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Early diagenetic carbonate precipitation and pore fluid migration in the Kimmeridge Clay of Dorset, England
Author(s) -
IRWIN HILARY
Publication year - 1980
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1980.tb01648.x
Subject(s) - diagenesis , geology , carbonate , dolomite , dolomitization , cementation (geology) , geochemistry , concretion , mineralogy , geomorphology , cement , chemistry , materials science , organic chemistry , facies , structural basin , metallurgy
ABSTRACT In the argillaceous sequence of Kimmeridge Clay a carbonate rich bed is composed of ferroan dolomite cement with varying amounts of excess CaCO 3 , and Fe 2+ substitution in the Mg 2+ sites. The isotopic and chemical compositions change symmetrically about the centre of the band proving that it grew by vertical accretion during diagenesis. Textural and isotopic evidence shows that growth centred on a horizon rich in primary carbonate which became dolomitized and assimilated during production of diagenetic carbonate. This accounts for the lateral extent of the concretion. Early central diagenetic carbonate was produced from organic matter by bacterial fermentation (δ 13 C =+0.59‰) and later marginal carbonate by abiotic breakdown, (δ 13 C tending towards — 2.73‰). δ 18 O values range from — 1.56 to — 4.46‰ because the dolomite precipitated during progressive burial. As burial increased, magnesium, whose dominant source was trapped seawater, became depleted while the relative availability of Fe 2+ , whose source was dominantly reduced detrital oxides, increased. Dolomitization and the source of diagenetic components for dolomite formation are discussed. Diffusion and pore fluid migration transported ions to the site of precipitation. Early cementation of the band served to influence pore fluid migration, but thereafter pore fluid migration controlled carbonate precipitation.