Tephra ring interpretation in light of evolving maar–diatreme concepts: Stracciacappa maar (central Italy)

Journal of Volcanology and Geothermal Research 308 (2015) 19–29

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Tephra ring interpretation in light of evolving maar–diatreme concepts: Stracciacappa maar (central Italy) Greg A. Valentine a,⁎, Gianluca Sottili b, Danilo M. Palladino c, Jacopo Taddeucci d a

Department of Geology, University at Buffalo 126 Cooke Hall, Buffalo, NY 14260, USA Istituto di Geologia Ambientale e Geoingegneria (IGAG)-CNR, Roma, Italy Dipartimento di Scienze della Terra, Sapienza-Università di Roma, Roma, Italy d Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy b c

a r t i c l e

i n f o

Article history: Received 16 June 2015 Accepted 7 October 2015 Available online 22 October 2015 Keywords: Maar Diatreme Phreatomagmatic Tephra ring Pyroclastic Sabatini

a b s t r a c t Maar–diatreme volcanoes in their simplest form are thought to result primarily from discrete, subsurface phreatomagmatic explosions. Tephra ring deposits around maar craters are commonly interpreted in terms of a framework wherein explosions begin at shallow levels and migrate downward, ejecting progressively deeper-seated lithic clasts that contribute to the tephra rings. Additionally, variations in grain size of the deposits are sometimes inferred to record different degrees of fragmentation, which are in turn related to variations in magma–water ratios at the explosion sites. Recent detailed studies of diatremes and of maar tephra rings suggest a different conceptual model wherein explosions can happen at various vertical and lateral locations within a diatreme during its eruptive lifetime, rather than being limited to simple downward migration. Experiments, numerical modeling, and field data indicate that most explosions deeper than about 200–250 m will not vent to the surface, but instead contribute to mixing within the diatremes. Arrival of deep-seated lithics at the surface is related to this mixing process. Experiments also indicate that explosion phenomena can result in a range of emplacement mechanisms and grain sizes in tephra rings, even in the absence of variations in explosion mechanisms (fragmentation efficiency). Here we document the tephra ring of Stracciacappa maar (central Italy), and interpret its eruption and emplacement history in terms of evolving conceptual models along with a comparison to more traditional interpretations. Because most elements of both conceptual models are viable, the work points to an important need to develop criteria to distinguish between the various inferred mechanisms. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Maar–diatremes are craters cut into the pre-eruption land surface. They are underlain by crudely funnel-shaped bodies of brecciated and subsided country rock, pyroclastic deposits, and small intrusions, that may extend hundreds of meters to ~ 2 km into the subsurface (White and Ross, 2011). Maar craters are surrounded by low profile tephra rings, and may be partly filled with post-eruption sediments. In their end-member form, maar–diatremes are generally thought to result from repeated subsurface phreatomagmatic explosions. These discrete explosions are responsible for the diatreme structures, which are significantly larger than the feeder dikes and small conduits that feed scoria cones, the non-phreatomagmatic cousins of many maar–diatremes (Keating et al., 2008; Friese et al., 2013; Geshi and Neri, 2014; Harp and Valentine, 2015). In detail, most maar–diatremes record some

⁎ Corresponding author at: University at Buffalo, 126 Cooke Hall, Buffalo, New York, 14260, U.S.A. E-mail addresses: [email protected] (G.A. Valentine), [email protected] (G. Sottili), [email protected] (D.M. Palladino), [email protected] (J. Taddeucci).

http://dx.doi.org/10.1016/j.jvolgeores.2015.10.010 0377-0273/© 2015 Elsevier B.V. All rights reserved.

non-phreatomagmatic activity such as Strombolian lava fountaining and effusion, often in the form of scoria fallout beds or scoria cones and lavas within the maar crater, but their overall structure is primarily the result of the subsurface phreatomagmatic explosions (Palladino et al., 2015). Maar–diatremes have long been interpreted within a framework, here referred to as conceptual model 1 (CM1), wherein the locus of subsurface phreatomagmatic explosions propagates downward with the deepening of a groundwater drawdown cone, as water is used and ejected by explosions (Lorenz, 1986). The diatremes widen due to slumping and subsidence of host material as their explosion loci, inferred in CM1 to occur at the bottoms of the diatremes (root zones), deepen (Lorenz, 1986; Lorenz and Kurszlaukis, 2007). Within this framework, the common stratigraphic-upward appearance of progressively deeper-seated lithic clasts in tephra ring deposits reflects the arrival of the diatreme root zone at the lithic source depths (e.g., Németh et al., 2001). Explosions at the root zone are assumed to vent at the surface and directly eject these increasingly deeper-seated lithics onto the tephra ring. Variations in grain size distributions of tephra ring deposits are inferred by many authors to reflect variations in the intensity of fragmentation during the phreatomagmatic

