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Ediacaran–Cambrian phosphorites from the western margins of Gondwana and Baltica

Author:
  • J. Javier Álvaro
  • Graham A. Shields-Zhou
  • Per Ahlberg
  • Sören Jensen
  • Teodoro Palacios
Publishing year: 2016
Language: English
Pages: 350-377
Publication/Series: Sedimentology
Volume: 63
Issue: 2
Document type: Journal article
Publisher: Wiley-Blackwell

Abstract english

The late Ediacaran–mid Cambrian occurrence s of phosphorites in the wes-
tern Mediterranean region (West Gondwana) and southern Sweden (north -
west Baltica) are related to the poleward drift of West Gondwana and the
northern drift of Baltica. As a result, these regions crossed subtropical and
temperate palaeolatitudes of the Southern Hemisphere, in which oceanic
upwelling and high organic productivity were the probable sources of phos-
phorus along their western margins. A parallel south-west/north-east migra-
tion trend of evaporites and delayed, time transgressive phosphorites point
to distal upwelling environments associated with shallower arid conditions.
Concentration of hardground-derived phosphate accumulations took place
by repeated alternations of low sedimentation rates and condensation,
in situ early diagenetic and microbially mediated precipitation of amorphous
and cryptocrystalline, carbonate-bearing francolite, winnowing and poly-
phase reworking of previously phosphatized skeletons and hardground-
derived clasts. Thicker phosphorites of economic importance display a
spiculate sponge-thromboid consortium that points to a distinct benthic
community developed on suboxic, slope to basinal substrates. For most
phosphorite samples an up to ten-fold enrichment in rare earth element
concentration relative to post-Archaean average Australian shale reflects sig-
nificant incorporation of rare earth elements and yttrium after deposition.
Considerable scavenging took place from anoxic pore waters, following
reductive dissolution of iron oxides as evidenced from the medium rare
earth element enrichment in most samples. This contrasts with the rare pre-
servation of primary sea water features, such as cerium depletion and
yttrium enrichment, in phosphatized thrombolites from the shallower realms
(for example, Montagne Noire). The data are consistent with phosphogenesis
after upwelling and transgression of nutrient-rich, suboxic–anoxic, ferrugi-
nous waters over more oxygenated, rifting settings and inland portions of an
evolving passive margin.

Keywords

  • Geology

Other

Published
  • ISSN: 0037-0746
Per Ahlberg
E-mail: per [dot] ahlberg [at] geol [dot] lu [dot] se

Professor

Lithosphere and Biosphere Science

+46 46 222 78 70

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