01

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692 694m I 7856 IV NE 35'22'30 RIDE ROE 50'7 698 IMORRO BAV NORTHI 700 47'30K 701 I

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E X PL AN ATION

39< 5 I 80'64K WA 6 Alluvial deposits Cobble-pebble gravel, sand, silt, and clay + v IO/2 u,' Ie 0 66 Sand dune deposits T3as 29 Aw J - Unconsolidated white to brown, windblown sand. Includes older stabilized 305. .M lze . ~ <(T6DE O sand dune deposits '+, + 8

M d ,/ Landslide deposits M ==, I 05e~p <3 Lithology dependent on source I 7 Td material white Pain ~J . OI nsexdnd. t 2 't

I

39<3 67 7 67 0 0 ~ Terrace deposits a 0 0 3 Unconsolidated 67 0 6 sand, silt, clay,and gravel. Marine and stream terrace deposits 01 0 Iu 8 ppvp UNCONFORMITY P P II P P ///rl p p o' <(r p OD ;08 M 4 44

/ <3 //I vvvv I 01 / J — ch ~+~ ):rr Paso Rabies('?) Formation vvvv 36 fi,,',' ++ 'OT >I ~~65/' 7 -: > I ', P- Td c Td feei Nkv /// 20' (Y , siltstone, claystone, and Monterey clast conglomerate. Poorly uoj(UOO sIO J v v <',/ to well consolidated. Partly or entirely marine 7(»e4$ Pll / '/ 7 - 67 11. i12, UNCONFORMITY ie 0 a 0 v 67 o v In o 0 / C0 6XOL r'IJ- P E JArj 0 Squire Member s P / J//~/ w<+ /CP Massive white to tan calcareous arkosic sandstone; more calcareous ond ) L 2/ d resistant near base; 75% to 80% quartz,15% to 20% feldspar, less than sI 0 5% mafic mineral grains. Fine-to )I-~~~ E 2 UJ / — I medium-grained; grains subrounded to AJ I subangular, moderately well sorted. Finer grained in where DedL / west, it grades ~~J into siltstone and claystone with some AU> yN E> sandstone. Contains Lyrr>pec/en m cerrasensis s I oedL I- (Gabb). Upper . Marine. Hall (1973) 0 0 E z 2 / 0 0 (5 = I e : <0[ Belleview Member /C7 /P+ Interbedded buff to gray claystone and fine -grained sandstone. Claystone '9 is spheroidally fractured. Upper Pliocene. 10 Marine. Hall (1973) I /I,' KJ ..1;' 'I '1'... 'I '3 7 1'i '< I: ':::-.: i'. I I I 0 ~e . R o-MD )- 0 ,I ~'I Gragg Member I Massive Well 44, 0 resistant to soft medium-grained sandstone. White, weathering \5'I I i C< xem V E buff; 65% quartz, 30% feldspar 4 /o clay, 1% mafic minerals Contains Lyropecren cerrosensis (GabbI. Upper Pliocene. Marine. Hall (1973) 0 7 E * Tpm >0 o»ou0 <~ ohx 7

7 ID0 ) )' IO I Tpes we / / .Tpe "Tpec V 0 0 / Miguelito and 0 0 / Edna Members Tpm, brown claystone and siltstone; beds 2 to 4 in. 0 0 p 2 g> thick, but commonly I/2 to 3 in.thick towards top and southwest porcelaneous shale opaline <3 shale 45')3 diatomite or diatomaceous shale; relatively soft well-bedded siltstone; + + + + + + C q. locally tuffaceous near base in north, absent southwest; locally bituminous + + + + + (bry) or friable sandstone; + 4 + + + + conglomerate locally near base. Claystone and k L siltstone beds grade laterally into sandstone toward east (Arroyo Grande +++++++ + + + + + AH quadrangle) Contains Arrr>monrereydnd (Osmont) 0 0»OQO CpNR.g // quartz, than 5%to 15% feldspar I DO Tpes,nonbituminous buff to gray sandy siltstone; where bituminous(bry) e~)r 03 /// /// E Tpec,conglomerate of predominantly hard clasts of fine-grained Obispo ~/ J// /// tuff (85% to 90 / ) and others of Franc i sean chert / /// // J/// Edna Member contains Asrrodrrpsis spd/iosds (Kew) and Patinopec/ 0 Tmts S Tmb I 0 Monterey Formation ' WR — s '; F ezdtn 5, IN - ( INN Tmc, resistant hard /35 Qg KK'~w ~g siliceous shale and interbedded chert. Color variable ~~ but commonly white and brown to and ( gray reddish-brown, weathering )3, chalky white. Brittle with subconchoidal fracture. Beds iQ I/2 to 6 in. thick, n<''NQ commonly I/2 to I in. thick. Laminations less than I/4 in. thick; color banding common, reddish-brown and black chert in west, brown and cream-colored chert in east. Commonly fractured and sheared. Locally mterlaminated and interbedded with IL IOO diatomaceous siltstone, Contains eI=JT perlitic glass and breccia LLI e~)r 62 Top, 0 Q Radiometric ages of tuff, 15.3 j: 0.5 to 16.5 L 0.8 m.y. (Turner,1970), DJ I Tmcaa 9 Upper Saucesian and lower Relizian ID 0 '>0 sosO son 7 V I 2<46r>I Iu: 2 Xello/I >I>010 Y

