The hydraulic geometry of ephemeral channels in a tectonically active landscape : north flank of the San Bernardino Mountains, California

Transcription

1 Lehigh University Lehigh Preserve Theses and Dissertatins 21 The hydraulic gemetry f ephemeral channels in a tectnically active landscape : nrth flank f the San Bernardin Muntains, Califrnia Sarah A. (Amie) Newland Lehigh University Fllw this and additinal wrks at: Recmmended Citatin Newland, Sarah A. (Amie), "The hydraulic gemetry f ephemeral channels in a tectnically active landscape : nrth flank f the San Bernardin Muntains, Califrnia" (21). Theses and Dissertatins. Paper 697. This Thesis is brught t yu fr free and pen access by Lehigh Preserve. It has been accepted fr inclusin in Theses and Dissertatins by an authrized administratr f Lehigh Preserve. Fr mre infrmatin, please cntact preserve@lehigh.edu.

2 Newland, Sarah A. THE HYDRAULIC GEOMETRY OF EPHEMERAL CH.ANNELS IN A TECTON.ICALLY ACTIVE... June 21

3 The hydraulic gemetry fephemeral channels in a tectnically active landscape: nrth flank f the San Bernardin Muntains, Califrnia by Sarah A. Newland A Thesis Presented t the Graduate and Research Cmmittee Of Lehigh University In Candidacy fr the Degree f Master f Science III Earth and Envirnmental Sciences Lehigh University May 4, 21

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5 Acknwledgments I first need t thank Frank fr his guidance and enthusiasm as an advisr and cnfidant. I am prud t be ne f Frank's first students at Lehigh. I truly believe he has helped me mature as a student and a gemrphlgist. Frank als fund mney t fund my research when ther surces fell thrugh; again, thanks. Partial funding fr my research was prvided by the Palmer Fund thrugh the EES department at Lehigh. I als need t thank Ed and Dave fr their helpful cmments and supprt; my thesis, and gelgic backgrund are better because f them. In additin, I need t thank Martha Eppes (Ph.D. candidate at the University f New Mexic) fr allwing me int her dissertatin. Her knwledge, excitement and help surveying in the field, as well as her insight t buy Otter Pps was invaluable. I need t thank my fellw graduate students fr supprting me during my tw years, bth at wrk and ut n the twn. Finally, I need t thank Justin Pearce fr his uncnditinal friendship and supprt these past tw years. Frank, thanks fr intrducing me t Justin! iii

6 Table f Cntents List f Tables List f Figures Preface v VI VB Abstract 1 Intrductin 2 Gelgic and Tectnic Setting 5 Fluvial Expressin f Active Tectnics 12 Methds 14 Results 19 Discussin 27 Cnclusins 37 References Cited 39 Appendix A. 41 Vita 12 iv

7 List f Tables Table 1. Calculated and measured aspects f hydraulic gemetry fr streams nrmal t the Cugar Buttes anticline. p.18 v

8 List ffigures Figure 1. Tectnic map f suthern Califrnia shwing majr p.4 faults and generalized muntainus tpgraphy. Figure 2. Map f the Lucerne Valley and nrthern flank f the p. 7 San Bernardin Muntains expressed as a shaded relief Digital Elevatin Mdel. Figure 3. Stratigraphic clumn shwing the gelgic units f p.9 the nrth flank f the San Bernardin Muntains near the twn f Lucerne Valley, CA. Figure 4. Gelgic map f Cugar Buttes anticline. p.11 Figure 5. Tpgraphic map fcugar Buttes anticline shwing p. 15 the lcatins f the six channels and reaches A thrugh E fr each channel. Figure 6. Schematic shwing a representative crss sectin fr p. 17 each reach (A-E) f each stream. Figure 7. Plt shwing the width t depth rati f a representative p.21 prtin feach channel reach fr East Middle wash. Figure 8. Tpgraphic map shwing prtins f Blackhawk and p.24 Wdlane washes as they encunter a nrthwest-trending uplift (dashed). Figure 9. Tpgraphic map shwing the initial stage f headward p.26 ersin int the anticline axis just west f Slick wash. Figure 1. Cartn blck diagram illustrating the grss changes in channel pattern mrphlgy fr ephemeral alluvial channels respnding t an active tectnic uplift. Figure 11. Gelgic map f Pitzer Buttes anticline. p.29 p.35 vi

9 Preface This paper details research leading t my master's thesis that was cmpleted during the summer f 2, near the twn f Lucerne Valley, Califrnia. This study is a cmpnent f a larger landscape evlutin study undertaken by Martha Cary Eppes, a Ph.D. candidate at the University f New Mexic wh has been wrking in this area as a part fher dissertatin. My thesis was written with the intent that it will be submitted t a gelgic jurnal, therefre it is nt in traditinal masters thesis frmat. Our prject's main bjective is t quantify the respnse fephemeral streams as they interact with an area f active tectnic uplift. This paper is ne f the first steps in remediating the lack f knwledge f ephemeral streams, especially in the cntext f active tectnics. Our ability t dcument bth quantitative and qualitative aspects f the hydraulic gemetry f these ephemeral streams prvides a initial step in understanding the imprtant prcesses that ccur in these channels, as well as prviding a tl t lcate ptential areas f tectnic uplift in ther arid r semi-arid landscapes. This paper begins by presenting the reasns why this particular field site was chsen fr ur study, fllwed by ur hypthesis. Ephemeral streams n the nrth flank f the San Bernardin Muntains will have a quantifiable, measurable respnse t changes in gradient impsed by actively rising structures. The gelgic setting is clearly laid ut, s that the reader will be spatially lcated, and understand the evlutin f the Transverse Ranges, as well as the stratigraphy that is key t this study. A brief descriptin f the evlutin f the sils n the piedmnt will key the reader in as t why the sils are s imprtant t the tpgraphic expressin f active tectnics, and why this study wrked s well in this lcatin. Vll

