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Time Lines, Time Scales, and Correlations — Getting It Right

Hannah, Judith L.*1; Stein, Holly J.2
(1) AIRIE Program, Geosciences, Colorado State University, Fort Collins, CO.
(2) Geological Survey of Norway, 7491 Trondheim, Norway.

Re-Os isotope dating of black shales offers a solid approach for tracking time through stratigraphic sequences, complementing U-Pb dating of ash layers for placing relative time into a chronometric framework. At basin scales, time lines refine correlations and document time-transgressive horizons. At global scales, radiometric time pins provide correlations where biostratigraphy is not definitive. Using examples from the Norwegian Arctic, East Greenland, and Western Newfoundland, we integrate chronostratigraphy with other stratigraphic controls to enhance knowledge of basin evolution (Xu et al. 2009, EPSL 288: 581-587; Georgiev et al. 2011, EPSL in press).

Re-Os chronology has a unique advantage - it is based on analysis of source rocks, the same units that reveal hydrocarbon potential and paleo-environmental conditions. But there are pitfalls, mostly tied to sampling. A Re-Os age is calculated by linear regression of data from multiple samples, assuming that all samples are the same age, have the same initial 187Os/188Os, and have neither gained nor lost Re or Os. Samples used for each age must contain sufficiently diverse organic compounds to generate a spread in 187Re/188Os, yet be sufficiently close stratigraphically to preclude variation in age and initial 187Os/188Os.

Thus, sampling is crucial in three fundamental ways:

(1) Know what is being analyzed. Shales are not monotonous accumulations; rather, they contain mm-scale variations in grain size, mineralogy, metal concentrations, amount and type of organic material, and depositional mechanism. Short hiatuses (depositional or erosional) and intervals of bioturbation may be subtle; detailed petrography is essential.

(2) Minimize stratigraphic interval sampled and size of each sample. Primary features are obliterated if samples are too large. Samples must be large enough to homogenize post-depositional intergranular exchange of Re or Os, but small enough to preserve 187Re/188Os variations. Each setting is unique; there are no magic threshold numbers.

(3) Avoid oxidation, which may occur at any time between deposition and sampling. Surficial weathering can be avoided by using drill core, man-made exposures, or outcrops where mechanical weathering outstrips chemical alteration. Inexpensive geochemical screening techniques may reveal cryptic disturbances in Re-Os systematics.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California