A new depositional model for the classical turbidite locality at San Clemente State Beach, California

Transcription

1 A new depsitinal mdel fr the classical turbidite lcality at San Clemente State Beach, Califrnia Hilari Camach, Cathy J. Busby, and Ben Kneller ABSTRACT The Micene turbidite system expsed in the beach cliffs at San Clemente State Beach, Califrnia, has been used by industry and academia alike as a field labratry. It has been used as an analg fr petrleum reservirs in the Ls Angeles Basin and ther areas. We interpret the turbidite system at San Clemente State Beach t represent the fill f a single turbidite channel that aggraded subvertically. This interpretatin is based n cnstructin f a detailed phtmsaic, mapping f three-dimensinal facies distributins, measurement f representative sectins, and cllectin f new palecurrent data; we als extended this analysis t utcrps nt previusly described in the literature. The channel had a minimum width f 1 km and a paletransprt directin tward the nrthwest. Our new palecurrent measurements indicate an average transprt directin f 321 fr the channel depsits, apprximately perpendicular t previusly published palecurrent data. Our palecurrent data are in agreement with the trend f the channel as defined by facies mapping. The lateral and vertical facies changes alng the sea cliffs at San Clemente are a result f interfingering between axial and marginal facies within a single turbidite channel. We interpret the turbidite channel t have been cut int a lw-gradient cntinental slpe (less than 1 ), rather than representing a channel n a submarine fan. We prpse that the Gllum channel system is a mdern analg f the turbidite system at San Clemente State Beach. INTRODUCTION The turbidite system expsed n the sea cliffs at San Clemente State Beach, Califrnia, has been used by industry and academia alike as a field labratry and an analg fr unpublished studies f subsurface turbidite reservirs. The sedimentary sectin at San Cpyright The American Assciatin f Petrleum Gelgists. All rights reserved. Manuscript received March 13, 2000;revised manuscript received August 6, 2001;final acceptance March 18, AUTHORS Hilari Camach Department f Gelgical Sciences, University f Califrnia, Santa Barbara, Building 526, Santa Barbara, Califrnia, 93106; camach@gel.ucsb.edu Hilari Camach received a B.Sc. degree in gelgy frm the Universidad de Granada, Spain, in 1992 and an M.S. degree frm Califrnia State University, Lng Beach. He is currently a Ph.D. candidate at the University f Califrnia, Santa Barbara, and is chief gelgist at Signal Hill Petrleum, Inc., in Lng Beach, Califrnia. His research interests are fcused n fluid flw in sedimentary basins, il generatin and migratin, and siliciclastic diagenesis. Cathy J. Busby Department f Gelgical Sciences, University f Califrnia, Santa Barbara, Building 526, Santa Barbara, Califrnia, 93106; cathy@gel.ucsb.edu Cathy Busby received her B.S. degree frm the University f Califrnia at Berkeley in 1977 and her Ph.D. frm Princetn University in She then jined the faculty at the University f Califrnia at Santa Barbara, where she has been a prfessr since As part f her research n the tectnics f sedimentary basins, she has published n turbidite facies architecture in utcrp, making cmparisns with mdern analgs and subsurface examples. She teaches turbidite curses at the graduate level and fr industry. Ben Kneller Institute fr Crustal Studies, 1140 Giruetz Hall, University f Califrnia, Santa Barbara, Califrnia, 93106; ben@crustal.ucsb.edu Ben Kneller gained his B.Sc. degree frm the University f Sheffield and his Ph.D. frm the University f Aberdeen, United Kingdm. He was frmerly n the faculty at the University f Leeds, where he initiated and, fr seven years, led the Turbidites Research Grup. He is nw a researcher at the University f Califrnia at Santa Barbara. He has applied a cmbinatin f traditinal facies-based field studies, experimental wrk, and thery t turbidite prcess sedimentlgy. He regularly cnsults and teaches shrt curses fr industry. AAPG Bulletin, v. 86, n. 9 (September 2002), pp

2 ACKNOWLEDGEMENTS This wrk was funded by grant number PRF AC8 frm the American Chemical Sciety t Cathy Busby and by a gift frm ARCO t Cathy Busby. Additinal supprt frm Signal Hill Petrleum, Inc., t Hilari Camach is als acknwledged. The article was greatly imprved by the cmments f William Mrris and the frmal reviewers W. Nrmark and J. Cleman. Thanks t Kari Bassett fr assisting Cathy Busby in shting the phtmsaic and making the earliest line drawings. Discussins in the field at San Clemente with Jeff Peakall are gratefully acknwledged. Clemente State Beach (Figure 1) cnsists f carse-grained t finegrained sandstnes, siltstnes, and mudstnes, as well as lesser cnglmerates. These upper Micene lwer Plicene depsits (Ingle, 1971) were infrmally referred t by Weser (1971) as the San Clemente lens f the Capistran Frmatin. Framinifera assemblages frm the Capistran Frmatin arund the Dana Pint area indicate midbathyal palewater depths f apprximately m (Ingle, 1971). Several previus studies have prpsed depsitinal mdels fr the sedimentary sequence at San Clemente (Weser, 1971; Walker, 1975; Hess, 1979; Clark and Pickering, 1996). All f these authrs interpreted the turbidite system at San Clemente t recrd depsitin in a deep-sea fan envirnment. These studies fcused n the analysis f vertical trends (ne-dimensinal [1-D] analysis) and n the tw-dimensinal (2-D) lateral facies distributin alng the utcrp (2-D analysis). Our study is based n a three-dimensinal (3-D) analysis f the utcrp, including utcrps nt previusly described in the literature (Figure 2), and the cnstructin, descriptin, and interpretatin f a detailed phtmsaic f the castal utcrp (Figures 3, 4). The detailed phtmsaic, cmbined with a new set f palecurrent data (Figure 5) and measured stratigraphic sectins (Figure 6), has allwed us t determine the 3-D lithfacies distributin f the turbidite system at San Clemente. We prpse that the turbidite system at San Clemente State Beach represents the fill f a single channel cut int a lw-gradient slpe (see the summary interpretatin at the base f Figure 4). The channel had a minimum width f abut 1 km and a paletransprt directin tward the nrthwest (Figure 7). PREVIOUS WORK 1544 Turbidite Mdel at San Clemente State Beach (Califrnia) Weser (1971) used vertical-sequence (1-D) analysis t interpret the turbidite system at San Clemente as the fill f a channel in the prximal part f a deep-water submarine fan (the upper fan f Nrmark [1970]). Weser (1971) prpsed that the submarine fan develped during the late Micene in a nrtheast-suthwest trending embayment lcated suth f the Ls Angeles Basin. Weser (1971) inferred that the sediment surce fr the submarine fan was lcated in the present-day ffshre tward the nrthwest. Weser s (1971) study cncentrated n the suth and central parts f the castal utcrp (sectins A, B, and C f Figure 2). Walker (1975) studied the 2-D architecture f the utcrps at San Clemente State Beach, cncentrating n the utcrps suth f the state beach parking lt (sectins A and B f Figure 2). On the basis f the lateral facies distributin, the presence f mudstne-siltstne drapes, vertical trends, and palecurrent data, Walker (1975) suggested that the turbidite system at San Clemente represents the fill f eight nested, laterally migrating channels within the suprafan area f a submarine fan (see Walker s [1975] interpretatin at the base f Figure 4). Walker (1975)

