Morphological and textural diversity of the Steens Basalt lava flows,Southeastern Oregon,USA: implications for emplacement style and nature of eruptive episodes |
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Authors: | Ninad R Bondre William K Hart |
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Institution: | (1) Nature Geoscience, Nature Publishing Group, 4 Crinan Street, London, N1 9XW, UK;(2) Department of Geology, Miami University, 114 Shideler Hall, Oxford, OH 45056, USA |
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Abstract: | This study focuses on Middle Miocene tholeiitic flood basalt lava flows from the Oregon Plateau, northwestern USA (Steens
Basalt), and is the first to comprehensively document and evaluate their morphology. Field observations of flows from several
sections within and proximal to the main exposures at Steens Mountain have been supplemented with textural and geochemical
data, and are used to offer preliminary insights into their emplacement. Compound pahoehoe flows of variable thickness appear
to be common throughout the study area, laterally and vertically. These tend to be plagioclase phyric and the morphology and
disposition of constituent flow lobes are quite similar to those from other provinces such as Hawaii and the Snake River Plain.
Classic a’a flows with brecciated upper and basal crusts are not abundant, but by no means uncommon. Flows with characters
different from typical pahoehoe and a’a are also common. Such flows display a range in morphology; flows with preserved upper
crusts but brecciated basal crusts, as well as those displaying well-developed flow-top breccias and preserved basal crusts
(rubbly pahoehoe) are observed. The Steens Basalt appears to display greater morphological and textural diversity at the outcrop
scale than that described for some other flood basalt provinces. The abundant compound pahoehoe flows (often rich in plagioclase
phenocrysts) were likely emplaced during slow but sustained eruptive episodes; their constituent lobes show clear evidence
for endogenous growth. The relatively aphyric flows with brecciated surfaces (including a’a) hint at higher strain rates and/or
higher viscosity, probably caused by higher effusion rates. A couple of sections are characterized by compositionally similar,
but morphologically different flows that were possibly part of the same eruption. While differences in pre-eruptive topography
could explain this, it is also possible that certain physical parameters changed substantially and abruptly during eruption
and that such changes were accompanied by differentiation processes within the plumbing system. It is possible that such observations
indicate temporal fluctuations within complex magmatic and eruptive systems, and deserve closer scrutiny. |
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Keywords: | Steens Basalt Lava flows Morphology Texture Emplacement Pahoehoe A’ a |
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