ELSEVIER Journal of Volcanology and Geothermal Research 78 (1997) 77-102

Pumice fall deposits of the western Vulsini Volcanoes ( central Italy)

Danilo M. Palladino * , Emanuela Agosta Dipartimento di Scienze della Terra, Uniuersit& di Ronm ‘La Sapienza’, P.le Aldo Moro 5, I-00185 Roma, Italy

Received 18 July 1996; accepted 3 December 1996

Abstract

Widely dispersed pyroclastic fall deposits are prominent among the volcanic products of the western part of the Vulsini Volcanic District (central Italy). In this paper we describe major pumice fall deposits in the area, that correlate to the early activity of the Latera Volcanic Complex (0.3-0.2 Ma). They represent excellent stratigraphic markers for a wide area mostly extending west and south of the present Latera caldera. Pumice fall deposits lower B and upper B are closely associated with major pyroclastic flow units of the Canino Eruptive Unit; pumice fall C caps the Canino products without relevant time hyatus. Fyroclastic falls D and E, the latter part of the minor Stenzano explosive eruption, belong to the stratigraphic interval between Canino and Famese eruptive units. Pumice full F is part of the initial products of the Famese eruptive sequence. We present detailed stratigraphy, depositional features, vertical and lateral variations of grain size, componentry and chemical composition in individual fallout beds. Source areas, as inferred from thickness and maximum pumice and lithic clast size distributions, are located in the area of the present-day Latera caldera. Tentative estimates of volume, column height and magma discharge rate indicate moderate-large to large Plinian eruptive events.

Keywords: Plinian eruptions; pyroclastic fall deposits; potassic volcanism; Vulsini volcanoes

1. Introduction tefiascone volcanic complexes (Vezzoli et al., 1987).

Widely dispersed pyroclastic fall deposits are prominent among the volcanic products of the Vulsini Volcanic District (VVD). The VVD, located in cen- tral Italy, is the northernmost part of the Quaternary potassic Roman Province (Fig. 1; Washington, 1906). The large-scale volcanic activity in the VVD oc- curred between 0.6 and 0.13 Ma and is related to five major volcanic complexes: tbe Paleo-Vulsini, Bolsena-Orvieto, Southern Vulsini, Latera and Mon-

Five major Subplinian to Plinian fall deposits, mostly occurring in the eastern part of the VVD, are de- scribed in detail by Nappi et al. (19941, who relate them to the activity of the Paleobolsena and Bolsena volcanic complexes, broadly corresponding to the Paleo-Vulsini and Bolsena-Orvieto complexes, re- spectively.

Remarkable pumice fall deposits are also found in the western part of the VVD and are the focus of this paper. The main volcanic landform of the western

* Corresponding author. Fax: +39-6-4454729; e-mail: dot-

trig@itcaspur.caspur.it.

VVD is the Latera caldera, whose eastern rim inter- sects the western part of the volcano-tectonic depres- sion partially filied by the Bolsena Lake (Lago di

0377~O273/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved.

PII SO377-0273(96)00107-2

78 D.M. Palladino, E. Agosta / Journal of Volcanology and Geothermal Research 78 (I 997) 77-102

o Aquapendsnts

lschia dl Castro

34 *a

15b *

16 *** 0 Ce”e’e

o Canino 6* 0

_CL edge ofvolcano-tectonic depression r - caldera rim

/ Odkrn

Arlena di Castro

42”25’00” N

Fig. 1. Sketch map of the study area (western half of the Vulsini Volcanic District). Key localities cited in the text are shown (0 ). The limits

of the exposed products of the Latera Volcanic Complex broadly coincide with Fiora River (F. Fiora) to the west and with Marta River (F. Marta) to the east. Numbered A locate the stratigraphic sections in Fig 2; t and 13 indicate sample localities for gram size and component

analyses and for chemical analyses, respectively. The top right inset shows the geographical setting of the Vulsini Volcanic District within

the Quatemary potassic volcanic districts of central Italy.

D.M. Palladino. E. Agosta/Joumal of Volcanology and Geothemd Research 78 (1997) 77-102 19

Bolsena, Fig. 1). The exposed volcanic deposits are mostly related to the activity of the Latera Volcanic Complex (LVC), dating back to 0.3-0.15 Ma (Vez- zoli et al., 1987, and references therein); however, southeast of the Latera caldera, the superposition of the activity of the Southern Vulsini, Latera and Montefiascone volcanic complexes, as well as of the Vito Volcano, resulted in an intricate stratigraphic record (Palladino et al., 1994).

Volumetrically, the eruptive products are domi- nantly pyroclastic deposits: of these, pyroclastic flow deposits comprise by far the largest volume among the exposed terrains and have been studied in detail by several authors in the last two decades (e.g., Sparks, 1975; Metzeltin and Vezzoli, 1983; Vezzoli et al., 1987; Palladino, 1992; Palladino and Valen- tine, 1995). Pyroclastic fall deposits, which are com- mon at various stratigraphic levels throughout the entire area, have been poorly considered by previous studies, with the exception of Sparks (1975), who briefly described some pyroclastic fall deposits of the VVD. Here we describe field aspects, grain size, componentry and chemical composition of major pumice fall deposits, and establish their stratigraphic position. We discuss implications for eruptive history and reconstruction of major explosive events in the area. We briefly mention minor pumice fall deposits occurring in the area.

2. Field description

We recognized seven major pyroclastic fall de- posits and informally named them by alphabetic letters from A to F. Fig. 2 illustrates their position in the stratigraphic framework of the western VVD. Six fall deposits are described in detail (lower B, upper B, C, D, E, F, in stratigraphic order), which are excellent stratigraphic markers throughout a wide area mostly extending south and west of the Latera caldera. These deposits all have typical features indi- cating origin by fallout (i.e., mantling of topography, relatively good sorting, angular shape of pumice clasts and absence of cross-stratification), which will be omitted in the description below. Commonly, these deposits are unconsolidated or mildly indurated where partially altered. The stratigraphic position, dispersal area and principal features of the deposits

are summarized in Table 1. The distributions of their thickness and of maximum lithic and pumice clast size are reported in Fig. 3.

In the following description, MP5 and ML5 are the average length in centimeters of long axis of the five largest pumice and lithic clasts, respectively; for each deposit clast size data were collected at a given height from the base, across a lateral distance of approximately 2 m. Therefore, MP5 and ML5 for each deposit at any exposure refer to a specific stratigraphic height rather than to the whole thick- ness. Average density of lithic clasts (mainly potas- sic lava types and subordinately sedimentary) is 2500 kg/m3; measured pumice density is reported in Table 1.

2.1. Pumice fall A

Pumice fall A overlies a thick paleosol developed on top of phonolitic to phono-tephritic lava flows related to the Southern Vulsini activity (Palladino et al., 1994) and it is capped by a locally dispersed pyroclastic sequence underlying the Canino Eruptive Unit (E.U.) (Canino Formation; Vezzoli et al., 1987). Therefore, fall A and the associated pyroclastic se- quence are the oldest exposed products of the LVC explosive activity [‘basal pyroclastic sequence (24)’ of Palladino et al., 19941. Owing to the scarcity of preserved outcrops the eruptive source can not be established.