3 55 Freeway 405 Freeway 73 Freeway Study Area Pacific Cast Hwy. 133 Freeway Laguna Beach 5 Freeway Ortega Hwy. 74 N Pacific Ocean Dana Pint Capistran Beach Pacific Cast Hwy. San Clemente Ave. Pic Orange C. San Dieg C. San Clemente State Park Figure 2 5 Freeway 0 5 kilmeters Figure 1. Lcatin f San Clemente State Beach, shwing area mapped in Figure 2. Camach et al. 1545

4 Figure 2. Lithfacies map f the San Clemente State Beach area, interpreted here as a nrthwest-trending turbidite channel cut int a lw-gradient cntinental slpe. Carsegrained depsits in the center f the channel are mapped as channel-axis facies, and finer grained channel-fill depsits n either side are mapped as channel-margin facies. Very fine grained, nnslumped depsits in expsures suth f the channel wall are mapped as lwgradient slpe depsits. Slid lines labeled A E shw the lcatin f phtmsaics and line drawings in Figures 3 and 4. Previusly published descriptins include sectins A C by Weser (1971), Hess (1979), and Clark and Pickering (1996) and sectins A and B by Walker (1975). There are n previusly published descriptins f sectins D and E. Dts labeled 1 5 indicate lcatins f measured sectins in Figure 6. E PACIFIC OCEAN N Channel-axis facies Channel-margin facies D Channel-margin facies C State Beach Parking Lt 5 4 B 5 Freeway Lw-gradient slpe facies A Paths Campgrund 100' km Cntact, dashed where inferred 20' Tpgraphic cnturs at 20 ft intervals inferred paletransprt directins that ranged frm nrthwest t suthwest (apprximately utward frm the cliffs) during channel migratin. Hess (1979) expanded Walker s (1975) wrk by incrprating the sectin nrth f the state beach parking lt int his study (sectin C in Figures 2, 4). Hess (1979) agreed with Walker s (1975) interpretatin f laterally migrating channels but interpreted the channel cmplex t have frmed in a mre prximal setting n an inner suprafan. Clark and Pickering (1996) applied architecturalelement analysis t the turbidite system at San Clemente, analgus t that develped fr fluvial systems by Miall (1985), and cncurred with Walker (1975) that the turbidite system at San Clemente State Beach represents a series f laterally migrating channels in the middle-fan area f a submarine fan. We previusly prpsed that the turbidite system at San Clemente State Beach represents the fill f a single channel cut int a lw-gradient slpe (Busby et al., 1998; Camach et al., 2000), and we presented ur phtmsaic interpretatin in an infrmal guidebk (Nilsen et al., 2000). We frmally publish ur results fr the first time here. OUTCROP DESCRIPTION We have cnstructed a detailed phtmsaic f the sea cliffs and cmbined it with the mapping f side can Turbidite Mdel at San Clemente State Beach (Califrnia)

5 SECTION A CHANNEL-AXIS FACIES Ersin surfaces t numerus t label r trace with cnfidence Massive t faintly laminated pebbly sandstnes with lad structures and siltstne blcks. Inland expsure f Tp f sectin 3 (Figure 6) cbbly and buldery Uncnfrmity sandstne ES-F ES-E ES-D Interfingering channel-axis and channel-margin facies Gully: thin-bedded sandstne and siltstne cut by ersin surfaces verlain by pebbly sandstne with siltstne clasts. Beds thicken and carsen inland t pebbly sandstnes with lad structures, amalgamatin surfaces, siltstne rafts, and cnvlute laminatins. ES-C CHANNEL-MARGIN FACIES ES-C LOW-GRADIENT SLOPE FACIES Beds thin tward and drape scur as sandstne-siltstne cuplets Massive, biturbated, silty t sandy mudstnes and lesser laminated siltstnes Sectin 1. Figure 6 ES-A ES-B Sectin 2, Figure 6 Light-clred medium- t fine-grained sandstne beds with planar laminatins and ripple crss-laminatins. Tb-c Buma divisins with cnvlute laminatins. Ersin surface Scurs A6 Figure 8A Restrms Siltstne-sandstne Ersin surface cuplets ES-C Base f sectin 3 (Figure 6) Nrth ftpath, Figure 2 Massive t faintly laminated thick pebbly sandstne beds bunded by ersin surfaces, minr interstratified thinbedded laminated, fine-grained sandstne and siltstne in graded beds. N vertical trends Multiple ersin surfaces. Lenticular beds due t wedging nt ersin surfaces and scuring. Ripple crss-laminatin indicates palecurrent directins f 323, 313, 288, 303, 360. A5 20 m. ES-B' ES-B ES-B' Suth ftpath (Figure 2) A3 A4 Channel wall Prechannel lw-gradient slpe depsits. Planar nnslumped, Trend: Dip: 21 N Cvered sedimentary rcks. Palecurrent Beds terminate abruptly against it thin bedded, fine-grained directins 298, 303, 268, 288, 313 Thin- t medium-bedded, fine- t medium-grained sandstne and minr siltstne with carsest and A2 thickest beds in center f expsure. Siltstnes and fine sandstnes heavily biturbated ES J ES I 10 m. 0 m. A1 ES G SECTION B SECTION B CHANNEL-MARGIN FACIES Flame structure indicates transprt directin f 303 Laminated clayey siltstne beds m thick. Crrelative with beds n sectin C. INTERFINGERING OF CHANNEL-MARGIN AND CHANNEL AXIS-FACIES TOWARD CHANNEL AXIS TOWARD CHANNEL EDGE CHANNEL-MARGIN FACIES Ripple crss-laminae 318, 318 Ersin surfaces t numerus t label ES-K Sectin 4, Figure 6 Gulch: scur surfaces dip steeply inland n faces perpendicular t beach cliff Parking Lt ES-J ES-J ES-G 20 m. Scur Massive pebbly sandstne with abundant blcks f laminated siltstne ES-I Nrmally graded, pebbly sandstnes B3 Slump B2 Lw-angle scur surfaces. Massive t Figure 8C parallel, riple, and cnvlute laminated sandstnes and granule sandstnes. Finer grained beds are heavily biturbated Sandstne beds thicken and carsen. Ta-c Buma divisins Lw-angle scur surfaces n the cliff face. Sandstne beds thicken and carsen tward channel axis. Fine-grained sandstnes and siltstnes are biturbated. Scurs dip tward channel axis Laminated t massive sandstnes and laminated siltstnes ES-H Interfingering ES-G Carse tail grading Massive cbbly sandstnes Figure 8B B1 LEGEND Thin-bedded sandstne ES-A, ES-B, etc. refer t ersin surfaces A1, A2, etc. refer t lcalities described in text Slpe depsits Medium- t thick-bedded sandstne Figure 3 Suthern half f the turbidite system at San Clemente (nrthern half shwn in Figure 4). Phtmsaic and line drawing f sea cliffs fr sectins A and B (lcatins shwn in Figure 2). Fr summary interpretatin, see Figure m. 0 m.

