N ; 5 . I vi /1.4?7' 3. , / O > ,1 U o Qm i 1+ #.,i L O O _c p ,n 1—:_,- U U QO .. ... �� ;a 14/II . �$ . I.*u, .I ' v, , J O U zi. Ii t.,111 , ,i C3? N t 411'1' } �y r1 .., 1.eray Nearshore Seismic Line The shallowest seismic line closest to shore off the landslide skirts the eastern side of Inspiration Point and traverses to the east (#3832 - 3838) (Fig. 12). This figure shows a • scanned record from both the 3.5KHz and 'Boomer" records for comparison of geological features. The original records of the 3.5 KHz profile are much better in showing definition within the overburden due to difficulties in scanning the original records. Bedrock probably basalt and associated hydrothermally altered shales and limestones form the seafloor with little to no sedimentary cover. Rock bottom extends to the east to #3834 where Holocene sediments overlap the bedrock as it looses relief, possibly due to a change in rock type and resistance of bedrock to erosion. The Holocene sediments have highly reflective internal bedding indicating cobble lenses,. Bedrock internal reflections along this easterly traverse are flat lying indicating the vessel was following the strike of the beds. This would indicate that the beds have a east-west strike. Traverses perpendicular to this one show a shallow dip to the south(see Fig. 11). Representative Profiles of Sediment Overlying Bedrock Because of the large number of crossing track lines and their similarity it would be • confusing to present each as a separate profile showing depth, sediment thickness over bedrock and underlying geological structure. We have therefore chosen a few profiles that contain features that represent the western,central and eastern sides of the seafloor seaward of the active landslide. The length of seismic profiles running north-south relative to the thickness of sediment mandates that we show these profiles with a high vertical exaggeration in order to place them within the written report and still be able to see the Holocene sediments. It is important to point out that the spacing between Station ID locations is not a constant distance but a constant time between navigation locations, a period of 15 seconds. However, the survey vessel was kept at a constant engine RPM which provides a nearly constant distance between Station ID locations. In general the distance is approximately 125 feet as the vessel averaged a speed of about 4 nautical miles per hour during the survey. Exact distances can be obtained by measuring the distance between stations located on the large scale map showing station ID numbers in Appendix D. Two profiles running parallel to the shore. One is a composite of east-west, nearshore tracks as we traversed to the next north-south line. The other us the closest track-line traversed along the 20 foot contour line,just south and seaward of areas of isolated rock outcrops that made a continuous vessel traverses closer to shore hazardous. Kelp beds ;'• attached to the rocky bottom also prevented us from towing our seismic gear closer to shore 25 #2759. This is an area just seaward of the Portuguese Bend Landslide. Also sediment • thickness is less than seven feet all along the traverse. High resolution 3.5KHz records did show there are strong layered and lenticular shaped horizontal bedding within the Holocene sediments covering the bedrock. These reflectors are probably cobble beds. West side N/S Profile #2494 to #3760 (Fig. 16) Starting nearshore this profile starts at the western edge of the landslide, skirts the eastern side of Inspiration Point and terminates approximately half way across the shelf at a depth of 140 feet. Nearshore Holocene sediment thins rapidly as one approaches Inspiration Point and skirts rock outcrops extending east of the Point. Kelp beds growing on isolated outcrops of bedrock or in areas with a thin overburden extend eastward from Inspiration Point out to the traverse. Traversing seaward sediments thicken and loose internal reflectors, indicating a more homogeneous structure than the nearshore area. Sediment thickens all the way to the shelf break (not shown in this traverse but seen on adjacent profiles not presented) forming a featureless sedimentary seafloor. Eastern side N/S Profile ID #3006 to #2902 (Fig.17) This profile suns south from the eastern edge of the active landslide. The inner portion has a thin cover of Holocene sediment with numerous lenses of acoustic reflectors, • interpreted as cobble beds. The traverse crosses the exposed bedrock ledges of the rocky bottom that forms the eastern margin of the survey area. The contact between the bedrock and the sediment lapping up on the bedrock high is sharp with several reentrants and well developed surge channels. The bedrock has a well developed kelp forest attached to its stable surface. Seaward of the rocky bottom the sediment thickens out to the shelf break at a depth of 320 feet. The sediment surface is smooth with almost