2.2. Pumice fall B

Pumice fall B crops out extensively all around the present Latera caldera, with the exception of the northeastern sector of the LVC. In the Cellere- Canino area, the pumice fall sequence being in- terbedded with a pumice flow deposit (Cellere pyro- elastic flow unit of the Canino E.U.; Palladino, 19921, two discrete fall units can be distinguished, named lower fall B and upper fall B. Conversely, where the pyroclastic flow was not emplaced, lower fall B passes upwards to upper fall B, the transition being marked by an abrupt grain size change. In the southeastern distal area (from Arlena di Castro as far as west of Tuscania), an intervening 3- to 20-cm- thick, fine-ash fallout layer, in some places bearing accretionary lapilli, indicates some eruptive break

80

Table 1

D.M. Palladino, E. Agosta / Joumul of Volcanology and Geothermal Research 78 (1997) 77-102

Stratigraphic position, dispersal area and principal features of the western VVD fall deposits

Fall Underlying deposits Overlying deposits Dispersal area unit

F

E

D

d,

cl

C

Upper B

Lower

B

A

Brown paleosol; locally, 5-6 cm, ash-

grained, cross-bedded, surge (Farnese-

Selva de1 Lamone); 35 cm ash beds

(Pitigliano area)

Co-eruptive Arlena di Castro flow unit Western half of LVC. as far as Fiora

of Farnese E.U. River

up to 2.4 m co-eruptive, faintly strati-

fied, ash deposits of Stenzano E.U.,

bearing accretionary lapilli

Below described pyroclastic sequence

on top of fall c,

Up to 80 cm ashy-pumiceous deposits, 35 cm ash grained, reworked deposits

partly altered to paleosol, resting on top containing pumice and lithic swarms,

of fall c underlying fall D

Several-meter-thick, reworked, ashy-

pumiceous deposits, coarse lithicrich

breccia, partially altered ash and pumice

layers bearing accretionary lapilli, on

top of Pianiano flow unit

Pianiano flow unit; locally separated

from fall B either by 20 cm coarse-ash

or by 30 cm reworked, ashy-pumiceous

deposits

Separated from fall D by 60 cm ash

beds; locally by 25 cm surge, 2 m

coarse lithic-rich breccia, with soil on

top, up to 80 cm ash and pumice beds

bearing accretionary lapilli

Up to 3 m ashy and pumiceous deposits,

including fall D, with dark brown pale-

osol on top

Cellere flow unit (Cellere-Canino area),

or lower fall B, or 3-20 cm fine ash

fallout overlying lower fall B (Arlena di

Castro-E of Tuscania)

Dark brown paleosol

Co-eruptive Pianiano flow unit or local

pumice flow units of Canino E.U.

Co-eruptive Cellere flow unit (Cellere-

Canino area), or transition to upper fall

B, or 3-20 cm fine ash fallout underly-

ing upper fall B (Arlena di Castro-E

of Tuscania)

Ash fallout, pumice- and litbic-lapilli

fallout, and ash flow deposits underlying

Canino E.U.

Paleosol on top of phonolitic to phono-

tephritic lava flows of Southern Vulsini

Co-eruptive, sanidine-rich pyroclastic

flow deposits of Stenzano E.U.

Separated from Stenzano E.U. by > 2

m reworked deposits, including a brown

paleosol

SW of Latera Caldera, as far as Canino

and Fiora River; W of Latera Caldera:

Selva de1 Lamone-Pitigliano area

Mainly exposed in the Cellere-Selva

de1 Lamone and limitedly in the Pi-

ansano-Marta River areas

SE of Latera Caldera: Piansano-Marta

River area

SW of Latera Caldera: Cellere-Farnese

area

SE of Latera Caldera, as far as E of

Tuscania; best exposed in Piansano-

Marta area

S and SE of Latera Caldera, as far as E

of Marta River

S sector of the LVC, as far as E of

Marta River; NW sector: Pitigliano-

Sorano

SE of Latera Caldera: Piansano-Marta

River area

between lower and upper fall B. A well-stratified pumice fall sequence, attaining a total thickness of about 7 m, underlies the pumice flows of the Canino E.U. along F.so Olpeta, immediately southwest of Latera caldera. It may be correlated to overall fall B, in which case representing its most proximal expo-

sure. Besides multiple grading, no evident partition between a lower and an upper deposit is observed at this locality.

The intimate association with the interbedded Cellere flow unit and with the overlying pumice flow units of the Canino E.U., as well as the similarity of

L&M. Pailadino, E. Agosta / Journal of Volcanology and Geothemal Research 78 (1997) 77-102 81

Thickness Bedding features and textures Pumice clasts Lithic clasts

Even > 80 cm in the Selva de1 Lamone-Pitigliano area; a few centimeters near So- rano and S of Famese

Maximum 30 cm S of Far- nese

Maximum 1.5 m just S of Selva de1 Lamone; up to 70 cm in the Piansano-Marta River area

Maximum 95 cm E of Pi- ansano; decreases towards SE

Maximum 1.15 m near Far- nese; rapidly decreases to- wards SW

Up to 1.9 m near Marta River; ambiguous distribu- tion

Up to 1.4 m in the Cellere area; decreases towards SE; anomalous thickness (1.15 m) way S of Arlena di Castro Up to 2 m near Cellere; 7 cm in the vicinity of Sorano; overall fall B * 7 m imme- diately SW of Latera Caldera

Maximum 40 cm at La Roc- chetta (E of Piansano)

Alternating pumice beds (up to 5) and ash beds; a single bed with multiple grading north- wards; overall grading normal to inverse moving in a clock-wise direction Rich in sanidine free crystals; pumice clast grading inverse (Famese-Ischia di Castro area> to normal (SW of Farnese) to inverse-normal (Canino area) Rhytmic grain size variations im- parting stratified appearance; up to 20 cm coarse ash layer in the middle of the deposit; inverse grading in both halves Includes up to six beds separated by thin ash layers

Normal grading of lithic clasts

Lower-middle part: normal to inverse grading to ungraded, from Marta to Arlena di Castro; upper part: inverse grading; lo- cally 2-3 cm ash layers upwards Massive appearance; ungraded to subtle inverse-normal to inverse grading, moving from W to E

Repeated grain size variations imparting well stratified appear- ance; overall inverse grading; lo- cally up to 50 cm ash fallout interbedded upwards Inverse-normal grading of pumice clasts in the middle-up-

per part

Light grey, bearing altered leucite; usually centimeter-sized; density of centimeter-sized clasts 700-900 kg/m3

White to light grey, rich in sani- dine; millimeter- to centimeter- sized; density of centimeter-sized clasts 5 1000 kg/m3

Coexisting light grey, vesicular and dark grey, poorly vesicular; millimeter- to centimeter-sized

Light grey; a few centimeters in size (MP5 = 4)

Light grey and dark grey; mil- limeter- to centimeter-sized

White to light grey; usually cen- Lava and subordinate sedimen- timeter-sized, sometimes > 1 tary (sandstones, marly lime- dm; density of centimeter-sized stones, clays); _ 10% by vol.; clasts 600-800 kg/m3 centimeter-sized, rarely 1 dm

White to light grey; usually cen- timeter-sized, sometimes > 1 dm; density of centimeter-sized clasts 600-800 kg/m3 White to light grey, millimeter- to a few centimeter-sized; den- sity of centimeter-sized clasts 6OC-800 kg/m3

Mostly lava; m 2% by vol. up- wards; m illim e ter- to centimeter-sized

Mostly lava; - 1% by vol. near the top; millimeter-sized

Light yellow; MP5 in the middle part varies from 6 cm (La Roc- chetta) to 4 (Piansano)

Mainly lava; w 5% by vol. near the top; millimeter- to centime- ter-sized (ML5 = 4)

Lava and sedimentary (sand- stones); 5-10% by vol.; millime- ter- to centimeter-sized

Mostly lava; < 5% by vol.; rarely > 2 cm

Erey lava (up to 45% by vol. in the Piansano-Marta River area), and sedimentary (2%); millime- ter- to centimeter-sized

Grey lava; up to 50% by vol.; millimeter- to centimeter-sized (ML5 = 4); especially enriched in the lowermost bed Dark grey lava; millimeter- to centimeter-sized

juvenile clasts, indicates that fall B is co-eruptive with those flow units and thus it is part of the Canino E.U. Pyroclastic falls 1 and 2 of Sparks (1975) likely

correspond to pumice falls lower and upper B, re- spectively .