6 SECTION C Channel-Axis Facies 20 m. 10 m. Ersin surfaces dip suth Gully shws the deepest scurs t be symmetrical Sandstne with dispersed pebbles ES-L 0 m. Inclined pebble laminatins C1 Siltstne blcks Cbble cnglmerate C4 Sectin characterized by massive, nnlaminated, pebbly t carse sandstnes. The pebbles are smetimes dispersed thrughut the sandstne and in ther cases the pebbles frm distinct layers. C3 Channel-Axis Facies: characterized by pebbly t carse sandstne C2 and cbble cnglmerate. The sandstnes are massive t faintly laminated with ccasinal gravel laminatins. Siltstne is uncmmn hwever siltstne blcks are cmmn. The siltstnes and fine sands are parallel-and cnvlute-laminated and frm discntinuus beds. Figure 8D Figure 8E and 8F SECTION D SECTION E Channel-Axis t Channel-Margin Facies transitin Channel-Margin Facies 20 m. Thick massive sandstnes with parallel laminatin Medium- t thick-bedded fine- t medium-grained sandstne expsed inland frm cliff Cvered Cvered Cvered Cvered Cvered Cvered Cvered Massive sand Massive t laminated sandstnes. The ersin surfaces are lined with rip-up clasts. D2 Interfigering channel-axis and channel-margin facies Channel margin Channel axis Sutheastern Channel wall 20 m. PATH FROM CAMPGROUND AT TOP OF VALLEY Gap 3 10 m. 0 m. Slpe depsits Sectin E Sectin D LEGEND Sectin C Sectin A Sectin B Walker's (1975) interpretatin f the turbidite system at San Clemente State Park (includes nly sectins A and B f ur study, shwn abve): interpreted as a cut perpendicular t a laterally migrating series f suprafan channels. Circled numbers indicate individual channels. Thin-bedded sandstne Slpe depsits ES-A, ES-B, etc. refer t ersin surfaces A1, A2, etc. refer t lcalities described in text Medium- t thick-bedded sandstne Figure 4 Nrthern half f the turbidite system at San Clemente (suthern half shwn in Figure 3). Phtmsaic and line drawing f sea cliffs fr sectins C E (lcatins shwn in Figure 2). Summary interpretatin als is shwn. PLEISTOCENE TERRACE PARKING LOT biturb. msts WIDE GULLY cglm. msts. biturb. cgls 40 m. LIFEGUARD PATH FROM CAMPGROUND AT TOP OF VALLEY biturb. msts biturb. msts. cgls. cgls. 20 m. lwer cliff edge biturb. msts. mainly massive ssts. mainly massive ssts.? 0 METERS PATH RESTROOMS PATH TUNNEL 450 prly 2 expsed 0 m. D1 Interfigering channel-axis and channel-margin facies Channel margin Channel axis Gap 1 Gap 2. PARKING LOT Thin- t medium-bedded sandstne and siltstne, biturbated. ES-N Thick-bedded, medium-t carse-grained, massive t parallel laminated sandstnes. Sme beds shw amalgamatin surfaces and cnvlute laminatins. Fine-grained sandstne, parallel laminated Palm Tree NEW SUMMARY INTERPRETATION OF SAN CLEMENTE TURBIDITE SYSTEM: Interpreted as an blique cut thrugh a single, vertically aggrading turbidite channel cut int lw-gradient slpe depsits Channel-Axis t Channel-Margin Facies transitin ES-M ES-N Cvered ES-O Thin- t medium-bedded sandstne Gap 4 10 m. Massive sand Ersin surface Channel margin Pebbly t carse-grained sandstnes Ersin surface Cvered 500 I m.

7 Mean Figure 5. Palecurrent data frm channel-axis facies, channelmargin facies, and slpe depsits, based n ripple crsslaminatins (lcatins shwn in Figure 3). Mean transprt directin is 321 fr channel depsits. This value is in agreement with measurements f the rientatin f the suth channel wall (pltted n Figure 3). This is als similar t the transprt directin inferred frm ripple crss-laminatin in the lw-gradient slpe facies, int which the channel cuts (als pltted n Figure 3). The radius f the diagram is prprtinal t the square rt f the frequency. Circular variance r 0.1. yns, measurement f sectins, and analysis f palecurrent data t determine the 3-D architecture f the turbidite system at San Clemente State Beach. We divide the phtmsaic int sectins A E (Figures 2, 3, 4). Sectin A Sectin A is the suthernmst utcrp shwn in the phtmsaic (Figure 2). Its suthern end lies apprximately 600 m sutheast f the state beach parking lt (Figure 2) at lcatin A1 in Figure 3. The sedimentary rcks at lcatin A1 are cmpsed f massive t very thin bedded laminated mudstne and siltstne and minr sandstne. The siltstnes are fine t very fine grained, with minr sandy siltstne. The massive intervals f mudstne and siltstne are heavily biturbated. The laminated intervals reflect a lack f biturbatin and, in sme cases, display slightly carser grain size than the biturbated intervals. The mntny f these depsits is interrupted nly by the presence f five distinct, flat-lying sandstne beds at 0.5, 2.5, 3.3, , and 6.2 m abve the base f the utcrp (clumn 1 in Figure 6). These beds are cmpsed f white t light-gray, fine- t medium-grained sandstne. Sme f the beds have planar and trugh crss-laminatin, representing Buma Tb c divisins. Crss-laminatins in several f the sandstne beds indicate palecurrent directins f 298, 313, 303, 268, and 288. The upper three sandstne beds, which are the thickest, lcally shw sft-sediment defrmatin structures that are cnfined t the interir f each f these beds, each f which has planar upper and lwer bundaries (Figure 3). There are n slump flds r slide scars within the siltstne sectin, which is flat bedded. Several samples were cllected frm this part f the sectin fr micrpalentlgical analysis, but all were barren. The fine-grained depsits at lcality A1 can be fllwed suthward beynd the area cvered by the phtmsaic fr mre than 500 m, with n change in lithlgy r sedimentary structures. The fine-grained depsits at the suthernmst end f sectin A are cut by a prminent ersin surface (labeled ES-A) at lcatin A2 in Figure 3, apprximately 10 m nrthwest f lcatin A1. The ersin surface strikes between N74 W and N64 W and dips 21 N. The sedimentary rcks that nlap the ersin surface are cmpsed f medium- t thin-bedded, medium- t fine-grained sandstnes and siltstnes. The fill f ES-A cntains numerus, small-scale, lw-angle ersin surfaces. There is n evidence f any vertical trend in grain size r bed thickness within the fill f ES-A (Figure 3). The fill f ES-A is in turn cut t the nrthwest by anther prminent ersin surface, labeled ES-B (lcatin A3 n Figure 3). Expsures d nt permit us t determine whether ES-B extends t ES-B at lcality A4 (Figure 3) r whether it lies deeper, making ES-B a higher ersin surface. In either case, the fllwing relatinships are bserved between ES-B/ES-B and verlying bedding: (1) the fill is carsest and mst thickly bedded abve the lwest part f the ersin surface; (2) bedding is cncrdant with the lwest part f the ersin surface, where it is apparently flat lying; (3) higher n the ersin surface, where that surface is steepest, it is nlapped by sandstne and siltstne that is finer grained and thinner bedded than beds lwer n the surface; and (4) the highest part f the ersin surface is draped by the finest grained and thinnest beds in the sectin. This lateral change is accmplished by abrupt thinning and fining f individual beds nt the ersin surface. There are, hwever, n vertical trends in bed thickness r grain size within the fill f ES-B. Camach et al. 1547