10 A sectin entitled "Fluvial Expressin f Active Tectnics" will prvide the reader with an understanding f the distinctin between ephemeral and perennial streams, and the difficulties in making cmparisns between the tw. The cncept fhydraulic gemetry is discussed, and utilized in describing the results frm previus research n perennial streams in active tectnic settings. Later, the Methds sectin describes hw the field area was divided, and hw we actually measured the aspects f hydraulic gemetry f these ephemeral channels. The Results and Discussin sectins are divided int three sub-tpics: channel pattern mrphlgy, transverse drainage develpment, and tectnic transitin. The channel pattern mrphlgy sectins describe the visible quantitative and qualitative aspects f hw the ephemeral streams respnd as they traverse actively uplifting structures. The transverse drainage develpment sectins describe and present data cllected that illustrates three phases f transverse drainages, and later presents a fi?del. t explain hw these drainages evlve. The tectnic transitin sectins put the specific field area int the larger perspective f the entire nrth flank f the San Bernardin Muntains, and the tectnic uplift fthe Transverse Ranges. The utility fephemeral channels as a tl t lcate areas f active tectnics that may have previusly been verlked is discussed. And finally, a gelgic map and bservatins f ephemeral streams are presented t illustrate that structures exist in ther areas f the piedmnt where the tpgraphic expressin f these structures may nt be preserved. This necessitates the cnsideratin that each individual blind thrust fault may be lnger than what is expressed tpgraphically as a fault-prpagatin-fld. viii

11 Abstract The nrth flank f the San Bernardin Muntains prvides an ideal field lcatin t investigate the respnse f ephemeral alluvial channels t pssibly active, Quaternary fault prpagatin flding. In this study, the primary respnse ftransverse streams t a fault-prpagatin-fld is t change their channel pattern mrphlgy frm a single channel t a braided channel immediately upstream f the fld axis, and then incise, preserving a terrace in the axis f the fld and preserving bars f sediment further dwnstream. This stream respnse is dcumented fr tpgraphically bvius faultprpagatin-flds, as well as fr flds that have nt been recgnized as structures. The Cugar Buttes anticline als prvides insight n the develpment ftransverse drainages; sme channels may be antecedent, whereas thers are clearly subsequent. Antecedent streams head in the muntain frnt and are spaced at wider intervals in cmparisn t subsequent streams that head n the alluvial fan surface. These bservatins help cnstrain a mdel fr fault/fld-ephemeral stream interactin that prves useful in identifying previusly unrecgnized active (?) Quaternary structures. The Cugar Buttes anticline extends 1 km further west beynd its current tpgraphic expressin; the respnse f the ephemeral channels allw the true length t.be measured. Because the streams are accurate recrders f the lcatins f tectnic defrmatin, I can use the streams t lcate areas f defrmatin alng the piedmnt, and gain sme understanding f the seismic hazards assciated with ptential ruptures alng these faults. Cllectively, these structures represent a cmplicated zne f strain partitining betweenthe Big Bear and the Mjave blcks. 1

12 Intrductin Blind thrust faults ccur in many cmpressinal tectnic settings, and they are capable f generating large damaging earthquakes, such as the 1994 M 6.7 Nrthridge and the 1987 M 6.1 Whittier Narrws earthquakes. Smetimes these features have n histric seismicity and n bvius gemrphic expressin, s their relative activity and assciated seismic hazards are unknwn. In suthern Califrnia there is previus excellent wrk, including Bullard and Lettis, (1993), Burbank and Verges, (1994), and Keller et al., (2), that have fcused n the active uplift and gemrphic respnse t the Wheeler anticline, a well-knwn actively grwing fld atp a blind thrust, that has a recurrence interval f 3 t 6 years (Keller, et al., 1998). Their research has pened the dr t subsequent investigatins fhw a grwing fld interacts with the fluvial system in an arid landscape. Hwever, cnvergent tectnic settings where blind thrusts cmmnly interact with fluvial systems in arid landscapes are nt restricted t suthern Califrnia. The apprach f using the gemrphic respnse f the fluvial system t help identify and understand tectnic defrmatin is equally applicable in humid settings with perennial streams, as it is in arid settings with ephemeral streams, such as suthern Califrnia. Gemrphlgists knw frm many perennial. alluvial channel studies (Ouchi, 1985; Schumm etal., 1987) that these channels d adjust t active tectnics. Cntemprary defrmatin tends t mdify channel gradients, and these gradient changes are accmmdated by measurable adjustment fchannel metrics such as sinusity. In cntrast, the respnse f ephemeral alluvial channels t active tectnics is much mre prly knwn. The prcesses and characteristics f ephemeral channels in arid settings 2

13 differ frm perennial channels in many ways. Mst ntably, ephemeral channels are characterized by bth fluvial and debris flw prcesses (Bull, 1997). These differences suggest that in respnse t tectnic defnnatin, ephemeral channels can respnd in a different manner than previusly dcumented fr perennial channels. Upwards f -3 % f the wrlds' active cmpressinal tectnic settings lie in arid and semi-arid lands (Mres and Twiss, 1995). Gemrphic analysis is the primary means f assessing the seismic hazard ptential, grwth, and activity f flds and structures within these settings (e.g. Bullard and Lettis, 1993; Keller et ai., 1998, 1999, 2). T date, there has nt been a majr study directly related t ephemeral channel respnse t active tectnics. Ifimprtant insights int cnvergent tectnic prcesses are t be made using gemrphic techniques, then it is clear that ephemeral channel-tectnic interactins must be defined and characterized. This paper details a field study cnducted alng the nrth flank f the San Bernardin Muntains f suthern Califrnia, near the twn f Lucerne Valley (Figure 1) that illustrates hw ephemeral channels interact with blind thrust faults and active tectnics in arid landscapes. The underlying premise in this study is that ephemeral streams shuld have a quantifiable, measurable respnse t subtle r large changes in gradient impsed by actively rising structures. First, I dcument that respnse in an area where tpgraphic criteria clearly indicate the lcatin f an active structure. Using this dcumentatin, I am then able t recgnize actively rising structures in adjacent areas where tpgraphic expressin is equivcal r absent.t date, ephemeral channels have been described primarily in qualitative tenns (Bull, 1991). This research seeks t 3

