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Origin of quartz cements in some sandstones from the Jurassic of the Inner Moray Firth (UK)
Author(s) -
VAGLE GRETE BLOCK,
HURST ANDREW,
DYPVIK HENNING
Publication year - 1994
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.1365-3091.1994.tb01411.x
Subject(s) - quartz , sponge spicule , geology , cementation (geology) , chalcedony , mineralogy , cement , materials science , composite material , paleontology
The extent of quartz cementation in shallow marine sandstones of the Brora Arenaceous Formation (Oxfordian) is closely related to the occurrence and abundance of Rhaxella perforata sponge spicules. Three cement morphologies are identified, chalcedonic quartz, microquartz and mesoquartz. Chalcedonic quartz forms matrix‐supported cements which preserve moulds of Rhaxella spicules. Chalcedonic quartz crystals have inequant development of crystal faces, on average 0·1 μm in diameter, and are the first formed cement and reveal homogeneous dark grey tones on the SEM‐CL/BEI. Microquartz forms 5–10 μm diameter crystals, which commonly grow on chalcedonic quartz substrates and show various grey tones under SEM‐CL/BEI. Mesoquartz crystals grow in optical continuity with their host grains, have >20 μm a ‐axial diameter crystals, and exhibit distinctly zoned luminescence. Although no opaline silica is preserved, the quartz cement is interpreted to have formed from an opaline precursor. Detrital quartz has an average δ 18 O composition of + 12·2‰ and mesoquartz (syntaxial overgrowth) has an average δ 18 O composition of +20·0‰. Estimates of the δ 18 O compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types; mixtures of the two give consistently higher δ 18 O compositions than mesoquartz, the higher estimate being +39·2‰. From oxygen isotope data the formation of quartz, microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71°C in marine derived pore waters. Organic and inorganic maturation data constrain the upper temperature limit to less than 60°C.