8 Legend Planar laminatins Ripple crss-laminatins Sft-sediment defrmatin Flame structures Siltstne clasts Planar-laminated t massive sandstne and siltstne Very thin bedded and laminated t massive siltstne Massive sandstne Massive siltstne Palecurrent Channel-axis facies Channel axis/channel margin transitin ES-D Channel-axis facies Channel-margin facies Channel-axis facies ES-C 1 m Lw-gradient slpe facies ES-B' Silt F. Sand M. Sand C. Sand Gravel Silt F. Sand M. Sand C. Sand Gravel Silt F. Sand M. Sand C. Sand Gravel Silt F. Sand M. Sand C. Sand Gravel Silt F. Sand M. Sand C. Sand Gravel 1548 Turbidite Mdel at San Clemente State Beach (Califrnia)

9 Figure 6. Representative measured sectins thrugh lw-gradient slpe facies, channel-margin facies, and channel-axis facies f the San Clemente turbidite system. The lw-gradient slpe facies cnsists f very thin bedded, planar-laminated t massive, biturbated siltstne, with minr ripple crss-laminated, fine-grained t medium-grained sandstne with lcal sft-sediment defrmatin structures. The channel-margin facies cnsists f thin-bedded t medium-bedded, planar-laminated, ripple crss-laminated and massive, silty medium-grained sandstnes and lesser thick bedded, massive, medium-grained t carse-grained sandstnes. The channel-axis facies cnsists f very thick bedded, nrmally graded, carse-grained sandstnes and lesser pebble cnglmerates, with Buma A and B divisins, and abundant basal scurs and amalgamatin surfaces. Althugh there are many graded beds, we recgnize n vertical trends thrugh multiple beds, such as fining- r thinning-upward sequences f beds; nr d we recgnize thickening- r thinningupward sequences f beds. ES-B, ES-C, ES-D ersin surfaces mapped n Figure 3. The cncrdant, lwer part f the fill f ES-B is shwn in the lwer half f measured sectin 2, belw ersin surface C (ES-C) (Figure 6). This part f the sectin cnsists f thin-bedded, fine-grained t mediumgrained, massive sandstne and siltstne interbeds and minr thick-bedded, medium-grained sandstne. The thick beds clearly thin and fine laterally tward ES-B and merge int the very thin bedded drapes high n the ersin surface. We refer t the very thin bedded drapes high n the ersin surface as sandstne-siltstne cuplets, because they cnsist f alternating very thin beds f fine-grained sandstne and siltstne. Measurements taken n ES-B shw it has a strike f N87 W and dips 22 N (Figure 3). Ersin surface C appears t be nearly hrizntal alng the cliff face suth f the suthern ftpath, but it steepens and cuts dwnsectin n the cliff face nrth f the ftpath (Figure 3). Hwever, the steepest apparent dips n ES-C are in the side canyn that cntains the suthern ftpath (Figure 3), where it dips E D C B A Channelaxis facies Lw-gradient slpe facies Channel-margin facies Figure 7. Schematic depsitinal mdel fr the turbidite system at San Clemente State Beach, based n the phtmsaic and linedrawing interpretatins f Figures 3 and 4. A single turbidite channel was cut int fine-grained sediments depsited n a lw-gradient cntinental slpe and filled by vertical aggradatin. The lw-gradient slpe depsits at the present-day level f expsure were all depsited befre the turbidite channel was cut. We interpret multiple ersin surfaces within the turbidite channel t recrd lateral shifting f the channel thalweg (nt separate turbidite channels) with time. Lateral shifting f the thalweg resulted in interfingering f channel-axis and channel-margin depsits. Heavy lines labeled A E shw apprximate lcatins f phtmsaics and line drawings presented in Figures 3 and 4, and a single, simplified sectin f the system is drawn t scale, with n vertical exaggeratin, at the bttm f Figure 4. Questin marks shwn utside the turbidite channel refer t the fact that any sediment that presumably existed abve the present-day cliff tps is nt preserved. Therefre, we cannt determine whether (1) ceval sedimentatin utside the turbidite channel was dminated by verbanking and building f sandy levees r (2) the channel was deep enugh (and/r turbidity currents dense enugh) t prevent verbanking, resulting in nging lw-gradient slpe mud sedimentatin utside the turbidite channel (see Figure 9). Camach et al. 1549

