Unbedded diatreme deposits reveal maar-diatreme-forming eruptive processes: Standing Rocks West, Hopi Buttes, Navajo Nation, USA
Diatremes provide partial records of how dyke-delivered magma, periodically interacting with water, produces the largest known cylindrical conduit structures of any volcano type. We address how pre-eruptive country rock is disrupted and redistributed to form the diatreme structure during an eruption by establishing the internal architecture of the unbedded diatreme at Standing Rocks West, and the volumes and sources of wall-rock within the diatreme and in a complementary tephra ring at Teshim, both in the Hopi Buttes volcanic field. The unbedded diatreme is dominantly exposed as a massif comprising multiple subvertical columns, interpreted as ephemeral-conduit deposits of well-mixed, poorly sorted, composite and juvenile pyroclast-rich deposits that truncate marginal layered deposits, plus a peripheral heterolithic country rock breccia. Wall-rock clasts are scarce in the massif and were mainly sourced from ~75 to 245 m depth below the paleosurface. By contrast, the tephra ring has abundant wall-rock fragments, dominantly from the upper 75 m of the pre-eruptive sedimentary sequence. The diatreme structure is interpreted to have been formed by many small-volume explosions fed by a low flux of basaltic magma. Late stage explosive activity was rooted mostly within pyroclastic debris at shallow to intermediate depths within the diatreme structure. This resulted in damped and shifting intra-diatreme explosions and jets that facilitated gradual mixing, recycling and remobilization of debris in the diatreme, with incremental addition of juvenile material and possibly a local rise in the crater floor.
KeywordsDiatreme Country rock breccia Monogenetic Fissure Hopi buttes volcanic field
Maar-diatreme volcanoes, unlike other small volcanoes, are formed by eruptions that excavate an open crater into the substrate. Most maar-diatreme-forming activity takes place below the land surface, producing a significant sub-conical sub-crater structure filled by below-ground deposits. These deposits occupy compound conduit structures, the largest found among all volcano types relative to their surface edifice. Based on common differences in geometry and internal architecture at different depths, maar-diatreme volcanoes are divided into three main volcano-structural levels: feeder dyke, diatreme structure, and tephra ring (Clement 1982; White and Ross 2011). The feeder dyke carries magma from its source toward the surface. The diatreme structure is specifically formed by fragmentation and mobilization of magma and country rock, and the primary deposits in it can be divided into those of a root zone and the diatreme (cf. White and Ross 2011). The tephra ring is the site of deposition for pyroclastic material ejected out of the maar crater onto the surrounding land surface.
Although maar-diatreme volcanoes worldwide of different magma compositions show generally very similar characteristics (White and Ross 2011), there is no consensus on what drives country rock excavation and how this volcano type evolves over the duration of an eruption to form a subterranean structure filled with well-mixed pyroclastic debris (e.g. Lorenz 1986; Sparks et al. 2006; Lorenz and Kurszlaukis 2007; Cas et al. 2008; Valentine 2012; Valentine and White 2012). It is important to better understand subvolcanic processes and the development of shallow plumbing systems in monogenetic volcanic fields, which are directly linked to surface activity including the inception of hazardous eruptions.
The data presented in this study were acquired by detailed volcanological mapping of vertical faces at metre-scale, with systematic lithological descriptions and quantitative clast counts of the individual mapped units, and by petrographic investigations of 42 representative samples from geological units identified in the diatreme massif. Clast counts, to determine component abundance, were acquired by 29 line counts (1 to 2 per unit) for 18 lapilli tuff units (see Electronic Supplement Materials (ESM) 1–3 for further details). They are supported by multi-scale image analysis and functional stereology on six samples that provide volumetric data from 0.5 mm to 150 cm by combining information from outcrop and thin-section images (after Jutzeler et al. 2012; see ESM 4–6 for further details). Volumes of the maar-diatreme volcano-structural levels, disrupted wall-rock, and wall-rock distribution within the volcano-structural levels were estimated using simple geometry calculations for an inverted cone. Information fed into the calculations comes from measurements of the diatreme at the current erosional levels, inferences based on other published maar-diatreme dimensions, and the relative proportions of wall-rock to juvenile material determined from clast counting (ESM 7 gives details).