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G.A. Valentine et al. / Journal of Volcanology and Geothermal Research 308 (2015) 19–29

explosions and/or intervening magmatic volatile-driven phases (e.g., Sottili et al., 2009; Jordan et al., 2013; van Otterloo et al., 2013) which in turn are often inferred to be related to magma–water ratios (e.g., Wohletz, 1983; Jordan et al., 2013). Some maars record a closing phase of magmatic volatile-driven activity in the form of lavas and/or scoria accumulations; these are interpreted within the framework of CM1 to result from the available groundwater having been used up. Recent work has revealed a somewhat different framework for phreatomagmatic maar–diatreme volcanism that we refer to as conceptual model 2 (CM2; Valentine and White, 2012). Field, experimental, and modeling studies of diatremes provide evidence for explosions at multiple levels during any given time window, which contribute to churning and mixing of diatreme fill through a combination of upward-directed debris jets and downward subsidence (McClintock and White, 2006; Ross and White, 2006; q et al., 2008a,b, 2013; Lefebvre et al., 2013; Andrews et al., 2014; Graettinger et al., 2014; Sweeney and Valentine, 2015; Valentine et al., 2015). Differently from the previous interpretations of maar–diatremes, the field evidence suggests that explosions are not restricted to the bottoms of diatremes and that many explosions do not vent at the surface. Tephra ring data on the abundances of lithics derived from various depths beneath maars support a view that only shallow (less than ~200–250 m deep) explosions vent to the surface (Valentine, 2012; Lefebvre et al., 2013), and this is supported by empirical relationships and modeling studies (Valentine et al., 2014; Sweeney and Valentine, 2015). Finally, experiments on buried explosions show that a wide range of deposit types and grain sizes can result simply by different combinations of explosion energy, depth, and crater shape effects on eruptive jets (Taddeucci et al., 2013; Graettinger et al., 2014, 2015, in press), even without any fundamental change in explosion or fragmentation mechanisms. In addition to a range of explosion depths, the lateral locations can shift within the footprint of the final maar crater, as evidenced by field studies (e.g., Valentine et al., 2011; Jordan et al., 2013), adding complexity to the tephra ring and diatreme. Individual magma batches might rise to the surface with little or no explosive interaction with water at any time during a maar–diatreme eruptive episode and even concurrently with phreatomagmatic explosions (e.g., Houghton et al., 1999; Valentine and Cortés, 2013), producing magmatic volatile-driven processes such as lava flows and sustained eruption columns. A third conceptual model for maar–diatreme development, which has arisen largely in the literature on diatremes of kimberlitic compositions (kimberlite pipes; e.g., Field and Scott Smith, 1999, and references therein; Sparks et al., 2006; Cas et al., 2008; Gernon et al., 2012), but has also been inferred for other compositions (e.g., Mattsson and Tripoli, 2011; Rausch et al., 2015), is that in most cases they are the result of rapid ascent of volatile rich magmas with little or no explosive interaction with groundwater. Workers who have favored magmatic volatile-driven explanations for kimberlite pipes commonly have two objections to phreatomagmatic models: (1) perceived absence of evidence for phreatomagmatic fragmentation at the scale of individual clasts, despite the broad similarities of lithofacies and structures between kimberlite pipes and “normal” diatremes (e.g., Lorenz, 1986; Kurszlaukis et al., 1998; White and Ross, 2011; Brown and Valentine, 2013; Kurszlaukis and Fulop, 2013); (2) the unlikelihood that all magmas of kimberlitic composition would erupt via phreatomagmatic processes. Brown and Valentine (2013) suggest, however, that this may be an artifact of preservation and of research emphasis on pipes for diamond resources, and that many (most?) kimberlites eruptions might not form deep diatremes and may be more akin to other monogenetic volcanic activity (e.g., Brown et al., 2012). For the purposes of this paper we focus on maar–diatremes that are dominated by phreatomagmatic explosions (but understanding that such an event might include phases of magmatic volatile-driven activity). We hypothesize that the presence of a maar–diatreme structure is in itself strong evidence of numerous, discrete, subsurface explosions, and that the structure is most easily explained by phreatomagmatic processes.

