Sedimentology (1995) 42, 783-804

Interpretation of neovolcanic versus palaeovolcanic sand grains: an example from Miocene deep-marine sandstone of the Topanga Group (Southern California) SALVATORE CRITELLI* and RAYMOND V. INGERSOLLt *Consiglio Nazionale delle Ricerche-Istituto di Ricerca per la Protezione Idrogeologica nell’ltalia meridionale ed insulare, Via G. Verdi 248, 87030 Roges di Rende (CS), Italy tDepartment of Earth and Space Sciences, University of California, Los Angeles, CA 90024-1 567, USA

ABSTRACT

Despite abundant data on volcaniclastic sand(stone), the compositional, spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented. One of the most intricate tasks in optical analysis of sand(stone) containing volcanic particles is to distinguish grains derived by erosion of ancient volcanic rocks (i.e. palaeovolcanic, noncoeval grains) from grains generated by active volcanism (subaqueous and/or subaerial) during sedimentation (neovolcanic, coeval grains).

Deep-marine volcaniclastic sandstones of the Middle Topanga Group of southern California are interstratified with 3000-m-thick volcanic deposits (both subaqueous and subaerial lava and pyroclastic rocks, ranging from basalt, andesite to dacite). These rocks overlie quartzofeldspathic sandstones (petrofacies 1) of the Lower Topanga Group, derived from deep erosion of a Mesozoic magmatic arc.

Changes in sandstone composition in the Middle Topanga Group provide an example of the influence of coeval volcanism on deep-marine sedimentation. Volcaniclastic strata were deposited in deep-marine portions of a turbidite complex (volcaniclastic apron) built onto a succession of intrabasinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks.

distinctive petrofacies (2-5). Directly overlying basalt and basaltic-andesite lava flows, petrofacies 2 is a pure volcanolithic sandstone, including vitric, microlitic and lathwork volcanic grains, and neovolcanic crystals (plagioclase, pyroxene and olivine). The abundance of quenched glass (palagonite) fragments suggests a subaqueous neovolcanic provenance, whereas sandstones including andesite and minor basalt grains suggest subaerial neovolcanic provenance. This petrofacies probably was deposited during syneruptive periods, testifying to provenance from both intrabasinal and extrabasinal volcanic events. Deposited during intereruptive periods, impure volcanolithic petrofacies 3 includes both neovolcanic (85%) and older detritus derived from plutonic, metamorphic and palaeovolcanic rocks. During post-eruptive periods, the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus (48-14%) and increase of plutonic-metamorphic and palaeovolcanic detritus.

The Upper Topanga Group (Calabasas Formation), conformably overlying the Middle unit, has dominantly plutoniclastic sandstone (petrofacies 6). Neovolcanic detritus is drastically reduced (4%) whereas palaeovolcanic detritus is similar to percentages of the Lower Topanga Group (petrofacies 1).

The Middle Topanga Group sandstones are vertically organized into four

0 1995 International Association of Sedimentologists 783

784 S. Critelli and R. V. Ingersoll

In general, the volcaniclastic contribution represents a well-defined marker in the sedimentary record. Detailed compositional study of volcaniclastic strata and volcanic particles (including both compositional and textural attributes) provides important constraints on deciphering spatial (extrabasinal vs. intrabasinal) and temporal relationships between neovolcanic events (pre-, syn-, inter- and post-eruptive periods) and older detritus.

INTRODUCTION

The evolutionary record of Earth’s processes (subaerial and subaqueous endogenic and exo- genic) preserved in the form of sedimentary rocks has been pivotal in palaeogeographical and palaeotectonic reconstructions of source/ basin systems. The value of petrographic analy- sis of clastic strata in deciphering the complex events that occur during mountain building and within adjacent depositional basins is reflected in the many attempts that have been made to refine models that relate tectonic setting and sand(stone) composition (e.g. Crook, 1974; Schwab, 1975; Dickinson, 1985, 1988; Valloni, 1985; Zuffa, 1987). More recently, research on sand(stone) composition has focused on com- plex factors that control composition, including geomorphic processes, climate, tectonics, depo- sitional environment and particular source rocks (e.g. Zuffa, 1987; Johnsson, 1993). More refined compositional studies of sand(stone) have focused on temporal and spatial deciphering of clastic particles (e.g. Zuffa, 1985, 1987). Carbon- ate and volcanic sand(stone) grains illustrate how deciphering of spatial and temporal prov- enance may be useful for correct palaeo- geographical reconstruction (e.g. Zuffa, 1987; Dickinson, 1988; Fontana, 1991; Marsaglia & Ingersoll, 1992).

Volcanic particles have particular geodynamic significance. Volcaniclastic sand(stone) (e.g. Fisher, 1961, 1966), including the entire spectrum of clastic materials composed in part or entirely of volcanic particles, has been the subject of intense research in both modern settings (particularly DSDPIODP investigations; e.g. Enkeboll, 1982; De Rosa et a]., 1986; Thornburg & Kulm, 1987; Mack & Jerzykiewicz, 1989; Marsaglia, 1991, 1993; Marsaglia & Ingersoll, 1992; Zuffa et al., 1993; Macaire et al., 1994) and ancient settings (e.g. Dickinson, 1982; Ingersoll, 1983; Garzanti, 1985; Cawood, 1990; Cather & Folk, 1991; Ingersoll & Cavazza, 1991; Lundberg, 1991; Critelli, 1993; Abbott et al., 1994; Cousineau, 1994).

Despite abundant data on volcaniclastic sand- (stone), the distinction between spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented. One of the most intricate tasks in optical analysis of volcaniclastic sand(stone) is the distinction of grains eroded from ancient volcanic rocks (i.e. palaeovolcanic, noncoeval grains) from grains generated by subaqueous and/or subaerial active volcanism during sedimentation (neovolcanic, coeval grains) ( e g Zuffa, 1985, 1987).

In terms of facies sequences, volcaniclastic sediments have received much attention, particu- larly in terrestrial depositional environments (e.g. Kuenzi et al., 1979; Vessel & Davies, 1981; Smith, 1987, 1988, 1991). Volcanic influences on sedi- mentation and volcanism-induced deposition are important constraints on dispersal patterns, trans- port and deposition of volcanic detritus (e.g. Kuenzi et al., 1979; Sigurdsson et al., 1980; Vessel & Davies, 1981; Carey & Sigurdsson, 1984; Fisher, 1984; Busby-Spera, 1985, 1988; Cole & DeCelles, 1991; Smith, 1991). In contrast to the relatively well-controlled record of fluviatile volcaniclastic sedimentation, it is difficult to monitor ’eruptive’ periods using deep-marine volcaniclastic strata.

Middle Miocene deep-marine strata of the Los Angeles Basin (southern California) illustrate the distinction between palaeovolcanic detritus derived from erosion of the Mesozoic magmatic arc and neovolcanic grains derived from sub- marine and subaerial lava flows and pyroclastic flows erupted during deposition. This paper utilizes petrological methods to discriminate neovolcanic and palaeovolcanic detritus, and tentatively to decipher syneruptive, intereruptive and post-eruptive processes during deposition of the Miocene Topanga Group.

GEOLOGICAL AND STRATIGRAPHIC SETTING

The Los Angeles Basin is a Neogene basin along the southern California coast; it is located at the northern end of the Peninsular Ranges, and

0 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

Interpretation of neovolcanic vs. palaeovolcanic sand grains 785

Fig. 1. Generalized geological map of the western and central Santa Monica Mountains, showing distribution and diagrammatic relations of the Topanga Group. Intrusive and hypabyssal rocks related to Conejo Volcanics are in black. Modified from Dibblee & Ehrenspeck (1993).

bounded on the north and east by the Transverse Ranges and on the west by the continental border- land (Fig. 1) (e.g. Biddle, 1991). The Los Angeles Basin is a small polyhistory basin developed within the rapidly evolving San Andreas trans- form zone (e.g. Atwater, 1970; Crowell, 1987; Wright, 1991; Nicholson et a]., 1994). Basin for- mation was initiated by transrotational rifting at about 18 Ma, followed by rapid subsidence, resulting in a deep water-filled basin [e.g. Crowell, 1987; Mayer, 1991). Sedimentation accelerated immediately following widespread submarine and subaerial volcanism at about 16 Ma. Deep-marine sedimentation occurred from Marine Miocene through Pliocene, fol- lowed by Pleistocene to present nonmarine sedimentation.

The basement rocks of the Los Angeles Basin consist of metamorphic, sedimentary, plutonic and volcanic rocks of Precambrian to Early Miocene age. General stratigraphy of the Los

Angeles Basin includes the Middle Miocene Topanga Group, the Middle to Upper Miocene Modelo (post-Topanga Group; Fig. l ) , Puente and Monterey formations, and the Upper Miocene to Pleistocene Capistrano, Repetto, Fernando and Pic0 formations (e.g. Blake, 1991).