10 steeply inland (t the east-nrtheast). The fill f ES-C in the side canyn frms the upper half f measured sectin 2 (Figure 6) and is thick-bedded, carse- t medium-grained sandstne and cnglmerate in highly ersively based beds, with numerus amalgamatin surfaces and lad structures. Nrmal grading is cmmn, with planar r cnvlute laminatin at the tps f beds. Many f the beds have basal shale clast cncentratins. On the cliff face near lcality A3 (Figure 3), ES-C appraches ES-B and becmes parallel with it, and the beds abve ES-C thin tward ES-C and drape it (Figure 3). Ersin surfaces becme s numerus nrth f the suthern ftpath (lcality A5 in Figure 3) that it is difficult t crrelate them acrss the side canyn (see ES-B and ES-C in Figure 3). These are in turn verlain by very clsely spaced ersin surfaces (ES- D, ES-E, ES-F). Ersin surfaces then becme s clsely spaced that we d nt attempt t label them individually between lcalities A5 and the nrthern end f sectin A (Figure 3). In fact, many beds are bunded bth belw and abve by ersin surfaces. These ersin surfaces shw maximum dips in expsures perpendicular t the cliff face. A representative sectin (clumn 3 in Figure 6) was measured alng the nrthern ftpath (Figure 3). The fill f ES-D, ES- E, ES-F, and s n cnsists largely f very thick bedded and lesser medium-bedded, medium-grained t carse-grained sandstne and cnglmerate, in graded beds that are largely massive, with planar laminatins near the tps f sme beds. This carse-grained lithfacies includes islated lad balls f cbble cnglmerate. Expsures alng the nrthern ftpath als include an example f a s-called flap f siltstne arrested in the prcess f being ripped up (Figure 8A). The very thick beds in the measured sectin f the nrthern ftpath (clumn 3 in Figure 6) frm part f a carse-grained lithfacies belt that can be mapped behind the cliffs directly int thick beds in the measured sectin f the suthern ftpath (clumn 2 in Figure 6), as shwn by the facies map f Figure 2. Based n ur mapping f ersin surfaces, hwever, the beds in clumn 3 in Figure 6 must be yunger than mst f the beds in clumn 2. Sectin B An ersin gap in the cliffs lies between sectins A and B, as shwn t scale in ur interpretive summary at the base f Figure 4. At the suthern end f sectin B (lcality B1) is a thick (1.5 m), massive, pebbly sandstne bed full f shale clasts ranging in size frm a few centimeters t greater than 1 m (Figure 8B). The rip-up clasts maintain their internal structure, shwing little defrmatin. The cmpsitin and internal sedimentary structures f the clasts are very similar t the sandstne-siltstne cuplets described in sectin A (Figures 3, 4). The base f the bed is erded int a thin-bedded sandstne-siltstne cuplet sectin that may be crrelative with the disrupted, thin-bedded sectin at lcality A6. The pebbly sandstne bed with shale clasts is verlain by a series f tw graded, cbble- t pebble-sandstne beds, 1 2 m thick. The upper f the tw beds is cut by an ersin surface (ES-G) that was filled by thinbedded sandstne-siltstne cuplets and was in turn cut by the ersin surface labeled ES-H. This surface is filled with a thick, massive sandstne that was in turn erded alng ES-I (Figure 3). That surface is in turn verlain and nlapped by a very thick bed and verlying medium beds f massive, carse-grained sandstne. These in turn are cut by ES-J, a lw-angle scur draped by thin-bedded sandstne-siltstne cuplets. This ersin surface is ne f many lw-angle ersin surfaces that prgressively steepen nrthward between ES-J and ES-K (Figure 3). The flatter segments f ersin surfaces are draped by sandstnesiltstne cuplets, whereas the steeper segments f ersin surfaces are nlapped by the sandstne-siltstne cuplets. Ersin surface K (ES-K in Figure 3) marks the beginning f a sectin, extending t the end f sectin B (lcality B3), where ersin surfaces are nt bvius alng the cliff face. They are bvius, hwever, in expsures perpendicular t the cliff face. Ersin surface K is nlapped and draped by sandstne-siltstne cuplets similar t thse belw ES-K, but they pass laterally (nrthward) int thick-bedded t very thick bedded sandstnes that extend t the nrthern end f sectin B (Figure 3). These thick-bedded t very thick bedded sandstnes ccur in intervals that alternate with intervals f medium-bedded, muddy siltstnes and fine-grained sandstnes (clumn 4 in Figure 6). The very thick bedded t thick-bedded sandstnes are cmmnly massive, but sme either cntain Buma Ta c divisins (Figure 8C) r are planar laminated. The medium-bedded, muddy siltstnes and finegrained sandstnes are dminated by Buma Tb c divisins. Sedimentary structures include lad structures, dish structures, and burrws that are cmmnly bedding parallel, althugh sme crss the beds. Siltstne clasts include gray mudstne in sme beds and 1550 Turbidite Mdel at San Clemente State Beach (Califrnia)