Although concepts of maar–diatreme volcanism are rapidly evolving, some of the previous concepts and ways of interpreting the volcanoes and their deposits remain viable, and in many, if not most, cases some elements of both conceptual models may play a role. For example, variations in grain size of tephra ring deposits could be due to differences in fragmentation efficiency or to differences in scaled depth and crater effects on eruptive jets, or some combination of both. One aspect of CM1 that can be ruled out, in our opinion, is the venting of deep (N 200–250 m below crater floor, approximately) explosions directly to the surface. In this paper we briefly describe a relatively simple maar and present and compare interpretations of its tephra ring deposits in light of both frameworks. The comparison points out the need to develop criteria that can be used, given sufficient exposure and data, to test which elements of CM1 versus CM2 are most applicable for a given case study. 2. Geologic setting and general characteristics of the Stracciacappa maar Stracciacappa maar is in the Sabatini Volcanic District of central Italy (Fig. 1; de Rita and Zanetti, 1986; Sottili et al., 2012). The land surface at the time of the Stracciacappa activity was that of a plateau capped by a b10 m thick lava; this lava is poorly exposed in the northern wall of the maar crater. Drill core data indicate that pre-Stracciacappa volcanic deposits (interbedded tuffs and lavas from nearby Sabatini and Vico volcanic centers) extend from the base of the capping lava to ~ 450 m depth. These are underlain by carbonates of Mesozoic–Cenozoic age to depths of at least 650 m (Sottili et al., 2012). The maar crater's diameter is ~1.5 km (measured from high points of tephra ring) and the tephra ring volume is ~0.1 km3. The crater rim rises ~50 m above the crater floor, but tephra ring deposits comprise just the upper half of that height; the lower half of the crater wall is poorly exposed and is presumably country rock made of older volcanic deposits. The crater floor is slightly lower than the surrounding landscape, and hosts about 50 m of lake sediments based upon drilling data (the crater lake was drained in the 19th century; Sottili et al., 2012). Tephra ring deposits are currently exposed in cross section in two radial road cuts (Fig. 1) and consist of two main sequences separated by a paleosol, indicating reoccupation of the site by distinct eruptive episodes separated by hundreds to thousands of years. The deposits above the paleosol have a radiometric age date of 97 ± 4 ka (Sottili et al., 2010). We focus only on the lower tephra ring sequence in this paper. Juvenile material in the lower sequence has ~7 vol.% phenocrysts (primarily leucite and clinopyroxene) and a bulk chemical composition of phonotephrite to tephriphonolite (Sottili et al., 2012). 3. Description and interpretation of emplacement processes 3.1. Stratigraphy The lower tephra ring sequence is exposed in a 23 m thick section (Fig. 2; see also de Rita and Zanetti, 1986). Although the base of the sequence is not currently exposed, previous excavations revealed a mature paleosol within ~2 m below the current exposures. The deposits consist mainly of stratified and cross-stratified tuffs and lapilli tuffs that can be grouped loosely into five units, described below. Clasts within the entire sequence are dominated by moderately vesicular scoria (60–97 wt.%; de Rita and Zanetti, 1986, and our data; vesicularity based upon qualitative visual estimates of volume fraction of vesicles in individual clasts, poorly vesicular – 0–40 vol.%, moderately vesicular – 40–60 vol.%, highly vesicular – N60 vol.%). Previous workers assumed that the scoria is a juvenile component (de Rita and Zanetti, 1986; Sottili et al., 2012), but we were unable to determine whether a given lapillus is juvenile for the particular explosion that emplaced it in the tephra ring (primary juvenile; White and Houghton, 2006), recycled juvenile material (from earlier phase in the monogenetic episode that formed

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Fig. 1. Simplified geologic map of the central portion of Sabatini Volcanic District, showing location of Stracciacappa maar and its eruptive products. Insets show distribution of Sabatini and other volcanic districts of central Italy, location within Italy, and locations of exposures and stratigraphic section.

the maar, or from a vesicular intrusion within the diatreme), or from pre-Stracciacappa lavas that had similar compositions. We also note a full gradation between vesicular and non-vesicular lapilli and ash grains of the same composition. Below we provide brief descriptions of the lower tephra ring deposits, along with interpretations of their emplacement mechanisms. This is followed by discussion of the potential relationship between the deposit characteristics and explosion and diatreme processes.

3.2. Units 1 and 2 Units 1 and 2 comprise the lowest 10 m of the exposed sequence (Fig. 2). Unit 1 consists of massive, decimeter-thick lapilli tuff beds of coarse ash to medium lapilli beds, and stratified and cross-stratified tuff beds of fine to coarse ash (Fig. 3A). In many cases, these different bed types form alternating pairs or bed sets, with each set having basal massive lapilli tuff overlain by stratified/cross-stratified tuff (Fig. 3B). Isolated small (b 15 cm) blocks of dense lava, probably derived from the plateau-capping unit of pre-Stracciacappa rocks, are present throughout Unit 1, but no other lithic block types were observed in it. Unit 2 has generally similar characteristics to Unit 1, but contains more abundant blocks of three different types: dense lava, preStracciacappa tuffs, and carbonate clasts. The base of Unit 2 is marked by two massive lapilli tuff-stratified tuff bed sets that contain coarse lapilli-size carbonate clasts (Fig. 3C), representing the first appearance of deeper-seated (N450 m) wall-rock clasts that is visible at outcrop scale (note that carbonate fragments do occur in the ash fraction at lower stratigraphic levels). Interestingly, the carbonate lithic content varies laterally within these two beds; the clasts are nearly absent at distances of 5–10 m from the site of their maximum concentrations

(Fig. 2). The top of Unit 2 corresponds to two beds, 40 and 60 cm thick, of relatively well-sorted scoria lapilli with scattered angular, dense lava lapilli. The two beds have massive internal structure and planar-parallel top and bottom contacts. Block sags are poorly developed or absent in Units 1 and 2. We interpret most of the deposits in Units 1 and 2 to have been emplaced by pyroclastic currents, possibly in some cases by ballistic curtain mechanisms (see Graettinger et al., 2015). Deposition from laterally moving flows is recorded by the abundant stratification to cross-stratification. Bed sets with coarse, massive lapilli tuff at the base and stratified to cross-stratified tuff on top likely record individual emplacement events (e.g., Dellino et al., 2008). These are similar to blast deposits (Belousov et al., 2007) and have been described elsewhere at tephra rings around maars (Valentine, 2012), and it is possible that these bed sets each record an explosion. Planar-parallel, relatively well-sorted lapilli beds that occur at the top of Unit 2 may record periods of fallout from sustained eruption columns. If so, it is likely that the sustained columns were related to magmatic volatile-driven eruption processes rather than discrete phreatomagmatic explosions.

