Discovery of a Trachyte Ignimbrite Sequence at Hualālai, Hawaii
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Bull Volcanol (2016)78:34 DOI 10.1007/s00445-016-1027-2 SHORT SCIENTIFIC COMMUNICATION Discovery of a trachyte ignimbrite sequence at Hualālai, Hawaii Thomas Shea1 & Jacqueline Owen2 Received: 2 December 2015 /Accepted: 29 March 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract Ignimbrites are common in many intraplate Keywords Pyroclastic density currents . Ignimbrites . ocean islands but have been missing from the known geo- Hawaii . Hualālai volcano . Trachyte logical record in Hawaii. During a recent field campaign, the remnants of a trachytic ignimbrite sequence have been discovered at Hualālai volcano, fortuitously preserved Introduction from subsequent basaltic lava flow cover. We provide a preliminary description of these deposits, as well as bulk Ignimbrites (here defined loosely as pyroclastic density cur- and glass chemical analyses to determine their potential rent BPDC^ deposits consisting dominantly of a poorly sorted relationship with other nearby trachytes from Pu’u mixture dominated by juvenile pumice and ash, e.g., Branney Wa’awa’a(PWW)andPu’u Anahulu (PA). The results and Kokelaar 2002) are widespread along arc, back-arc, and suggest that these ignimbrites are from neither PWW nor continental intraplate regions that erupt evolved magmas (e.g., PA, but instead may relate to trachytes that are found as phonolites, trachytes, rhyolites). Large ignimbrites have also maar wallrock blocks some 20 km distant. Therefore, de- been identified at several oceanic intraplate settings, including spite being rare overall in Hawaii, the ignimbrites—and at the Canary Islands (Freundt and Schmincke 1995), the more generally trachytes—were probably widespread Azores (e.g., Duncan et al. 1999), and Cape Verde (Eisele around Hualālai. Compared to other intraplate ocean et al. 2015). Along the Canary Islands, this type of volcanic islands, the combination of a fast-moving plate, high mag- activity has produced many hundreds of cubic kilometer of ma supply, and eruption rates underneath Hawaiian volca- deposits. Evolved magmas typically erupt in intraplate ocean- noes may explain the scarcity of ignimbrites preserved at ic islands during post-shield volcanism. Their origin is often the surface. Their presence at Hualālai could reflect unusu- attributed to fractional crystallization (±replenishment) of al conditions of edifice stress during the transition from alkalic basalts that produces phonolites or trachytes, and/or shield to post-shield volcanism. mixing of basalts with assimilated sediments or crustal intru- sions, which typically yields rhyolites (Freundt and Schmincke Editorial responsibility: K.V. Cashman 1995; Freundt-Malecha et al. 2001;HansteenandTroll2003; Edgar et al. 2007;Sigmarssonetal.2013). Electronic supplementary material The online version of this article In Hawaii, exposed volcanic sequences involving evolved (doi:10.1007/s00445-016-1027-2) contains supplementary material, which is available to authorized users. magmas are comparatively rare at the surface. Exceptions in- clude the voluminous Pu’u Anahulu-Pu’uWa’awa’atrachyte * Thomas Shea flow and cone association at Hualālai, Hawaii (Stearns and [email protected] Macdonald 1946), trachyte domes and flows on West Maui (e.g., Pu’u Koae, Mt Eke, Pu’u Launiupoko, Stearns and Macdonald 1942;Velde1978), and the rhyodacitic Mt. 1 Department of Geology and Geophysics, SOEST, University of Kuwale flow on Oahu (Van der Zander et al. 2010). To date, Hawaii, 96822 Honolulu, HI, USA however, no ignimbrites have been identified, in contrast with 2 Lancaster Environment Centre, Lancaster University, Lancaster, UK other intraplate sites such as the Canary Islands. Pyroclastic 34 Page 2 of 8 Bull Volcanol 78:34 (2016) density current deposits recognized at Kilauea are distinct in During recent field work, we identified a previously being basaltic and lithic-rich/juvenile-poor, with an inferred undescribed tephra sequence on top of the Pu’u Anahulu flow. phreatomagmatic origin (McPhie et al. 1990; Swanson et al. This sequence was preserved from erosion and weathering at 2012). In this brief communication, we describe the first dis- only a few sites (nine locations are described here, cf. Fig. 1), covery of ignimbrite deposits in Hawaii, at Hualālai volcano. although the base of the deposit still blankets the flatter por- We briefly speculate on the potential for other similar deposits tions of some of the Pu’u Anahulu flow lobes. The thickest to be buried underneath the recent lava flow cover and the section (preserving the most comprehensive record of the ig- hazard implications of such eruptions for Hawaii. nimbrite deposits) was trenched about 1.5 km from the nearby Pu’uWa’awa’a trachyte cone (location 1) (Fig. 2a). The ignimbrite deposits were subdivided into four major eruptive units (EU1-EU4) based on abrupt changes in Description of deposits lithofacies relationships