no relief. Center N/S Profile ID #6007 to #6095 (Fig. 18) This profile runs south from the center of the ave landslide shown in Figure 1 and Appendix A out to the shelf break at 240 feetY4Sediment thickens seaward from a thin blanket near shore to the center of the traverse and then thins toward the shelf break. The sediment surface is featureless with almost not relief. Bedrock under the Holocene cover has a low relief. A well defined fault zone cuts the traverse at Station ID #6042 to #6044. Bedding within the bedrock dips in opposite directions at the fault contact, probably representing drag folds. • 27 • • • . _ _ SI_ ;, v. 3.5KHz seismic record 5 t 10-8 m Holocene sediment 15 ;°Basalt sill S.ea Floor 19 feet deep _ s i sa t x .4..:_ J,• il''r'J ,{ a .� r r�i + if . a� :, a .4S • r• i �i -•. �•...• . 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"a•.-•.-.w'waatc- .e,,,,....^,,,,,,,,.'. .. .eewN^• axe"w .: ,ms, r..,es.......,...<..w."w'vreixY,.eee.,enx�°`.rarx::-,»d°t'0+amt?aa�e'°'"r':'°ew'v�?•�rer'°.�"ti°.. ,,,nr,,..,m".sms-'` l e,>o. 'ae..,. ".' .svg.,••.-"=" *. ter..m.aa..^a'.ae '...w,,,,,,sm_',v,•.a<e�.. veaw.w. ... .-.... --4". >... tae. .., .....<.... e,r.,.no.�: ..<ze»-........R..».,.r ",.,"wm•, .. aa-.am,,. ..p-.........47444.1........4'.44•19111.1....g..„. 1.11t .-.,,..........ax..... • .'..a.a„ er"44••• or•ta, ".«ae...,.w ... .n a 'eebr..w:.»_...S .."aF'...'''„caw+ "a`/44. "...."w"" ..., ,..r . ......,4_.1.4.,,,..... .411.41......./.411.1............."141.111""4""'"4 1 .,»+,..,. .. ..> _''xw-a +y,.ww'"e'i°b'.. . ,yam •a»,..., .ra.ww,we• .ne'"'.<'•'w 4. 1" �r'.s•. .•"s�+'+uicwa°°' 'f`.,,s' 411 ...a.a a 0.,,f`ve....MeekoreNe>:uee �e...MA.93.1"'".. .. 4 r• .,.. .. ....., a xa... ':..i.+....w "... M.ANr.�w s. areee. .i-.,. ''-d."...eda'r#' s 4a.wa...i.,A^r`<ce..a�<. Boomer seismic record Figure it.West to east traverse approximately 300 feet seaward of the toe of the landslide. Apparent dip of beds is zero indicat��ne the vessel was going along the strike of bedrock bedding. A traverse �)0° to this one (I ig.11) showed beds had an appa• rent dip of 13° to the north. True Dip Bearing would be 015 at a dip of 13°. 0• none •4181000 4152000 i _' y my m, ny.., ,E ^nn �s_t_- 1 4,01 ---I 4172000 4173000 4174000 4175000 4176000 4177000 00 4179000 4019000• rn.a l h— 4010000 NTH, ( /Kt;dnl ! rt,,,i',,-.42 ep lit:46 o `D d .., 40191100, nol5ono, alt.s fd I I ' \ I ''''' 1 I , r { ,ree •\ I - -fir'` 1 �l 1 t 1• 5,-- ( 95 ilr I , j },1 J ,,� '1.....,:.,_ 4,,,I ''''.;,-\C 1�t 1' ". ..,%"',...07's .n� \ `, I o r `�ic:, '� ' " ;, , 4017000: 4017000. 1 Beni?'",,r+ i i Tee P F3enH ` �.' (r,J,�� .}.W,„, L. dsllde \ i,, ,f • W�4V`i I .\ \ ` 1. kk ','',,,,...e. 4018000. 4016000.' r7psiahQelrmle 100 j `a4 .� � 75 i �. \ 1 r 1 "”' t � \ f \ �t 111 100, ' `it \� 4015000 4015000, i ; 'It11 , 125. j4 25 N \.\, 4014000. AOIAOOO.i 4013000. ,o 4018000. ,,\ 75 4012000. .. — 125, _.100 I - .- __4----- >:4012000• /1172000 4170000 4174000 4175000 4175000 4177000 4175000 4170000 4150000 4181000 4182000 • • .. West Near Shore East-West Traverse East s l , . I 1 _ 1 Ip e I 2r. WsAtr i I I � t i 1A I r i o i i i I f i. _ T9 TJ'- 20 `... ... r- �..... ...... .1... �.::.:. ..._ ..... ...... t Malate >a Hol000ns Q 'Holocona sodlment &b 25 ill.y" est aafrosk Station ID Figure 13.Offshore seismic profile ID#5040 to 5081 showing Holocene sediment cover relative to the Palos Verdes Portuguese Send Landslide.Holocene sediments,contours end depths are In feet. 0 4172000 4173000 41(4000 41/5000 4176000 417. • 4178000 4179000 4180000 4181000 4182000 — 0 ._..t.. .. __-.�_ _.__.,.,y_..28 — _ __-_—� 4019000 1 C Ftwa • 4019000.C:!1--- tea::; � ��b a k M yr;n: Fei14 \ VIS.. < 11.0 1'i,rU •p yUt1 " .•l .S`f"4>•. �. 4018000 25— - 1.a --.:-=-\ ,_} i ' -,-..4 4 ,- ,: '.'1,-,- -.=.- \.\.:%.e4"�=, ' ---&,;-...4,1=7;1 �14. i";\ "rs\\• - 4017000. 4017000. �� w 1? rj d �tour. ate :-,�.: ° :,\ Y ` (INik\3 :� , 50 _ \ SH • 4016000. crock bouan 'N..'• ,.- 4016000. 1.F •,. F ry`3: 100 v4w1 ,..1')1 -,,,,,,tP,''''''' ,,, i ''Z ` 4015000. 4016000, N \ ({� \ L • 125..,._ •\ti\i /, 4�s: ! 4014000. 4014000. - _..v --t .... Perry Landslide \ \ Profiles \, r-zz —``- 4013000. 4013000.- \\. - .� ' 125 100 7S f + —— 4012000. 4012000. -1. /1 72000 4173000 4174000 4175000 4176000 4177000 4178000 4179000 4180000 4181000 4182000 ,\ Depth• Diagonal SW-NL ID 3488-3532 section across survey area , Holocene Sediment \ NE fl Bedrock \ 3000 feet — -- — — - a SL m i i L0 0 a t1j { 40.0 is i .. ; 1 :. ' 1 1 i. - : 60.0 l I r - -. L {: 180.0 Bedrock ° , 1 Holocene sediment 100.0 = 120.0 8 140.0 1 lif1llIfillii iligli ; ; ; ; ; ; ; -• W ; N N N 5m � m Station ID Figure 14. Offshore seismic profile*3488 to*3532 showing Holocene sediment cover over bedrock relative to the Portuguese Bend Landslide. .F.Dill.,995 Holocene sediments, contours and depth measure ments are In feet. P F oMari95 •72000 4173000 4174000 4175000 4176000 4177000 4179000 4180000 4181000 4182000 Joo. ;(,._______t_ _--,--- -v...,,,,-7---- --,-- -F I PBI.6 A C g1111—r- ---11- - PI*64;.au I FT,03 — 4019000 •' .. I -N s' ".....