Carbonate Cementation of Granular and Fracture Porosity: Implications for the Cenozoic Hydrologic Development of the Peru Continental Margin

Suess, Erwin and Thornburg, T. M. (1990) Carbonate Cementation of Granular and Fracture Porosity: Implications for the Cenozoic Hydrologic Development of the Peru Continental Margin Proceedings of the Ocean Drilling Program: Scientific Results, 112 . pp. 95-109. DOI 10.2973/

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The evolution of pore fluids migrating through the forearc basins, continental massif, and accretionary prism of the Peru margin is recorded in the sequence of carbonate cements filling intergranular and fracture porosities. Petrographic, mineralogic, and isotopic analyses were obtained from cemented clastic sediments and tectonic breccias recovered during Leg 112 drilling. Microbial decomposition of the organic-rich upwelling facies occurs during early marine diagenesis, initially by sulfate-reduction mechanisms in the shallow subsurface, succeeded by carbonate reduction at depth. Microcrystalline, authigenic cements formed in the sulfate-reduction zone are 13C-depleted (to -20.1%c PDB), and those formed in the carbonate-reduction zone are 13C-enriched (to +19.0%c PDB). Calcium-rich dolomites and near-stoichiometric dolomites having uniformly heavy S180 values (+2.7 to +6.6%c PDB) are typical organic decomposition products. Quaternary marine dolomites from continental-shelf environments exhibit the strongest sulfate-reduction signatures, suggesting that Pleistocene sea-level fluctuations created a more oxygenated water column, caused periodic winnowing of the sediment floor, and expanded the subsurface penetration of marine sulfate. We have tentatively identified four exotic cement types precipitated from advected fluids and derived from the following diagenetic environments: (1) meteoric recharge, (2) basalt alteration, (3) seafloor venting and (4) hypersaline concentration. Coarsely crystalline, low-magnesium (Lo-Mg) calcite cements having pendant and blocky-spar morphologies, extremely negative S180 values (to -7.5%o PDB), and intermediate S13C values (-0.4%c to +4.6%c PDB) are found in shallow-marine Eocene strata. These cements are evidently products of meteoric diagenesis following subaerial emergence during late Eocene orogenic movements, although the strata have since subsided to greater than 4,000 m below sea level. Lo-Mg calcite cements filling scaly fabrics in the late Miocene accretionary prism sediments are apparently derived from fluids having lowered magnesium/calcium (Mg/Ca) and 18Q/16Q ratjos; such fluids may have reacted with the subducting oceanic crust and ascended through the forearc along shallow-dipping thrust faults. Micritic, high-magnesium (Hi-Mg) calcite cements having extremely depleted 513C values (to -37.3%c PDB), and a benthic fauna of giant clams (Calyptogena sp.) supported by a symbiotic, chemoautotrophic metabolism, provide evidence for venting of methane-charged waters at the seafloor. Enriched 5180 values (to +6.6%c PDB) in micritic dolomites from the continental shelf may be derived from hypersaline fluids that were concentrated in restricted lagoons behind an outer-shelf basement ridge, reactivated during late Miocene orogenesis.

Document Type: Article
Keywords: Carbonate cementaion, Cenozoic, Peru Continental Margin
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Refereed: No
DOI etc.: 10.2973/
ISSN: 1096-7451
Projects: ODP
Date Deposited: 13 May 2019 12:32
Last Modified: 13 May 2019 12:32

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