Standing Rocks West volcanic architecture
Diatreme remnant geometry
Heterolithic country rock breccia
The country rock breccia is the oldest unit preserved in the diatreme structure (Fig. 2), and is cross-cut by the massif rocks and a NW–SE trending, poorly preserved, moderately to highly welded spatter dyke (cf. Lefebvre et al. 2012). The breccia is eroded to the present land surface and partly covered by scree. The breccia characteristically has a mixture of metre-sized blocks of Moenave Formation sandstone and Owl Rock Member wall-rocks sourced from shallower depths. No pyroclasts are apparent within the breccia.
Diatreme massif structure and componentry
The diatreme massif consists of two main types of pyroclastic rock: (1) subvertical lapilli tuff and tuff breccia columns that comprise most of the massif’s volume; and (2) layered lapilli tuff and tuff breccia (Fig. 2). Based on the subvertical perimeter of the diatreme massif, and visible contacts spanning ~10 m of topography, we infer that the external massif contacts are sharp, curvi-planar to slightly undulating, and subvertical to possibly upward flaring (~ 80°).
Subvertical lapilli tuff and tuff breccia columns
The lapilli tuff column deposits are considered typical of unbedded diatreme deposits (“lower diatreme” in White and Ross 2011) comprising inhomogeneously distributed and generally poorly sorted wall-rock clasts and pyroclasts. Individual units here are structureless to domainal, and matrix-supported to locally framework-supported. The units mostly lack fabric but some units have rare, crude and often steep alignment of elongate clasts. Decimetre- to metre-scale domains defined by differences in component abundance and grain size occur in several units, including local concentrations of blocks and bombs. Rare units contain isolated, discontinuous, millimetre- to decimetre-scale, internally massive, graded and cross-laminated layers, defined by a different grain size and/or component abundance compared to the host unit. These layered domains are often convolute and steeply dipping (~50–85°). In addition, some columns are cross-cut by narrower, discontinuous, irregular, subvertical lapilli tuff to tuff breccia domains that are distinguished by their framework-supported texture, coarser grain size, highly domainal grain size distribution, low matrix and high cement contents, better sorting, and relative wall-rock and juvenile abundance.
Layered lapilli tuff and tuff breccia
Four layered lapilli tuff units, cross-cut by subvertical lapilli tuff columns, occur as thin screens (less than a few metres thick) along the massif’s northern and eastern margins (Figs. 2 and 5). The largest preserved vestige (unit A33; ~125 m perimeter) occurs as several 10 to 50 m long remnants confined by country rock breccia or wall-rock. It is a layered, matrix-supported, closely packed, well-mixed, wall-rock-rich lapilli tuff and tuff breccia with a pink clastic matrix interpreted as Bidahochi Formation mudstone (Fig. 5d). The metre-scale layers, shallowly dipping predominantly towards the massif centre, are crudely defined by wall-rock clast abundance and size and have diffuse contacts (Figs. 4a and 5d). The wall-rock clast population differs from the other diatreme infill types in that Owl Rock and upper Petrified Forest Member clasts are relatively abundant. Rare country rock-rich beds exhibit normal grading (Fig. 5d).
Two layered, juvenile-rich, matrix-supported, closely packed lapilli tuff and tuff breccia units have similar component type and abundance to the lapilli tuff columns. Unit A35 truncates unit A33, and unit A38 overlies the lapilli tuff columns (Fig. 5a–c). The crude, discontinuous, metre-scale thick pyroclast-rich layers are marked by the abundance of blocks and bombs, and diffuse contacts. They dip at shallow angles toward the massif centre and have planar to irregular lenticular geometries (Fig. 5a–c).
By contrast, unit A14, a thin screen on the southeast massif margin (Fig. 2, 3c,d), comprises country rock-rich, matrix- to framework-supported tuff breccia, intercalated with matrix-supported, pyroclast-rich lapilli tuff (Fig. 5e). Layers are metre-scale in thickness and irregular, with lenticular geometries. The tuff breccia layers dominantly comprise blocks (~65–85 %; average size 20 cm, maximum 2 m), with each layer derived mostly from a single stratigraphic unit up section (i.e. the Moenave Formation or Owl Rock Member; Fig. 5e); there are rare flattened bombs moulded to their bases (Fig. 5f). The blocks are set in a finer-grained and juvenile-rich matrix similar to the juvenile-rich lapilli tuff layers and neighbouring lapilli tuff columns.