Lower fall B represents the initial products of the

Canino E.U. The well-stratified appearance and the scarcity of lithic clasts make it clearly distinguish- able among the pyroclastic fall deposits in the LVC.

Average density of centimeter-sized pumice clasts varies from 600 to 800 kg/m3 with decreasing clast size. Although irregular data distribution does not

82 D.M. Palladino, E. Agosta/ Journal of Volcanology and Geothemml Research 78 (1997) 77-102

D.M. Palladino, E. Agosta/ Joumul of Volcanology and Geothermal Research 78 (1997) 77-102 83

allow us to draw detailed isopach and isopleth maps, areal distribution, thickness variation and pumice and lithic clast size (Fig. 3a) indicate the vent position within the present southwestern sector of the Latera caldera and the dispersal axis broadly directed south- wards.

Upper fall B is clearly distinguishable from lower B, its coarser, uniform grain size imparting a mas- sive appearance. The range of measured densities of centimeter-sized pumice clasts is the same as lower B. Areal distribution, thickness variation and pumice and lithic clast size (Fig. 3a) suggest the same vent location as for lower B, while the dispersal axis is broadly directed southeast of the present Latera caldera. Moving from west to east across the disper- sal area, subtle clast grading patterns vary from ungraded to inverse-normal to overall inverse, sug- gesting counterclockwise rotation of the dispersal axis during the late stage of the eruptive phase.

2.3. Pumice fall C

Southeast of Latera caldera, pumice fall C directly overlies the most widespread pumice flow deposit of the Canino E.U. (Pianiano pyroclastic flow unit; Palladino and Valentine, 1995), apparently without significant time break. However, in the Marta- Capodimonte area, where the Pianiano pyroclastic flow unit is lacking, fall C is separated from underly- ing fall B by locally reworked, ashy-pumiceous de- posits indicating an eruptive break.

Available thickness distribution is unrelated to a plausible vent area, the maximum thickness occur- ring near the Marta River (F. Marta, Fig. 3b). Unfor- tunately, the stratigraphic level corresponding to fall C is not exposed further to the east. Note that the exposed thicknesses seem unaffected by erosional process, the top surface being usually flat and capped by ashy deposits. On the other hand, maximum lithic and pumice clast size, measured towards the top of the deposit, regularly decreases from Latera caldera towards the southeast (Fig. 3b). Therefore, in spite of ambiguous thickness distribution, clast size likely

indicates the location of the source area in the south- eastern part of the present Latera caldera.

Lithic clast size and abundance slightly increase upwards. Pumice clasts, besides the inverse grading in the uppermost part of the deposit, impart different grading patterns in different areas: moving from Marta to Arlena di Castro, an overall variation from normally graded to inverse to ungraded is observed in the lower-middle part of the deposit, suggesting clockwise rotation of the dispersal axis during the early and middle stages of the eruptive phase.

2.4. Pumice fall c,

This pumice fall bed occurs southwest of Latera caldera and has been distinguished from fall C by the presence of darker-coloured pumices and by its en- richment in dark grey, normally graded lava lithic clasts. Stratigraphic contacts with fall C were not found. However, fall c, is separated from the prod- ucts of the Canino eruption by a several-meter-thick, partly reworked sequence indicating significant time interval.

Thickness data are included in Fig. 3b. MP5 and ML5 both reach a maximum of 6 cm and decrease towards the southwest (Fig. 3b), consistently with thickness.

2.5. Pyroclastic fall d,

We briefly mention a stratified fallout sequence exposed in the Piansano-Marta River area including several lithic-rich pumice beds, separated by thin ash layers. A few decimeter-thick, ash-grained, reworked deposits, containing pumice and lithic swarms, sepa- rate fall d, from an overlying lithic-rich fallout bed with massive appearance, likely correlating to fall D (see below).

2.6. Pyroclastic fall D

Pyroclastic fall D is characterized by coexisting light grey, vesicular pumice fragments and subordi-

Fig. 2. Selected stratigraphic sections showing the relationships among pumice fall deposits and the other early volcanic deposits of the

Latera Volcanic Complex. See Fig. 1 for location of the sections. 37 = S.Rocco; 52 = F.so la Nova; 46 = Rovine di Castro; 31 = P.te di

Stenzano; 60 = La Chiusetta; 16 = Cellere; 2 = La Rocchetta; 10 = Pian di Vito. cp = phonolitic lava flow.

84 D.M. Pa&&no, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-302

u tschia di Castro

’ >50; 3k1

oJ3ansano 43, 12; 2/-q 3/2

115, 40; 3/1,4/2; b I

l 130; 3/<1 40, 2J WI, w *NO; l/Cl

l i

55, 130; 451,8/5 115; 312 p; 45125, &Q 4/3,513.

I3(4/3 &?5, 2,412, fj&

Arlena di Castro

; ;:; proposed vent location i40, 70: S/3 40, 25; 2/<1,X .

-CL edge of volcano-tectonic depression l 100, 4/2 /

L-L caldera rim

OLkrn

l >60. 6/3 30, 50; 2/<1,4

so, LLj; 5/2,8/3 isi a Tuscania 1

Fig. 3. Maps of the thickness and of maximum lithic and pumice clast size distribution of fall B (a), C and cI (bb), D cc>, E (d) and F (e).

For each measured locality, the fist number (in italics) refers to the thickness (in cm) and it is followed by MP5/MLS data (in cm). In (a)

data on lower fall B are followed by data on upper fall B (underlined). The thickness datum between brackets along F.so Olpeta refers to

overall fall B. In (b) * refer to fall C and * to fall c,. In (e) clast size data Points for fall F refer to the lower bed south of Selva de1

Lamone and to the upper part of the deposit north of it. Methods for measurements are discussed in the text.

D.M. Pall&no, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-102 85

’ lschia di Castro

8; 312 * * 15; 3/3 l 75; 616 120; IL?6

OePiansano

IS; 213 o Cellere ‘50; 915 145;

130;?/6 ‘130; 7~4

l -110 lCv5 115 190; 614

o Canino l 25; 212 .40

ci

0 Arlena di Castro . 70

proposed vent location

.35 , 50; 412

Tuscan/a

45; 31; 70; 411 .

lschia di Castro

40; 3/3 .

O,Piansano

45; 412 .30; 312

20; 313 . .40; 312 *IlO; 513

25; 413 o Cellere .

: Gy proposed vent location

-L_I calderarim

O_ Jkm o Canino 0

Arlena di Castro

Fig. 3 (continued).

86 D.M. Palladino, E. Agosta/Joumal of Volcanology and Geothermal Research 78 (1997) 77-102

I Fall E

4; 211 .

l >8

s@ tip

22; 612 o

(5; 311 r/L/ ’ l 25; 612 ’ lschia di Castro 10; 214 l

l 22; 512 .29; 412

l 19; 513

l 25; 312 ,30; 412

‘28; 612 iI 4y2 Cellere l 10

2;?; 4/l .I7; 3k1

. 20 Id; 3/c], o Camno

014; 314 l IO

,” T ; proposed vent location $Ei Fig. 3 (continued).

nate dark grey, poorly vesicular fragments, and by a coarse-ash layer occurring in the middle of the de- posit. In the area from Piansano to Marta River a single bed is recognized, which is notably enriched in grey lava lithic clasts (up to 45% by volume). Thickness and maximum clast size data sets (Fig. 3c) suggest a possible vent location in the southwestern sector of the present Latera caldera.