3.3. Unit 3 Unit 3 (Fig. 2) is characterized by stratified and cross-stratified tuffs with alternating fine-ash rich and coarse-ash to fine-lapilli rich horizons. Wavy structures are abundant due to a combination of dune forms and well-developed impact sags beneath fine lapilli to medium blocks (Fig. 4A,B). Coarse lapilli and blocks include dense lavas, older tuffs, and carbonate clasts. Older tuff blocks often occur in isolated lenticular clusters, and carbonate clasts tend to be concentrated in a few layers, but are sparse in most. In detail, many beds occur in coarse-fine

G.A. Valentine et al. / Journal of Volcanology and Geothermal Research 308 (2015) 19–29

*S1 20 m

15 m

*S3

10 m

*S4

Unit 4

+

*

*

*S8

+

**

+

*

*

Unit 2, ~5 m thick, similar to underlying unit but generally coarser grained with dominant grain sizes in coarse ash to small lapilli range. Includes some beds up to 50 cm thick with sharp parallel top and bottom contacts, and well-sorted lapilli (subangular to subrounded scoria with subordinate angular lava clasts). Blocks are scattered or occur in clusters or lenses, and include dense lava and older tuffs. ML = 30, 14, 25 cm.

*S10

*

+

+

*S5

*S7 *S9

*

Unit 3, ~7.5 m thick, composed of alternating decimeterto centimeter-thick horizons of fine-medium ash and coarse ash to small lapilli. Unit contains abundant thin, low angle cross beds to cross lamination. Wavy bed forms and block and lapilli sags abundant. Coarser horizons are lenticular laterally over 1-5 m. Contacts between fine (underlying) and coarse (overlying) horizons have millimeter- to centimeter-size sags and load structures. Dominant material is scoriaceous mafic clasts, which are mostly subrounded in the lapilli size range. Blocks are mainly dense lava with scattered older tuff and carbonate clasts. ML - 40, 55, 25 cm (all dense lavas). Blocks tend to occur in clusters or lenses.

*S6

5m

+

Unit 4, 80 cm thick, lapilli- and block-rich tuff, crudely stratified to massive. Blocks are angular to subrounded, the latter coated with a ~1 cm rind of fine ash. Clasts are supported in coarse-ash scoriaceous matrix. ML = 30, 30, 35 cm (dense lavas).

*S2

*

*

Unit 5

Unit 5, ~4 m thick, composed mainly of medium ash with isolated coarse lapilli and blocks. Massive structure with faint bedding and low-angle cross bedding defined by thin layers of small lapilli. Scattered coarse lapilli are rounded to angular and consist mainly of dense lava or finely vesicular scoria and sparse limestone clasts. ML = 70, 60, and 220 cm, the largest lying upon sagged contact with underlying unit. All block-sized clasts are dense mafic lava.

Unit 3

covered

*+

*

Unit 2

22

+

Stratigraphically lowest occurrence of carbonate clasts in coarse lapilli and block sizes occurs at this level.

*+* **

*

**

++

*

*S11

Unit 1, ~5 m thick, composed of alternating decimeterto centimeter-thick horizons of fine-medium ash and coarse ash to small lapilli. Unit contains abundant thin, low angle cross beds to cross lamination. Blocks are subangular dense lava, ML = 13, 8, 14 cm.

0

mafic clasts (incl. scoria) of medium lapilli to blocks <15 cm carbonate lithics ML

maximum lithic clasts

*

+

-2 0 2 4 0 4 8 12 16 Median size (φ) wt% carbonate clasts (in >2 mm fractions)

base not exposed

volcanic lithic blocks

* +

Unit 1

+

stratification and cross stratification (thin beds to laminae) wavy bed form

S10 Sample number, this study *

Sample location, this study

+

Approximate sample location, median size from de Rita and Zanetti (1986)

block/bomb sag

Fig. 2. Simplified stratigraphic column, median grain size, and component data for the lower tephra ring sequence at Stracciacappa maar. Median size data include samples from de Rita and Zanetti (1986). Sorting values range from 1.5 to 2.1 (phi units), and do not vary systematically with mean grain size. Component data shows weight percentage of carbonate clasts in the lapilli and larger size fractions (note that sandstone and shale clasts are also present in a few samples, but are extremely rare). Tie lines connect sample pairs S7-S8 and S9-S10; each pair was collected in a single bed about 5 m apart. Note lack of systematic variation in sedimentary clast content. Other components include scoria and dense mafic clasts, with full gradation between them (unclear which of the denser clasts are juvenile Stracciacappa products or from similar preceding volcanic deposits), pre-Stracciacappa pyroclastic rocks (always b2 wt.%), and trace quantities of siliciclastic sedimentary rocks. Granulometry obtained by dry sieving, and componentry data by sorting and weighing grains of different types within individual sieve fractions.

bed sets, similar to Units 1 and 2, but are finer-grained and thinner (Fig. 4C). We interpret Unit 3 to have been emplaced mainly by dilute pyroclastic density currents. Where the coarse-fine bed sets occur they likely

record individual density currents related to explosions. Blocks and coarse lapilli followed ballistic paths and sag structures indicate impact at a range of angles, even approaching subvertical. The deposits are relatively fine-grained compared to the rest of the lower tephra ring

G.A. Valentine et al. / Journal of Volcanology and Geothermal Research 308 (2015) 19–29