Juvenile and composite pyroclasts
All units of the massif contain juvenile and composite pyroclasts, ranging from extremely fine ash to bombs, with the majority of fine lapilli-size or smaller. The term “pyroclast” here is used for all primary volcaniclasts, formed by the fragmentation of magma by any volcanic event, including explosions that produced a crater or/and debris-jet activity, rockfall, and dyke injections. We justify this usage because the final emplacement of these primary clasts within the massif occurred from jets or fallback (White and Houghton 2006). These events took place during the single eruption inferred to have formed this volcano probably over days to years as observed for historical monogenetic eruptions (White and Ross 2011; calculated by Lorenz and Kurszlaukis 2007; other small-volume basaltic volcanoes, Valentine and Gregg 2008). We consider that deposits within a diatreme structure are primary unless demonstrably reworked by sedimentary processes, and apply the primary volcaniclastic terminology recommended by White and Houghton (2006).
Definitions of the five types of juvenile and composite pyroclasts classified based on internal structure and componentry
Homogeneous, massive particle consisting of groundmass ±phenocrysts
Heterogeneous juvenile particle with fluidal domains of different groundmass crystallinity and vesicularity; the most obvious are those of mingled microlitic sideromelane with either transitional or tachylite
Cored (cf. Fisher and Schmincke 1984)
Single proportionately large lithic clast or juvenile clast enclosed in one or more partial to complete rim(s) of juvenile material
Loaded (cf. Rosseel et al. 2006)
Proportionately small particles (lithic, juvenile) within juvenile pyroclast; pyroclast may be domainal, show internal welding
Fluidal clast with domains of interstitial matrix from the host deposit; includes fragments of peperite, i.e. from a zone of magma-host mingling on a millimetre- to centimetre scale (cf. McClintock and White 2006), but can also include clasts with sediment within vesicles or as domains between agglutinated clasts
Recycled juvenile and composite pyroclasts
Generic term for grains with evidence for repeated ejection of particles
Primary volcaniclasts did not escape vent or conduit at time they formed
= “recycled” of Houghton and Smith (1993); clasts have fallen into vent and been ejected again. Clasts may be abraded, broken, or coated.
Particles fell back into debris within a vent and were later entrained in a second eruptive jet or plume; cycle may repeat
A composite clast containing one (cored) or more (loaded) “recycled juvenile” clasts from the same eruption
Primary or reused pyroclasts that fell back into a vent and were incorporated into coherent magma subsequently fragmented to form new clasts (Rosseel et al. 2006 “recapitulated”; Lefebvre et al. 2012)
Between framework grains are clastic matrix (grains < 0.5 mm), cement and pore space. Generally, the massif deposits include varied amounts of cement and open pore space is rare. The matrix of the massif deposits mostly consists of juvenile pyroclasts, which are locally well-resolved to sizes smaller than 30 μm (Fig. 8f) and subordinate sandstone, siltstone and mudstone clasts, detrital quartz grains, single crystals of clinopyroxene, olivine and spinel, and unidentified, opaque microcrystalline material. By contrast, rare wall-rock-rich units contain a pale pink matrix similar to clayey Bidahochi Formation or Petrified Forest Member mud, and have a higher detrital quartz content. The cement in all exposed units consists of multiple generations of carbonate, phyllosillicate, and zeolite, which vary across millimetre- to several metre-scale domains throughout the massif irrespective of infill type. In many places these cements demonstrably replaced the clastic matrix.
Teshim tephra ring as a tephra ring proxy for SRW
The Teshim tephra ring, located in the northeastern part of the HBVF, was chosen as a proxy for the now eroded SRW tephra ring because it was previously studied by White (1991), and in general seems to be representative of tephra rings within the HBVF. Unlike some maars in the field, Teshim is a relatively simple, circular maar and its tephra ring strata are completely preserved where overlain by lava from a neighbouring volcano. It comprises a low, broad apron with a maximum height of 15 m proximal to the 850 m diameter maar crater and extends ~1 km away where the distal deposits terminate. White (1991) observed that the tephra ring consists of decimetre- to metre-thick lapilli tuff deposits, both cross-laminated and unbedded and interpreted as formed by pyroclastic density currents (ESM 12; White 1991, Fig. 2, p. 241). These deposits show an overall decrease in wall-rock to juvenile clast ratio up section with the lower deposits dominant in Bidahochi Formation mud and the upper deposits containing a comparatively high abundance of Moenave Formation sandstone clasts.