The Topanga Group in the Santa Monica Mountains rests conformably on Oligocene to Lower Miocene, nonmarine to shallow-marine strata of the Sespe and Vaqueros formations, or directly on Palaeocene and Cretaceous deep- marine strata (pre-Topanga Group; Fig. 1). The Topanga Group is up to 6100 m in thickness and is divided into three formations, the Topanga Canyon Formation, the Conejo Volcanics and the Calabasas Formation (Yerkes & Campbell, 1979; Fritsche, 1993). More recently, Dibblee & Ehrenspeck (1993) subdivide the Topanga Group into three parts, the Lower Topanga Group, the Middle Topanga Group (including the Conejo Volcanics) and the Upper Topanga Group.

0 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

786 S. Critelli and R. V. Ingersoll

$3 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

Interpretation of neovolcanic vs. palaeovolcanic sand grains 787

The Lower Topanga Group (Topanga Canyon Formation) is Relizian and partly Saucesian in age (20-16 Ma), including dominantly shallow-marine conglomerate, sandstone and mudrock (Flack, 1993). This formation, which pre-dates the Conejo volcanism, is interpreted as consisting of predomi- nantly wave-dominated coastal deposits in the lower part of the succession and river-dominated, deltaic coastal and fluvial deposits through the middle and upper parts of the succession (Flack, 1993).

This formation is overlain by the Conejo Volcanics, which range in age from 15-5 f 0.8 to 13.9f0.4Ma (Weigand & Savage, 1993). The Conejo Volcanics are up to 3000m thick in the western Santa Monica Mountains but are markedly thinner in the east. They consist of subaqueous basal and basaltic-andesite lava, sub- aerial andesite and dacite lava and pyroclastic rocks; abundant hypabyssal intrusions of similar composition intrude the volcanics (Williams, 1983; Dibblee & Ehrenspeck, 1993; Weigand & Savage, 1993). Very thin to thick sandstone bodies are interstratified with the volcanic rocks and represent turbidite deposition during syn-, inter- and immediately post-eruptive periods. These strata are included within the Middle Topanga Group (Dibblee & Ehrenspeck, 1993).

The upper unit of the Topanga Group, the Calabasas Formation (Yerkes & Campbell, 1979), is up to 1200 m thick, and is mostly Luisian and Mohnian in age (13-10Ma). It consists of deep- marine and shallow-marine sandstone and con- glomerate; it locally unconformably overlies the Conejo Volcanics and Middle Topanga Group (Dibblee & Ehrenspeck, 1993).

The lower part of the Conejo Volcanics is pri- marily composed of subaqueous basaltic, and basaltic-andesite flows, flow breccias and hyalo- clastic breccia (Fig. 2A). Interbedded basaltic vol- caniclastic sandstone represents syneruptive (Fig. 2B) and intereruptive deep-marine sedimentation in response to basaltic eruptions. These rocks are

basalt and basaltic-andesite containing dominantly plagioclase (An73-54 based on probe microanalysis; Weigand & Savage, 1993) phenocrysts; hyper- sthene, augite and olivine are minor. The ground- mass contains plagioclase, augite, glass and opaque minerals. Basaltic glass is devitrified and contains sideromelane altered to palagonite.

Subaerial andesite and basalt flows, autoclastic flow breccias, and dacite lava and pyroclastic flows represent the uppermost units of the Conejo Volcanics (Williams, 1983; Dibblee & Ehrenspeck, 1993; Weigand & Savage, 1993). Subaerial andesite and basaltic-andesite lavas contain dominantly plagioclase (An5548), hypersthene and augite phenocrysts; the groundmass includes plagioclase, glass and opaque minerals. Subaerial dacite has felsitic texture and contains dominantly plagio- clase (An35-25), opaque minerals, glass and rare quartz (Weigand & Savage, 1993). Associated sedi- mentary rocks include pure and impure synerup- tive, intereruptive (Fig. 2C) and post-eruptive volcanolithic and quartzofeldspathic sandstones with variable amounts of neovolcanic detritus (Fig. 2D-H). Overlying sandstone and conglomer- ate have abundant volcanogenic detritus, repre- senting post-eruptive marine sedimentary strata.

SANDSTONE PETROLOGY

Fifty-three unaltered medium to coarse sandstone samples were selected for thin-section analysis, covering the entire sedimentary succession: 11 from the Lower Topanga Group (Topanga Canyon Formation), 36 from the Middle Topanga Group, and six from the Upper Topanga Group (Calabasas Formation). Five-hundred points were counted by one of us (S.C.) for each thin section (etched and stained for plagioclase and potassium feldspar) according to the Gazzi-Dickinson method (Ingersoll et a]., 1984; Zuffa, 1985, 1987). Point- count results are tabulated in Table 1, and re- calculated in Tables 2 and 3. Grain parameters (Table 1) and recalculated parameters (Tables 2

Fig. 2. Exposure of deep-marine volcaniclastic sandstones of the Topanga Group: (A) hyaloclastic andlor broken basaltic pillow breccia in upper part of submarine basaltic sequence (loc. Calabasas Quadrangle), overlain by (B) syneruptive basaltic sandstone strata; (C) intereruptive deep-marine volcaniclastic mass-flow strata, including white siliceous shale extraclast (loc. Malibu Beach Quadrangle); (D) sandstone succession directly overlying basaltic lava flow and breccia includes syn-, inter- and post-eruptive strata. In this section, we collected sandstone samples from cV13 to CV21; strata from c V l 6 to CV21 are post-eruptive turbidite sandstones (circle indicates hammer; loc. Cold Canyon Road, Malibu Beach Quadrangle); (E) neo-volcanic basaltic clast in massive post-eruptive sandstone strata (loc. Thousand Oaks Quadrangle); (F) and (G) massive post-eruptive sandstone including abundant basaltic and andesitic large-floating clasts (loc. Thousand Oaks Quadrangle); (H) submarine post-eruptive debris-flow deposit including neovolcanic (basalt, andesite and dacite), palaeovolcanic, plutonic-metamorphic clasts and reworked shelfal bioclasts (loc. Cold Canyon Road, Malibu Beach Quadrangle).

8 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

Tab

le 1. C

ateg

orie

s use

d fo

r san

dsto

ne p

oint

-cou

nts o

f fra

mew

ork

grai

ns a

nd a

ssig

ned

grai

ns in

reca

lcul

ated

plo

ts. C

rite

ria

for t

empo

ral a

nd te

xtur

al su

bdiv

isio

n of

vol

cani

c gr

ains

are

thos

e of

Dic

kins

on (

1970

), Zu

ffa

(198

7) a

nd M

arsa

glia

(19

93).

r.f.

=roc

k fr

agm

ent.

Roc

k fr

agm

ent r

ecal

cula

tions

are

thos

e of

Cri

telli

& L

e Pe

ra (

1994

).

~ Qm

. QP,

F,

K, P

. L

para

met

ers

Cou

nted

par

amet

ers

Ass

igne

d A

ssig

ned

para

met

ers

para

met

ers

toL

and

R

to L

v R

ecal

cula

ted

para

met

ers

Qua

rtz

(Qt=

Qm

+Qp)

Q

uart

z (s

ingl

e cr

ysta

ls)

Poly

crys

talli

ne q

uart

z C

hert

Q

uart

z in

met

amor

phic

r.f

. Q

uart

z in

plu

toni

c r.f

. Q

uart

z in

plu

toni

c or

gne

issi

c r.f

. C

alci

te re

plac

emen

t on

qua

rtz

Feld

spar

s (F

=K+P

) K

-fel

dspa

r (s

ingl

e cr

ysta

ls)

K-f

elds

par

in m

etam

orph

ic r

.f.

K-f

elds

par

in p

luto

nic

r.f.

K-f

elds

par i

n pl

uton

ic o

r gn

eiss

ic r

.f.

Cal

cite

repl

acem

ent

on k

-fel

dspa

r Pl

agio

clas

e (s

ingl

e cr

ysta

ls)

Plag

iocl

ase

in m

etam

orph

ic r

.f.

Plag

iocl

ase

in p

luto

nic

r.f.

Plag

iocl

ase

in p

luto

nic

or g

neis

sic

r.f.

Cal

cite

rep

lace

men

t on

Pla

gioc

lase

M

icas

and

den

se m

iner

als

Mic

as (

sing

le c

ryst

als)

M

icas

in p

luto

nic

or g

neis

sic

r.f.

Mic

as i

n m

etam

orph

ic r

.f.

Den

se a

nd o

paqu

e m

iner

als

(sin

gle

crys

tals

) D

ense

min

eral

s in

plu

toni

c r.f

. D

ense

min

eral

s in

met

amor

phic

r.f.

Pala

eovo

lcan

ic g

rain

s Q

uart

z in

vol

cani

c r.f

. K

-fel

dspa

r in

volc

anic

r.f.