23

A

talus

talus

B

talus

talus carbonate poor

C carbonate rich

carbonate poor

carbonate rich

Fig. 3. (A) Photograph of the lower few meters (Unit 1) of the Stracciacappa tephra ring sequence, showing generally planar-parallel large-scale stratification. Scale is 1 m. (B) Close-up of same outcrop. Large-scale stratification is composed in detail of doublets with massive, fines-poor lapilli tuffs overlain by ash-rich, thinly bedded to laminated and cross-laminated tuffs. Brackets show three examples of these doublets (bed sets), which likely each record emplacement during an individual explosion. (C) Close up showing two bed sets that mark the lowest occurrence of carbonate lithic clasts in the tephra ring sequence. The carbonate lithic content varies laterally within each bed set over distances of a few meters. Lower bed set (including massive lower part and stratified upper part) is ~60 cm thick. Scale is ~70 cm.

sequence, and some emplacement units may have been damp, but the occurrence of low profile dune forms (e.g., Fig. 4B) also indicates a lack of grain cohesion such as during deposition from a dry bed load for some of the beds. 3.4. Units 4 and 5 Units 4 and 5 form the upper part of the lower tephra ring sequence (Figs. 2 and 5A). Unit 4 comprises ~80 cm of lapilli- and block-rich tuff (Fig. 5B). The unit is dominantly massive with only poorly developed stratification, and coarsens upward from its basal 20 cm that is lapilli

dominated. Despite its overall coarse-grained character, a coarse-ash matrix supports the dominant lapilli and blocks. Poorly developed sag structures (difficult to define due to the poor stratification of the deposit) underlie some larger blocks, and the basal contact of Unit 4 is undulatory, probably due to loading by rapid sedimentation onto the relatively fine-grained deposits of Unit 3. Dense mafic clasts dominate lithics in Unit 4, with angular carbonate clasts only being present in the lapilli size range. Lava blocks range from angular to subrounded, and the latter are sometimes coated with a rind of ash (Fig. 5C). In contrast to Unit 4, Unit 5 is composed almost entirely of massive to stratified and cross-stratified ash (Fig. 5A), with stratification being

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G.A. Valentine et al. / Journal of Volcanology and Geothermal Research 308 (2015) 19–29

A

A

Unit 5 Unit 4

B

Unit 3 40 cm

B

sags dune crest migration

Unit 4

50 cm

Unit 3

C

C

lava block

40 cm Fig. 4. (A) General character of Unit 3, with dune forms and impact sags contributing to an overall wavy appearance. Measuring stick is 1 m. (B) Detail showing impact sags related to lapilli and small blocks, and low angle dune form with migration away from crater. (C) Detail of coarse-fine bed sets (examples partly enclosed in dashed lines).

defined by centimeter-thick layers of relatively coarser ash or fine lapilli compared to the dominant fine to medium ash. Although the unit is dominated by ash, it contains scattered carbonate lapilli and large (up to 2.2 m) dense lava blocks, the largest in the lower tephra ring sequence. Impact sags are difficult to define, but the largest block is at the base of Unit 5 and penetrates into underlying Unit 4. Units 4 and 5 have unique characteristics in the lower tephra ring sequence. Unit 4 approaches a tuff breccia, with its poor sorting, vague stratification, undulatory base due to rapid loading of its substrate, and crude impact sags beneath the largest blocks suggesting a combination of lateral flow and ballistic emplacement with very high sedimentation rate. Unit 5, on the other hand, appears to have been emplaced by multiple, relatively fine-grained, dilute pyroclastic currents that may have been concurrent with occasional ejection of very large and dense blocks. This is discussed further below.

20 cm

ashy rind

Fig. 5. (A) Upper part of lower tephra ring sequence at Stracciacappa maar. Units 4 and 5 contain scattered very large blocks, and Unit 5, although its overall character is fine-grained and dominated by massive to vaguely stratified ash, contains the largest blocks in the sequence (up to 2.2 m). Scale is 1 m. (B) Detail of Unit 4, showing undulating basal contact, lower ~20 cm of fine- to medium-lapilli above which the deposits coarsen to a matrix supported lapilli- and block-rich tuff (tuff breccia). (C) Ash rind coating dense lava block in Unit 4. The rind is composed of concentric layers of ash. The surrounding Unit 4 material is otherwise dominated by fine- to medium lapilli.

4. Discussion—explosion phenomena and diatreme processes recorded in the tephra ring We discuss two main aspects of the lower tephra ring sequence in light of conceptual model 2, which are compared with interpretations that typically arise within the framework of conceptual model 1 in Table 1. The two aspects are the range of deposit facies as a function of

G.A. Valentine et al. / Journal of Volcanology and Geothermal Research 308 (2015) 19–29

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Table 1 Comparison of interpretations of characteristics of Stracciacappa maar. Characteristic

Conceptual Model 1 (CM1)

Massive to crudely stratified, coarse lapilli- to block-rich tuff Stratified and cross-bedded and laminated tuff

Poor fragmentation, emplacement by concentrated PDCs

First appearance of carbonate lithic clasts

Lateral variation of carbonate lithic content in individual beds Alternating cross-stratified tuffs and scoria fall beds Abundant subangular to subrounded scoria in stratified and cross-stratified tuffs Dense, angular to subrounded mafic clasts