11 black carbnaceus mudstne in ther beds. A sftsediment slump invlving several beds is present at lcality B2 (Figure 3). This slumped hrizn is als lcally present but thicker behind the cliffs at the nrthern end f sectin B (clumn 5 in Figure 6). This measured sectin shws that sandstnes thicken and carsen very rapidly behind the cliff. A flame structure in clumn 5 in Figure 6 suggests a palecurrent directin f 303. Sectin C Expsures between sectin B and sectin C are semicntinuus arund the state beach parking lt, and the medium-bedded sectin at the base f the cliff at lcality C1 (Figure 4) appears t be crrelative with the medium-bedded sectin n the suth side f the parking lt at lcality B3 (Figure 3). The upper part f the cliff at lcality C1, hwever, cntains carse-grained sandstnes, pebbly sandstnes, and cnglmerates in very thick, amalgamated beds; these are nt present n the suth side f the state beach parking lt. The carse-grained, very thick bedded strata lie abve a prnunced ersin surface (ES-Lin Figure 4), and they dminate sectin C. The bedding in sectin C is characterized by curved, nnparallel, and, in sme cases, lenticular beds (Figure 4). There are abundant ersin and amalgamatin surfaces. The apparent dips f the ersin surfaces are bth t the suth and nrth; hwever, suth-dipping ersin surfaces becme mre dminant tward the nrthern end f sectin C (Figures 4, 8D). Ersin surfaces are cmmnly lined with pebble stringers (Figure 8D) r with siltstne clasts (Figure 8E, F). The sandstne-cnglmerate beds are cmmnly massive, and mst are nrmally graded, althugh sme cntain pebbles evenly dispersed thrughut the bed (lcality C2 in Figure 4). Discntinuus pebble trains lcally define a crude laminatin within sme beds (e.g., lcalities C3 and C4 in Figure 4). The sandstne-cnglmerate beds shw lad structures where they verlie mediumbedded, fine-grained sandstnes and siltstnes (Figure 8E, F). Excellent expsures, perpendicular t the cliff face, are present in the side canyn at the nrthern end f sectin C, and the deepest scurs ccur there. Within the nrmally graded sands and gravels f the channel-axis facies f sectin C (as well as sectin B) are areas r entire beds f sandstne r pebbly sandstne cntaining subrunded t angular clasts f siltstne, ranging in size frm a few centimeters t rafts mre than a meter in length (Figure 8B). These may cnstitute discrete beds r lenses f matrix-supprted siltstne-clast cnglmerate r frm gradatinally bunded patches rich in siltstne clasts within therwise structureless r nrmally graded sandstne r pebbly sandstne. The siltstne within the clasts is similar t siltstnes in the lcal sequence. The biggest gap in ur phtmsaic (300 m) lies nrth f sectin C, where the utcrp is cvered by vegetatin and has been degraded by the real estate develpment f the area. There is n utcrp at all fr the suthern third f this distance, and the nrthern tw-thirds has spradic, pr utcrp alng the small gully shwn in Figure 2. These utcrps cnsist f medium- t thick-bedded, fine- t medium-grained sandstnes. Beachward f these sandstnes is a very prly expsed sectin f mudstnes and siltstnes that is unlikely t be in place, because basketball curts, fences, and cncrete walls are slipping dwn this slpe. Sectin D The suthernmst end f sectin D (lcality D1 in Figure 4) cntains the nly thin- t medium-bedded siltstnes and sandstnes in sectin D. These are interstratified with very thick bedded, pebbly sandstnes (lcality D1). This sequence is cut by a prminent, deep ersin surface (ES-M in Figure 4). The ersin surface is nlapped by at least ne bed, which thins tward the nlap, and this bed is in turn apparently dwnlapped by three mre beds. The tp f these three beds has been partly remved by ES-N, which appears t merge suthward with ES-M (Figure 4). The cliff face frm lcality D1 t lcality D2 has numerus water seeps that result in grwth f algae and mss, s sedimentary textures and structures are very prly displayed. Fresh rckfall fragments at the base f the cliff shw that the sands are planar laminated, with uncmmn crss-laminatins. Bedding between lcalities D1 and D2 is relatively parallel and cntinuus. The rest f sectin D t the nrth f lcality D2, in cntrast, has extremely lenticular bedding. The sectin f lenticular beds lies abve an ersin surface at the nrthern end f the utcrp (ES-O in Figure 4). The lenticular beds are dminantly massive, with lcal planar r cnvlute laminatins, and are very thick, amalgamated beds with basal scurs. Siltstne clasts up t 0.5 m are mre abundant here than anywhere else at San Clemente (Figure 4). These clasts are bth dispersed thrugh the beds and Camach et al. 1551

12 Figure 8. Outcrp features f the San Clemente turbidite system. The psitins f these phtgraphs are pltted n Figures 3 and 4. (A) Incrpratin f siltstne substrate int the base f a high-density turbidity current, apparently frzen int place befre any dwnstream transprt ccurred (channel-axis facies in sectin A in Figure 3) (B) Large siltstne clasts within massive, pebbly sandstne f the channel-axis facies (sectin B in Figure 3). This bed is verlain by tw nrmally graded cnglmerate t sandstne beds (bases marked by arrws). Cntinued. cncentrated abve basal scurs. The beds belw the ersin surface at the nrthern end f sectin D (ES- O) are nt well expsed in the sea cliff but are well expsed inland, where they cnsist f medium- t thick-bedded, fine- t medium-grained sandstnes. These beds are als discntinuusly expsed in the 150 m gap between sectin E and sectin F, which is largely cvered by huses Turbidite Mdel at San Clemente State Beach (Califrnia)

13 Figure 8. Cntinued. (C) Buma Tb and Tc divisins in thick-bedded sandstne, verlain by laminated silty sandstne; frm the channelmargin facies near the nrthern end f sectin B (Figure 3). Yardstick fr scale. (D) Carsegrained sandstne and pebble t cbble cnglmerate f the channel-axis facies (sectin C in Figure 4). Sandstnes and cnglmerates are stratified t massive and cmmnly nrmally graded. Scur surfaces are numerus (sme pinted ut by arrws). Fine-grained sandstnes and siltstnes are uncmmn, except as intraclasts. Cntinued. Sectin E The nrthern third f the utcrp shwn in sectin E (Figure 4) is presently cvered by a slide that frmed during the El Niñ strms f 1998, after we sht ur phtmsaic. Sectin E is cmpsed f thin- t medium-bedded, fine- t carse-grained sandstne, siltstne, and clayey siltstne. This sectin is very similar t the part f sectin A between lcalities A3 and A5 and t the part f sectin B between ES-J and ES-K. As in these sectins, numerus lw-angle ersin surfaces are present, but in sectin E, these dip suthwestward, in cntrast t the nrthward dip f ersin surfaces in sectins A and B. LITHOFACIES DISTRIBUTION AND PALEOCURRENT DATA We recgnize three main lithfacies, interpreted t represent slpe, channel-margin, and channel-axis depsits, based n ur 3-D facies analysis. The prechannel, Camach et al. 1553

14 Figure 8. Cntinued. Clse-up (E) and detailed (F) views a f channel-axis facies, shwing uncmmn ccurrence f interstratified fine-grained sandstne and siltstne (sectin C in Figure 4). Pebbles and cbbles are cmmnly cnfined t the flrs f scurs (e.g., abve the vertical arrws in E), althugh sme ccur as layers within beds (e.g., alng the hrizntal layers). Cntinuus layers f siltstne intraclasts (e.g., marked with a slid line belw it) prbably frmed by disruptin f siltstne beds by turbulent flws. The bject in E is apprximately 58 cm tall. lw-gradient slpe facies ccurs in the suthernmst part f sectin A (Figure 3). The channel-margin facies ccurs in parts f sectins A, B, D, and E (Figures 3, 4). The channel-axis facies ccupies all f sectin C and als parts f sectins A and B (Figures 3, 4). We shw here that the lithfacies distributin (Figure 2), tgether with the palecurrent data (Figure 5), indicate that the turbidite channel has a sutheast-nrthwest trend, with a paletransprt directin tward the nrthwest. Lw-Gradient Slpe Depsits (Prechannel) We cncur with Weser (1971) that the siltstnes and mudstnes that extend at least 500 m suthward frm the suthern end f sectin A (Figure 3) represent slpe depsits, but we infer them t represent lwgradient slpe depsits rather than the depsits typical f high-gradient cntinental slpes. The slpe sectin lacks sft-sediment defrmatin structures, such as slump-flded sectins, slide scars, r slide blcks, sug Turbidite Mdel at San Clemente State Beach (Califrnia)