Interpretation and discussion
Significance of lapilli tuff columns
Detailed mapping substantiates the observation of White (1991) that the massif consists predominantly of multiple, commonly cross-cutting, subvertical lapilli tuff columns (Figs. 2 and 3), also recognized at many other unbedded diatreme localities worldwide (e.g. several South African kimberlites, Clement 1982; Coombs Hills, Antarctica, Ross and White 2006; Maegok volcano, Korea, Kwon and Sohn 2008). The SRW columns are interpreted as superimposed ephemeral-conduit deposits that formed from many individual active “debris-jets” (dispersed pyroclasts and wall-rock clasts, gases ± liquid water droplets). The jets were sourced from multiple discrete explosion sites, and propagated as expanding cavities through pre-existing pyroclastic debris in the diatreme; the later events left the strongest imprints (as inferred elsewhere by McClintock and White 2006 and Ross et al. 2008a, b). We interpret their irregular distribution to indicate shifting fragmentation sites that were not all located along a feeder dyke whose position was confined by country rock, and instead shifted both laterally and vertically within pyroclastic debris of the diatreme from earlier phases of eruption (e.g. Houghton and Nairn 1991; Valentine 2012). Intrusion of dykes into diatreme infill is consistent with observations elsewhere of dykes that have intruded into unconsolidated pyroclastic material with irregular morphology, small offshoots, and marginal peperite (e.g. Hooten and Ort 2002; McClintock and White 2006; Befus et al. 2009).
Gradual mixing, recycling and remobilization of pyroclastic debris within the diatreme
Non-bedded deposits, grain-by-grain mixture of wall-rock clasts from several different country rock stratigraphic levels, and many generations of juvenile pyroclasts within individual lapilli tuff columns are features shared by some other diatremes (e.g. Clement 1982; White and Ross 2011). The mixing process was not a sorting one, because fines remain prominent throughout the diatreme deposits except where obscured or replaced during diagenesis. Based on the componentry data for the lapilli tuff columns (Fig. 4) and the internal architecture of the massif, mixing must have been gradual and facilitated by multiple, small-volume explosions that repeatedly transported and emplaced small volumes of pyroclastic debris vertically and laterally including debris from earlier explosions (Valentine and White 2012).
The interpretation of multiple discrete fragmentation events (e.g. Ross and White 2006), rather than deposition from the waning of an eruption that had been dominated by sustained discharge of material (e.g. Sparks et al. 2006; Gernon et al. 2008; Porritt et al. 2008) is based on the following observations in combination. (1) The unbedded diatreme margins and the inner southeast region consist of multiple, randomly distributed, subvertical columns and planar domains that are inferred to make up most of the unbedded massif, as also seen in plan view at Coombs Hills (Ross and White 2006). Column boundaries are not mappable on the weathered upper surface of the massif, but deposit component mixtures and grain size appear indistinguishable from those in identified columns elsewhere in the massif. (2) Individual lapilli tuff columnar units were emplaced at different times, recorded by cross-cutting relationships where one column is partly enclosed by another, and contacts that truncate local clast alignments. (3) Individual columnar units have small volumes relative to that of the massif. (4) The lapilli tuff columns contain common recycled material. Further support that the maar-diatreme-forming eruptions involved repeated small-volume events, typical of other small-volume volcanoes (as shown by Valentine et al. 2012), includes the multiple decimetre- to metre-thick lapilli tuff layers comprising the tephra rings within the field (White 1991; Vazquez and Ort 2006; Newkirk 2009) and observations of such activity at rare historical eruptions over days to weeks (e.g. summarised in White and Ross 2011).
Small-scale heterogeneities seen at SRW have also been documented in other unbedded diatreme deposits and interpreted to indicate incomplete homogenization of within-structure deposits during an eruption (e.g. Clement 1982; Ross and White 2006; Brown et al. 2008; Kwon and Sohn 2008). Low-angle bedding and cross-lamination must have originally formed on a depositional surface open to the sky, such as the syn-eruptive crater floor or tephra ring; thus the randomly oriented, steeply dipping beds surrounded by unbedded pyroclastic deposits are not interpreted as in situ deposits. Some isolated layers may have been incorporated during closure of debris-jet cavities (cf. Ross et al. 2008a), which may have also caused local collapse of layered diatreme deposits.