Plag

iocl

ase

in v

olca

nic

r.f.

Vol

cani

c li

thic

with

mic

roli

tic

text

ure

Vol

cani

c li

thic

wit

h la

thw

ork

text

ure

Vol

cani

c li

thic

wit

h vi

tric

text

ure

Vol

cani

c li

thic

wit

h fe

lsiti

c gr

anul

ar te

xtur

e V

olca

nic

lith

ic w

ith

fels

itic

seri

ate

text

ure

Oth

er l

ithi

c gr

ains

M

etav

olca

nic

lith

ic

Phyl

lite

Fine

-gra

ined

Sch

ist

Silt

ston

e Sh

ale

Neo

volc

anic

gra

ins

Qua

rtz

in n

eo-v

olca

nic

r.f.

Plag

iocl

ase

(sin

gle

euhe

dral

cry

stal

) Pl

agio

clas

e in

neo

-vol

cani

c r.f

. D

ense

min

eral

in

neo-

volc

anic

r.f.

Sing

le n

eo-v

olca

nic

dens

e m

iner

al

Vol

cani

c li

thic

wit

h m

icro

liti

c te

xtur

e V

olca

nic

lith

ic w

ith

lath

wor

k te

xtur

e V

olca

nic

lith

ic w

ith

vitr

ic t

extu

re

Vol

cani

c li

thic

wit

h fe

lsit

ic g

ranu

lar

text

ure

Vol

cani

c li

thic

wit

h fe

lsit

ic se

riat

e te

xtur

e

Rs

Rm

Rg

Rm

pala

eo-R

v pa

laeo

-Rv

pala

eo-R

v pa

laeo

-Lv

pala

eo-L

v pa

laeo

-Lv

pala

eo-L

v pa

laeo

-Lv

LVm

Lm

Lm

Ls

Ls

neo-

Rv

neo-

Rv

neo-

Rv

neo-

Lv

neo-

Lv

neo-

Lv

neo-

Lv

neo-

Lv

Lvm

i Lv

l L

w

LVf

Lvf

Qm

FLt p

lot Qm

= m

onoc

ryst

alli

ne q

uart

z F=

feld

spar

s IK

-fel

dspa

r+Pl

agio

clas

e)

Lt=

apha

niti

c lit

hic

grai

ns+p

olyc

ryst

alli

ne q

uart

z (Q

p)

QtF

L pl

ot

Qt=

mon

ocry

stal

line

qua

rtz

(Qm

)+po

lycr

ysta

llin

e qu

artz

(QP)

F

= fe

ldsp

ars

(K-f

elds

par c

Plag

iocl

ase)

L=

apha

niti

c li

thic

gra

ins

Qm

KP

plot

Qm

= m

onoc

ryst

alli

ne q

uart

z K

= K

-fel

dspa

r P

= pl

agio

clas

e

pala

eo-K

pal

aeod

neo

-P p

lot

pala

eo-K

= K

-fel

dspa

r pa

laeo

P=

mon

ocry

stal

line

pla

gioc

lase

and

coa

rse

plag

iocl

ase

in p

luto

nic,

met

amor

phic

and

pa

laeo

volc

anic

pha

neri

tic

frag

men

ts

neo-

P= si

ngle

euh

edra

l pla

gioc

lase

and

coa

rse

plag

iocl

ase

in n

eovo

lcan

ic p

hane

riti

c fr

agm

ents

QpL

vmLs

m p

lot

Qp=

poly

crys

talli

ne q

uart

z L

vm=

pala

eovo

lcan

ic (

pala

eo-L

v), n

eovo

lcan

ic (n

eo-L

v) an

d m

etav

olca

nic

(Lvm

) lit

hic

grai

ns

Lsm

= se

dim

enta

rty

(Ls)

and

met

amor

phic

(Lm

) lit

hic

grai

ns

LmLv

Ls,L

m+L

s ne

o-Lv

pal

aeo-

Lv p

lots

L

m=

met

amor

phic

(Lm

) and

met

avol

cani

c (L

vm) l

ithi

c gr

ains

Lv

= ne

ovol

cani

c (n

eo-L

v) a

nd p

alae

ovol

cani

c (p

alae

o-Lv

) lit

hic

grai

ns

Ls=

sedi

men

tary

lith

ic g

rain

s ne

o-L

v=ne

ovol

cani

c li

thic

gra

ins

pala

eo-L

v= p

alae

ovol

cani

c li

thic

gra

ins

Lw

Lvm

iLvl

, L&

vmiL

vl,

Lw

Lvm

i+L

vl+L

vf n

eocr

ysta

ls p

lots

L

w=

vitr

ic p

alae

ovol

cani

c (p

alae

o-Lv

l +vi

tric

neo

volc

anic

(neo

-Lv)

gra

ins

Lvm

i= m

icro

litic

pal

aeov

olca

nic

(pal

aeo-

Lvl

+mic

roli

tic n

eovo

lcan

ic (n

eo-L

vl g

rain

s Lv

l= la

thw

ork

pala

eovo

lcan

ic (p

alae

o-L

v)+l

athw

ork n

eovo

lcan

ic (n

eo-L

v) g

rain

s Lv

f= fe

lsiti

c se

riat

e an

d gr

anul

ar p

alae

ovol

cani

c (p

alae

o-Lv

l+fe

lsiti

c se

nate

and

gra

nula

x ne

ovol

cani

c (n

eo-L

v) g

rain

s ne

o-

crys

tals

=sin

gle

neov

olca

nic

quar

tz, f

elds

pars

and

den

se m

iner

als

and

sam

e m

iner

als

in

phan

erit

ic n

eovo

lcan

ic fr

agm

ents

(ne

o-R

v)

RgR

vRm

, Rg+

Rm

neo

-Rv

pala

eo-R

v pl

ots

Rg=

coa

rse

quar

tz, f

elds

pars

, mic

as a

nd d

ense

min

eral

s in

pl

uton

ic a

nd g

neis

sic

phan

erit

ic f

ragm

ents

R

v= ap

hani

tic

volc

anic

gra

ins

(Lv)

+coa

rse q

uart

z, fe

ldsp

ars,

mic

as a

nd d

ense

min

eral

s in

pal

aeov

olca

nic

(pal

aeo-

Rv)

and

neo

volc

anic

(ne

o-R

vl p

hane

riti

c fr

agm

ents

and

dens

e m

iner

als

in n

eo-v

olca

nic

phan

erit

ic f

ragm

ents

ne

o-R

v= a

phan

itic

neo

-vol

cani

c gr

ains

(neo

-Lvl

+coa

rse

quar

tz, f

elds

pars

, mic

as

Lvm

i Lv

l L

w

pala

eo-v

olca

nic

phan

erit

ic f

ragm

ents

Lv

f Lv

f

pala

eo-R

v= ap

hani

tic

pala

eo-v

olca

nic

grai

ns (p

alae

o-Lv

)+co

arse

qua

rtz

and

feld

spar

s In

Rm

= ap

hani

tic

met

amor

phic

gra

ins

(Lm

lcco

arse

qua

rtz,

feld

spar

s,

mic

as a

nd d

ense

min

eral

s in

met

amor

phic

pha

neri

tic

frag

men

ts

Tab

le 2

. R

ecal

cula

ted

mod

al p

oint

cou

nt d

ata

for

the

Top

anga

Gro

up s

ands

tone

s. S

ee T

able

1 fo

r ex

plan

atio

n of

par

amet

ers.

X=

mea

n; S

D=

stan

dard

dev

iati

on.

CI W W

cn 6

6 a R

l-5

5'

3

Qm

F Lt

%

QtF

LX

Qm

K P

X

Qp

Lvm

Lsm

%

LmLv

Ls%

pa

laeo

-K p

alae

o-P

Neo

-P%

R

gRV

Rm

Q,

Qm

F

Lt

Qt

F L

Qm

K

P

Qp

Lvm

Lr

m

Lm

Lv

Ls

pdae

o-K

pa

laen

-P

neo-

P Rg

Rv

Rm

PI

F Lv

iL

RgIR

~~

~ ~

Pew

ofac

kes 1

(Low

er T

opan

ga

TC

I 44

54

T

CZ

42

48

T

C3

43

51

T

C4

40

52

TC

5 44

46

T

C6

33

60

TC

7 1

3

75

TC

8 34

55

T

C9

46

43

Tc

ln

51

41

TC

ll

52

38

X 40

51

Gro

up o

x T

opan

ga C

anyo

n

10

43

6

44

10

46

7 34

1

2

14

11

34

11

47

n 45

n 41

Form

atio

n)

50

5 48

9

51

5 52

7

46

n 60

6

75

11

55

11

43

10

47

46

46

44

49

35

14

39

51

56

44

1 12

17

15

2 0 9

19

36

40

51

41

39

43

35

50

41

1

6

58

38

9 15

1

3

20

17

1

5

10

15

29

44

39

41

36

34

50

76

46

20

2

1

19

39

i 1

6 83

80

100

90

1 1

0 49

50

43

26

44

42

42

44

34

42

17

9n

21

14

25

1

9

15

9

29

35

33

31

29

66

61

90

75

50

51

42

50

56

25

30

63

55

56

45

52

26

33

37

40

44

38

34

72

67

0 5 0 17

14

0 0 0

17

27

24

36

33

24

12

24

58

83

73

76

64

67

76

76

42

nn

56

57

71

66

80

74

73

62

85

14

14

13

5

30

29

16

10

25

9 3 35

19

1 10 1 0 0 0

+1 n in

in

n

0.83

0.