--\.-_\_--\. 4, *"`...-, -‘ „... , '''N.. .. 11.5ii 4 17"..: ie“I E 0 ..1 tlitt \ ,I.,:8i,..,1.", 'pp I 4018000.1 . . -, , 4018000. 1 • .- -"k-:.. - ; '-' '.. _, KG.,•:•6 ' ''',:',) -.5•'.----:-Iit•-•', 'I-III ,./..=-1-;:::::71-- .-----) i_Al . .----. -/,).•:-.,,,,'L''.2? .1.,„_ 25-------.........._, , ' I 6 ----,,,...„-::.---.. ..-„,.K. ---.... ..•-•-_-,.. ... 4.., :,,,,,,,,, -„,,...... : -. 017000 \ _.., /i \ 1,, *.-.4,j_--_--•••11___ -.:44.- - ---....t4‘4-. ,t.. .-''.(•...%„,,'.- \,\%..1.k 4017000. ' "......._ \ I f .. ..""t'Tt-#.1-•. ,' , ,..cs.,\. wN, i •..._ .".",47t•... f) - .. \..s. •,.;::, . P.••. N-. ' 4016000.- \ '- V '.g.; - 4016000. 75--.....„ , . . ,.‘ ---\--..... \t-t - . ' '',,,:"-x Na.".1.,,,,,..•,-pp,mt. „''',--------.,-..•- I_ -...., --..., 1 „• ,.....:::,.......sinie prom. .\ ,,, ...„....., •.,.....,„,..„_,,,:, .,Wri...Z.,03'• • • -. .. . .E...r.iv1 r z . \. -- -.,_____.___-,,,.....,.. ..,.. . _ 1(S1 , 100.,. Joarchiggelcin ; •-....\ . 4015000. - , ''et:\\ N t 4015000. 'N..- . . - . t ' - 25- 125.. X \._. .\ \ \ c - , • •••‘: .3-..- N, \. 4014000.- ---, •.,-;\\. - 4014000. ---\ . . . . , , . . •, / . . .N \ I .- ..,/ '- -,, . --1 . , erry Land , ‘, ' —P . „ •, .S., - 4013000.- ---S ."---..,, ----'',.50 N .........--, - 4013000. •/ N. .\ / NN. / „., \\\ '---.... ---, ,.. . N-...tx, - . 76 4012000. /4 1 100 I NNI, 401200D. 4172000 4173000 4174000 4175000 4176000 4177000 4178000 4179000 4180000 4181000 4182000 NiNi,. .N-N West // Shallow Profile(composite)-Inspiration Point to east side of Portuguese Bend Landslide East LI Bedrock . / Station ID / n Sediment(ft)' , ,• Z.' ° ( ' ' , '' ' • " ' ' ' •' ' ' - ''""---77 r-'''-'''' ' ' '''' '''''''''''-'-`-'-''—r--7-'-''''''-''''''''77777:4:7:77:17777:7•7'717.77"- 1 0 .c inspiration ,i........-__.___ __------_ - - -- -- - a --- 4•••1;-1,;:.;.;;::i,:i.n,•::-.-'i.l?;•,l,--.. -'..?:-:-`“ ,4Z::-•.,,, '-;•.,• ••••••..• . . . . .• •:..,. ..... . -.,--. •••••; • i. ; • ;::;::•;1-''..'::;•.;:... 5 Point ::.:.''..'.",:':::E,'••::::',.,'::•-',:•':.',:'.''''.':':..:'.,..„.-::;-.*;•:..'•.•.•::•''.....„‘i..:'-' ' :-. • •, 5 -aBasalt Holoe Sednt Holocene Sediment oidet, ' - cli to cetlime c cp (contains horizontal cobble beds') o . . -6 15 i _ . . .. .. • Bedrock , 20 • - 20 Bedrock r". Bedrock,below basalt sill shows southeasterly dip in the seismic records (2 2525 6- CO CO CO CO CO CO CO CO ;7- ;7' :-Jt' ;-1* CP' •:/* Th V' CV c\J C\I CV C\I CID (.0 CO (0 CO CO CO CO CO CO CO CO CO C/) ....,- - __ _ • • 2200 feet ____ _Figure 15.15. Composite profile of shallow traverses closest to shore showing thin Holocene overburden over bedrock. Note that bedrock forms the bottom in several areas, near Inspiration Point on the west, in the central area off the eastern side of the Landslide and off basalt and dolostone natural groins on shore. • • 4172000 4173000 4174000 4175000 4019000 _-774.. )4,---•---‘,.1 . r•-;:,v, -----.:.:-,,I4 --- 4- .... ''•,;:.;,.. :1 4176000 -I- ,,,4„417A70' 4178000 --1- ' C.' •,,,r' •rE Z: 4179000 i 4180000 4181000 rn.s 4187000 4019000 • .. ..., 2 MO, MI" ' '' '.--Asle.in•-5=3--"':... ‘`.. ., .„! •0 rn ., - .. 4018000. 4018000 ,4•0 2 A / - -- -- ;'_,"- ` 7.71- ''' 1. , \\\ I, ((c j, s,,,,,...„„.,... ,•7"--•- 22.-,.. :--,4,-- c '.----, ''.'- '-z,r-", '.,, 25 . ) Ins ,,-,-...,-...,1-:.7,,`<-44. 4 j--4 ,.r! ,}4. L- , I . ,.','.:\A.-3' s' 1 -\ "t44.....- 4!1- '-'---J"-• VT ' • 'Y ' '', 4017000. - / '/( s' , 04,--..,:-.7:...: !%,ic:s7-11 ' , _. ,....,,j., , , 4017000. -11 „,...,- • • •,,,,,,.; '',:', '`•••:•• -4\, 44,,,,\ • Aft' . i #3760 - 'N46...,,e' • ,,,,„...,'... \\, ,. „ • I4S 2 4 f-' 4 - ri s'-',.' • , 4016000 •,. \ . • IT' -1 4016000 75- . _,.r \ ....\....\, , \ s?..s,:.,.. _,,,. ..-...., • • . ,„ ____ '--\ (L ''' '''s•'''''':',--,„,,,,.., i -------., Western seismic 4.• •.. '\ ••- ,z---:,,,,, ---:s-7-: ,„ -.-..„ .,.- .. . . , \_. --=_.z..•_.:.--4-,= 4015000. profile %.....4 , . 4., ,,,i.,:-/\\ ''.: ,\'f( • . N\_, r . . \\\ L ,,1_,- \\:-..i.._\\ -1 00 N- -4...., , 4015000. \_, l,, r \ -6'•"-..( \ NN•'\'‘', --?5 \ ' -,----' 125-, • , ----... N N.1. 4014000. „ . 1 .k.. 4014000. \,,4- 1 N 1:-• \ .- ---.., .,... - ,t .” ---7 - '"-•., ,, N s Perry Extended Ale,. .4-7-, '''' -\ Land Profiles N .. 4‘, ••.-.\•,,•_.;__" - ---- :'' '--:'" . 4013000.- ' 4 .., •.... - ' 50 N 1 4013000 .' I , • , 1 • __---- .IP'#2494 "'4, , ', 125 100 v _.„, ,,..,75 4012000. - -----4- " ,• < "-- 4. N. 4172000 4173000 4174000 4175000 4176000 4177000 4178000 4179000 41800nn 4181000 4182000 N .---- --- .• Western NS Profile-2494-3764 N Bedrock exposures on seafloor , v -.- vv.-- 5300 feel ediecant to Inspiration Point -1,-- •-•• 0";-- ---T----r---•--7-r-,--T-• • • • 7-7-7---,----,------, • --- -7--1-•--",-- 1 • -, • T- 7 T5"" , • , ---,-- , T—T- r-I' ' • "--'- i 0 201 CoNsto hods In I lolorene I 20 - , 40 El' -6 "ii, ,,,,,,,,,..,,,• e 60 .2 .:, -'• .,"1. 