14 121f ' 12( ' ' 114,' 36fOO'" 119(' 118(' 117(' 116(' 115[ 36fOO' i '" BAKERSFIELD 35fOO' SANTA BARBARA.j:. 34fOO',34fOO' PACIFIC OCEAN 33fOO' 33fOO' Q I,,,EjOkm l" L-... "," 114J ' 121fOO 12fOO' 119fOO' 118fOO' 117fOO' 116fOO' 115fOO' Figure 1. Tectnic map fsuthern Califrnia shwing maj r faults and generalized muntainus tpgraphy. Area ffigure 2 is shwn by the bx in the Lucerne Valley: Mdified frm Miller, 1987,

15 quantitatively characterize ephemeral channels in the cntext f active tectnics, and thus fill a majr gap in the understanding f these fluvial systems. The paper will discuss the specific suite f field methds necessitated by this particular setting, present sme quantitative and qualitative data n channel metrics, define the range f channel behavirs in this setting, and draw sme insights int fault and fld behavir in this specific cnvergent setting. Frm this study we gain a better understanding f the seismic hazards in Lucerne Valley, because the fluvial system has cmpelled us t cnsider a series f blind thrusts that may be lnger than previusly believed. The study als has implicatins fr the develpment and significance f transverse drainages, the behavir f ephemeral channels as they respnd t active uplifts, and the cmplex strain partining acrss structures marking the transitin frm the Big Bear blck t the Mjave blck. Gelgic and Tectnic Setting Suthern Califrnia tectnics are dminated by the right-lateral San Andreas Fault Zne. As the San Andreas Fault cmpletes its "big bend" frm suth f Ls Angles t Santa Barbara, the right-lateral mtin causes a brad band f transpressin, acrss which the Transverse Ranges are uplifting (Figure 1). The large crustal blck that is bund t the suth by the Transverse Ranges, and t the nrth by the left-lateral Garlck Fault Zne is the Mjave blck. This blck is disrupted with nrthwest-trending right-lateral strike slip faults synthetic t the San Andreas, that are estimated t accmmdate between 9 and 23% f the relative mtin between the Pacific and Nrth American Plates (Suthern Califrnia Earthquake Center, 21). Sme f these nrthwest-trending faults 5

16 are presently seismically active, as demnstrated by the 1992 magnitude 7.3 Landers earthquake that ruptured prtins f the Jhnsn Valley,Landers, Camp Rck and Hmestead Valley faults (Figure 1). The nrth flank f the San Bernardin Muntains marks a cmplex transitin between the nrthward-driven shrtening f the Transverse Ranges and the right-lateral strike-slip ffset acrss nrthwest-striking faults in the Mjave blck (Figure 1). The field area fr this study is lcated alng the piedmnt f the nrth flank f the San Bernardin Muntains, the range that marks the suthern bundary f the Mjave desert. The piedmnt receive n average 5 inches f precipitatin a year, with the majrity received during the winter mnths. The study site encmpasses an area beginning just suth f the twn f Lucerne Valley, CA, and extending 1 km east, t the edge fthe Blackhawk landslide (Figure 2). Elevatin varies between 3 feet at the nrthern edge and 37 feet at the suthern edge f the study site. The tpgraphic presence ffault cred anticlinal structures alng the piedmnt, and reginal recrd f seismic activity suggest that the nrth flank has recently experienced tectnic defrmatin. The San Bernardin Muntains are believed t have been uplifted in tw distinct phases (Meisling and Weldn, 1989). The first phase began in the late Micene t earliest Plicene (between 9.5 and 4.1 Ma) as crystalline rcks f the ancestral San Bernardin Muntains were thrust suthward alng the Squaw Peak thrust system. Predating, and cncurrent with the first phase f uplift, the reginal drainage system flwed frm nrth t suth, draining the Mjave blck directly acrss what is nw the San Bernardin 6

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18 Muntains. Prly cnslidated cntinental redbed siltstnes and sandstnes fthe Old Wman Springs Frmatin (Meisling and Weldn, 1989) were depsited acrss the study area during the Micene and Plicene by these drainages (Figure 3). Scattered clasts f Mjave blck prvenance basalt depsited in the Old Wman Springs Frmatin attest t the nrthern prvenance f these sediments. Prtins f the Old Wman Springs Frmatin have subsequently been rewrked int yunger fan depsits, allwing the Mjave blck basalt clasts t be fund n the surface tday. The main prtin f the mdern San Bernardins, the Big Bear blck, was uplifted during the late Plicene and early Pleistcene (between 2. and 1.5 Ma) alng the Nrth Frntal Thrust System. This thrust system is cmprised f a series f suth-dipping reverse faults that bund the nrth flank f the San Bernardin Muntains and frm a 7 km lng escarpment. This new phase f thrusting is evidenced by the reversal f the Old Wman Springs Frmatin drainage and accmpanying depsitin f the Cushenbury Springs Frmatin (Figure 3). This mstly early Pleistcene unit is a gray and tan carse fanglmerate and sandstne shed nrthward frm the actively uplifting Transverse Ranges (Shreve, 1968; Meisling and Weldn, 1989). Frm the San Bernardin escarpment, alluvial fans (pwell and Matti, 1999) spill ut frm the muntain frnt nt the piedmnt, and discnfrmably verlie the Cushenbury Springs Frmatin. These fan units range in age frm middle Pleistcene t Hlcene, and interfinger with basin depsits in Lucerne Valley (Figures 1 and 2). 8

19 7 Alluvial Fan Units Middle Pleistcene t Hlcene (J) 1- Q) +"" Q) 6 5 E 4 c:.- (J) (J) Q) c: 3.-.c: I- 2 - Cushenbury Springs Frmatin Early Pleistcene Ma vertebrate fssils Old Wman Springs Frmatin Micene and Plicene 1 San Bernardin Mtns Crystalline Basement Figure 3. Stratigraphic clumn shwing the gelgic units f the nrth flank f the San Bernardin Muntains near the twn flucerne Valley, CA (mdified frm Meisling and Weldn, 1989). Scale is in meters. 9