73

0.76

0.37

0.

40

0.44

0.

38

0 34

0.72

0.

67

0.94

0.

73

u 38

0.

44

0.53

* 0

.20

1.00

1.00

1.00

1.

00

1.00

i 0

-00

0.95

0.

96

0.97

0-

94

0.96

0.

92

0.97

0.

96

0.96

-t

0.02

0.97

0.56

0.

56

0.71

0.

84

0-63

0.74

0.

73

0-62

0.

51

0.71

0.

67

10-11

0.02

0.01

n.80

0.00

0.64

0.

67

0.76

0

-88

0.

76

0.42

0.

48

0.45

0.

67

1 0

.16

1.00

1.00

1

00

0.

99

1 0.

03

0.76

0.69

0.

54

0.74

0.74

0.

67

0-6

9

0.71

* 0

.08

0.94

0.83

0.70

9 12

17

24

9 4 13

17

1

3

14

+

6

0 0 0 0 0

i0

6

12

49

21

36

1 18

0 0 0

0 0 f

0

5 4 2 4 4 7

6 27

94

73

0 7 0 4 1

6

0 0 0 0 0

+O

0 1 n 1 n

20

12

lo

14

i5

97

99

100

100

99

+I

85

95

71

89

85

87

83

90

86

i7

87

n 10

9 1

2

53

5

3

41

1 11

3

41

38

51

i 10 17

22

2

1

17

19

f

3

6 9 8 1

2

no

83

74

78

79

*4

23

23

17

+6

n 5 0 0 1 1

3

15

10

1

9

23

15

1

9

15

21

16

1

7

*4

34

66

50

1 2

2

100

100

100

100

100

f0

93

94

98

95

96

90

94

96

95

*3

9n

55

56

43

1 17

0 0 0 0 0 1

0

5 3 3 5

38

-t 2

0

44

53

100

100

100

100

100

10

95

96

97

94

96

92

97

96

97

96

+1

52

55

33

48

0 45

0

67

0 1

16

+O

9 9

1

6 88

0

100

4 94

f

5

f6

46

30

52

31

n m

u

21

48

26

2n

32

38

30

44

31

43

35

32

21

48

33

38

1

11

+n

53

71

68

+ 1

0 2 1 0 0 1 11

6 2 9

22 7 7

SD

+11

+1

0

Petr

ofac

ies

2 (M

iddl

e To

pang

a c

v7

3

17

CV

13

4 22

C

V35

1

21

CV

36

0 17

f 1

6 0 6 0 0 2

f3

24

17

I G

roup

) 80

74

83

79

i4

78

1 G

roup

) 48

52

61

57

61

56

56

52

72

57

+7

4 1

n 0.

00

0.0

1

i 0

-01

2 1

2

X

2 19

SD

+

2

+3

P

ehof

acie

s 3

(Mid

dle T

opao

ga

CV

14

19

33

cv1

5

19

29

c

v4

3

15

24

cv

44

22

2

1

CV

45

16

23

CV

46

17

27

cv

47

19

25

C

V48

17

31

C

V50

14

1

4

X

18

25

SD

i3

16

20

20

15

23

16

19

19

15

in

33

29

24

21

23

27

25

31

47

51

61

56

81

55

56

50

71

57

*

7

2 2 0 1

0 3 0 3 1 1

*I

0.06

0.

02

0.09

0-

22

0.07

0.

07

31

46

26

30

26

33

31

29

i8

4 6 3

4

8 3 5 3 5 f

2

2 0 0 0 0 i

0

2 3 3

10

12

7

0.10

0.

12

0.07

0.

09

1 0.06

4 3 14

25

f

6

9 1

6

in

+3

4

f2

Pe

trof

acie

s 4

(Mid

dle

Top

anga

Gro

up)

CV

23

44

31

25

46

31

23

58

22

20

10

77

13

1

8

81

1

52

39

9 33

48

19

0.48

0.

81

03

3

CV

24

42

34

24

44

32

24

56

18

26

7 79

14

17

n2

1

4

1

47

12

2n

52

20

0.59

0.

82

0.28

3

CV

27

47

31

22

48

31

21

60

21

19

5

73

22

28

69

3 52

47

1

29

45

2n

0-48

0.

69

0.29

ZI,

cv

zn

43

31

26

44

31

25

58

19

23

6 78

16

I8

8

0

2 45

4

3

12

16

63

21

0.

54

0.80

0.

16

cv2

9

45

30

25

46

30

24

60

23

17

3 78

19

32

65

3

57

34

9 22

47

31

0.

44

0.65

0.

22

6 2 C

V30

51

23

26

52

23

25

69

1

8

13

5

78

17

14

80

58

39

cv3

2

41

30

29

42

30

2n

59

22

19

2 93

5

12

86

52

35

3 50

4

1

9 2n

52

20

0.

50

0.77

0.

27

;D +

3

+4

i

2

i3

1

4

12

+

4

12

+

4

i3

16

f5

1

7

17

1

2

16

1

5

14

f

6

17

1

6

+0-

06

10

.07

1

0.0

6

2' C

V34

47

25

28

4

7

25

28

65

16

1

9

1 85

14

1

7

78

5 45

44

11

36

47

1

7

0.54

0.

78

0-35

a

5 80

1

5

20

77

45

29

26

46

29

25

60

20

20

Pet

rafa

cies

5 (

Mid

dle-

Upp

er T

opan

ga G

roup

) C

V3

45

44

11

46

44

10

50

17

33

11

74

1

5

24

76

0 34

54

1

2

55

24

21

0.66

0.

76

0.55

d

cv

4

47

44

9 47

44

9

51

26

23

2 6

8

30

46

52

0 5

3

46

1 69

13

1

8

0.47

0

52

0

69

C

V5

40

49

11

42

49

9 45

19

36

14

70

1

6

25

75

0 35

62

3

38

41

21

0.65

0.

75

0.3~

cv8

38

47

IS

39

47

1

4

44

19

37

CV

l0

45

39

16

46

39

15

55

25

20

8

74

in

23

73

4 56

43

1

54

31

15

0.

45

0.73

0.

53

CV

16

43

44

13

45

44

11

49

23

28

14

68

18

30

68

2

45

50

5

38

40

22

0.55

0.

68

0.37

0

CV

17

44

41

1

5

47

41

1

2

52

27

21

17

59

24

31

67

2

57

43

o 58

25

17

0.

43

0.67

0-

57

Q

CV

19

50

39

11

52

39

9

55

24

21

23

52

25

49

473

3 53

47

0

57

17

26

0.47

0.

48

0.56

C

V20

54

34

1

2

56

34

10

61

23

16

1

5

64

21

48

50

2 59

41

0

63

1

5

22

0.41

0.

50

0.62

C

VZ

l 52

38

1

0

54

38

8

57

24

19

18

62

20

27

68

5 55

44

1

45

27

28

0.45

0.

68

0.44

C

V22

39

44

17

40

44

16

46

22

32

8 69

23

28

72

0

41

47

12

51

37

1

2

0.59

0.

72

0.51

X

47

4.9

13

49

40

11

53

22

25

12

67

2

1

34

63

3 4

8

49

3 50

29

21

0.

52

0.63

0.

48

0 2 34

60

cV

9

51

36

13

53

36

11

59

26

15

11

68

2

1

29

69

63

36

6

37

50

13

5

77

18

29

71

55

45

0

64

11

25

0.45

0-

30

0.53

7

CV

18

49

41

10

51

41

8

54

25

21

20

52

28

56

30

14

CV

31

50

35

15

dl

35

14

60

20

20

4 63

33

41

57

2 5

0

49

1 27

36

37

0

50

0.57

0.

27

2.

CAI

46

41

13

48

41

11

53

26

21

1

3

48

39

48

48

4 56

44

0

57

16

27

0.44

0

48

0-

56

Q

CA

5 5

6

35

9 56

35

9

61

25

14

7 66

27

4

1

59

0 64

36

0

67

16

17

0.

36

05

9

0.67

2.

4 31

65

4

35

42

23

0.69

0.

