40Ar/39Ar whole-rock data constraints on Acadian diagenesis and Alleghanian cleavage in the Martinsburg formation, eastern Pennsylvania

American Journal of Science
By: , and 

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Abstract

A comparison of 40Ar/39Ar age spectra of whole-rock mudstone and slate samples from the Ordovician Martinsburg Formation at Lehigh Gap, Pennsylvania and stratigraphic and thermal constraints support Alleghanian age for regional slaty cleavage and a late Acadian age for diagenesis in these rocks. Age spectra from mud-stones have a sigmodal shape, with slopes that climb steeply from apparent Mesozoic ages to intermediate saddle regions with Devonian apparent ages, and then climb steeply again to Late Proterozoic apparent ages. The steps with these oldest apparent ages are interpreted to be dominated by late Proterzoic detrial muscovite. The saddle region of the mudstone samples gives very Late Silurian to earliest Devonian ages, which are maximum ages of diagenetic micas and which eliminate a Taconic age for the cleavage. The ages of saddle regions of the slate samples constraining cleavage-forming muscovite is <~375. This is the maximum age of this mica and requires an Alleghanian age for the cleavage. These age constraints are supported by ages of individual mica components calculated with knowledge of the total gas ages and mass fractions of the micas and by predictions from thermal modeling. We conclude that the Taconic orogeny in the Martinsburg Formation in eastern Pennsylvania was a very mild event. Not only is the cleavage in these rocks not Taconic in age, but even the mild (~100C) diagenetic growth of illite was Silurian of younger. Thus the Taconic event in these rocks in limited to loading of lass than about 3 km.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title 40Ar/39Ar whole-rock data constraints on Acadian diagenesis and Alleghanian cleavage in the Martinsburg formation, eastern Pennsylvania
Series title American Journal of Science
DOI 10.2475/ajs.296.7.766
Volume 296
Issue 7
Year Published 1996
Language English
Publisher International Earth Science Journal
Contributing office(s) Eastern Geology and Paleoclimate Science Center
Description 23 p.
First page 766
Last page 788
Country United States
State Connecticut, Delaware, Maryland, Pennsylvania, New York, New Jersey
Other Geospatial Martinsburg Formation
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