Open AccessRe‐Os age and depositional environment for black shales from the Cambrian‐Ordovician boundary, Green Point, western Newfoundland
Author(s) -
Tripathy Gyana Ranjan,
Hannah Judith L.,
Stein Holly J.,
Yang Gang
Publication year - 2014
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1002/2013gc005217
Subject(s) - geology , ordovician , sedimentary depositional environment , global boundary stratotype section and point , laurentia , baltica , isochron , paleontology , oil shale , gondwana , geochemistry , radiometric dating , trace element , authigenic , diagenesis , biostratigraphy , structural basin
Chemical and isotopic signatures for black shales serve as potential proxies for reconstruction of paleoenvironmental conditions. Here we bring Rock‐Eval, major and trace element and Re‐Os isotopic data together to examine the environmental record at the Cambrian‐Ordovician Global Stratotype Section and Point (GSSP) at Green Point in western Newfoundland, Canada. The Green Point shales are oil mature and contain Type II organic material of marine origin. A Re‐Os isochron for the shales provides the first radiometric age for shale deposition at the GSSP at 484 ± 16 Ma, with an initial 187 Os/ 188 Os ratio of 0.74 ± 0.05 (Model 3 age; MSWD = 21; n = 13; 2 σ uncertainties). Factor analysis of the geochemical data set shows association of most trace elements with total organic carbon (TOC) and S contents, ensuring an authigenic origin for most elements and hence, their validity for evaluating the paleo‐redox state. Relatively high‐enrichment factors for redox‐sensitive elements (e.g., Re, U, and Mo) compared to average shale, but comparatively low enrichment compared to modern Black Sea sediments, suggest deposition in anoxic, but not euxinic waters. Comparison of Lower Ordovician shale geochemistry data sets at a global scale leads us to suggest that anoxic conditions and warm oceanic regimes were restricted to the margins of Laurentia and Baltica, whereas depositional basins with colder waters (e.g., Avalonia and Gondwana) were less reducing. These outcomes underscore the important role of paleogeography in regulating oceanic conditions and marine life.