15 gesting that a lw-gradient, rather than a high-gradient, cntinental slpe is represented. Only the three thickest sandstne beds shw any sft-sediment defrmatin, and that defrmatin is internal t each bed (Figures 3, 6). These prbably frmed by liquefactin penecntempraneus with depsitin and d nt recrd whlesale slumping f a sectin. Significant slump flding appears t ccur n cntinental slpes with gradients greater than 1. Fr example, Kenyn et al. (1978) reprted abundant slump flds n slpes between 3 and 5 in the Bay f Biscay (nrthern Spain), and Lewis (1971) reprted slumps n slpes between 1 and 4 ff Hawkes Bay Land District in New Zealand. The slpe depsits were depsited belw wave base, because they shw n evidence f wave-generated sedimentary structures and they have a majr turbidite channel cut int them, supprting the interpretatin that they are deep-water depsits. Palebathymetric data are nt available frm San Clemente State Beach, unfrtunately, because ur samples are barren, pssibly due t disslutin in the utcrp. The same lithfacies in the Capistran Frmatin at Dana Pint (Figure 1), hwever, has yielded benthic framinifera assemblages that indicate depsitin at bathyal water depths (Ingle, 1971). At the current level f ersin, it is nt pssible t cmpletely discard the pssibility that the San Clemente channel was leveed, because n time-equivalent sediments are preserved abve the mdern sea cliffs utside the turbidite palechannel and prbably nly a small part f the entire thickness f channel fill is preserved. The fine-grained depsits at the suthern end f sectin A (Figure 3) predate the channel, which is cut int them. We interpret these t be slpe depsits, rather than levee depsits, fr tw reasns. First, ne f the key diagnstic features f levee depsits is the presence f slumps directed away frm the channel (Mrris and Busby-Spera, 1990). All types f channels may have slumps directed inward tward the channel axis, because bank undercutting is cmmn; levees, hwever, have psitive depsitinal relief abve the rest f the fan surface (r basin flr surface), s they als cllapse utward (e.g., Kenyn et al., 1995). Slump flds and slide scars are absent frm the slpe depsits f the San Clemente sectin. Secnd, mdern and ancient levee depsits shw palecurrents directed at sme angle away frm the main channel axis. Hisctt et al. (1997a, p. 76) estimated paleflw directins f verspill depsits in the levees f the Amazn Fan channel, based n the anistrpy f magnetic susceptibility f elngated silt-size t sand-size particles, and cncluded that paleflw directins are bradly away frm the main channel axis but with cnsiderable dispersin, partly because f tpgraphic cmplexities and partly because f verspill frm neighbring channel bends. Our data, in cntrast, shw lw dispersin (r 0.1) and are parallel t palecurrent measurements frm depsits within the channel. We cnclude that the fine-grained sectin represents slpe, rather than levee, depsits. Anther pssibility is that the fine-grained depsits at the suthern end f sectin A (Figure 3) represent basin-flr depsits, but we wuld nt expect the turbidite channel that is cut int these depsits t cntain cbbles in its fill if it had a nearly hrizntal axial gradient. Turbidite channels can carry sand n basin flrs with gradients f 0.2 (Carter and Carter, 1996), but we are nt aware f any basin-flr turbidite channels with cbbles in their fill. Finally, ne culd argue that the fine-grained depsits are significantly lder than the turbidite channel that is cut int them and, therefre, culd have frmed in a cmpletely different depsitinal envirnment than that f the turbidite channel. Hwever, the fine-grained sectin appears t have been unlithified when the turbidite channel was cut int it, as shwn by small-scale irregularities alng the ersin cntact, as well as the absence f sandstne blcks in the channel fill (criteria used by Mrris and Busby-Spera [1988]). In summary, we cnclude that the fine-grained depsits at the suthern end f San Clemente State Beach accumulated n a lw-gradient slpe that was t gentle t prduce slumps but steep enugh fr turbidity currents t transprt gravel. Channel-Margin Depsits We interpret the ersin surface at the suthern end f sectin A (ES-A, lcatin A2 in Figure 3) t represent the suthern wall f a single turbidite channel whse fill extends t the nrthern end f the sectin described in this article (sectin E in Figure 4). There is a very marked cntrast in lithlgy acrss the turbidite channel wall, with siltstne belw and sandstne abve, and numerus sandstne beds nlapping the channel wall. All f the ther ersin surfaces in the sectin are clsely spaced surfaces that have similar rck types belw and abve them (dminantly sandstne) and are filled with ne t several beds that thin and drape nt the ersin surface. We interpret these t recrd lcalized, temprary episdes f scuring by relatively ersive turbidity currents within a much Camach et al. 1555

16 larger turbidite channel that was verall aggradatinal in character. We d nt interpret them t be channel walls. Our usage f scurs vs. channels fllws the widely accepted definitin f channel as a lng-term pathway fr sediment transprt (Mutti and Nrmark, 1987, p. 9). The channel-margin facies is recgnized as fine- t medium-grained, thin- t medium-bedded siltstnes and sandstnes, with ersin surfaces dipping nrthwest in the suthern channel-margin depsits (sectins A and B in Figure 3) and ersin surfaces dipping suthwest in the nrthern channel-margin depsits (sectins D and E in Figure 4). The nrthern channel wall is nt expsed, s we can nly prvide a minimum estimate f channel width (1 km). If the abundant, large siltstne clasts in the nrthern part f the channel-margin facies (sectin E in Figure 4) were derived by bank cllapse f the nrthern channel wall, hwever, it prbably did nt lie far away, and ur minimum estimate is clse t the true channel width. Channel-Axis Facies The channel-axis facies is distinguished by its carse grain size (carse sand t cbbles), very thick bedding, amalgamatin surfaces, lack f siltstne interbeds, and the subhrizntal and symmetrical gemetry f ersin surfaces (sectin C in Figure 4; Figure 8D, E). The channel-axis facies gradatinally interfingers with the channel-margin facies ver lateral distances f 100 m r less (sectins A, B, and C in Figures 3, 4). The map-scale irregularities in the channel axis channel margin cntacts (Figure 2) are als apparent in the sea cliffs (Figures 3, 4) and prbably result frm minr, lateral migratin f the turbidite channel thalweg as the channel aggraded. Palecurrent Data Palecurrent directins frm intrachannel depsits, as well as prechannel slpe depsits, are all apprximately the same (Figure 5). Our palecurrent measurements (n 14), taken frm ripple crss-laminatin and ne flame structure, range frm 270 t 3. Fr channel depsits, the average transprt directin is 321, and where bth the channel sediments and slpe depsits are cnsidered, the average transprt directin is 311. This average transprt directin is in agreement with ur measurements f the rientatin f the suthern channel wall (286 and 296 ). Walker (1975) and Hess (1979) btained palecurrent directins similar t urs frm ripple crss-laminatins frm prechannel and channel-fill rcks. Hwever, mst f Walker s (1975) palecurrent measurements used apparent dips f ersin surfaces n the cliff face, which actually dip int the cliff face; these yielded palecurrent directins that are perpendicular t ur measurements. Discussin We see n vertical trends, in terms f bed thickness r grain size, n ur phtmsaic line drawings (Figures 3, 4) r n ur measured sectins (Figure 6). We d nt recgnize the thickening- and carsening-upward sequences f beds reprted by Walker (1975) and Hess (1979) and interpreted t represent lbe prgradatin. Nr d we recgnize the thinning- and fining-upward sequences f beds that they inferred t recrd channelabandnment cycles (Walker, 1975; Hess, 1979). Thinner/finer and thicker/carser beds alternate withut any vertical pattern, althugh rapid lateral changes ccur between the channel-margin facies and the channel-axis facies. Walker (1975) interpreted the sectin t represent a series f eight laterally migrating channels, separated by what he termed siltstne-mudstne drapes (see Walker s [1975] lateral sectin and interpretatin, reprduced at the base f Figure 4). We d nt recgnize siltstne-mudstne drapes, althugh we d recgnize sandstne-siltstne cuplets that thin tward and nlap r drape sme f the ersin surfaces (in the channel-margin facies in Figures 3, 4). We suggest that the sandstne-siltstne cuplets represent depsitin frm the lw-density part f the turbidity currents at higher levels within the main channel (channel margin) at the same time that the high-density (basal) part f the turbidity currents mved thrugh lwer parts f the channel (channel-axis facies; see the fllwing prcess sedimentlgy sectin). The scurs lined by sandstnesiltstne cuplets are nt channels; instead, they are scurs within the main channel, and they were cut by mre energetic flws that lcally passed dwn the channel. The nly sizable gap in gd expsure thrugh the San Clemente turbidite system lies between sectins C and D, as described previusly. If ur interpretatin is crrect and the prly expsed mudstnes alng the beach represent mdern slumps cvering sandstnes that spradically utcrp in the small inland gully (Figure 2), then gap 3 (summary interpretatin in Figure 4) can be mapped as part f the channel axis channel margin transitin. Alternatively, if the mudstnes and 1556 Turbidite Mdel at San Clemente State Beach (Califrnia)