71

0.34

0 3

cV33

44

40

16

45

40

15

52

1

5

33

7 79

1

4

25

71

SD

+5

1

4

i3

i5

1

4

f3

15

+

4

17

1

6

+8

1

6

+I1

11

3

i4

+1

1 1

8

14

1

12

1

12

+

6

iO.1

0 +

0-1

3

10

.12

2

Pch

ofac

ies

6 IU

pper

Top

anga

Gro

up o

r C

alab

asas

For

mat

ion)

CA

2 52

35

1

3

55

35

10

60

26

14

19

45

36

5

6

40

4 63

33

1

63

13

24

0.

34

0.40

0-

62

CA

3 41

44

15

42

44

14

48

29

23

6

47

47

47

43

10

56

44

o 55

in

27

0.

44

0.43

0,

53

CA

4 53

38

9

55

38

7 57

24

1

9

I7

39

44

65

32

3 56

44

0

67

8 25

0.

44

0-32

0.

67

CA

6

53

36

11

55

36

9 59

28

13

23

49

2n

41

51

8

69

31

0 5

3

24

23

0.31

0-

51

SO

38

12

52

38

10

56

27

17

14

49

37

50

45

5

61

39

0 60

16

24

0.

39

0.46

::;

: 2

*6

f

0

+6

1

5

f4

+

0.06

1

0-0

9

10

07

SO

1

6

+4

+

2

+6

i

4

*2

1

5

f2

1

4

17

1

9

16

i9

19

14

1

6

u

co

W

u

w

0

bP m I m

u

(D

0

Tab

le 3.

Rec

alcu

late

d m

odal

poi

nt c

ount

dat

a of

the

vol

cani

c pa

rtic

les

for t

he T

opan

ga G

roup

san

dsto

nes.

See

Tab

le 1

for

expl

anat

ion

of p

aram

eter

s. X

=mea

n;

SD-s

tand

ard

devi

atio

n.

Rgt

Rm

neo

Rv

pdeo

Rv%

L

w L

vmi+

vl+v

f neo

*%

Lvv

Lvm

l L"

l%

Lvm

, LV

l L"f%

L

m+L

s neo

Lv

pale

aLv%

~

LVV

Lvm

+vl+

vf

neoC

ry

L""

L"m

1 LV

l L"

ml

Lvl

Lvf

Lm

tLs

neoL

v pa

laea

L"

Rg

+b

*W

R"

pale

oRv

Pet

roia

cm 1

(L

ower

Top

anga

Gro

up o

r T

opan

ga C

anyo

n Fo

rmat

ion

TC

1 0

100

0 T

CZ

0

100

0 T

C3

0

100

0 T

C4

0

100

0 T

C5

33

67

0 T

C6

19

8

1

0 T

C7

4 96

0

TC

8 0

100

0 T

C9

27

73

0 T

Cl0

1

8

82

0 T

Cll

0

100

0 X

9

91

0 SD

1 13

+ 1

3

*O

cv

7

20

63

17

CV

13

49

29

22

cv

35

12

66

22

C

V36

8

74

18

X

22

58

20

SD

f 1

9

1 2

0 1

3

cv

14

34

47

19

C

V15

53

29

1

8

cv

43

40

42

1

8

cv

44

37

51

1

2

cv

45

55

29

1

6

CV

46

33

49

18

c

v4

7

39

44

17

cv

48

45

35

20

C

V50

55

34

11

X

43

40

17

SD

19

*

8

+3

cv

23

9

77

14

cv

z4

5

77

18

c

v2

7

45

51

4 c

v2

8

37

48

15

c

v2

9

24

59

17

CV

30

62

34

4 C

V32

23

64

1

3

cv

34

23

65

12

X

29

59

1

2

SD

f 1

9 * 1

5

+5

cv

3

8 47

45

c

v4

8

80

12

c

v5

3

79

18

CV

8 7

69

24

c

v9

10

85

5

CV

lO

14

67

19

cv

16

14

48

38

c

v1

7

11

89

0 cv

18

0 1

00

0

cv

19

14

72

1

4

cvzo

9

91

0 C

VZ

l 23

72

5

cv

22

8

55

37

CV

31

57

39

4 c

v3

3

33

55

12

X 15

69

16

SD

f 1

4 1 18

f 1

5

CA

I 19

8

1

0 C

A2

25

70

5 C

A3

42

58

0 C

A4

36

64

0 C

A5

17

83

0

CA

6 37

63

0

X 29

7

0

1

SD

* 10

+ 10

i

2

Petr

ofac

ies

2 (M

iddl

e Ta

pang

a G

roup

)

Peh

afac

ies

3 [M

iddl

e Ta

pang

a G

roup

)

Petr

afac

ies 4

(M

iddl

e Ta

pang

a G

roup

)

Petr

ofac

ies 5

(Mid

dle-

Upp

er T

opan

ga G

roup

)

Peho

faci

es 6

(U

pper

Top

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Interpretation of neovolcanic vs. palaeovolcanic sand grains 791

Q Qm

P

Lsm

Fig. 3. QFL, QmKP, QpLvmLsm, LmLvLs and RgRvRm triangles of Topanga Group sandstones (see Table 1 for explanation of symbols]. Numbers are sandstone petrofacies; polygons are one standard deviation on either side of mean; shaded polygons are volcanic-rich petrofacies and arrows indicate evolution from syn- (Z), inter- (3) to post-eruptive (4, 5) petrofacies for the Middle and Upper Topanga Group.

Rm

and 3) are those of Dickinson (1970, 1985), Zuffa (1985, 1987). Marsaglia & Ingersoll (1992) and Critelli & Le Pera (1994). Twenty samples were also investigated by XRD and SEM-EDAX to recognize diagenetic minerals and to collect detailed information about neovolcanic grains. We subdivided the volcanic detritus using tex- tural (e.g. Marsaglia, 1993) and temporal (e.g. Zuffa, 1987) criteria. Volcanic detritus was sub- divided into palaeovolcanic and neovolcanic grains (Table 1). Palaeovolcanic grains (grains derived from erosion of ancient volcanic terrane) are dominantly altered, rounded, and have the same grain size as nonvolcanic detritus. Neo- volcanic grains (grains derived from active volcan- ism during sedimentation) have a generally fresh

appearance, generally larger grain size than the associated other terrigenous clasts, and are repre- sented by both lithic grains and single neocrysts (euhedral neo-plagioclase and mafic minerals).

Previous studies of the lower part of the Topanga Group sandstones document a quartzo- feldspathic composition (e.g. Lane, 1989; Critelli et al., 1995), reflecting their plutoniclastic prov- enance from the root of the Mesozoic magmatic arc (e.g. Dickinson, 1982; Ingersoll, 1983). In the present study, sandstones of the Topanga Group have been petrostratigraphically subdivided into six distinctive petrofacies (Figs 3 and 4) that record the key palaeogeographical, palaeogeo- dynamic and volcanotectonic evolution of the western Los Angeles Basin.

0 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

792 S . Critelli and R. V. Ingersoll

paleo-K Rg +Rm

paleo -P I paleo-RV

Lm+Ls Lvv

+VI +Vf C r y s t a l s

Lvv Lvf

Lvmi Lvl Lvmi Lvl Fig. 4. Palaeo-K palaeo-P neo-P, Rg+Rm neo-Rv palaeo-Rv, Lm+Ls neo-Lv palaeo-Lv triangles subdivide framework grains using temporal criteria (palaeo- vs. neo-grains); LwLvmi+vl+vf neo-Crystals, .LvvLvmiLvl and LvfLvmiLvl triangles subdivide volcanic grains using textural attributes (vitric, Lvv; microlitic, Lvmi; lathwork, Lvl, and felsitic, Lvf).

Lower Topanga Group P/F=0.67). Aphanitic lithics include abundant metasedimentary and volcanic grains (Lm43 f 1 7 Lv53 f 20 Ls4 f 6). Volcanic lithics are repre- Petrofacies 1

The quartzofeldspathic sandstone (Qm40 f 11 sented dominantly by felsitic grains, and subordi- F51 Ifr 10 Lt9 f 2) of the Lower Topanga Group nate microlitic and lathwork grains, representing (Topanga Canyon Formation) contains abundant palaeovolcanic detritus derived from Mesozoic plagioclase grains (Qm44 i 12 K17 i 6 P39 f 16; arc-related volcanic terranes (e.g. Ingersoll, 1983).

0 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

Interpretation of neovolcanic vs. palaeovolcanic sand grains 793

Phaneritic rock fragments (distributed in Qm, K, P, micas and dense minerals; Table 1; see Critelli & Le Pera, 1994; Critelli et ~ l . , 1995) include abundant plutonic and subordinate volcanic and metamorphic fragments (e.g. Rg68 f 10 Rv14 f 5 Rm18f 10; Fig. 3). Plutonic rock fragments include primarily plagioclase-bearing rocks (e.g. diorite and tonalite) in the lower Topanga Canyon Formation and K-feldspar-bearing intrusive rocks (e.g. granodiorite and granite) in the upper Topanga Canyon Formation.