The complexity of juvenile clasts observed at SRW suggests substantial remobilization, mixing and recycling of materials within the diatreme. Here, we consider recycled grains (Table 2) to include “reused” ones that have simply dropped back into the vent debris and then moved again (“recycled” clasts of Houghton and Smith 1993), and “reborn” ones that involved capture of an earlier-formed pyroclast into coherent magma, which was then fragmented again with the old pyroclast inside it (“recapitulated” clasts of Rosseel et al. 2006).
The origins of “recycled reborn” composite grains are incompletely understood, but involve delivery of original pyroclasts back into sites where they become incorporated into coherent, subsequently fragmented, magma. Such movement and mixing of debris within the diatreme can occur: (1) at and near the explosion site when debris driven into the opening jet-cavity sediments into it; (2) stratigraphically above the explosion site, when overlying debris is pushed up from below during cavity growth then flows inward and downwards during cavity collapse to capture mixed tephra during closure of the cavity walls (Lorenz and Kurszlaukis 2007; Ross et al. 2008b). To form the composite fragments, either coherent magma of a dike or pond, or juvenile pyroclasts sufficiently hot to weld back into coherent magma must be available in the mixing zone. This debris can include collapse material derived from crater floor or maar-rim ejecta (Lorenz 1986; Houghton and Smith 1993; McClintock and White 2006; Lorenz and Kurszlaukis 2007; Valentine 2012). Along with pyroclastic debris, condensed water from deflating debris-jet cavities, or from lofted debris that falls back into the syn-eruptive crater, may also be recycled back into the diatreme; this provides accessible water not directly coupled to wall-rock aquifer transmissivity or saturation (White and McClintock 2001; Valentine and White 2012).
All massif deposits at SRW have evidence for single- or multi-stage recycling of pyroclastic debris, as follows. (1) There are common ‘loaded’ composite pyroclasts containing dispersed older pyroclasts and/or wall-rock clasts from different stratigraphic levels. (2) Composite pyroclasts are abundant, and include kernels that are themselves ‘loaded’ pyroclasts, as well as multiply rimmed varieties. (3) There are “hypabyssal” holocrystalline lithic clasts that were broken from the feeder dyke or/and from dikes that extended from it into the diatreme fill. (4) Convolute domains comprising Bidahochi Formation mud with dispersed pyroclasts are sparsely present. (5) Some deposit domains have several different pyroclast types that were mixed on a grain-to-grain scale, with varied levels of grain palagonitization. (6) Wall-rock clasts, typically millimetre- to centimetre-size, from the Moenave and upper Chinle Formations are now hosted within the massif deposits tens to ~200 m below their original stratigraphic positions.
Many of the wall-rock clasts dispersed within discrete units throughout the diatreme originated from higher stratigraphic levels and are variably (but weakly) thermally altered, indicating that they experienced diverse events during emplacement. Wall-rock failure results in deposition either onto the syn-eruptive crater floor, or within an ephemeral cavity (Kurszlaukis and Barnett 2003; Brown et al. 2009). Wall-rock clasts within the diatreme structure can subside during debris-jet activity (e.g. White and Ross 2011) or be transported upward during emplacement of debris-jet deposits (Ross and White 2006).
Evidence for low magma flux during diatreme formation
Relatively common composite pyroclasts within the juvenile-rich lapilli tuff columns comprising the SRW massif suggest low magma flux during diatreme formation because in a high-flux eruption most clasts are carried high in a plume or dispersed in lateral currents, thus becoming unavailable for any form of recycling in the vent. The paucity of wall-rock clasts (<10 %, mostly from higher stratigraphic levels) implies regions of considerable “substitution” of wall-rock material during diatreme formation, and also of repeated magma fragmentation away from the walls of the diatreme structure (Ross and White 2006; Valentine and White 2012). The fact that composite, mostly loaded, pyroclasts are common demonstrates that the repeated explosions were accompanied by significant recycling. However, determining the volumes of recycled pyroclasts versus primary juvenile pyroclasts involved in an explosion is exceptionally difficult, because different simple pyroclast generations can be chemically indistinguishable, particle reuse leaves little or no signature in lithified deposits, and textural inhomogeneities are common in conduits (Houghton and Gonnermann 2008). At other localities where differentiation is possible, the proportion of juvenile pyroclasts added during a fragmentation event appears to be low when there is recycling in the form of reuse, about 10–30 % (Houghton and Smith 1993; Lorenz et al. 2002; Ross and White 2006). Multiple small-volume eruptive bursts are also reflected by the multiple thin tephra ring layers (White 1991; Vazquez and Ort 2006), and are consistent with expected flux of magma through thin (<1 m) feeder dykes (Hooten and Ort 2002; Lorenz and Kurszlaukis 2007). Thus, it is likely that each event produced a small volume of pyroclasts, implying a low rate of magma supply, which is consistent with the general behavior of small-volume monogenetic mafic volcanoes with magma fluxes on the order of a cubic metre per second (Valentine and Perry 2006; Valentine and White 2012).
