Abstract

Unusually thick accumulations of diatomite and diatomaceous mud- and siltstones occur throughout the Mio-Pliocene along the Pacific margin. These extensive micro-fossil assemblages represent an important biostratigraphic resource, record of environmental conditions, and are economically relevant as unconventional reservoirs, sources and seals. Most depositional models for diatomaceous sediments tend to emphasize diatom productivity sufficient to overcome siliciclastic dilution and reflect a low-energy depositional regime with passive settling of diatoms through the water column. However, the diatomaceous sediments of the Pisco Basin, Peru, have been observed in longitudinally bedded, wavy, non-parallel laminated deposits, interpreted as hummocky-swaley cross-stratification. Other outcrop observations include tempestite-like sequence stacking, normal grading, and syndepositional soft-sediment deformation, suggesting a bedload orgin for the Pisco diatomite under a combined-flow regime and storm-like conditions. Such diatomite can also be found associated with fossil marine vertebrate burial. This study sought to investigate these findings at the level of micro-texture though thin section imaging, SEMs of freshly parted rock surfaces, and micro-CT modeling. Notable textures observed include graded micro-couplets containing a normally graded xii silt component and reverse graded diatomaceous component, predominantly observed in hummocky-swaley cross-stratified diatomite and swale drapings. In a stacked channel complex of interlaminated diatomite-siltstone, wave-enhanced sediment gravity flows were found as a graded triplet capped in purest diatomite. These suggest a bedload origin for much of the Pisco’s diatomaceous sediments, and mirror textures previously derived in mudstone flume experiments, further suggesting that diatoms may entrain, flocculate, and accumulate in a manner effectively similar to clays. These findings also call into question the diatomite necessarily reflects a low-energy to passive depositional regime. Likewise, with respect to the Pisco Basin, it remains probable that much of its fossil content encased in diatomaceous sediments met their fate under a tractional regime rather than due to the slow-settling of diatoms and their components.

LLU Discipline

Geology

Department

Geology

School

School of Medicine

First Advisor

Kevin E. Nick

Second Advisor

V. Leroy Leggitt

Third Advisor

Gina Roque-Torres

Degree Name

Master of Science (MS)

Degree Level

M.S.

Year Degree Awarded

2020

Date (Title Page)

4-2020

Language

English

Library of Congress/MESH Subject Headings

Peru; Diatomite

Type

Thesis

Page Count

xii, 66 p.

Digital Format

PDF

Digital Publisher

Loma Linda University Libraries

Usage Rights

This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.

Collection

Loma Linda University Electronic Theses and Dissertations

Collection Website

http://scholarsrepository.llu.edu/etd/

Repository

Loma Linda University. Del E. Webb Memorial Library. University Archives

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