2.7. Pumice fall E

Fall E belongs to the Stenzano E.U., that is related to a minor explosive eruption of the early activity of the LVC, predating the Famese eruption (Farnese Formation; Vezzoli et al., 1987). Its distin- guishing feature is given by the high content in free sanidine crystals (even > 25% by weight in the 2-4

D.M. Palladino, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-102

Fall F

/ (2 Pitigliano

5; 714

.

.88; 613

60; 5/s Y”““““““‘ ‘“““““V

.

ti

Latera

. 85; 514 Caldera

l 18; 312

l 20; 313

’ lschia di Castro

II; 312

5; 2/l o Cellere

; ; ; proposed vent location

(e)

Fig. 3 (continued).

mm matrix fraction, Fig. 5). Centimeter-sized pumice clasts approach a density of 1000 kg/m3 or slightly more with increasing degree of porphyricity.

Thickness reaches a maximum of 30 cm south of Famese, setting a NNE-SSW-trending dispersal axis (Fig. 3d). Maximum Iithic and pumice clast measure- ments (Fig. 3d) are consistent with a vent position within the southwestern area of the present caldera.

Subtle pumice clast grading varies from inverse (Famese-Ischia di Castro area), to normal (south- west of Famese) to inverse-normal (Canino area).

2.8. Pumice fall F

Pumice fall F represents the initial products of the Famese E.U. (Famese Formation; Vezzoli et al., 1987) in the western part of the LVC, where it

88 D.M. Palladino, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-102

(a) Lower fall B (W Lower fall B Cellere (16) - 200 cm thick Pian di Vito (10) - 80 cm thick

50 50 height from base: height from base:

16 8 4 2 lmm 500 250 125 62!.um 16 8 4 2 lmm 500 250 125 62~1x1

+ grain size - U- grain size -

03 Upper fall B WI Upper fall B

F.so di Marano (34) - 90 cm thick

height from base:

Pian di Vito (10) - 115 cm thick

height from base:

16 8 4 2 lmm 500 250 125 62pm

+ grain size -

W Fall C

La Rocchetta (2) - 145 cm thick

16 8 lmm 500 250 125 62~1x1

Q- grain size -

16 8 4 2 lmm 500 250 125 62pm

U- grain size -

(4 Fall C C. Parri (8) - 190 cm thick

50 height from base:

O-38 cm 38-76 cm 76-l 14 cm 114-152 cm 152-190 cm

16 8 4 2 lmm 500 250 125 62pm

O- grain size -

Fig. 4. Grain size variation with stratigraphic height (cm from the base) in individual fall deposits. For fall E samples are representative of

the whole thickness. Sample localities arc reported in Fig. 1.

(9) Fall D P.te di Stenzano (31) - 115 cm thick

4n height from base:

16 8 4 2 lmm 500 250 125 62pm

4- grain size-

0) Fall F (i) Fall F P.gio Bottinello (53) - 85 cm thick Rovine di Castro (46) - 40 cm thick

5n , 50 height from base: height from base:

16 8 4 2 lm 500 250 125 62pm 16 8 4 2 lmm 500 250 125 62~

-3- grain size - d-grain size-

I u-0 Fall E

16 8 4 2 l= 500 250 125 62pm

Q- grain size -

D.M. Palladino, E. Agosta / Journal of Volcanology and Geothemml Research 78 (1997) 77-102 89

Fig. 4 (continued).

underlies the main pumice flow unit of that eruption (Arlena di Castro flow unit; Palladino and Valentine, 19951. Its distinctive feature is the occurrence of leucite, altered to analcite, both in pumice clasts and as free crystals. The upper surface of the deposit locally (i.e., south of Pitigliano) appears truncated and injected by pyroclastic material from the overly- ing pumice flow unit.

Southwest of Latera caldera fall F is made up of alternating pumice beds (up to 4-5) and minor ash beds. This partition gradually disappears northwards,

where a single bed occurs, even though subtle multi- ple grading still imparts a stratified appearance. Be- sides inverse to inverse-normal grading within indi- vidual beds, overall pumice and lithic clast grading

through fall F gradually changes from normal to inverse moving in a clock-wise direction, that is, coarsest clasts occur in the lowermost, thicker bed in the area south of Selva de1 Lamone, whereas they occur in the middle bed in the area just north of Selva de1 Lamone, and then in the upper part of the deposit further to the north (Pitigliano area). Density of centimeter-sized pumice clasts ranges from 700 to 900 kg/m3 with decreasing size.

Thickness variation is broadly symmetric relative to a E-W-trending dispersal axis (Fig. 3e). MP5 and ML5 data distributions (Fig. 3e) poorly correspond to thickness distribution: e.g., pumice and lithic clasts are notably coarse even in the thin deposit of the Sorano area. Both sets of data, however, are broadly

90 D.M. Palladino, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-102

consistent with a vent area located in the northwest- em part of the present Latera caldera.

2.9. Minor pumice fall deposits

After the Farnese eruption, small-scale pumice fall deposits occur locally in the stratigraphic record of the LVC and are briefly mentioned in this section.

A millimeter-sized pumice fall deposit, bearing altered leucite, marks the base of the Sovana E.U. (Sovana Formation; Vezzoli et al., 1987) in the western and northern sectors of the LVC, between Selva de1 Lamone and Acquapendente. Its maximum thickness reaches only 5 cm.

Remnants of another pumice fall deposit, with millimeter-sized pumice clasts, occur locally at the base of the main pyroclastic flow deposit of the Sovana E.U. (Piansano flow unit; Palladino and Valentine, 19951, e.g., between Acquapendente and Castell’Ottieri (thickness of 5 cm) and between Far- nese and P.te S. Pietro (thickness up to 10 cm>.

Furthermore, the upper Sovana eruptive sequence includes an up to 70-cm-thick pyroclastic fall de- posit, exposed just south of Selva de1 Lamone, which is characterized by centimeter-sized, altered-leucite- bearing grey pumice and black scoria clasts, and abundant lithic lapilli (tens of volume percents), enclosed in a matrix rich in sanidine and altered leucite crystals. This passes upwards to a 35-cm-thick ash fallout deposit and then to paleosol.

In the succeeding Sorano E.U. (Sorano Forma- tion; Vezzoli et al., 1987) a millimeter-sized-pumice fall deposit, up to 10 cm thick, overlies a basal, lo-cm-thick ash bed, between Piansano and Bolsena Lake. In the same area, the younger Grotte di Cas- tro-Onano eruptive sequence (Palladino et al., 1994) begins with a lo-cm-thick, millimeter- to centimeter-sized pumice fall bed, followed by green- ish, accretionary-lapilli-bearing ash bed sequence.

Finally, the several-meter-thick pumice fall se- quence, exposed in a relatively small area around the locality of Case Collins (between Pitigliano and Latera caldera rim), belongs to the post-caldera vol- canics of the Pitigliano Formation (Vezzoli et al., 1987). Due to its limited areal extent compared to thickness, this pumice fall sequence is likely related with a former pumice cone structure.

3. Grain size and componentry

3.1. Field and laboratory approach

Grain size analyses have been performed on bulk samples taken at regular intervals through the whole thickness of pumice fall deposits lower B (two sites, ten samples), upper B (two sites, ten samples), C (two sites, ten samples), D (one site, five samples) and F (two sites, six samples). For fall E (four sites, four samples), one sample per site is sufficiently representative of the deposit for its limited thickness and relatively uniform grain size.