5t)'''!: ' I -- -W-yo—'..'•,Ai--'-r. i cF...,' . •,. 83 7 , - — 7 0 . v " ,-•,.1-; ‘ ao too itotopliosedtment ,. 1v 20.21 i ri Benton' - ,q' 5 1. U'"e".f.5`'..t..k :Seismic words show south-easterly ,:f4. , -•„?y,„itz--16 jf:_,,,,'. ,, ; 'e*,.•J`. ,, ;r-A' f' " . faC-;,,, i . 2 _.. dipping beds under basalt -krit,7,A14144a- ,- .,4,-,., ,i,,,,,.,s, ,-,,,.„ 4-,, y ..- T). 160 _ • - 1 I lolocene sediment ... VA' -vmc:i.--? 'IVA:fgrem".4.1".'''''"c1°:;•:P A,„,,-',-1!k,a';''.:44,f,,,'f(''..AM,,,:,,,..:4-7tcrlantirsaomazettlail.: .5.& M J . •ser'45,...s,',..V,,,,,,41,t,r.:-,„ ,4;. ...,,, •.',;.....:,..a .?„,4,..,....j,,,,,;,.4t1.,:vi.,,,,,..,77,,,.,-.„,..J1 Beth.„k 4,•;,5..-f--,,,,,,,,,, v,',4-4,4,,W4;'-ttkl--.r. ` ,. . ....Z.:S1,:..,itir.7.g.,,=.5,:;„m,,y4.45,1;iy,r 160 u 1 --r ,rriv=k6:-Attypirgnow-ggfifoeipeopm4,930Litvam-,"04.;' )0*.f_frro%','"-f,R.`';'1,;.h--,:1;,'4'.:'''.'• ijaitAgrA:'';1;:iilitr4riAft c) _...—,—•-,,,;;• o J.• • •:•g• :g.''.;:s „ •„M , 0, 4. .+. $0.' fulVb...f ,,,'4,. ' .' , ZW,g,,,;,.,. ltriir 1.,i3,4 ,,,t,, ‘ .•, 4`'• ',,,,,'. *,(1...... ..j.,;;),eAf.,'?g;S;g0.%.:14r..,;XY;,+'.^1.'1 1 Bo 180 44 • `F,4,4 ,.,.a.4,'• .4 4,tbera,M. 4..,...-,,,,,.....val..+,.. ,., AVf.S. . g... ; •,• .1.*,./1.....,,S11_,M.441.MT4.1',0 Nit,I..4"Ori. /lit,;(ar z,...,,,...;.,,.., 0).,,,,,,<0.,•`+f c.,,, „•,r„,„!:, , .r,,,;,r tir_,V+1,,,,Ittf4 Ira•trg,,-1.0fr.40:11.;:r,l't:.„,:,";;; 600,014,0;011;;„;1,:.'.,:.'"Az..., .,..,..„..;.i.i c.f.„JIM-„?kli4V4-;....j:,p,;:r•" ts,54,.,', ;•i4;',:', -;;t;'4,;,,',,4875101r3743M COI.'•Wrir$:Rt iiSi 4,gr2r;:#"4:1-4V.i b;,.;;TWO:, V.,Fr4, ;'t •',,.;.4.,voliz,..,,,:tvit.,' •-i..'., lusw.130,NAtilm,F>fte,rto,,,:::', c'tg, ,IYInPR, P, `,41F, K; '1.1 .tz vats.1 .1', KIK, ,„ n T, „, ,,, ,,, ,,, ,,, r. ngin P, T, v.', P, ,':-., -, Station ID Figure 16.Western side of Portuguese Bend Landslide.Traverse runs north-south along the eastern margin of Inspiration Point. Cobbles and coarse sand are abundant near shore between stations 2562 and 2552 that would make vibrocoring in this area difficult.Sediment thickens seaward and the sediment bottom is featureless with little relief.Note:large vertical exaggeration (5.3 to 1). 4172000 4173000 4174000 4175000 4176000 4177000 08000 4179000 4180000 4181000 418200• 1110 �_ .._ y. zB• — ---- — 1 4019000 4019000 r �/ lC ;#u., A { rrt�� rr1•e (r 1.113‘.p1 E T�� r / \ )) 4 o ov �+ rw+.+ /Ir; < *tiv 70.. 4018000. 4, 4018000. , 11APP.tr f Kr:.. 25 J J/7it a ..' , \{•151 1 _r4 .1 '. �t r Y; i .Th) .. ---. - . ,, s ,. z 4017000. \\� `lir '-1 \ 3O 50 „ -#•#..... /I \. „'ti I:... \ #..\ ., 401600. 4(716000. + 4' M • # 75- +�.0 j7`� f 1, ,0 l \� \ 4015000. 4015000. W \ \\17,.. (� \ �. 'L, 125- e f -25 . ! �\\,l ` ` \ 4014000. w stNS...o aoa� 'ffe t \ l 4014000. .S � < _ ..- i Land P Noe ..------:1-, 4 it 1Z Z ��L�. 4013000. .j 50 4013000. S\'f . 125 #2968.A , 100T 7b 1. +— — \""....,4,0120W. 4172000 4173000 4174000 4175000 4176000 177000 4178000 4179000 4180000 4181000 4182000 \. N. East side North South profile 2902- 3024East side of P.Berl North South landslide.-__- � 1.._.__.. ..._...___ —...-_.. 8100 feet\ 0 ----- - ._. .._..,.—._._.__ N295U Holocene has numerous cobble beds near slrofn 40 40 _ fie,,...-. ., Et1dOf prvule on roup j 1 80 E .. - i I I i .. 120 I i g120 13 Holocene sectIment I { so15 1 Iso ❑liediot:k I i i .. �_. Bed roek . I . . . _i. 1 j. .. 200 V a, I 1 ' {i _ I 1 I I 4 240 Holocene Sediment X I I . i i i { _ 4 240 a _.r. I I i ; I 1 I 1 1 ; j I I m i i � , i { i { 1 MU 280 % ' 1 J ' -1 1 i I I I 1 r 1 1 i 1 ` 1 { 1 I 1 1. . 1 t f 1 { I i ; I �, { ago ' 1 I I /;. 1 Ip I I I I j I I ' I 3 I Iy 1 �I y� I yg �3 g g g gg g azu A A 1 1 iC iC iC EQ ; A 'i ••A O A A A A 'A A� i X1 A A ftl % A A R! A A ti l A l R % i l lig i i'3 A 8)4/on It) Figure 17.North-south profile from shelf break to the eastern side of the Portuguese Bend Landslide.Note that track on map does not go to the end of the profile.Vertical exaggeratiob+ is 5.2. . iiii- 04172000 4173000 4174000 4175000 4176000 4177000 Mir 4179000 4180000 4181000 4182000 • why A 9 --i, x esu-i,3 . 4019000 — _-\r) t. it ,... • j F�2 r FFFKKK' I•IS J 11� L..; �`'"•\--.. �\ _ ';,, ,'-'?'-=. : �' 4018000. 4018000.I' \,- -- , �jy��/1 ( / -Nom' :�` :/ �'�, - 4017000. 4017000. - `\..__ "'l�. il.. q hr.., \ ��. / fid \. 116007 " �\'\\\ i'�y.„ : ,--, eu,d i,�n dada 1 'l 4016000. ' ti \` `, \ .. `' )\ 4016000. 75._ timir/wap ''�, \ '`--.. ,--.7A.1j �.. - `t.'p] 4015000. W ` ��.n} ,\�\ \`�- \ .�.i1 4015000. \4-,,,4 i \ Y \--25 \' \ \ ' 125. �� C. ��_ ;1 \ 4014000. 4014000. " \ 1, I Eastern NS Seismic - �� ` 1. Profile #6007-6096 -_ ) 1 r- \ , -\ \ Perry Extended 1 Lend Profiles 4013000. 4013000. 50 \ L. v I. 'v 125 \ _.._.- - -------... -- // 100 $6071 75 .-- -- I ----I--....- - ----\12000. 4012000.I-----• -- t—------------- .