Field relationships and stable isotope geochemistry of concretions from James Ross Island, Antarctica

D PIRRIE, J.D. Marshall

Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid


Early diagenetic (precompactional) concretions are abundant throughout the Cretaceous-Tertiary Marambio Group Larsen Basin, Antarctica. Four distinct concretion types are recognised: (1) spherical-subspherical concretions; (2) sheet concretions; (3) fossil-nucleated concretions; and (4) concretionary burrow networks. All concretion types have a micritic to microsparry variably non-ferroan to ferroan calcite cement. Stable isotope analyses show a wide spread in both delta-O-18 and delta-C-13. Delta-C-13 values are typically negative, ranging between -3.38 and -39.15 parts per thousand (PDB) (usually -16 to -30 parts per thousand). Delta-O-18 ranges between -1.28 and -13.81 parts per thousand (PDB) with most of the values between -5 and -10 parts per thousand.

The delta-C-13 signature is interpreted to represent carbon sourced from sulphate reduction and/or methane oxidation, with minor input from shell dissolution, and is consistent with a shallow burial, early diagenetic origin. A single mudstone hosted concretion has a delta-O-18 composition indicative of precipitation of carbonate from seawater. The low delta-O-18 signatures in the sandstone- and siltstone-hosted concretions are possibly due to early diagenetic modification of the pore water composition through volcaniclastic mineral dissolution/reprecipitation reactions and perhaps through input of meteoric water. Concretion distribution is related to (a) changes in sedimentation rate and (b) the dominance of diffusion on concretion cementation.

Iaith wreiddiolSaesneg
Tudalennau (o-i)137-150
Nifer y tudalennau14
CyfnodolynSedimentary Geology
Rhif cyhoeddi3-4
Dynodwyr Gwrthrych Digidol (DOIs)
StatwsCyhoeddwyd - Mai 1991

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