Middle Topanga Group

The Middle Topanga Group includes deep- marine sedimentary strata (Middle Topanga Formation), and deep-marine, shallow-marine and subaerial volcanic lava flows and pyroclastic rocks (Conejo Volcanics). Turbidite strata of the Middle Topanga Group are interstratified with volcanic lava flows and cover the volcanic succes- sions. These sandstone strata were derived from both coeval volcanic rocks (syneruptive and intereruptive periods), and distant extrabasinal sources (syn-, inter- and post-eruptive periods; Figs 3 and 4).

Petrofacies 2

The sandstones of this petrofacies are medium- to coarse-grained turbidite beds (0.1-0.3 m thick) interbedded with basaltic, basaltic-andesitic and andesitic rocks. The volcanolithic sandstone of this petrofacies (Qm2 & 2 F19 f 3 Lt79 f 4) have only few per cent quartz; plagioclase is the domi- nant feldspar (P/F=0.99) and the volcanic lithic grains are the only aphanitic detritus represented (LmO LvlOO LsO). The volcanic detritus (NeoLv/ L=1.00) includes vitric (Lv%Lvv ranges from 11 to 63%), microlitic (Lv%Lvmi, 3243%) and lathwork (Lv%Lvl, 3-54%) grains (mean Lvv28 Lvmi36 Lv136) (Fig. 4). The percentage ranges of the three volcanic textures suggest that neo- volcanic detritus is closely linked to the compo- sitional character of the erupted volcanics (basalt to andesite; Fig. 5). Vitric lithics are dominantly devitrified basaltic glass (Fig. 5A) including sideromelane altered to palagonite (Fig. 5B). These grains were derived from submarine basal- tic lava flows; microlitic and lathwork lithics include plagioclase and pyroxene microlites and vitric groundmass (Fig. 5C-E); felsitic lithics (Fig. 5F) are represented by large phenocrysts of plagioclase (An55-75), olivine relics, augite, hyper- sthene and opaque minerals in vitric and seriate

groundmass. Glassy basalt lithics are brown, pal- agonitized due to rapid quenching of magma by water (e.g. Cas & Wright, 1987), devitrified, and altered to clay minerals. Single neovolcanic crystals include pyroxene (Fig. 5G), plagioclase and opaque minerals.

Interstitial and secondary components are volcanogenic matrix, clay (Fig. 5H), calcite, dolo- mite, colloform silica, zeolites, authigenic albite and phyllosilicate.

Petrofacies 3

The sandstones of this petrofacies, directly over- lying petrofacies 2, are impure volcanolithic (Qm18 i 3 F25 & 6 Lt57 f 7), including abundant plagioclase (P/F = 0.71) and volcanic lithic grains (Lm4 Lv96 Ls0). The volcanic lithics (Figs 6 and 7) include abundant vitric (Fig. 6A), microlitic (Fig. 7A-C), and subordinate lathwork (Fig. 6B,C) and felsitic (Fig. 6D) grains (Lvv53 Lvmi33 Lv114; Lvmi68 Lv126 Lvfe6). Neovolcanic grains are dominant (NeoLv/L= 0.93) with respect to palaeo- volcanic (PalaeoLv/L = 0.02) and nonvolcanic grains (Lm+Ls/L=0.05). The total source-rock recalculation of both phaneritic and aphanitic detritus suggests an increase of granitoids and metamorphic detritus with respect to petrofacies 2 (Rg9 f 6 Rv86 f 7 Rm5 f 2). This increase in the nonvolcanic contribution includes increased K-feldspar content (Palaeo-K29 Palaeo-P33 Neo-P38; Fig. 4).

Petrofacies 4

The sandstones of this petrofacies are quartzo- feldspathic (Qm45 f 3 F29 i 4 Lt26 f 2), having abundant monocrystalline quartz and equal pro- portions of plagioclase and K-feldspar (P/F=0.50). Aphanitic lithic grains have abundant volcanic grains and subordinate metasedimentary grains (Lm20 Lv77 Ls3). Neovolcanic grains are abun- dant (NeoLv/L=O.72), whereas palaeovolcanic grains are rare (PalaeoLv/L=O.06). Neovolcanic grains include pyroclastic grains (Fig. 6E,F). Recalculation of both phaneritic and aphanitic fragments (Rg28 Rv52 Rm20) documents increased granitoid and metamorphic detritus and a drastic reduction in the neovolcanic detri- tus (Rg+Rm48 f 7 NeoRv48 f 7 PalaeoRv4 f 2). These sandstones have intense diagenetic signals including compaction, authigenic clay, dissol- ution of silicate framework grains, crystallization of calcite and dolomite cement, and authigenic quartz and silica (Fig. 7D).

0 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

794 S. Critelli and R. V. Ingersoll

0 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

Interpretation of neovolcanic vs. palaeovolcanic sand grains 795

Petrofacies 5

Rapid cessation of volcanic activity is recorded in the upper Middle Topanga Group, with depo- sition of quartzofeldspathic sandstone (Qm47 & 5 F40 f 4 Lt13 f 3). It contains abundant mono- crystalline quartz, increased polycrystalline quartz and abundant feldspars (P/F=0.52). Aphanitic lithic grains include abundant metasedimentary and volcanic grains (Lm34 Lv63 Ls3). The neo- volcanic detritus is much reduced (Lm+Ls37 NeoLv29 PalaeoLv34) with respect to the inter- eruptive petrofacies 4. The volcanic lithics are rep- resented by abundant felsitic palaeovolcanic grains (Lvmi20 LvllO Lvfe70), and rock fragments include abundant granitoid detritus (Rg50 Rv29 Rm21).

Upper Topanga Group

Petrofacies 6

A significant change in composition occurs at the base of the Calabasas Formation; the sandstone is quartzofeldspathic (Qm50 f 6 F38 f 4 Lt12 f 2), including abundant quartz and K-feldspar (Qm56 K27 P17), and minor plagioclase (P/F=0.39). Aphanitic lithics include abundant metasedimen- tary and volcanic grains (Lm50 Lv45 Ls5). Vol- canic lithics include minor lathwork (Fig. 6G) and abundant felsitic (Fig. 6H) (Lvmi6 Lvl3 Lvfe91) grains, interpreted as palaeovolcanic grains. Neo- volcanic detritus is minor (NeoLv/L=O-11 f 0.08) and is represented by vitric and microlitic vol- canics (Lvv83 Lvmill Lv16). Abundant phaneritic rock fragments include plutonic, and minor metamorphic and volcanic (Rg60 Rv16 Rm24; Rg+Rm84 NeoRv4 PalaeoRvl2) rocks.

DISCUSSION

Provenance of Topanga Group Sandstones

Modal analyses of Topanga Group sandstones define source terranes comprising plutonic-

metamorphic, older (palaeo) volcanic and coeval (neo) volcanic detritus, whereas sedimentary detritus is rare (Figs 3 and 4). The influence of intrabasinal and extrabasinal tectonics during the initial rift stage (e.g. Crowell, 1987), and submarine and subaerial volcanism within the rapidly evolving Los Angeles Basin and surround- ing areas, is revealed by the spatial- and time- dependent petrofacies distribution of the Topanga sandstones. Several significant petrological parameters and diagrams (Figs 3, 4 and 8 ) are used to illustrate various grouping of petrofacies. Classic triangular plots (Figs 3 and 4) show evol- utionary trends. Triangular plots of Fig. 4 and two-component diagrams of Fig. 8 emphasize volcanic particles. We use textural attributes and temporal subdivisions of volcanic particles to combine criteria expressed by Zuffa (1985, 1987) and Marsaglia (1991, 1993) to show evolutionary trends and relationships.