� 4172000 4173000 4174000 4175000 4176000 4177000 4178000 4179000 4180900 4181000 4182000 End of track ,Af) • on map#6071 - ) N North South Profile 6007-6095 Cr') JQ �r tl Eastern side of P.Bend Landslide \ North Sough _ _ _ _ 6000 feet -- -- Jj 4 , to ..::. 1 � .. , i I .t iHi f , t? � _ — , 100 z � k 2 Y ` ' ; 1 E i . 1. -1- 1 Y L..L_,.- ■Bedrock 150 v 150 Ho 200 s g 200 m 0 _ 250 1 I 1 1 1 1 i 1 1 i 1 ir fci iri 1 I 1 1 1 1 ci § § §Itirg § § li 250 Station ID • • Figure 18.NS profile running south from the eastern edge of the Portuguese Bend Landslide.All of the traverse was over a Holocene covered bedrock,however the traverse skirts the western edge of a rocky bottom that forms the eastern edge of the survey area.Note that the profile extends beyond the boundary of the location map.Vertical exaggeration 3.6 to 1. Profile Summary All profiles,those presented and those seen on the original survey records show there is a thin cover of coarse sediment with cobble beds nearshore. This Holocene sedimentary body looses internal reflectors approximately 1000 feet offshore in depths beyond 40 feet. Holocene sediments thickens seaward out to a depth of approximately 200 feet where sediment transporting currents are no longer capable of carrying near shore derived sediments to the Shelf s outer edge. The major east-west fault that crosses the area can be seen on all north-south traverses. Although there is an indication of vertical displacement of bedding within bedrock at the fault. this is not reflected in the overlying Holocene cover. Undisturbed sediment with flat lying internal bedding overlying the disturbed areas in underlying bedrock indicates that the fault has not been active since being covered by Holocene sediment, approximately 8000 to 5000 years ago. Prior to that period this was dry land with no marine sedimentation. Traverses made seaward of Portuguese Bend Landslide are similar to those made in the 1989 Abalone Cove survey using the same equipment. Therefore it was possible to combine data and construct a map relating major landslides affecting the south shore of Palos Verdes Peninsula. Comparative Studies • Abalone Cove Survey Many of the reflective acoustic discontinuities found during our 1989 side-scan and seismic survey of Abalone Cove Survey are also seen in this Portuguese Bend survey records. Therefore it is useful to review previous findings, relating cored sediments to acoustic reflectors, because they can help in the interpretation of the records from Portuguese Bend where cores have not been taken. The findings from the Abalone Cove Survey also provide information on the feasibility of vibrocoring and whether any additional subbottom information could be obtained by vibrocoring in Portuguese Bend. The Abalone Cove Survey was designed to determine if bottom topography or sedimentary features seen in seismic records indicated recent movement along landslide glide planes in bedrock or in the Holocene overburden covering bedrock in Abalone Cove. A thick blanket of Holocene marine sediments covers most of the bedrock of this area. Truncated bedrock bedding forms a gently seaward sloping rock surface that shows this region was subjected to nearshore erosion prior to or during the last rise of sea level. The sedimentary blanket seen in the seismic records must have been deposited since this flooding and must therefore be less than 3,000 years old, probably much younger. High • resolution seismic records show that acoustic reflectors in the Holocene sediments are lenticular in shape and concentrated in zones that run parallel to the present shoreline. We 35 a projected zone of weakness within weathered basalt that has been encountered in onshore • core holes. The seismic and core evidence indicate that the present active landslide terminates at the shoreline along the northeastern margin of Abalone Cove and does not extend offshore. Bioturbation and continued low energy wave activity smooth out storm generated relief after sediment transport ceased and sedimentary boundaries became less distinct. Both side scan-sonar and diving observations showed that there is no surface expression of landslide activity seaward of the present shoreline exposure of the slide plane. However, because of the inability to use boat supported side-scan and seismic instrumentation shoreward of the surf zone along with the dense kelp beds over the nearshore rocky bottom the seismic survey could not cover shallow areas with enough detail to positively conclude that there has not been ancient movement along sub-bottom acoustic discontinuities within the bedrock out to depths of 15 feet, the minimum safe depth to operate a boat in this area. However the lack of exposed cobbles ridges or any indication of exposed beds thrust to the seafloor from below is strong evidence that Holocene sediments have been stable since the flooding of the region during the Holocene rise of sea level. Undistorted acoustic reflections (that coring showed to be buried cobble beds) are strong evidence against • offshore recent landslide movement for at least the past 3000 years. Divers found outcrops of basalt in the central part of Abalone Cove, where a set of linear ridges appears to trend into an abrupt break in bottom type. This would indicate that the western margin of the "ancient landslide" described in Ehlig (1987) is stable and rock outcrops are not displace Miocene sedimentary rock carried seaward by an old landslide slip plane. Although this boundary, has a similar trend and lines up with a suspected ancient landslide boundary and valley on land, (Robert Stone, et al., 1987) it does not appear to be active at this time and is probably a sediment filled channel formed during lowered sea level. However, important questions remained after the survey. We could not determine from the data collected if past landslide movements extended offshore prior to Holocene deposition. We know this was a time when the larger ancient landslide covering much of the region shoreward of the Abalone Cove Slide was active. Most importantly we could not determine without offshore drilling if planes of weakness within the bedrock of the region resulting from these ancient landslides could be reactivated and displace all proposed remedial mitigation efforts planned for the Abalone Cove Area. It was submitted to the Rancho Palos Verdes Abalone Cove Technical Panel to provide • them with a more complete picture of the offshore geology of the region and the nature of 37 of two seismic lines showing Holocene sediments and 5) a geological interpretation map ibased on the seismic stratigraphy of the region. Concentrated off the Will Rogers Beach State Park seaward of an area known as Castle Rock, seismic traverse lines extended 2 miles offshore to a point where it became clear that the geology being recorded was not related to land stratigraphy. Traverses parallel to the shoreline were run between the juncture of Tuna Canyon on the west to just east of the juncture of Sunset Boulevard and Pacific Coast Highway a total distance of about 2.5 miles. It became apparent early in the survey that the nearshore area north of a reported offshore fault exhibited a poorly defined and chaotic stratigraphy with little coherent structure. This type of acoustic record is characteristic of geological formations that have suffered extensive fault and/or landslide movement which brings about destruction of well defined primary depositional features. This lack of structure within bottom sediments extended seaward (south) almost to a projected east-west trending offshore fault identified as the "Malibu Coast Fault" on maps of the region and show a line of gas and oil seeps along the fault to the west of our study area. A preliminary reconnaissance of the onshore geology prior to the survey showed that the sedimentary rocks forming the cliffs and shoreline exposures of bedrock are chaotic and • highly disturbed by thrust fault and landslide movements; similar/4o those at Portuguese Bend. In areas where our seismic traverses crossed the projected trend of the Malibu Coast Fault zone, both echo-sounder and seismic records show what appears to be bubble screens, both in the water column and within the sediment. This phenomena indicates that the geological structures of the region are leaking gas from fractured underlying petroliferous formations similar to areas reported by Greene and Kennedy (1986) on the Malibu Coast Fault further to the west and that this fault related release of gas extends into our area of study. Our survey indicates that there may be two east-west trending offshore fault zones one crossing the sea floor along the 60 to 70 ft. contour and one along the outer limits of the survey in an area cut by a buried channel system. The outer fault zone has tilted terraces along cut and fill structures within the buried channel,indicating movement prior to the area being covered by Holocene sediments. What is striking however,is the lack of topographic relief seaward of the present coastline which rises abruptly at the shoreline to a series of uplifted and tilted coastal terraces. Viewed from sea, there is a notable tilting of the two mesas, Parker and Castellammare, showing that this region has a long history of tectonic • displacement. This is not surprising, noting its close proximity to the large thrust faults that , 39 Acknowledgments • Special thanks are extended to Perry Ehlig City Geologist for Rancho Palos Verdes, who provided valuable advice and corrections to the final report along with historical records, reports, and land maps for our study. Charles Abbott and Don Wilson of Charles Abbott Associates, Inc. provided guidance throughout the study. Dr. Robert Walker of the Corps of Engineers provided suggestions and comments that greatly improved the final report. Neil and Kyle Marshall, and Karel Zabloudil provided logistical support both at sea and in the office from EcoSystems Management Inc. Their