Conceptual Model 2 (CM2)

Near optimal scaled depth explosion, emplacement by ballistic curtain Fine fragmentation, emplacement by dilute PDCs Deeper-than-optimal scaled depth explosion, pre-existing crater effects, emplacement by expulsion-driven dilute PDCs Arrival of downward-propagating diatreme root zone at Arrival of upward-mixed domains (debris jet deposits) rich in carbonate aquifer, direct ejection of deep carbonate lithics carbonate lithics to depths of ~200 m or less, ejection by shallow explosion Not addressed Maintenance of some coherence (partial mixing) of a debris jet deposit during its ejection by shallow explosion Fluctuations in water availability to conduit Fluctuations in magma flux and/or water flux, and/or differing styles of activity at different vents within crater Interaction of vesiculated magma with water Recycling of scoria deposits from earlier (mainly magmatic volatile-driven?) eruptive phases within upper diatreme and ejection by shallow explosions Brecciation and ejection of older volcanic rocks Brecciation and ejection of solidified juvenile intrusions within diatreme (in addition to CM1 mechanism)

subsurface explosion phenomena, and the occurrences of lithic clasts as indicators of diatreme processes. 4.1. Explosion phenomena and the tephra ring Deposits in the lower tephra ring sequence range from stratified and cross-stratified tuffs and lapilli tuffs to massive lapilli- and block-rich tuffs. Most of the deposits are interpreted to have been deposited from a transport system involving lateral flow, although at least two beds (top of Unit 2) are likely to have been deposited by fallout from sustained eruption columns. Experiments with subsurface explosions show that explosion phenomena and emplacement of tephra ring deposits depend very strongly on the combination of explosion depth and energy, and on topography over the explosion locus (Graettinger et al., 2014, 2015, in press). The depth–energy combination is traditionally parameterized as scaled depth, Dsc = d · E−1/3 (d is the physical depth of an explosion center and E is the explosion energy; Dsc has units of m J− 1/3; e.g., Houser, 1969; Goto et al., 2001; Graettinger et al., 2014). Explosions that occur at or near Dsc ≈ 0.004 m J− 1/3, which is the scaled depth for optimum crater excavation (Sonder et al., 2015), produce the most extensive proximal deposits of relatively poorly sorted debris, forming a positive landform (tephra ring) that grades laterally to a continuous sheet (medial deposits) and eventually to isolated clasts (distal deposits; Graettinger et al., 2015). Deposition of the proximal and at least part of the medial facies occurs via a ballistic curtain, i.e., an eruption jet that collapses with simultaneous outward spreading. At shallower scaled depths (Dsc b 0.004 m J−1/3) less material is excavated and the material is thrown large distances on ballistic trajectories as separate clasts, so that the distal facies dominates the dispersal with proximal and medial only very close to the resulting (small) crater. Between 0.004 b Dsc b 0.008 m J−1/3 ejection velocities and jet spread angles become progressively smaller and the deposits are dominated by proximal facies with decreasing lateral extent (Taddeucci et al., 2013; Graettinger et al., 2015) until Dsc ≈ 0.008 m J−1/3, which marks the approximate threshold for fully contained (non-erupting) explosions (Valentine et al., 2014; Sonder et al., 2015). At Stracciacappa's lower tephra ring sequence the coarser deposits such as Unit 4 could be evidence of explosions near the optimal scaled depth and deposition from ballistic curtains, with extremely rapid sedimentation from a spreading and collapsing jet while larger clasts follow relatively independent ballistic paths. For most phreatomagmatic explosions the optimal scaled depth corresponds to physical depths less than about 100 m beneath the crater floor at the time of a given explosion (Valentine et al., 2014). The lateral extent of a ballistic curtain deposit depends upon the explosion energy and on its scaled depth (Graettinger et al., 2015). If the explosion that produced Unit 4 occurred beneath the center of the crater, the distance of the exposed deposits is

