Journal
PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY
Volume 411, Issue -, Pages 267-280Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.palaeo.2014.07.005
Keywords
Strontium isotope stratigraphy; Danish Upper Oligocene-Miocene succession
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One hundred and fifty-six Sr-87/Sr-86 analyses have been performed on 129 samples from 18 outcrops and boreholes in Oligocene-Miocene deposits from Jylland, Denmark. These analyses were mainly conducted on mollusc shells but foraminiferal tests, Bolboforma and one shark tooth were also analysed. The main purpose of the study is to compare the ages of the Danish succession suggested by the biostratigraphic zonation on dinoflagellate cysts (Dybkjmr and Piasecld, 2010) with the ages based on analyses of the Sr-87/Sr-86 composition of marine calcareous fossils in the same succession. Analyses of samples from the Danish Brejning, Vejle Fjord, Klintinghoved, Arnum, Odderup, Hodde, Ornhej and Gram formations gave ages between 25.7 My (late Oligocene) and 10.3 My (late Miocene). The Sr isotope ages from the lower part of the succession, i.e. Brejning to Odderup formations, agree with the age estimates based on biostratigraphy. However, the Sr-87/Sr-86 ratios of fossil carbonates from the middle-upper Miocene, Hodde to Gram succession consistently indicate ages older than those recorded by biostratigraphy. Post-depositional processes as an explanation for this offset are inconsistent with good preservation of shell material and little reworking. A palaeoenvironmental cause for the observed mismatch is therefore indicated. Search for geological events that could explain the older ages obtained by Sr isotope compositions have not led to any conclusions and we had recognised the same problem in earlier reports and communications. We conclude that this is a general and possibly global, middle-late Miocene problem that has to be reconsidered and explained geologically. (c) 2014 The Authors. Published by Elsevier EN.
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