Fragmentation sites appear to have lain mainly within the middle parts of the western portion of the diatreme, below the current exposure level, during the formation of massif deposits. This is inferred because there were no feeder dykes observed within the massif, there is little wall-rock material from below the massif, and the individual lapilli tuff columns comprising the massif are interpreted as conduit remnants that extend below the current surface. The massif deposits are interpreted to have formed in the main part of the eruption represented here, after emplacement of the almost juvenile-free country rock breccia. Each eruptive burst from closely spaced sites within earlier-formed diatreme debris would have added an increment of primary juvenile pyroclast material and helped recycle older clastic material, leading to high concentrations of juvenile material in the massif. Such enrichment, particularly in recycled pyroclasts, is not a predicted result of explosion sites that progressively excavate downward into wall-rock (e.g. Lorenz and Kurszlaukis 2007).
Distribution of wall-rock debris within a maar-diatreme volcano
Surface and diatreme dimensions, and volume calculations for Standing Rocks West volcano
Volume of maar-diatreme volcano-structural levels
Diatreme structure (total volume of disrupted wall-rock)
Country rock breccia
Volume of disrupted individual wall-rock units of the maar-diatreme structure
Owl Rock Mbr
Petrified Forest and Shinarump Mbr
Volume of wall-rock within the maar-diatreme pyroclastic deposits:
Country rock breccia
Wall-rock volume budget
Total wall-rock accounted for
Total wall-rock not accounted for (distal deposits)
Magma volume budget
The total volume of wall-rock disrupted by formation of the diatreme structure should equal the sum of wall-rock still in the diatreme plus that in the tephra ring and distal fall deposits just after the eruption; the crater is empty by definition. We calculate (Table 3) that ~70 % of the total disrupted wall-rock volume was ejected from the diatreme structure with ~45 % in the tephra ring and ~30 % returned to or remaining within the diatreme; 25 % is unaccounted for and interpreted to have been dispersed downwind in unmapped and probably unpreserved fall deposits. Distal fall deposits from historical maar-diatreme eruptions extend kilometres to tens of kilometres from their sources (e.g. Houghton and White 2000; Lorenz and Kurszlaukis 2007); a 40 km × 10 km fall deposit with volume 31 × 105 m3 would average less than 1 cm thick and be easily overlooked even in young deposits.
Distribution in the diatreme and tephra ring of fragments from different wall-rock strata
Comparison of calculated disrupted wall-rock volumes with volumes of wall-rock in diatreme
Volume of disrupted wall-rock (×105 m3)
Volume of disrupted wall-rock units in the tephra ring (× 105 m3)
Volume of disrupted wall-rock units in the country rock breccia (×105 m3)
Volume of disrupted wall-rock units in the diatreme massif (×105 m3)
Volume of disrupted wall-rock units not accounted for (×105 m3)
Owl Rock Mbr
Wall-rock units below the Owl Rock Mbr
Clasts in both the SRW massif and in the country rock breccia are mostly derived from parts of the country rock stratigraphy that originated above the present erosional surface (depth interval ~75–245 m), but from deeper levels than observed in the Teshim tephra ring (<75 m). Most massif and country rock breccia clasts are from the Moenave Formation (54 %) and the Owl Rock Member (31 %). The wall-rock-fragment population of the massif differs from that of the country rock breccia in containing both rare clasts from the Bidahochi Formation, and from strata originating below ~245 m. About 40 % of the disrupted Moenave Formation remained in the diatreme as wall-rock collapse deposits (country rock breccia), with less than 5 % incorporated into the pyroclast-rich massif. In addition, most Owl Rock Member debris remained within the diatreme at or below its original stratigraphic level. Some wall-rock material from depth may have been transported upwards beyond current exposure levels (Fig. 9) and deposited within the diatreme structure, but those deposits are not preserved, leaving the material unaccounted for.