700–800 m from their “vent,” which might imply a relatively large energy and an explosion depth approaching 100 m beneath the crater floor. Although exposure is limited, Unit 4 appears sheet-like and thus might be medial facies within the definitions of Graettinger et al. (2015a). Other deposits in the lower tephra ring sequence might also be related to medial ballistic curtain facies, specifically the bed sets that are prominent in Units 1 and 2. In these cases the coarse, massive basal bed of each bed set records rapid sedimentation from the medial ballistic curtain, while the stratified and cross-stratified, finer-grained upper bed records the waning deposition of fines with a bed load component of emplacement (Fig. 6). However, the bed sets in Units 1 and 2 were likely produced by explosions with greater-than-optimal scaled depth, and/or from lower-energy explosions at optimal scaled depths, which did not excavate as much or produce as much material in their ballistic curtains compared to the explosion for Unit 4. Topography above an explosion locus can strongly affect the spread angle and directivity of an erupting jet. Explosions beneath relatively flat terrain produce the widest spread and farthest-extending ballistic curtains for a given scaled depth and energy. If explosions occur beneath the center of a crater and are deeper than their optimal scaled depths, the jets are vertically focused, and can form narrow columns of debris that mainly collapse back into the crater (Taddeucci et al., 2013; Graettinger et al., 2014, 2015). As the poorly sorted mixtures collapse into the crater they displace gas (air and erupted gases) laterally, and the gas is capable of carrying some of the fine particles outward with it. This expulsion process can trigger dilute, relatively fine-grained density currents that travel out of the crater and that, in nature, would be a dilute pyroclastic density current (pyroclastic surge or base surge; Fisher and Waters, 1970; Graettinger et al., 2014, 2015). Experiments show that in the latter case of focused jets, even though most of the erupted material falls back into the crater, isolated outsized ballistic clasts are ejected out of the crater (e.g., Valentine et al., 2012). Although not discussed further here, it is worth noting that explosions that are not beneath a crater center or flat crater floor, but rather beneath a crater wall, can produce inclined jets with asymmetric deposit lobes (Valentine et al., 2015). Some of the finer-grained deposits at Stracciacappa can be interpreted in terms of crater effects on eruptive jets. For example, Unit 5, which is dominantly stratified and cross-stratified tuff, may record expulsion-driven dilute pyroclastic density currents (Fig. 6). Such currents could record explosions beneath a deep crater, potentially after the inferred near-optimal-scaled depth and higher-energy explosion that deposited Unit 4 via ballistic curtain processes. The large isolated lava blocks in Unit 5, which seem incongruous with its otherwise finegrained nature, are consistent with experimental observations of scattered outsized ballistic clasts during expulsion-driven density currents. Lower in the sequence, Unit 3 is relatively finer-grained than

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EJECTION

DEPOSITION

Units 1 & 2 ballistic curtain

explosion site maar diameter

single explosion crater

Unit 3 expulsion-driven PDC

Unit 4

Unit 5

Fig. 6. Cartoon illustration of explosions that might have produced individual beds or bed sets within the units shown in Fig. 2 (see also Graettinger et al., 2014, 2015). Black dots represent lapilli and blocks; gray shade represents cloud of mainly ash (darkness proportional to ash concentration). Cartoons on left show eruptive jets at maximum height, and cartoons on the right show the jets during deposition. In Units 1 and 2 (ballistic curtain dominated, but probably slightly deeper than optimal scaled depth), deposits consist of bed sets with a coarse, massive lapilli tuff layer overlain by stratified and cross-stratified tuff layer, and isolated blocks. Not shown are possible phases of sustained, magmatic volatile-driven activity that produced likely tephra fall layers in the upper part of Unit 2. Unit 3, consisting of ash-rich stratified and cross-stratified beds with isolated blocks and associated sag structures, might have been deposited from expulsion-driven pyroclastic density currents (PDCs) as vertically focused jets collapsed back into crater. Note that such explosions combine deeper-than-optimal scaled depth and ejection through a pre-existing crater, and the prevalence of fallback into the crater results in a very shallow depression (see Graettinger et al., 2014). Unit 4, a massive lapilli- and block-rich tuff, might have resulted from a relatively high energy, near-optimal scaled depth explosion with deposition from a ballistic curtain mechanism. Such an explosion would produce a deep crater, which might have focused jets during explosions that produced the ashrich, stratified and cross-stratified Unit 5 from expulsion-driven PDCs. Note that even in such a case, isolated ballistics also result from the explosions. During the entire eruptive episode, each explosion probably produced a crater that was smaller than the final maar crater. The figure illustrates the possibility of laterally shifting explosion sites, but limited exposure precludes any quantification of this at Stracciacappa. Explosion sites are shown as relatively deep or shallow to represent scaled depth, but in reality this parameter depends both on depth and energy of an explosion. The inferred underlying diatreme is not shown for simplicity; similarly, we show only vertically directed explosion jets here and neglect the possibilities for inclined jets (Valentine et al., 2015; Graettinger et al., in press).

G.A. Valentine et al. / Journal of Volcanology and Geothermal Research 308 (2015) 19–29

underlying units, has abundant dune forms as evidence for deposition from dilute pyroclastic density currents, and also contains outsized ballistic blocks. It might have been deposited by expulsion-driven currents (Fig. 6) caused by vertically focused jets in the crater(s) formed during deposition of Units 1 and 2. We note that the craters produced by individual explosions likely were not as large as the maar that we see today. Rather, each explosion probably made a crater with diameter of ~150 m or less, but these craters were ephemeral and the final maar crater is the result of many of these overlapping ephemeral craters (e.g., Jordan et al., 2013; Valentine et al., 2015) and is overprinted by post-eruptive sediments. 4.2. Diatreme processes and the tephra ring We assume that eruptive episodes such as the one recorded in the lower tephra ring sequence at Stracciacappa may have durations of weeks to years and with varying magma fluxes, analogous with other monogenetic volcanic episodes such as at scoria cones (Wood, 1980). During an episode batches of magma continue to arrive, often following complex pathways as the feeder dike(s) transitions upward from relatively competent country rock, through the root zone (here used to refer to the transition from base of diatreme to feeder dike or dikes), into the loose debris of the developing diatreme. Some magma batches may stall and form variably shaped and sized intrusions within a diatreme and with no explosive magma–water interactions (e.g., Valentine and van Wyk de Vries, 2014), others reach the surface without appreciable interaction with water and possibly feed lava flows and magmatic

