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Title:Origin and stratigraphic significance of limestone discontinuity surfaces, Ordovician (Whiterockian) of Nevada
Author(s):Siewers, Fredrick Deschweinitz
Doctoral Committee Chair(s):Sandberg, Philip A.
Department / Program:Geology
Discipline:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Geology
Abstract:Discontinuity surfaces in Ordovician (Whiterockian) limestones of central and southern Nevada have been studied to: (1) establish new, general criteria for their recognition in Lower Paleozoic limestones; (2) constrain factors influencing their development in various depositional settings; (3) evaluate their use in stratigraphic correlations; and (4) examine their chronostratigraphic and sequence stratigraphic significance. The discontinuities studied include coastal paleokarst surfaces, submarine hardgrounds, and submarine omission surfaces, differentiated by their surface features, facies associations, and, in some examples, geochemical relationships. Coastal paleokarst surfaces in Lower Paleozoic limestones are distinguishable from submarine hardgrounds by slight depletions ($\le1\perthous$) in $\delta\sp{18}$O content of grain and cement components below surfaces relative to $\delta\sp{18}$O content of similar components in overlying lithologies.
In the Ordovician limestones examined, discontinuity surfaces formed whenever synsedimentary lithification and erosion were essentially in equilibrium. With the exception of omission surfaces, discontinuity lithification occurred by early marine cementation, driven either by wave and tidal pumping or by oceanic current pumping. Locally, lithification was controlled by the degree of substrate stability, mineralogical composition of the grain components, and the depositional porosity-permeability of the sediments. Regionally, discontinuity surface formation was governed by relative sea level fluctuations which regulated the absolute space available for sediment accumulation (accommodation space). Variations in accommodation space, in turn, controlled processes of near-surface porewater circulation and lithification, influenced the degree of synsedimentary sea floor erosion, and regulated cyclic phosphogenesis in some deep-water settings.
Results indicate that discontinuity surfaces enhance correlations between sections only after a well-established biostratigraphic zonation is in place. Chronostratigraphically significant discontinuities parallel biozone boundaries or occur in roughly equivalent abundances within zones. Diachronous surfaces may demonstrably converge into single discontinuity surfaces in other sections. Diachronous surfaces appear, in part, to be a function of topographic irregularities within the depositional system.
Spatially widespread facies successions bounded by closely-spaced discontinuity surfaces define an "accommodation minimum zone" which is interpreted as the stratigraphic expression, in the study area, of the Sauk-Tippecanoe unconformity. Contrary to some proposed interpretations of Ordovician sea level history, closely spaced discontinuity surfaces suggest a relative sea level lowstand during the latest Ibexian-earliest Whiterockian prior to an Early Whiterockian relative sea level rise.
Issue Date:1995
Type:Text
Language:English
URI:http://hdl.handle.net/2142/20273
Rights Information:Copyright 1995 Siewers, Fredrick Deschweinitz
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9522175
OCLC Identifier:(UMI)AAI9522175


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