20 INTENTIONAL SECOND @ Alluvial Fan Units Middle Pleistcene (j) Cl)... Cl) 6 5 E c 4 (J) (J) Cl) C..:::t:. 3 u...c I- QI QI QI QI @ @ QI III It It CIl It CD QI Early Pleistcene -_ Ma vertebrate fssils Old Wman Springs Frmatin Micene and Plicene 1 San Bernardin Mtns Crystalline Basement Figure 3. Stratigraphic clumn shwing the gelgic units f the nrth flank f the San Bernardin Muntains near the twn f Lucerne Valley, CA (mdified frm Meisling and Weldn, 1989). Scale is in meters. 9

21 Uplift fthe San Bernardins cntinues tday, as evidenced by many Hlcene fault scarps alng the piedmnt. The thrust frnt appears t have prpagated nrthward, away frm the Nrth Frntal Thrust System as a cmplex zne f blind, r recently emergent suth-dipping thrust faults. Thrust faults are ften manifest as fault prpagatin flds in the medial piedmnt, where alluvial fans are ften warped int anticlinal ridges. Incisin thrugh these anticlines by transverse drainages expses underlying Cushenbury Springs and Old Wman Springs Frmatins. The mst distal f these anticlines, including the Cugar Buttes and Pitzer Buttes anticlines, represent sme f themst yuthful expressins f shrtening acrss the San Bemardins (Figure 2). The Helendale fault, a nrthwest-trending right-lateral fault f the Mjave blck trends rughly perpendicular t the Nrth Frntal Thrust System and strikes directly thrugh the twn f Lucerne Valley, CA. Althugh these tw faults intersect, it is unclear which actually dminates. It is believed that these tw fault systems might bth be active (Sptila and Sieh, 2). East f the Helendale fault, the muntain frnt is cmpsed primarily f Palezic marbles and meta-sediments that are expsed as rf pendants t Meszicplutns, resulting in a carbnate prvenance fr the mid-pleistcene alluvial fans mantling the piedmnt in frnt f this part f the muntain range. A calcic sil, interpreted t be 3-7+ ka has develped n these limestne fans (Eppes et al., 2). The sil has a 2-3 meter thick, cemented petrcalcic hrizn that is highly resistant t ersin. In places, the sil has been erded dwn t the petrcalcic hrizn, which is nw uncnfrmably verlain by yunger fan depsits. Atthe Cugar Buttes anticline (Figure 4), the petrcalcic hrizn is at the surface where it frms and prtects 1

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23 the flanks f the anticline frm ersin (Eppes et al., 2). In cntrast, the muntain frnt west f the Helendale fault is dminated by Meszic granitic bathliths. Alluvial fans inthis prtin f the piedmnt are cmpsed primarily f granitic clasts that have decmpsed int grus. The majrity f fan units in this prtin f the piedmnt are characterized by the develpment f sils that are interpreted t be less than 2 ka which are characterized by reddened hrizns, with less than stage I calcic hrizn develpment (Eppes, etai., 1998). These sils differ greatly frm the sils develped n the limestne fan, mst ntably by the presence f uncnslidated grussy sand in the upper 2 cm f the silpfile, and the lack f a resistant petrcalcic hrizn. The genesis f this sil results in dramatically different gemrphic and ersinal behavir f the surface f the granitic fans with respect t the limestne fans, mainly due t the absence f a resistant hrizn (Eppes, et ai., 2). The ersin and mdificatin f granitic fan surfaces is mre extensive than that bserved n the limestne fans. Defrmatin, where present alng this prtin fthe piedmnt, is nt bvius frm an investigatin f tpgraphy alne. Fluvial Expressin f Active Tectnics This study fcuses n a series f ephemeral streams that traverse the Cugar Buttes anticline and cntinue nrth int the Mjave blck. These streams head n bth the alluvial fans f the piedmnt and within the muntain frnt. Based upn the results frm many previus studies f perennial alluvial channels (as reviewed in Schumm, 2) I anticipate seeing a respnse in the ephemeral channels as they encunter the actively rising fld. Althugh the majrity f previus research has fcused upn 12

24 dcumenting the respnse t active tectnics fr perennial channels, I enter this research with the wrking hypthesis that ephemeral channels will als respnd, but in an unknwn manner. The respnse is unknwn because ephemeral and perennial channels have vastly different discharge and sediment transprt characteristics. A perennial alluvial channel is free t mutually adjust its dimensins, shape, pattern, and gradient in respnse t external factrs such as climate, tectnics, r sediment supply. The perennial channel bed and banks are cmpsed f the material transprted by the river under prevailing flw cnditins (Schumm, et ai., 1987). As discharge increases dwnstream, the depth f the channel increases, and the gradient f the stream decreases, bth prprtinally t discharge (Lepld and Maddck, 1953). In cntrast t the perennial streams, the depth f ephemeral streams in semi-arid regins will increase less rapidly and gradient will decrease less rapidly in the dwnstream directin, due t the high water lss and increase in suspended sediment cncentratin in the dwnstream directin (Schumm, et ai., 1987). Additinally, ephemeral streams in arid regins tend t becme clgged with debris flw lbes, the result f frequent hypercncentrated flws, and debris flws during times when the stream has discharge. These debris lbes are usually very carse grained, armr the channel, and thus greatly increase the respnse time fthe channel t adjust t a tectnically-induced change in gradient. These differences in sediment transprt render it unlikely that ephemeral channels will respnd by making the same subtle changes in channel sinusity that has been dcumented fr lw gradient perennial channels. I prceed n the premise that ephemeral channel respnse will be expressed primarily by changes in bth channel hydraulic gemetry (Lepld and Maddck, 1953), 13

25 and in general channel pattern. Hydraulic gemetry is an expressin f the balance between frm and prcess in the fluvial system, and includes channel metrics such as: width, depth, velcity, sinusity, and average channel bed grain size. These metrics mutually adjust in respnse primarily t changes in discharge, and secndarily t changes in slpe. Slpe refers t bth channel slpe and valley slpe, in which the sinusity f the channel is the cmpensatin between the tw. Ouchi (1983) dcumented a useful example f channel hydraulic gemetry and channel pattern changes fr a braided, perennial stream that encunters a zne f uplift. Upstream f the uplift the channel tends t aggrade, experience a thalweg shift, and have many submerged bars. In the axis f uplift, the channel tends t incise, have single bars, and frms terraces. And dwnstream f the uplift, the channel tends t aggrade. Methds Six transverse channels were chsen t cver the alng strike expsure f the Cugar Buttes anticline. Frm east t west they are Blackhawk wash, Dubleknb wash,. East Middle wash, Middle wash, Slick wash and Cntrl wash (Figure 5). Each stream was divided int 5 reaches, A thrugh E, each apprximately.5 km in length, with reaches A and B being upstream f the fld, D and E dwnstream f the fld, and reach C crrespnding t the axis f the fld. The channels were divided in this manner s that each reach wuld crrespnd t a particular lcatin relative t the fld axis; distal upstream (A), prximal upstream (B), the fld axis (C), prximal dwnstream (D), and distal dwnstream (E). 14