Both the SRW diatreme massif, and the Teshim tephra ring contain mostly wall-rock material that originated from above Standing Rocks West’s present exposure level, with little from the deeper strata. This indicates that the majority of the wall-rock within the diatreme arrived at its present location by moving downward, and supports the interpretation that space generated at shallow depths facilitates wall-rock collapse. The scarcity of Bidahochi Formation within the diatreme suggests most was ejected early on and remained at or near the surface, though some could be ‘hidden’ as clay, sand and silt grains in clastic matrix of different SRW units.
The downward tapering diatreme shape does not, on its own, explain the observed paucity of disrupted wall-rock from below. Wall-rock clasts in the tephra ring and diatreme at the level of observation (~300 m depth) that were sourced from below ~245 m represent only ~10 % by volume of the diatreme structure below that depth. If conventional diatreme geometries are assumed, wall-rock was disrupted to depths below ~700 m (Fig. 9; Table 3), so about 400 m of deep stratigraphy appears under-represented. Possible explanations are that the diatreme structure rapidly narrows below the level of SRW exposure, which seems unlikely, or that material from depth was rarely transported significantly upwards. The latter is likely as large volumes from these depths are unknown to the authors at higher elevations elsewhere in the field. This implies that most of the diatreme below the current exposure level is dominated by locally derived country rock-rich breccia. If this is the case, there must have been several main sites of fragmentation with one, ejecting material for the tephra ring, lying above current exposure levels, a second producing pyroclast-rich material, lying somewhat below the current level and forming the massif, and another one deeper, generally below 300 m, breaking and mixing material that only occasionally reached shallower levels. Explosions generated below 300 m contributed to growth of the diatreme structure, but provided only scarce debris to higher levels and did not involve large volumes of magma.
The difference in wall-rock proportions between the diatreme and tephra ring, and the high proportion of pyroclasts in the diatreme massif, indicates that the tephra ring does not provide a full record of a maar-diatreme’s eruptive history. The tephra ring was built largely by early, shallow explosions, perhaps later added to by deeper ones with much higher energies (e.g. Ross et al. 2008b; Valentine and White 2012). By contrast, the diatreme massif is inferred to reflect later, deeper stages of ‘eruption’, when explosive energy was typically insufficient to eject material from depth onto the palaeosurface. The country rock breccia predates the massif, and comprises material almost entirely dropped down from higher levels. Taken as a whole, we infer that most products of SRW’s ‘eruptive’ activity remained within the diatreme structure. The overall estimated magma volume is low (~7 × 106 m3; Table 3), as at other small-volume volcanoes (e.g. Brown et al. 2012; Mattsson 2012; Valentine 2012).
The well-exposed Standing Rocks West (SRW) diatreme, together with the tephra ring proxy (Teshim) offer valuable insight into the emplacement of well-developed, unbedded, juvenile-rich diatremes deposits. SRW was formed by multiple, small explosive events, mostly confined to the diatreme structure, fed by a low flux of magma. There was no direct link between the volcano-structural “levels” (i.e. root zone, diatreme and tephra ring) and the sites of fragmentation, transportation and deposition. In other words, the root zone, as the connection between the feeder dyke and diatreme, was not the main and only site of fragmentation, the diatreme did not solely act as a conduit and collapse structure, and the tephra ring is not the repository of deposits from all explosive events.
We gratefully acknowledge M. Jutzeler for his guidance and use of his image analysis Excel spreadsheets, and A. Proussevitch for processing the data using his statistical functional stereology software. We thank B. Lefebvre for invaluable field assistance, P-S Ross, R. Brown and S. Moss for their thoughtful comments on an earlier version of the manuscript, and G. Valentine, B. Brand and an anonymous reviewer for constructive feedback on this version. This work was supported by a Society of Economic Geologists Student Grant, Sigma Delta Epsilon Graduate Women in Science Fellowship, Commonwealth PhD Scholarship to N. Lefebvre and GNS Science funding to J.D.L. White. Field work on the Navajo Nation was conducted under a permit from the Navajo Nation Minerals Department. Any persons wishing to conduct geological investigations on the Navajo Nation must first apply for, and receive, a permit from the Navajo Nation Minerals Department, P.O. Box 1910, Window Rock, Arizona 86515, USA telephone: 01 (928) 871–6587.
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