Sieve analyses at l-phi intervals have been per- formed on the size fraction finer than 16 mm. The fraction coarser than 16 mm is not considered statis- tically significant because of the small amount of clasts, and therefore all the reported analyses refer to the fraction finer than 16 mm normalized to 100. This represents almost the whole grain size distribu- tion (with the exception of upper fall B). On the other hand, the weight percentage of the fraction coarser than 16 mm has been included in determin- ing median grain size and sorting parameters after Inman (1952). Further analysis of the population finer than 62 pm, that usually represents less than 10% by weight of the bulk sample, is not reported, since the original content and grain size distribution of the fine ash fraction are significantly affected by the sieving process itself.

Component analyses were performed on the 2-4 mm and the 250-500 pm grain size fractions. The weight percent of pumice and lithic fragments, of sanidine, altered leucite (analcite) and mafic (clino- pyroxene + dark mica + opaques) free crystals has been determined after hand-picking in the 2-4 mm fraction. Volume percent of the above components has been obtained by grain counting (1000 grains) in the 250-500 pm fraction.

Most significant grain size and component fea- tures are summarized below. Sample localities are in Fig. 1. Vertical variations in grain size distribution and componentry in individual fall beds are reported in Figs. 4 and 5, respectively. Table 2 reports median grain size (Md,) and sorting ( u,,> parameters.

3.2. Fallout deposit characteristics

The studied fallout deposits are quite distinctive as to their overall grain size characteristics (sum-

D.M. Palladino, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-102

Lower fall B Celkre (16) - ‘200 cm thick

2-4 mm

Lower fall B Cellme (16) - 200 cm thick

250-500 um

i

hei& from base Lower fall B

F’ian di Vito (10) - 80 cm thick 2-4 mm

WI

hei& from base Lower fall B

Pirm di Vito (10) - 80 cm thick 250-500 w

height from base Upper fall B

F.so di Marano (34) - 90 cm thick 2-4 mm

height from base Upper fall B

Fso di Marano (34) - 90 cm thick 250-500 w

height from base Upper fall B

F’iandiVico(lO)-115cmthick n 2-4 mm

height from base

Upper fall B Pian di Vito (10) - 115 cm thick

250-500 um

91

height from base. height from base

92 D.M. Palladino, E. Agosta / Journal of Volcanology and Geothermal Research 78 (1997) 77-102

Fall C La Rocchetta (2) - 145 cm thick

2-4 mm

height from base Fall C

C. Pani (8) - 190 cm thick 2-4 mm

O-38 cm height from base

Fall E 2-4 mm

34 1% 46

locality

Fall F P.gio Boainello (53) - 85 cm thick

2-4 mm

height from base

(j) Fall C La Rocchetta (2) - 145 cm thick

250-500 um

height from base Fall C

C. Pani (8) - 190 cm thick 250-500 l.tm

O-38 cm 38.76~11 76-114em 114-152em 1%19Ocm height from base

Fall E 250-500 WI

34 15b 46 locality

Fall F P.gio Bottinello (53) _ 85 cm thick

250-500 w

height from base

D.M. Palladino, E. Agosta/ Joumul of Volcanology and Geothermal Research 78 (1997) 77-102 93

(4) Fall F Ravine di Castro (46) - 40 cm thick

height from base

Fall F Ravine di Castro (46) - 40 cm thick

height from base

Fig. 5. Component variation with stratigraphic height (cm from the base) in individual fall deposits. For fall E samples are representative of

the whole thickness. Two grain size fractions are reported for each sample: the 2-4 mm fraction (left side; wt.%) and the 250-500 pm

fraction (right side; vol.%). Sample localities are reported in Fig. 1.

marized by Md, and o@ parameters in Table 2) and their internal variations.

The grain size distribution and Md, show that lower fall B is normally graded in the relatively proximal site, Cellere, and reversely graded in the distal site, Pian di Vito. However, at the latter locality, despite overall reverse grading, the abun- dance of finer fractions increases upwards: the top- most sample, indeed, has an additional mode in the tine ash fraction. In the proximal site the grain size approaches a Rosin distribution in the lower part of the deposit, while in the distal site it changes up- wards from approximately Gaussian to Rosin-type (Fig. 4). Overall, sorting improves from proximal to distal locations, as is typical in fallout deposits (e.g., Fisher, 1964; Walker and Croasdale, 1971).

Upper fall B is characterized by relatively uni- form grainsize. An odd feature, however, is the increasing median grain size (Md,) from relatively proximal (F.so di Marano) to distal location (Pian di Vito), in the lower and middle parts of the deposit.

In the 2-4 mm size fraction, lithic and sanidine contents increase vertically through fall B at the distal locality of Pian di Vito (Fig. 5). Also note that in this fraction the lithic content of lower fall B decreases from proximal to distal location, while the lithic content of upper fall B increases. Moreover, the lithic fragments in lower fall B tend to concen- trate distally in the 250-500 pm fraction. In this size fraction an upward increase of sanidine content

is also evident in lower fall B at the proximal site of Cellere.

Fall C is characterized by rhythmic vertical grain size variations. Md, values (Table 2) oscillate simi- larly in the two sampled localities and indicate that at any stratigraphic height fall C at La Rocchetta is coarser than at Casale Parri. This is an important point in the question of the vent area for this deposit, suggesting transport toward the east-southeast.

Vertical trends in free crystal content also parallel each other in the two localities, in both size frac- tions, providing an excellent means of deposit corre- lation. Overall, the amount of sanidine and mafic crystals significantly decreases from La Rocchetta to Casale Parr-i, supporting the inferred ESE direction of transport.

The grain size distribution of fall D at P.te di Stenzano locality is typically unimodal; overall in- verse grading of the deposit is quite evident.

Component analysis of fall D (not reported) proved to be impractical, the distinction between dark grey, poorly vesicular, juvenile fragments and accidental lava fragments being uncertain (unless crushing grains). As a whole, dark grey fragments reach up to 30-40 wt.% in the 2-4 mm size fraction. Sanidine content is 0.2-0.9 wt.% and 6-16 vol.% in the 2-4 mm and 250-500 pm fractions, respec- tively. Mafic free crystal content increases upwards from 1 to 6 vol.% in the 250-500 pm fraction.

Fall E becomes regularly finer grained and better

94 D.M. Pall&no, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-102

Table 2

Variation of median grain size (Md& and sorting (u@,) parameters (after Inman, 1952) with stratigraphic height (cm from the base) in

individual fall deposits. For fall E samples are representative of the whole thickness

Fall F

P.gio Bottinello (53) Rovine di Castro (46)

Height O-20 20-40 40-50 a 50-85 O-20 33-40

%J 1.80 1.80 3.45 2.30 1.70 1.75

M&i - 1.4 -1.0 -0.1 - 1.7 - 1.5 - 1.8

FaUE

(31) (34) (15b) (46)

% 1.90 1.90 1.50 1.30

Md, -0.7 -0.3 -0.2 -0.2

Fall D

P.te di Stenzano (3 1)

Height O-23 23-46 46-69 69-92 92-115

% 1.50 1.65 1.50 1.90 1.80

Md, -0.1 -1.2 - 1.6 - 1.7 - 1.5

FallC

La Rocchetta (2) C. Parri (8)

Height O-29 29-58 58-87 87-116 116-145 O-38 38-76 76-114 114-152 152-190

% 1.20 1.25 1.40 1.55 1.50 1.75 1.80 1.40 1.60 1.55

Md, -2.8 -3.0 -2.6 -2.3 -2.7 - 2.4 -2.5 -2.3 -2.0 -2.4

Upper fall B

F.so di Marano (34) Pian di Vito (10)