26 1 s s 4 2 East Middle wash l,a,.",>---111: BOO 12 Dubleknb wash Blackhawk wash 18 -:-,Jt':-;;;'_--i. -+"'"' 8 Middle wash 4 8 -VI 14 4 A B C D 8t., SOO Slick wash s -SOT A B C D -1 I, I -1-6 SOO 1 HmO 2 II 1 km 4t 2 Cntrl wash ::t,,, D II I A B c!.\., 15 ' _5 6 1,. Figure 5. Tpgraphic map fcugar Buttes anticline shwing the lcatins fthe six channels and reaches A thrugh E fr each channel. Map is 1:24, with a cntur interval f2 feet. Lngitudinal prfiles fr each channel are als shwn. The upper line is the interfluve prfile, and the lwer line is the channel prfile. Scale is in meters.

27 A high-reslutin survey f each channel and interfluve adjacent t the channel was made using a Skkia laser thedliite. Frm these surveys, lngitudinal prfiles were pltted t illustrate the gradient fthe channel nrmal t the rising structure. The interfluve prfiles fr Middle and East Middle washes appear t have a duble peaked anticline, hwever, it is merely the develpment f a strike-parallel drainage in the easily erdable Cushenbury Springs and Old Wman Springs Frmatins that have been expsed in the cre f the anticline. A representative crss sectin f the entire valley width was als surveyed fr each reach (Figure 6). Frm these crss sectins, the width (w) and depth (d) f the channel was measured, allwing the width t depth rati (w:d) t be calculated (Table 1). w:d =wid (1). The width t depth calculatin is a gd example f a prblem that I faced while trying t quantify these channels. In the field, it is ften very ambiguus where the channel bed stps and the fan surface begins. Channel gemetry is s variable, that it is difficult t decide where discharge is actually channeled. In an effrt t be cnsistent, the width f the channel is defined by an bservable change frm a sandy channel bttm t a carser fan surface, recrded in field ntes taken while surveying. The depth is defined by the difference in elevatin between the lwest elevatin in the channel and the elevatin f the clsest flat fan surface. 16

28 Dl""" \ l -Ctrl wash 5 meters Figure 6. Schematic shwing a representative crss sectin fr each reach (A-E) feach stream. Dubleknb wash extends t reach F in rder t capture the channel respnse as it crsses a nrthwest-trending structure in reach E.

29 TABLE 1. CALCULATED AND MEASURED ASPECTS OF HYDRAULIC GEOMETRY FOR STREAMS NORMAL TOTHE COUGAR BUTTES ANTICLINE n trend decrease increase decrease increase Cntrl Slick Middle East Duble- Black- Middle knb hawk sinusity A B C D E entire average grain size (in mm) debris lbe in 65.8 reach B B C D w:d A B dns* C D E dns* *dns =did nt survey. 18

30 Channel sinusity (P), the length f the surveyed channel (Ie) divided by the length f the valley (Iy), was calculated fr entire streams, as well as fr individual reaches (Table 1): P =Idly (2). Using a strategy inspired by the Wlman methd (Lepld et ai., 1964), the grain size f the alluvium frm the bed f Blackhawk and Middle washes was sampled (Table 1). The Wlman methd prvides a sampling strategy that prduces an bjective measure f a channel's average bed grain size. Using this methd, the B-axis f 1 sediment grains frm a single reach f the channel is measured. A measuring tape is placed lngitudinally in the channel, and the grain that is nearest every.5 m interval n the tape is measured. The number reprted is simply the mathematical average fthe 1 grains measured. Results The Cugar Buttes anticline The Cugar Buttes anticline (Figures 2 and 4) was chsen fr this study because it clearly is an area f recenttectnic uplift. Based upn the age f the defrmed stratigraphy, this fld has been active in the late Quaternary. The Cugar Buttes anticline is tpgraphically expressed fr 2 lan, and has an amplitude f 2 m at the center f the fld that decreases t 12 m alng strike t the east and west. The wavelength f the fld varies frm 16 t 2 m. Dips n the backlimb range frm 2 up t 55, while dips n the frelimb range frm 2 t 26, with steeper dips generally near the center f the fld. Bth limbs are capped and supprted by a resistant 2-3 m thick petrcalcic hrizn. The Old Wman Springs and Cushenbury Springs Frmatins are expsed in the cre f the 19

31 fld. The fld cre als expses a nrth verging thrust fault that ranges in dip between 19 and 25. The petrcalcic hrizn frms a well-defined stratigraphic markerbed that allws direct bservatin f fault ffset. At fairly regular intervals (apprximatelylt7ry.2 km) in the central prtin f Cugar Buttes anticline, the axis f the fld has been cut by transverse ephemeral channels which have incised 15-2 m. These drainages head either n the alluvial fan itself, r immediately in the muntain frnt, and d nt vary in drainage basin area by mre than ne rder f magnitude. The Quaternary stratigraphy and structures are mapped in detail in Figure 4. A pht crss-sectin f the incisin int the fld by Middle wash shws the stratigraphy, the fault plane, and the 15 m f relief that exists between the crest f the fld and the channel bttm. Channel pattern mrphlgy Apart frm any ther measurable aspect fhydraulic gemetry, the bservable change in channel pattern mrphlgy is the mst distinct distinguishing feature between adjacent reaches f each channel as they traverse the Cugar Buttes anticline. A large increase in w:d values frm reach A t B, a sharp decrease in reach C, and a slw increase frm D t E was a general trend fund fr each stream and best expressed in East Middle wash (Figure 7). The verall mrphlgy f the channel, as seen in the phts f reaches A, Band C (Figure 7) crrbrates the w:d variatins. Reach A is generally a small single channel apprximately.5 t 1 m wide, incised 1 t 2 m int the fan surface. Reach B is distinctly braided, with many active channels, each abut 1 m wide, and a much wider valley bttm. This reach usually cntains active bars and debris flw lbes, and is the strage site fr a large wedge f sediment. Once the stream enters 2.

