Carbonate cements constrain the burial history of the Portledge-Peppercombe Permian outlier, North Devon

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Authors:Burley, S. D.; Cornford, C.
Author Affiliations:Primary:
BG Technology, Loughborough, United Kingdom
IGI, United Kingdom
Volume Title:Geoscience in South-west England
Volume Authors:Williams, Brian J., editor
Source:Proceedings of the Ussher Society, Vol.9(Part 3), p.188-202; Annual conference of the Ussher Society, Bideford, United Kingdom, Jan. 1998, edited by Brian J. Williams. Publisher: Ussher Society, Bristol, United Kingdom. ISSN: 0566-3954
Publication Date:1998
Note:In English. 31 refs.; illus., incl. sects., 4 plates, 6 tables, geol. sketch map
Summary:A sequence of carbonate cements is described for the Permo-Triassic coarse clastic fill of the Portledge-Peppercombe half-graben, North Devon. The cements comprise sparry calcites in conglomerates with high intergranular volumes (IGVs), a variety of microsparry pedogenic nodules and rhizoconcretions, and later dolomite-ankerite cements in conglomerates with low IGVs. Fluid inclusions within sparry calcite have variable liquid to vapour ratios (LVR) and measured homogenisation temperatures (Th) of between 59°C and 148°C, indicating resetting due to stretching. If the lowest Th values record minimum trapping temperatures, then oxygen isotopes indicate these cements formed from an evolved water with a δ180 composition of 0 per mil to +10 per mil SMOW. These sparry calcites are interpreted to be calcretes that have recrystallised at moderate subsurface temperatures. Fluid inclusions in dolomite-ankerite have consistent LVRs and values that record minimum Th values of 69°C, these being the minimum temperatures attained by the sediments. Assuming formation at these temperatures, oxygen isotope compositions indicate that the dolomites precipitated from water of δ180 composition of +2 per mil to +3 oer mil SMOW. The aqueous fluids in the calcite inclusions are of low salinity, approximating to 0.5 wt%NaCl equivalents, but are moderately saline in the dolomite-ankerite cements as determined from depression of freezing points. Thus early diagenetic cements and later dolomite-ankerite both yield minimum precipitation temperatures of ca 60-70°C in the redbed fill of the Portledge-Peppercombe half-graben. These temperatures are consistent with the prediction of burial history and thermal modelling if the Mesozoic. Tertiary thicknesses measured from well and seismic data in the main Bristol Channel graben and from adjacent onshore areas are projected as the former cover for the study area. An elevated palaeo-heat flow and/or an hydrothermal flux through these Permo-Triassic redbeds as a means of attaining the predicted temperatures are considered and rejected. The thermal geo-history modelling places the timing of major uplift and erosion as an Oligo-Miocene event post-dating the Lundy intrusion.
Subsections:Sedimentary petrology
Subjects:Calcitization; Calcrete; Carbonate rocks; Carbonatization; Cement; Cenozoic; Diagenesis; Faults; Fluid inclusions; Geologic thermometry; Half grabens; Inclusions; Isotopes; Mesozoic; Miocene; Models; Neogene; O-18; Oligocene; Outliers; Oxygen; Paleogene; Paleozoic; Permian; Precipitation; Recrystallization; Rhizoliths; Sedimentary rocks; Sedimentary structures; Sediments; Stable isotopes; Tertiary; Triassic; Unconformities; Uplifts; England; Europe; Great Britain; South-West England; United Kingdom; Western Europe; North Devon; Peppercombe England; Portledge England
Coordinates:N510000 N510000 W0041800 W0041800
Abstract Numbers:99M/3259
Record ID:1999047753
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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