The Topanga Group succession represents sedi- mentation during initial rifting of the Los Angeles Basin (eg. Crowell, 1987; Biddle, 1991; Wright, 1991) in response of the rapidly evolving San Andreas transform zone. Initial thinning by stretching of lithosphere in the Los Angeles Basin occurred at about 22 Ma, where transgressive nearshore and shelf sediments of the Vaqueros Formation rest on fluvial red beds of the Sespe Formation (Crowell, 1987). Initial-rift stage in the Los Angeles Basin occurred at about 20Ma, where pre-eruptive sandstone of the Topanga Canyon Formation were deposited from 20 to 16 Ma, followed by eruptive periods (represented by Conejo volcanism and deep-marine sandstone) of the Middle Topanga Group, from 16 to 13 Ma, and post-cessation eruptive periods (Upper Topanga Group) from 13 to 10Ma. The deep- marine volcaniclastic apron is confined within the Middle Topanga Group (eruption-related sandstones), where it rests on quartzofeldspathic plutoniclastic sandstone (petrofacies 1) of the Lower Topanga Group (pre-eruption sandstones)

~~

Fig. 5 . Photomicrographs of the syneruptive (2) sandstone petrofacies. (A) Palagonitic basaltic glass (Lvv) grains; (B) detail of palagonitic basaltic glass grain; (C) basaltic lathwork grain (Lvl), represented by plagioclase (An6s-,s) in brown glass groundmass; (D) basaltic-andesite microlitic grains (Lvmi), represented by plagioclase phenocrysts in a black glass groundmass; (E) intereruptive sandstone having abundant vitric (Lvv, arrow indicates palagonitic basaltic glass grains) and lathwork (Lvl) grains; (F) syneruptive andesitic sandstone, including felsitic grains (Lvf), represented by abundant plagioclase (Ans,,+,,) in a seriate groundmass; (G) pyroxene crystals (Px) in syneruptive basaltic sandstone; (H) interstitial components include haematite-goetite (h), pore-lining of neoforming clay minerals (cl) and poikilotopic calcite (c, partially replacing neovolcanic plagioclase, neo-P); framework includes quartz (a, palaeo-plagioclase (palaeo-P), lathwork (Lvl), microlitic (Lvmi) and vitric (Lvv) volcanics; vitric volcanic grain is devitrified and altered to clay minerals (cl). Scale bars=0.2 mm; (A,B,C,E,H) plane-polarized light; (D,F,G) crossed nicols.

@ 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

796 S . Critelli and R. V. Ingersoll

0 1995 International Association of Sedirnentologists, Sedimentology, 42, 783-804

Interpretation of neovolcanic vs. palaeovokanic sand grains 797

-

Fig. 7 . Scanning-electron photomicrographs of volcaniclastic sandstone petrofacies. (A) Syneruptive volcanolithic sandstone having basaltic grains and authigenic calcite cement (ca); (B) back-scattered images of a palagonitic basaltic grain; (C) intereruptive volcanolithic petrofacies having both abundant basaltic glass grains (dark grains) and nonvolcanic grains (white grains); (D) authigenic quartz (aQ) and crystobalite (c) filling pore space.

and is unconformably covered by deep-marine to shallow-marine quartzofeldspathic (plutoniclas- tic) sandstone (petrofacies 6) of the upper Topanga Group (cessation eruptions sandstones) (Fig. 9).

Extrusion of submarine to subaerial basalts and andesite lavas formed pure volcanolithic sand- stones (petrofacies 2) deposited during synerup-

tive periods. Impure volcanolithic sandstones (petrofacies 3), having 85% of remobilized neo- volcanic contribution, represent sandstone strata deposited during intereruptive periods. Sand- stone strata of the upper part of the Middle Topanga Group, organized into two distinctive petrofacies (4 and 5), have progressive decreases in neovolcanic (%neo-Rv) Contribution from 48

Fig. 6. Photomicrographs of inter- (petrofacies 3) and post-eruptive (petrofacies 4-6) sandstone petrofacies. (A) General view of immediately post-eruptive glass-rich (brown grains) sandstone; (B) large microlitic and lathwork basaltic grains; (C) typical post-eruptive sandstone texture including oversized basaltic lithic and smaller nonvol- canic grains; (D) felsitic seriate (Lvf) dacite neovolcanic grain represented by plagioclase, minor quartz in a silicic groundmass; (E) large pyroclastic grain, including nonvolcanic material (arrow); (F) pyroclastic perlitic grain with plagioclase crystals and minor quartz in a silicic vitric groundmass; (G) and (H) palaeovolcanic grains within (G) Lower Topanga Group, having lathwork texture (palaeo-Lvl), and (H) Upper Topanga Group (Calabasas Formation) having felsitic texture [palaeo-Lvf). Scale bars= 0.2 mm; (A,B,C,E,F) plane-polarized light; (D,G,H) crossed nicols.

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798 S. Critelli and R. V. Ingersoll

PI F 2

0 5

0.5 LV/L

2 Rv/R

P I F 2

neo-Lv/L

r-7 paleo-Lv/L

2 neo-Lv/L

L

PIF

paleo- Lv/L

1-1 RS+Rrn/ R 1

neo -Rv/ R

1 Lrn+Ls+ paleo-Lv/L

2 neo Lv/L

to 14% (represented by basalt, andesite and dacite lava and pyroclastic grains), representing deposition during immediately post-eruptive periods.

Syneruptive vs. intereruptive vs. post-eruptive deposits

Models for the development of volcaniclastic aprons in continental and marine environments include rapid volcanically induced sedimen-

Fig. 8. Two-component diagrams of representative parameters (P/F, LvIL, Rg/R, RvIR, neo-LvIL, palaeo-Lv/L, Rg + Rm/R, neo-Rv/R, Lm+Ls+palaeoLv/L). Shaded areas indicate evolutionary trend from syn- (z), inter- (3) to post-eruptive (4, 5) petrofacies for the Middle and Upper Topanga Group.

tation (e.g. Fisher, 1984; Cas & Wright, 1987; Cas, 1989; Fisher & Smith, 1991; Smith, 1991). Distinc- tion between syneruptive, intereruptive and post- eruptive periods and related deposits may be useful in understanding changes in geomorphic and sedimentological style and sediment compo- sition. These aspects are well documented in fluvio-deltaic environments ( e g Kuenzi et al., 1979; Vessel & Davies, 1981; Smith, 1987, 1988, 1991), where sedimentary response to volcanic activity is instantaneous; this approach has not

0 1995 International Association of Sedimentologists, Sedimentology, 42, 783-804

Interpretation of neovolcanic vs. palaeovolcanic sand grains 799

Fig. 9. Detrital mode evolution of the Topanga Group (using both phaneritic and aphanitic grains) from pre-eruptive period of the initial rifting (20-16 Ma), eruptive period (16-13 Ma) to cessation of the eruptive events (13-10 Ma). Rg (granitoid), Rm (metamorphic), neo-Rv (neo-volcanic) and palaeo-Rv (palaeo-volcanic) grains.

been very well studied in deep-marine environ- ments. Various styles of volcanism, both subaerial and/or subaqueous, can produce a wide range of pyroclastic, hydroclastic, autoclastic and epiclas- tic materials, which can be transferred to deep- marine basins to form distinctive sandstone strata.

Despite the large body of literature on volcani- cally induced deep-marine sedimentation (e.g. Fiske & Matsuda, 1964; Cas, 1979; Carey & Sigurdsson, 1980, 1984; Sigurdsson ef al., 1980; Wright & Mutti, 1981; Fisher, 1984; Busby-Spera, 1985; Kokelaar et al., 1985; White & Busby-Spera, 1987; Cole & Stanley, 1994), it is difficult to interpret sequential volcaniclastic strata in terms of syn-, inter- and post-eruptive periods. Specific textural and compositional characteristics of syn-, inter- and post-eruptive nonmarine sedimentary strata have been described by Smith (1991). Syneruptive fluvial strata are likely to be com- posed primarily of monolithological sediment, because it is closely connected with instan- taneous eruptions that considerably dilute non- volcanic contributions. Sand-rich facies are the norm. Intereruptive fluvial strata tend to have gravel bedload (Smith, 1991).

In deep-marine environments, syneruptive deposits are monolithic volcaniclastic rocks. The composition characteristics of the volcanic

0 1995 International Association of Sedimentologists, Sedimen

detritus in these strata are largely dependent on eruptive style. Fragmentation of intrabasinal basaltic and andesitic lava flows tend to produce volcanolithic sandstone, including abundant vitric, microlitic and lathwork volcanic lithics; subaerial and shallow-marine pyroclastic erup- tions tend to produce syneruptive deep-marine ash turbidites, vitric-rich and/or crystal-rich (‘crystal tuffs’) volcaniclastic rocks (e.g. Keller et al., 1978; Wright & Mutti, 1981; Cas, 1983; Carey & Sigurdsson, 1984; Fisher, 1984; Cas & Wright, 1987).

If volcaniclastic (neovolcanic) sedimentation in a deep-marine basin represents a well-defined interval in the stratigraphic record where non- volcanic detritus is the norm, then intereruptive deep-marine volcaniclastic strata could be ‘impure’ in composition. During post-eruptive periods, deep-marine volcaniclastic strata show a progressive decrease of neovolcanic detritus with respect to the syn- and intereruptive periods. The post-eruptive volcanic detritus includes a large spectrum of clast types, depending on the sequence of erupted volcanic products.

The volcaniclastic sequence of the Middle Topanga Group records a series of volcanic events and related deep-marine deposition. The detailed petrostratigraphy of Middle Topanga Group sand- stone shows close relationships with extrusion

tology, 42, 783-804

800 S. Critelli and R. V. Ingersoll

of volcanic products of the Conejo Volcanics. Petrofacies 2-5 corresponds to syneruptive, inter- eruptive and post-eruptive periods. Petrofacies 2 has pure basaltic, basaltic-andesitic (subaqueous neovolcanic provenance) and andesitic and dacitic (subaerial neovolcanic provenance) sandstones documenting syneruptive deposition. Petrofacies 3-5 [impure volcanolithic (3) and quartzofelds- pathic (4, 5 ) sandstones] illustrate progressive re- duction in neovolcanic detritus (R%neoRv), from 85, 48 to 14%, respectively. In these sandstones, neovolcanic detritus has a wide range of grain types, including basaltic, andesitic and dacitic lava fragments and pyroclastic fragments.

