Elutriation pipes in ignimbrites: An analysis of concepts based on the Huichapan Ignimbrite, Mexico

Abstract

The upper portion of the Huichapan Ignimbrite (UHI), central Mexico, is a non-welded massive ignimbrite characterized by large and well-developed fines-poor elutriation pipes. We conclude that the most important pipe-forming process in the Huichapan Ignimbrite was deposition-triggered aggregative-type segregation, and this process is enhanced by positive pore-pressure feedback during the rapid settling of the pyroclastic material. In experiments, pumice concentration zones and elutriation pipes have been formed in separated portions of the deposits. Contrary to the experimental results, in the UHI both pumice concentration zones and elutriation pipes have been observed alongside one another within the deposit. We interpret this association to have formed during time periods of unusually high deposition rate within a long-lived, sustained pyroclastic density current system. Elutriation pipes were formed because of low shearing and vertical channeling of gases, and the pumice concentration zones formed because of pumice segregation during hindered settling. Nevertheless, the deposition rate was so high that the segregated pumice was “frozen” before it could bypass, within the deposit, right next to the elutriation pipes. We present a classification scheme of five types of elutriation pipes based on our field observations in the UHI and a review of field and experimental work from other authors: 1) tubular pipes; 2) irregular tubular pipes; 3) anastomosing pipes; 4) pinch-out pipes; 5) deflected pipes. Irregular tubular pipes are the most common in the volcanic geological record and they form because of deposition-triggered aggregative-type segregation and/or post-depositional upward fluid flow. Likely, anastomosing elutriation pipes are a unique product of deposition-triggered aggregative-type segregation. A comparison between the lower and upper portion of the Huichapan Ignimbrite leads to the conclusion that deposition-triggered pipe formation is enhanced by poor sorting, low shearing, and presence of large clasts (>−5Φ, 32 mm). Pipe-free ignimbrites in the volcanic geological record likely come from relatively better-sorted dispersions with few coarse clasts, but more importantly, strong shearing during deposition may be the main processes that diminish pipe formation.