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volatile-driven explosive activity. The former case can provide a supply of juvenile clasts that can be broken up and ejected by later explosions (recycled juvenile clasts). The case of magma rising to the surface without phreatomagmatic interaction is recorded in Stracciacappa tephra ring by the two scoria lapilli beds at the top of Unit 2, inferred by us to record fallout from sustained eruption columns. Other magma batches do explode in the subsurface due to interaction with water, and diatreme processes are strongly influenced by these explosions occurring at multiple levels and lateral locations during the eruptive lifetime of a maar–diatreme (Valentine and White, 2012). The explosions result in churning and mixing within the diatreme, but generally only erupt if they occur at depths less than about ~200–250 m (Valentine et al., 2014; Sweeney and Valentine, 2015). We see evidence of these churning and mixing processes at Stracciacappa. Although not quantified, clasts in the lower tephra ring have varying degrees of rounding (Fig. 2), which is probably related to milling within the diatreme. This can result from eruption of material that collapses back into the crater and is later ejected by a subsequent explosion, or it can occur mainly in the subsurface due to debris jet motion. Blocks coated with concentrically layered ash rinds (e.g., Fig. 5C) might record repeated churning in an ash-rich environment such as in the diatreme or during repeated eruption in moist, ash-rich explosion jets, each time accreting a concentric ash layer and falling back into the crater, until finally being ejected onto the tephra ring. Coarse lapilli and block-size carbonate clasts (derived from N450 m depth) and pre-Stracciacappa tuff blocks first appear at the base of Unit 2, although they are present at low concentrations in finer size

A

~100 m

debris jet deposit

B

Fig. 7. Cartoon of the upper part of a maar–diatreme, showing a debris jet deposit composed of deep-seated clasts mixed with other diatreme fill in a vertical domain (A) prior to a subsequent explosion, and (B) during an explosion illustrating how a debris jet deposit would be preserved during jet expansion, which would be followed by collapse as a ballistic curtain and deposition of a tephra ring unit with variable lithic composition.

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fractions in the middle of Unit 1 (Fig. 2). Their abundances vary nonsystematically in the stratigraphy, in layers that are otherwise dominated by scoria lapilli and lava clasts, and laterally within individual layers (Fig. 3C). These observations are consistent with upward mixing of material in debris jets (Ross et al., 2008a,b; Sweeney and Valentine, 2015). Successive subsurface explosions brecciated the deep-seated material and injected it upward. Each explosion involved in this step-wise mixing probably had a different energy, and they occurred at different depths and lateral locations, such that originally monomict breccia domains were broken down and diluted with other clast types as they were mixed upward, as seen in experiments (Graettinger et al., 2014; Valentine et al., 2015). Inferred debris jet deposits at exposed diatremes have the shapes of irregular vertical columns, a few meters to a few tens of meters wide (e.g., Ross and White, 2006; Lefebvre et al., 2013). Intrabed lateral variability of lithic clasts within individual beds at Stracciacappa may record that a debris jet deposit was part of the volume disrupted and ejected by a shallow explosion, and part of the debris jet's compositional identity was preserved during ejection and deposition (Fig. 7). Most explosions probably affected only portions of the heterogeneous diatreme. Thus, variability from bed to bed reflects the intersection of explosive disruption of individual (or groups of) debris jet deposits, but the dominance of shallow-derived material (scoria and lava) in the deposits is consistent with explosion loci that were relatively shallow (see also Valentine, 2012; Lefebvre et al., 2013). The fact that carbonate clasts are mainly in the lapilli and small block size range, while the largest lithic clasts in a given horizon are always dense lavas, might be an artifact of the carbonates being more fractured in their original units, or because they are relatively weaker than the lavas and broke down more easily within the diatreme, or because they experienced more churning and milling in the diatreme as they were transported upwards from their depth of origin compared to lavas that originated near the surface. As pointed out by Graettinger et al. (2014), the presence of carbonate clasts in the tephra ring can only be taken as an indicator that the diatreme extends to at least the clasts' depth of origin. The absence of deeper-seated materials does not mean that the diatreme bottomed within these carbonates, nor does their first appearance within the tephra ring record the arrival of the root zone at the carbonate depth of origin. 5. Conclusions Conceptual models 1 and 2 provide two frameworks for interpreting Stracciacappa deposits and indeed, previous work by de Rita and Zanetti (1986) and to some extent by Sottili et al. (2012) resulted in interpretations based on CM1. The conceptual models in fact can lead to somewhat different inferences (Table 1), which in turn can affect derivative products such as geophysical interpretations and hazard assessments. Many of the different inferences are not mutually exclusive, given our current understanding. Some aspects of CM1 are not ruled out by CM2, such as the possibility that fine-grained deposits record differences in fragmentation efficiency in addition to the possibility that the differences are due to eruptive jet dynamics, indicating future work needs to focus on developing criteria for determining which interpretation best explains a given deposit. Because individual parameters, for example ash morphology, might not be conclusive on their own, it is likely that the criteria will involve multiple parameters related to particle types and sizes, deposit geometry, and evidence related to explosion depths. Acknowledgments GAV's contribution was supported by a visiting professorship at Sapienza-Università di Roma, and by US National Science Foundation (Grant EAR-1420455). We thank Alison Graettinger, Simone Jordan, and an anonymous reviewer for useful comments on earlier versions of the manuscript.

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