Neovolcanic vs. palaeovolcanic and intrabasinal vs. extrabasinal grains

The evolutionary record of volcanic chains pre- served in the form of sedimentary rocks has been pivotal in developing palaeotectonic and palaeo- geographical models, particularly along active margins (Dickinson, 1982, 1988). Nonetheless, these rocks have been rarely used to monitor the volcanic record on a detailed scale (e.g. Zuffa, 1987; Marsaglia & Ingersoll, 1992). Zuffa (1985, 1987, 1991) described criteria for subdivision of volcanic grains derived from ancient volcanic rocks (palaeovolcanic, dominantly subaerial) or generated by active volcanism (neovolcanic), either by subaerial or by subaqueous activity during deep-marine sedimentation.

Our conclusions, combining textural criteria with origin and temporal criteria to obtain detailed information about neovolcanic vs. palaeovolcanic, subaerial vs. subaqueous, syn- eruptive vs. intereruptive vs. post-eruptive inter- pretation of the volcaniclastic detritus in Topanga Group sandstones, are illustrated in Figs 8 and 9. 1 Palaeovolcanic detritus represents 14% of the combined phaneritic and aphanitic grain popu- lation (R%palaeo-Rv) in the oldest petrofacies; it disappears or is very rare in syneruptive (Z), intereruptive (3) and immediately post-eruptive (4) sandstone petrofacies (0-1-4%, respec- tively), and returns to the same percentages in post-eruptive sandstone petrofacies (5 and 6; respectively, 14 and 12%). 2 Palaeovolcanic detritus is dominantly repre- sented by rounded, and partially altered felsitic granular and seriate grains, having the same grain size as nonvolcanic detritus. 3 Neovolcanic detritus is dominant in synerup- tive, intereruptive and immediately post-eruptive petrofacies (2-4) ranging from 100 to 48%

(NeoRv%), whereas it is drastically reduced to 14-12% in post-eruptive petrofacies ( 5 , 6). 4 The syneruptive volcanolithic petrofacies (2) in- cludes strata derived from both coeval intra- basinal and coeval extrabasinal volcanism. Intra- basinal volcanolithic sandstones are represented by vitric grains (dominantly quenched glass and palagonitic sideromelane glass, brown, black and orange glass), microlitic grains plagioclase-dominant, hypersthene, augite and rare olivine) and lathwork grains plagi- oclase, hypersthene and augite phenocrysts, and vitric or microlitic groundmass), derived from basaltic and andesitic rocks. Extrabasinal volcano- lithic sandstones are represented by microlitic and lathwork grains derived from andesite and basalt. 5 Post-eruptive quartzofeldspathic petrofacies (4-6) have a wide range of volcanic particles, including dominantly basalt, andesite and dacite fragments.

Provenance of volcanic particles based on their texture and composition A well-focused analysis of volcanic clast popu- lations based on their textural characteristics may be useful in deciphering the contributions of different kinds of volcanic suites during volcani- clastic deposition. Previous workers (Dickinson, 1970; Ingersoll & Cavazza, 1991; Lundberg, 1991; Marsaglia, 1991, 1993; Marsaglia & Ingersoll, 1992) used volcanic grain textures to distinguish different tectonic settings and varied volcanic provenance. Lvv Lvmi Lvl and Lvf Lvmi Lvl diagrams (Marsaglia, 1991, 1993) are sensitive indicators of volcanic provinces and tectonic setting. Andesitic provenance has microlitic-rich sandstone, whereas basaltic provenance has dominantly lathwork lithics. The Lvf Lvmi Lvl triangle for syn- and intereruptive Topanga sand- stones shows an elongate field on the Lvmi Lvl side, whereas post-eruptive sandstones have major felsitic grains (Fig. 4).

Based on the Lvf Lvmi Lvl and Lvv Lvmi Lvl triangles, petrofacies 2 and 3 resemble basaltic sands (e.g. Marsaglia, 1993) more than andesitic or felsitic sands (e.g. Marsaglia, 1991; Marsaglia & Ingersoll, 1992). They plot in the same field as the Hawaiian basaltic beach sand (Marsaglia, 1993). Post-eruptive petrofacies (4, 5 ) have a more complicated trend: petrofacies 4 is lathwork- dominated, suggesting basaltic sources, whereas petrofacies 5 has more felsitic grains suggesting influence of palaeovolcanic sources.

The eruption-related sandstones generally have unique interstitial characteristics, including

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Interpretation of neovolcanic vs. palaeovolcanic sand grains 801

abundant volcaniclastic matrix, authigenic clays (clay rims and clay coats), pore-lining and pore- filling zeolite, silica, clay minerals and chlorite (e.g. Davies et al., 1979: Galloway, 1979; Surdam & Boles, 1979; Mathisen, 1984).

CONCLUSIONS

Deep-marine sandstones of the Topanga Group provide detailed information on the distribution of volcanogenic sediments and record basinal sedimentary response during syn-, inter- and post-eruptive periods. Topanga Group sandstones preserve a complete record from pre-eruption sediments derived from dissection of Mesozoic arc-related terranes to volcaniclastic sandstones interstratified and overlying volcanic eruptives (volcaniclastic apron), and a return to Mesozoic arc provenance after cessation of the volcanic activity (Fig. 9).

Sandstone composition varies systematically in response to changes in palaeovolcanic and neo- volcanic subaerial and subaqueous sources, defin- ing six distinctive petrofacies. The petrofacies distribution is controlled by syndepositional vol- canism (Fig. 9). Pre-eruptive sandstone (Lower Topanga Group) is represented by abundant plu- tonic (%Rg = 68%), metamorphic (%Rm= 18%) and palaeovolcanic (%Rv= 14%) detritus derived from nearby mountain chains (i.e. San Gabriel Mountains).

The eruption-related sands (Fig. 9) were deposited as a deep-marine turbidite complex (volcaniclastic apron) on subaqueous basaltic lava and on the steep flanks of subaerial andesite to dacite volcanic centres.

During syneruptive periods, pure volcanolithic sandstones became interstratified with volcanic deposits. Pure basaltic sandstones, having abun- dant devitrified basaltic glass, overlie intrabasinal basalt and basaltic andesite. Subaerial andesitic lava provided pure syneruptive andesite sands, having more microlitic and lathwork volcanic grains.

During intereruptive periods, impure volcano- lithic sands were closely related to erupted materials, both basalt and andesite, but were mixed with non-neovolcanic detritus. During post-eruptive periods, neovolcanic detritus drastically decreased from 48 to 14% (R%neo-Rv).

The close relationships between erupted materials and characteristics of the neovolcanic grains within the framework of eruption-related

sandstones may be compared with palaeovolcanic grains derived from ancient arc-related terranes. These palaeovolcanic grains are primarily felsitic, rounded, altered and partially recrystallized and have the same grain size as nonvolcanic grains. Neovolcanic grains include abundant microlitic, lathwork and vitric basaltic grains, are angular, fresh and mostly coarser than nonvolcanic detri- tus, and include variable percentages of single euhedral mafic and plagioclase crystals. The eruption-related sandstones generally also have distinctive interstitial materials, including abun- dant volcaniclastic matrix, and pore-lining and pore-filling authigenic clays, zeolite and silica.

In conclusion, this study of the Topanga Group provides an example of the use of sandstone detrital modes in interpreting deep-marine syn-, inter- and post-eruptive deposition. Documenta- tion of the compositional, spatial, temporal and textural characteristics of volcanic grains may be used to constrain palaeogeographical and palaeo- tectonic reconstructions of source/basin systems on a detailed scale.

ACKNOWLEDGMENTS

We are grateful to Cathy Busby, Ben Castellana, Emilia Le Pera, Peter Rumelhart and Guido Ventura for their constructive discussion and comments, and Sedimentology reviewers Ron Cole, Guy Plint and Gian Gaspare Zuffa. We thank F. Falco (SEM/EDAX analysis): E. Barrese (XRD analysis): R. Alkaly (thin sections). S.C. was supported by a C.N.R.-N.A.T.O. Advanced Fellowship at the University of California, Los Angeles. Work supported by funds of the C.N.R., grant to the CNR-Istituto di Ricerca per la Protezione Idrogeologica, Roges di Rende (CS), Italy (Project: Sedimentation, Denudation Processes and Uplift Rates in Neogene to Quaternary Orogenic Belts: Implications for Tectonic Setting and Regional Geology; Resp. S. Critelli),

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