review of the material as it was being prepared for this report were invaluable. We also want to thank the secretaries at CAA for providing us with support when presenting the preliminary results of this report. The public works crew and park ranger of Cabrillo Park were extremely helpful and provided a safe over night storage of our valuable equipment and boat during the survey. • • 41 Fischer, P. J. and Rudat, J. H., 1987a, Late Quaternary Seismic stratigraphy and shelf deposits of the San Pedro to Oceanside Shelf. (ed.) Fischer, P. J., in: Geology of the Palos Verdes Peninsula and San Pedro Bay, Volume and Guidebook No. 55, Pacific. Sec. SEPM., Los Angeles, CA, p. 79-90. Fischer, P. J., Kreutza, P.A., Morrison, R. L., Rudat, J. H., and Young, M. 1983, Study on Quaternary Shelf Deposits (Sand and Gravel) of Southern California. Dept. of Boating and Waterways, State of California, Sacramento, CA, 66 p. Fischer, P. J., Rudat, J. H., Patterson, R. T. H., Darrow, A. C., and Simila, G., 1987b, The Palos Verdes Fault Zone: Onshore to Offshore. (ed.) Fischer, P. J., in: Geology of the Palos Verdes Peninsula and San Pedro Bay, Volume and Guidebook No. 55, . Pacific. Sec. SEPM., Los Angeles, CA, p. 91-133. Heilman, J. A., Allen, C. R., and Nordquist, J. M., 1973, Seismicity of the southern California region, 1 ;January 1932 to 31 December, 1972. California Institute of Technology, Division of Geological and Planetary Sciences, Contribution No. 2385. Jahns, R. H. and Vonder-Lin4 K., 1973, Space-time relationships of land sliding on the southerly side of the Palos Verdes Hills, California. Special Pub. Association. Engineering Geologists, p. 123-138. Kerr, P. E., and Drew, I. M., 1969, Clay mobility in Portuguese Bend California. Bull. California Division of Mines and Geology, Short Contribution, p. 3-16. • Lee, W.H.K., U. Yerkes, R. F., and Simirenko, M., 1979, Recent earthquake activity and focal mechanisms in the western Transverse Ranges, California. US Geological Survey Circular 779-A, p.1-26. Legg, M. R., 1986, Earthquake epicenters and selected fault plane solutions of the Mid- southern California continental margin, in, H. Gary Greene and Michael P. Kennedy (eds.), California Continental Margin Geologic Map Series, Mid-southern California Continental Margin Area 2 of 7, Map nos. 2A.,2 B, 2C,and 2D, California Department of Conservation, Mines and Geology and US Geological Survey Publication. Leighton and Associates, 1974, Geotechnical investigations, proposed regional park Abalone Cove,Palos Verdes Peninsula Los Angeles,California. 3 plates, 33 p. Merriam, R., 1960, Portuguese Bend Landslide, Palos Verdes Hills, California, Journal. of Geology, v. 68, p. 140-152. Robert Stone and Associates, 1979, Geotechnical investigations of Abalone Cove Landslide, Rancho Palos Verdes, Los Angeles County, California. Final Report submitted to City of Rancho Palos Verdes, California., Job No. 1370-00. 54 p. Shepard, F. P., 1973, Submarine Geology. Harper and Row, New York, 517 p. Shepard, F. P., and Dill, R. F., 1966, Submarine canyons and the sea valleys. Rand McNally, New York, 381 p. • Slosson, J. E. and Havens, G. W., 1987, Mitigation rather than litigation of the Abalone Cove Landslide. ed: Fischer, R J., in: Geology of the Palos Verdes Peninsula and San 43 Field Methods and System Description Seismic Survey system The survey was conducted from a 25 foot survey vessel specially equipped for shallow water seismic surveys (Fig.4)The boat was launched from the boat ramp at Marina del Rey approximately 8 miles south of the survey site. The survey system is specially designed to work in nearshore areas where larger vessels can not safely obtain data. Two seismic systems were used in the survey to obtain the detail needed to identify anomalous bedding or sedimentary features related to offshore slumping, landslide displacement or fault activity; 1.) a High-Resolution, Subbottom Profiler(HRSBP) and 2.) a "Boomer." This equipment package allows subbottom structures just below the sea floor surface to be examined in detail by high frequency acoustic energy and the deeper structures by lower frequency sound energy which does not give the detail but has the capability of deeper penetration.By operating both systems at the same time and recording returns on the same chart we could compare, in real time, both deeper structures from the "Boomer" with the structures in the overburden above bedrock. This system was used successfully in surveys off Palos Verdes Peninsula south of the study area (Dill, 1989; Dill et. al., 1990; and Dill and Slosson, 1993) and was used in this survey because it optimizes the ability to interpret all subbottom features that might be related to faulting and landslide displacements. Seismic traverses were made at a constant RPM that gave us and average speed of 4.5 knots. Recording was on a dual channel Precision Graphics Recorder (PGR) thus permitting a direct correlation