The Pacoima Area


(1)THE PACOT. '1IA AREA Rob~rt. Lloyd 'l !'isht3r. a. t. T.. ' 1+9.

(2) FRON'r I 8PT : !:•Jfi;. /. DEDICATION This report ls. dedicate~. to the ?harisees ,. who thou3ht t1.1 ey would be heard fo.r. tl-ielr much-s9eaki.n3..

(3) A.BS TRACT. The Pacoima area cons i sts of Miocene (sediments and basalts) rocks underlain by a quartz diorite basement complex of Jurassic age.. The strati-. granhi_c units range from 25 feet to more than 600 feet in thickness.. The. lower sediments are arkosic land- laid deposits whi4-e the Modelo formation, shallow marine in orig in, consists of thinly bedded sandstones, shal es, vo l canic ash and cal careous members.. An andesitic mass, possibly a volcanic. plug, outcrops near the summit of one of the hills. An anticline ; and possibly a.n associated syncline, has been developed by compressional folding.. Small scale contortion is l ocally exposed.. ional dip is northward, decreasing from south t o north in the area. i s common, most faults trending nearly north-south.. Reg-. Faulting. Movement along these. faults ranges from a few feet to several hundred feet . There are no important mineral deposits in the area. baseme nt rocks has been abandoned.. Quarrying of the.

(4) TABLE. O~. CO ~ T~~TS. Introdu c tion Geo3 rap hy Index ma.p .I+. Geologic Structure Struct ur e sections Colu~nar section Field ;_)'1-loto sra _:-)11s. 9 10. Geolo 51c History. 11. 8.

(5) ( 1). INTRODUCTION The field work upon which the following report is based was undertaken at the instigation of Professor Richard H. J a hns of the Division of Geological Sciences, California Institute of Technology.. The region had been map-. ped topofraphically by the United States Geological Survey and investigated geologi.cally by several C . I. T. field geology classes. The area mapped consists of approximately one-half of a square mile, tvrn miles southeast of San Fernando in the -;)uadrangle.. 1. Pacoima,. California.,. Osborne Boulevard, connecting U. S. Highway 99 and State Highway. 118 borders the area on the east and provides easy access to the exposures. The field mapping was carried out by two-man. parties walking over the Contacts were traced and mapped on a 1: 6000 base map, with an aerial. area.. photograph used as an aid to location.. Station location. ~-1as. by pacing , in-. terpolation from r.ihotographed surface features and by triangulation where aerial photograph coverage was lacking . Apparent dip-and- strike of contact - contour anomalies along the basement sediments contact are due to changed topography, a result of quarrying operations subsequent to the topographic mapping.. Since aerial coverage of. this locality was very good, the writer ignored the mapped topography and deDended solely on the aerial photograph for location at these points.. The. area was mappe d during the months of F arch, April and Hay, 1948 with a total of t e n days spent in the field. The writer wishes to Brovm.. acknm·~ledge. assistance in field work given by D. R.. !:Ciss Shirley Chapman rendered valuable aid in the editing of copy.. The writer is especially grateful to her for the support and encouragement she freely gave for the duration of the proje c t..

(6) .{ ~ >. GEOGRAPHY. 'T'he latitude and longitude of the Pacoima area are 34 22'. 1,:. resnecti.vely.. 0. 17 1 N and 118. The area rrtapoed comprises o.54 square mile.. range from 975 feet at the southern end to 1300 feet on Peak Abu.. 0. Altitude s Local. reli ef is great, being most extreme at the southern face of the hills.. Over-. all relief is small . Water drains dowri gulleys and ravine s , some passing into Hanson Dam, most going off to the south via irrigation ditche s .. All of the streams are. intennittent. The vegetation consists of yuccas , live oak, wail:t- high chaparral, elderberries a nd numerous trees in man-made groves.. Jackrabbits, quail, redwing. blackbirds, orioles, snakes and ground squirrels comprise the fauna . Two highways border the area, whil e it is crossed by dirt roads and telephone lines.. Quarry buildings, small residences, a neighboring dam and. an airport complete the culture. Dominant topograohic features are low, mature hills separated by broad, aJlu:v i :\lm-filled valleys.. Rocks are best exposed in quarry faces and a long. road c nts. The area cnrrently support s t >vo industries , air travel and f lower growing. the. Horses graze on the hills. ma.~or. nuarrying of the basement rock was f ormerly. industry but these operations have ceased..