17 siltstnes alng the beach at gap 3 are nt mdern slide depsits, then these depsits may represent (1) a mud plug develped in the turbidite channel during a perid f abandnment, (2) a slide sheet r blck depsited within the turbidite channel, r (3) interchannel depsits, between tw adjacent channels represented by sectins D and E and by sectins A, B, and C, respectively. We, hwever, believe that the sandstnes expsed in the gully behind the sea cliff are in place and that the mudstnes and siltstnes in gap 3 (Figure 4) are the result f prnunced landslide activity alng San Clemente sea cliffs (Abbtt, 1999). PROCESS SEDIMENTOLOGY We cncur with Walker s (1975) suggestin that the lateral equivalence f drapes high n the ersin surfaces, with thicker sands lwer n these surfaces, is a result f flw stratificatin. Sand was mainly transprted lw in the flws and was mre cnfined tward the bathymetrically lwer channel axis, whereas silt was carried in mre unifrm suspensin. We see n reasn t invke different explanatins, such as temprary channel abandnment (Walker, 1975), as an explanatin fr drapes; the surfaces that appear t be draped thrughut the height f the present-day utcrp likely pass belw utcrp level int thick sands, just as many thers d within the utcrp. Significant surfaces f ersin within the channelmargin facies are cmmnly separated by mre beds than they are in the channel-axis facies, indicating that ersin higher n the channel sides was a cmparatively infrequent ccurrence. Within the channel axis, in cntrast, all f the larger flws (r at least thse that left a depsitinal recrd) were initially ersinal, because virtually all thick beds in the channel-axis facies have significant ersin surfaces at their bases. The presence f pebbles and cbbles cnfined t the flrs f scurs and f islated lad balls f cnglmerate indicates that there was cnsiderable bypassing f gravelly material dwn the axial part f the channel during initial (ersinal r nndepsitinal) stages f flw. Mst f these carse-grained depsits shw nrmal grading, suggesting that the initiatin f depsitin frm each flw in the channel axis was triggered by waning f the current, which then infilled scurs created by the same current during its earlier stages. Siltstne clast rich beds are restricted t the channel axis (see the preceding descriptin f sectin C), which indicates that the flws (r parts f flws) frm which they were depsited were very dense. It seems unlikely that rafts f silt culd be transprted significant distances by rlling r sliding alng the bed. This implies that the transprt mechanism invlved supprt by frictinal strength f the sandy matrix. Hwever, the mechanical prperties f such high-cncentratin sandy dispersins argue against sandy debris flw as a lng-distance transprt mechanism n lw slpes (e.g., Hisctt et al., 1997b; Kneller and Buckee, 2000). We interpret these depsits as a result f a transient transprt/depsitinal regime invlving a high-density, mbile layer f sand at the lwer flw bundary (a scalled quick bed), pssibly underging laminar flw, and supprted by dispersive pressure and upward displacement f water (Kneller and Branney, 1995). Such high-cncentratin layers may develp at the base f strngly depsitinal currents (Middletn, 1967; Vrlijk and Suthard, 1997). They are supplied by sediment that falls ut f suspensin frm the verlying current, and they in turn supply sediment t the bed by prgressive freezing at the base. They are driven by the residual mmentum f sediment that enters frm abve and by shear frm the verriding flw. We suggest that these layers may lcally cut dwn int the substrate by injectin r plwing (Branney et al., 1990), incrprating rafts f silt and transprting them a shrt distance dwnstream befre part f the layer carrying the silt clasts freezes due t frictinal frces. The incrpratin f silt substrate is lcally recrded by freezing f the layer during the prcess f injectin (Figure 8A) befre any dwnstream transprt has ccurred. We d nt cnsider the siltstne clast rich sandstne beds t be the result f chesinless r sandy debris flw (cf. Shanmugam, 1996), because, in ur interpretatin, the high-density layers that prduced them were generated and driven by an verlying turbidity current. Sand was likely transprted mst f the distance frm the shelf, suspended within the turbidity current; the high-density layer int which siltstne clasts were incrprated nly came int existence as the flw began t cllapse. This mechanism is feasible n small r even zer slpes and is cnsistent with the implied frmatin f high-density basal layers thrugh high suspended-sediment fallut rates. The presence f massive depsits within the channel axis (indicating high sediment-lad fallut rates) and weakly develped tractin structures elsewhere all suggest rapidly waning flws (Kneller and McCaffrey, 1995). At least ne majr phase f channel incisin ccurred t prduce the main bunding ersin surface Camach et al. 1557