between the type of signals being received at different depths of penetration and resolution. Operating at 3.5 KHz the HRSBP system allowed detailed observation of sedimentary structures and continuity of beds within the overburden. In several areas the bedding of the underlying bedrock was also observed on the 3.5 KHz. system. Dual channel recording permitted us to compare,on the same record, the Holocene sediments internal structures defined by the 3.5 KHz soundings with structures seen in the underlying bedrock. structures seen on the "Boomer" records. In several areas where overburden thicknesses exceeded the resolution of the 3.5 KHz system the lower frequency system permitted determination of depth to bedrock. The recorder sweep rate of 1/4 second provided a depth scale for the "Boomer" of 200 meters, with scale lines every 20 meters. The 3.5 KHz recorded every 1/8th second giving a full scale depth range of 100 meters with scale lines every 10 meters. Navigational fix marks were placed on the chart every 15 • seconds and numbered to correspond with the fix locations provided by the MiniRanger 45 calibrate the instruments and take in account variations of sound velocity in the survey area. • The bar check is a standard offshore survey calibration method. It is done by lowering a reflecting metal plate over the side beneath the transducer with a calibrated steel wire cable. Soundings recorded on the recorder tape at depths of 10, 20, 30 and 40 feet are used to measure the distance between the outgoing signal and the echo return to calibrate the system.This value is then used to determine water depth, and acoustic reflectors within the sediment and underlying bedrock. The sound velocity within the sediment overburden and the underlying bedrock are higher than in the water column velocities therefore a velocity correction must be made for the decreased travel time to obtain corrected thicknesses. In this study we used the average values of sound velocity obtained for sediments and rock types off southern California by Moore (1969) and Hamilton (1971). The Moore and Hamilton values were obtained by using a diver operated probe that measured seawater velocity between two transducers at a fixed distance of one meter. The probe was then pushed into the sediment and allowed to come to equilibrium and the velocity of sound then measured in the sediment between the transducers. Values taken in situ by probes were later checked using sediment cores taken at the underwater sites and retrieved for laboratory determinations. Additional measurements were made using sonabuoys to measure refraction velocities in the southern • • California region in both shallow and deep water. The average velocity for unconsolidated California shelf sediments is 1700 m/sec. Thus for our values we apply a velocity correction of thickness measured on the chart (t ) x 1.17 = overburden thickness. For the underlying rock we used a correction based on an average of 2110 msec for the sound velocity in Neogene sedimentary rocks given in Moore (1969) and used,by Fischer and Rudat (1987) in their report on the sediments and structure of the San Pedro and Santa Monica Shelf. By using the same values we will be able to compare values of depths reported in other offshore surveys and calculated rock thicknesses in areas to the east of this survey. Thicknesses of penetration were made by measuring the thickness of reflective beds that were continuous over several fix locations, using the first arrival of an echo from the subsurface rock/sediment interface to locate and define a bed for measuring and using (ts ) x 1.45 =corrected rock thickness. The later value was also used to determine apparent dip by measuring the distance along the bottom between a fix where continuous beds were truncated by the sediment/rock erosion surface and tracing them down dip over a given distance along the bottom determined by a subsequent fix on the traverse. The tangent of the distance across the bottom and the depth to the top of the bed followed down dip over this distance gives the apparent angle of dip for the acoustic reflector. It is assumed that the 47 Plots of the survey lines were constructed using Surfer (v 4.0) PC software and a Hewlett Packard 7576 plotter. The navigational data along with measurements of depth and overburden thickness were placed as a text file in an Excel software spread sheet that could be read using Surface IIF, v 2.5.1 , a mapping and plotting software for the Macintosh developed by the Kansas Geological Survey. Using Surface III'M plots as a base for map construction, Adobe Illustrator 5.5 (a EPS graphics program) and Canvas 3.5.3 were used to construct the final maps and charts in this report. Seismic records and data sheets obtained on this survey have been turned over to CAA. INC. for distribution. • • 49 Appendix D - Profiles of sediment thickness over bedrock off the Portuguese Bend Landslide. 1 • 53