Height O-18 18-36 36-54 54-72 72-90 O-23 23-46 46-69 69-92 92-115

% 2.20 2.00 2.50 2.55 2.65 1.95 2.70 2.75 2.15 1.65

Mdm -2.7 - 2.5 -2.3 - 2.6 -2.7 -3.3 -3.7 - 3.2 - 2.2 -2.7

Lower fall B

Cellere (16) Pian di Vito (10)

Height O-40 40-80 80-120 120-160 180-200 b O-16 16-32 32-48 48-64 64-80

% 1.75 1.65 3.00 3.40 2.60 0.85 1.10 1.40 1.35 3.70

Md, -1.1 -1.0 -0.6 + 0.9 +2.7 -0.3 - 1.0 -0.8 - 1.6 - 1.7

a Interbedded coarse ash bed.

b Ash fallout bed on top of the pumiceous sequence.

sorted with increasing distance from the inferred

vent area. It is well distinctive because of its crystal- rich nature. Component distribution is quite homoge- neous in the 2-4 mm size fraction at different loca- tions (besides a slight decrease in lithic and sanidine contents at the distal one>, while free crystals tend to concentrate in the 250-500 pm fraction from proxi- mal to distal localities.

The grain size distribution of fall F at a relatively proximal locality (P.gio Bottinello) approaches a

Rosin type in the lower part, and becomes polimodal

upwards, with increasing proportion of fines. Note that the sample from the 40-50 cm height interval represents an interbedded coarse ash bed. Rosin-type distributions are also typical of the relatively distal site of Rovine di Castro, where the deposit is slightly better sorted.

The proportion of lithic and crystal fragments in the 2-4 mm size fraction increases upwards at the more proximal locality, whereas it decreases up-

D.M. Palkdino, E. Agosta/.Journd of Volcanology and Geothermal Research 78 (1597) 77-102 95

Table 3 Major-element pumice composition by xRF analyses of lower fall B (LB6, Tesscmo; LB 10, F’ian di Vice; LB 16, Gem), upper fall B (IJB~O, pian di vice; UB16, Cellere; UB34, F.so di Marano~, fall C (C2, La Rocchetta), fall D (D3td, P.te di Stcnza~ dark, poorly vesicular juvenile clasts; D311, P.te di Stenzano, light pumice clasts), fall E (E31, P.te di Stenzano; E46, Rovine di Castro), fall F (F46,

Rovine di Castro; F52, F.so La Nova; F53, Pgio Bottinello)

LB6 LB10 LB16 UBlO UB16 uB34 c2 D31d D311 E31 E46 F46 F52 F53

SiO,

TiO 2

A1203

Fe203,

MnO

MgO CaO Na,O

K2O

p20,

LO1

58.34 59.90 58.74 58.09 57.85 60.03 58.56 58.44 57.63 61.41 59.87 55.84 53.74 54.10

0.31 0.30 0.34 0.32 0.34 0.31 0.34 0.55 0.49 0.50 0.49 0.46 0.48 0.48

19.70 19.00 19.90 20.09 21.34 19.11 19.69 17.07 16.73 18.54 17.04 16.37 18.86 18.99

2.59 2.52 2.69 2.95 2.99 2.73 3.28 4.13 3.61 3.10 3.31 5.46 5.01 4.42

0.15 0.17 0.14 0.15 0.14 0.15 0.13 0.14 0.16 0.11 0.13 0.13 0.10 0.11

0.45 0.32 0.46 0.37 0.47 0.34 0.64 1.00 0.83 0.60 0.77 0.95 1.10 1.11

1.92 1.89 1.79 1.81 1.69 1.99 2.27 3.86 3.42 2.41 2.86 4.59 4.95 4.97

3.90 3.87 3.09 3.07 3.26 4.04 3.31 2.99 2.31 2.43 2.80 2.92 2.91 2.93

7.34 7.96 6.48 7.86 6.84 8.19 8.69 9.96 10.05 8.09 9.33 8.25 7.66 7.72

0.18 0.10 0.18 0.22 0.11 0.15 0.26 0.19 0.12 0.08 0.12 0.14 0.32 0.32

5.13 4.93 6.13 5.71 4.97 2.94 3.74 1.67 4.64 3.68 3.28 4.90 4.81 4.81

Total 100.01 100.96 99.94 100.64 100.00 99.98 100.91 100.00 99.99 100.95 100.00 100.01 99.95 99.94

Sample label numbers refer to sample localities indicated in Fig. 1. t = FeO,, reported as Fe,O,.

wards at the more distal one. It is worth noting the wide occurrence of leucite, always altered to anal- cite, in the 250-500 pm fraction.

4. Compositional features

Typical mineralogical assemblages of juvenile pumice clasts, as well as of free crystal fragments, are given by ubiquitous sanidine, by far the most abundant phase, and clinopyroxene, frequently asso- ciated with scarce dark mica and opaques. Leucite, generally altered to analcite, occurs only in fall F, for the first time in the LVC pyroclastic succession. Pumice clasts are usually scarcely porphyritic (e.g., millimeter-sized crystals attain < 2 vol.%), with the notable exception of fall E (even > 10 vol.%). Total crystal content determined in artificially crushed pumices is - 10 wt.% in fall B and _ 20 wt.% in fall F.

Major- and selected trace-element contents were obtained from composite samples (at least five pumice clasts) collected at a single stratigraphic height. Sample localities are indicated in Fig. 1. Data for falls lower B, upper B, C, E and F are reported in Tables 3 and 4. For fall D, only the major-element composition is available (Table 3). Major-element compositions were obtained by XRF analyses; trace

elements were determined by Atomic Absorption Spectrometry (AAS).

Pumices are mostly trachytic in the TAS diagram (Fig. 6; Le Bas et al., 1986); however, fall F shifts towards the latitic field, fall D towards the phonolitic one. Such compositions broadly match those of co- eruptive pyroclastic flow units (e.g., Conticelli et al., 1987; Landi, 1987; Palladino et al., 1994, and unpub lished analyses). A temporal decrease in silica con- tent significantly occurs from lower fall B to fall F, with the exception of fall E. Note that the composi- tion of light, vesicular fragments in fall D matches that of dark, poorly vesicular fragments (but for LOI).

Trace-element composition has been determined on 61 samples, collected at regular intervals through the stratigraphic height of individual fall deposits (lower fall B - 30 samples, three sites; upper fall B - fifteen samples, three sites; fall C - ten sam- ples, one site; fall E - one sample; fall F - five samples, one site). Trace-element contents vary slightly within individual fall deposits, revealing no systematic trends (Fig. 7); some lateral variation is only shown by Rb content in lower fall B. Composi- tional ranges support correlation of fallout deposits. On the other hand, significant variations exist among different fall deposits, which provide an additional tool for their characterization. In particular, Sr, Ba

96 D.M. Palladino, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-102

Table 4

Selected trace-element composition by Atomic Absorption Spec-

trometry (AAS) analyses of pumice clasts (ppm) and correspond- ing stratigraphic height interval in individual fall deposits (cm from the base)