that could be recognized between dif- ferent outcrops. The grouping into different eruptive units re- The Pu’u Anahulu (PA) trachyte flow on the north flank of flects each inferred flow cycle. The Pu’u Anahulu flow under- Hualālai is a topographically prominent feature (≤260 m from neath the tephra sequence usually consists of holocrystalline base to top) that has been linked to the nearby trachyte pyroclastic (microlites and microphenocrysts) trachyte blocks 10–50 cm cone of Pu’uWa’awa’a (PWW; Fig. 1; Stearns and Macdonald in size, enclosed within either (1) a weathered mixture of ash 1946; Cousens et al. 2003; Shamberger and Hammer 2006). The to coarse ash-sized trachyte grains and soil, or (2) a basal Pu’u Anahulu and Pu’uWa’awa’a trachytes are >100 ka (Clague pumice-bearing ignimbrite (massive lapilli tuff lithofacies, 1987; Cousens et al. 2003), and by far the oldest subaerial mLT) containing disrupted lenses of soil, which are interpreted Hualālai rocks exposed (Hualālai’s next youngest exposed flows have been remobilized by the ignimbrite (Figs. 2a and 3c). This are ∼13 ka, Moore et al. 1987). ignimbrite (eruptive unit EU1) has a fine ash matrix and a low KO MK ML >11ka ML 3-5ka HL ML KIL HL 5-11ka Huehue 8 group Rift zone 9 6 7 5 Pu’u Anahulu flow Hualālai Waha Pele group 4 5 km 3 ML 1.5-3ka 2 Legend HL >11ka Mauna Loa Hualālai (ML) (HL) basalts alkali basalts Pu’u Wa’awa’a cone 1 ML 1859 Hualālai vents <13 ka Pu’u Wa’awa’a-Pu’u Anahulu HL 3-5ka Trachytes HL 1.5-3ka Other Hualālai wells cones trachyte locations N Minimum inferred extent of Pu’u Anahulu flow Hualālai summit Ignimbrite outcrops 2 km Fig. 1 Geological setting of Hualālai ignimbrites. (Left) Digital elevation wells in which trachyte was recovered (BHuehue group^) (cf. Cousens model of Hualālai volcano with prominent trachyte exposures (Pu’u et al. 2003). (Right) Geological map of the Pu’uWa’awa’a-Pu’u Anahulu Wa’awa’a cone and Pu’u Anahulu flow) on northern flank. Yellow stars area (modified from Sherrod et al. 2007). Only certain areas of the Pu’u show the location of other trachyte blocks found as lithics in more recent Anahulu trachyte remain uncovered by <13 ka post-shield activity. basaltic products (BWaha Pele group^), red stars mark the location of Numbers refer to main stratigraphic locations described in the text Bull Volcanol (2016)78:34 Page 3 of 8 34 cm a LITHOFACIES DESCRIPTION b massive pumice lapilli tuff pumice lapilli unit soil coarse ash/fine lapilli unit dark-clast-rich unit Post-ignimbrite deposits unit w/ color gradient trachyte blocks/flows basaltic tephra SSS soil/weathered material Basal bed comprises coarse iii strongly indurated mLT/paleosoil pumice ash with variable Location 1 150 EU4 reddish coloration both at SSS diffuse stratification mpLT bottom and top. Grades into weathered ignimbrite/paleosoil. EU4 Legend diffuse cross stratification marked cross stratification Unit consists of tripartite basal erosive surface mLT-mLTpip lithofacies with grainsize 2 variations (slight normal then SSS gradual transition inverse grading), with top bed iii EU3 indurated, surrounded by dark tuff that propagates into top ignimbrite via pipe structures. EU3 Top third of massive lapilli tuff iii facies is indurated. 100 dsLT iii 5 iii Base of unit is a coarse pumice SSS 7 3 iii SSS 8 ash-rich series of beds iii displaying diffuse stratification iii SSS SSS mLT and bedding, with iii iii EU2b pinching-swelling. Like units EU5? below, grades into ignimbrite EU2b iii dspLT with increasing induration 9 iii towards the top. SSS iii iii Unit marked by pinch and swell mLT cross-stratified beds with highly iii variable grainsize and a few 4 pumice-rich beds. High SSS 50 EU2a iii 6 iii abundance of dark/dense iii xsLT material overall. Grades into EU2a ignimbrite towards the middle. SSS iii Ignimbrite unit with slight iii iii reverse grading of pumice iii iii iii grains and frequent ash/tuff iii iii lenses with different lithologies iii mLT-lensT EU1 in lower half (pumice ash, obsidian+pumice ash, soil pods). Change in color from EU1 brown at base to light grey/yellowish, and to tan at top. 0 Indurated towards the top. SSS SSS SSS SSS SSSSSS SSS SSS SSS Pu’u Anahulu trachyte flow PA Location 1 Fig. 2 a Stratigraphic column of pyroclastic density current deposits at explanation). b Variations in PDC stratigraphy across the different location 1. Lithofacies nomenclature used follows Branney and Kokelaar locations, ordered arbitrarily by distance from the Hualālai summit. (2002), m = massive, LT = lapilli-tuff or lapilli-ash, p = pumice, Stratigraphy and lithofacies are here represented as simplified patterns, ds = diffuse-stratified, xs = cross-stratified, lensL = pumice lapilli lens. along with information pertaining to lithofacies contact, state of Lithofacies were grouped into eruptive units EU1-EU4 (see text for induration, and stratification abundance of pumice lapilli that reach ∼3–4 cm in size at location distinctive from other units due to slight changes in color 1.