32

33 the fld (reach C), it transfrms again t a single channel apprximately 3 t 6 m wide. Reaches C and D are string large amunts f alluvium like reach B, hwever they are ding s n either side f the channel as vegetated bars r terraces. In the hinge f the fld the terraces can be up t 1 m higher than the active channel bttm, but further dwnstream in the lwest stretch f reach C and the entirety f reach D, the terraces and bars tend t nly be 2 r 3 cm abve the channel bttm. The vegetatin n these terraces usually cnsists fsmall t medium sized shrubs and creste bushes that likely act t stabilize these landfrms. ReachD is similar t reach C, hwever the channel is usually nly incised int the fan surface 2 t 5 m, rather than 1 t 2 m as inreach C. Like reach D, reach E is als a single channel apprximately 2 t 4 m wide, but the vegetated bars have disappeared, and the channel is nly incised.5 t 1 m int the fan surface. Tectnic transitin The changing channel pattern mrphlgy respnse f these ephemeral channels is nt limited t the uplift f Cugar Buttes anticline. Nrthwest-trending faults synthetic t the Helendale fault,strike acrss the distal prtins f the fans and may accmmdate sme f the shrtening acrss the San Bernardin-Mjave blck transitin. Instead f creating an east-west trending fault-prpagatin-fld similar t the Cugar Buttes anticline, defrmatin appears t be taking advantage f the pre-existing nrthwesttrending structural grain f the Mjave blck, creating uplifts with this nrthwest Mjave blck rientatin. The defrmatin is expressed as small nrthwest-trending anticlinal (r mnclinal) uplifts that have nt yet expsed lder units r a fault plane, and are nt 22

34 always recgnized as structures (pwell and Matti, 1999). Based upn tpgraphy alne, these uplifts wuld be characterized as.3 t 1 km.in length and stand 3 t 1 meters abve the fan surface (Figure 8). Tw streams, Blackhawk and Wdlane washes, interact with ne such uplift 3 km nrtheast f the Cugar Buttes anticline (Figure 5). Blackhawk wash is merely deflected arund the structure, but Wdlane wash cuts the structure. The lngitudinal prfile f bth channels shws the disparity in gradient between the tw as Wdlane crsses the axis. In additin t changes in gradient, these channels als adjust their channel pattern mrphlgy. Pht C (Figure 8) shws a representative reach f Blackhawk wash, which is apprximately 6 m wide, flatbttmed, lacking any vegetated bars, and incised nly 1's f cm int the fan surface. In cmparisn, pht D (Figure 8) shws that Wdlane wash changes frm a wide, flatbttmed channel.5 km upstream f the fld, similar t Blackhawk wash, t a braided channel with active and vegetated bars immediately upstream and int the axis f the fld (crrespnding t reaches Band C f Cugar Buttes anticline). The respnse f Wdlane wash suggests that the main adjustment made by these ephemeral streams t an uplift, whether it be these small nrthwest-trending uplifts r Cugar Buttes anticline, is t change channel pattern mrphlgy t a braided pattern, temprarily string sediment in the reach immediately upstream f the fld and in the fld axis itself. Transverse drainage develpment In additin t capturing the respnse f these ephemeral channels t changes in tectnically altered gradients, the Cugar Buttes anticline als prvides an ideal field lcatin t explre the develpment f transverse drainages. A space-fr-time 23

35

36 substitutin can be perfrmed alng strike f Cugar Buttes anticline, because structural and stratigraphic ffset inthe fld's center is greater than at its tips. The gemrphlgy prvides evidence that the smaller fld amplitude at each end f the anticline represents yunger defrmatin, rather than less structural ffset, as a result ffault tip prpagatin. Alng the trace f the fld, channels at different stages f breeching the anticline can be bserved. Sme lcatins appear t be "established", that is, the path f the stream that cuts the fld is relatively straight, the canyn is lo's f meters wide and deeply incised, and yunger generatins finset fan lbes are able t use these gaps t get acrss the uplift. In cntrast, ther lcatins have streams that appear t have just recently breeched the anticline axis. That is, the canyn is nt very wide r deep, and the channel width remains relatively small. And still in ther lcatins, small headward-erding channels are visible n the dwnstream side f the fld. These headward-erding channels nly have the runff generated n the frelimb f the fld, but the cllectin f discharge is aided by the impermeable petrcalcic hrizn that is at r very clse t the surface. In ther wrds, the drainage basin area is very small, but the petrcalcic hrizn limits infiltratin, and funnels any runff int the headward-erding channel, increasing its ability t erde. Because n watergap exists in the fld in these lcatins, the drainage that is cllected n the fan surface upstream f the fld is ften frced t flw parallel t the fld until it can find a gap t get thrugh. Figure 9 shws three tpgraphic maps that were surveyed in the field shwing an example f each f these three lcatins; (A) illustrates a headward-erding channel, (B) illustrates a stream that has recently breeched the anticline, and (C) illustrates a channel that is "established". Numerus factrs such as the aridity f the climate, the amplitude f Cugar Buttes anticline, and the presence f 25