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II Sandstone I Vaqueros gA Tan, gray, or dark reddish-brown sandstone. Poorly to well erin<>s (Dali) I 0 Cl and Chldmys cf. C sespeensis (Arnold), Zemorrian. Marine on)/3 )/NO3 ed'O=-- 0 e + + AJ + ++ 0 eTdf Q,ex I o 3 ', OJJ/00= Td,porphyritic dacite; —Morro Rock complex, 50% of rock s)iuJ I» JJ composed of phenocrysts with a typical composition of 65% strongly AJ ~~ 15< zoned andesine, 15% biotite and clay,10% hornblende, 5% partially //g +g z resorbed E> quartz,and 5% magnetite, apatite,and zeolites The ground- ed'':.:. I 9 e <>L ~~ + D ( Potassium-argon age determinations of dacite: Morro Rock 22,1 + 0,9 my.; , 26.4 j: 0.8 my.; near ,2634 08 m.y., I Turner (1968) and Turner, et al. (1970). Refugian and Zemorrian Lu dJ 96 56 12'3-

'I 'I 15 1 I I vvv /r r rl / Unnamed sedimentary rocks See>0 A'olsi Interbedded white, gray,tan, 60% to 70% brown or dark greenish-brown (7> 6 hard arkosic wacke sandstone and 30'/0 to 40% greenish-brown or 4 O D black claystone and siltstone, conglomerate with pebbles and cobbles of UJ Na D D T black chert and quartzite. Massive to moderately well-bedded. Sandstone O commonly not as altered, sheared, or weathered as Franciscan graywacke, 'J J UOOO 100 9 NK3 4 I- grain boundaries more distinct. Thick-bedded sandstone composed of 50 /o I <5 I

56 idols; Teoces of fold KJgwz -..-.:: >f/»"/ / sch Os CB) axes north of "rf )D Jfme 2/lr / this fault zone .Kdgwi:. ~~ r / Jt +wh ~v colnmonlyhave J J'J trends that are Franciscan rocks rr J N45 BooW,and KJgwz,interbedded very fine-grained graywacke orclaystone and greenish- "N ~N/ronddark ahois ..'!///QN ...i-. .. ".S. traces of fold ,6<0' ~-, Qt IM -;: M.. 373....[M,,I<..Ewcp( 85 55 -<)Erne brown medium-grained graywacke composed of 60 /o to 70% quartz, 'JO U000 IOO m c 98 axes south of O ~ the fauh zone 15'/0 to 25% plagioclase,2% to 5% K-feldspar, 2% to 5% biotite, IO/o ILJ ond within the rock fragments of dark gray siltstone. Exotic blocks absent O Il/J'/ / Monterey Formation KJgwi, mostly dark brown massive graywacke and greenish-brown or I- (' J'/ / and older arnis ILI Jr Ver (///( (( J 56 are east-west or gray micrograywacke or claystone. Bedding commonly chaotic. Exotic to the northeast blocks rare or absent, otherwise similar to KJgwz O These units of presumed Jurassic or cretaceous oge ore petrologically abun- O )~ J J/ similar and have been differentiated only on the basis of relative dance or absence of exotic blocks and the degree of shearing. The units O //// // Er of tectonism or degrees of tectonism apparently represent different ages V> 0> 3 5 E DLNL 0( ~ Jv, weathered diabase and basalt associated with red chert (ch) and O

SIO I QQQKIO ~ '. re .-/ I k4Q C'693 IOL / AI— GEOLOGIC SYMBOLS 'V ve( .0'J<:98 / V /Iz Contact AJ. c ck / Dashed where approximately located, queried where doubtful 62 Point Son Luis IO ,r >, c(0- High-<)ngle fault 'x(J Dashed where approximately located or inferred;dotted where concealed IOL ' 96 55 queried where presence is doubtful. Saw-teeth on upper plate. Dip o 72 750 fault plane between 30 and 80 '56 '692 I 'I) 55

/ 7 I"'77I '777 Anticline nestd5nl Showing axis at surface. Dashed where approximately located;dotted Rde 6 4 where concealed IO "~c Ln 02 3/- 6 I 0 c c 'k) 72 3 Syncline 0 0 70 Showing axis at surface. Dashed where approximately located; dotted 0 0 762 369100M N 0 0 35'08'37" I where concealed Iu n< 696 1 GOO FEET 697 35 08'30" 123 52'30" 150 50 702 (9 30~ Base map from U. S Geologicaf Survey SCALE 1 ?-<000 The geology along the cc:stline and the Miguelito Member-sand dune Horizontal Inclined Vertical Morro Bay South and Port San Luis contact north of Coon Creek, and the Miguelito Member- Franciscan quadrangles, 1965, I: r 0 JP IX'65 Strike and dip of beds 24,000. F 5 rocks contact southwest of Osos Creek were mapped by W. G. Ernst ' 01'I and C. A. Hall, 1968 The remainder of the Morro Bay South and 30JW Port San Luis quadrangles was mapped by C. A. Hall, 1968-1972. Strike and dip of flow banding Interpretation of geology shovn in cross sections is that of C.A. Hall. CAME Geology drafted by M. J. Guenther. This direction of landslide movement QUADRANGLE LOCATION investigation was supp

Abandoned well drilled for oil Siltstone —- ———-— Tuff —a —a —a- Marker beds REFERENCES CITED Holi C A Jr ig73, Geology of the ArroyO Grande quadrangle, San Luis QP'HE and sheet GIEQLQGZC MQIRIRQ Div. Mines Geology Map MAP ISLAY : California SQttTTH AND PQIRZ SAN Z.ttTZS Obispo County, 24. (Scale I:24,000) lorn-argon dante~ co~c~~~ fora