--> Abstract: Re-Os Time Lines Crack Stratigraphic Puzzles in the Barents and Adjacent Basins, by Judith L. Hannah and Holly J. Stein; #90130 (2011)

Datapages, Inc.Print this page

Re-Os Time Lines Crack Stratigraphic Puzzles in the Barents and Adjacent Basins

Judith L. Hannah1 and Holly J. Stein2
1Geological Survey of Norway, Trondheim, Norway.
2AIRIE Program, Colorado State University, Fort Collins, CO.

The complex interplay of tectonism and sedimentation across the Barents Sea and adjacent basins confounds basin-scale correlations. Re-Os geochemistry of organic carbon-rich shales helps crack the puzzles by establishing time-lines directly tied to other stratigraphic constraints. For example, in the Barents Sea, Middle Triassic black shales defined as lithostratigraphic equivalents, are shown to be temporally correlated [1]. Small differences in Re-Os ages suggest that overlying delta and pro-delta sediments advanced from the southeast. Similarly, identical Re-Os ages and initial 187Os/188Os, and strikingly high 187Re/188Os ratios all confirm correlation of Upper Permian shales between East Greenland and the Mid-Norwegian shelf [2].

Re-Os geochronology also refines Earth’s time scale by calibrating biostratigraphy. Ammonoid zones in the Botneheia Formation show that the Blanknuten source rocks span most of the Anisian and Ladinian stages (Middle Triassic). Re-Os ages approach 240 Ma at the Anisian-Ladinian stage boundary [1], verifying correlation of biozones between Boreal and Tethyan realms despite the absence of common species. These new age constraints support the Alternate Triassic time scale proposed in Ogg et al. [3]. New Re-Os ages bracketing the Ladinian-Carnian stage boundary similarly support the Alternate Triassic time scale - and an incredible shrinking Ladinian.

Examples from Archean to Cretaceous show that Re-Os shale chronology works. But under what conditions? Oxidized material must be avoided, but how much exposure is too much? Hydrocarbon expulsion does not disturb the shale chronometer, but what about introduced exotic hydrocarbons? Sampling strategies for Re-Os shale geochronology demand a delicate balance on a fulcrum of geologic information. Necessary decisions include the type of material sampled (composition, mineralogy, texture, and structure), the proximity of the sampled interval to lithologic variations, the span of the sampled stratigraphic interval, and the size of sample homogenized for a single isotopic analysis. We contend that there are no firm rules for these decisions. Rather, each section must be evaluated on its own merit, using lithology, permeability, heterogeneity, quality of organic material and other related geologic factors. We will demonstrate how by example.
References:
[1] G. Xu et al. (2009) Earth and Planetary Science Letters, v. 288, p. 581-587.
[2] S. Georgiev et al. (2010) Geochimica et Cosmochimica Acta, v. 74, no. 12, Sup. 1, p. A324.
[3] Ogg et al. (2008) The Concise Geological Time Scale, Cambridge University Press, 177 p.
Supported by Petromaks contract 180015/S30 to NGU in partnership with the AIRIE Program. Thanks to SINTEF, GEUS, and NPD for core access, G. Xu and S. Georgiev for analyses and endless insights, and B. Bingen, H.M. Weiss, A. Mørk, S. Piasecki, and B. Lundschien for sampling assistance, geologic knowledge, and open discussion.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.

������������