37

38 the very resistant petrcalcic hrizn all cntribute t the difficultly f these ephemeral channels actually making it acrss the Cugar Buttes anticline. But clearly these channels have traversed the fld. Middle, Dubleknb and Blackhawk washes head in the muntain frnt, and are mst likely aided by the higher discharge prduced frm the slightly larger drainage basin area. Hwever Cntrl, Slick and East Middle washes head n the alluvial fan surface, and are still able t traverse Cugar Buttes anticline, even with a smaller drainage basin area. Discussin Channel pattern mrphlgy The data dcuments ephemeral alluvial channel respnse t active tectnics in an arid setting. In sme aspects, the channel's respnse is similar t perennial channels. Initially ne might believe that ephemeral alluvial channels will respnd t active tectnics in a manner similar t perennial channels, by cntinually making adjustments t the channel's hydraulic gemetry. Hwever, because the ephemeral channels are limited in the time that they are able t mve sediment, due t the small windw that th stream actually has discharge, their respnse times are mst likely lnger than perennial channels. Earlier I stated that the main adjustment made by the streams as they encunter the uplift f Cugar Buttes anticline is t change their channel pattern mrphlgy. All f the channels that crss the Cugar Buttes anticline respnd t this uplift by becming braided and string sediment upstream f the fld in reach B (smetimes reaching up t 5 meters upstream f the fld axis). Als, they incise and preserve a terrace in reach C, 27

39 dwnstream f the fld hinge. This series f adjustments in the channel pattern mrphlgy are summarized in a cartn blck diagnun (Figure 1). Whereas perennial channels aggrade dwnstream f the uplift, these ephemeral channels widen and return t the grade f the fan; a behavir perhaps influenced by the dwnstream lss f discharge. Althugh each stream makes the same adjustments in their channel patterns, sme channels have a greater magnitude f change. Amng the channels, sme are chked with carse sediment in reach B, while thers have fewer and smaller bars and debris flw lbes in this reach. This distinctin is mst likely related t the size f the drainage basin abve the fld, and thus, the amunt f sediment supplied t each stream. Streams that head in the muntain frnt have a greater magnitude f change in channel pattern mrphlgy, and ften have carser debris flw lbes in reach B, when cmpared t streams that head n the alluvial fans. The changes that are bserved are much mre subtle fr streams that head n the alluvial fas. I. When making bservatins fstream reaches t identify areas f tectnic defrmatin, ne must be aware f drainage basin size abve the defrmatin, and the surce (and size} f sediment that is being supplied t the stream. The mst cnsistent respnse bserved amngst these ephemeral channels is a change t a braided, aggrading pattern upstream f an uplift. Yet, the physical ratinale fr this respnse is unknwn. Perhaps the aggradatin is a respnse t the change in gradient assciated with the fld. Alternatively, it culd be a respnse t the physical barrier f the fld and bttlfmeck prduced by the narrw width f the gap cut in the fld. In this specific lcatin, it is difficult t quantify hw much f the channel pattern change is a respnse t the tectnic change in channel gradient versus hw much is a respnse t 28

40 N \!I = creste bush =terrace Figure 1. Cartn blck diagram illustrating the grss changes in channel pattern mrphlgy fr ephemeral alluvial channels respnding t an active tectnic uplift.

41 the tpgraphic barrier physigallytrapping the weggef sediment, althugh the tw are mre r less related. Besides the streams that crss the Cugar Buttes anticline and Wdlane wash as it crsses a nrthwest-trending anticlinal uplift, anther stream clearly underges similar changes in channel pattern in respnse t crssing an uplift. Reaches D, E and F f Dubleknb wash shw changes in channel pattern mrphlgy as this stream encunters anther nrthwest-trending uplift just nrth f the Cugar Buttes anticline (Figure 5, dashed line). As the channel encunters this structure, the channel respnds by slightly aggrading in reach D, the reach immediately upstream f the structure. In the axis f this structure (reach E), the channel incises int the fan surface, and preserves a vegetated terrace. Finally, after the stream passes any influence f uplift, reach F returns t a wide single channel nly incised.5 m int the fan surface (Figure 6). Again, I see the same series f changes in channel pattern mrphlgy in reaches D, E and F f Dubleknb wash as I see in streams transverse t the Cugar Buttes anticline, as well as in Wdlane wash. Thus, it appears that the changes in channel pattern mrphlgy illustrated in Figure 1 are cmmn t any f these ephemeral channels that encunter an uplift. Transverse drainage develpment The existence f transverse drainages demnstrates that ephemeral channels in an arid setting, and with small drainage basin areas are able t traverse the tpgraphic barrier and resistant petrcalcic hrizn f the Cugar Buttes anticline. Hwever, it is still unclear what mechanisms the channels use t traverse the fld. One mdel t explain the develpment f the transverse drainages invlves a prgressin between the three 3

42 "stages" fdevelpment illustrated in Figure 9. Initially a headward-erding channel fnns n the frelimb f the anticline (Figure 9 A). Given enugh time, it breeches the axis fthe anticline, gains the drainage basin area that previusly was deflected by the tpgraphic barrier, and rapidly incises (Figure 9 B). At its final stage fdevelpment, the channel has deeply incised and created an "established" path thrugh the axis fthe anticline (Figure 9 C). Alternatively, these three stages fdevelpment may nt be linked at all. In sme lcatins, these streams culd have been flwing alng their present curse lng befre the Cugar Buttes anticline began t uplift. As the anticline develped, these channels merely incised at a rate t keep up with the uplift f the fld. These streams are antecedent (Figure 9 C) and are the primary means fbreeching the anticline. Later, headward-erding channels develp, and nce they breech the anticline, they becme the subsequent streams (Figure 9 A and B) representing the secndary means fbreeching the anticline. It is difficult t prve which mdel is crrect, but at the Cugar Buttes anticline the develpment f transverse drainages mst likely invlves a cmbinatin f the tw mdels. The streams that head in the muntain frnt are prbably antecedent because they have slightly mre discharge, and have mre pwer t keep up with the rising fld. The streams that head n the alluvial fan are prbably subsequent, and breech the anticline in lcatins brn frm headward-erding channels. This distinctin can be quantified alng strike f the Cugar Buttes anticline because the spacing f drainages appears t be a diagnstic characteristic between antecedent and subsequent streams. The streams that head in the muntain frnt, which are believed t be antecedent, have an average spacing f.6 kin, with values ranging between.2 and 1.2 kin. Alternatively, 31

