Post on 31-Mar-2023
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
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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.
0 1995 International Association of Sedimentologists, Sedirnenfology, 42, 783-804
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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