Epicontinental ironstone accumulation during the end-Ordovician glaciation and extinction events
Loading...
Authors
Malone, Jackson
Date
Type
thesis
Language
eng
Keyword
Ordovician , Biogeochemical cycling , Ironstone , Paleoceanography , Glaciation , Mass extinction
Alternative Title
Abstract
The Upper Ordovician Neda Formation, Midwest, USA, is composed of hematitic
dolostone and ironstone that rest conformably on crinoid and bryozoan wackestone to packstone
of the underlying Maquoketa Formation. Deposition occurred within the Maquoketa Sea south of
the Transcontinental Arch between 15 and 20°S. Where not eroded, the top of the Neda
Formation is a subaerial laterite that formed as sea-level fell during the Hirnantian Glaciation.
This unconformity is the Ordovician-Silurian boundary and records a regression of at least 100 m
and nearly two million years of exposure.
Lithofacies associations indicate deposition began on a storm-dominated ramp during a
minor transgression, that through coastal upwelling emplaced an anoxic water mass, shutting
down carbonate production. Upwelling of ferruginous and euxinic bottom waters derived from
the Iapetus Ocean produced depauperate, Fe-rich mudstone and granular ironstone. Mixing with
oxygenated surface water is interpreted to have precipitated Fe-(oxyhydr)oxide in the water
column that accumulated as Fe-rich mud on the seafloor. In shallower environments, advection
of Fe away from the upwelling front and Fe-redox pumping beneath the sediment-water interface
produced granular ironstone. Granular ironstone is composed of abundant coated Fe grains
preserving both concentric, redox-aggraded cortical layers and those that are erosively truncated,
which record periods of exhumation, reworking, and reburial beneath the seafloor into the zone
of authigenic precipitation.
It is becoming clear that sedimentologic and geochemical data suggest that Paleozoic
ironstones are the product of upwelling ferruginous water masses rather than Fe derived from
continental weathering. An increase in the equator-to-pole temperature gradient and concomitant
reorganization of thermohaline circulation during Hirnantian Glaciation likely intensified coastal
upwelling and drew ferruginous seawater into the Maquoketa Sea to produce ironstone. Such
expansion of anoxic conditions supports recent research suggesting the onset of shallow anoxia
in other basins contributed to the second pulse of the end-Ordovician mass extinction.
Widespread anoxia probably persisted into the early Silurian.
Description
Citation
Publisher
License
Queen's University's Thesis/Dissertation Non-Exclusive License for Deposit to QSpace and Library and Archives Canada
ProQuest PhD and Master's Theses International Dissemination Agreement
Intellectual Property Guidelines at Queen's University
Copying and Preserving Your Thesis
This publication is made available by the authority of the copyright owner solely for the purpose of private study and research and may not be copied or reproduced except as permitted by the copyright laws without written authority from the copyright owner.
ProQuest PhD and Master's Theses International Dissemination Agreement
Intellectual Property Guidelines at Queen's University
Copying and Preserving Your Thesis
This publication is made available by the authority of the copyright owner solely for the purpose of private study and research and may not be copied or reproduced except as permitted by the copyright laws without written authority from the copyright owner.