43 the subsequent streams that head n the alluvial fans have an average spacing f.2 lan, with values ranging between.1 and.4 lan. The greater spacing f the antecedent streams represents the larger drainage area f the muntain frnt basins, and the smaller spacing f the subsequent streams represents the spacing n the frelimb fthe fld f the headward-erding channels. In ther wrds, a distance fapprximately.2 lan culd be the threshld needed befre the headward-erding channel is able t cllect enugh discharge t be able t breech the anticline. TectniC Significance The shrtening assciated with the uplift fthe San Bernardin Muntains is accmmdated in the cmplicated zne ffaults and flds between the Big Bear and Mjave blcks (Figure 1). As shrtening cntinues, the defrmatin is prgressively taking advantage f pre-existing nrthwest-trending Mjave blck structures. Althugh the Cugar Buttes anticline generally trends east-west, the mre distal structures all trend nrthwest. This suggests that the transitin between the Big Bear blck f the San Bemardins, and the Mjave blck is apprtined ver a span f at least 4 lan, which is the distance between the Cugar Buttes anticline and the furthest distal nrthwesttrending mncline. This field study prvides an pprtunity t gain insights n the rates f defrmatin alng the nrth flank f the San Bernardin Muntains, as well as the ptential magnitude f earthquakes resulting frm rupture n blind thrust faults such as the Cugar Buttes anticline. I am able t d this because the respnse f these ephemeral channels can be used as a tl t lcate znes f tectnic defrmatin in arid r semi-arid 32

44 landscapes. Alng the'nrth flank f the San Bemardins, the stream respnse and gemrphlgy cmpels the cnsideratin f a wider zne f shrtening than is bserved in the tpgraphic expressin f the Cugar Buttes anticline. Besides lcating areas f tectnic defrmatin, the streams als are able t shw that the length f any individual structure is greater than what wuld be interpreted frm tpgraphic maps r aerial phts. Fr example, any tpgraphic expressin n a 2' cntur map fcugar Buttes anticline stps just west f Slick wash (tpgraphic cnturs f Figure 5). Hwever, the behavir f Cntrl wash, and the cnvexity in the interfluve lngitudinal prfile shws that indeed, the anticline extends at least that far west (Figure 5). Based upn the expressin n the tpgraphic map, Cugar Buttes anticline extends nly 2 km, but if the streams are faithful recrders f the active structure, in fact the anticline extends at least 3 km. The magnitude f an earthquake is highly crrelated t the surface length f the rupture that generates it (Wells and Cppersmith, 1994). Applying this relatinship t the Cugar Buttes anticline wuld predict an earthquake f maximum magnitude 5,4 fr a fault 2 km in length, the tpgraphic extent fthe fld, but an earthquake fmaximum magnitude 5.7 fr a fault 3 Ian length, the minimum length f the fld. This study was strategically designed t dcument the respnse f ephemeral channels t tectnic uplift in an area where the uplift is bvius in the tpgraphy. Once this respnse has been dcumented, this tl can be applied in ther lcatins where the effect f defrmatin is nt s clear in the tpgraphy. In this respect, it is useful t investigate that prtin f the piedmnt that is west f the Helendale fault. On the alluvial fans cmpsed f granitic grus, I suspect that fault-prpagatin-flds exist, but tpgraphic anticlines are absent because f the lack f a resistant petrcalcic hrizn as 33

45 well as mre effective ersin and mdificatin f fan surfaces. The differences in parent material f the fan, and the sil that develps, creates a huge disparity with respect t the limestne dminated fans in the manifestatin f tpgraphy, and des nt allw any tpgraphic expressin f the Pitzer Buttes anticline (details fund in Eppes et al., in prep.). A detailed gelgic map f the fans near Pitzer Buttes (Figure 11) reveals warped Tertiary and Quaternary beds I term the Pitzer Buttes anticline. Changes in channel pattern mrphlgy as Pitzer Buttes wash encunters the Pitzer Buttes anticline are similar t changes alng the channels transverse t the Cugar Buttes anticline. The stream begins as a single channel 2 m wide, nly slightly incised int the fan surface,. then the valley widens t accmmdate many individual active channels, and finally the stream returns t a single channel that is string sediment as small vegetated bars. This respnse crrespnds t reaches A, Band C f streams that crss the Cugar Buttes anticline, with the nly majr difference being the lack f a vegetated terrace in reach C. Althugh similar changes in channel pattern mrphlgy are present, these changes are muted and mre difficult t distinguish, because the entire stream system is cnstantly inundated with the easily erdable fine grssy sediment frm the fan surface. Carse debris lbes are present in reach B, immediately upstream fthe fld, hwever they are stranded 2 m abve the current channel elevatin. I believe that these debris lbes were depsited immediately after the fld was uplifted, in respnse t the tpgraphic barrier f the fld, as well as the change in gradient as the stream encuntered the fld. Hwever, because these granitic fans and their sils ffer n resistance t ersin, Pitzer Buttes wash has since incised thrugh the fan material, and 34

46

47 int the easily erdable Cushenbury Springs in the fld's cre, creating an anticlinal valley which is 2 meters lwer than the previus fan elevatin (Eppes, et al., in prep.). Since the carse cbbles f the debris lbes armr the surface and are difficult t transprt by fluvial prcesses, the debris lbes are left stranded abve the current channel elevatin. This suggests that changes in channel pattern similar t thse fund at the Cugar Buttes anticline did exist, befre the anticline was erded away. It appears that this cmmn series f changes in channel pattern are independent f the lithlgy f the alluvial fans that the ephemeral channels are flwing acrss. Althugh the Pitzer Buttes anticline is nt bvius frm tpgraphy alne, the respnse f the streams as well as the map patterns attest t its existence. This suggests that fault-prpagatin-flds assciated with the cntinued uplift f the San Bernardin Muntains may be lcated in many mre areas f the piedmnt than previusly believed. It is difficult t say much abut the activity f fault-prpagatin-flds such as the Cugar Buttes and Pitzer Buttes anticlines, but they bth appear t have been active in the late Quaternary and might be grwing by lateral prpagatin. Ifthis is true, returning t the Wells and Cppersmith (1994) relatin, the ptential rupture alng ne fthese faults culd result in a much larger earthquake than previusly believed. I d nt intend t make inferences f the seismic hazards that are pssible in Lucerne Valley, but instead I wish t draw attentin t the pssibility that the thrust faults assciated with the uplift f the San Bernardin Muntains are f a larger scale than previusly thught. 36