Height km) Ni Co Cu Cr Zn Pb Sr Rb Ba

Lower fall B: Tessennano 6

o-13 19 25 8 8 33 104 82 776 114

13-26 20 23 10 10 29 105 85 716 170

26-39 16 22 8 12 36 103 79 811 41

39-52 16 22 6 7 34 120 79 779 65

52-65 16 25 6 12 37 98 79 563 64

65-78 19 27 5 4 64 111 n.d. n.d. 97

78-91 22 33 8 10 33 102 88 597 114

91-104 24 30 8 10 32 99 93 795 82

104-l 17 22 25 10 10 37 138 97 700 81

117-130 16 29 10 10 31 353 100 764 147

Lower fall B: Pian di Vito 10 O-8 25 75 6 6 78 413 166 937 109

8-16 18 45 11 7 84 127 269 907 107

16-24 26 39 6 6 74 154 172 972 70

24-32 29 44 4 6 77 139 163 955 71

32-40 28 58 5 5 88 149 164 957 49

40-48 26 62 5 3 64 124 157 766 92

48-56 22 67 5 5 66 131 146 926 48

56-64 28 68 7 5 62 118 217 862 136

64-72 32 55 4 7 77 140 175 955 81

72-80 30 52 6 3 73 119 166 891 99

Lower fall B: Cellere I6 O-20 24 47 8 4 53 98 169 899 111

20-40 51 62 7 7 82 93 157 898 84

40-60 18 76 6 10 77 115 178 874 111

60-80 24 64 8 8 58 179 203 860 121

80-100 36 47 6 6 67 148 203 869 138

100-120 39 74 7 8 98 165 177 1184 82

120-140 18 93 4 2 69 118 245 1137 65

140-160 22 61 7 6 79 147 205 1013 116

160-180 27 69 6 3 75 138 165 943 70

1 SO-200 33 71 4 4 84 141 154 1023 65

Upperfali B: Pian di Vito IO O-23 24 98 5 3 66 138 195 1023 51

23-46 28 41 5 3 72 147 149 922 60

46-69 27 73 3 3 60 115 216 857 107

69-92 32 79 7 5 59 98 161 781 41

92- 115 39 48 6 4 84 189 169 994 63

Upperfall B: Cellere 16 O-28 27 63 7 6 95 127 159 928 52

28-56 31 77 11 7 78 121 179 883 70

56-84 24 68 6 4 76 128 196 962 49

84-112 33 74 6 5 71 112 260 950 93

112-140 39 65 8 6 63 114 165 870 39

Table 4 (continued)

Height (cm) Ni Co Cu Cr Zn Pb Sr Rb Ba

Upper fall B: F.so di Marano 34

O-18 28 71 4 4 65 108 265 925 57

18-36 30 83 4 3 70 131 167 965 60

36-54 28 62 4 4 56 125 201 968 78

54-72 30 74 4 2 66 147 178 960 49

72-90 37 74 4 4 67 112 178 946 97

Fall C: La Rocchetta 2

o-14.5 36 107 11 10 72 102 449 659 504

14.5-29 29 63 4 4 72 94 481 880 309

29-43.5 14 59 4 4 62 104 541 867 391

43.5-58 35 43 4 4 62 106 501 881 348

58-72.5 26 66 5 4 61 101 483 844 345

72.5-87 27 43 5 5 71 137 480 861 96

87-101.5 20 42 4 3 34 58 386 579 183

101.5-116 28 88 4 5 65 92 446 879 295

116-130.5 36 80 6 7 60 94 641 872 489

130.5-145 32 45 6 6 63 95 620 872 625

Fall E: Stenzano 31

o-15 29 31 n.d. n.d. 60 225 223 972 82

Fall F: F.so L.a Nova 52

o-14 41 39 9 6 73 120 585 318 832

14-28 41 38 9 6 69 138 610 322 657

28-42 40 42 8 4 74 129 601 402 487

42-56 39 33 10 6 69 114 548 546 706

56-70 40 30 10 6 72 119 573 541 487

Sample localities are indicated in Fig. 1. n.d. = not determined.

and Rb contents define a temporal variation trend from lower fall B to fall F, again with the exception of fall E (Fig. 8).

The reported trace-element variation, consistently with the silica trend, may indicate the tapping of a compositionally normally zoned magma chamber during early LVC activity. Apparently the magma batch erupted in the early stage of the Canino erup- tion (falls lower and upper B and interbedded Cellere pyroclastic flow unit) was relatively homogeneous in composition, while significant changes were revealed as deeper levels of the magma reservoir were tapped in the late stage of the Canino eruption or in a new eruptive cycle (fall 0.

5. Volumes and eruptive conditions

The calculation of fallout tephra volume and of eruptive parameters requires information on isopach

D.M. Palladino, E. Agosta/ Joumd of Volcanology and Geothemal Research 78 (1997) 77-102 97

o 1owerB

n upperB A c e D CI E . F

Rhyolite

51 55 59 63 67 71

Si02 wt %

Fig. 6. Plot of pumice composition in the classificative Total Alkali Silica diagram (Le Bas et al., 1986). Data from Table 3.

and isopleth geometry and on location of source vent area (Walker, 1973; Wilson et al., 1978; Carey and Sparks, 1986; Pyle, 1989; Fierstein and Nathenson, 1992).

In most of our cases isopach locations are poorly constrained, because of irregular thickness distribu- tion, lack of proximal exposures and erosion of deposits beyond the borders of the VVD. Further- more, the inferred vent area and related volcanic structures were obliterated by the collapse of the Latera caldera. Tentative isopachs, however, are broadly consistent with maximum lithic and pumice clast isopleth maps in indicating the source vent areas in the central part of the LVC, corresponding to the present-day Latera caldera and immediate vicinity. This agrees with vent areas inferred from the co-eruptive pyroclastic flow units of the LVC. Falls B, D and E erupted from the area correspond- ing to the southwestern sector of the caldera, while fall F erupted from the northwestern sector (Fig. 3). A notable exception is represented by fall C, whose thickness and clast size data sets give contradictory indications. However, the vent area, inferred from clast size distribution and grain size analyses, is likely located in the southeastern sector of the Latera caldera.

We consider the indications from maximum clast size data more reliable for this purpose for the following reason. Maximum clast sizes, measured at comparable stratigraphic height in a given deposit, reflect energetic conditions in the ‘umbrella’ region of the eruption column at a specific moment of the eruptive event, while the total thickness at any local- ity (even if unaffected by post-depositional changes) is the end result of a time interval of the order of several hours, during which energetic conditions may be highly variable. For example, changing dominant wind direction and intensity (either temporal varia- tions of the wind profile or variations related to oscillating column height) might be responsible for rotation of the dispersal axis and for variation of the downwind range, and thus influence the accumula- tion of pyroclasts at a given locality. This is also evidenced by different orientation of dispersal axis resulting from isopach and isopleth maps of the studied deposits. In most of these (e.g., falls upper B, C, E and F), evidence of rotating d&per& axis is suggested by lateral variation of clast grading pat- terns, as noted above.

Moreover local factors (i.e., wind conditions, to- pography of depositional substrate, post-depositional changes) may play a major role in determining thick-

98 D.M. Palladino. E. Agosta/Joumal of Volcanology and Geothermal Research 78 (1997) 77-102

ness distribution, whereas they likely affect to a transport. In fact, if we plot the above MP5 and ML5 minor extent lithic and pumice clast size distribution, data versus distance from possible source vent (not which is then more strictly related to large-scale reported), there still exists significant inverse correla-

fall lower B fall lower B Cellere Cellere

cm 200 ,

1 +x 0 I

180.

160-

100

120 140!

\

80

60 -

40

20 !

,/"

0 200 400 600 600 1000 1200 rmm

-&I +Pb * Sr -Rb - cu + Cr * Ni -co -- zn

fall lower B fall lower B Tessennano Tessennano

cm 130 /+ ll7- UY _,._ _ _ ,p

104. @I

_-D 7

04 0 200 400 600 600 1000 1200

pm 0 20 40 60 60 100 120

-&I +Pb *sr -Rb - cu + Cr * Ni - co - Zn

fall lower B fall lower B Plan dl Vlco Pian di Vito

cm 80

72

64

56

46

40

32

24

16

6

0 200 400 600 600 1000 Pm

]

1 ,200 0 20 40 60 60 100 120

-88 +Pb * sr -Rb - cu + Cr * Ni - co - zn

cm 200

160

160

140

120

100

60

60

40

20

1

J

Fig. 7. Plots of selected trace-element abundance vs. stratigraphic height km from the base) in individual pumice fall deposits. Data from

Table 4.