(8) ( '+). GgoLOGIC STRUCTURE. Unconformities The Shirley sediment - Basement complex contact is a\1unconformity, ,tniocen:t. sediments lying u pon Jurassic igneous rocks.. It is a depositional con-. tact over about one - half of its extent, becoming a fault contact in the southeast part of t he area The other contacts are chiefly depositional in character, though the Modelo-Broadwell contact may be a fault contact at the or west ends.. The. basalt layers were extended essentially conformably on the arkose surfaces. The small ''island" of Brassnuts basalt surr ounded by Shirley sediments is probably a remnant of a stream channel filling in the eroded .Shirley surface. Folding Folding i n this area was due mostly to compression acting in a nearly · east- west d i rection .. This is indicated by a broad anticline, whose axis bears. about N 10° 1·! through the mid-portion of the area mapped.. Its presence is in-. ferred solely from dip-and- strike changes since folding in the basalt and secl.Lmentary u n:lts can't be observed .. It apparent l y plunges into the Eodelo. sediments, possibly giving way to an even broader syncline or basin .. Location. of these features is not precise, since strike-and-dip data is not consistent even over very small sectors .. This anticline is post- Ifode lo in age.. Ad-. ditional folding, very limited in extent , occurred as the andesite mass cut through Like Hill.. Sma ll scale contortion is exposed in the road cuts along. Osborne Boulevard. Dip-and-str ike variation in the stratigraphic units, upon which the above hypothesis is based, may have come about through faulting or renewed intrusion in the basement complex.. Hovrever, the author b e lieves that the above. described features a re dlle to compression rather than to the se cc:rnses..

(9) --. \ :J ). (. Faulting Faulting has been present as a companion to folding as a topography modifying agent in the Pacoima area.. There are about twenty mappable faults,. trending in a general north-south direction, v:hose traces are modified by surface irregularities. types.. Displacement has been of the dip - slip and strike-s lip. No overall system may be postulated, but adjacent faults show similar. characters, so the pattern may b e broken down into seven major group s. 'l'he major faults in the area are Frankie 1 s fault and Friday Fault, 1•;hich meet to form a fault wedge .. Frankie 1 s Fault .appears as a fault scarp in the. l arge st qiiarry, having a strike of N 25 E and a dip of 66° N"!.. Northeast-. ward its position is inferred by marked thinn·i ng of the S hirley sediments and v.ri.desoread occurrence of the basement complex.. To'riday E' ault, whose posi-. ti.on must be similarljr i nferred, joins it about 700 feet :'.:SE of Peak Abu. 'f' he auth0r beli.eves this wedge was elevated subsequen t to the nearby Eayfield sedLnent faulting and that the thin Shi rley sediments capping the wedge were eroded off, l eaving basement exposed far u p the valley.. .A light, cream col-. ored calcareous rock occurs on and just beneath the ground surface a long Frankie 's Fault trace in the valley.. This is probably tufa depos ite d from. carbon dioxide - bearing springs along the fault contact.. Po ssibly associat-. ed 17ith this moveme nt is the a pparent motion southward o:f a block of Shirley 1. s ediments now extending into the centra l quarry flo or . The four faults on the south flank of Like Hill tre nd nearly north and south.. These steep faults, possibly associated the formation of. the formation of the andesitic plug or dike.. Displacement has been down on. the east side on the t wo outside faults, vvhile a block has been dro pped be-tween the two center faults.. The west e rn-most fault of this group is p as-. sibly a part of the l arger fault immediately to the south, thoug h it is l e ss.

(10) ( 6). likely that the other faults may be similarly correlated.. An alternate in-. terpretation, based solely on strike-and-dip data, is that the fault and contact lines along the Eodelo-Broadwell boundary should be interchanged, vfith the assumption made that the fault pattern is due to East-1·.rest compressional forces.. Field evidence is unconclusive on this point, but the author. believes that this fault system owes its origin to radial forces associated with emplacement of the andesitic mass. Three steep, NNr.; trending faults have offset the rfayfield sediments in the saddle 1000 feet west of Osborne Boulevard.. Displ aceme nt was probably. stril<:e-slip in nature, and amount of offset must be inferred as about 100 :feet The I1a y field, Brassnuts and Shirley formations have been offset by t wo nearly _parallel faults bounding a block n ear the west edge of the area mapped. The block has moved south-eastward or downward 10 - 25 feet.. Possibly one of. t hese faults is a continuation of the N70P trending fault in the nearby granite diorite, though no fourth fault can be found cutting the basement rocks. Minor offsets of the Brassnut-s-Shirley contact occur farther east. A fourth system offsets the Hayfield sediments northeast of Ai;r.port . Ridge, where a block of Broadwell and :Mayfield rocks was dropped 6-10 feet. Subsequently, an arcuate section of Broadwe ll basalt dropped several feet, abr uptly terminating the 3rassnuts basalts and burying a section of the Mayfield sediments. High angle reverse faulting has re-exposed Hayfield sediments in a single locality just northeast of the above area, indicating minor north-south compression.. The ad,jacent r i dge and valley have been cut by numerous faults which,. however, affe cted only the S roadwell basalts..