D.M. Palladirw, E. Agosta/ Journal of Volcanology and Geothermal Research 78 (1997) 77-102 99

fall upper B F.so di Marano

\

O-

0 200 400 600

PPm

_

cm 140 ,

- Ba + Pb * Sr

fall upper B Cellere

d 600 1000

- Rb

1 1200

1

\

“2:1:, 26 64 56 0 t f ) ( , , , /

0 200 400 600 600 1000 1200

PPm

- Ba + Pb + Sr - Rb

fall upper B Pian di Vito

600 PPm

60 PDm

- Ba + Pb * sr - Rb - cu + cr -*- Ni - co - zn

fall upper B F.so di Marano

0 20 40 60 60 100 120

DPrn

- cu + Cr + Ni - co * zn

fall upper B Cellere

OL

0 20 40 60 60 100 120 mm

- cu + Cr + Ni - co - Zn

fall upper B Pian di Vito

Fig. 7 (continued).

tion for most deposits (that is pumice and lithic clast thickness poorly inversely correlates with distance sizes decrease with increasing distance), supporting from the vent. the above location of the vent areas. Conversely, Tephra volume calculations for these fall deposits

100 D.M. Palladino, E. Agosta / Joumal of Volcanology and Geothermal Research 78 (1997) 77-l 02

fall F fall F F.ao la Nova F.ao la Nova

66

42 !

04 I

0 200 400 600 300 1000 1200

Pm

-aa -Pb - sr -Rb

fall C La Rocohetta

pm

- cu - Cr - Ni - co - zn

fall C La Rocchetta

0 200 400 600 800 1000 1200 pm

2

0 20 40 60 30 100 120

-3a -Pb * Sr -Rb - cu +- Cr + Ni - co * Zn

Fig. 7 (continued).

have been carried out by applying the method of Fierstein and Nathenson (1992). In dealing with ancient pyroclastic fall deposits, however, a large uncertainty in drawing isopachs in distal areas must be taken into account, due to the scarcity or absence of distal exposures. Indeed, our isopach area mea- surements yield conservative volume estimates. The tephra volumes obtained for western VVD pumice fall deposits (Table 5) range from 0.02 km3 (fall E) to 1.25 km3 (overall fall B). For fall C and fall D, whose data points do not allow us to reconstruct a complete set of isopachs, a crude estimate of their minimum volume has been obtained by comparison of their area1 extent and average thickness with the other deposits.

Estimates of column height were obtained follow- ing Carey and Sparks (1986). These authors have pointed out how results can be somewhat dependent on how the clast size data are collected in the field

and analyzed. Indeed, we consider it incorrect to average clast size measurements taken at different height in the deposit at any locality, as they corre- spond to different times of deposition and therefore different energetic conditions in the column/wind field. Moreover, as noted by Carey and Sparks (1986), variation in wind direction would lead to overestimates of column height. In this work, maxi- mum clast data were collected at specific strati- graphic height in each deposit, likely corresponding to the climax of the sustained column phase. Our estimates, which then represent maximum column height, range from 18 to 23 km (Table 5). Elongate isopleth patterns indicate influence by strong cross- winds for all cases. Consequent estimates of peak magma discharge rates, following Wilson et al. (19781, range in the order of lo’-lo* kg/s (Table 5).

For the above reasons, these deposits can not be

D.M. Palkulino, E. Ago&a/Journal of Volcanology and Geothermal Research 78 (1997) 77-102 101

loo0 , I 6. Concluding remarks

0 loo 200 300 400 500 600 700

Sr @pm)

Fig. 8. Plots of Sr vs. Ba and Rb contents. Data from Table 4

strictly classified on the basis of the dispersal/frag- mentation plot (Walker, 1973). However, we note that the area enclosed by the l/100 of maximum thickness isopach line is definitely larger than 500 km2, reported as the limit between Subplinian and Plinian fields, for all deposits.

Moreover, the above estimates of volume of ejecta and column height indicate moderate-large to large, Plinian eruptive events, with volcanic explosivity index (VEI) between 3 and 4 (Newhall and Self, 1982).

Table 5 Eruptive parameters for western VVD pumice fallout episodes. See text for discussion

Deposit Source area Volume Column height MDR (km3) (km) (kg/s)

Fall F NW Latera Caldera 0.52 21 6.0 x 10’ Fall E SW Latera Caldera 0.02 18 2.5 x 10’ Fall D SW Latera Caldera > 0.07 18 2.5 x 10’ Fall C SE LateraCaldera > 0.15 ? ? Fall B SW Latera Caldera 1.25 23 1.0x 108

A D.R.E. (dense rock equivalent) density of 2500 kg/m3 has been used to calculate magma discharge rate (MDR).

Detailed stratigraphic relationships, depositional features and grain size, component and chemical analyses of major pumice fall deposits of the western VVD provide their characterization as excellent stratigraphic markers and allow us to relate them to specific eruptions of the early known activity of the LVC (- 0.3-0.2 Ma).

Lower fall B resulted from an early, prolonged sustained eruption column phase of the Canino erup- tion. To the progressive rise of the eruption column, an intermittent phase followed, with time intervals long enough to enable settling of fine ash. We suggest that the Cellere pyroclastic flow originated during the time break before sustained eruption col- umn phase resumed (upper fall B), rather than by temporary/localized collapse from continuous sus- tained column. The following phase, during which the eruption column reached its highest level and upper fall B was deposited, represents the energetic climax of the eruption, leading to the generation of the main pyroclastic flows (e.g., Pianiano flow unit). Fall C may be either related to the final stage of the Canino eruption or to a succeeding event, but this is difficult to assess. Falls c,, d,, D and E demonstrate repeated episodes of Plinian-style activity separated by significant repose periods. In particular, fall E represents a relatively intense, short-duration erup- tive phase of the Stenzano eruption, involving rela- tively small volume of magma. Finally, fall F, which accounts for the last documented example of major Plinian-type activity in the history of the LVC, is another example of deposition from sustained ernp- tion column preceding the formation of major pyro- elastic flows, possibly originated by column collapse (e.g., Arlena di Castro flow unit).

In conclusion, violent explosive activity leading to the emplacement of important pumice fallout de- posits repeatedly took place in the span of time that includes the early major pyroclastic flow-forming eruptions of the LVC (i.e., Canino and Farnese eruptions). Subsequent LVC activity was essentially characterized by non-Pliniau type explosive erup- tions dominated by widespread pyroclastic flows (i.e., Sovana, Sorano and Grotte di Castro eruptions, Vez- zoli et al., 1987). We suggest that this major change in eruptive style and conditions is related to a major

102 D.M. Palladino, E. Agosta/Joumal of Volcanology and Geothermal Research 78 (1997) 77-102

change in the feeding/eruptive system, such as the onset of caldera collapse episodes. This is supported by the occurrence of ‘co-ignimbrite’ coarse lithic-rich volcanic breccias in the upper stratigraphic record of the LVC (i.e., in the Sovana and Onano eruptive units; Marsella et al., 1987), commonly associated with caldera-forming eruptions (e.g., Druitt and Sparks, 1984 Walker, 1985).

Acknowledgements

We gratefully thank Grant Heiken for reviewing the manuscript, and Greg A. Valentine and Eric Baer for very helpful comments on an early version of the paper.

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