(11) Joints Joint systems are developed in the quartz diorite, a vo lcanic ash member of the Hodelo formation and in the ande site plug. The basement joints are about equally developed, striking N50'.{ and N53E and dipping 55° NE and 80° SE respectively .. If these are due to compression,. they indicate an :Sast-T;'e st direction of compression. In the volcanic ash, one excellently developed ~joint set strikes N 70° E and dins 72° SS, v;;hile a subordinate set strike s N5°'.·! and dips 80° S~·L. How-. ever, the author does not believe that these joints have structural significa.nce but merely were forme d during the alteration and lithification o f the ash beds.. Thlrdl~y, the plug ' s joints, striking N 4 H and N 89 E and dipping 69° S\~/ and 47°. m,,r. respectively, were formed during the cooling of the extruded andesite..

(12) STRUCTURE. SECTIONS. 1200. 1100. \. \. \. 1--~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~rooo. A. Shirley. BY-oa.d<-ve//. Modelo. A'. IZ50. !/50. 1050. B. Snirle-;t. Horlzonta.I scale Vertica I sea.le. / " = 500 / I " -::./67 '.

(13) SECTION. COLUMNAR UnC: t:. L o/o rpj. Name. ThiCKness (a V'?}). U11 i. 'f: Allu v iu,.,.,_. ,. Miocene. Modelo. -. - -- ----~~ .;r;r ~ ·. ~ - ..'. : ·~. 7·. ).~:-:-.-:~·. ,,,,,... .,, ..... !-". -. -. -. frn.. . Thi.n/y - !J£>dde c/ mrarlne .sands t:ones_, s )t ~ /e>s, t:k. .s->i . Fos S'i./t -te>ro<A. s .. Ande si.t:i.c. €00. pl""'. -. ::: j{ : : : : • .• ·• •• : : : :. I! .ii :~...................... -·~--~ --'.'- .:.:...:..·...:.. - -·..:..·:...: ..' ••::. .. --:. ·.:.·... .:..:.:. :·~·.. :·:.:..: ·:··:. I. r Broad<..ue.11. Ba .Sa It$ . l\4v. d r/ot..v (?)<Ander blocKy cinnamon. t> 240 '. 20'. Ma.. r.ove dQ. /oi.. da. /. ;~t}:l~~·?~·~:~: . ... ....~.. . ... ... .. ... ;-~ :;/ ~~: ~~~·.}:.·. Sht.' rley. and.. a. 1"n'} g-. .. No.I'? · >?'I a y i ne sa.n a s tone . 8o(J./c/er con9lon-i er a t; e s .. G..-a>''- - cli.oYi."te; pe91'1f'ra.-t i. t. e d U <e.r . S'ch i r -t;. 75 ,. 250'.

(14) FIELD PHOTOGRAPHS. Fra.n.1<<.:e's Fa.u./t; fa..ce , i.n center quarry .. Be c1c1ec1. .sediments. e x pose d. L.11.. or. Osb or n.e-. -the. Bl v d. exposed. /Vlod e /o r oa d. f m-.,. c~t:. ..

(15) \ I I ). ~OLOGIC. HISTORY. Jura ssic to Miocene l.. Intrusion of granite diorite int o previously formed rocks .. 2.. Introduction of differentiated intrusives and p egmatites.. 3.. Removal of pre-intrusion rocks; leveling of intrusives' surface.. Miocene to Recent. 4.. Uplift (?) in source areas. erates and arkoses.. 5.. Volcanism.. 6.. R.enewec;l. terrestial deposition.. 7.. 1. 8.. Subme rge nce by s e a .. 9.. Broa d warping: fo r mation of a nt icline.. ro1cani sM.. Deposition of terrestial Shirley cong lom-. Extrusion of Brassnuts basalt in t wo separate flows. Mayfield .formation.. Broadwell basalts in t wo flows . Deposition of ma rine M:idelo formation continuous.. 10.. For'11ation of Andesi_te ol u g . de oos i ts.. 11.. Dissection.. l2.. F ithdrawal o f the sea .. 13.. Quaternary alluvium formed in va lleys.. Faulting of Ha Jrfield sediments.. Associated f aulting .. Volc a nism .. Zlevation of Fr a nkie-Friday f a ult wedg e . Continued dissection.. Ash.


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